Esquivel-Sirvent, Raul; Schatz, George
2014-03-01
The theory of generalized van der Waals forces by Lifshtz when applied to optically anisotropic media predicts the existence of a torque. In this work we present a theoretical calculation of the van der Waals torque for two systems. First we consider two isotropic parallel plates where the anisotropy is induced using an external magnetic field. The anisotropy will in turn induce a torque. As a case study we consider III-IV semiconductors such as InSb that can support magneto plasmons. The calculations of the torque are done in the Voigt configuration, that occurs when the magnetic field is parallel to the surface of the slabs. The change in the dielectric function as the magnetic field increases has the effect of decreasing the van der Waals force and increasing the torque. Thus, the external magnetic field is used to tune both the force and torque. The second example we present is the use of the torque in the non retarded regime to align arrays of nano particle slabs. The torque is calculated within Barash and Ginzburg formalism in the nonretarded limit, and is quantified by the introduction of a Hamaker torque constant. Calculations are conducted between anisotropic slabs of materials including BaTiO3 and arrays of Ag nano particles. Depending on the shape and arrangement of the Ag nano particles the effective dielectric function of the array can be tuned as to make it more or less anisotropic. We show how this torque can be used in self assembly of arrays of nano particles. ref. R. Esquivel-Sirvent, G. C. Schatz, Phys. Chem C, 117, 5492 (2013). partial support from DGAPA-UNAM.
Five-body van der Waals interactions
Han, Jianing
2017-06-01
We report on the five-body repulsive and attractive van der Waals interactions between the strongly dipole-dipole coupled Rydberg states. Compared to four-body van der Waals interactions, five-body van der Waals interactions show more energy levels and more potential wells caused by avoided crossings. This research bridges the few-body physics and many-body physics. Other disciplines, such as chemistry, biology, and medical fields, will also benefit from better understanding van der Waals interactions.
Dynamical and non-additive atomic van der Waals phases
Impens, François; Neto, Paulo A Maia
2013-01-01
We investigate dynamical corrections to the van der Waals phase induced by the non-unitary evolution of atomic waves propagating near a perfectly conducting surface. These corrections reflect the interplay between field retardation effects and the dynamics of the external atomic degrees of freedom. The dynamical atom-surface interaction phase shift contains both local and non-local contributions. We derive the local contributions and show that they are equivalent to coarse-graining the instantaneous van der Waals potential over the time scale corresponding to the round-trip travel time of light between atom and surface. We show that the non-local phase contributions are non-additive, and propose to use this property in a multiple-path van der Waals interferometer in order to isolate them from the standard (and much larger) quasi-static van der Waals phase.
Aruna Rajagopal
2014-10-01
Full Text Available In the context of extended phase space, where the negative cosmological constant is treated as a thermodynamic pressure in the first law of black hole thermodynamics, we find an asymptotically AdS metric whose thermodynamics matches exactly that of the Van der Waals fluid. We show that as a solution of Einstein's equations, the corresponding stress energy tensor obeys (at least for certain range of metric parameters all three weak, strong, and dominant energy conditions.
Rememorant a Mies van der Rohe
Gómez Montenegro, Carlos
1999-01-01
Imatges del primer gratacels de Mies Van Der Rohe a través dels mitjans tècnics actuals. Imágenes del primer rascacielos de Mies Van Der Rohe através de los medios técnicos actuales. Peer Reviewed
Professor Willem van der Angeliaan / Aive Sarjas
Sarjas, Aive
2010-01-01
Läänemaal Koluvere hooldekodus kord nädalas tegevusteraapia teenust osutavast väikesest šetlandi tõugu ruunast Willem van der Angeliaanist ning tegevusterapeudi Moonika Salumaa tegevusteraapias kasutatavatest teooriatest ja meetoditest
Professor Willem van der Angeliaan / Aive Sarjas
Sarjas, Aive
2010-01-01
Läänemaal Koluvere hooldekodus kord nädalas tegevusteraapia teenust osutavast väikesest šetlandi tõugu ruunast Willem van der Angeliaanist ning tegevusterapeudi Moonika Salumaa tegevusteraapias kasutatavatest teooriatest ja meetoditest
Van der Waals Interactions and Exciton Condensation
Handel, P. H.; Kittel, C.
1971-01-01
It is shown that the van der Waals interaction can lead at low temperatures to a condensed state of excitons with properties in qualitative agreement with the observations of exciton droplets. Our calculation gives a binding energy of the correct sign and magnitude for the exciton condensate. In a diclectric medium, the strong enhancement of the exciton polarizability leads to a giant van der Waals interaction, and this interaction appears to make possible a condensed exciton phase. PMID:16591958
Integrable extended van der Waals model
Giglio, Francesco; Landolfi, Giulio; Moro, Antonio
2016-10-01
Inspired by the recent developments in the study of the thermodynamics of van der Waals fluids via the theory of nonlinear conservation laws and the description of phase transitions in terms of classical (dissipative) shock waves, we propose a novel approach to the construction of multi-parameter generalisations of the van der Waals model. The theory of integrable nonlinear conservation laws still represents the inspiring framework. Starting from a macroscopic approach, a four parameter family of integrable extended van der Waals models is indeed constructed in such a way that the equation of state is a solution to an integrable nonlinear conservation law linearisable by a Cole-Hopf transformation. This family is further specified by the request that, in regime of high temperature, far from the critical region, the extended model reproduces asymptotically the standard van der Waals equation of state. We provide a detailed comparison of our extended model with two notable empirical models such as Peng-Robinson and Soave's modification of the Redlich-Kwong equations of state. We show that our extended van der Waals equation of state is compatible with both empirical models for a suitable choice of the free parameters and can be viewed as a master interpolating equation. The present approach also suggests that further generalisations can be obtained by including the class of dispersive and viscous-dispersive nonlinear conservation laws and could lead to a new type of thermodynamic phase transitions associated to nonclassical and dispersive shock waves.
Boon, E.F.; Boot, J.; Vahl, L.; Van Eldik Thieme, H.C.A.; Blok, H.
1959-01-01
Samenvatting van de toespraak door Prof.ir, R. van Hasselt bij de officiele opening van vier nieuwe laboratoriavan de Afdeling der Werktuigbouwkunde Boon, E.F. en Boot, J.: het Laboratorium voor Chemische Werktuigen Vahl, L.: het Laboratorium voor Koudetechniek Van Eldik Thieme, H.C.A.: het
Walter van der Cruijsen / Walter van der Cruijsen ; interv. Tilman Baumgärtel
Cruijsen, Walter van der
2006-01-01
1997. a. Berliinis tehtud intervjuu hollandi maali, võrgu- ja installatsioonikunstniku Walter van der Cruijseniga (sünd. 1958), kes kuulub Euroopa ühe edukama 1990-ndatel loodud võrguprojekti De Digitale Stad Amsterdam loojate hulka
Walter van der Cruijsen / Walter van der Cruijsen ; interv. Tilman Baumgärtel
Cruijsen, Walter van der
2006-01-01
1997. a. Berliinis tehtud intervjuu hollandi maali, võrgu- ja installatsioonikunstniku Walter van der Cruijseniga (sünd. 1958), kes kuulub Euroopa ühe edukama 1990-ndatel loodud võrguprojekti De Digitale Stad Amsterdam loojate hulka
Van der Waals forces and spatial dispersion
2009-01-01
A version of the Green's functions theory of the Van der Waals forces which can be conveniently used in the presence of spatial dispersion is presented. The theory is based on the fluctuation-dissipation theorem and is valid for interacting bodies, separated by vacuum. Objections against theories acounting for the spatial dispersion are discussed.
Women Screenwriters : Paula van der Oest
Willemsen, Steven; Nelmes, Jill; Selbo, Jule
2015-01-01
Contribution discussing the work of screenwriter and director Paula van der Oest (Laag-Soeren, 1965), who has been prolific in Dutch film and television from the 1990s up to the present, and whose screenplays are characterized by an empathetic take on societal and interpersonal themes.
Cluster ions and van der Waals molecules
Smirnov, Boris M
1992-01-01
This review discusses current ideas in the physics and chemistry of cluster ions and Van der Waals molecules as well as presenting numerical data on their parameters and the processes involving them. It is also a detailed reference on basic data relating to many species.
Thermodynamics of Van der Waals Fluids with quantum statistics
Redlich, Krzysztof
2016-01-01
We consider thermodynamics of the van der Waals fluid of quantum systems. We derive general relations of thermodynamic functions and parameters of any ideal gas and the corresponding van der Waals fluid. This provides unambiguous generalization of the classical van der Waals theory to quantum statistical systems. As an example, we apply the van der Waals fluid with fermi statistics to characterize the liquid-gas critical point in nuclear matter. We also introduce the Bose-Einstein condensation in the relativistic van der Waals boson gas, and argue, that it exhibits two-phase structure separated in space.
Thermal response in van der Waals heterostructures
Naidu Gandi, Appala; Alshareef, Husam N.; Schwingenschlögl, Udo
2017-01-01
We solve numerically the Boltzmann transport equations of the phonons and electrons to understand the thermoelectric response in heterostructures of M2CO2 (M: Ti, Zr, Hf) MXenes with transition metal dichalcogenide monolayers. Low frequency optical phonons are found to occur as a consequence of the van der Waals bonding, contribute significantly to the thermal transport, and compensate for the reduced contributions of the acoustic phonons (increased scattering cross-sections in heterostructures), such that the thermal conductivities turn out to be similar to those of the bare MXenes. Our results indicate that the important superlattice design approach of thermoelectrics (to reduce the thermal conductivity) may be effective for two-dimensional van der Waals materials when used in conjunction with intercalation.
Thermal response in van der Waals heterostructures
Gandi, Appala
2016-11-21
We solve numerically the Boltzmann transport equations of the phonons and electrons to understand the thermoelectric response in heterostructures of M2CO2 (M: Ti, Zr, Hf) MXenes with transition metal dichalcogenide monolayers. Low frequency optical phonons are found to occur as a consequence of the van der Waals bonding, contribute significantly to the thermal transport, and compensate for the reduced contributions of the acoustic phonons (increased scattering cross-sections in heterostructures), such that the thermal conductivities turn out to be similar to those of the bare MXenes. Our results indicate that the important superlattice design approach of thermoelectrics (to reduce the thermal conductivity) may be effective for two-dimensional van der Waals materials when used in conjunction with intercalation. © 2016 IOP Publishing Ltd.
van der Waals Heterostructures Grown by MBE
Hinkle, Christopher
In this work, we demonstrate the high-quality MBE heterostructure growth of various layered 2D materials by van der Waals epitaxy (VDWE). The coupling of different types of van der Waals materials including transition metal dichalcogenide thin films (e.g., WSe2, WTe2, HfSe2) , insulating hexagonal boron nitride (h-BN), and topological insulators (e.g., Bi2Se3) allows for the fabrication of novel electronic devices that take advantage of unique quantum confinement and spin-based characteristics. The relaxed lattice-matching criteria of van der Waals epitaxy has allowed for high-quality heterostructure growth with atomically abrupt interfaces, allowing us to couple these materials based primarily on their band alignment and electronic properties. We will discuss the impact of sample preparation, surface reactivity, and lattice mismatch of various substrates (sapphire, graphene, TMDs, Bi2Se3) on the growth mode and quality of the films and will discuss our studies of substrate temperature and flux rates on the resultant growth and grain size. Structural and chemical characterization was conducted via reflection high energy electron diffraction (RHEED, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning tunneling microscopy/spectroscopy (STM/S), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Experimentally determined band alignments have been determined and compared with first-principles calculations allowing the design of novel low-power logic and magnetic memory devices. Initial results from the electrical characterization of these grown thin films and some simple devices will also be presented. These VDWE grown layered 2D materials show significant potential for fabricating novel heterostructures with tunable band alignments and magnetic properties for a variety of nanoelectronic and optoelectronic applications.
Dr Jacob van der Land, marine biologist extraordinary
Bruggen, van A.C.
2001-01-01
This contribution is an attempt to sketch the life and works of Dr Jacob van der Land, curator of worms and chief marine biologist of the National Museum of Natural History, on the occasion of his official retirement. Born in 1935, Jacob van der Land read biology at Leiden University (1958-1964), wh
Dr Jacob van der Land, marine biologist extraordinary
Bruggen, van A.C.
2001-01-01
This contribution is an attempt to sketch the life and works of Dr Jacob van der Land, curator of worms and chief marine biologist of the National Museum of Natural History, on the occasion of his official retirement. Born in 1935, Jacob van der Land read biology at Leiden University (1958-1964),
Nadja Louwerse
2010-03-01
Full Text Available Dutch nautical terms in Russian. Anna Croiset van der Kop versus Reinder van der Meulen This article explores the controversy in the beginning of the 20th century between de Dutch slavists Anna Croiset van der Kop and Reindert van der Meulen, author of the book De Hollandsche Zee- en Scheepstermen in het Russisch (Dutch Nautical Terms in Russian. After publication, she reacted with an undeniably negative and unusually elaborate review (72 pages!, written not in Dutch, but in Russian and published in one of the most leading, scientific series in Russia. Van der Meulen has never directly reacted to this review: in 1959 he published a supplement to his earlier work from 1909: Nederlandse woorden in het Russisch (Dutch words in Russian, not mentioning Croiset van der Kops name, nor her review. Obviously he felt hurt by her merciless criticism. The article focuses on the motives underlying her strong reaction. It will be argued that her severe criticism was not really inspired by Van der Meulen’s subject, but by his failing research methods. Furthermore, it will be pointed out that this review seemed to offer her an opportunity to measure up to her Dutch colleagues and to present herself internationally as a slavist. On the basis of the now available data it is hard to say, which motives really brought Croiset van der Kop to write her review. Investigation of her archives, preserved in St- Petersburg, and until now hardly studied, may perhaps reveal her real motives.
Interpretation of van der Waal density functionals
Hyldgaard, Per; Schröder, Elsebeth
2014-01-01
The nonlocal correlation energy in the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92, 246401 (2004); Phys. Rev. B 76, 125112 (2007); Phys. Rev. B 89, 035412 (2014)] can be interpreted in terms of a coupling of zero-point energies of characteristic modes of semilocal exchange-correlation (xc) holes. These xc holes reflect the internal functional in the framework of the vdW-DF method [Phys. Rev. B 82, 081101(2010)]. We explore the internal xc hole components, showing that they share properties with those of the generalized-gradient approximation. We use these results to illustrate the nonlocality in the vdW-DF description and analyze the vdW-DF formulation of nonlocal correlation.
Synchronization law for a van der Pol array.
Peles, Slaven; Wiesenfeld, Kurt
2003-08-01
We explore the transition to in-phase synchronization in globally coupled oscillator arrays, and compare results for van der Pol arrays with Josephson junction arrays. Our approach yields in each case an analytically tractable iterative map; the resulting stability formulas are simple because the expansion procedure identifies natural parameter groups. A third example, an array of Duffing-van der Pol oscillators, is found to be of the same fundamental type as the van der Pol arrays, but the Josephson arrays are fundamentally different owing to the absence of self-resonant interactions.
The van der Waals fluid and its role in cosmology
Jantsch, Rudinei C. S.; Christmann, Marcus H. B.; Kremer, Gilberto M.
2016-01-01
We analyze the properties of a generic cosmological fluid described by the van der Waals equation-of-state. Exact solutions for the energy-density evolution are found as implicit functions of the scale factor for a flat Friedmann-Robertson-Walker (FRW) spacetime. The possible values of the free parameter in the van der Waals equation are selected a posteriori, in accordance with asymptotic behaviors that are physically relevant. The stability of the model against small perturbations is studied through the hydrodynamic perturbations of the fluid for the relevant cases. It is found that a van der Waals fluid seems appropriate to describe noneternal inflationary scenarios.
The van der Waals fluid and its role in cosmology
Jantsch, Rudinei C S; Kremer, Gilberto M
2016-01-01
We analyze the properties of a generic cosmological fluid described by the van der Waals equation of state. Exact solutions for the energy density evolution are found as implicit functions of the scale factor for a flat Friedmann-Robertson-Walker space-time. The possible values of the free parameter in the van der Waals equation are selected \\emph{a posteriori}, in accordance with asymptotic behaviors that are physically relevant. The stability of the model against small perturbations is studied through the hydrodynamic perturbations of the fluid for the relevant cases. It is found that a van der Waals fluid seems appropriate to describe non-eternal inflationary scenarios.
The van der Waals force and gravitational force in matter
Zhang, Lei
2013-01-01
It was thought that the van der Waals force and gravitational force were distinct. Now a model is used to describe the attraction between macroscopic objects according to van der Waals interaction. The force between two objects with thermal equilibrium deviates from the law of universal gravitation slightly, and the gravity on the earth is explained approximately. We argue that the gravitational force is the van der Waals force actually. In other words, the gravitational force and mass are related to the quantum fluctuations of electron clouds in atoms, and these parameters are dictated by dielectric susceptibility.
Strong Van der Waals force in the hadron physics
Sawada, T
2000-01-01
Possible strong Van der Waals interaction between hadrons is searched in the P-wave amplitude of the pi-pi scattering by removing the cut of the two-pion exchange spectrum as well as the unitarity cut.
(AJST) ANALYSIS OF VAN DER WAAL EQUATION NEAR THE ...
Department of Physics, University of Nairobi, P.O. Box 30197, 00100 Nairobi ... some additional critical parameters and presents the ideal gas temperature as a lower limit to the van der ... sufficiently low pressures, the relations among these.
Lifshitz theory of van der Waals pressure in dissipative media
Zheng, Yi
2010-01-01
We derive a first--principles method of determining the van der Waals or Casimir pressure in a dissipative and dispersive planar multilayered system by calculating the Maxwell stress tensor in a fictitious layer of vacuum, that is eventually made to vanish, introduced in the structure. This is illustrated by calculating the van der Waals pressure in a thin film with dissipative properties embedded between two semi--infinite media.
Nano Electronics on Atomically Controlled van der Waals Quantum Heterostructures
2015-03-30
Final 3. DATES COVERED (From - To) 14 Aug 13 to 13 Feb 15 4. TITLE AND SUBTITLE Nano Electronics on Atomically Controlled van der Waals...OMB control number. 1. REPORT DATE 14 MAY 2015 2. REPORT TYPE Final 3. DATES COVERED 14-08-2013 to 13-02-2015 4. TITLE AND SUBTITLE Nano ...AOARD Grant 134122 “ Nano Electronics on Atomically Controlled van der Waals Quantum Heterostructures” 3/30/2015 Name of Principal
Modified Van der Waals equation and law of corresponding states
Zhong, Wei; Xiao, Changming; Zhu, Yongkai
2017-04-01
It is well known that the Van der Waals equation is a modification of the ideal gas law, yet it can be used to describe both gas and liquid, and some important messages can be obtained from this state equation. However, the Van der Waals equation is not a precise state equation, and it does not give a good description of the law of corresponding states. In this paper, we expand the Van der Waals equation into its Taylor's series form, and then modify the fourth order expansion by changing the constant Virial coefficients into their analogous ones. Via this way, a more precise result about the law of corresponding states has been obtained, and the law of corresponding states can then be expressed as: in terms of the reduced variables, all fluids should obey the same equation with the analogous Virial coefficients. In addition, the system of 3 He with quantum effects has also been taken into consideration with our modified Van der Waals equation, and it is found that, for a normal system without quantum effect, the modification on ideal gas law from the Van der Waals equation is more significant than the real case, however, for a system with quantum effect, this modification is less significant than the real case, thus a factor is introduced in this paper to weaken or strengthen the modification of the Van der Waals equation, respectively.
Van der Waals Force Assisted Heat Transfer
Sasihithlu, K.; Pendry, J. B.; Craster, R. V.
2017-02-01
Phonons (collective atomic vibrations in solids) are more effective in transporting heat than photons. This is the reason why the conduction mode of heat transport in nonmetals (mediated by phonons) is dominant compared to the radiation mode of heat transport (mediated by photons). However, since phonons are unable to traverse a vacuum gap (unlike photons), it is commonly believed that two bodies separated by a gap cannot exchange heat via phonons. Recently, a mechanism was proposed [J. B. Pendry, K. Sasihithlu, and R. V. Craster, Phys. Rev. B 94, 075414 (2016)] by which phonons can transport heat across a vacuum gap - through the Van der Waals interaction between two bodies with gap less than the wavelength of light. Such heat transfer mechanisms are highly relevant for heating (and cooling) of nanostructures; the heating of the flying heads in magnetic storage disks is a case in point. Here, the theoretical derivation for modelling phonon transmission is revisited and extended to the case of two bodies made of different materials separated by a vacuum gap. Magnitudes of phonon transmission, and hence the heat transfer, for commonly used materials in the micro- and nano-electromechanical industry are calculated and compared with the calculation of conduction heat transfer through air for small gaps as well as the heat transfer calculation due to photon exchange.
Simon van der Meer (1925-2011)
2011-01-01
Simon van der Meer was a true giant of modern particle physics, though a gentle one. His contributions to accelerator science remain vital for the operation of accelerators such as the LHC today. Simon was an electrical engineer who grew up in The Hague, moving on to Delft University to study electrical engineering. After a short stint with Philips, he came to CERN in 1956, just two years after the lab opened, and remained with us until his retirement in 1990. Simon was an incredibly inventive man. When confronted with a problem, he would sink into deep reflection, rarely emerging until he had a solution. One of us, Steve Myers, remembers him as a man who did not suffer fools gladly, and who was extremely taciturn. Simon would never use two words where one would suffice. But that one word would invariably be the right one. Simon is best known for his contribution to the SPS collider project, for which he was awarded the Nobel Prize, jointly with Carlo Rubbia, in 1984. Stochastic cooling, the in...
Van der Waals Interactions in Aspirin
Reilly, Anthony; Tkatchenko, Alexandre
2015-03-01
The ability of molecules to yield multiple solid forms, or polymorphs, has significance for diverse applications ranging from drug design and food chemistry to nonlinear optics and hydrogen storage. In particular, aspirin has been used and studied for over a century, but has only recently been shown to have an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin are degenerate in terms of lattice energy, kinetic effects have been suggested to determine the metastability of the less abundant form II. Here, first-principles calculations provide an alternative explanation based on free-energy differences at room temperature. The explicit consideration of many-body van der Waals interactions in the free energy demonstrates that the stability of the most abundant form of aspirin is due to a subtle coupling between collective electronic fluctuations and quantized lattice vibrations. In addition, a systematic analysis of the elastic properties of the two forms of aspirin rules out mechanical instability of form II as making it metastable.
THE PORTRAITS OF SIMON VAN DER STEL, FIRST GOVERNOR ...
van Riebeeck, and the other that of his first wife, Maria. Quevellerius (Fig ... Jan Weenix (attributed), Portrait of Simon van der Stel and his son Willem Adriaan. Canvas, 112 x ... In the distance we see five natives of a dark skin colour, dressed in ...
Beschrijving der ontwikkelingsstadia van Thosea asigna V.E.
Eecke, van R.
1930-01-01
Namens den Directeur van het Deli-Proefstation te Medan, Sumatra, werd mij de hieronder volgende beschrijving der ontwikkelingsstadia van Thosea asigna v. E., eene Limacodide, beschreven in de Zool. Meded. Mus. Leiden XII, p. 113, pl. XII, f. 10, 1929, toegezonden. Ei: de eieren worden afgezet tegen
Materials perspective on Casimir and van der Waals interactions
Woods, L. M.; Dalvit, D. A. R.; Tkatchenko, A.; Rodriguez-Lopez, P.; Rodriguez, A. W.; Podgornik, R.
2016-10-01
Interactions induced by electromagnetic fluctuations, such as van der Waals and Casimir forces, are of universal nature present at any length scale between any types of systems. Such interactions are important not only for the fundamental science of materials behavior, but also for the design and improvement of micro- and nanostructured devices. In the past decade, many new materials have become available, which has stimulated the need for understanding their dispersive interactions. The field of van der Waals and Casimir forces has experienced an impetus in terms of developing novel theoretical and computational methods to provide new insights into related phenomena. The understanding of such forces has far reaching consequences as it bridges concepts in materials, atomic and molecular physics, condensed-matter physics, high-energy physics, chemistry, and biology. This review summarizes major breakthroughs and emphasizes the common origin of van der Waals and Casimir interactions. Progress related to novel ab initio modeling approaches and their application in various systems, interactions in materials with Dirac-like spectra, force manipulations through nontrivial boundary conditions, and applications of van der Waals forces in organic and biological matter are examined. The outlook of the review is to give the scientific community a materials perspective of van der Waals and Casimir phenomena and stimulate the development of experimental techniques and applications.
Evolutionary design of interfacial phase change van der Waals heterostructures.
Kalikka, Janne; Zhou, Xilin; Behera, Jitendra; Nannicini, Giacomo; Simpson, Robert E
2016-10-27
We use an evolutionary algorithm to explore the design space of hexagonal Ge2Sb2Te5; a van der Waals layered two dimensional crystal heterostructure. The Ge2Sb2Te5 structure is more complicated than previously thought. Predominant features include layers of Ge3Sb2Te6 and Ge1Sb2Te4 two dimensional crystals that interact through Te-Te van der Waals bonds. Interestingly, (Ge/Sb)-Te-(Ge/Sb)-Te alternation is a common feature for the most stable structures of each generation's evolution. This emergent rule provides an important structural motif that must be included in the design of high performance Sb2Te3-GeTe van der Waals heterostructure superlattices with interfacial atomic switching capability. The structures predicted by the algorithm agree well with experimental measurements on highly oriented, and single crystal Ge2Sb2Te5 samples. By analysing the evolutionary algorithm optimised structures, we show that diffusive atomic switching is probable by Ge atoms undergoing a transition at the van der Waals interface from layers of Ge3Sb2Te6 to Ge1Sb2Te4 thus producing two blocks of Ge2Sb2Te5. Evolutionary methods present an efficient approach to explore the enormous multi-dimensional design parameter space of van der Waals bonded heterostructure superlattices.
Brane cosmology with a van der Waals equation of state
Kremer, G M
2004-01-01
The evolution of a Universe confined onto a 3-brane embedded in a five-dimensional space-time is investigated where the cosmological fluid on the brane is modeled by the van der Waals equation of state. It is shown that the Universe on the brane evolves in such a manner that three distinct periods concerning its acceleration field are attained: (a) an initial accelerated epoch where the van der Waals fluid behaves like a scalar field with a negative pressure; (b) a past decelerated period which has two contributions, one of them is related to the van der Waals fluid which behaves like a matter field with a positive pressure, whereas the other contribution comes from a term of the Friedmann equation on the brane which is inversely proportional to the scale factor to the fourth power and can be interpreted as a radiation field, and (c) a present accelerated phase due to a cosmological constant on the brane.
A cartography of the van der Waals territories.
Alvarez, Santiago
2013-06-28
The distribution of distances from atoms of a particular element E to a probe atom X (oxygen in most cases), both bonded and intermolecular non-bonded contacts, has been analyzed. In general, the distribution is characterized by a maximum at short E···X distances corresponding to chemical bonds, followed by a range of unpopulated distances--the van der Waals gap--and a second maximum at longer distances--the van der Waals peak--superimposed on a random distribution function that roughly follows a d(3) dependence. The analysis of more than five million interatomic "non-bonded" distances has led to the proposal of a consistent set of van der Waals radii for most naturally occurring elements, and its applicability to other element pairs has been tested for a set of more than three million data, all of them compared to over one million bond distances.
The Economics of van der Waals Force Engineering
Pinto, Fabrizio
2008-01-01
As micro-electro-mechanical system (MEMS) fabrication continues on an ever-decreasing scale, new technological challenges must be successfully negotiated if Moore's Law is to be an even approximately valid model of the future of device miniaturization. Among the most significant obstacles is the existence of strong surface forces related to quantum mechanical van der Waals interatomic interactions, which rapidly diverge as the distance between any two neutral boundaries decreases. The van der Waals force is a contributing factor in several device failures and limitations, including, for instance, stiction and oscillator non-linearities. In the last decade, however, it has been conclusively shown that van der Waals forces are not just a MEMS limitation but can be engineered in both magnitude and sign so as to enable classes of proprietary inventions which either deliver novel capabilities or improve upon existing ones. The evolution of van der Waals force research from an almost exclusively theoretical field in quantum-electro-dynamics to an enabling nanotechnology discipline represents a useful example of the ongoing paradigm shift from government-centered to private-capital funded R&D in cutting-edge physics leading to potentially profitable products. In this paper, we discuss the reasons van der Waals force engineering may lead to the creation of thriving markets both in the short and medium terms by highlighting technical challenges that can be competitively addressed by this novel approach. We also discuss some notable obstacles to the cultural transformation of the academic research community required for the emergence of a functional van der Waals force engineering industry worldwide.
Dr Jacob van der Land, marine biologist extraordinary
Bruggen, van, A.
2001-01-01
This contribution is an attempt to sketch the life and works of Dr Jacob van der Land, curator of worms and chief marine biologist of the National Museum of Natural History, on the occasion of his official retirement. Born in 1935, Jacob van der Land read biology at Leiden University (1958-1964), where he obtained his Ph.D. in 1970 on a treatise on the Priapulida under the supervision of Prof. Dr L.D. Brongersma. In 1964 he was appointed curator of worms in the museum. Later on he took over l...
Eglon Hendrik van der Neer (1635/36 - 1703) : Zijn leven en werk
Schavemaker, E.
2009-01-01
Eglon van der Neer. His Life and His Work In his own time Eglon van der Neer was highly successful. His acclaim endured throughout the eighteenth century. Nowadays he is rated as a second-rate master. Van der Neer was born in Amsterdam in 1635 or 1636 as the son of the now more famous landscape pain
Van der Waals forces in pNRQED
Shtabovenko, Vladyslav [Physik-Department, Technische Universität München, James-Franck-Str. 1, 85748 Garching (Germany)
2016-01-22
We report on the calculation of electromagnetic van der Waals forces [1] between two hydrogen atoms using non-relativistic effective field theories (EFTs) of QED for large and small momentum transfers with respect to the intrinsic energy scale of the hydrogen atom. Our results reproduce the well known London and Casimir-Polder forces.
Šveitsi hommage Mies van der Rohele / Kai Lobjakas
Lobjakas, Kai, 1975-
1999-01-01
Šveitsi väikelinna Buschi ehitatud voolava ruumilahendusega maja meenutab Mies van der Rohe kujundatud saksa paviljoni maailmanäitusel Barcelonas. Materjalideks betoon, puit, klaas. Loodud on palju erineva kõrgusega pindu maja ja seda ümbritseva müüri vahel. Interjööris punakas tammepuit. Arhitekt: Peter+Christian Frei, Architekten.
Higher-accuracy van der Waals density functional
Lee, Kyuho; Murray, Éamonn D.; Kong, Lingzhu
2010-01-01
We propose a second version of the van der Waals density functional of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate semilocal exchange functional and the use of a large-N asymptote gradient correction in determining the vdW kernel. The predicted binding energy...
Comment on the Paper by van der Smagt
Schwaninger, Markus
2006-01-01
The article comments on a paper by van der Smagt on the social construction of shared realities and models. According to the theory of social constructivism, collective cognition is perceived as a social process by which such creation and representation often take place. The research focuses on the difference between causation and constitution.
Šveitsi hommage Mies van der Rohele / Kai Lobjakas
Lobjakas, Kai, 1975-
1999-01-01
Šveitsi väikelinna Buschi ehitatud voolava ruumilahendusega maja meenutab Mies van der Rohe kujundatud saksa paviljoni maailmanäitusel Barcelonas. Materjalideks betoon, puit, klaas. Loodud on palju erineva kõrgusega pindu maja ja seda ümbritseva müüri vahel. Interjööris punakas tammepuit. Arhitekt: Peter+Christian Frei, Architekten.
The Lyapunov exponents of the Van der Pol oscillator
Grasman, J.; Verhulst, F.; Shih, S.D.
2005-01-01
Lyapunov exponents characterize the dynamics of a system near its attractor. For the Van der Pol oscillator these are quantities for which an approximation should be at hand. Similar to the asymptotic approximation of amplitude and period, expressions are derived for the non-zero Lyapunov exponent
Een concentratieprobleem bij B.J. van der Walt
S. Griffioen
2010-07-01
Full Text Available B.J. van der Walt’s concentration problem This contribution probes the concept of secularism, a key notion of B.J. van der Walt’s “Transforming power” (2007. It is found that Van der Walt’s interpretation of secularism rests on a double assumption. The first assumption is that human nature is intrinsically religious. Humans cannot live without putting their trust in something. The second is that this religious nature manifests itself in “concentrated” ways, rather than dispersing itself over a plurality of objects. These assumptions in tandem explain why Van der Walt holds the view that atheism, agnosticism and even overt indifference in matters of faith are at heart propelled by convictions that share the main features of positive religions. It also explains why he assumes that all these convictions tend towards one and the same goal: to gain dominance in the public realm. This article is sympathetic towards the first assumption, and skeptical towards the second. It is argued that the “concentration- thesis” fails to do justice to world and life-views that obviously do not claim total allegiance. To illustrate this point it turns to the phenomenon of “multiple religious participation”, as well as to different strands within contemporary humanism. It concludes that the main problem may well be that secular culture has little to offer to satisfy the innate religious drive in humankind.
Mies van der Rohe preemia 2011 / Karen Jagodin
Jagodin, Karen, 1982-
2011-01-01
20. juunil antakse Barcelonas inglise arhitektile Sir David Chipperfieldile Berliini Neues Museumi restaureerimise eest üle 2011. aasta Mies van der Rohe arhitektuuripreemia. Nimetatud võidutöö viis suuremat konkurenti, varem preemia pälvinud hooneid ja nende autoreid. Sir David Chipperfieldist, Berliini Neues Muuseumi restaureerimisest. Loetletud preemia nominendid Eestist
Mies van der Rohe preemia 2011 / Karen Jagodin
Jagodin, Karen, 1982-
2011-01-01
20. juunil antakse Barcelonas inglise arhitektile Sir David Chipperfieldile Berliini Neues Museumi restaureerimise eest üle 2011. aasta Mies van der Rohe arhitektuuripreemia. Nimetatud võidutöö viis suuremat konkurenti, varem preemia pälvinud hooneid ja nende autoreid. Sir David Chipperfieldist, Berliini Neues Muuseumi restaureerimisest. Loetletud preemia nominendid Eestist
Graphene on metals: A van der Waals density functional study
Vanin, Marco; Mortensen, Jens Jørgen; Kelkkanen, Kari André;
2010-01-01
We use density functional theory (DFT) with a recently developed van der Waals density functional (vdW-DF) to study the adsorption of graphene on Co, Ni, Pd, Ag, Au, Cu, Pt, and Al(111) surfaces. In contrast to the local-density approximation (LDA) which predicts relatively strong binding for Ni...
Electromagnetically induced transparency with controlled van der Waals interaction
Wu, Huaizhi; Shen, Li-Tuo; Chen, Rong-Xin; Yang, Zhen-Biao; Zheng, Shi-Biao
2014-01-01
The transmission of light through cold Rydberg atoms controlled by a second laser beam under the condition of the electromagnetically induced transparency (EIT) has been shown to exhibit highly optical nonlinearity. Here we study this effect with two individually addressed Rydberg atoms under the influence of the interatomic van der Waals interaction. We derive an effectively atomic Raman transition model that can potentially overcome the limits of applications for EIT with atoms of the ladder-type level configuration. By probing one of the atoms, we observe four doublets of absorption induced by the Autler-Townes (AT) splitting and van der Waals interaction. In particular, we find that the EIT center keeps unshifted compared with the case of interatomic interaction free, which demonstrated that the interference among the multiple transition channels is basically destructive. The EIT with controlled Rydberg-Rydberg interaction among few atoms provides a versatile tool for engineering the propagation dynamics ...
Poisson-Boltzmann Calculations: van der Waals or Molecular Surface?
Pang, Xiaodong; Zhou, Huan-Xiang
2013-01-01
The Poisson-Boltzmann equation is widely used for modeling the electrostatics of biomolecules, but the calculation results are sensitive to the choice of the boundary between the low solute dielectric and the high solvent dielectric. The default choice for the dielectric boundary has been the molecular surface, but the use of the van der Waals surface has also been advocated. Here we review recent studies in which the two choices are tested against experimental results and explicit-solvent calculations. The assignment of the solvent high dielectric constant to interstitial voids in the solute is often used as a criticism against the van der Waals surface. However, this assignment may not be as unrealistic as previously thought, since hydrogen exchange and other NMR experiments have firmly established that all interior parts of proteins are transiently accessible to the solvent.
Persistent hysteresis in graphene-mica van der Waals heterostructures.
Mohrmann, Jens; Watanabe, Kenji; Taniguchi, Takashi; Danneau, Romain
2015-01-09
We report the study of electronic transport in graphene-mica van der Waals heterostructures. We have designed various graphene field-effect devices in which mica is utilized as a substrate and/or gate dielectric. When mica is used as a gate dielectric we observe a very strong positive gate voltage hysteresis of the resistance, which persists in samples that were prepared in a controlled atmosphere down to even millikelvin temperatures. In a double-gated mica-graphene-hBN van der Waals heterostructure, we found that while a strong hysteresis occurred when mica was used as a substrate/gate dielectric, the same graphene sheet on mica substrate no longer showed hysteresis when the charge carrier density was tuned through a second gate with the hBN dielectric. While this hysteretic behavior could be useful for memory devices, our findings confirm that the environment during sample preparation has to be controlled strictly.
Van der Waals explosion of cold Rydberg clusters
Faoro, R; Archimi, M; Masella, G; Valado, M M; Arimondo, E; Mannella, R; Ciampini, D; Morsch, O
2015-01-01
We report on the direct measurement in real space of the effect of the van der Waals forces between individual Rydberg atoms on their external degrees of freedom. Clusters of Rydberg atoms with inter-particle distances of around 5 {\\mu}m are created by first generating a small number of seed excitations in a magneto-optical trap, followed by off-resonant excitation that leads to a chain of facilitated excitation events. After a variable expansion time the Rydberg atoms are field ionized, and from the arrival time distributions the size of the Rydberg cluster after expansion is calculated. Our experimental results agree well with a numerical simulation of the van der Waals explosion.
Goden en mensen. De godsdienstwetenschap van Gerardus van der Leeuw 1890-1950
Hofstee, Willem
1997-01-01
This study examines Van der Leeuw's science of religion as it developed in the years between the two world wars. In the first chapter I give a theoretical justification for the questions I raise and provide a short historical introduction to the development of the history of religion in the Netherla
Van der Waals interaction between two crossed carbon nanotubes
Zhbanov, Alexander I.; Pogorelov, Evgeny G.; Chang, Yia-Chung
2008-01-01
The analytical expressions for the van der Waals potential energy and force between two crossed carbon nanotubes are presented. The Lennard-Jones potential for two carbon atoms and the method of the smeared out approximation suggested by L.A. Girifalco were used. The exact formula is expressed in terms of rational and elliptical functions. The potential and force for carbon nanotubes were calculated. The uniform potential curves for single- and multi- wall nanotubes were plotted. The equilibr...
Van der Waals interactions of parallel and concentric nanotubes
Schroder, Elsebeth; Hyldgaard, Per
2003-01-01
For sparse materials like graphitic systems and carbon nanotubes the standard density functional theory (DFT) faces significant problems because it cannot accurately describe the van der Waals interactions that are essential to the carbon-nanostructure materials behavior. While standard implementations of DFT can describe the strong chemical binding within an isolated, single-walled carbon nanotube, a new and extended DFT implementation is needed to describe the binding between nanotubes. We ...
Dynamic changes of phase in a van der Waals fluid
Hagan, R.; Serrin, J.
1984-03-01
This paper gives sufficient conditions to guarantee the existence of a shock layer solution connecting two different equilibrium states in a van der Waals fluid. In particular, the equilibrium states can belong to two different phases of the fluid. The constitutive laws come from a modified Korteweg theory which is compatible with the Clausius Duhem inequality. The Clausius Duhem inequality in turn gives rise to a Liapunov function. The main mathematical tool is the LaSalle invariance principle.
Impulsive Homoclinic Chaos in Van der Pol Jerk System
丁玉梅; 张琪昌
2010-01-01
A 3D continuous autonomous chaotic system is reported, which contains a cubic term and six system parameters. Basic dynamic properties of the new Van der Pol Jerk system are studied by means of theoretical analysis and numerical simulation. Based on the Silnikov theorem, the chaotic characterisitics of the dynamic system are discussed. Using Cardano formula and series solution of differential equation, eigenvalue problem and the existence of homoclinic orbit are studied. Furthermore, a rigorous proof for th...
Van der Waals and Casimir-Polder interactions between neutrons
Babb James F.
2016-01-01
Full Text Available We investigate the van der Waals interaction between neutrons using the theory of Casimir and Polder, wherein the potential for asymptotically large separations falls off as the inverse seventh power, and compare it to the similar interaction between a neutron and a proton, for which the asymptotic interaction falls off as the inverse fourth power. Modifications of the formalism to extend the validity to smaller separations using dynamic electric and magnetic dipole polarizability data are discussed.
Structure and dynamics of small van der Waals complexes
Loreau, J. [Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles (ULB) CP 160/09, 1050 Brussels (Belgium)
2014-10-06
We illustrate computational aspects of the calculation of the potential energy surfaces of small (up to five atoms) van der Waals complexes with high-level quantum chemistry techniques such as the CCSD(T) method with extended basis sets. We discuss the compromise between the required accuracy and the computational time. Further, we show how these potential energy surfaces can be fitted and used in dynamical calculations such as non-reactive inelastic scattering.
Approximate transformations for van der Pol-type equations
R. K. Gazizov
2006-01-01
Full Text Available Classification of van der Pol-type equations with respect to admitted approximate transformation groups transforming a small parameter is given. It is shown that approximate symmetries transforming the small parameter as well as the usual approximate symmetries can be used for approximate integration (e.g., by method of successive reduction of order of ordinary differential equations with a small parameter.
Scaling Laws for van der Waals Interactions in Nanostructured Materials
Gobre, Vivekanand; Tkatchenko, Alexandre
2014-03-01
Van der Waals (vdW) forces originate from interactions between fluctuating multipoles in matter and play a significant role in the structure and stability of nanostructured materials. Many models used to describe vdW interactions in nanomaterials are based on a simple pairwise-additive approximation, neglecting the strong electrodynamic response effects caused by long-range fluctuations in matter. We develop and utilize an efficient microscopic method to demonstrate that vdW interactions in nanomaterials act at distances greater than typically assumed, and can be characterized by different scaling laws depending on the dimensionality and size of the system. Specifically, we study the behaviour of vdW interactions in single-layer and multilayer graphene, fullerenes of varying size, single-wall carbon nanotubes and graphene nanoribbons. As a function of nanostructure size, the van der Waals coefficients follow unusual trends for all of the considered systems, and deviate significantly from the conventionally employed pairwise-additive picture. We propose that the peculiar van der Waals interactions in nanostructured materials could be exploited to control their self-assembly.
Disaini ja rahvaste saatusest / Daniel Van der Velden ; interv. Kristjan Mändmaa
Van der Velden, Daniel
2008-01-01
17.-27. X 2008 oli Eesti Kunstiakadeemias näitus "Marsilt. Omaalgatuslikud projektid graafilises disainis". Kuraatorid: tšehhi disainerid Adam Machacek ja Radim Peshko. 17. X esinesid EKA saalis loengutega James Goggin Londoni firmast Practice ja Daniel van der Velden Hollandi disainikollektiivist Metahaven. 16. V osales D. van der Velden Eesti Disainiinstituudi korraldatud kohabrändingu-teemalises ümarlauas. D. van der Velden disaini, poliitika ja kohabrändingu tulevikust, Metahaveni disainitöödest
Disaini ja rahvaste saatusest / Daniel Van der Velden ; intervjueerinud Kristjan Mändmaa
Van der Velden, Daniel
2011-01-01
17.-27. X 2008 oli Eesti Kunstiakadeemias näitus "Marsilt. Omaalgatuslikud projektid graafilises disainis". Kuraatorid: tšehhi disainerid Adam Machacek ja Radim Peshko. 17. X esinesid EKA saalis loengutega James Goggin Londoni firmast Practice ja Daniel van der Velden Hollandi disainikollektiivist Metahaven. 16. V osales D. van der Velden Eesti Disainiinstituudi korraldatud kohabrändingu-teemalises ümarlauas. D. van der Velden disaini, poliitika ja kohabrändingu tulevikust, Metahaveni disainitöödest
Disaini ja rahvaste saatusest / Daniel Van der Velden ; intervjueerinud Kristjan Mändmaa
Van der Velden, Daniel
2011-01-01
17.-27. X 2008 oli Eesti Kunstiakadeemias näitus "Marsilt. Omaalgatuslikud projektid graafilises disainis". Kuraatorid: tšehhi disainerid Adam Machacek ja Radim Peshko. 17. X esinesid EKA saalis loengutega James Goggin Londoni firmast Practice ja Daniel van der Velden Hollandi disainikollektiivist Metahaven. 16. V osales D. van der Velden Eesti Disainiinstituudi korraldatud kohabrändingu-teemalises ümarlauas. D. van der Velden disaini, poliitika ja kohabrändingu tulevikust, Metahaveni disainitöödest
Disaini ja rahvaste saatusest / Daniel Van der Velden ; interv. Kristjan Mändmaa
Van der Velden, Daniel
2008-01-01
17.-27. X 2008 oli Eesti Kunstiakadeemias näitus "Marsilt. Omaalgatuslikud projektid graafilises disainis". Kuraatorid: tšehhi disainerid Adam Machacek ja Radim Peshko. 17. X esinesid EKA saalis loengutega James Goggin Londoni firmast Practice ja Daniel van der Velden Hollandi disainikollektiivist Metahaven. 16. V osales D. van der Velden Eesti Disainiinstituudi korraldatud kohabrändingu-teemalises ümarlauas. D. van der Velden disaini, poliitika ja kohabrändingu tulevikust, Metahaveni disainitöödest
k Sequences of Generalized Van der Laan and Generalized Perrin Polynomials
Kaygisiz, Kenan
2011-01-01
In this paper, we present k sequences of Generalized Van der Laan Polynomials and Generalized Perrin Polynomials using Genaralized Fibonacci and Lucas Polynomials. We give some properties of these polynomials. We also obtain generalized order-k Van der Laan Numbers, k sequences of generalized order-k Van der Laan Numbers, generalized order-k Perrin Numbers and k sequences of generalized order-k Perrin Numbers. In addition, we examine the relationship between them.
Carvacrolreiches Oregano-Öl als Futterzusatz: Interview met Hans van der Mheen
Grohs, Birgit; Mheen, van der H.J.C.J.
2011-01-01
In dit interview met Hans van der Mheen van het PPO-agv te Lelystad worden de mogelijkheden van het gebruik van etherische olie gewonnen uit oregano; origanum vulgare en origanum heracleoticum, geteeld in Nederland besproken. De olie kan gebruikt worden in voer voor pluimvee en varkens. Gezocht word
van der Waals Equation of State and Solution Theories%van der Waals状态方程与溶液理论
刘国杰; 黑恩成
2007-01-01
阐明了van der Waals状态方程与溶液理论间的密切关系.由van der Waals状态方程中的内压力和自由体积简捷地导得Scatchard-Hildebrand正规溶液理论、无热溶液理论和Flory-Huggins聚合物溶液理论.表明这些理论都是建立在van der Waals流体的基础上,为进一步改进这些理论提供了一条切实可行的思路.
Van der Waals interaction between two crossed carbon nanotubes.
Zhbanov, Alexander I; Pogorelov, Evgeny G; Chang, Yia-Chung
2010-10-26
The analytical expressions for the van der Waals potential energy and force between two crossed carbon nanotubes are presented. The Lennard-Jones potential between pairs of carbon atoms and the smeared-out approximation suggested by L. A. Girifalco (J. Phys. Chem. 1992, 96, 858) were used. The exact formula is expressed in terms of rational and elliptical functions. The potential and force for carbon nanotubes were calculated. The uniform potential curves for single- and multiwall nanotubes were plotted. The equilibrium distance, maximal attractive force, and potential energy have been evaluated.
Analytical solution for Van der Pol-Duffing oscillators
Kimiaeifar, A. [Department of Mechanical Engineering, Shahid Bahonar University of Kerman, Kerman (Iran, Islamic Republic of); Saidi, A.R. [Department of Mechanical Engineering, Shahid Bahonar University of Kerman, Kerman (Iran, Islamic Republic of)], E-mail: saidi@mail.uk.ac.ir; Bagheri, G.H.; Rahimpour, M. [Department of Mechanical Engineering, Shahid Bahonar University of Kerman, Kerman (Iran, Islamic Republic of); Domairry, D.G. [Department of Mechanical Engineering, Noshirvani Institute of Technology, Babol (Iran, Islamic Republic of)
2009-12-15
In this paper, the problem of single-well, double-well and double-hump Van der Pol-Duffing oscillator is studied. Governing equation is solved analytically using a new kind of analytic technique for nonlinear problems namely the 'Homotopy Analysis Method' (HAM), for the first time. Present solution gives an expression which can be used in wide range of time for all domain of response. Comparisons of the obtained solutions with numerical results show that this method is effective and convenient for solving this problem. This method is a capable tool for solving this kind of nonlinear problems.
Simon van der Meer in the AA Control Room
CERN PhotoLab
1984-01-01
Simon van der Meer, spiritus rector of the Antiproton Accumulator, in the AA Control Room. Inventor of stochastic cooling, on which the AA was based, and of the magnetic horn, with which the antiprotons were focused, he also wrote most of the software with which the AA was controlled, and spent uncountable numbers of hours in this chair to tickle the AA to top performance. 8 months after this picture was taken, he received, in October 1984, the Nobel prize, together with Carlo Rubbia, the moving force behind the whole Proton-Antiproton Collider project that led to the discovery, in 1983, of the W and Z intermediate bosons.
Van der Woude syndrome with unusual developmental disorders
Vijay Kumar Ambaldhage
2014-01-01
Full Text Available Van der Woude syndrome is a rare developmental disorder characterized by a combination of congenital lower lip pits, cleft palate alone or with cleft lip in some cases, and hypodontia. It occurs in approximately 2% of the population with facial clefts. Its prevalence varies from one in 100,000 to 200,000 births. This case report details a unique case of Vander Woude syndrome in a seven-year-old girl and her mother, with median lip pits, cleft lip and cleft palate, missing lateral incisors, and retained deciduous teeth, along with transposition of teeth.
Spherical and hyperspherical harmonics representation of van der Waals aggregates
Lombardi, Andrea; Palazzetti, Federico; Aquilanti, Vincenzo; Grossi, Gaia; Albernaz, Alessandra F.; Barreto, Patricia R. P.; Cruz, Ana Claudia P. S.
2016-12-01
The representation of the potential energy surfaces of atom-molecule or molecular dimers interactions should account faithfully for the symmetry properties of the systems, preserving at the same time a compact analytical form. To this aim, the choice of a proper set of coordinates is a necessary precondition. Here we illustrate a description in terms of hyperspherical coordinates and the expansion of the intermolecular interaction energy in terms of hypersherical harmonics, as a general method for building potential energy surfaces suitable for molecular dynamics simulations of van der Waals aggregates. Examples for the prototypical case diatomic-molecule-diatomic-molecule interactions are shown.
Casa 50' x 50': Ludwig Mies van der Rohe
Mantovani Genari, Eduardo
2007-01-01
El trabajo se enfoca hacia el análisis arquitectónico del material producido por el arquitecto [Mies van der Rohe] y sus colaboradores. Este análisis, realizado a partir de la información recopilada, tiene por objeto aclarar el orden interno del proyecto de la Casa 50' x 50', comprender la jerarquía de las decisiones y determinar los elementos básicos y sus criterios de relación. Se propone un acercamiento sensible y directo a la obra de arquitectura, a su realidad física y constructiva, basa...
Field Effect in Graphene-Based van der Waals Heterostructures
Stradi, Daniele; Papior, Nick Rübner; Hansen, Ole
2017-01-01
Stacked van der Waals (vdW) heterostructures where semiconducting two-dimensional (2D) materials are contacted by overlaid graphene electrodes enable atomically thin, flexible electronics. We use first-principles quantum transport simulations of graphene-contacted MoS2 devices to show how...... the transistor effect critically depends on the stacking configuration relative to the gate electrode. We can trace this behavior to the stacking-dependent response of the contact region to the capacitive electric field induced by the gate. The contact resistance is a central parameter and our observation...
Van der Woude syndrome: Management in the mixed dentition
Sonahita Agarwal
2013-01-01
Full Text Available This article presents the case of a patient with Van der Woude syndrome treated with orthodontic and orthopedic intervention in the mixed dentition stage. The patient had a bilateral cleft of the lip and alveolus accompanied by lip pits on the lower lip. Intra-orally, there was bilateral anterior and posterior cross-bite with a collapsed maxilla. The maxillary transverse deficiency was managed with orthopedic expansion and the second phase of treatment involved secondary alveolar bone grafting followed by retention with functional regulator-3. The mild maxillary retrognathia and deficient lip support was managed with dental compensation.
Van der Waals phase transition in the framework of holography
Zeng, Xiao-Xiong
2015-01-01
Phase structure of the quintessence Reissner-Nordstr\\"{o}m-AdS black hole is probed with the nonlocal observables such as holographic entanglement entropy and two point correlation function. Our result shows that, as the case of the thermal entropy, both the observables exhibit the similar Van der Waals-like phase transition. To reinforce the conclusion, we further check the equal area law for the first order phase transition and critical exponent of the heat capacity for the second order phase transition. We also discuss the effect of the state parameter on the phase structure of the nonlocal observables.
Jean le Clus-Theron
2012-11-01
Full Text Available Hierdie artikel bied ’n ondersoek na die werk van Strijdom van der Merwe, die gedagte van aardkuns en die kunstenaarsboek in die konteks van die historiese en eietydse kunsmark. Die stelling dat ‘indien iets kuns genoem kan word, kan dit ook verkoop word’ word gebruik as vertrekpunt – ook in die geval van kunsvorme wat skynbaar ‘onmoontlik’ is om te verkoop, soos aardkuns. Hierdie artikel besin oor die werk van Strijdom van der Merwe, die bekendste Suid Afrikaanse aardkunstenaar, binne die ietwat ongewone konteks (vir aardkuns van die Suid-Afrikaanse kunsmark. Hy is ’n suksesvolle voltydse aardkunstenaar; nogal ’n uitsonderlike verskynsel binne die Suid-Afrikaanse kunsomgewing. Aardkuns is allerweë in die 1960s beskou as ’n ‘onmoontlike’ kunsvorm om te verkoop, en hierdie artikel ondersoek die geldigheid van hierdie stelling vir vandag. Die navorsingsmetode van die artikel is semi-gestruktureerde onderhoude en ’n literatuurstudie. My argument is, met verwysing na Van der Merwe se werk, dat die stelling ‘indien iets kuns genoem kan word, dit verkoopbaar is’ waar mag wees, maar met die moontlike voorbehoud van ‘verkoopbaar – op die een of ander manier’.
van der Waals interaction energy and disjoining pressure at small separation.
White, Lee R
2010-03-01
The divergence of the van der Waals interaction energy E(132)(L) between plane half-spaces 1 and 2 separated by medium 3 as the separation distance L tends to zero is naively thought of as due to the overlap of the atomic polarization centers. It follows that it may therefore be prevented by properly allowing for the finite size of the atomic species which would prevent the overlap. The distance cutoff model is a simple example of such a modification. The present paper demonstrates that this is not ultimately the origin of the divergence and, that although finite atomic dimensions would alleviate the embarrassment, non-overlap does not properly address the thermodynamic restriction that pertains to the interaction energy. By allowing in an albeit approximate way for the wavelength dependence of the material dielectric response functions epsilon(i xi, k) which arise naturally in the modern Lifshitz theory for this interaction, a form for the van der Waals energy and the corresponding disjoining pressure may be derived which obey the thermodynamic constraint and remove the divergence as L-->0. The energy and disjoining pressure in this new model are compared with the classic non-retarded results and the length cutoff model.
Franckeite as a naturally occurring van der Waals heterostructure
Molina-Mendoza, Aday J.; Giovanelli, Emerson; Paz, Wendel S.; Niño, Miguel Angel; Island, Joshua O.; Evangeli, Charalambos; Aballe, Lucía; Foerster, Michael; van der Zant, Herre S. J.; Rubio-Bollinger, Gabino; Agraït, Nicolás; Palacios, J. J.; Pérez, Emilio M.; Castellanos-Gomez, Andres
2017-01-01
The fabrication of van der Waals heterostructures, artificial materials assembled by individual stacking of 2D layers, is among the most promising directions in 2D materials research. Until now, the most widespread approach to stack 2D layers relies on deterministic placement methods, which are cumbersome and tend to suffer from poor control over the lattice orientations and the presence of unwanted interlayer adsorbates. Here, we present a different approach to fabricate ultrathin heterostructures by exfoliation of bulk franckeite which is a naturally occurring and air stable van der Waals heterostructure (composed of alternating SnS2-like and PbS-like layers stacked on top of each other). Presenting both an attractive narrow bandgap (<0.7 eV) and p-type doping, we find that the material can be exfoliated both mechanically and chemically down to few-layer thicknesses. We present extensive theoretical and experimental characterizations of the material's electronic properties and crystal structure, and explore applications for near-infrared photodetectors. PMID:28194037
Equations of State: From the Ideas of van der Waals to Association Theories
Kontogeorgis, Georgios; Economou, Ioannis G.
2010-01-01
The ideas of van der Waals have resulted to cubic equations of state like Soave–Redlich–Kwong (SRK) and Peng–Robinson (PR) which are widely used in the petroleum and chemical industries. It is often thought that the range of applicability of van der Waals-type models is limited to mixtures...
Mihkelson paljastas van der Lindeni ärihuvid Venemaal / Helga Koger
Koger, Helga, 1945-
2007-01-01
Riigikogu EL asjade komisjoni esimees Marko Mihkelson tutvustas materjale, mis viitavad hiljuti Eestit kritiseerinud Euroopa Nõukogu Parlamentaarse Assamblee presidendi Rene van der Linderi ärihuvile Venemaal. Mihkelsoni sõnade järgi võivad van der Lindenil olla majandushuvid seoses Vladimiri oblasti Sobinski rajooni rajatava tööstuspargiga
Mihkelson paljastas van der Lindeni ärihuvid Venemaal / Helga Koger
Koger, Helga, 1945-
2007-01-01
Riigikogu EL asjade komisjoni esimees Marko Mihkelson tutvustas materjale, mis viitavad hiljuti Eestit kritiseerinud Euroopa Nõukogu Parlamentaarse Assamblee presidendi Rene van der Linderi ärihuvile Venemaal. Mihkelsoni sõnade järgi võivad van der Lindenil olla majandushuvid seoses Vladimiri oblasti Sobinski rajooni rajatava tööstuspargiga
Bartholomeus van der Helst (1613-1670): een studie naar zijn leven en zijn werk
van Gent, J.F.J.M.
2011-01-01
Bartholomeus van der Helst was a leading portrait painter in the Northern Netherlands in the 17th century. This monograph reconstructs his career and his circle of patrons based on the surviving works and documents. Van der Helst was born in Haarlem around 1613. In the early 1630s he moved to
E. Oliver
1992-01-01
Full Text Available Reverend Dirk van der Hoff's contribution to the development of Christian National Education in the Zuid-Afrikaansche Republiek Today, Afrikaners are closely associated with the Christian National system of education. This system has a long and interesting history and many people have been involved in its development. This article shows the vital and essential role that Reverend Dirk van der Hoff, the first resident minister of religion in the ZAR, played in the structuring of CNE.
Implication of Two-Coupled Differential Van der Pol Duffing Oscillator in Weak Signal Detection
Peng, Hang-hang; Xu, Xue-mei; Yang, Bing-chu; Yin, Lin-zi
2016-04-01
The principle of the Van der Pol Duffing oscillator for state transition and for determining critical value is described, which has been studied to indicate that the application of the Van der Pol Duffing oscillator in weak signal detection is feasible. On the basis of this principle, an improved two-coupled differential Van der Pol Duffing oscillator is proposed which can identify signals under any frequency and ameliorate signal-to-noise ratio (SNR). The analytical methods of the proposed model and the construction of the proposed oscillator are introduced in detail. Numerical experiments on the properties of the proposed oscillator compared with those of the Van der Pol Duffing oscillator are carried out. Our numerical simulations have confirmed the analytical treatment. The results demonstrate that this novel oscillator has better detection performance than the Van der Pol Duffing oscillator.
Cosmological models described by a mixture of van der Waals fluid and dark energy
Kremer, G M
2003-01-01
The Universe is modeled as a binary mixture whose constituents are described by a van der Waals fluid and by a dark energy density. The dark energy density is considered either as the quintessence or as the Chaplygin gas. The irreversible processes concerning the energy transfer between the van der Waals fluid and the gravitational field are taken into account. This model can simulate: (a) an inflationary period where the acceleration grows exponentially and the van der Waals fluid behaves like an inflaton; (b) an inflationary period where the acceleration is positive but it decreases and tends to zero whereas the energy density of the van der Waals fluid decays; (c) a decelerated period which corresponds to a matter dominated period with a non-negative pressure; and (d) a present accelerated period where the dark energy density outweighs the energy density of the van der Waals fluid.
{InSe}/{GaSe} heterointerfaces prepared by Van der Waals epitaxy
Lang, O.; Klein, A.; Schlaf, R.; Löher, T.; Pettenkofer, C.; Jaegermann, W.; Chevy, A.
1995-01-01
Epitaxial films of layered substrates can be prepared onto layered substrates even for large lattice mismatch, when the growth is attempted with the Van der Waals surfaces opposing each other (Van der Waals epitaxy). Thin epitaxial InSe(GaSe) films are prepared onto ultrahigh vacuum (UHV) cleaved GaSe(InSe) Van der Waals (0001) surfaces. The films and the heterointerface are characterized by photoelectron spectroscopy, electron diffraction and scanning tunneling microscopy (STM). High quality and stoichiometric films are obtained by direct InSe(GaSe) evaporation from a Knudsen cell at sample temperatures between 520 and 620 K. Despite a 6% lattice mismatch the deposited films are oriented with their c- and α-axis to the hexagonal substrate. The growth mostly follows the Frank-Van der Merwe mode. This rather ideal growth behaviour is related to the specific properties of the Van der Waals plane which contains no dangling bonds.
Flexible ferroelectric element based on van der Waals heteroepitaxy.
Jiang, Jie; Bitla, Yugandhar; Huang, Chun-Wei; Do, Thi Hien; Liu, Heng-Jui; Hsieh, Ying-Hui; Ma, Chun-Hao; Jang, Chi-Yuan; Lai, Yu-Hong; Chiu, Po-Wen; Wu, Wen-Wei; Chen, Yi-Chun; Zhou, Yi-Chun; Chu, Ying-Hao
2017-06-01
We present a promising technology for nonvolatile flexible electronic devices: A direct fabrication of epitaxial lead zirconium titanate (PZT) on flexible mica substrate via van der Waals epitaxy. These single-crystalline flexible ferroelectric PZT films not only retain their performance, reliability, and thermal stability comparable to those on rigid counterparts in tests of nonvolatile memory elements but also exhibit remarkable mechanical properties with robust operation in bent states (bending radii down to 2.5 mm) and cycling tests (1000 times). This study marks the technological advancement toward realizing much-awaited flexible yet single-crystalline nonvolatile electronic devices for the design and development of flexible, lightweight, and next-generation smart devices with potential applications in electronics, robotics, automotive, health care, industrial, and military systems.
Engineering Low Dimensional Materials with van der Waals Interaction
Jin, Chenhao
Two-dimensional van der Waals materials grow into a hot and big field in condensed matter physics in the past decade. One particularly intriguing thing is the possibility to stack different layers together as one wish, like playing a Lego game, which can create artificial structures that do not exist in nature. These new structures can enable rich new physics from interlayer interaction: The interaction is strong, because in low-dimension materials electrons are exposed to the interface and are susceptible to other layers; and the screening of interaction is less prominent. The consequence is rich, not only from the extensive list of two-dimensional materials available nowadays, but also from the freedom of interlayer configuration, such as displacement and twist angle, which creates a gigantic parameter space to play with. On the other hand, however, the huge parameter space sometimes can make it challenging to describe consistently with a single picture. For example, the large periodicity or even incommensurability in van der Waals systems creates difficulty in using periodic boundary condition. Worse still, the huge superlattice unit cell and overwhelming computational efforts involved to some extent prevent the establishment of a simple physical picture to understand the evolution of system properties in the parameter space of interlayer configuration. In the first part of the dissertation, I will focus on classification of the huge parameter space into subspaces, and introduce suitable theoretical approaches for each subspace. For each approach, I will discuss its validity, limitation, general solution, as well as a specific example of application demonstrating how one can obtain the most important effects of interlayer interaction with little computation efforts. Combining all the approaches introduced will provide an analytic solution to cover majority of the parameter space, which will be very helpful in understanding the intuitive physical picture behind
Chaos of the Relativistic Forced van der Pol Oscillator
Ashkenazy, Yu; Horwitz, L P
1997-01-01
A manifestly relativistically covariant form of the van der Pol oscillator in 1+1 dimensions is studied. We show that the driven relativistic equations, for which x and t are coupled, relax very quickly to a pair ofidenticaldecoupled equations, due to a rapid vanishing of the ``angular momentum'' (the boost in 1+1 dimensions). A similar effect occurs in the damped driven covariant Duffing oscillator previously treated. This effect is an example of entrainment, or synchronization (phase locking), of coupled chaotic systems. The Lyapunov exponents are calculated using the very efficient method of Habib and Ryne. We show a Poincare map that demonstrates this effect and maintains remarkable stability in spite of the inevitable accumulation of computer error in the chaotic region. For our choice of parameters, the positive Lyapunov exponent is about 0.242 almost independently of the integration method.
Thermohydrodynamics of boiling in a van der Waals fluid.
Laurila, T; Carlson, A; Do-Quang, M; Ala-Nissila, T; Amberg, G
2012-02-01
We present a modeling approach that enables numerical simulations of a boiling Van der Waals fluid based on the diffuse interface description. A boundary condition is implemented that allows in and out flux of mass at constant external pressure. In addition, a boundary condition for controlled wetting properties of the boiling surface is also proposed. We present isothermal verification cases for each element of our modeling approach. By using these two boundary conditions we are able to numerically access a system that contains the essential physics of the boiling process at microscopic scales. Evolution of bubbles under film boiling and nucleate boiling conditions are observed by varying boiling surface wettability. We observe flow patters around the three-phase contact line where the phase change is greatest. For a hydrophilic boiling surface, a complex flow pattern consistent with vapor recoil theory is observed.
Critical fluctuations in models with van der Waals interactions
Vovchenko, V; Gorenstein, M I; Poberezhnyuk, R V; Stoecker, H
2016-01-01
Particle number fluctuations are considered within the van der Waals (VDW) equation, which contains both attractive (mean-field) and repulsive (eigenvolume) interactions. The VDW equation is used to calculate the scaled variance of particle number fluctuations in generic Boltzmann VDW system and in nuclear matter. The strongly intensive measures $\\Delta[E^*,N]$ and $\\Sigma[E^*,N]$ of the particle number and excitation energy fluctuations are also considered, and, similarly, show singular behavior near the critical point. The $\\Delta[E^*,N]$ measure is shown to attain both positive and negative values in the vicinity of critical point. Based on universality argument, similar behavior is expected to occur in the vicinity of the QCD critical point.
Heterostructures based on inorganic and organic van der Waals systems
Gwan-Hyoung Lee
2014-09-01
Full Text Available The two-dimensional limit of layered materials has recently been realized through the use of van der Waals (vdW heterostructures composed of weakly interacting layers. In this paper, we describe two different classes of vdW heterostructures: inorganic vdW heterostructures prepared by co-lamination and restacking; and organic-inorganic hetero-epitaxy created by physical vapor deposition of organic molecule crystals on an inorganic vdW substrate. Both types of heterostructures exhibit atomically clean vdW interfaces. Employing such vdW heterostructures, we have demonstrated various novel devices, including graphene/hexagonal boron nitride (hBN and MoS2 heterostructures for memory devices; graphene/MoS2/WSe2/graphene vertical p-n junctions for photovoltaic devices, and organic crystals on hBN with graphene electrodes for high-performance transistors.
A crossover in anisotropic nanomechanochemistry of van der Waals crystals
Shimamura, Kohei [Department of Physics, Kumamoto University, Kumamoto 860-8555 (Japan); Collaboratory for Advanced Computing and Simulations, Department of Physics and Astronomy, Department of Computer Science, Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-0242 (United States); Graduate School of System Informatics, Kobe University, Kobe 657-8501 (Japan); Misawa, Masaaki [Department of Physics, Kumamoto University, Kumamoto 860-8555 (Japan); Collaboratory for Advanced Computing and Simulations, Department of Physics and Astronomy, Department of Computer Science, Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-0242 (United States); Li, Ying [Argonne Leadership Computing Facility, Argonne National Laboratory, Argonne, Illinois 60439 (United States); Kalia, Rajiv K.; Nakano, Aiichiro; Vashishta, Priya [Collaboratory for Advanced Computing and Simulations, Department of Physics and Astronomy, Department of Computer Science, Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-0242 (United States); Shimojo, Fuyuki [Department of Physics, Kumamoto University, Kumamoto 860-8555 (Japan)
2015-12-07
In nanoscale mechanochemistry, mechanical forces selectively break covalent bonds to essentially control chemical reactions. An archetype is anisotropic detonation of layered energetic molecular crystals bonded by van der Waals (vdW) interactions. Here, quantum molecular dynamics simulations reveal a crossover of anisotropic nanomechanochemistry of vdW crystal. Within 10{sup −13} s from the passage of shock front, lateral collision produces NO{sub 2} via twisting and bending of nitro-groups and the resulting inverse Jahn-Teller effect, which is mediated by strong intra-layer hydrogen bonds. Subsequently, as we transition from heterogeneous to homogeneous mechanochemical regimes around 10{sup −12} s, shock normal to multilayers becomes more reactive, producing H{sub 2}O assisted by inter-layer N-N bond formation. These time-resolved results provide much needed atomistic understanding of nanomechanochemistry that underlies a wider range of technologies.
de Souza, André Rodrigues; Petrocelli, Iacopo; Lino-Neto, José; Santos, Eduardo Fernando; Noll, Fernando Barbosa; Turillazzi, Stefano
2016-01-01
Recent studies have reported incipient morphological caste dimorphism in the Van der Vecht organ size of some temperate Polistes paper wasps. Whether species other than the temperate ones show a similar pattern remains elusive. Here, we have studied some Neotropical Polistes species. By comparing females collected through the year, we showed caste related differences in the size of the Van der Vecht organ in P. ferreri (body size corrected Van der Vech organ size of queens = 0.45 ± 0.06, workers = 0.38 ± 0.07 mm2, p = 0.0021), P. versicolor (body size corrected Van der Vech organ size of queens = 0.54 ± 0.11, workers = 0.46 ± 0.09 mm2, p = 0.010), but not P. simillimus (body size corrected Van der Vech organ size of queens = 0.52 ± 0.05, workers = 0.49 ± 0.06 mm2, p = 0.238). Therefore, it seems that queens and workers of some Neotropical Polistes have diverged in their ontogenic trajectory of the Van der Vecht organ size, providing clear evidence for incipient morphological caste dimorphism. As Polistes are distributed mostly in the tropics, we propose that physical caste differences may be widespread in the genus. Also, we highlight that morphological divergence in the queen–worker phenotypes may have started through differential selection of body structures, like the Van der Vecht organ. PMID:27167514
Concurrent Van der Woude syndrome and Turner syndrome: A case report
Los, Evan; Baines, Hayley; Guttmann-Bauman, Ines
2017-01-01
Most cases of Van der Woude syndrome are caused by a mutation to interferon regulatory factor 6 on chromosome 1. Turner syndrome is caused by complete or partial absence of the second sex chromosome in girls. We describe a unique case of the two syndromes occurring concurrently though apparently independently in a girl with Van der Woude syndrome diagnosed at birth and Turner syndrome at 14 years 9 months. Short stature was initially misattributed to Van der Woude syndrome and pituitary insuf...
Effect of surface roughness on van der Waals and Casimir-Polder/Casimir attraction energies
Makeev, Maxim A.
2017-09-01
A theoretical model is devised to assess effects of surface roughness on dispersion interactions between macroscopic bodies, bounded by self-affine fractal surfaces and separated by a vacuum gap. The rough-surface profiles are described statistically by the saturation values of surface width and the correlation lengths; i.e., in terms of experimentally measurable quantities. The model devised takes into account the separation distance-dependent nature of dispersive interactions. The case of non-retarded van der Waals interactions, known to operate at smaller separation distances between the bodies, and that of retarded attractions, operative at larger separation length-scales, are treated separately in this work. Analytical formulae for the roughness corrections are deduced for the two aforementioned types of attractions. The model is employed to compute roughness corrections to interactions between an extended body, bounded by a self-affine surface, and: a) a point-like adherent; and b) a planar half-space. Furthermore, the roughness-induced corrections to dispersive interaction energies between half-spaces, both bounded by self-affine surfaces, are obtained under assumption that the corresponding surface profiles are not correlated. The predictions of the model are compared with some previously reported theoretical studies and available experimental data on the theme of dispersive adhesion between macroscopic bodies.
NATO Advanced Research Workshop on Dynamics of Polyatomic Van der Waals Complexes
Janda, Kenneth
1991-01-01
This publication is the Proceedings of the NATO Advanced Research Workshop (ARW) on the Dynamics of Polyatomic Van der Waals Molecules held at the Chateau de Bonas, Castera-Verduzan, France, from August 21 through August 26, 1989. Van der Waals complexes provide important model problems for understanding energy transfer and dissipation. These processes can be described in great detail for Van der Waals complexes, and the insight gained from such studies can be applied to more complicated chemical problems that are not amenable to detailed study. The workshop concentrated on the current questions and future prospects for extend ing our highly detailed knowledge of triatomic Van der Waals molecule dynamics to polyatomic molecules and clusters (one molecule surrounded by several, or up to sev eral tens of, atoms). Both experimental and theoretical studies were discussed, with particular emphasis on the dynamical behavior of dissociation as observed in the dis tributions of quantum states of the dissociatio...
Jazõk do demokrati ne dovedjot / Max van der Stoel
Stoel, Max van der, 1924-2011
1999-01-01
OSCE vähemusrahvuste ülemkomissari Max van der Stoeli pöördumine Lennart Meri poole seoses riigikeele oskuse nõude sisseviimisega Riigikogu valimise ja kohaliku omavalitsuse volikogu valimise seadusesse
ENPA juht usub Vene arengusse / Rene van der Linden ; interv. Erkki Bahovski
Linden, Rene van der
2006-01-01
Vt. ka Postimees : na russkom jazõke 2. juuni lk. 7. Euroopa Nõukogu Parlamentaarse Assamblee president Rene van der Linden Euroopa Nõukogu (EN) rollist, Venemaa ja Eesti arengutest EN-i liikmena. Lisa: Euroopa parlamentide kogu
Jazõk do demokrati ne dovedjot / Max van der Stoel
Stoel, Max van der, 1924-2011
1999-01-01
OSCE vähemusrahvuste ülemkomissari Max van der Stoeli pöördumine Lennart Meri poole seoses riigikeele oskuse nõude sisseviimisega Riigikogu valimise ja kohaliku omavalitsuse volikogu valimise seadusesse
ENPA juht usub Vene arengusse / Rene van der Linden ; interv. Erkki Bahovski
Linden, Rene van der
2006-01-01
Vt. ka Postimees : na russkom jazõke 2. juuni lk. 7. Euroopa Nõukogu Parlamentaarse Assamblee president Rene van der Linden Euroopa Nõukogu (EN) rollist, Venemaa ja Eesti arengutest EN-i liikmena. Lisa: Euroopa parlamentide kogu
The scholar and the state in search of van der Waerden
Soifer, Alexander
2015-01-01
Bartel Leendert van der Waerden made major contributions to algebraic geometry, abstract algebra, quantum mechanics, and other fields. He liberally published on the history of mathematics. His 2-volume work Modern Algebra is one of the most influential and popular mathematical books ever written. It is therefore surprising that no monograph has been dedicated to his life and work. Van der Waerden’s record is complex. In attempting to understand his life, the author assembled thousands of documents from numerous archives in Germany, the Netherlands, Switzerland and the United States which revealed fascinating and often surprising new information about van der Waerden. Soifer traces Van der Waerden’s early years in a family of great Dutch public servants, his life as professor in Leipzig during the entire Nazi period, and his personal and professional friendship with one of the great physicists Werner Heisenberg. We encounter heroes and villains and a much more numerous group in between these two extremes. ...
Bart Karstens
2010-09-01
Full Text Available The totality of man. The anthropological approach to psychiatry in the work of Lammert van der Horst (1893-1978 The anthropological approach was one of the new approaches to psychiatry that emerged in the interbellum. In the Netherlands professor Van der Horst (VU-university Amsterdam and the municipal University of Amsterdam was its most prominent proponent. The general idea of the anthropological approach was to integrate the various ways of knowing then available. A psychiatric disease was seen as the result of a failure in the self-realisation of the individual person. This required to consider all relevant aspects relating to the patient’s existence. How to tailor these ideas to concrete forms of diagnosis and methods of treatment was no easy matter and Van der Horst devoted himself all his life to this task. He first sought to classify man in three or four types of character inspired by the works of Heymans and Kretschmer. Then he tried to give the specific human aspect its place in psychiatry by introducing a ‘pneumatic’ dimension in his analysis of persons. He also connected this dimension to Calvinism, the church he belonged to. In the 1940’s he made a turn towards existentialism and tried to connect this philosophy to anthropological psychiatry. In spite of its fragmentary appearance I believe it is possible to discern a degree of continuity in the work of Van der Horst. The concern with the specifically human was always central to him. Moreover Van der Horst saw no strict divide between addressing questions in psychiatry and thinking about the greater questions of life which provides an explanation for his meandering thoughts. The dissertation of J.H. van den Berg which appeared in 1946 offers an interesting contrast to Van der Horst. To Van den Berg the anthropological approach was no more than a method best developed by Binswanger. Van den Berg tested this method and concluded that the approach could offer hermeneutic
Van der Waals interactions and Photoelectric Effect in Noncommutative Quantum Mechanics
LI Kang; CHAMOUN Nidal
2007-01-01
We calculate the long-range Van der Waals force and the photoelectric cross section in a noncommutative setup. It is argued that non-commutativity effects could not be discerned for the Van der Waals interactions. The result for the photoelectric effect shows deviation from the usual commutative one, which in principle can be used to put bounds on the space-space non-commutativity parameter.
van der Waals energy under strong atom-field coupling in doped carbon nanotubes
Bondarev, Igor; Lambin, Philippe
2004-01-01
Using a unified macroscopic QED formalism, we derive an integral equation for the van der Waals energy of a two-level atomic system near a carbon nanotube. The equation is valid for both strong and weak atom-vacuum-field coupling. By solving it numerically, we demonstrate the inapplicability of weak-coupling-based van der Waals interaction models in a close vicinity of the nanotube surface.
Modified van der Pauw method based on formulas solvable by the Banach fixed point method
2012-01-01
We propose a modification of the standard van der Pauw method for determining the resistivity and Hall coefficient of flat thin samples of arbitrary shape. Considering a different choice of resistance measurements we derive a new formula which can be numerically solved (with respect to sheet resistance) by the Banach fixed point method for any values of experimental data. The convergence is especially fast in the case of almost symmetric van der Pauw configurations (e.g., clover shaped samples).
van der Waals energy under strong atom field coupling in doped carbon nanotubes
Bondarev, I. V.; Lambin, Ph.
2004-10-01
Using a unified macroscopic QED formalism, we derive an integral equation for the van der Waals energy of a two-level atomic system near a carbon nanotube. The equation is valid for both strong and weak atom-vacuum-field coupling. By solving it numerically, we demonstrate the inapplicability of weak-coupling-based van der Waals interaction models in a close vicinity of the nanotube surface.
to different joint combinations and the Sharp/van der Heijde radiographic score
Ejbjerg, Bo Jannik; Vestergaard, Aage; Jacobsen, Søren;
2005-01-01
To compare 2 magnetic resonance imaging (MRI) approaches and radiographic evaluation according to the Sharp/van der Heijde method with respect to sensitivity to change in joint destruction in patients with rheumatoid arthritis (RA).......To compare 2 magnetic resonance imaging (MRI) approaches and radiographic evaluation according to the Sharp/van der Heijde method with respect to sensitivity to change in joint destruction in patients with rheumatoid arthritis (RA)....
José O Valderrama
2010-01-01
Full Text Available El autor presenta la conferencia que dictó Johannes Diderik van der Waals cuando recibió el premio Nobel de física el año 1910. Este trabajo es una contribución a las celebraciones que se realizan alrededor del mundo para conmemorar los cien años desde que van der Waals recibiera el máximo galardón. La conferencia resume en forma simple pero detallada algunos de sus principales logros; en particular la ecuación de estado, el principio de estados correspondientes y la teoría de mezclas. Estas pioneras ideas de van der Waals han influenciado efectivamente varias áreas de la ciencia, pero en particular estos tres conceptos, y que fueron los que motivaron su merecido premio Nobel. Este artículo concluye que aún después de 100 años del Nobel y más de 125 de la Tesis doctoral, y a pesar de muchos otros extraordinarios logros en la física y en la termodinámica, los conceptos de van der Waals siguen más vigentes que nunca.The author presents the conference that van der Waals delivered when he received the Nobel Prize for Physics in 1910. This work is a contribution to the several commemorating activities that are be-ing organized around the world to celébrate the 100 years since van der Waals was awarded the máximum prize. The conference summarizes some of the main achievements and in particular the equation of state, the principie of corresponding states and the theory of mixtures. These pioneering ideas of van der Waals have influenced several áreas of physics and thermodynamics but in particular these three concepts, which motivated his merited Nobel Prize. The paper concludes that even after a hundred years since the Nobel and more than 125 years since the doctoral thesis, and despite of many other extraordinary advances in physics and thermodynamics, the concepts of van der Waals continué more valid than ever.
Density-functional calculation of van der Waals forces for free-electron-like surfaces
Hult, E.; Hyldgaard, P.; Rossmeisl, Jan;
2001-01-01
A recently proposed general density functional for asymptotic van der Waals forces is used to calculate van der Waals coefficients and reference-plane positions for realistic low-indexed Al surfaces. Results are given for a number of atoms and molecules outside the surfaces, as well as for the in......A recently proposed general density functional for asymptotic van der Waals forces is used to calculate van der Waals coefficients and reference-plane positions for realistic low-indexed Al surfaces. Results are given for a number of atoms and molecules outside the surfaces, as well...... as for the interaction between the surfaces themselves. The densities and static image-plane positions that are needed as input in the van der Waals functional are calculated self-consistently within density-functional theory using the generalized-gradient approximation, pseudopotentials, and plane waves. This study...... shows that the van der Waals density functional is applicable to realistic surfaces. The need for physically correct surface models, especially for open surfaces, is also illustrated. Finally the parameters for the anisotropic interaction of O-2 with Al are calculated....
Curious and Interesting——Products from Van Eijk & Van der Lubbe%奇趣设计
夏天
2009-01-01
@@ 来自荷兰的Van Eijk & Van der Lubbe是近些年来业界最受瞩目的设计组合之一.用设计师自己的话说,他们的设计总是趋向于提出问题,将人们生活中熟知的事物,例如生活用品或是陈设等,重新设计改良.
Molecular interactions in particular Van der Waals nanoclusters
Jungclas, Hartmut; Schmidt, Lothar [Marburg Univ. (Germany). Chemistry Dept.; Komarov, Viacheslav V.; Popova, Anna M. [Marburg Univ. (Germany). Chemistry Dept.; Lomonosov Moscow State Univ. (Russian Federation). Skobeltzin Inst. of Nuclear Physics
2017-04-01
A method is presented to analyse the interaction energies in a nanocluster, which is consisting of three neutral molecules bound by non-covalent long range Van der Waals forces. One of the molecules (M{sub 0}) in the nanocluster has a permanent dipole moment, whereas the two other molecules (M{sub 1} and M{sub 2}) are non-polar. Analytical expressions are obtained for the numerical calculation of the dispersion and induction energies of the molecules in the considered nanocluster. The repulsive forces at short intermolecular distances are taken into account by introduction of damping functions. Dispersion and induction energies are calculated for a nanocluster with a definite geometry, in which the polar molecule M{sub 0} is a linear hydrocarbon molecule C{sub 5}H{sub 10} and M{sub 1} and M{sub 2} are pyrene molecules. The calculations are done for fixed distances between the two pyrene molecules. The results show that the induction energies in the considered three-molecular nanocluster are comparable with the dispersion energies. Furthermore, the sum of induction energies in the substructure (M{sub 0}, M{sub 1}) of the considered nanocluster is much higher than the sum of induction energies in a two-molecular nanocluster with similar molecules (M{sub 0}, M{sub 1}) because of the absence of an electrostatic field in the latter case. This effect can be explained by the essential intermolecular induction in the three-molecular nanocluster.
Van der Waals torque induced by external magnetic fields
Esquivel-Sirvent, R; Palomono-Ovando, M
2010-01-01
We present a method for inducing and controlling van der Waals torques between two parallel slabs using a constant magnetic field. The torque is calculated using the Barash theory of dispersive torques. In III-IV semiconductors such as $InSb$, the effect of an external magnetic field is to induce an optical anisotropy, in an otherwise isotropic material, that will in turn induce a torque. The calculations of the torque are done in the Voigt configuration, with the magnetic field parallel to the surface of the slabs. As a case study we consider a slab made of calcite and a second slab made of $InSb$. In the absence of magnetic field there is no torque. As the magnetic field increases, the optical anisotropy of $InSb$ increases and the torque becomes different from zero, increasing with the magnetic field. The resulting torque is of the same order of magnitude as that calculated using permanent anisotropic materials when the magnetic fields is close to 1 T.
Van der Waals torque induced by external magnetic fields
Esquivel-Sirvent, R.; Cocoletzi, G. H.; Palomino-Ovando, M.
2010-01-01
We present a method for inducing and controlling van der Waals torques between two parallel slabs using a constant magnetic field. The torque is calculated using the Barash theory of dispersive torques. In III–IV semiconductors such as InSb, the effect of an external magnetic field is to induce an optical anisotropy, in an otherwise isotropic material, that will in turn induce a torque. The calculations of the torque are done in the Voigt configuration, with the magnetic field parallel to the surface of the slabs. As a case study we consider a slab made of calcite and a second slab made of InSb. In the absence of magnetic field there is no torque. As the magnetic field increases, the optical anisotropy of InSb increases and the torque becomes different from zero, increasing with the magnetic field. The resulting torque is of the same order of magnitude as that calculated using permanent anisotropicmaterials when the magnetic fields is close to 1 T.
Spatially Correlated Disorder in Epitaxial van der Waals Heterostructures
Laanait, Nouamane; Zhang, Zhan; Schleputz, Christian; Liu, Ying; Wojcik, Michael; Myers-Ward, Rachael; Gaskill, D. Kurt; Fenter, Paul; Li, Lian
The structural cohesion of van der Waals (vdW) heterostructures relies upon a cooperative balance between strong intra-layer bonded interactions and weak inter-layer coupling. The confinement of extended defects to within a single vdW layer and competing interactions introduced by epitaxial constraints could generate fundamentally new structural disorders. Here we report on the presence of spatially correlated and localized disorder states that coexist with the near perfect crystallographic order along the growth direction of epitaxial vdW heterostructure of Bi2Se3/graphene/SiC grown by molecular beam epitaxy. With the depth penetration of hard X-ray diffraction microscopy and high-resolution surface scattering, we imaged local structural configurations from the atomic to mesoscopic length scales, and found that these disorder states result as a confluence of atomic scale modulations in the strength of vdW layer-layer interactions and nanoscale boundary conditions imposed by the substrate. These findings reveal a vast landscape of novel disorder states that can be manifested in epitaxial vdW heterostructures. Supported by the Wigner Fellowship program at Oak Ridge Nat'l Lab.
Thermionic Energy Conversion Based on Graphene van der Waals Heterostructures
Liang, Shi-Jun; Liu, Bo; Hu, Wei; Zhou, Kun; Ang, L. K.
2017-01-01
Seeking for thermoelectric (TE) materials with high figure of merit (or ZT), which can directly converts low-grade wasted heat (400 to 500 K) into electricity, has been a big challenge. Inspired by the concept of multilayer thermionic devices, we propose and design a solid-state thermionic devices (as a power generator or a refrigerator) in using van der Waals (vdW) heterostructure sandwiched between two graphene electrodes, to achieve high energy conversion efficiency in the temperature range of 400 to 500 K. The vdW heterostructure is composed of suitable multiple layers of transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2 and WSe2. From our calculations, WSe2 and MoSe2 are identified as two ideal TMDs (using the reported experimental material’s properties), which can harvest waste heat at 400 K with efficiencies about 7% to 8%. To our best knowledge, this design is the first in combining the advantages of graphene electrodes and TMDs to function as a thermionic-based device. PMID:28387363
Consistent van der Waals radii for the whole main group.
Mantina, Manjeera; Chamberlin, Adam C; Valero, Rosendo; Cramer, Christopher J; Truhlar, Donald G
2009-05-14
Atomic radii are not precisely defined but are nevertheless widely used parameters in modeling and understanding molecular structure and interactions. The van der Waals radii determined by Bondi from molecular crystals and data for gases are the most widely used values, but Bondi recommended radius values for only 28 of the 44 main-group elements in the periodic table. In the present Article, we present atomic radii for the other 16; these new radii were determined in a way designed to be compatible with Bondi's scale. The method chosen is a set of two-parameter correlations of Bondi's radii with repulsive-wall distances calculated by relativistic coupled-cluster electronic structure calculations. The newly determined radii (in A) are Be, 1.53; B, 1.92; Al, 1.84; Ca, 2.31; Ge, 2.11; Rb, 3.03; Sr, 2.49; Sb, 2.06; Cs, 3.43; Ba, 2.68; Bi, 2.07; Po, 1.97; At, 2.02; Rn, 2.20; Fr, 3.48; and Ra, 2.83.
LA CASA CON PATIO EN MIES VAN DER ROHE / House with patio from Mies van der Rohe
José Altés Bustelo
2013-05-01
Full Text Available RESUMEN Una revisión del tema Casa con Patio, con especial atención a los planteamientos desarrollados por Mies van der Rohe durante sus últimos años en Berlín, a propósito del conocido dibujo denominado “Casa con tres patios”. Se explora su pensamiento arquitectónico a partir de los datos historiográficos de su trayectoria, más los que se pueden deducir del análisis de los dibujos y croquis previos del arquitecto. El conjunto de proyectos en que aparece el tema es muy amplio durante ese tiempo y los dibujos conservados ofrecen múltiples reflexiones. Sobre ese soporte se examinan conceptos relativos a la definición espacial y formal, donde aparecen cuestiones que ligan esa definición con los sistemas y materiales constructivos. Malla estructural, muro, pared de vidrio y patio vividero, vinculados necesariamente a sus respectivas cualidades tectónicas, adquieren el sentido de materiales de proyecto utilizados para definir un particular modo de entender la arquitectura en el que el usuario está presente siempre como destinatario de la misma. Finalmente, se plantea la utilidad didáctica del análisis de esos dibujos para el progreso del conocimiento en el proyecto arquitectónico.SUMMARY A review of the theme, House with Patio, with special reference to the plans developed by Mies van der Rohe during his last years in Berlin, in relation to the well–known drawing called: “House with three patios”. His architectural thoughts are explored through the historiographic data of his career, plus those that can be deduced from the analysis of the architect’s previous drawings and sketches. The group of projects, in which the theme appears is prolific from that time, and the preserved drawings offer multiple opportunities for reflection. Concepts on that medium are examined, relative to spatial and formal definition, where questions appear that link that definition with the constructive systems and materials. Structural mesh
Hyldgaard, Per; Rahman, Talat S.
2012-10-01
The past decade has seen a dramatic rise in interest in exploring the role that van der Waals (vdW) or dispersion forces play in materials and in material behavior. Part of this stems from the obvious fact that vdW interactions (and other weak forces, such as Casimir) underpin molecular recognition, i.e., nature's approach to search for a match between genes and anti-genes and hence enable biological function. Less obvious is the recognition that vdW interactions affect a multitude of properties of a vast variety of materials in general, some of which also have strong technological applications. While for two atom- or orbital-sized material fragments the dispersive contributions to binding are small compared to those from the better known forms (ionic, covalent, metallic), those between sparse materials (spread over extended areas) can be of paramount importance. For example, an understanding of binding in graphite cannot arise solely from a study of the graphene layers individually, but also requires insight from inter-sheet graphene vdW bonding. It is the extended-area vdW bonding that provides sufficient cohesion to make graphite a robust, naturally occurring material. In fact, it is the vdW-bonded graphite, and not the all-covalently bonded diamond, that is the preferred form of pure carbon under ambient conditions. Also important is the understanding that vdW attraction can attain a dramatic relevance even if the material fragments, the building blocks, are not necessarily parallel from the outset or smooth when viewed in isolation (such as a graphene sheet or a carbon nanotube). This can happen if the building blocks have some softness and flexibility and allow an internal relative alignment to emerge. The vdW forces can then cause increasingly larger parts of the interacting fragments to line up at sub-nanometer separations and thus beget more areas with a sizable vdW bonding contribution. The gecko can scale a wall because it can bring its flexible hairs
Van der Waals-like behaviour of charged black holes and hysteresis in the dual QFTs
Mariano Cadoni
2017-05-01
Full Text Available Using the rules of the AdS/CFT correspondence, we compute the spherical analogue of the shear viscosity, defined in terms of the retarded Green function for the stress-energy tensor for QFTs dual to five-dimensional charged black holes of general relativity with a negative cosmological constant. We show that the ratio between this quantity and the entropy density, η˜/s, exhibits a temperature-dependent hysteresis. We argue that this hysteretic behaviour can be explained by the Van der Waals-like character of charged black holes, considered as thermodynamical systems. Under the critical charge, hysteresis emerges owing to the presence of two stable states (small and large black holes connected by a meta-stable region (intermediate black holes. A potential barrier prevents the equilibrium path between the two stable states; the system evolution must occur through the meta-stable region, and a path-dependence of η˜/s is generated.
On van der Waals friction between half-spaces at low temperature.
Barton, Gabriel
2011-09-07
We determine, in the van der Waals regime (neglecting retardation and relativistic effects), the frictional power loss P and the drag force F = P/u per unit area between two Drude-modelled half-spaces, with surface plasmon frequency ω(S) and realistically weak dissipation, separated by a gap of width ζ, and constrained to uniform parallel motion with relative speed u. The calculation uses only textbook-level adiabatic and perturbative methods of nonrelativistic quantum mechanics. The initial temperature is taken to be low, with τ ≡ k(B)T/ħω(S) 1. But at fixed nonzero τ, as v rises from zero, P is at first dominated by a temperature-dependent component proportional to u(2)T(2)/ζ(4). The assumptions of the model as regards the half-spaces are satisfied by most quantum-electrodynamics-based approaches, whose results in the nonretarded limit should therefore be the same as ours. We also find that the frequency distribution of the friction-induced energy increments is not thermal, suggesting that in this respect the Huttner-Barnett theory (which we use to describe dissipative materials) needs further refinement.
Maria van der Hoeven, the Netherlands minister for education, culture and science, visited CERN
maximilien Brice
2005-01-01
On 21 April, the Netherlands Minister for Education, Culture and Science, Mrs Maria van der Hoeven, was welcomed to CERN by the Director-General, Robert Aymar, and the Chief Scientific Officer, Jos Engelen. Minister van der Hoeven visited the ATLAS installations, the LHC tunnel and the magnet assembly and test hall before meeting a group of young scientists from the Netherlands. Picture 05 : from left to right, Frank Linde, Director of the Netherlands National Institute for Nuclear Physics and High Energy Physics (NIKHEF), Jos Engelen, CERN's Chief Scientific Officer, Maria van der Hoeven, Netherlands Minister for Education, Culture and Science, and Herman Ten Kate, Head of the ATLAS magnet project, visiting the ATLAS assembly hall.Picture 09 ; Here she talks with, from left to right, Jos Engelen, CERN's chief scientific officer, Peter Jenni, the ATLAS spokesman, Herman Ten Kate, head of the ATLAS magnet project, and Frank Linde, director of the Netherlands National Institute for Nuclear Physics and High Ener...
Van der Waals interaction torque and force between dielectrically anisotropic layered media
Lu, Bing-Sui
2016-01-01
We analyse the van der Waals interaction for a pair of dielectrically anisotropic plane-layered media interacting across a dielectrically isotropic medium. We investigate the van der Waals torque and force for the following cases: (i) a pair of single anisotropic layers, (ii) a single anisotropic layer interacting with a multilayered slab consisting of alternating anisotropic and isotropic layers, and (iii) a pair of multilayered slabs each consisting of alternating anisotropic and isotropic layers, looking at the cases where the optical axes lie parallel and/or perpendicular to the plane of the layers. For the first case, the optical axes of the oppositely facing anisotropic layers of the two interacting slabs generally have an angular mismatch, and within each multilayered slab the optical axes may either be the same, or undergo constant angular increments across the anisotropic layers. In particular, we examine how the behaviors of the van der Waals torque and force can be "tuned" by adjusting the layer th...
Van der Waals interaction between a microparticle and a single-wall carbon nanotube
Blagov, E V; Mostepanenko, V M
2007-01-01
The Lifshitz-type formulas describing the free energy and the force of the van der Waals interaction between an atom (molecule) and a single-wall carbon nanotube are obtained. The single-wall nanotube is considered as a cylindrical sheet carrying a two-dimensional free electron gas with appropriate boundary conditions on the electromagnetic field. The obtained formulas are used to calculate the van der Waals free energy and force between a hydrogen atom (molecule) and single-wall carbon nanotubes of different radia. Comparison studies of the van der Waals interaction of hydrogen atoms with single- and multi-wall carbon nanotubes show that depending on atom-nanotube separation distance the idealization of graphite dielectric permittivity is already applicable to nanotubes with only two or three walls.
Concurrent Van der Woude syndrome and Turner syndrome: A case report
Los, Evan; Baines, Hayley; Guttmann-Bauman, Ines
2017-01-01
Most cases of Van der Woude syndrome are caused by a mutation to interferon regulatory factor 6 on chromosome 1. Turner syndrome is caused by complete or partial absence of the second sex chromosome in girls. We describe a unique case of the two syndromes occurring concurrently though apparently independently in a girl with Van der Woude syndrome diagnosed at birth and Turner syndrome at 14 years 9 months. Short stature was initially misattributed to Van der Woude syndrome and pituitary insufficiency associated with clefts before correctly diagnosing Turner syndrome. We discuss the prevalence of delayed diagnosis of Turner syndrome, the rarity of reports of concurrent autosomal chromosome mutation and sex chromosome deletion, as well as the need to consider the diagnosis of Turner syndrome in all girls with short stature regardless of prior medical history. PMID:28228961
Simon van der Meer (1925-2011): A modest genius of accelerator science
Chohan, Vinod C
2011-01-01
Simon van der Meer was a brilliant scientist and a true giant of accelerator science. His seminal contributions to accelerator science have been essential to this day in our quest for satisfying the demands of modern particle physics. Whether we talk of long base-line neutrino physics or antiproton-proton physics at Fermilab or proton-proton physics at LHC, his techniques and inventions have been a vital part of the modern day successes. Simon van der Meer and Carlo Rubbia were the first CERN scientists to become Nobel laureates in Physics, in 1984. Van der Meer's lesser-known contributions spanned a whole range of subjects in accelerator science, from magnet design to power supply design, beam measurements, slow beam extraction, sophisticated programs and controls.
Professor Bennie van der Walt: a bridge between white Afrikaners and black Africans
Y. Turaki
2010-07-01
Full Text Available This article honours Professor Bennie van der Walt as a bridge builder between white Afrikaners and black Africans as well as a renowned Christian scholar. Historical Western colonialism in South Africa divided its citizens against each other by means of white racism and apartheid. The whites in general were pitched against the blacks on the basis of white racism and its doctrine of apartheid. This doctrine of separation of races kept the white Afrikaners from the Bantu Africans. However, apartheid as a form of political, social, cultural and religious racism is now history in South Africa. The role which Professor Van der Walt played in bridging the gap between this racial divide is highly commendable and needs to be acknowledged and appreciated, hence the primary objective of this article in honour of his 71st birthday. Furthermore, the article discusses the immense contributions of Professor Bennie van der Walt to Christian scholarship in Africa.
Van der Waals interactions and the limits of isolated atom models at interfaces.
Kawai, Shigeki; Foster, Adam S; Björkman, Torbjörn; Nowakowska, Sylwia; Björk, Jonas; Canova, Filippo Federici; Gade, Lutz H; Jung, Thomas A; Meyer, Ernst
2016-05-13
Van der Waals forces are among the weakest, yet most decisive interactions governing condensation and aggregation processes and the phase behaviour of atomic and molecular matter. Understanding the resulting structural motifs and patterns has become increasingly important in studies of the nanoscale regime. Here we measure the paradigmatic van der Waals interactions represented by the noble gas atom pairs Ar-Xe, Kr-Xe and Xe-Xe with a Xe-functionalized tip of an atomic force microscope at low temperature. Individual rare gas atoms were fixed at node sites of a surface-confined two-dimensional metal-organic framework. We found that the magnitude of the measured force increased with the atomic radius, yet detailed simulation by density functional theory revealed that the adsorption induced charge redistribution strengthened the van der Waals forces by a factor of up to two, thus demonstrating the limits of a purely atomic description of the interaction in these representative systems.
Bell's palsy before Bell: Cornelis Stalpart van der Wiel's observation of Bell's palsy in 1683.
van de Graaf, Robert C; Nicolai, Jean-Philippe A
2005-11-01
Bell's palsy is named after Sir Charles Bell (1774-1842), who has long been considered to be the first to describe idiopathic facial paralysis in the early 19th century. However, it was discovered that Nicolaus Anton Friedreich (1761-1836) and James Douglas (1675-1742) preceded him in the 18th century. Recently, an even earlier account of Bell's palsy was found, as observed by Cornelis Stalpart van der Wiel (1620-1702) from The Hague, The Netherlands in 1683. Because our current knowledge of the history of Bell's palsy before Bell is limited to a few documents, it is interesting to discuss Stalpart van der Wiel's description and determine its additional value for the history of Bell's palsy. It is concluded that Cornelis Stalpart van der Wiel was the first to record Bell's palsy in 1683. His manuscript provides clues for future historical research.
Observation of scalar nuclear spin-spin coupling in van der Waals molecules
Ledbetter, Micah; Bagno, Alessandro; Tran, Nhan; Romalis, Michael
2011-01-01
Scalar couplings between covalently bound nuclear spins are a ubiquitous feature in nuclear magnetic resonance (NMR) experiments, imparting valuable information to NMR spectra regarding molecular structure and conformation. Such couplings arise due to a second-order hyperfine interaction, and, in principle, the same mechanism should lead to scalar couplings between nuclear spins in unbound van der Waals complexes. Here, we report the first observation of scalar couplings between nuclei in van der Waals molecules. Our measurements are performed in a solution of hyperpolarized ${\\rm ^{129}Xe}$ and pentane, using superconducting quantum interference devices to detect NMR in 10 mG fields, and are in good agreement with calculations based on density functional theory. van der Waals forces play an important role in many physical phenomena, and hence the techniques presented here may provide a new method for probing such interactions.
Cohesive properties of noble metals by van der Waals-corrected Density Functional Theory
Ambrosetti, Alberto
2016-01-01
The cohesive energy, equilibrium lattice constant, and bulk modulus of noble metals are computed by different van der Waals-corrected Density Functional Theory methods, including vdW-DF, vdW-DF2, vdW-DF-cx, rVV10 and PBE-D. Two specifically-designed methods are also developed in order to effectively include dynamical screening effects: the DFT/vdW-WF2p method, based on the generation of Maximally Localized Wannier Functions, and the RPAp scheme (in two variants), based on a single-oscillator model of the localized electron response. Comparison with results obtained without explicit inclusion of van der Waals effects, such as with the LDA, PBE, PBEsol, or the hybrid PBE0 functional, elucidates the importance of a suitable description of screened van der Waals interactions even in the case of strong metal bonding. Many-body effects are also quantitatively evaluated within the RPAp approach.
Many-body effects in the van der Waals-Casimir interaction between graphene layers
Sarabadani, Jalal; Naji, Ali; Asgari, Reza; Podgornik, Rudolf
2011-10-01
Van der Waals-Casimir dispersion interactions between two apposed graphene layers, a graphene layer and a substrate, and in a multilamellar graphene system are analyzed within the framework of the Lifshitz theory. This formulation hinges on a known form of the dielectric response function of an undoped or doped graphene sheet, assumed to be of a random-phase-approximation form. In the geometry of two apposed layers, the separation dependence of the van der Waals-Casimir interaction for both types of graphene sheets is determined and critically compared with some well-known limiting cases. In a multilamellar array, the many-body effects are quantified and shown to increase the magnitude of the van der Waals-Casimir interactions.
Lu, Bing-Sui; Podgornik, Rudolf
2016-01-01
We analyze van der Waals interactions between two rigid polymers with sequence-specific, anisotropic polarizabilities along the polymer backbones, so that the dipole moments fluctuate parallel to the polymer backbones. Assuming that each polymer has a quenched-in polarizability sequence which reflects, for example, the polynucleotide sequence of a double-stranded DNA molecule, we study the van der Waals interaction energy between a pair of such polymers with rod-like structure for the cases where their respective polarizability sequences are (i) distinct and (ii) identical, with both zero and non-zero correlation length of the polarizability correlator along the polymer backbones in the latter case. For identical polymers, we find a novel $r^{-5}$ scaling behavior of the van der Waals interaction energy for small inter-polymer separation $r$, in contradistinction to the $r^{-4}$ scaling behavior of distinct polymers, with furthermore a pronounced angular dependence favoring attraction between sufficiently ali...
Concurrent Van der Woude syndrome and Turner syndrome: A case report
Evan Los
2017-01-01
Full Text Available Most cases of Van der Woude syndrome are caused by a mutation to interferon regulatory factor 6 on chromosome 1. Turner syndrome is caused by complete or partial absence of the second sex chromosome in girls. We describe a unique case of the two syndromes occurring concurrently though apparently independently in a girl with Van der Woude syndrome diagnosed at birth and Turner syndrome at 14 years 9 months. Short stature was initially misattributed to Van der Woude syndrome and pituitary insufficiency associated with clefts before correctly diagnosing Turner syndrome. We discuss the prevalence of delayed diagnosis of Turner syndrome, the rarity of reports of concurrent autosomal chromosome mutation and sex chromosome deletion, as well as the need to consider the diagnosis of Turner syndrome in all girls with short stature regardless of prior medical history.
Concurrent Van der Woude syndrome and Turner syndrome: A case report.
Los, Evan; Baines, Hayley; Guttmann-Bauman, Ines
2017-01-01
Most cases of Van der Woude syndrome are caused by a mutation to interferon regulatory factor 6 on chromosome 1. Turner syndrome is caused by complete or partial absence of the second sex chromosome in girls. We describe a unique case of the two syndromes occurring concurrently though apparently independently in a girl with Van der Woude syndrome diagnosed at birth and Turner syndrome at 14 years 9 months. Short stature was initially misattributed to Van der Woude syndrome and pituitary insufficiency associated with clefts before correctly diagnosing Turner syndrome. We discuss the prevalence of delayed diagnosis of Turner syndrome, the rarity of reports of concurrent autosomal chromosome mutation and sex chromosome deletion, as well as the need to consider the diagnosis of Turner syndrome in all girls with short stature regardless of prior medical history.
Van der Waals universality in homonuclear atom-dimer elastic collisions
Giannakeas, P
2016-01-01
The universal aspects of atom-dimer elastic collisions are investigated within the framework of Faddeev equations. The two-body interactions between the neutral atoms are approximated by the separable potential approach. Our analysis considers a pure van der Waals potential tail as well as soft-core van der Waals interactions permitting us in this manner to address the universally general features of atom-dimer resonant spectra. In particular, we show that the atom-dimer resonances are solely associated with the {\\it excited} Efimov states. Furthermore, the positions of the corresponding resonances for a soft-core potentials with more than 5 bound states are in good agreement with the corresponding results from an infinitely deep pure van der Waals tail potential.
Measurement of van-der-Waals interaction by atom trajectory imaging
Thaicharoen, N; Raithel, G
2015-01-01
We study the repulsive van der Waals interaction of cold rubidium $70S_{1/2}$ Rydberg atoms by analysis of time-delayed pair correlation functions. After excitation, Rydberg atoms are allowed to accelerate under the influence of the van der Waals force. Their positions are then measured using a single-atom imaging technique. From the average pair correlation function of the atom positions we obtain the initial atom-pair separation and the terminal velocity, which yield the van der Waals interaction coefficient $C_{6}$. The measured $C_{6}$ value agrees well with calculations. The experimental method has been validated by simulations. The data hint at anisotropy in the overall expansion, caused by the shape of the excitation volume. Our measurement implies that the interacting entities are individual Rydberg atoms, not groups of atoms that coherently share a Rydberg excitation.
Van der Waals Interactions among Alkali Rydberg Atoms with Excitonic States
Zoubi, Hashem
2015-01-01
We investigate the influence of the appearance of excitonic states on van der Waals interactions among two Rydberg atoms. The atoms are assumed to be in different Rydberg states, e.g., in the $|ns\\rangle$ and $|np\\rangle$ states. The resonant dipole-dipole interactions yield symmetric and antisymmetric excitons, with energy splitting that give rise to new resonances as the atoms approach each other. Only far from these resonances the van der Waals coefficients, $C_6^{sp}$, can be defined. We calculate the $C_6$ coefficients for alkali atoms and present the results for lithium by applying perturbation theory. At short interatomic distances of several $\\mu m$, we show that the widely used simple model of two-level systems for excitons in Rydberg atoms breaks down, and the correct representation implies multi-level atoms. Even though, at larger distances one can keep the two-level systems but in including van der Waals interactions among the atoms.
Li, Yupeng; Kim, Hyung-Ick; Wei, Bingqing; Kang, Junmo; Choi, Jae-Boong; Nam, Jae-Do; Suhr, Jonghwan
2015-08-01
The local buckling behavior of vertically aligned carbon nanotubes (VACNTs) has been investigated and interpreted in the view of a collective nanotube response by taking van der Waals interactions into account. To the best of our knowledge, this is the first report on the case of collective VACNT behavior regarding van der Waals force among nanotubes as a lateral support effect during the buckling process. The local buckling propagation and development of VACNTs were experimentally observed and theoretically analyzed by employing finite element modeling with lateral support from van der Waals interactions among nanotubes. Both experimental and theoretical analyses show that VACNTs buckled in the bottom region with many short waves and almost identical wavelengths, indicating a high mode buckling. Furthermore, the propagation and development mechanism of buckling waves follow the wave damping effect.The local buckling behavior of vertically aligned carbon nanotubes (VACNTs) has been investigated and interpreted in the view of a collective nanotube response by taking van der Waals interactions into account. To the best of our knowledge, this is the first report on the case of collective VACNT behavior regarding van der Waals force among nanotubes as a lateral support effect during the buckling process. The local buckling propagation and development of VACNTs were experimentally observed and theoretically analyzed by employing finite element modeling with lateral support from van der Waals interactions among nanotubes. Both experimental and theoretical analyses show that VACNTs buckled in the bottom region with many short waves and almost identical wavelengths, indicating a high mode buckling. Furthermore, the propagation and development mechanism of buckling waves follow the wave damping effect. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr03581c
Mudd, Garry W; Svatek, Simon A; Hague, Lee; Makarovsky, Oleg; Kudrynskyi, Zakhar R; Mellor, Christopher J; Beton, Peter H; Eaves, Laurence; Novoselov, Kostya S; Kovalyuk, Zakhar D; Vdovin, Evgeny E; Marsden, Alex J; Wilson, Neil R; Patanè, Amalia
2015-07-01
High broad-band photoresponsivity of mechanically formed InSe-graphene van der Waals heterostructures is achieved by exploiting the broad-band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near-infrared to the visible spectrum. © 2015 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
A high-pressure van der Waals compound in solid nitrogen-helium mixtures
Vos, W. L.; Finger, L. W.; Hemley, R. J.; Hu, J. Z.; Mao, H. K.; Schouten, J. A.
1992-01-01
A detailed diamond anvil-cell study using synchrotron X-ray diffraction, Raman scattering, and optical microscopy has been conducted for the He-N system, with a view to the weakly-bound van der Waals molecule interactions that can be formed in the gas phase. High pressure is found to stabilize the formation of a stoichiometric, solid van der Waals compound of He(N2)11 composition which may exemplify a novel class of compounds found at high pressures in the interiors of the outer planets and their satellites.
Solution of the Duffing-van der Pol oscillator equation by a differential transform method
Mukherjee, Supriya [Department of Mathematics, Swami Vivekananda Institute for Science and Technology, South Gobindapur, P S - Sonarpur, Kolkata 700145, West Bengal (India); Roy, Banamali [Department of Mathematics, Bangabasi Evening College, 19 Rajkumar Chakraborty Sarani, Kolkata 700009, West Bengal (India); Dutta, Sourav, E-mail: supriya_ju@yahoo.com, E-mail: banamaliroy@yahoo.co.in [Department of Electrical Engineering, Jadavpur University, Kolkata 700032, West Bengal (India)
2011-01-15
In this paper, we have tried to implement a relatively new exact series method of solution, known as the differential transform method, for solving one of the widely studied and challenging equations in nonlinear dynamics, the Duffing-van der Pol oscillator equation. Chandrasekar et al (2004 J. Phys. A 37 4527) showed that the force-free Duffing-van der Pol oscillator is completely integrable for a specific parametric choice, and they derived a general solution for this parametric choice. The results of this paper are in sufficient agreement with those of Chandrasekar et al.
Anisotropic spheres with Van derWaals-type equation of state
S Thirukkanesh; F C Ragel
2014-07-01
We study static spherically symmetric space-time to describe relativistic compact objects with anisotropic matter distribution and derive two classes of exact models to the Einstein–Maxwell system with a modified Van derWaals equation of state. We motivate a Van derWaals-type equation of state to physically signify a high-density domain of quark matter, and the generated exact solutions are shown to contain several classes of exact models reported previously that correspond to various physical scenarios. Geometrical analysis shows that the physical quantities are well behaved so that these models may be used to describe anisotropic charged compact spheres.
A modified van der Pol equation with delay in a description of the heart action
Zduniak Beata
2014-12-01
Full Text Available In this paper, a modified van der Pol equation is considered as a description of the heart action. This model has a number of interesting properties allowing reconstruction of phenomena observed in physiological experiments as well as in Holter electrocardiographic recordings. Our aim is to study periodic solutions of the modified van der Pol equation and take into consideration the influence of feedback and delay which occur in the normal heart action mode as well as in pathological modes. Usage of certain values for feedback and delay parameters allows simulating the heart action when an accessory conducting pathway is present (Wolff-Parkinson-White syndrome.
Near-Unity Absorption in van der Waals Semiconductors for Ultrathin Optoelectronics.
Jariwala, Deep; Davoyan, Artur R; Tagliabue, Giulia; Sherrott, Michelle C; Wong, Joeson; Atwater, Harry A
2016-09-14
We demonstrate near-unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (<15 nm) van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of the key criteria to enable TMDCs as potential candidates to achieve high optoelectronic efficiency.
Nanoscale Tail Aggregation in Ionic Liquids： Roles of Electrostatic and van der Waals Interactions
赵海清; 石锐; 王延颋
2011-01-01
Nanoscale spatial heterogeneity in ionic liquids is formed by the aggregation of cationic tail groups. The electrostatic interactions between polar groups and the collective van der Waals interactions between nonpolar tail groups both contribute to the formation of tail domains, but the degrees of their contributions were unknown. In this work, by applying a strong external electric field to effectively overpower the electrostatic interactions between polar groups, we have determined that the tail aggregation is majorly attributed to the electrostatic interactions and the van der Waals interactions only have minor influence on the spatial heterogeneity phenomenon of ionic liquids.
Using computation to teach the properties of the van der Waals fluid
Swendsen, Robert H.
2013-10-01
The calculation of the thermodynamic properties of the van der Waals fluid is not trivial and most of its properties are rarely discussed because of mathematical difficulties. I describe a numerical approach that produces the full thermodynamic behavior of the van der Waals fluid with little effort. The numerical approach is particularly useful for showing the behavior of the specific heat, the isothermal compressibility, and the coefficient of thermal expansion at and near the critical point. The results of these computations show some surprising properties and give new insights into the mean-field description of the liquid-gas transition.
Competition of van der Waals and chemical forces on gold–sulfur surfaces and nanoparticles
Reimers, Jeffrey R.; Ford, Michael J.; Marcuccio, Sebastian M.
2017-01-01
Chemists generally believe that covalent and ionic bonds form much stronger links between atoms than the van der Waals force does. However, this is not always so. We present cases in which van der Waals dispersive forces introduce new competitive bonding possibilities rather than just modulating...... traditional bonding scenarios. Although the new possibilities could arise from any soft–soft chemical interaction, we focus on bonding between gold atoms and alkyl or arylsulfur ligands, RS. Consideration of all the interactions at play in sulfur-protected gold surfaces and gold nanoparticles is necessary...
Oscillation death in a coupled van der Pol–Mathieu system
Madhurjya P Bora; Dipak Sarmah
2013-10-01
We report an investigation of the oscillation death (OD) of a parametrically excited coupled van der Pol–Mathieu (vdPM) system. The system can be considered as a pair of harmonically forced van der Pol oscillators under a double-well potential. The two oscillators are coupled with a cubic nonlinearity. We have shown that the system arrives at an OD regime when coupling strength crosses a threshold value at which the system undergoes saddle-node bifurcation and two limit cycles coalesce onto a fixed point of the system. We have further shown that this nonautonomous system possesses a centre manifold corresponding to the OD regime.
The hot pick-up technique for batch assembly of van der Waals heterostructures
Pizzocchero, Filippo; Gammelgaard, Lene; Jessen, Bjarke Sørensen
2016-01-01
The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces bet...
La Torre, Alessandro; Gimenez-Lopez, Maria del Carmen; Fay, Michael W; Lucas, Carlos Herreros; Brown, Paul D; Khlobystov, Andrei N
2015-06-01
Transmission electron microscopy studies on the assembly and growth of gold nanoparticles on carbon nanotubes supported on few-layer graphene and amorphous carbon reveal a competition between van der Waals forces and electrostatic interactions, enabling controlled positioning and sizing of adsorbed nanoparticles at the nanochannels formed between the carbon nanotube and the few-layer graph-ene surface.
Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope
Govyadinov, Alexander A.
2017-07-14
Van der Waals materials exhibit intriguing structural, electronic, and photonic properties. Electron energy loss spectroscopy within scanning transmission electron microscopy allows for nanoscale mapping of such properties. However, its detection is typically limited to energy losses in the eV range-too large for probing low-energy excitations such as phonons or mid-infrared plasmons. Here, we adapt a conventional instrument to probe energy loss down to 100 meV, and map phononic states in hexagonal boron nitride, a representative van der Waals material. The boron nitride spectra depend on the flake thickness and on the distance of the electron beam to the flake edges. To explain these observations, we developed a classical response theory that describes the interaction of fast electrons with (anisotropic) van der Waals slabs, revealing that the electron energy loss is dominated by excitation of hyperbolic phonon polaritons, and not of bulk phonons as often reported. Thus, our work is of fundamental importance for interpreting future low-energy loss spectra of van der Waals materials.Here the authors adapt a STEM-EELS system to probe energy loss down to 100 meV, and apply it to map phononic states in hexagonal boron nitride, revealing that the electron loss is dominated by hyperbolic phonon polaritons.
Control of excitons in multi-layer van der Waals heterostructures
Calman, E. V., E-mail: ecalman@gmail.com; Dorow, C. J.; Fogler, M. M.; Butov, L. V. [Department of Physics, University of California at San Diego, La Jolla, California 92093-0319 (United States); Hu, S.; Mishchenko, A.; Geim, A. K. [School of Physics and Astronomy, University of Manchester, Manchester M13 9PL (United Kingdom)
2016-03-07
We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of MoS{sub 2} and hexagonal boron nitride. The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.
Molecular interpretation of nonclassical gas dynamics of dense vapors under the van der Waals model
Colonna, P.; Guardone, A.
2006-01-01
The van der Waals polytropic gas model is used to investigate the role of attractive and repulsive intermolecular forces and the influence of molecular complexity on the possible nonclassical gas dynamic behavior of vapors near the liquid-vapor saturation curve. The decrease of the sound speed upon
Molecular van der Waals Space and Topological Indices from the Distance Matrix
Seiman Corina
2004-12-01
Full Text Available A comparative study of 36 molecular descriptors derived from the topologicaldistance matrix and van der Waals space is carried out within this paper. They arepartitioned into 16 generalized topological distance matrix indices, 11 topologicaldistance indices known in the literature (seven obtained from eigenvalues/eigenvectors ofdistance matrix, and 9 van der Waals molecular descriptors. The generalized topologicaldistance indices, kÃŽÂ´ÃŽÂ» (ÃŽÂ» = 1 Ã¢Â€Â“ 3, k = 1 Ã¢Â€Â“ 4, are introduced in this work on the basis ofreciprocical distance matrix. Intercorrelation analysis reveals that topological distanceindices mostly contain the same type of information, while van der Waals indices can bebound to the shape or the size of molecules. Furthermore, we found that topologicaldistance indices are good for describing molecular size, and they may be viewed as bulkparameters. The most accurate QSPR models for predicting boiling point of alkanes arebased on some of the generalized, eigenvalues/eigenvectors topological distance indicesand the van der Waals descriptors of molecular size.
Kinetic Roughening and Material Optical Properties Influence on Van der Waals/Casimir Forces
van Zwol, P. J.; Palasantzas, G.
Atomic force microscopy measurements and force theory calculations using the Lifshitz theory show that van der Waals/Casimir dispersive forces have a strong dependence on surface roughness and material optical properties. It is found that at separations below 100 nm the roughness effect is
Influence of dielectric properties on van der Waals/Casimir forces in solid-liquid systems
van Zwol, P. J.; Palasantzas, G.; De Hosson, J. Th. M.
In this paper, we present calculations of van der Waals/Casimir forces, described by Lifshitz theory, for the solid-liquid-solid system using measured dielectric functions of all involved materials for the wavelength range from millimeters down to subnanometers. It is shown that even if the
Molecular interpretation of nonclassical gas dynamics of dense vapors under the van der Waals model
Colonna, P.; Guardone, A.
2006-01-01
The van der Waals polytropic gas model is used to investigate the role of attractive and repulsive intermolecular forces and the influence of molecular complexity on the possible nonclassical gas dynamic behavior of vapors near the liquid-vapor saturation curve. The decrease of the sound speed upon
A van der Waals Equation of State for a Dilute Boson Gas
Deeney, F. A.; O'Leary, J. P.
2012-01-01
An equation of state of a system is a relationship that connects the thermodynamic variables of the system such as pressure and temperature. Such equations are well known for classical gases but less so for quantum systems. In this paper we develop a van der Waals equation of state for a dilute boson gas that may be used to explain the occurrence…
Beam-Beam effects at the CMS BRIL van-der-Meer scans
CMS Collaboration
2017-01-01
The CMS Beam Radiation Instrumentation and Luminosity Project (BRIL) is devoted to the simulation and measurement of luminosity, beam conditions and radiation fields in the CMS Experiment at CERN. The project is engaged in operating and developing new detectors, compatible with the high luminosity experimental environments at the LHC. BRIL operates several detectors based on different physical principles and technologies. The detectors are calibrated using van-der-Meer scans to measure the luminosity that is a fundamental quantity of the LHC beam. In van-der-Meer scans the count rate in a detector is measured as a function of the distance between beams in the plane perpendicular to beam direction, to extract the underlying beam overlap area. The goal of the van-der-Meer scans is to obtain the calibration constant for each luminometer to be used at calibration then in physics data taking runs. The note presents the overview of beam-beam effects at the van-der-Meer scan and the corresponding corrections that sh...
The Hopf-van der Pol system: Failure of a homotopy method
Meijer, H.G.E.; Kalmár-Nagy, T.
2012-01-01
The purpose of this article to provide an explicit example where continuation based on the homotopy method fails. The example is a one-parameter homotopy for periodic orbits between two well-known nonlinear systems, the normal form of the Hopf bifurcation and the van der Pol system. Our analysis sho
On the history of van der Waerden's theorem on arithmetic progressions
Tom C. Brown
2001-12-01
Full Text Available In this expository note, we discuss the celebrated theorem known as ``van der Waerden's theorem on arithmetic progressions", the history of work on upper and lower bounds for the function associated with this theorem, a number of generalizations, and some open problems.
Van der Waals Type Model for Neutron-Proton Elastic Scattering at High Energies
Aleem, F.
1980-12-01
The most recent measurements of the angular distribution and total cross-section for neutron-proton elastic scattering between 70< pL <400 GeV/c with squared four momentum transfer -t ≤ 3.6 (GeV/c)2 have been explained using Van der Waals type model.
Theoretical investigation of van der Waals forces between solid surfaces at nanoscales
Kudryavtsev, Y.V.; Gelinck, E.R.M.; Fischer, H.R.
2009-01-01
A theoretical investigation of van der Waals forces acting between two solid silicon surfaces at separations from zero to approximately 20 nm is presented. We focused our efforts on the analysis of different factors that can cause deviations from the classical pressure-distance dependence p ∼ 1/D3.
Influence of ultrathin water layer on the van der Waals/Casimir force between gold surfaces
Palasantzas, G.; Svetovoy, V. B.; van Zwol, P. J.
In this paper we investigate the influence of ultrathin water layer (similar to 1-1.5 nm) on the van der Waals/Casimir force between gold surfaces. Adsorbed water is inevitably present on gold surfaces at ambient conditions as jump-up-to contact during adhesion experiments demonstrate. Calculations
Theoretical investigation of van der Waals forces between solid surfaces at nanoscales
Kudryavtsev, Y.V.; Gelinck, E.R.M.; Fischer, H.R.
2009-01-01
A theoretical investigation of van der Waals forces acting between two solid silicon surfaces at separations from zero to approximately 20 nm is presented. We focused our efforts on the analysis of different factors that can cause deviations from the classical pressure-distance dependence p ∼ 1/D3.
van der Waals Density Functional Theory vdW-DFq for Semihard Materials
Peng, Qing; de, Suvranu
There are a large number of materials with mild stiffness, which are not as soft as tissues and not as strong as metals. These semihard materials includes energetic materials, molecular crystals, layered materials, and van der Waals crystals. The integrity and mechanical stability are mainly determined by the interactions between instantaneously induced dipoles, the so called London dispersion force or van der Waals force. It is challenging to accurately model the structural and mechanical properties of these semihard materials in the frame of density functional theory where the non-local correlation functionals are not well known. Here we propose a van der Waals density functional named vdW-DFq to accurately model the density and geometry of semihard materials. Using β-cyclotetramethylene tetranitramine as a prototype, we adjust the enhancement factor of the exchange energy functional with generalized gradient approximations. We find this method to be simple and robust over a wide tuning range when calibrating the functional on-demand with experimental data. With a calibrated value q = 1 . 05 , the proposed vdW-DFq method shows good performance in predicting the geometries of 11 common energetic material molecular crystals and 3 typical layered van der Waals crystals. The authors would like to acknowledge the generous financial support from the Defense Threat Reduction Agency (DTRA) Grant # HDTRA1-13-1-0025.
Cort van der Linden (1846-1935) : Minister-president in oorlogstijd : een politieke biografie
Hertog, Johannes Paul den
2007-01-01
This political biography analyzes the political influence of, and methods used by, P.W.A. Cort van der Linden (1846-1935), Dutch Prime-Minister from 1913 to 1918. While he was a Professor of Economics he developed a view of liberalism based on German idealism which also included a progressive use o
Bonded Paths and van der Waals Interactions in Orpiment, As2S3
Gibbs, Gerald V.; Wallace, Adam F.; Zallen, Richard; Downs, R. T.; Ross, Nancy L.; Cox, David F.; Rosso, Kevin M.
2010-06-17
Bond critical properties and bond paths have been calculated for the thioarsenide molecular crystal orpiment, As2S3. In addition to the intramolecular As-S bond paths and van der Waals As-S and S-S bond paths within the layers, intermolecular S-S, As-S and As-As van der Waals paths exist between the layers. The S-S bond paths between the layers are identified with the main interlayer restoring forces responsible for the vibrational internal-mode splitting and the low frequency rigid layer modes previously documented in infrared and Raman studies of orpiment. These S-S bond paths are comparable with those calculated for orthorhombic native sulfur and the As4Sn (n = 3,4,5) molecules for several arsenide molecular crystals. The As-S bond paths show that the two nonequivalent arsenic atoms are each coordinated by a highly distorted octahedral array of sulfur atoms. The octahedra consist of three As-S intramolecular bonded interactions and three longer van der Waals interactions (two intramolecular and one intermolecular). One of the arsenic atoms is also coordinated by an arsenic atom in an interlayer As-As bonded interaction. Laplacian isosurface envelopes calculated for the arsenic and sulfur atoms are comparable with those calculated for native arsenic and orthorhombic sulfur. The intermolecular As-S bond paths connect Lewis acid domains on arsenic and an Lewis base domains on sulfur. Van der Waals interactions are traditionally defined as attractive interactions other than those ascribed to bond formation. However, theoretical evidence and arguments, as well as the connection between the bond paths and the vibrational spectra, indicate that the van der Waals interactions in orpiment are directed bonded interactions in the Slater sense. The experimental bond lengths for the As-S and S-S bonded interactions decrease nonlinearly with the increasing value of the electron density at the bond critical point, concomitant with a decrease in the bonded radii of arsenic and
Between Looking and Making: Unravelling Dom Hans van der Laan’s Plastic Number
Caroline Voet
2016-01-01
Full Text Available Between 1920 and 1991, the Dutch Benedictine monk and architect Dom Hans van der Laan (1904–91 developed his own proportional system based on the ratio 3:4, or the irrational number 1.3247. . ., which he called the plastic number. According to him, this ratio directly grew from discernment, the human ability to differentiate sizes, and as such would be an improvement over the golden ratio. To put his theories to the test, he developed an architectural language, which can best be described as elementary architecture. His oeuvre — four convents and a house — is published on an international scale. His buildings have become pilgrimage sites for practicing architects and institutions that want to study and experience his spaces. His 1977 book 'Architectonic Space: Fifteen Lessons on the Disposition of the Human Habitat', translated into English, French, German and Italian, still inspires architects today, as does his biography, 'Modern Primitive', written by the architect Richard Padovan in 1994. But beyond the inspiration of his writings and realisations, the actual application of the plastic number in Van der Laan’s designs is unclear. Moreover, Van der Laan’s theories seem to be directed towards one goal only: to present the plastic number as the only possible means by which eminent architecture can be achieved, making them a target for suspicion and critique. To understand and evaluate Van der Laan’s application of the plastic number, this paper approaches it as a practical design tool. It analyses its genealogy and defines its key concepts. From that framework, Van der Laan’s architectonic space is interpreted as a design methodology that combines antique tectonic theories reminiscent of writers from Plato to Vitruvius with more recent atectonic approaches towards space through experience and movement.
"Neue Liebe, neues Leben" - Ein Goethe-Gedicht in der Vertonung durch Ludwig van Beethoven
Körndle, Franz
2001-12-01
Full Text Available Ludwig van Beethoven composed two versions of Goethe's poem „Neue Liebe, neues Leben" in 1798/99 and 1810 respectively. A comparison of both versions reveals a change in the composer's intention. At first in several passages he followed the more traditional technique of transferring single words of the text directly into musical figures. Later on he seemed more intent on expressing the general mood and eliminated some of the rather simplistic details in favour of a more subjective rendering of the poems content.
[de] Ludwig van Beethoven vertonte das Goethe-Gedicht „Neue Liebe, neues Leben" in zwei Fassungen 1798/99 und 1810. Der Vergleich der beiden Fassungen ergibt, daß sich in der Zeit zwischen diesen beiden Fassungen die Vorstellungen des Komponisten von der musikalischen Umsetzung teilweise gewandelt haben müssen. Zunächst scheint Beethoven eher nach der traditionellen Méthode einer direkten Übertragung von Begriffen aus dem Text komponiert zu haben. Später war er dann aber eher an einer subjektiven Darstellung des Inhalts interessiert und drângte drängte zu einfach gerate Einzelheiten zugunsten der - nunmehr freilich recht differenziert gestalteten - Gesamtstimmung zurück.
Imidacloprid of niet? (interview met Sjef van der Steen)
Kleis, R.; Steen, van der J.J.M.
2014-01-01
Is bijensterfte het werk van neonicotinoïden? Ja, zeggen en denken velen. Volgens hen zijn de neo’s rechtstreeks verantwoordelijk voor de hoge sterfte onder bijen. Nee, toont Wagenings onderzoek aan. Intussen lijkt het met de sterfte voorzichtig de goede kant op te gaan.
Imidacloprid of niet? (interview met Sjef van der Steen)
Kleis, R.; Steen, van der J.J.M.
2014-01-01
Is bijensterfte het werk van neonicotinoïden? Ja, zeggen en denken velen. Volgens hen zijn de neo’s rechtstreeks verantwoordelijk voor de hoge sterfte onder bijen. Nee, toont Wagenings onderzoek aan. Intussen lijkt het met de sterfte voorzichtig de goede kant op te gaan.
In-situ epitaxial growth of graphene/h-BN van der Waals heterostructures by molecular beam epitaxy.
Zuo, Zheng; Xu, Zhongguang; Zheng, Renjing; Khanaki, Alireza; Zheng, Jian-Guo; Liu, Jianlin
2015-10-07
Van der Waals materials have received a great deal of attention for their exceptional layered structures and exotic properties, which can open up various device applications in nanoelectronics. However, in situ epitaxial growth of dissimilar van der Waals materials remains challenging. Here we demonstrate a solution for fabricating van der Waals heterostructures. Graphene/hexagonal boron nitride (h-BN) heterostructures were synthesized on cobalt substrates by using molecular beam epitaxy. Various characterizations were carried out to evaluate the heterostructures. Wafer-scale heterostructures consisting of single-layer/bilayer graphene and multilayer h-BN were achieved. The mismatch angle between graphene and h-BN is below 1°.
Novoa, Juan J.; Lafuente, Pilar; Mota, Fernando
1998-07-01
The hydrogen bond nature of angular C-H⋯O contacts is examined to determine when these contacts are better classified as hydrogen bonds or as Van der Waals bonds. To classify the bond we propose to look at the nature of the intermolecular bond critical point present in the electron density of the complex containing the bond. The physics behind this approach is explained using a qualitative orbital overlap model aimed at describing the main changes in the electronic density of the complex produced by the C-H⋯O bending.
van der Waals interaction of a neutral atom with the surface of a metal or dielectric nanosphere
Schmidt, Regine; Chormaic, Sile Nic [Photonics Centre, Tyndall National Institute, Prospect Row, Cork (Ireland); Minogin, Vladimir G, E-mail: s.nicchormaic@ucc.ie [Institute of Spectroscopy Russ. Ac. of Sciences, 142190 Troitsk, Moscow region (Russian Federation)
2011-01-14
We analyse the van der Waals interaction of a neutral atom with the internal and external surfaces of either a metal or dielectric nanosphere. We derive closed analytical equations for the van der Waals interaction energy using an electrostatic approximation and show that the energy increases or decreases as a function of the atom's distance from the surface, depending on the surface curvature. For concave spherical surfaces, the van der Waals energy can increase by up to a factor of 6, while for convex surfaces it decreases by as much as a factor of 2, when compared to that obtained for a flat surface. The derived analytical equations are very simple and can be used for a comparison between theory and experimental measurements of the van der Waals constant, C{sub 3}.
Effects of the van der Waals Force on the Dynamics Performance for a Micro Resonant Pressure Sensor
Lizhong Xu
2016-01-01
Full Text Available The micro resonant pressure sensor outputs the frequency signals where the distortion does not take place in a long distance transmission. As the dimensions of the sensor decrease, the effects of the van der Waals forces should be considered. Here, a coupled dynamic model of the micro resonant pressure sensor is proposed and its coupled dynamic equation is given in which the van der Waals force is considered. By the equation, the effects of the van der Waals force on the natural frequencies and vibration amplitudes of the micro resonant pressure sensor are investigated. Results show that the natural frequency and the vibrating amplitudes of the micro resonant pressure sensor are affected significantly by van der Waals force for a small clearance between the film and the base plate, a small initial tension stress of the film, and some other conditions.
Hirotani, Jun; Ikuta, Tatsuya; Nishiyama, Takashi; Takahashi, Koji
2013-01-16
Interfacial thermal transport via van der Waals interaction is quantitatively evaluated using an individual multi-walled carbon nanotube bonded on a platinum hot-film sensor. The thermal boundary resistance per unit contact area was obtained at the interface between the closed end or sidewall of the nanotube and platinum, gold, or a silicon dioxide surface. When taking into consideration the surface roughness, the thermal boundary resistance at the sidewall is found to coincide with that at the closed end. A new finding is that the thermal boundary resistance between a carbon nanotube and a solid surface is independent of the materials within the experimental errors, which is inconsistent with a traditional phonon mismatch model, which shows a clear material dependence of the thermal boundary resistance. Our data indicate the inapplicability of existing phonon models when weak van der Waals forces are dominant at the interfaces.
Universal curves for the van der Waals interaction between single-walled carbon nanotubes.
Pogorelov, Evgeny G; Zhbanov, Alexander I; Chang, Yia-Chung; Yang, Sung
2012-01-17
We report very simple and accurate algebraic expressions for the van der Waals (VDW) potentials and the forces between two parallel and crossed carbon nanotubes. The Lennard-Jones potential for two carbon atoms and the method of the smeared-out approximation suggested by Girifalco were used. It is found that the interaction between parallel and crossed tubes is described by two universal curves for parallel and crossed configurations that do not depend on the van der Waals constants, the angle between tubes, and the surface density of atoms and their nature but only on the dimensionless distance. The explicit functions for equilibrium VDW distances, well depths, and maximal attractive forces have been given. These results may be used as a guide for the analysis of experimental data to investigate the interaction between nanotubes of various natures.
Spectroscopic measurement of the titanium-helium van der Waals molecule: TiHe
Quiros, Nancy; Tariq, Naima; Weinstein, Jonathan
2016-05-01
Atoms that are weakly bound by the van der Waals (vdW) interaction are known as van der Waals molecules. The existence and formation of vdW molecules is favorable at low temperatures due to their weak binding energy. We have used laser ablation and helium buffer gas cooling to create the exotic vdW diatomic molecule made of titanium (Ti) and helium (He). TiHe molecules were detected through laser-induced-fluorescence spectroscopy closely blue-detuned from the a3F2 --> y3F3 atomic Ti transition at 25227 cm-1. Measurements of the binding energy of TiHe were obtained by studying its equilibrium thermodynamic properties. It is believed the molecules are formed from the constituent cold atoms through three-body recombination. Progress towards measuring the three-body recombination rate coefficient will be discussed. This material is based upon work supported by National Science Foundation under Grant No. PHY 1265905.
Wu, Jun; Gygi, François
2012-06-01
We present a simplified implementation of the non-local van der Waals correlation functional introduced by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] and reformulated by Román-Pérez et al. [Phys. Rev. Lett. 103, 096102 (2009)]. The proposed numerical approach removes the logarithmic singularity of the kernel function. Complete expressions of the self-consistent correlation potential and of the stress tensor are given. Combined with various choices of exchange functionals, five versions of van der Waals density functionals are implemented. Applications to the computation of the interaction energy of the benzene-water complex and to the computation of the equilibrium cell parameters of the benzene crystal are presented. As an example of crystal structure calculation involving a mixture of hydrogen bonding and dispersion interactions, we compute the equilibrium structure of two polymorphs of aspirin (2-acetoxybenzoic acid, C9H8O4) in the P21/c monoclinic structure.
Simon van der Meer and his legacy to CERN and particle accelerators
Chohan, Vinod
2012-01-01
Simon van der Meer was a brilliant scientist and a true giant in the field of accelerators. His seminal contributions to accelerator science are essential to this day in our quest to satisfy the demands of modern particle physics. Whether we are talking of long-baseline neutrino physics or antiproton-proton physics at CERN and Fermilab, or proton-proton physics at the LHC, his techniques and inventions have been a vital and necessary part of modern-day successes. Simon van der Meer and Carlo Rubbia were the first CERN scientists to become Nobel laureates in Physics in 1984. His less well-known contributions spanned a whole range of subjects in accelerator science from magnet design to power supply design, beam measurements, slow beam extraction, sophisticated programs, and controls.
Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan
2016-08-01
This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling-dominated transport to thermionic-dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The use of layered van der Waals heterostructures has two advantages: (a) thermionic transport barriers can be tuned by changing the number of layers, and (b) thermal conductance across these non-covalent structures is very weak. The phonon thermal conductance of the present van der Waals heterostructure is found to be 4.1 MW m-2 K-1 which is one order of magnitude lower than the lowest value for that of covalently-bonded interfaces. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices. This study shows that layered van der Waals structures have great potential to be used as solid-state energy-conversion devices.This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the
Two dimensional graphene nanogenerator by coulomb dragging: Moving van der Waals heterostructure
Zhong, Huikai; Li, Xiaoqiang; Wu, Zhiqian; Zhang, Shengjiao; Xu, Zhijuan [Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027 (China); Chen, Hongsheng; Lin, Shisheng, E-mail: shishenglin@zju.edu.cn [Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027 (China); State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027 (China)
2015-06-15
Harvesting energy from environment is the current focus of scientific community. Here, we demonstrate a graphene nanogenerator, which is based on moving van der Waals heterostructure formed between graphene and two dimensional (2D) graphene oxide (GO). This nanogenerator can convert mechanical energy into electricity with a voltage output of around 10 mV. Systematic experiments reveal the generated electricity originates from the coulomb interaction induced momentum transfer between 2D GO and holes in graphene. 2D boron nitride was also demonstrated to be effective in the framework of moving van der Waals heterostructure nanogenerator. This investigation of nanogenerator based on the interaction between 2D macromolecule materials will be important to understand the origin of the flow-induced potential in nanomaterials and may have great potential in practical applications.
Two dimensional graphene nanogenerator by coulomb dragging: Moving van der Waals heterostructure
Zhong, Huikai; Li, Xiaoqiang; Wu, Zhiqian; Zhang, Shengjiao; Xu, Zhijuan; Chen, Hongsheng; Lin, Shisheng
2015-06-01
Harvesting energy from environment is the current focus of scientific community. Here, we demonstrate a graphene nanogenerator, which is based on moving van der Waals heterostructure formed between graphene and two dimensional (2D) graphene oxide (GO). This nanogenerator can convert mechanical energy into electricity with a voltage output of around 10 mV. Systematic experiments reveal the generated electricity originates from the coulomb interaction induced momentum transfer between 2D GO and holes in graphene. 2D boron nitride was also demonstrated to be effective in the framework of moving van der Waals heterostructure nanogenerator. This investigation of nanogenerator based on the interaction between 2D macromolecule materials will be important to understand the origin of the flow-induced potential in nanomaterials and may have great potential in practical applications.
Formation and dynamics of van der Waals molecules in buffer-gas traps
Brahms, Nathan; Zhang, Peng; Kłos, Jacek; Forrey, Robert C; Au, Yat Shan; Sadeghpour, H R; Dalgarno, A; Doyle, John M; Walker, Thad G
2011-01-01
We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxations are too slow to affect trap lifetimes. However, helium-3-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag3He molecules, using ab initio calc...
Exfoliation of natural van der Waals heterostructures to a single unit cell thickness
Velický, Matěj; Toth, Peter S.; Rakowski, Alexander M.; Rooney, Aidan P.; Kozikov, Aleksey; Woods, Colin R.; Mishchenko, Artem; Fumagalli, Laura; Yin, Jun; Zólyomi, Viktor; Georgiou, Thanasis; Haigh, Sarah J.; Novoselov, Kostya S.; Dryfe, Robert A. W.
2017-02-01
Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeite's layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeite's electronic band structure to rationalize the experimental results.
Cl-intercalated graphene on SiC: Influence of van der Waals forces
Cheng, Yingchun
2013-01-01
The atomic and electronic structures of Cl-intercalated epitaxial graphene on SiC are studied by first-principles calculations. By increasing the Cl concentration, doping levels from n-type to slightly p-type are achieved on the SiC(0001) surface, while a wider range of doping levels is possible on the SiC(0001̄) surface. We find that the Cl atoms prefer bonding to the substrate rather than to the graphene. By varying the Cl concentration the doping level can be tailored. Consideration of van der Waals forces improves the distance between the graphene and the substrate as well as the binding energy, but it is not essential for the formation energy. For understanding the doping mechanism the introduction of non-local van der Waals contributions to the exchange correlation functional is shown to be essential. Copyright © EPLA, 2013.
Finite-size nanowire at a surface: Unconventional power laws of the van der Waals interaction
Makhnovets, K. A.; Kolezhuk, A. K.
2017-09-01
We study the van der Waals interaction of a metallic or narrow-gap semiconducting nanowire with a surface, in the regime of intermediate wire-surface distances (vF/c )L ≪d ≪L or L ≪d ≪(c /vF)L , where L is the nanowire length, d is the distance to the surface, and vF is the characteristic velocity of nanowire electrons (for a metallic wire, it is the Fermi velocity). Our approach, based on the Luttinger liquid framework, allows one to analyze the dependence of the interaction on the interplay between the nanowire length, wire-surface distance, and characteristic length scales related to the spectral gap and temperature. We show that this interplay leads to nontrivial modifications of the power law that governs van der Waals forces, in particular to a nonmonotonic dependence of the power-law exponent on the wire-surface separation.
Van der Waals interactions in rare-gas dimers: The role of interparticle interactions
Chen, Yu-Ting; Chai, Jeng-Da
2015-01-01
We investigate the potential energy curves of rare-gas dimers with various ranges and strengths of interparticle interactions (nuclear-electron, electron-electron, and nuclear-nuclear interactions). Our investigation is based on the highly accurate coupled-cluster theory associated with those interparticle interactions. For comparison, the performance of the corresponding Hartree-Fock theory, second-order Moller-Plesset perturbation theory, and density functional theory is also investigated. Our results reveal that when the interparticle interactions retain the long-range Coulomb tails, the nature of van der Waals interactions in the rare-gas dimers remains similar. By contrast, when the interparticle interactions are sufficiently short-range, the conventional van der Waals interactions in the rare-gas dimers completely disappear, yielding purely repulsive potential energy curves.
Effects of a periodic drive and correlated noise on birhythmic van der Pol systems
Yonkeu, R. Mbakob; Yamapi, R.; Filatrella, G.; Tchawoua, C.
2017-01-01
This paper considers the dynamics of a van der Pol birhythmic oscillator submitted both to colored noise and harmonic excitation. Applying the quasi-harmonic assumption to the corresponding Langevin equation we derive an approximated Fokker-Planck equation, that is compared with the results of computer simulations. We thus derive both the effects of the correlation time and the harmonic excitation on the parameter space where birhythmicity appears. In this region, we find that the multi-limit-cycle van der Pol oscillator reduces to an asymmetric bistable system where the sinusoidal drive intensity plays the role of asymmetric parameter, and noise can lead to stochastic bifurcations, consisting in a qualitative change of the stationary amplitude distribution. Under both influence of noise and harmonic excitation, the dynamics can be well characterized through the concepts of pseudo-potential, that regulates the low noise Arrhenius-like behavior.
Stochastic Bifurcations induced by correlated Noise in a Birhythmic van der Pol System
Yonkeu, R Mbakob; Filatrella, G; Tchawoua, C
2015-01-01
We investigate the effects of exponentially correlated noise on birhythmic van der Pol type oscillators. The analytical results are obtained applying the quasi-harmonic assumption to the Langevin equation to derive an approximated Fokker-Planck equation. This approach allows to analytically derive the probability distributions as well as the activation energies associated to switching between coexisting attractors. The stationary probability density function of the van der Pol oscillator reveals the influence of the correlation time on the dynamics. Stochastic bifurcations are discussed through a qualitative change of the stationary probability distribution, which indicates that noise intensity and correlation time can be treated as bifurcation parameters. Comparing the analytical and numerical results, we find good agreement both when the frequencies of the attractors are about equal or when they are markedly different.
Stochastic bifurcations in a bistable Duffing-Van der Pol oscillator with colored noise.
Xu, Yong; Gu, Rencai; Zhang, Huiqing; Xu, Wei; Duan, Jinqiao
2011-05-01
This paper aims to investigate Gaussian colored-noise-induced stochastic bifurcations and the dynamical influence of correlation time and noise intensity in a bistable Duffing-Van der Pol oscillator. By using the stochastic averaging method, theoretically, one can obtain the stationary probability density function of amplitude for the Duffing-Van der Pol oscillator and can reveal interesting dynamics under the influence of Gaussian colored noise. Stochastic bifurcations are discussed through a qualitative change of the stationary probability distribution, which indicates that system parameters, noise intensity, and noise correlation time, respectively, can be treated as bifurcation parameters. They also imply that the effects of multiplicative noise are rather different from that of additive noise. The results of direct numerical simulation verify the effectiveness of the theoretical analysis. Moreover, the largest Lyapunov exponent calculations indicate that P and D bifurcations have no direct connection.
Electrical and optical properties of SnS2/WSe2 van der Waals Heterojunction FETs
Zubair, Ahmad; Nourbakhsh, Amirhasan; Dresselhaus, Mildred; Palacios, Tomas
Two dimensional crystals based on atomically thin films of transition metal dichalcogenides offer an exciting platform for various optoelectronic applications. Their unique crystal properties make them particularly attractive for van der Waals heterostructures which open up an additional degree of freedom to tailor the material properties into new physics and device applications. In this work, we explore, for the first time, the optoelectronic properties of van der Waals SnS2/WSe2 heterojunction. WSe2 is an ambipolar semiconductor while SnS2 is an n-type wide bandgap semiconductor. We use the pickup and dry transfer methods to fabricate SnS2/WSe2 heterojunction transistors (hetero-FETs). We observe negative differential transconductance in the SnS2/WSe2 hetero-FET. Also, the heterostructure couples strongly to incident light and shows high photovoltaic responsivity which can find applications in nano-devices such as photo-detectors and solar cells.
Steady dynamics of exothermic chemical wave fronts in van der Waals fluids
Dumazer, G.; Antoine, C.; Lemarchand, A.; Nowakowski, B.
2009-12-01
We study the steady dynamics of an exothermic Fisher-Kolmogorov-Petrovsky-Piskunov chemical wave front traveling in a one-dimensional van der Waals fluid. The propagating wave is initiated by a nonuniformity in reactant concentration contrary to usual combustion ignition processes. The heat release and activation energy of the reaction play the role of control parameters. We recently proved that the propagation of an exothermic chemical wave front in a perfect gas displays a forbidden interval of stationary wave front speeds [G. Dumazer, M. Leda, B. Nowakowski, and A. Lemarchand, Phys. Rev. E 78, 016309 (2008)]. We examine how this result is modified for nonideal fluids and determine the effect of the van der Waals parameters and fluid density on the bifurcation between diffusion flames and Chapman-Jouguet detonation waves as heat release increases. Analytical predictions are confirmed by the numerical solution of the hydrodynamic equations including reaction kinetics.
Bonding in Mercury-Alkali Molecules: Orbital-driven van der Waals Complexes
Dieter Cremer
2008-06-01
Full Text Available The bonding situation in mercury-alkali diatomics HgA (2ÃŽÂ£+ (A = Li, Na, K, Rb has been investigated employing the relativistic all-electron method Normalized Elimination of the Small Component (NESC, CCSD(T, and augmented VTZ basis sets. Although Hg,A interactions are typical of van der Waals complexes, trends in calculated De values can be explained on the basis of a 3-electron 2-orbital model utilizing calculated ionization potentials and the De values of HgA+(1ÃŽÂ£+ diatomics. HgA molecules are identified as orbital-driven van der Waals complexes. The relevance of results for the understanding of the properties of liquid alkali metal amalgams is discussed.
Holographic Van der Waals-like phase transition in the Gauss–Bonnet gravity
He, Song, E-mail: hesong17@gmail.com [Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Mühlenberg 1, 14476 Golm (Germany); State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190 (China); Li, Li-Fang, E-mail: lilf@itp.ac.cn [Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190 (China); Zeng, Xiao-Xiong, E-mail: xxzeng@itp.ac.cn [State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190 (China); School of Material Science and Engineering, Chongqing Jiaotong University, Chongqing 400074 (China)
2017-02-15
The Van der Waals-like phase transition is observed in temperature–thermal entropy plane in spherically symmetric charged Gauss–Bonnet–AdS black hole background. In terms of AdS/CFT, the non-local observables such as holographic entanglement entropy, Wilson loop, and two point correlation function of very heavy operators in the field theory dual to spherically symmetric charged Gauss–Bonnet–AdS black hole have been investigated. All of them exhibit the Van der Waals-like phase transition for a fixed charge parameter or Gauss–Bonnet parameter in such gravity background. Further, with choosing various values of charge or Gauss–Bonnet parameter, the equal area law and the critical exponent of the heat capacity are found to be consistent with phase structures in temperature–thermal entropy plane.
Gajewski, Byron; Price, Larry R; Bott, Marjorie
2015-01-01
Sijtsma and van der Ark present a broad set of models and methods for reliability estimation, and their discussion of similarities and differences provides clear information for nurse researchers to move forward in their instrument development projects. In particular, we applaud the authors' clear exposition of the factor analytic model and its utility for providing a framework for unifying reliability and validity. However, we do not want to be constrained only to the point estimates. We also need to ascertain the uncertainty in the point estimate-usually in the form of a 95% confidence interval-or, as the Bayesians refer to, a credible interval. Another issue not discussed by Sijtsma and van der Ark is conditional standard errors of measurement along the score scale measuring latent traits or true scores. In our response, practical tools for estimating intervals and a brief discussion of conditional standard errors of measurement are presented.
Van der Waals force: a dominant factor for reactivity of graphene.
Lee, Jong Hak; Avsar, Ahmet; Jung, Jeil; Tan, Jun You; Watanabe, K; Taniguchi, T; Natarajan, Srinivasan; Eda, Goki; Adam, Shaffique; Castro Neto, Antonio H; Özyilmaz, Barbaros
2015-01-14
Reactivity control of graphene is an important issue because chemical functionalization can modulate graphene's unique mechanical, optical, and electronic properties. Using systematic optical studies, we demonstrate that van der Waals interaction is the dominant factor for the chemical reactivity of graphene on two-dimensional (2D) heterostructures. A significant enhancement in the chemical stability of graphene is achieved by replacing the common SiO2 substrate with 2D crystals such as an additional graphene layer, WS2, MoS2, or h-BN. Our theoretical and experimental results show that its origin is a strong van der Waals interaction between the graphene layer and the 2D substrate. This results in a high resistive force on graphene toward geometric lattice deformation. We also demonstrate that the chemical reactivity of graphene can be controlled by the relative lattice orientation with respect to the substrates and thus can be used for a wide range of applications including hydrogen storage.
van der Waals binding and band structure effects in graphene overlayers and graphane multilayers
Hyldgaard, Per; Rohrer, Jochen
2011-03-01
We study graphene formation (by selective Si evaporation) and adhesion on SiC surfaces as well as stacking and binding of graphane multilayers using a number of versions of the van der Waals Density Functional (vdW-DF) method and plane-wave density functional theory calculations. For the graphene/SiC systems and for the graphane multilayers we document that the bonding is entirely dominated by van der Waals (vdW) forces. At the same time we find that dispersive forces acting on the layers produce significant modifications in the graphene and graphane band structure. We interpret the changes and discuss a competition between wave function hybridization and interaction with the charge enhancement (between the layers) that results from density overlap. Supported by Svenska Vetenskapsrådet VR #621-2008-4346.
Holographic Van der Waals-like phase transition in the Gauss-Bonnet gravity
He, Song; Li, Li-Fang; Zeng, Xiao-Xiong
2017-02-01
The Van der Waals-like phase transition is observed in temperature-thermal entropy plane in spherically symmetric charged Gauss-Bonnet-AdS black hole background. In terms of AdS/CFT, the non-local observables such as holographic entanglement entropy, Wilson loop, and two point correlation function of very heavy operators in the field theory dual to spherically symmetric charged Gauss-Bonnet-AdS black hole have been investigated. All of them exhibit the Van der Waals-like phase transition for a fixed charge parameter or Gauss-Bonnet parameter in such gravity background. Further, with choosing various values of charge or Gauss-Bonnet parameter, the equal area law and the critical exponent of the heat capacity are found to be consistent with phase structures in temperature-thermal entropy plane.
Van der Waals and Casimir interactions between atoms and carbon nanotubes
Klimchitskaya, G. L.(Central Astronomical Observatory at Pulkovo of the Russian Academy of Sciences, 196140, St. Petersburg, Russia); Blagov, E. V.; Mostepanenko, V. M.
2008-01-01
The van der Waals and Casimir interactions of a hydrogen atom (molecule) with a single-walled and a multiwalled carbon nanotubes are compared. It is shown that the macroscopic concept of graphite dielectric permittivity is already applicable for nanotubes with only two or three walls. The absorption of hydrogen atoms by a nanotube at separations below one nanometer is considered. The lateral force due to exchange repulsion moves the atom to a position above the cell center, where it is absorb...
Randrianandrasana, Michel; Wei, Xueyong; Lowe, David
2010-01-01
The global and local synchronisation of a square lattice composed of alternating Duffing resonators and van der Pol oscillators coupled through displacement is studied. The lattice acts as a sensing device in which the input signal is characterised by an external driving force that is injected into the system through a subset of the Duffing resonators. The parameters of the system are taken from MEMS devices. The effects of the system parameters, the lattice architecture and size are discussed.
戎海武; 徐伟; 王向东; 孟光; 方同
2002-01-01
The principal resonance of Van der Pol-Duffing oscillator to combined determin istic and random parametric excitations is investigated. The method of multiple scales was used to determine the equations of modulation of amplitude and phase. The behavior, stability and bifurcation of steady state response were studied. Jumps were shown to occur under some conditions. The effects of damping, detuning , bandwidth, and magnitudes of deterministic and random excitations are analyzed. The theoretical analysis were verified by numerical results.
无
2011-01-01
In this paper,we propose a backstepping approach for the synchronization and control of modified Van-der Pol Duffing oscillator circuits.The method is such that one controller function that depends essentially on available circuit parameters that is sufficient to drive the two coupled circuits to a synchronized state as well achieve the global stabilization of the system to its regular dynamics.Numerical simulations are given to demonstrate the effectiveness of the technique.
Stability analysis for periodic solutions of the Van der Pol-Duffing forced oscillator
Cui, Jifeng; Liang, Jiaming; Lin, Zhiliang
2016-01-01
Based on the homotopy analysis method (HAM), the high accuracy frequency response curve and the stable/unstable periodic solutions of the Van der Pol-Duffing forced oscillator with the variation of the forced frequency are obtained and studied. The stability of the periodic solutions obtained is analyzed by use of Floquet theory. Furthermore, the results are validated in the light of spectral analysis and bifurcation theory.
Period-Doubling Cascades and Strange Attractors in Extended Duffing-Van der Pol Oscillator
YU Jun; PAN Wei-Zhen; ZHANG Rong-Bo
2009-01-01
The dynamical behavior of the extended Dulling-Van der Pol oscillator is investigated numerically in detail. With the aid of some numerical simulation tools such as bifurcation diagrams and Poincaré maps, the different routes to chaos and various shapes of strange attractors are observed. To characterize chaotic behavior of this oscillator system, the spectrum of Lyapunov exponent and Lyapunov dimension are also employed.
Njah, A.N. [Department of Physics, University of Agriculture, P.M.B 2240, Abeokuta (Nigeria)], E-mail: njahabdul@yahoo.com; Vincent, U.E. [Department of Physics, Olabisi Onabanjo University, P.M.B 2002, Ago-Iwoye (Nigeria)], E-mail: ue_vincent@yahoo.com
2008-09-15
This paper presents chaos synchronization between single and double wells Duffing-Van der Pol (DVP) oscillators with {phi}{sup 4} potential based on the active control technique. The technique is applied to achieve global synchronization between identical double-well DVP oscillators, identical single-well DVP oscillators and non-identical DVP oscillators, consisting of the double-well and the single-well DVP oscillators, respectively. Numerical simulations are also presented to verify the analytical results.
Theoretical Study of the Pyridine-Helium van der Waals Complexes
v, Hubert; Henriksen, Christian; Fernandez, Berta
2015-01-01
In this study we evaluate a high-level ab initio ground-state intermolecular potential-energy surface for the pyridine–He van der Waals complex, using the CCSD(T) method and Dunning’s augmented correlation consistent polarized valence double-ζ basis set extended with a set of 3s3p2d1f1g midbond f...
Stolyarov, Maxim; Aytan, Ece; Bloodgood, Matthew; Salguero, Tina T.; Balandin, Alexander A.
2016-09-01
The continuous downscaling of interconnect dimensions in combination with the introduction of low-k dielectrics has increased the number of heat dissipation, integration and reliability challenges in modern electronics. As a result, there is a strong need for new materials that have high current-carrying capacity for applications as nanoscale interconnects. In this presentation, we show that quasi-one-dimensional (1D) van der Waals metals such as TaSe3 have excellent breakdown current density exceeding that of 5 MA/cm2. This value is above that currently achievable in conventional copper or aluminum wires. The quasi-1D van der Waals materials are characterized by strong bonds along one dimension and weak van der Waals bonds along two other dimensions. The material for this study was grown by the chemical vapor transport (CVT) method. Both mechanical and chemical exfoliation methods were used to fabricate nanowires with lateral dimensions below 100 nm. The dimensions of the quasi-1D nanowires were verified with scanning electron microscopy (SEM) and atomic force microscopy (AFM). The metal (Ti/Au) contacts for the electrical characterization were deposited using electron beam evaporation (EBE). The measurements were conducted on a number of prototype interconnects with multiple electric contacts to ensure reproducibility. The obtained results suggest that quasi-1D van der Waals metals present a feasible alternative to conventional copper interconnects in terms of the current-carrying capacity and the breakdown current-density. This work was supported, in part, by the SRC and DARPA through STARnet Center for Function Accelerated nanoMaterial Engineering (FAME).
Van der Waals equation of state revisited: importance of the dispersion correction.
de Visser, Sam P
2011-04-28
One of the most basic equations of state describing nonideal gases and liquids is the van der Waals equation of state, and as a consequence, it is generally taught in most first year undergraduate chemistry courses. In this work, we show that the constants a and b in the van der Waals equation of state are linearly proportional to the polarizability volume of the molecules in a gas or liquid. Using this information, a new thermodynamic one-parameter equation of state is derived that contains experimentally measurable variables and physics constants only. This is the first equation of state apart from the Ideal Gas Law that contains experimentally measurable variables and physics constants only, and as such, it may be a very useful and practical equation for the description of dilute gases and liquids. The modified van der Waals equation of state describes pV as the sum of repulsive and attractive intermolecular interaction energies that are represented by an exponential repulsion function between the electron clouds of the molecules and a London dispersion component, respectively. The newly derived equation of state is tested against experimental data for several gas and liquid examples, and the agreement is satisfactory. The description of the equation of state as a one-parameter function also has implications on other thermodynamic functions, such as critical parameters, virial coefficients, and isothermal compressibilities. Using our modified van der Waals equation of state, we show that all of these properties are a function of the molecular polarizability volume. Correlations of experimental data confirm the derived proportionalities.
Measuring anisotropic resistivity of single crystals using the van der Pauw technique
2015-01-01
Anisotropy in properties of materials is important in materials science and solid-state physics. Measurement of the full resistivity tensor of crystals using the standard four-point method with bar shaped samples requires many measurements and may be inaccurate due to misalignment of the bars along crystallographic directions. Here an approach to extracting the resistivity tensor using van der Pauw measurements is presented. This reduces the number of required measurements. The theory of the ...
van der Waals epitaxy and photoresponse of two-dimensional CdSe plates
Zhu, Dan-Dan; Xia, Jing; Wang, Lei; Li, Xuan-Ze; Tian, Li-Feng; Meng, Xiang-Min
2016-06-01
Here we demonstrate the first growth of two-dimensional (2D) single-crystalline CdSe plates on mica substrates via van der Waals epitaxy. The as-synthesized 2D plates exhibit hexagonal, truncated triangular and triangular shapes with the lateral size around several microns. Photodetectors based on 2D CdSe plates present a fast response time of 24 ms, revealing that 2D CdSe is a promising building block for ultrathin optoelectronic devices.
Moire superlattice effects in graphene/boron-nitride van der Waals heterostructures
Wallbank, John R.; Chen, Xi; Fal' ko, Vladimir I. [Department of Physics, Lancaster University, Lancaster (United Kingdom); Mucha-Kruczynski, Marcin [Department of Physics, University of Bath (United Kingdom)
2015-06-15
Van der Waals heterostructures of graphene and hexagonal boron nitride feature a moire superlattice for graphene's Dirac electrons. Here, we review the effects generated by this superlattice, including a specific miniband structure featuring gaps and secondary Dirac points, and a fractal spectrum of magnetic minibands known as Hofstadter's butterfly. (copyright 2015 by WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)
Van der Waals epitaxial double heterostructure: InAs/single-layer graphene/InAs.
Hong, Young Joon; Yang, Jae Won; Lee, Wi Hyoung; Ruoff, Rodney S; Kim, Kwang S; Fukui, Takashi
2013-12-17
Van der Waals (vdW) epitaxial double heterostructures have been fabricated by vdW epitaxy of InAs nanostructures on both sides of graphene. InAs nanostructures diametrically form on/underneath graphene exclusively along As-polar direction, indicating polarity inversion of the double heterostructures. First-principles and density functional calculations demonstrate how and why InAs easily form to be double heterostructures with polarity inversion.
First-principles study of two-dimensional van der Waals heterojunctions
Hu, Wei; Yang, Jinlong
2015-01-01
Research on graphene and other two-dimensional (2D) materials, such as silicene, germanene, phosphorene, hexagonal boron nitride (h-BN), graphitic carbon nitride (g-C3N4), graphitic zinc oxide (g-ZnO) and molybdenum disulphide (MoS2), has recently received considerable interest owing to their outstanding properties and wide applications. Looking beyond this field, combining the electronic structures of 2D materials in ultrathin van der Waals heterojunctions has also emerged to widely study th...
Van der Waals density functional study of water binding in metal-organic frameworks
Lee, Kyuho; Smit, Berend; Neaton, Jeffrey B.
2013-03-01
Metal-organic frameworks (MOFs) are promising candidate materials for gas storage, gas separation and catalysis. However, MOFs are vulnerable to humid air and effective surface area drops dramatically on an exposure to water. In this theoretical study, we investigate the interaction of single water molecule with MOF-74 on different binding sites by using van der Waals density functionals. We also explore how different type of metal cations affect the interaction.
Van der Waals Type Model and Structure in π-p Elastic Scattering at High Energies
Aleem, F.
1982-10-01
The most recent measurement of the angular distribution for π-p elastic scattering at pL =50 and 200 GeV/c which show a structure near -t ≈ 4(GeV/c)2, with squared four momentum transfer -t extended to 10(GeV/c)2, and the total cross section data for 50 ≤ pL ≤ 370 GeV/c have been simultaneously explained by using Van der Waal's type model.
The role of van der waals interaction on quantum-mechanical tunneling
Takayanagi, Toshiyuki; Kurosaki, Yuzuru [Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Tokai Research Establishment
1998-10-01
We present three-dimensional quantum cumulative reaction probabilities for the F + H{sub 2}, D{sub 2}, and HD reactions with a special emphasis on resonances associated with quasi-bound states localized in the reactant van der Waals region of the potential energy surface. The accurate ab initio potential surface of Stark and Werner and the less accurate 5SEC-W surface developed by Truhlar and co-workers have been employed. (author)
Effect of van der Waals interactions on the structural and elastic properties of black phosphorus
Appalakondaiah, S.; Vaitheeswaran, G.; Lebègue, S.;
2012-01-01
The structural and elastic properties of orthorhombic black phosphorus have been investigated using first-principles calculations based on density functional theory. The structural parameters have been calculated using the local density approximation (LDA), the generalized gradient approximation...... (GGA), and with several dispersion corrections to include van der Waals interactions. It is found that the dispersion corrections improve the lattice parameters over LDA and GGA in comparison with experimental results. The calculations reproduce well the experimental trends under pressure and show...
Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures
Pierucci, Debora; Henck, Hugo; Naylor, Carl H.; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E.; Dappe, Yannick J.; Bertran, François; Fèvre, Patrick Le; Johnson, A. T. Charlie; Ouerghi, Abdelkarim
2016-06-01
Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design.
Application of the van der Pauw method for samples with holes
Oh, Donggeon; Ahn, Cheongeung; Kim, Minchul; Park, Eung-Kyu; Kim, Yong-Sang
2016-12-01
Modifications of the original van der Pauw relation were suggested recently. The methods are applicable to samples with a hole, unlike the original van der Pauw relation, but it takes too much time and effort to apply the methods to samples with high randomness. So in this paper we suggest two generalizations of the van der Pauw method which are applicable for two-dimensional homogeneous systems with a finite number of holes. Both methods rely on obtaining a crucial constant of the system, ν. The first method involves setting the probes far from each other while conducting the experiment using a sample with a small hole, approximating a relation that gives ν as a linear function of the area ratio of the hole only. The second method produces an identical sample of known resistivity and thickness to obtain ν, which is believed to be dependent on the geometrical properties only. Unlike the earlier methods, which entailed complex procedures, little in the way of measurements and computation is needed for the new methods. The methods will be very useful in electric experiments or industries which need to measure the resistivity of samples.
Spooky correlations and unusual van der Waals forces between gapless and near-gapless molecules
Dobson, John F; Angyan, Janos G; Liu, Ru-Fen
2016-01-01
We consider the zero-temperature van der Waals interaction between two molecules, each of which has a zero or near-zero electronic gap between a groundstate and the first excited state, using a toy model molecule ( equilateral H3) as an example. We show that the van der Waals energy between two groundstate molecules falls off as D^(-3) instead of the usual D^(-6) dependence, when the molecules are separated by distance D: We show that this is caused by perfect "spooky" correlation between the two fluctuating electric dipoles. The phenomenon is related to, but not the same as, the "resonant" interaction between an electronically excited and a groundstate molecule introduced by Eisenschitz and London in 1930. It is also an example of "type C van der Waals non- additivity" recently introduced by one of us ( Int. J. Quantum Chem. 114, 1157 (2014)). Our toy molecule H3 is not stable, but symmetry considerations suggest that a similar vdW phenomenon may be observable, despite Jahn-Teller effects, in molecules with ...
Lu, Bing-Sui; Naji, Ali; Podgornik, Rudolf
2015-06-01
We analyze van der Waals interactions between two rigid polymers with sequence-specific, anisotropic polarizabilities along the polymer backbones, so that the dipole moments fluctuate parallel to the polymer backbones. Assuming that each polymer has a quenched-in polarizability sequence which reflects, for example, the polynucleotide sequence of a double-stranded DNA molecule, we study the van der Waals interaction energy between a pair of such polymers with rod-like structure for the cases where their respective polarizability sequences are (i) distinct and (ii) identical, with both zero and non-zero correlation length of the polarizability correlator along the polymer backbones in the latter case. For identical polymers, we find a novel r-5 scaling behavior of the van der Waals interaction energy for small inter-polymer separation r, in contradistinction to the r-4 scaling behavior of distinct polymers, with furthermore a pronounced angular dependence favoring attraction between sufficiently aligned identical polymers. Such behavior can assist the molecular recognition between polymers.
Underwater adhesion of abalone: The role of van der Waals and capillary forces
Lin, A.Y.M., E-mail: albertlin22@yahoo.com [Materials Science and Engineering Program, University of California, San Diego, La Jolla, CA 92093 (United States); Brunner, R. [Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093 (United States)] [Department of Nanoengineering, University of California, San Diego, La Jolla, CA 92093 (United States); Chen, P.Y. [Materials Science and Engineering Program, University of California, San Diego, La Jolla, CA 92093 (United States); Talke, F.E. [Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093 (United States)] [Center for Magnetic Recording Research, University of California, San Diego, La Jolla, CA 92093 (United States); Meyers, M.A. [Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093 (United States)] [Department of Nanoengineering, University of California, San Diego, La Jolla, CA 92093 (United States)] [Materials Science and Engineering Program, University of California, San Diego, La Jolla, CA 92093 (United States)
2009-08-15
The observation of the pedal foot of the red abalone Haliotis rufescens reveals the presence of micrometer-scaled setae terminating in nanometer-sized cylindrical fibrils, with some resemblance to those found on the gecko foot. Atomic force microscopy (AFM) pull-off force measurements on a single seta are compared with theoretical estimates for van der Waals attraction obtained through the Johnson-Kendall-Roberts (JKR) equation, approximately 600 nN, and show agreement. The use of the JKR equation is justified through an analysis of the shape of the fibril extremities (parabolic) as well as their diameter ({approx}200 nm). Measurements under varying humidity conditions indicate that additional capillary interactions play a role, since the pull-off force increases with humidity. It is proposed that both van der Waals and capillary forces play a role in the attachment mechanism of H. rufescens, effectively enabling suction to reach its theoretical limit. Bulk pull-off force measurements on entire live animals yield an average detachment stress of 115 kPa, consistent with theoretical estimates. The setae and nanoscale fibril terminations enable compliance to surfaces with a variety of roughnesses, effectively sealing the interface, in addition to providing capillary and van der Waals forces.
Van der Pol and the history of relaxation oscillations: toward the emergence of a concept.
Ginoux, Jean-Marc; Letellier, Christophe
2012-06-01
Relaxation oscillations are commonly associated with the name of Balthazar van der Pol via his paper (Philosophical Magazine, 1926) in which he apparently introduced this terminology to describe the nonlinear oscillations produced by self-sustained oscillating systems such as a triode circuit. Our aim is to investigate how relaxation oscillations were actually discovered. Browsing the literature from the late 19th century, we identified four self-oscillating systems in which relaxation oscillations have been observed: (i) the series dynamo machine conducted by Gérard-Lescuyer (1880), (ii) the musical arc discovered by Duddell (1901) and investigated by Blondel (1905), (iii) the triode invented by de Forest (1907), and (iv) the multivibrator elaborated by Abraham and Bloch (1917). The differential equation describing such a self-oscillating system was proposed by Poincaré for the musical arc (1908), by Janet for the series dynamo machine (1919), and by Blondel for the triode (1919). Once Janet (1919) established that these three self-oscillating systems can be described by the same equation, van der Pol proposed (1926) a generic dimensionless equation which captures the relevant dynamical properties shared by these systems. Van der Pol's contributions during the period of 1926-1930 were investigated to show how, with Le Corbeiller's help, he popularized the "relaxation oscillations" using the previous experiments as examples and, turned them into a concept.
Study of van der Waals bonding and interactions in metal organic framework materials.
Zuluaga, Sebastian; Canepa, Pieremanuele; Tan, Kui; Chabal, Yves J; Thonhauser, Timo
2014-04-02
Metal organic framework (MOF) materials have attracted a lot of attention due to their numerous applications in fields such as hydrogen storage, carbon capture and gas sequestration. In all these applications, van der Waals forces dominate the interaction between the small guest molecules and the walls of the MOFs. In this review article, we describe how a combined theoretical and experimental approach can successfully be used to study those weak interactions and elucidate the adsorption mechanisms important for various applications. On the theory side, we show that, while standard density functional theory is not capable of correctly describing van der Waals interactions, functionals especially designed to include van der Waals forces exist, yielding results in remarkable agreement with experiment. From the experimental point of view, we show examples in which IR adsorption and Raman spectroscopy are essential to study molecule/MOF interactions. Importantly, we emphasize throughout this review that a combination of theory and experiment is crucial to effectively gain further understanding. In particular, we review such combined studies for the adsorption mechanism of small molecules in MOFs, the chemical stability of MOFs under humid conditions, water cluster formation inside MOFs, and the diffusion of small molecules into MOFs. The understanding of these phenomena is critical for the rational design of new MOFs with desired properties.
Li intercalation in graphite: A van der Waals density-functional study
Hazrati, E.; de Wijs, G. A.; Brocks, G.
2014-10-01
Modeling layered intercalation compounds from first principles poses a problem, as many of their properties are determined by a subtle balance between van der Waals interactions and chemical or Madelung terms, and a good description of van der Waals interactions is often lacking. Using van der Waals density functionals we study the structures, phonons and energetics of the archetype layered intercalation compound Li-graphite. Intercalation of Li in graphite leads to stable systems with calculated intercalation energies of -0.2 to -0.3 eV/Li atom, (referred to bulk graphite and Li metal). The fully loaded stage 1 and stage 2 compounds LiC6 and Li1 /2C6 are stable, corresponding to two-dimensional √{3 }×√{3 } lattices of Li atoms intercalated between two graphene planes. Stage N >2 structures are unstable compared to dilute stage 2 compounds with the same concentration. At elevated temperatures dilute stage 2 compounds easily become disordered, but the structure of Li3 /16C6 is relatively stable, corresponding to a √{7 }×√{7 } in-plane packing of Li atoms. First-principles calculations, along with a Bethe-Peierls model of finite temperature effects, allow for a microscopic description of the observed voltage profiles.
The hot pick-up technique for batch assembly of van der Waals heterostructures.
Pizzocchero, Filippo; Gammelgaard, Lene; Jessen, Bjarke S; Caridad, José M; Wang, Lei; Hone, James; Bøggild, Peter; Booth, Timothy J
2016-06-16
The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled production of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices, we found semiclassical mean-free paths up to 0.9 μm, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures.
Spooky correlations and unusual van der Waals forces between gapless and near-gapless molecules
Dobson, John F.; Savin, Andreas; Ángyán, János G.; Liu, Ru-Fen
2016-11-01
We consider the zero-temperature van der Waals (vdW) interaction between two molecules, each of which has a zero or near-zero electronic gap between a ground state and the first excited state, using a toy model molecule (equilateral H3) as an example. We show that the van der Waals energy between two ground state molecules falls off as D-3 instead of the usual D-6 dependence, when the molecules are separated by distance D. We show that this is caused by a perfect "spooky" correlation between the two fluctuating electric dipoles. The phenomenon is related to, but not the same as, the "resonant" interaction between an electronically excited and a ground state molecule introduced by Eisenschitz and London in 1930. It is also an example of "type C van der Waals non-additivity" recently introduced by one of us [J. F. Dobson, Int. J. Quantum Chem. 114, 1157 (2014)]. Our toy molecule H3 is not stable, but symmetry considerations suggest that a similar vdW phenomenon may be observable, despite Jahn-Teller effects, in molecules with a discrete rotational symmetry and broken inversion symmetry, such as certain metal atom clusters. The motion of the nuclei will need to be included for a definitive analysis of such cases, however.
Hogenbirk, M.; Kestemont, M.
2013-01-01
Der ystorien bloeme is a rhymed Middle Dutch adaptation (ca. 1300?) of the Legenda aurea by Jacobus de Voragine. The anonymous author identifies himself as a ‘clerk’. The ystorien only survive in a single - possible incomplete - early fourteenth-century copy in the convoluted manuscript Leiden, Univ
van't Hoff-van der Waals osmotic pressure and energy transformers
Zener, Clarence; Levenson, William
1983-01-01
We find the van't Hoff relations between osmotic pressure, freezing point depression, and boiling point elevation provide a clue on how, by using salt solutions, one may lower the cost of extracting power from low-grade heat sources. In particular, the ratio of 7 between the heat of evaporation and the heat of freezing of pure water suggests a chemical system that raises 7-fold the temperature difference between heat source and heat sink, while decreasing by the same factor the heat flux. Hea...
Spontaneous doping on high quality talc-graphene-hBN van der Waals heterostructures
Mania, E.; Alencar, A. B.; Cadore, A. R.; Carvalho, B. R.; Watanabe, K.; Taniguchi, T.; Neves, B. R. A.; Chacham, H.; Campos, L. C.
2017-09-01
Steady doping, added to its remarkable electronic properties, would make graphene a valuable commodity in the solar cell market, as energy power conversion could be substantially increased. Here we report a graphene van der Waals heterostructure which is able to spontaneously dope graphene (p-type) up to n ~ 2.2 × 1013 cm-2 while providing excellent charge mobility (μ ~ 25 000 cm2 V-1 s-1). Such properties are achieved via deposition of graphene on atomically flat layered talc, a natural and abundant dielectric crystal. Raman investigation shows a preferential charge accumulation on graphene-talc van der Waals heterostructures, which are investigated through the electronic properties of talc/graphene/hBN heterostructure devices. These heterostructures preserve graphene’s good electronic quality, verified by the observation of quantum Hall effect at low magnetic fields (B = 0.4 T) at T = 4.2 K. In order to investigate the physical mechanisms behind graphene-on-talc p-type doping, we performed first-principles calculations of their interface structural and electronic properties. In addition to potentially improving solar cell efficiency, graphene doping via van der Waals stacking is also a promising route towards controlling the band gap opening in bilayer graphene, promoting a steady n or p type doping in graphene and, eventually, providing a new path to access superconducting states in graphene, predicted to exist only at very high doping.
Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals
Gong, Cheng; Li, Lin; Li, Zhenglu; Ji, Huiwen; Stern, Alex; Xia, Yang; Cao, Ting; Bao, Wei; Wang, Chenzhe; Wang, Yuan; Qiu, Z. Q.; Cava, R. J.; Louie, Steven G.; Xia, Jing; Zhang, Xiang
2017-06-01
The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications. In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin-Wagner theorem; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering, or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr2Ge2Te6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.
Local Probe Spectroscopy of Two-Dimensional van der Waals Heterostructures
Yankowitz, Matthew Abraham
A large family of materials, collectively known as "van der Waals materials", have attracted enormous research attention over the past decade following the realization that they could be isolated into individual crystalline monolayers, with charge carriers behaving effectively two-dimensionally. More recently, an even larger class of composite materials has been realized, made possible by combining the isolated atomic layers of different materials into "van der Waals heterostructures", which can exhibit electronic and optical behaviors not observed in the parent materials alone. This thesis describes efforts to characterize the atomic-scale structural and electronic properties of these van der Waals materials and heterostructures through scanning tunneling microscopy measurements. The majority of this work addresses the properties of monolayer and few-layer graphene, whose charge carriers are described by massless and massive chiral Dirac Hamiltonians, respectively. In heterostructures with hexagonal boron nitride, an insulating isomorph of graphene, we observe electronic interference patterns between the two materials which depend on their relative rotation. As a result, replica Dirac cones are formed in the valence and conduction bands of graphene, with their energy tuned by the rotation. Further, we are able to dynamically drag the graphene lattice in these heterostructures, owing to an interaction between the scanning probe tip and the domain walls formed by the electronic interference pattern. Similar dragging is observed in domain walls of trilayer graphene, whose electronic properties are found to depend on the stacking configuration of the three layers. Scanning tunneling spectroscopy provides a direct method for visualizing the scattering pathways of electrons in these materials. By analyzing the scattering, we can directly infer properties of the band structures and local environments of these heterostructures. In bilayer graphene, we map the electrically
Henry constants in polymer solutions with the van der Waals equation of state
Bithas, Sotiris; Kalospiros, Nikolaos; Kontogeorgis, Georgios
1996-01-01
The simple der Waals equation of state, as extended to polymer systems, is applied to the correlation and prediction of Henry constants in polymer solutions comprising five polymers and many nonpolar and polar solvents, including supercritical gases. The correlation achieved with one adjustable...... parameter is satisfactory, with typical errors within the experimental uncertainty and comparable to those with the more complex Perturbed Hard Chain Theory-based equations of state with the same number of adjustable parameters. A predictive scheme for calculating Henry constants is also presented, which...... is a corresponding-states correlation for a dimensionless Henry constant defined based on the van der Waals equation of state. Satisfactory results-often close to the ones from the one-parameter correlation-are obtained for all systems investigated in this work. Compared with literature models that have been applied...
Van der Waals-coupled electronic states in incommensurate double-walled carbon nanotubes
Liu, Kaihui; Jin, Chenhao; Hong, Xiaoping; Kim, Jihoon; Zettl, Alex; Wang, Enge; Wang, Feng
2014-10-01
Non-commensurate two-dimensional materials such as a twisted graphene bilayer or graphene on boron nitride, consisting of components that have no finite common unit cell, exhibit emerging moiré physics such as novel Van Hove singularities, Fermi velocity renormalization, mini Dirac points and Hofstadter butterflies. Here we use double-walled carbon nanotubes as a model system for probing moiré physics in incommensurate one-dimensional systems, by combining structural and optical characterizations. We show that electron wavefunctions between incommensurate inner- and outer-wall nanotubes can hybridize strongly, contrary to the conventional wisdom of negligible electron hybridization due to destructive interference. The chirality-dependent inter-tube electronic coupling is described by one-dimensional zone folding of the electronic structure of twisted-and-stretched graphene bilayers. Our results demonstrate that incommensurate van der Waals interactions can be important for engineering the electronic structure and optical properties of one-dimensional materials.
Atomically thin resonant tunnel diodes built from synthetic van der Waals heterostructures
Lin, Yu-Chuan; Ghosh, Ram Krishna; Addou, Rafik; Lu, Ning; Eichfeld, Sarah M.; Zhu, Hui; Li, Ming-Yang; Peng, Xin; Kim, Moon J.; Li, Lain-Jong; Wallace, Robert M.; Datta, Suman; Robinson, Joshua A.
2015-06-01
Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2-WSe2-graphene and WSe2-MoS2-graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.
Influence of van der Waals forces on the adsorption structure of benzene on silicon
Johnston, Karen; Lundqvist, Bengt I; Nieminen, Risto M
2008-01-01
Two different adsorption configurations of benzene on the Si(001)-(2x1) surface, the tight-bridge and butterfly structures, were studied using density functional theory. Several exchange and correlation functionals were used, including the recently developed vdW-DF functional, which accounts for the effect of van der Waals forces. In contrast to the PBE, revPBE and other GGA functionals, the vdW-DF functional finds that, for most coverages, the adsorption energy of the butterfly structure is greater than that of the tight-bridge structure.
Bond paths and van der Waals interactions in orpiment, As2S3.
Gibbs, G V; Wallace, A F; Zallen, R; Downs, R T; Ross, N L; Cox, D F; Rosso, K M
2010-06-17
The calculated electron density distribution for orpiment, As(2)S(3), reveals that As-S, S-S, and As-As bond paths are associated with the experimental interlayer directed bonded interactions detected in a combined infrared and Raman study. The successful modeling of the infrared- and Raman-determined interlayer bonded interactions together with bond paths and the structuralization of a variety of inorganic molecules in terms of "key-lock" bond path mainstays support the argument that van der Waals forces in inorganic molecular crystals are directional.
Lag Synchronization in Coupled Multistable van der Pol-Duffing Oscillators
Dawid Dudkowski
2014-01-01
Full Text Available We consider the system of externally excited identical van der Pol-Duffing oscillators unidirectionally coupled in a ring. When the coupling is introduced, each of the oscillator’s trajectories is on different attractor. We study the changes in the dynamics due to the increase in the coupling coefficient. Studying the phase of the oscillators, we calculate the parameter value for which we obtain the antiphase lag synchronization of the system and also the bifurcation values for which we observe qualitative changes in the dynamics of already synchronized system. We give evidence that lag synchronization is typical for coupled multistable systems.
Period-doubling cascades of canards from the extended Bonhoeffer-van der Pol oscillator
Sekikawa, Munehisa, E-mail: sekikawa@dove.kuee.kyoto-u.ac.j [Department of Electrical Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo, Kyoto, 615-8510 (Japan); Inaba, Naohiko [Department of Electronics and Bioinformatics, Meiji University, 1-1-1 Higashi-Mita, Tama-ku, Kawasaki, 214-8571 (Japan); Yoshinaga, Tetsuya [Institute of Health Biosciences, The University of Tokushima, 3-18-15 Kuramoto-cho, Tokushima, 770-8509 (Japan); Hikihara, Takashi [Department of Electrical Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo, Kyoto, 615-8510 (Japan)
2010-08-09
This Letter investigates the period-doubling cascades of canards, generated in the extended Bonhoeffer-van der Pol oscillator. Canards appear by Andronov-Hopf bifurcations (AHBs) and it is confirmed that these AHBs are always supercritical in our system. The cascades of period-doubling bifurcation are followed by mixed-mode oscillations. The detailed two-parameter bifurcation diagrams are derived, and it is clarified that the period-doubling bifurcations arise from a narrow parameter value range at which the original canard in the non-extended equation is observed.
Electro-photo modulation of the fermi level in WSe2/graphene van der Waals heterojunction
Sun, Honghui; Yang, Hang; Fang, Liang; Zhang, Jiangwei; Wang, Zhiyuan; Jiang, Tian
2017-04-01
We report an electro-photo double modulation of the fermi level in a WSe2/graphene heterojunction. The heterojunction exhibits high ION/IOFF ratio ( 103) in transfer characteristic in dark and distinct rectification behavior in output characteristic under light illumination, respectively. Time-dependent photoresponse reveals that the heterojunction has a considerable potential in the application of photodetection. Interestingly, an exotic current peak is observed in transfer characteristic under light illumination. This novel behavior is attributed to the tunable fermi level at the WSe2/graphene heterojunction by electro-photo double modulation. The results may be helpful to develop tunable photovoltaic optoelectronics based on van der Waals heterojunctions.
From Canards of Folded Singularities to Torus Canards in a Forced van der Pol Equation
Burke, John; Desroches, Mathieu; Granados, Albert;
2016-01-01
to the intermediate- and high-frequency forcing regimes and show that the forced van der Pol possesses torus canards instead. These torus canards consist of long segments near families of attracting and repelling limit cycles of the fast system, in alternation. We also derive explicit formulas for the parameter......-frequency periodically driven slow/fast systems with two fast variables and one slow variable which possess a non-degenerate fold of limit cycles. The analytic techniques used herein rely on geometric desingularisation, invariant manifold theory, Melnikov theory, and normal form methods. The numerical methods used...
Moerz, Sebastian T
2015-01-01
Bovine heart cytochrome c has been immobilized into the mesoporous silica host material SBA-15 in both its native folded and urea-unfolded state. The comparison of the two folding states' behavior casts doubt on the commonly used explanation of cytochrome c adsorption, i.e. the electrostatic interaction model. A detailed investigation of the protein binding as a function of pH and ionic strength of the buffer solution reveals the complex nature of the protein-silica interaction. Electrostatic interaction, van der Waals forces and entropic contributions by counterion release each contribute to adsorption on the silica pore walls.
Droplet evaporation in one-component fluids: Dynamic van der Waals theory
Teshigawara, Ryohei; Onuki, Akira
2008-01-01
In a one-component fluid, we investigate evaporation of a small axysymmetric liquid droplet in the partial wetting condition on a heated wall at $T\\sim 0.9 T_c$. In the dynamic van der Waals theory (Phys. Rev. E {\\bf 75}, 036304 (2007)), we take into account the latent heat transport from liquid to gas upon evaporation. Along the gas-liquid interface, the temperature is nearly equal to the equilibrium coexisting temperature away from the substrate, but it rises sharply to the wall temperature...
Field effect in graphene-based van der Waals heterostructures: Stacking sequence matters.
Stradi, Daniele; Papior, Nick; Hansen, Ole; Brandbyge, Mads
2017-03-06
Stacked van der Waals (vdW) heterostructures where semi-conducting two-dimensional (2D) materials are contacted by overlayed graphene electrodes enable atomically-thin, flexible electronics. We use first-principles quantum transport simulations of graphene- contacted MoS2 devices to show how the transistor effect critically depends on the stacking configuration relative to the gate electrode. We can trace this behavior to the stacking-dependent response of the contact region to the capacitive electric field induced by the gate. The contact resistance is a central parameter and our observation establishes an important design rule for ultra-thin devices based on 2D atomic crystals.
Van der Waals coefficients for alkali metal clusters and their size dependence
Arup Banerjee; Manoj K Harbola
2006-02-01
In this paper we employ the hydrodynamic formulation of time-dependent density functional theory to obtain the van der Waals coefficients 6 and 8 of alkali metal clusters of various sizes including very large clusters. Such calculations become computationally very demanding in the orbital-based Kohn-Sham formalism, but are quite simple in the hydrodynamic approach. We show that for interactions between the clusters of the same sizes, 6 and 8 scale as the sixth and the eighth power of the cluster radius, respectively, and approach their classically predicted values for the large size clusters.
Variations of the Lifshitz-van der Waals force between metals immersed in liquids
Esquivel-Sirvent, R
2010-01-01
We present a theoretical calculation of the Lifshitz-van der Waals force between two metallic slabs embedded in a fluid, taking into account the change of the Drude parameters of the metals when in contact with liquids of different index of refraction. For the three liquids considered in this work, water, $CCl_3F$ and $ CBr_3F$ the change in the Drude parameters of the metal imply a difference of up to 15% in the determination of the force at short separations. These variations in the force is bigger for liquids with a higher index of refraction.
Rance, Graham A; Marsh, Dan H; Bourne, Stephen J; Reade, Thomas J; Khlobystov, Andrei N
2010-08-24
We have demonstrated that ubiquitous van der Waals forces are significant in controlling the interactions between nanoparticles and nanotubes. The adsorption of gold nanoparticles (AuNPs) on nanotubes (MWNTs) obeys a simple quadratic dependence on the nanotube surface area, regardless of the source of AuNPs and MWNTs. Changes in the geometric parameters of the components have pronounced effects on the affinity of nanoparticles for nanotubes, with larger, more polarizable nanostructures exhibiting stronger attractive interactions, the impact of which changes in the following order MWNT diameter > AuNP diameter > MWNT length.
Stability of Multidimensional Phase Transitions in a Steady van der Waals Flow
Shuyi ZHANG
2008-01-01
In this paper,the author studies the multidimensional stability of subsonic phase transitions in a steady supersonic flow of van der Waals type.The viscosity capillarity criterion (in "Arch.Rat.Mech.Anal.,81(4),1983,301-315") is used to seek physical admissible planar waves.By showing the Lopatinski determinant being non-zero,it is proved that subsonic phase transitions are uniformly stable in the sense of Majda (in "Mem.Amer.Math.Soc.,41(275),1983,1-95") under both one dimensional and multidimensional perturbations.
Hopf Bifurcations of a Stochastic Fractional-Order Van der Pol System
Xiaojun Liu
2014-01-01
Full Text Available The Hopf bifurcation of a fractional-order Van der Pol (VDP for short system with a random parameter is investigated. Firstly, the Chebyshev polynomial approximation is applied to study the stochastic fractional-order system. Based on the method, the stochastic system is reduced to the equivalent deterministic one, and then the responses of the stochastic system can be obtained by numerical methods. Then, according to the existence conditions of Hopf bifurcation, the critical parameter value of the bifurcation is obtained by theoretical analysis. Then, numerical simulations are carried out to verify the theoretical results.
Controlled Synthesis of Organic/Inorganic van der Waals Solid for Tunable Light-Matter Interactions.
Niu, Lin; Liu, Xinfeng; Cong, Chunxiao; Wu, Chunyang; Wu, Di; Chang, Tay Rong; Wang, Hong; Zeng, Qingsheng; Zhou, Jiadong; Wang, Xingli; Fu, Wei; Yu, Peng; Fu, Qundong; Najmaei, Sina; Zhang, Zhuhua; Yakobson, Boris I; Tay, Beng Kang; Zhou, Wu; Jeng, Horng Tay; Lin, Hsin; Sum, Tze Chien; Jin, Chuanhong; He, Haiyong; Yu, Ting; Liu, Zheng
2015-12-16
High-quality organic and inorganic van der Waals (vdW) solids are realized using methylammonium lead halide (CH3 NH3 PbI3 ) as the organic part (organic perovskite) and 2D inorganic monolayers as counterparts. By stacking on various 2D monolayers, the vdW solids exhibit dramatically different light emissions. Futhermore, organic/h-BN vdW solid arrays are patterned for red-light emission. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Atomically thin resonant tunnel diodes built from synthetic van der Waals heterostructures
Lin, Yu-Chuan
2015-06-19
Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2–WSe2–graphene and WSe2–MoS2–graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.
Balakrishnan, Nilanthy; Kudrynskyi, Zakhar R.; Fay, Mike W.; Mudd, Garry W.; Svatek, Simon A; Makarovsky, Oleg; Kovalyuk, Zakhar D.; Eaves, Laurence; Peter H. Beton; Patanè, Amalia
2014-01-01
Room temperature electroluminescence from semiconductor junctions is demonstrated. The junctions are fabricated by the exfoliation and direct mechanical adhesion of InSe and GaSe van der Waals layered crystals. Homojunction diodes formed from layers of p- and n-type InSe exhibit electroluminescence at energies close to the bandgap energy of InSe (Eg= 1.26 eV). In contrast, heterojunction diodes formed by combining layers of p-type GaSe and n-type InSe emit photons at lower energies, which is ...
Xia, Congxin, E-mail: xiacongxin@htu.edu.cn; Xue, Bin; Wang, Tianxing; Peng, Yuting [Department of Physic, Henan Normal University, Xinxiang 453007 (China); Jia, Yu [School of Physics and Engineering, Zhengzhou University, Zhengzhou 450052 (China)
2015-11-09
The electronic characteristics of arsenene-graphene van der Waals (vdW) heterostructures are studied by using first-principles methods. The results show that a linear Dirac-like dispersion relation around the Fermi level can be quite well preserved in the vdW heterostructures. Moreover, the p-type Schottky barrier (0.18 eV) to n-type Schottky barrier (0.31 eV) transition occurs when the interlayer distance increases from 2.8 to 4.5 Å, which indicates that the Schottky barrier can be tuned effectively by the interlayer distance in the vdW heterostructures.
Microwave probes Dipole Blockade and van der Waals Forces in a Cold Rydberg Gas
Teixeira, R Celistrino; Nguyen, Thanh Long; Cantat-Moltrecht, T; Raimond, Jean-Michel; Haroche, S; Gleyzes, S; Brune, M
2015-01-01
We show that microwave spectroscopy of a dense Rydberg gas trapped on a superconducting atom chip in the dipole blockade regime reveals directly the dipole-dipole many-body interaction energy spectrum. We use this method to investigate the expansion of the Rydberg cloud under the effect of repulsive van der Waals forces and the breakdown of the frozen gas approximation. This study opens a promising route for quantum simulation of many-body systems and quantum information transport in chains of strongly interacting Rydberg atoms.
Silicene on MoS2: role of the van der Waals interaction
Jiajie Zhu,
2015-10-13
We demonstrate for silicene on MoS2 substrate the limitations of the predictive power of first principles calculations based on van der Waals density functional theory. Only the optB86b-vdW functional is found to give reasonable agreement with experimental results on structural properties, while for all other investigated functionals the interlayer interaction is underestimated or the charge redistribution at the interface is not described correctly so that the predicted electronic structure is qualitatively wrong. © 2015 IOP Publishing Ltd.
Feedback control and adaptive synchronization of chaotic forced Bonhoeffer-van der Pol oscillators
Kontchou, E W Chimi; Fotsin, H B [Laboratoire d' Electronique, Departement de Physique, Faculte des Sciences, Universite de Dschang, B P 67 Dschang (Cameroon); Woafo, P [Laboratory of Modelling and Simulation in Engineering and Biological Physics, Faculty of Science, University of Yaounde I, Box 812, Yaounde (Cameroon)], E-mail: hbfotsin@yahoo.fr
2008-04-15
This paper deals with chaos control and synchronization in forced Bonhoeffer-van der Pol (FBVP) oscillators. The state equations of the model are first established and the stability is analysed. A feedback control strategy for stabilizing the chaotic dynamics on a periodic orbit of the phase space is investigated. Adaptive synchronization of two FBVP oscillators, based on parameter estimation and a nonlinear observer approach, is also investigated. It appears that a particular unknown parameter of the model can be estimated, which gives the possibility of recovering information through chaotic masking. An application in secure communications is presented.
Weber F
1999-01-01
Full Text Available Problemstellung: Untersuchung der Wirkung einer Therapie mit Bisoprolol und Nifedipin auf die Herzfrequenzvariabilität (HRV von Patienten mit koronarer Herzerkrankung und stabiler Angina pectoris. Patienten und Methodik: Die HRV wurde im Frequenzbereich über 24 Stunden bei 32 Patienten der Total Ischemic Burden Bisoprolol Study (TIBBS gemessen, welche sich im Einjahresverlauf nach Randomisierung einer Bypassoperation unterzogen. 12 Patienten erhielten 1 x 10 mg/d Bisoprolol, 20 Patienten erhielten 2 x 20 mg/d Nifedipin retard. Folgende Parameter wurden bestimmt: Mittleres Normalschlag-Normalschlag-Intervall (NN-m, Total Power (TP, High Frequency Power (HF, Low Frequency Power (LF, Very Low Frequency Power (VLF und Ultra Low Frequency Power (ULF. Ergebnisse: Bisoprolol führte zu einem Anstieg von NN-m über die gesamten 24 Stunden. Es bewirkte gleichfalls einen HRV-Anstieg insbesondere am Tage. Nifedipin führte zu einer signifikanten Abnahme der HRV besonders während der Nacht. Schlußfolgerung: Der als prognostisch günstig anzusehende HRV-Anstieg wurde überwiegend unter Bisoprolol gesehen. Nifedipin führte auch in der retardierten Form zu einer HRV-Abnahme. Dies kann ein Indikator für neurohumorale Aktivierung und sympathovagale Dysbalance sein.
CARLOS EDUARDO CAVALCANTI
2000-03-01
Full Text Available Relatamos os casos de dois irmãos, com quatro e seis anos de idade, com achados característicos da síndrome de Van Der Knaap. Discutimos os seus aspectos clínicos e radiológicos, assim como suas peculiaridades. Comparamos aos dados da literatura e analisamos os possíveis mecanismos etiopatogênicos envolvidos.We report on two brothers, aged four and six years-old, evidencing the Van Der Knaap syndrome. Clinical and radiological aspects are discussed as well their peculiarities. Data are compared with related literature, as etiopathogenic mechanisms possibly involved.
Blagov, E V; Mostepanenko, V M
2005-01-01
The Lifshitz theory of the van der Waals force is extended for the case of an atom (molecule) interacting with a plane surface of an uniaxial crystal or with a long solid cylinder or cylindrical shell made of isotropic material or uniaxial crystal. For a microparticle near a semispace or flat plate made of an uniaxial crystal the exact expressions for the free energy of the van der Waals and Casimir-Polder interaction are presented. An approximate expression for the free energy of microparticle- cylinder interaction is obtained which becomes precise for microparticle-cylinder separations much smaller than cylinder radius. The obtained expressions are used to investigate the van der Waals interaction between hydrogen atoms (molecules) and graphite plates or multiwall carbon nanotubes. To accomplish this the behavior of graphite dielectric permittivities along the imaginary frequency axis is found using the optical data for the complex refractive index of graphite for the ordinary and extraordinary rays. It is ...
Li, Jing-Lun; Hu, Xue-Jin; Han, Yong-Chang; Cong, Shu-Lin
2016-09-01
We construct a simple model to calculate the trimer bound state energy ET(n ) and three-body recombination rate K30 of three identical bosons with van der Waals interaction without using any two- or three-body fitting parameter. Using this simple model, we investigate the influence of the van der Waals finite-range effect on ET(n ) and K30. Our calculation shows that the finite-range effect leads to the ground trimer state energy ET(0 ) not crossing the atom-dimer threshold, and the scaled three-body recombination rate K30/a4 deviating from the universal three-body theory. The results of our simple model agree within a few percent with other theoretical works with van der Waals interaction and also the experimental data.
Image method in the calculation of the van der Waals force between an atom and a conducting surface
Souza, Reinaldo de Melo e; Sigaud, C; Farina, C
2012-01-01
Initially, we make a detailed historical survey of van der Waals forces, collecting the main references on the subject. Then, we review a method recently proposed by Eberlein and Zietal to compute the dispersion van der Waals interaction between a neutral but polarizable atom and a perfectly conducting surface of arbitrary shape. This method has the advantage of relating the quantum problem to a corresponding classical one in electrostatics so that all one needs is to compute an appropriate Green function. We show how the image method of electrostatics can be conveniently used together with the Eberlein and Zietal mehtod (when the problem admits an image solution). We then illustrate this method in a couple of simple but important cases, including the atom-sphere system. Particularly, in our last example, we present an original result, namely, the van der Waals force between an atom and a boss hat made of a grounded conducting material.
Image method in the calculation of the van der Waals force between an atom and a conducting surface
de Melo e Souza, Reinaldo; Kort-Kamp, W. J. M.; Sigaud, C.; Farina, C.
2013-05-01
After a brief survey of van der Waals forces, we review a method recently proposed by Eberlein and Zietal to compute the dispersion van der Waals interaction between a neutral but polarizable atom and a perfectly conducting surface of arbitrary shape. This method has the advantage of relating the quantum problem to a corresponding classical one in electrostatics in an enlightening way so that all one needs is to compute an appropriate Green function. We show how the image method of electrostatics can be conveniently used together with the Eberlein and Zietal method (when the image solution is known). We then illustrate this method in some simple but important cases, including the atom-sphere system. Finally, we present an original result for the van der Waals force between an atom and a boss hat made of a grounded conducting material.
Kumar Vishal; Saurabh K Agrawal; Subir Das
2016-01-01
In this paper, we have discussed the local stability of the Mathieu–van der Pol hyperchaotic system with the fractional-order derivative. The fractional Routh–Hurwitz stability conditions were provided and were used to discuss the stability. Feedback control method was used to control chaos in the Mathieu–van der Pol system with fractional-order derivative and after controlling the chaotic behaviour of the system the synchronization between the fractional-order hyperchaotic Mathieu–van der Pol system and controlled system was introduced. In this study, modified adaptive control methods with uncertain parameters at various equilibrium points were used. Also the analysis of control time with respect to different fractional-order derivatives is the key feature of this paper. Numerical simulation results achieved using Adams–Boshforth–Moulton method show that the method is effective and reliable.
Novel Approach for a van der Pol Oscillator in the Continuous Time Domain
Junaid Ali Khan; Muhammad Asif Zahoor Raja; IJaz MansoorQureshi
2011-01-01
We investigate the continuous time domain numerical treatment of a van der Pol oscillator, applying the trial solution as an artiScial feed-forward neural network model containing unknown adjustable parameters. The optimization of the network is performed by simulated annealing in an unsupervised method. The proposed scheme is tested successfully by its application in both non-stiff and stiff conditions. Its reliability and effectiveness is validated through comprehensive statistical analyses. The obtained results are in good agreement with the classical RK45 method.%We investigate the continuous time domain numerical treatment of a van der Pol oscillator,applying the trial solution as an artificial feed-forward neural network model containing unknown adjustable parameters.The optimization of the network is performed by simulated annealing in an unsupervised method.The proposed scheme is tested successfully by its application in both non-stiff and stiff conditions.Its reliability and effectiveness is validated through comprehensive statistical analyses.The obtained results are in good agreement with the classical RK45 method.
An equal area law for the van der Waals transition of holographic entanglement entropy
Nguyen, Phuc H
2015-01-01
The Anti-de Sitter-Reissner-Nordstrom (AdS-RN) black hole in the canonical ensemble undergoes a phase transition similar to the liquid-gas phase transition. i.e. the isocharges on the entropy-temperature plane develop an unstable branch when the charge is smaller than a critical value. It was later discovered that the isocharges on the entanglement entropy-temperature plane also exhibit the same van der Waals-like structure. In this paper, we present numerical results which sharpen this similarity between entanglement entropy and black hole entropy, by showing that both of these entropies obey Maxwell's equal area law. Moreover, we checked this for two disk-shaped entangling regions of different sizes, and the conclusion seems to be valid regardless of the region's size. We checked the equal area law for AdS-RN in 4 and 5 dimensions, so that the conclusion seems to hold for any dimension. Finally, we also checked that the equal area law holds for a similar, van der Waals-like transition of the dyonic black ho...
Droplet spreading driven by van der Waals force: a molecular dynamics study
Wu Congmin [Department of Mathematics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon (Hong Kong); Qian Tiezheng [Department of Mathematics and Joint KAUST-HKUST Micro/Nanofluidics Laboratory, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon (Hong Kong); Sheng Ping, E-mail: maqian@ust.h [Department of Physics and William Mong Institute of Nano Science and Technology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon (Hong Kong)
2010-08-18
The dynamics of droplet spreading is investigated by molecular dynamics simulations for two immiscible fluids of equal density and viscosity. All the molecular interactions are modeled by truncated Lennard-Jones potentials and a long-range van der Waals force is introduced to act on the wetting fluid. By gradually increasing the coupling constant in the attractive van der Waals interaction between the wetting fluid and the substrate, we observe a transition in the initial stage of spreading. There exists a critical value of the coupling constant, above which the spreading is pioneered by a precursor film. In particular, the dynamically determined critical value quantitatively agrees with that determined by the energy criterion that the spreading coefficient equals zero. The latter separates partial wetting from complete wetting. In the regime of complete wetting, the radius of the spreading droplet varies with time as R(t){approx}{radical}t, a behavior also found in molecular dynamics simulations where the wetting dynamics is driven by the short-range Lennard-Jones interaction between liquid and solid.
Formation and dynamics of van der Waals molecules in buffer-gas traps.
Brahms, Nathan; Tscherbul, Timur V; Zhang, Peng; Kłos, Jacek; Forrey, Robert C; Au, Yat Shan; Sadeghpour, H R; Dalgarno, A; Doyle, John M; Walker, Thad G
2011-11-14
We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, (3)He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag(3)He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag(3)He, and report on our spectroscopic search for Ag(3)He, which produced a null result.
Understanding corrosion inhibition with van der Waals DFT methods: the case of benzotriazole.
Gattinoni, Chiara; Michaelides, Angelos
2015-01-01
The corrosion of materials is an undesirable and costly process affecting many areas of technology and everyday life. As such, considerable effort has gone into understanding and preventing it. Organic molecule based coatings can in certain circumstances act as effective corrosion inhibitors. Although they have been used to great effect for more than sixty years, how they function at the atomic-level is still a matter of debate. In this work, computer simulation approaches based on density functional theory are used to investigate benzotriazole (BTAH), one of the most widely used and studied corrosion inhibitors for copper. In particular, the structures formed by protonated and deprotonated BTAH molecules on Cu(111) have been determined and linked to their inhibiting properties. It is found that hydrogen bonding, van der Waals interactions and steric repulsions all contribute in shaping how BTAH molecules adsorb, with flat-lying structures preferred at low coverage and upright configurations preferred at high coverage. The interaction of the dehydrogenated benzotriazole molecule (BTA) with the copper surface is instead dominated by strong chemisorption via the azole moiety with the aid of copper adatoms. Structures of dimers or chains are found to be the most stable structures at all coverages, in good agreement with scanning tunnelling microscopy results. Benzotriazole thus shows a complex phase behaviour in which van der Waals forces play an important role and which depends on coverage and on its protonation state and all of these factors feasibly contribute to its effectiveness as a corrosion inhibitor.
Van der Waals and resonance interactions between accelerated atoms in vacuum and the Unruh effect
Lattuca, M.; Marino, J.; Noto, A.; Passante, R.; Rizzuto, L.; Spagnolo, S.; Zhou, W.
2017-08-01
We discuss different physical effects related to the uniform acceleration of atoms in vacuum, in the framework of quantum electrodynamics. We first investigate the van der Waals/Casimir-Polder dispersion and resonance interactions between two uniformly accelerated atoms in vacuum. We show that the atomic acceleration significantly affects the van der Waals force, yielding a different scaling of the interaction with the interatomic distance and an explicit time dependence of the interaction energy. We argue how these results could allow for an indirect detection of the Unruh effect through dispersion interactions between atoms. We then consider the resonance interaction between two accelerated atoms, prepared in a correlated Bell-type state, and interacting with the electromagnetic field in the vacuum state, separating vacuum fluctuations and radiation reaction contributions, both in the free-space and in the presence of a perfectly reflecting plate. We show that nonthermal effects of acceleration manifest in the resonance interaction, yielding a change of the distance dependence of the resonance interaction energy. This suggests that the equivalence between temperature and acceleration does not apply to all radiative properties of accelerated atoms. To further explore this aspect, we evaluate the resonance interaction between two atoms in non inertial motion in the coaccelerated (Rindler) frame and show that in this case the assumption of an Unruh temperature for the field is not required for a complete equivalence of locally inertial and coaccelerated points of views.
Structural and quantum properties of van der Waals cluster near the unitary regime
Lekala, M. L.; Chakrabarti, B.; Haldar, S. K.; Roy, R.; Rampho, G. J.
2017-07-01
We study the structural and several quantum properties of three-dimensional bosonic cluster interacting through van der Waals potential at large scattering length. We use Faddeev-type decomposition of the many-body wave function which includes all possible two-body correlations. At large scattering length, we observe spatially extended states which exhibit the exponential dependence on the state number. The cluster ground state energy shows universal nature at large negative scattering length. We also find the existence of generalized Tjon lines for N-body clusters. Signature of universal behaviour of weakly bound clusters can be observed in experiments of ultracold Bose gases. We also study the spectral statistics of the system. We calculate both the short-range fluctuation and long-range correlation and observe semi-Poisson distribution which interpolates the Gaussian Orthogonal Ensemble (GOE) and Poisson statistics of random matrix theory. It indicates that the van der Waal cluster near the unitary becomes highly complex and correlated. However additional study of P (r) distribution (without unfolding of energy spectrum) reveals the possibility of chaos for larger cluster.
Thermal transport in van der Waals solids from first-principles calculations
Lindroth, Daniel O.; Erhart, Paul
2016-09-01
The lattice thermal expansion and conductivity in bulk Mo and W-based transition metal dichalcogenides are investigated by means of density functional and Boltzmann transport theory calculations. To this end, a recent van der Waals density functional (vdW-DF-CX) is employed, which is shown to yield excellent agreement with reference data for the structural parameters. The calculated in-plane thermal conductivity compares well with experimental room-temperature values, when phonon-phonon and isotopic scattering are included. To explain the behavior over the entire available temperature range one must, however, include additional (temperature independent) scattering mechanisms that limit the mean free path. Generally, the primary heat carrying modes have mean free paths of 1 μ m or more, which makes these materials very susceptible to structural defects. The conductivity of Mo- and W-based transition metal dichalcogenides is primarily determined by the chalcogenide species and increases in the order Te-Se-S. While for the tellurides and selenides the transition metal element has a negligible effect, the conductivity of WS2 is notably higher than for MoS2, which may be traced to the much larger phonon band gap of the former. Overall, the present study provides a consistent set of thermal conductivities that reveal chemical trends and constitute the basis for future investigations of van der Waals solids.
Droplet spreading driven by van der Waals force: a molecular dynamics study
Wu, Congmin
2010-07-07
The dynamics of droplet spreading is investigated by molecular dynamics simulations for two immiscible fluids of equal density and viscosity. All the molecular interactions are modeled by truncated Lennard-Jones potentials and a long-range van der Waals force is introduced to act on the wetting fluid. By gradually increasing the coupling constant in the attractive van der Waals interaction between the wetting fluid and the substrate, we observe a transition in the initial stage of spreading. There exists a critical value of the coupling constant, above which the spreading is pioneered by a precursor film. In particular, the dynamically determined critical value quantitatively agrees with that determined by the energy criterion that the spreading coefficient equals zero. The latter separates partial wetting from complete wetting. In the regime of complete wetting, the radius of the spreading droplet varies with time as R(t) ∼ √t, a behavior also found in molecular dynamics simulations where the wetting dynamics is driven by the short-range Lennard-Jones interaction between liquid and solid. © 2010 IOP Publishing Ltd.
An introduction to the Dieterici Equation and the van der Waal Equation
Sheldon, John
2003-11-01
The derivation of the ideal gas law by using the kinetic theory of gases is usually presented in an undergraduate physics thermodynamics texts and physical chemistry texts. Following these derivations is the introduction of nonideal effects and the empirical equations of state: the van der Waals equation and the Dieterici equation. These are sometimes are simply given without comment as to the origin of the terms in them. An introduction to a "derivation" of these equations, appropriate for the undergraduate thermodynamics course, is given herein. Empirical equations are not rigorously derived, but rather they are invented, the so-called derivation simply serves to make the empirical terms appear reasonable.The barometric equation is exploited to get an expression for the effective attractive molecular forces. The differential form of the barometric is derived using kinetic theory, then from the barometric equation we get the Dieterici Equation an expansion of the Dieterici Equation, yields the van der Waals Equation of state. The relationship between the empirical constants is also discussed
Band Alignment and Minigaps in Monolayer MoS2-Graphene van der Waals Heterostructures.
Pierucci, Debora; Henck, Hugo; Avila, Jose; Balan, Adrian; Naylor, Carl H; Patriarche, Gilles; Dappe, Yannick J; Silly, Mathieu G; Sirotti, Fausto; Johnson, A T Charlie; Asensio, Maria C; Ouerghi, Abdelkarim
2016-07-13
Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS2 and the graphene layer lattice impose a superperiodic potential.
Exciton–polaritons in van der Waals heterostructures embedded in tunable microcavities
Dufferwiel, S.; Schwarz, S.; Withers, F.; Trichet, A. A. P.; Li, F.; Sich, M.; Del Pozo-Zamudio, O.; Clark, C.; Nalitov, A.; Solnyshkov, D. D.; Malpuech, G.; Novoselov, K. S.; Smith, J. M.; Skolnick, M. S.; Krizhanovskii, D. N.; Tartakovskii, A. I.
2015-01-01
Layered materials can be assembled vertically to fabricate a new class of van der Waals heterostructures a few atomic layers thick, compatible with a wide range of substrates and optoelectronic device geometries, enabling new strategies for control of light–matter coupling. Here, we incorporate molybdenum diselenide/hexagonal boron nitride (MoSe2/hBN) quantum wells in a tunable optical microcavity. Part-light–part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20 meV for a single MoSe2 monolayer, enhanced to 29 meV in MoSe2/hBN/MoSe2 double-quantum wells. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized. PMID:26446783
Interfactant-mediated quasi-Frank-van der Merwe growth of Pb on Si(111)
Schmidt, Th.; Bauer, E.
2000-12-01
The influence of interfactants (Au, Ag) on the growth of Pb on Si(111) is studied by low-energy electron microscopy in the temperature range from 260 K to 460 K. On the Si(111)-(7×7) surface Pb grows in the Stranski-Krastanov mode, on the Si(111)-(3×3)R30°-Au and on the Si(111)-(6×6)-Au surface in the quasi-Frank-van der Merwe (layer-by-layer) mode. On the Si(111)-(3×3)R30°-Ag surface the growth mode changes from layer-by-layer below 300 K to the Stranski-Krastanov mode above 300 K. The temperature dependence of the growth cannot be explained by thermodynamics but is governed by kinetics. The analysis of the maximum island density in terms of the atomistic nucleation theory gives acceptable values for nucleus size and energies only in the layer-by-layer growth regime. In the Stranski-Krastanov growth regime abnormal values are obtained that are attributed to high cluster mobility on the initial two-dimensional layer. The conditions leading to quasi-Frank-van der Merwe growth are discussed.
Direct measurement of the van der Waals interaction between two single atoms
Béguin, Lucas; Chicireanu, Radu; Lahaye, Thierry; Browaeys, Antoine
2013-01-01
We report on the direct measurement of the van der Waals interaction between two isolated, single Rydberg atoms separated by a controlled distance of a few micrometers. By working in a regime where the single-atom Rabi frequency of the laser used for excitation to the Rydberg state is comparable to the interaction energy, we observe a \\emph{partial} Rydberg blockade, whereby the time-dependent populations of the various two-atom states exhibit coherent oscillations with several frequencies. A quantitative comparison of the data with a simple model based on the optical Bloch equations allows us to extract the van der Waals energy, and to observe its characteristic $C_6/R^6$ dependence. The magnitude of the measured $C_6$ coefficient agrees well with an \\emph{ab-initio} theoretical calculation, and we observe its dramatic increase with the principal quantum number $n$ of the Rydberg state. Our results not only allow to test an important physical law, but also demonstrate a degree of experimental control which o...
Kelkkanen, Kari André; Lundqvist, Bengt; Nørskov, Jens Kehlet
2011-01-01
The ubiquitous van der Waals (vdW) force, particularly discernible in weak adsorption, is studied on noble and transition metals. In calculations with the vdW density functional (DF) [ M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)], the atomic structure near the adsorption site is systematic......The ubiquitous van der Waals (vdW) force, particularly discernible in weak adsorption, is studied on noble and transition metals. In calculations with the vdW density functional (DF) [ M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)], the atomic structure near the adsorption site...
van der Waals-Tonks-type equations of state for hard-hypersphere fluids in four and five dimensions.
Wang, Xian-Zhi
2004-04-15
Recently, we developed accurate van der Waals-Tonks-type equations of state for hard-disk and hard-sphere fluids by using the known virial coefficients. In this paper, we derive the van der Waals-Tonks-type equations of state. We further apply these equations of state to hard-hypersphere fluids in four and five dimensions. In the low-density fluid regime, these equations of state are in good agreement with the simulation results and existing equations of state.
CERN PhotoLab
1984-01-01
CERN's 1984 Nobel prizewinners Carlo Rubbia (left) and Simon van der Meer, who were awarded the prize for their roles in discovering the W+, W- and Z0 particles, the carriers of Nature's weak force. Carlo Rubbia's work allowed CERN's Super Proton Synchrotron (SPS) to collide protons and antiprotons, while Simon van der Meer's technical virtuosity made the project possible.
化存才; 陆启韶
2001-01-01
用量级平衡方法分析一类时变分岔问题，得出正反两个方向的分岔转迁滞后且具有对称性. 将Duffing-Van der Pol 振动方程转化为对该时变分岔的反馈控制问题，得出在适当的条件下，Duffing-Van der Pol 振子的相图是一动态滞后环.%A class of time-dependent bifurcation problem has been analyzed by using the methodof scaling balance. It is concluded that the bifurcation transitions in positive and negative directionsare both delayed and have symmetry. The Duffing-Van der Pol oscillation equation has beenchanged into a feedback control problem of the time-dependent bifurcation. Under certainconditions, the phase diagram of the Duffing-Van der Pol oscillator is a dynamical hysteresiscycle.
From Canards of Folded Singularities to Torus Canards in a Forced van der Pol Equation
Burke, John; Desroches, Mathieu; Granados, Albert; Kaper, Tasso J.; Krupa, Martin; Vo, Theodore
2016-04-01
In this article, we study canard solutions of the forced van der Pol equation in the relaxation limit for low-, intermediate-, and high-frequency periodic forcing. A central numerical observation made herein is that there are two branches of canards in parameter space which extend across all positive forcing frequencies. In the low-frequency forcing regime, we demonstrate the existence of primary maximal canards induced by folded saddle nodes of type I and establish explicit formulas for the parameter values at which the primary maximal canards and their folds exist. Then, we turn to the intermediate- and high-frequency forcing regimes and show that the forced van der Pol possesses torus canards instead. These torus canards consist of long segments near families of attracting and repelling limit cycles of the fast system, in alternation. We also derive explicit formulas for the parameter values at which the maximal torus canards and their folds exist. Primary maximal canards and maximal torus canards correspond geometrically to the situation in which the persistent manifolds near the family of attracting limit cycles coincide to all orders with the persistent manifolds that lie near the family of repelling limit cycles. The formulas derived for the folds of maximal canards in all three frequency regimes turn out to be representations of a single formula in the appropriate parameter regimes, and this unification confirms the central numerical observation that the folds of the maximal canards created in the low-frequency regime continue directly into the folds of the maximal torus canards that exist in the intermediate- and high-frequency regimes. In addition, we study the secondary canards induced by the folded singularities in the low-frequency regime and find that the fold curves of the secondary canards turn around in the intermediate-frequency regime, instead of continuing into the high-frequency regime. Also, we identify the mechanism responsible for this
Sacred History And Sacred Texts In Early Judaism : A Symposium In Honour Of A.s. Van Der Woude
Bremmer, J.N.; García Martínez, F.
1992-01-01
After a long and distinguished career Adam S. van der Woude will retire on Reformation Day (October 31) from the Chair of Old Testament Studies and Intertestamental Literature of the Faculty of Theology of the Rijksuniversiteit Groningen. In order not to let this occasion go unnoticed, the Center of
Johnston, Karen; Kleis, Jesper; Lundqvist, Bengt
2008-01-01
Two different adsorption configurations of benzene on the Si(001)-(2×1) surface, the tight-bridge and butterfly structures, were studied using density functional theory. Several exchange and correlation functionals were used, including the recently developed van der Waals density functional (vdW-...
Fotsin, H.B. [Faculte des Sciences, Universite de Dschang, B.P. 067 Dschang (Cameroon)]. E-mail: hbfotsin@yahoo.fr; Woafo, P. [Faculte des Sciences, Universite de Yaounde I, B.P. 812 Yaounde (Cameroon)
2005-06-01
This paper considers the problem of adaptive synchronization and parameter identification of an uncertain chaotic oscillator. Using recent results on adaptive control, we design a controller which enables both the synchronization of two unidirectionally coupled modified Van der Pol-Duffing oscillators and the estimation of unknown parameters of the drive oscillator.
Quist, W.J.
2015-01-01
Prof. Jacobus Alida van der Kloes (1845-1935) was appointed teacher in building materials at the “Polytechnische school” of Delft in 1882. From 1905 until his retirement in 1915 he was promoted to full professor on the subject of knowledge and research of building materials at the “Technische
D'onorio de Meo, Marco; Oh, Suhk Kun
1992-07-01
The problem of applying Wolff's cluster algorithm to anisotropic classical spin models is resolved by modifying a part of the Wolff algorithm. To test the effectiveness of our modified algorithm, the spin-van der Waals model is investigated in detail. Our estimate of the dynamical exponent of the model is z=0.19+/-0.04.
Zwol, P.J. van; Palasantzas, G.
2010-01-01
Theory calculations using the Lifshitz theory and atomic force microscopy force measurements show that Casimir/van der Weals dispersive forces have a strong dependence on material optical properties and surface roughness. At separations below 100 nm the roughness effect is manifested through a
Dumlich, Heiko; Gegg, Michael; Reich, Stephanie [Fachbereich Physik, Freie Universitaet Berlin, Arnimallee 14, 14195 Berlin (Germany); Hennrich, Frank [Institut fuer Nanotechnologie, Karlsruhe Institute of Technology, 76021 Karlsruhe (Germany)
2011-11-15
We study the binding strength and intertube distance of carbon nanotube bundles in dependence of their structure (chirality) with van der Waals density functional theory. We try to understand the bundling and debundling process of nanotube bundles and test whether an influence of chirality exists. The distance between the nanotubes in the bundles vary only in a small range within 3.3 and 3.4A, without any chiral angle and diameter dependence. We find an increase of van der Waals energy per length with increasing diameter of the nanotubes (E{sub vdW}{sup length}{proportional_to} d or{radical}(d)) for tubes with diameters between 5.6 and 27.3A, but no obvious correlation between the chirality of the tubes and the van der Waals energy per length. The van der Waals energy per atom is decreasing with increasing tube diameter (E{sub vdW}{sup atom}{proportional_to} 1/d or1/{radical}(d)). (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)
Quist, W.J.
2015-01-01
Prof. Jacobus Alida van der Kloes (1845-1935) was appointed teacher in building materials at the “Polytechnische school” of Delft in 1882. From 1905 until his retirement in 1915 he was promoted to full professor on the subject of knowledge and research of building materials at the “Technische Hooges
Aleem, F.
1980-03-01
The most recent measurements of the angular distribution in proton-proton elastic scattering at sqrt{s}=27.4, 45 and 62GeV with squared four momentum transfer, -t, extending up to 14(GeV/c)2, have been explained using Van der Waals type model.
Nonlocal van der Waals functionals: The case of rare-gas dimers and solids
Tran, Fabien
2013-01-01
Recently, the nonlocal van der Waals (vdW) density functionals [M. Dion, H. Rydberg, E. Schroeder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)] have attracted considerable attention due to their good performance for systems where weak interactions are important. Since the physics of dispersion is included in these functionals, they are usually more accurate and show less erratic behavior than the semilocal and hybrid methods. In this work, several variants of the vdW functionals have been tested on rare-gas dimers (from He2 to Kr2) and solids (Ne, Ar, and Kr) and their accuracy compared to standard semilocal approximations supplemented or not by an atom-pairwise dispersion correction [S. Grimme, J. Antony, S. Ehrlich, and H. Krieg, J. Chem. Phys. 132, 154104 (2010)]. An analysis of the results in terms of energy decomposition is also provided.
Encapsulation of organic molecules in carbon nanotubes: role of the van der Waals interactions
Dappe, Y. J.
2014-02-01
Carbon nanotubes are fascinating nano-objects not only from a fundamental point of view but also with respect to their remarkable properties, holding great potential in new materials design. When combined with organic molecules, these properties can be enhanced or modulated in order to fulfil the demand in domains as diverse as molecular electronics, biomaterials or even construction engineering, to name a few. To adequately conceive these hybrid materials it is essential to fully appreciate the nature of molecule-carbon nanotube interactions. In this review, we will discuss some relevant fundamental and applied research done on encapsulated molecules in carbon nanotubes. We will particularly focus on the weak and van der Waals interactions which rule the molecule-tube coupling. Therefore a small state of the art on the theoretical methods used to describe these interactions is presented here. Then, we will discuss various applications of molecular encapsulation, where we will consider structural, magnetic, charge transfer and transport, and optical properties.
Román-Pérez, Guillermo; Soler, José M
2009-08-28
We present an efficient implementation of the van der Waals density functional of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], which expresses the nonlocal correlation energy as a double spatial integral. We factorize the integration kernel and use fast Fourier transforms to evaluate the self-consistent potential, total energy, and atomic forces, in O(NlogN) operations. The resulting overhead, for medium and large systems, is a small fraction of the total computational cost, representing a dramatic speedup over the O(N(2)) evaluation of the double integral. This opens the realm of first-principles simulations to the large systems of interest in soft matter and biomolecular problems. We apply the method to calculate the binding energies and the barriers for relative translation and rotation in double-wall carbon nanotubes.
Graphene/h-BN/ZnO van der Waals tunneling heterostructure based ultraviolet photodetector.
Wu, Zhiqian; Li, Xiaoqiang; Zhong, Huikai; Zhang, Shengjiao; Wang, Peng; Kim, Tae-ho; Kwak, Sung Soo; Liu, Cheng; Chen, Hongsheng; Kim, Sang-Woo; Lin, Shisheng
2015-07-27
We report a novel ultraviolet photodetector based on graphene/h-BN/ZnO van der Waals heterostructure. Graphene/ZnO heterostructure shows poor rectification behavior and almost no photoresponse. In comparison, graphene/h-BN/ZnO structure shows improved electrical rectified behavior and surprising high UV photoresponse (1350AW(-1)), which is two or three orders magnitude larger than reported GaN UV photodetector (0.2~20AW(-1)). Such high photoresponse mainly originates from the introduction of ultrathin two-dimensional (2D) insulating h-BN layer, which behaves as the tunneling layer for holes produced in ZnO and the blocking layer for holes in graphene. The graphene/h-BN/ZnO heterostructure should be a novel and representative 2D heterostructure for improving the performance of 2D materials/Semiconductor heterostructure based optoelectronic devices.
Strain induced piezoelectric effect in black phosphorus and MoS2 van der Waals heterostructure.
Huang, Le; Li, Yan; Wei, Zhongming; Li, Jingbo
2015-11-10
The structural, electronic, transport and optical properties of black phosphorus/MoS2 (BP/MoS2) van der Waals (vdw) heterostructure are investigated by using first principles calculations. The band gap of BP/MoS2 bilayer decreases with the applied normal compressive strain and a semiconductor-to-metal transition is observed when the applied strain is more than 0.85 Å. BP/MoS2 bilayer also exhibits modulation of its carrier effective mass and carrier concentration by the applied compressive strain, suggesting that mobility engineering and good piezoelectric effect can be realized in BP/MoS2 heterostructure. Because the type-II band alignment can facilitate the separation of photo-excited electrons and holes, and it can benefit from the great absorption coefficient in ultra-violet region, the BP/MoS2 shows great potential to be a very efficient ultra-violet photodetector.
Accurate treatment of nanoelectronics through improved description of van der Waals Interactions
Kelkkanen, Kari André
W-DF functional are too small, as the exchange approximation used is too repulsive. With the vdW-DF and other functionals that account for vdW forces, the total isomer energies are minimized in molecular configurations, which are compact, and in which many hydrogen bonds (HBs) can be described as distorted......This thesis emerges from a patented idea to utilize intentionally structured sur- faces and differences in adsorption strengths to self-assemble some source mate- rial into nanoelectronic components, and ends up in the heated debate regarding structure of ambient water . It investigates the role...... and relevance of van der Waals (vdW) forces in molecular surface adsorption and water through density- functional theory (DFT), using the exchange-correlation functional vdW-DF [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)] and developments based on it. Results are first computed for adsorption with vd...
Thermal electron attachment to oxygen and van der Waals molecules containing oxygen
Shimamori, Hiroshi; Fessenden, Richard W.
1981-01-01
Thermal electron attachment to O2 has been studied for pure O2 (16O2 and 18O2), O2-N2, O2-CO, and O2-n-C4H10 (16O2 and 18O2) systems at temperatures from ˜330 down to 78° K using pulse radiolysis and microwave conductivity. For pure O2, O2-N2, and O2-CO mixtures, the electron attachment rates showed three-body pressure dependences at all temperatures over the pressure range studied (PO2<10Torr, PN2<60 Torr, PCO<40 Torr). The three-body rate constant of 16O2 decreases from ˜2.4×10-30 cm6 molecule-2 sec-1 at 330 °K to about 0.9×10-30 cm6 molecule-2 sec-1 at ˜ 140 °K but unexpectedly increases again to about 1.7×10-30 cm6 molecule-2 sec-1 at 79 °K. Similarly, the three-body rate constant of 18O2 decreases from 5.1×10-30 cm6 molecule-2 sec-1 at 300 °K to 1.8×10-30 cm6 molecule-2 sec-1 at ˜110 °K but increases to 2.3×10-30 cm6 molecule-2 sec-1 at 80 °K. The three-body rate constant of N2 shows a more dramatic monotonic increase from 0.9×10-31 cm6 molecule-2 sec-1 at 300 °K to 9.4×10-31 cm6 molecule-2 sec-1 at 78 °K. In the case of CO, the three-body rate constant appears to have a very shallow minimum around 170 °K and again increases with further decrease of temperature. Since theory predicts a simple decrease in rate constant with reduced temperature, an extra contribution to the rate constant which increases with lowered temperature is evident. Electron attachment to the van der Waals molecules (O2)2, (O2ṡN2), and (O2ṡCO) is proposed to account for this behavior. It has been found that the dependence of the excess rate on temperature follows rather closely the predicted concentration of van der Waals molecules. Qualitatively, this observation suggests that the rate constant for electron attachment to the van der Waals molecules is only weakly dependent on temperature. The estimated rate constants for this attachment are nearly two orders of magnitude larger than for O2 itself. A discussion of possible reasons for this large increase is given
Van der Waals potential and vibrational energy levels of the ground state radon dimer
Sheng, Xiaowei; Qian, Shifeng; Hu, Fengfei
2017-08-01
In the present paper, the ground state van der Waals potential of the Radon dimer is described by the Tang-Toennies potential model, which requires five essential parameters. Among them, the two dispersion coefficients C6 and C8 are estimated from the well determined dispersion coefficients C6 and C8 of Xe2. C10 is estimated by using the approximation equation that C6C10 / C82 has an average value of 1.221 for all the rare gas dimers. With these estimated dispersion coefficients and the well determined well depth De and Re the Born-Mayer parameters A and b are derived. Then the vibrational energy levels of the ground state radon dimer are calculated. 40 vibrational energy levels are observed in the ground state of Rn2 dimer. The last vibrational energy level is bound by only 0.0012 cm-1.
Coulomb expansion of a van der Waals C60 solid film
薛其坤; 厉建龙; 孙牧; 陆华; T.Hashizume; Y.Hasegawa; K.Ohno; Y.Kawazoe; T.Sakurai; H.Kamiyama; H.Shinohara
2000-01-01
Scanning tunneling microscopy study revealed a van der Waals C60, solid film with 13% room-temperature lattice expansion on the GaAs(001) 2×4 surface. The mechanism involves fundamental Coulomb interaction due to charge transfer from the GaAs substrate. Theoretical calculation determines the charge transfer to be 1.76 electrons per C60 molecule. Oriented at its (110) crystallo-graphic axis this film also distinguishes itself from those formed on all other semiconductor and metal substrates where only the low-energy (111) hexagonal packing of C60 molecules was developed. It is shown that this is due to the one-dimensional confinement effect of the anisotropic substrate, which may have the prospect of controlling crystal growth.
Asfin, Ruslan E; Buldyreva, Jeanna V; Sinyakova, Tatyana N; Oparin, Daniil V; Filippov, Nikolai N
2015-02-07
Non-intrusive spectroscopic probing of weakly bound van der Waals complexes forming in gaseous carbon dioxide is generally performed at low pressures, for instance in supersonic jets, where the low temperature favors dimers, or in few-atmosphere samples, where the signature of dimers varying as the squared gas density is entangled with the dominating collision-induced absorption. We report experimental and theoretical results on CO2 dimers at very high pressures approaching the liquid phase. We observe that the shape of the CO2-dimer bands undergoes a distinctive line-mixing transformation, which reveals an unexpected stability of the dimers despite the collisions with the surrounding particles and negates the common belief that CO2 dimers are short-lived complexes. Our results furnish a deeper insight allowing a better modeling of CO2-rich atmospheres and provide also a new spectroscopic tool for studying the robustness of molecular clusters.
Van Der Waals Heterostructures between Small Organic Molecules and Layered Substrates
Han Huang
2016-09-01
Full Text Available Two dimensional atomic crystals, like grapheme (G and molybdenum disulfide (MoS2, exhibit great interest in electronic and optoelectronic applications. The excellent physical properties, such as transparency, semiconductivity, and flexibility, make them compatible with current organic electronics. Here, we review recent progress in the understanding of the interfaces of van der Waals (vdW heterostructures between small organic molecules (pentacene, copper phthalocyanine (CuPc, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA, and dioctylbenzothienobenzothiophene (C8-BTBT and layered substrates (G, MoS2 and hexagonal boron nitride (h-BN. The influences of the underlying layered substrates on the molecular arrangement, electronic and vibrational properties will be addressed.
Stability and dynamics of a controlled van der Pol-Duffing oscillator
Ji, J.C. [School of Mechanical Engineering, University of Adelaide, SA 5005 (Australia)] e-mail: jcji@mecheng.adelaide.edu.au; Hansen, C.H. [School of Mechanical Engineering, University of Adelaide, SA 5005 (Australia)] e-mail: chansen@mecheng.adelaide.edu.au
2006-04-01
The trivial equilibrium of a van der Pol-Duffing oscillator under a linear-plus-nonlinear feedback control may change its stability either via a single or via a double Hopf bifurcation if the time delay involved in the feedback reaches certain values. It is found that the trivial equilibrium may lose its stability via a subcritical or supercritical Hopf bifurcation and regain its stability via a reverse subcritical or supercritical Hopf bifurcation as the time delay increases. A stable limit cycle appears after a supercritical Hopf bifurcation occurs and disappears through a reverse supercritical Hopf bifurcation. The interaction of the weakly periodic excitation and the stable bifurcating solution is investigated for the forced system under primary resonance conditions. It is shown that the forced periodic response may lose its stability via a Neimark-Sacker bifurcation. Analytical results are validated by a comparison with those of direct numerical integration.
Miwadinou, C H; Monwanou, A V; Orou, J B Chabi
2013-01-01
This paper considers nonlinear dynamics of plasma oscillations modeled by a forced modified Van der Pol-Duffing oscillator. These plasma oscillations are described by a nonlinear differential equation of the form $ \\ddot{x}+ \\epsilon (1 +{x}^{2}){\\dot{x}} + x+ \\alpha \\epsilon{x}{\\dot{x}} + {\\beta}x^{2}+\\gamma x^{3}= F\\cos{\\Omega t}.$ The amplitudes of the forced harmonic, superharmonic and subharmonic oscillatory states are obtained using the harmonic balance technique and the multiple time scales methods. Bifurcation sequences displayed by the model for each type of oscillatory states are performed numerically through the fourth order Runge- Kutta scheme. The influences of the differents parameters and of amplitude of external forced have been found.
Difference resonances in a controlled van der Pol-Duffing oscillator involving time delay
Ji, J.C. [Faculty of Engineering, University of Technology, Sydney, Level 20, Building 1, PO Box 123, Broadway, NSW 2007 (Australia)], E-mail: jin.ji@uts.edu.au; Zhang, N.; Gao Wei [Faculty of Engineering, University of Technology, Sydney, Level 20, Building 1, PO Box 123, Broadway, NSW 2007 (Australia)
2009-10-30
A non-resonant interaction of two Hopf bifurcations may appear after the trivial solution of a controlled van der Pol-Duffing oscillator without external excitation loses its stability, when two critical time delays corresponding to two Hopf bifurcations have the same value. In the vicinity of the non-resonant Hopf bifurcations, the presence of a periodic excitation in the controlled oscillator can induce difference resonances in the forced response, when the forcing frequency and the frequencies of the two Hopf bifurcations satisfy certain relationships. It is found that the frequency response curves of the controlled system under difference resonances are an isolated closed curve. The difference resonance response may admit two stable motions on a three-dimensional torus consisting of three frequencies. Illustrative examples are given to show the quasi-periodic motions.
Cavitation inception of a van der Waals fluid at a sack-wall obstacle
Kaehler, G; Gonnella, G; Lamura, A
2016-01-01
Cavitation in a liquid moving past a constraint is numerically investigated by means of a free-energy lattice Boltzmann simulation based on the van der Waals equation of state. The fluid is streamed past an obstacle and, depending on the pressure drop between inlet and outlet, vapor formation underneath the corner of the sack-wall is observed. The circumstances of cavitation formation are investigated and it is found that the local bulk pressure and mean stress are insufficient to explain the phenomenon. Results obtained in this study strongly suggest that the viscous stress, interfacial contributions to the local pressure, and the Laplace pressure are relevant to the opening of a vapor cavity. This can be described by a generalization of Joseph's criterion that includes these contributions. A macroscopic investigation measuring mass flow rate behavior and discharge coefficient was also performed. As theoretically predicted, mass flow rate increases linearly with the square root of the pressure drop. However,...
Le Quang, T.; Cherkez, V.; Nogajewski, K.; Potemski, M.; Dau, M. T.; Jamet, M.; Mallet, P.; Veuillen, J.-Y.
2017-09-01
We have investigated the electronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs), namely trilayer WSe2 and monolayer MoSe2, deposited on epitaxial graphene on silicon carbide, by using scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum. Depending on the number of graphene layers below the TMD flakes, we identified variations in the electronic dI/dV(V) spectra measured by the STM tip: the most salient feature is a rigid shift of the TMD spectra (i.e. of the different band onset positions) towards occupied states by about 120 mV when passing from bilayer to monolayer underlying graphene. Since both graphene phases are metallic and present a work function difference in the same energy range, our measurements point towards the absence of Fermi-level pinning for such van der Waals 2D TMD/Metal heterojunctions, following the prediction of the Schottky-Mott model.
Bounds of Falk and Bruch in the low temperature Van der Waals model
Lee, M.H.; Kim, I.M. (Georgia Univ., Athens (USA). Dept. of Physics)
For many-body systems, the susceptibility and the fluctuation in the long-range order can be different. There are bounds on the ratio of the susceptibility to the fluctuation due to Falk and Bruch, according to which the two correlation functions cannot be entirely independent. For the spin van der Waals model, we obtain exact expressions for the susceptibility and fluctuation in the low temperature region. We find that the susceptibilty are the same in the XY-like regime of the model but they are different in the Ising-like regime. In both cases the bounds do not merge. Thus, the bounds alone are not sufficient to explain the relative behavior of the susceptibility and fluctuation.
Winther, Kirsten Trøstrup; Thygesen, Kristian Sommer
2017-01-01
precise magnitude is non-trivial to predict because of the non-local nature of the screening in quasi-2D crystals. Moreover, the effect is not captured by effective single-particle methods such as density functional theory. Here we present an efficient and general method for calculating the band gap......The idea of combining different two-dimensional (2D) crystals in van der Waals heterostructures (vdWHs) has led to a new paradigm for band structure engineering with atomic precision. Due to the weak interlayer couplings, the band structures of the individual 2D crystals are largely preserved upon...... formation of the heterostructure. However, regardless of the details of the interlayer hybridisation, the size of the 2D crystal band gaps are always reduced due to the enhanced dielectric screening provided by the surrounding layers. The effect can be significant (on the order of electron volts) but its...
Solutions of the Force-Free Duffing-van der Pol Oscillator Equation
Najeeb Alam Khan
2011-01-01
Full Text Available A new approximate method for solving the nonlinear Duffing-van der pol oscillator equation is proposed. The proposed scheme depends only on the two components of homotopy series, the Laplace transformation and, the Padé approximants. The proposed method introduces an alternative framework designed to overcome the difficulty of capturing the behavior of the solution and give a good approximation to the solution for a large time. The Runge-Kutta algorithm was used to solve the governing equation via numerical solution. Finally, to demonstrate the validity of the proposed method, the response of the oscillator, which was obtained from approximate solution, has been shown graphically and compared with that of numerical solution.
Formation of van der Waals molecules in buffer-gas-cooled magnetic traps [corrected].
Brahms, N; Tscherbul, T V; Zhang, P; Kłos, J; Sadeghpour, H R; Dalgarno, A; Doyle, J M; Walker, T G
2010-07-16
We predict that a large class of helium-containing cold polar molecules form readily in a cryogenic buffer gas, achieving densities as high as 10(12) cm(-3). We explore the spin relaxation of these molecules in buffer-gas-loaded magnetic traps and identify a loss mechanism based on Landau-Zener transitions arising from the anisotropic hyperfine interaction. Our results show that the recently observed strong T(-6) thermal dependence of the spin-change rate of silver (Ag) trapped in dense (3)He is accounted for by the formation and spin change of Ag(3)He van der Waals molecules, thus providing indirect evidence for molecular formation in a buffer-gas trap.
On the van der Pauw's method applied to the measurement of low thermal conductivity materials.
Morales, C; Flores, E; Bodega, J; Leardini, F; Ferrer, I J; Ares, J R; Sánchez, C
2016-08-01
The electrical van der Pauw's method has recently been extended to measure the thermal conductivity of different elements and compounds. This technique provides an easy way to determine the sample in-plane thermal conductivity by avoiding the influence of the thermal contact resistances. However, the reported calculated error values appear to be underestimated when dealing with the materials with low thermal conductivity (conduction heat losses through the thermo-resistance wires as well as the resulting modification of the sample temperature map. Both phenomena lead to experimental values of the sample thermal conductivity, which are systematically higher than the tabulated ones. The magnitude of this systematic error is ∼100% dealing with the samples of macroscopic dimensions, and low thermal conductivity indicated that the obtained accurate measurements can be quite challenging.
Experiment and computation: a combined approach to study the van der Waals complexes
Surin L.A.
2017-01-01
Full Text Available A review of recent results on the millimetre-wave spectroscopy of weakly bound van der Waals complexes, mostly those which contain H2 and He, is presented. In our work, we compared the experimental spectra to the theoretical bound state results, thus providing a critical test of the quality of the M–H2 and M–He potential energy surfaces (PESs which are a key issue for reliable computations of the collisional excitation and de-excitation of molecules (M = CO, NH3, H2O in the dense interstellar medium. The intermolecular interactions with He and H2 play also an important role for high resolution spectroscopy of helium or para-hydrogen clusters doped by a probe molecule (CO, HCN. Such experiments are directed on the detection of superfluid response of molecular rotation in the He and p-H2 clusters.
Enhanced van der Waals epitaxy via electron transfer-enabled interfacial dative bond formation
Xie, Weiyu; Wang, Gwo-Ching; Bhat, Ishwara; Zhang, Shengbai
2016-01-01
Enhanced van der Waals (vdW) epitaxy of semiconductors on layered vdW substrate is identified as the formation of dative bonds. For example, despite that NbSe2 is a vdW layered material, first-principles calculations reveal that the bond strength at CdTe-NbSe2 interface is five times as large as that of vdW interaction at CdTe-graphene interface. The unconventional chemistry here is enabled by an effective net electron transfer from Cd dangling-bond states at CdTe surface to metallic non-bonding NbSe2 states, which is a necessary condition to activate the Cd for enhanced binding with Se.
Li, Wei; Wang, Tian-Xing; Dai, Xian-Qi; Wang, Xiao-Long; Ma, Ya-Qiang; Chang, Shan-Shan; Tang, Ya-Nan
2017-04-01
Using density functional theory calculations, we investigate the electronic properties of arsenene/graphene van der Waals (vdW) heterostructures by applying external electric field perpendicular to the layers. It is demonstrated that weak vdW interactions dominate between arsenene and graphene with their intrinsic electronic properties preserved. We find that an n-type Schottky contact is formed at the arsenene/graphene interface with a Schottky barrier of 0.54 eV. Moreover, the vertical electric field can not only control the Schottky barrier height but also the Schottky contacts (n-type and p-type) and Ohmic contacts (n-type) at the interface. Tunable p-type doping in graphene is achieved under the negative electric field because electrons can transfer from the Dirac point of graphene to the conduction band of arsenene. The present study would open a new avenue for application of ultrathin arsenene/graphene heterostructures in future nano- and optoelectronics.
Free-Standing GaMnAs Nanomachined Sheets for van der Pauw Magnetotransport Measurements
Jae-Hyun Lee
2016-12-01
Full Text Available We report on the realization of free-standing GaMnAs epilayer sheets using nanomachining techniques. By optimizing the growth conditions of the sacrificial Al0.75Ga0.25As layer, free-standing metallic GaMnAs (with ~6% Mn microsheets (with TC ~85 K with integrated electrical probes are realized for magnetotransport measurements in the van der Pauw geometry. GaMnAs epilayer needs to be physically isolated to avoid buckling effects stemming from the release of lattice mismatch strain during the removal of the AlGaAs sacrificial layer. From finite element analysis, symmetrically placed and serpentine-shaped electrical leads induce minimal thermal stress at low temperatures. From magnetotransport measurements, changes in magnetic anisotropy are readily observed.
The Van der Waals interaction of the hydrogen molecule an exact local energy density functional
Choy, T C
1999-01-01
We verify that the van der Waals interaction and hence all dispersion interactions for the hydrogen molecule given by: W''= -{A/R^6}-{B/R^8}-{C/R^10}- ..., in which R is the internuclear separation, are exactly soluble. The constants A=6.4990267..., B=124.3990835 ... and C=1135.2140398... (in Hartree units) first obtained approximately by Pauling and Beach (PB) [1] using a linear variational method, can be shown to be obtainable to any desired accuracy via our exact solution. In addition we shall show that a local energy density functional can be obtained, whose variational solution rederives the exact solution for this problem. This demonstrates explicitly that a static local density functional theory exists for this system. We conclude with remarks about generalising the method to other hydrogenic systems and also to helium.
On the van der Pauw's method applied to the measurement of low thermal conductivity materials
Morales, C.; Flores, E.; Bodega, J.; Leardini, F.; Ferrer, I. J.; Ares, J. R.; Sánchez, C.
2016-08-01
The electrical van der Pauw's method has recently been extended to measure the thermal conductivity of different elements and compounds. This technique provides an easy way to determine the sample in-plane thermal conductivity by avoiding the influence of the thermal contact resistances. However, the reported calculated error values appear to be underestimated when dealing with the materials with low thermal conductivity (temperature. The causes of this underestimation are investigated in this communication and it has been found that they are due to the drastic influence of conduction heat losses through the thermo-resistance wires as well as the resulting modification of the sample temperature map. Both phenomena lead to experimental values of the sample thermal conductivity, which are systematically higher than the tabulated ones. The magnitude of this systematic error is ˜100% dealing with the samples of macroscopic dimensions, and low thermal conductivity indicated that the obtained accurate measurements can be quite challenging.
Van der Waals-Casimir-Polder interaction of an atom with a composite surface
Eizner, Elad; Henkel, Carsten
2012-01-01
We study the dispersion interaction of the van der Waals and Casimir-Polder (vdW-CP) type between a neutral atom and the surface of a metal by allowing for nonlocal electrodynamics, i.e. electron diffusion. We consider two models: (i) bulk diffusion, and (ii) diffusion in a surface charge layer. In both cases the transition to a semiconductor is continuous as a function of the conductivity, unlike the case of a local model. The relevant parameter is the electric screening length and depends on the carrier diffusion constant. We find that for distances comparable to the screening length, vdW-CP data can distinguish between bulk and surface diffusion, hence it can be a sensitive probe for surface states.
Lu, Bing-Sui; Naji, Ali; Podgornik, Rudolf
2016-01-01
We consider the van der Waals interaction between a pair of polymers with quenched heterogeneous sequences of local polarizabilities along their backbones, and study the effective pairwise interaction energy for both stiff polymers and flexible Gaussian coils. In particular, we focus on the cases where the pair of polarizability sequences are (i) distinct and (ii) identical. We find that the pairwise interaction energies of distinct and identical Gaussian coils are both isotropic and exhibit the same decay behavior for separations larger than their gyration radius, in contradistinction to the orientationally anisotropic interaction energies of distinct and identical stiff polymers. For both Gaussian coils and stiff polymers, the attractive interaction between identical polymers is enhanced if the polarizability sequence is more heterogeneous.
Continuatio naturae. La arquitectura monástica de Dom Hans van der Laan
Sainz Gutiérrez, Victoriano
2009-01-01
En un texto de carácter autobiográfico redactado hacia el final de su vida, Hans van der Laan señalaba, al recordar sus años de infancia y adolescencia, que el principal objetivo en esos años cruciales para su formación había sido el conocimiento de la naturaleza: «Lento pero seguro escribe, me adentraba en la vida de la naturaleza, siendo consciente de que había sido creada por Dios. Mi hermana mayor, la que murió durante la guerra, tuvo como profesora, tanto en la enseñanza primaria como en...
Miceli, Giacomo; Pasquarello, Alfredo
2016-01-01
We investigate the structural properties of liquid water at near ambient conditions using first-principles molecular dynamics simulations based on a semilocal density functional augmented with nonlocal van der Waals interactions. The adopted scheme offers the advantage of simulating liquid water at essentially the same computational cost of standard semilocal functionals. Applied to the water dimer and to ice Ih, we find that the hydrogen-bond energy is only slightly enhanced compared to a standard semilocal functional. We simulate liquid water through molecular dynamics in the NpH statistical ensemble allowing for fluctuations of the system density. The structure of the liquid departs from that found with a semilocal functional leading to more compact structural arrangements. This indicates that the directionality of the hydrogen-bond interaction has a diminished role as compared to the overall attractions, as expected when dispersion interactions are accounted for. This is substantiated through a detailed a...
Van der Pol and the history of relaxation oscillations: toward the emergence of a concept
Ginoux, Jean-Marc
2014-01-01
Relaxation oscillations are commonly associated with the name of Balthazar van der Pol via his eponymous paper (Philosophical Magazine, 1926) in which he apparently introduced this terminology to describe the nonlinear oscillations produced by self-sustained oscillating systems such as a triode circuit. Our aim is to investigate how relaxation oscillations were actually discovered. Browsing the literature from the late 19th century, we identified four self-oscillating systems in which relaxation oscillations have been observed: i) the series dynamo machine conducted by G\\'erard-Lescuyer (1880), ii) the musical arc discovered by Duddell (1901) and investigated by Blondel (1905), iii) the triode invented by de Forest (1907) and, iv) the multivibrator elaborated by Abraham and Bloch (1917). The differential equation describing such a self-oscillating system was proposed by Poincar\\'e for the musical arc (1908), by Janet for the series dynamo machine (1919), and by Blondel for the triode (1919). Once Janet (1919)...
The T-shaped KrI2(ion-pair states) van der Waals complexes
Baturo, V. V.; Kevorkyants, R.; Lukashov, S. S.; Poretsky, S. A.; Pravilov, A. M.; Zhironkin, A. I.
2017-09-01
The T-shaped KrI2(E0g+) van der Waals complexes have been observed and studied for the first time. Analysis of the luminescence excitation spectra as well as luminescence spectra themselves in the spectral ranges, where the I2(E0g+ → B0u+, D0u+ → X0g+, β1g → A1u and D‧2g → A‧2u) transitions can occur, has been carried out. Branching ratios of the iodine molecule state formation and vibronic level population, as well as the potential parameters of the KrI2(E,vE = 0-8, nE) complexes have been determined. Binding energies of the KrI2(X0g+,vX = 0, nX = 0 and B0u+, vB = 19, nB = 0) complexes have been also estimated.
Díez, A; Largo, J; Solana, J R
2006-08-21
Computer simulations have been performed for fluids with van der Waals potential, that is, hard spheres with attractive inverse power tails, to determine the equation of state and the excess energy. On the other hand, the first- and second-order perturbative contributions to the energy and the zero- and first-order perturbative contributions to the compressibility factor have been determined too from Monte Carlo simulations performed on the reference hard-sphere system. The aim was to test the reliability of this "exact" perturbation theory. It has been found that the results obtained from the Monte Carlo perturbation theory for these two thermodynamic properties agree well with the direct Monte Carlo simulations. Moreover, it has been found that results from the Barker-Henderson [J. Chem. Phys. 47, 2856 (1967)] perturbation theory are in good agreement with those from the exact perturbation theory.
Improving the accuracy of the nonlocal van der Waals density functional with minimal empiricism.
Vydrov, Oleg A; Van Voorhis, Troy
2009-03-14
The nonlocal van der Waals density functional (vdW-DF) captures the essential physics of the dispersion interaction not only in the asymptotic regime but also for a general case of overlapping fragment densities. A balanced treatment of other energetic contributions, such as exchange, is crucial if we aim for accurate description of various properties of weakly bound systems. In this paper, the vdW-DF correlation functional is modified to make it better compatible with accurate exchange functionals. We suggest a slightly simplified construction of the nonlocal correlation, yielding more accurate asymptotic C(6) coefficients. We also derive a gradient correction, containing a parameter that can be adjusted to suit an exchange functional of choice. We devise a particularly apt combination of exchange and correlation terms, which satisfies many important constraints and performs well for our benchmark tests.
Quantum transport across van der Waals domain walls in bilayer graphene
Abdullah, H. M.; Van Duppen, B.; Zarenia, M.; Bahlouli, H.; Peeters, F. M.
2017-10-01
Bilayer graphene can exhibit deformations such that the two graphene sheets are locally detached from each other resulting in a structure consisting of domains with different van der Waals inter-layer coupling. Here we investigate how the presence of these domains affects the transport properties of bilayer graphene. We derive analytical expressions for the transmission probability, and the corresponding conductance, across walls separating different inter-layer coupling domains. We find that the transmission can exhibit a valley-dependent layer asymmetry and that the domain walls have a considerable effect on the chiral tunnelling properties of the charge carriers. We show that transport measurements allow one to obtain the strength with which the two layers are coupled. We perform numerical calculations for systems with two domain walls and find that the availability of multiple transport channels in bilayer graphene significantly modifies the conductance dependence on inter-layer potential asymmetry.
Exploring the van der Waals Atom-Surface attraction in the nanometric range
Fichet, M; Yarovitski, A; Todorov, P; Hamdi, I; Maurin, I; Saltiel, S; Sarkisyan, D; Gorza, M P; Bloch, D; Ducloy, M; Fichet, Mich\\`{e}le; Dutier, Gabriel; Yarovitski, Alexander; Todorov, Petko; Hamdi, Ismah\\`{e}ne; Maurin, Isabelle; Saltiel, Solomon; Sarkisyan, David; Gorza, Marie-Pascale; Bloch, Daniel; Ducloy, Martial
2006-01-01
The van der Waals atom-surface attraction, scaling as C3 z-3 for z the atom-surface distance, is expected to be valid for ~ 1-1000 nm, covering 8-10 orders of magnitudes in the interaction energy. Thanks to a Cs vapor nanocell, we analyze the spectroscopic modifications induced by the atom-surface attraction on the 6P3/2->6D5/2 transition. The C3 value, extracted independently for various thicknesses ranging from 40 nm to 130 nm, is found to be independent of the thickness. It agrees, but only within a factor of 2, with an elementary theoretical prediction, whose validity is discussed.
韦萍
2007-01-01
Van der Hoeve综合征即蓝色巩膜一脆骨（耳聋）综合征。太田痣（nevus of ota）即先天性眼皮肤黑色素细胞增多痣，又称Ota综合征，是围绕眼眶、眼睑及眉部皮肤的一种蓝痣，伴同侧结膜、巩膜及葡萄膜弥散性痣。遗传方式多为常染色体显性遗传，但也有隐性者。笔者于2006年8月诊治了1例该病患者。
Bifurcation threshold of the delayed van der Pol oscillator under stochastic modulation.
Gaudreault, Mathieu; Drolet, François; Viñals, Jorge
2012-05-01
We obtain the location of the Hopf bifurcation threshold for a modified van der Pol oscillator, parametrically driven by a stochastic source and including delayed feedback in both position and velocity. We introduce a multiple scale expansion near threshold, and we solve the resulting Fokker-Planck equation associated with the evolution at the slowest time scale. The analytical results are compared with a direct numerical integration of the model equation. Delay modifies the Hopf frequency at threshold and leads to a stochastic bifurcation that is shifted relative to the deterministic limit by an amount that depends on the delay time, the amplitude of the feedback terms, and the intensity of the noise. Interestingly, stochasticity generally increases the region of stability of the limit cycle.
1-D Van der Waals Foams Heated by Ion Beam Energy Deposition
Zylstra, A; Barnard, J J; More, R M
2010-03-19
One dimensional simulations of various initial average density aluminum foams (modeled as slabs of solid metal separated by low density regions) heated by volumetric energy deposition are conducted with a Lagrangian hydrodynamics code using a van der Waals equation of state (EOS). The resulting behavior is studied to facilitate the design of future warm dense matter (WDM) experiments at LBNL. In the simulations the energy deposition ranges from 10 to 30 kJ/g and from 0.075 to 4.0 ns total pulse length, resulting in temperatures from approximately 1 to 4 eV. We study peak pressures and temperatures in the foams, expansion velocity, and the phase evolution. Five relevant time scales in the problem are identified. Additionally, we present a method for characterizing the level of inhomogeneity in a foam target as it is heated and the time it takes for a foam to homogenize.
1-D Van der Waals Foams Heated by Ion Beam Energy Deposition
Zylstra, A. B.; Barnard, J. J.; More, R. M.
2009-12-23
One dimensional simulations of various initial average density aluminum foams (modeled as slabs of solid metal separated by low density regions) heated by volumetric energy deposition are conducted with a Lagrangian hydrodynamics code using a van der Waals equation of tate (EOS). The resulting behavior is studied to facilitate the design of future warm dense matter (WDM) experiments at LBNL. In the simulations the energy deposition ranges from 10 to 30 kJ/g and from 0.075 to 4.0 ns total pulse length, resulting in temperatures from approximately 1 o 4 eV. We study peak pressures and temperatures in the foams, expansion velocity, and the phase evolution. Five relevant time scales in the problem are identified. Additionally, we present a method for characterizing the level of inhomogeneity in a foam target as it is heated and the time it takes for a foam to homogenize.
Philippe G. LeFloch
2000-12-01
Full Text Available This paper deals with the so-called p-system describing the dynamics of isothermal and compressible fluids. The constitutive equation is assumed to have the typical convexity/concavity properties of the van der Waals equation. We search for discontinuous solutions constrained by the associated mathematical entropy inequality. First, following a strategy proposed by Abeyaratne and Knowles and by Hayes and LeFloch, we describe here the whole family of nonclassical Riemann solutions for this model. Second, we supplement the set of equations with a kinetic relation for the propagation of nonclassical undercompressive shocks, and we arrive at a uniquely defined solution of the Riemann problem. We also prove that the solutions depend $L^1$-continuously upon their data. The main novelty of the present paper is the presence of two inflection points in the constitutive equation. The Riemann solver constructed here is relevant for fluids in which viscosity and capillarity effects are kept in balance.
Turbulent mixing of a slightly supercritical Van der Waals fluid at Low-Mach number
Battista, Francesco; Casciola, Carlo Massimo
2014-01-01
Supercritical fluids near the critical point are characterized by liquid-like densities and gas-like transport properties. These features are purposely exploited in different contexts ranging from natural products extraction/fractionation to aerospace propulsion. Large part of studies concerns this last context, focusing on the dynamics of supercritical fluids at high Mach number where compressibility and thermodynamics strictly interact. Despite the widespread use also at low Mach number, the turbulent mixing properties of slightly supercritical fluids have still not investigated in detail in this regime. This topic is addressed here by dealing with Direct Numerical Simulations (DNS) of a coaxial jet of a slightly supercritical Van der Waals fluid. Since acoustic effects are irrelevant in the Low Mach number conditions found in many industrial applications, the numerical model is based on a suitable low-Mach number expansion of the governing equation. According to experimental observations, the weakly superc...
Nucleotide insertion initiated by van derWaals interaction during polymerase beta DNA replication
Andrew Das Arulsamy
2013-09-01
We present here an unambiguous theoretical analyses and to show that the exclusive biochemical reaction involved in a single nucleotide insertion into the DNA primer can be efficiently tracked using the renormalized van derWaals (vdW) interaction of a stronger type, the Hermansson blue-shifting hydrogen bond effect, and the Arunan composite hydrogen-vdW bond. We find that there are two biochemical steps involved to complete the insertion of a single base (cytosine) into the 3' end of a DNA primer. First, the O3' (from a DNA primer) initiates the nucleophilic attack on P (from an incoming dCTP), in response, O3 (bonded to P) interacts with H' (bonded to O3'). These interactions are shown to be strongly interdependent and require the forming and breaking of P—O and H—O covalent bonds, which in turn imply that we do not need any external energy supply.
A self-consistent GW approach to the van der Waals potential for a helium dimer.
Shoji, Toru; Kuwahara, Riichi; Ono, Shota; Ohno, Kaoru
2016-09-21
van der Waals interaction between two helium (He) atoms is studied by calculating the total energy as a function of the He-He distance within the self-consistent GW approximation, which is expected to behave correctly in the long wavelength limit. In the Born-Oppenheimer (BO) approximation, the pair potential curve has its minimum value at 2.87 Å, which is somewhat larger than the local density approximation result, 2.40 Å, and is closer to previous quantum chemistry results. The expectation value for the interatomic distance, calculated by solving the Schrödinger equation for the two nuclei problem using the BO potential energy curve, is 30 Å, which is smaller but of the same order as previous experimental and theoretical results.
Van der Woude syndrome: Report of two cases with supplementary findings
Chandramani B More
2013-01-01
Full Text Available Van der Woude syndrome (VWS is a rare developmental disorder with an autosomal dominant inheritance and variable expressivity, occurring in about 1 of every 1,00,000-2,00,000 people. This syndrome is remarkably variable. It is characterized by orofacial manifestations like lower lip pits, cleft lip and/or cleft palate, hypodontia, cleft or bifid uvula, syngnathia, narrow high arched palate, ankyloglossia and hyper nasal voice. We report two interesting cases of VWS with characteristic orofacial features and an unusual additional finding of bilateral commissural pits. The purpose of this article is to facilitate understanding of etio-pathogenesis, clinical manifestations, role of genetic counseling and with special emphasis on commissural pits as an additional feature in VWS.
Thioarsenides: A case for long-range Lewis acid-base-directed van der Waals interactions
Gibbs, Gerald V.; Wallace, Adam F.; Downs, R. T.; Ross, Nancy L.; Cox, David F.; Rosso, Kevin M.
2011-04-01
Electron density distributions, bond paths, Laplacian and local energy density properties have been calculated for a number of As4Sn (n = 3,4,5) thioarsenide molecular crystals. On the basis of the distributions, the intramolecular As-S and As-As interactions classify as shared bonded interactions and the intermolecular As-S, As-As and S-S interactions classify as closed-shell van der Waals bonded interactions. The bulk of the intermolecular As-S bond paths link regions of locally concentrated electron density (Lewis base regions) with aligned regions of locally depleted electron density (Lewis acid regions) on adjacent molecules. The paths are comparable with intermolecular paths reported for several other molecular crystals that link aligned Lewis base and acid regions in a key-lock fashion, interactions that classified as long range Lewis acid-base directed van der Waals interactions. As the bulk of the intermolecular As-S bond paths (~70%) link Lewis acid-base regions on adjacent molecules, it appears that molecules adopt an arrangement that maximizes the number of As-S Lewis acid-base intermolecular bonded interactions. The maximization of the number of Lewis acid-base interactions appears to be connected with the close-packed array adopted by molecules: distorted cubic close-packed arrays are adopted for alacránite, pararealgar, uzonite, realgar and β-AsS and the distorted hexagonal close-packed arrays adopted by α- and β-dimorphite. A growth mechanism is proposed for thioarsenide molecular crystals from aqueous species that maximizes the number of long range Lewis acid-base vdW As-S bonded interactions with the resulting directed bond paths structuralizing the molecules as a molecular crystal.
Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit
Huang, Bevin; Clark, Genevieve; Navarro-Moratalla, Efrén; Klein, Dahlia R.; Cheng, Ran; Seyler, Kyle L.; Zhong, Ding; Schmidgall, Emma; McGuire, Michael A.; Cobden, David H.; Yao, Wang; Xiao, Di; Jarillo-Herrero, Pablo; Xu, Xiaodong
2017-06-01
Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin-valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.
An oscillation free shock-capturing method for compressible van der Waals supercritical fluid flows
Pantano, C.; Saurel, R.; Schmitt, T.
2017-04-01
Numerical solutions of the Euler equations using real gas equations of state (EOS) often exhibit serious inaccuracies. The focus here is the van der Waals EOS and its variants (often used in supercritical fluid computations). The problems are not related to a lack of convexity of the EOS since the EOS are considered in their domain of convexity at any mesh point and at any time. The difficulties appear as soon as a density discontinuity is present with the rest of the fluid in mechanical equilibrium and typically result in spurious pressure and velocity oscillations. This is reminiscent of well-known pressure oscillations occurring with ideal gas mixtures when a mass fraction discontinuity is present, which can be interpreted as a discontinuity in the EOS parameters. We are concerned with pressure oscillations that appear just for a single fluid each time a density discontinuity is present. The combination of density in a nonlinear fashion in the EOS with diffusion by the numerical method results in violation of mechanical equilibrium conditions which are not easy to eliminate, even under grid refinement. A cure to this problem is developed in the present paper for the van der Waals EOS based on previous ideas. A special extra field and its corresponding evolution equation is added to the flow model. This new field separates the evolution of the nonlinear part of the density in the EOS and produce oscillation free solutions. The extra equation being nonconservative the behavior of two established numerical schemes on shocks computation is studied and compared to exact reference solutions that are available in the present context. The analysis shows that shock conditions of the nonconservative equation have important consequence on the results. Last, multidimensional computations of a supercritical gas jet is performed to illustrate the benefits of the present method, compared to conventional flow solvers.
Density, structure, and dynamics of water: the effect of van der Waals interactions.
Wang, Jue; Román-Pérez, G; Soler, Jose M; Artacho, Emilio; Fernández-Serra, M-V
2011-01-14
It is known that ab initio molecular dynamics (AIMD) simulations of liquid water at ambient conditions, based on the generalized gradient approximation (GGA) to density functional theory (DFT), with commonly used functionals fail to produce structural and diffusive properties in reasonable agreement with experiment. This is true for canonical, constant temperature simulations where the density of the liquid is fixed to the experimental density. The equilibrium density, at ambient conditions, of DFT water has recently been shown by Schmidt et al. [J. Phys. Chem. B, 113, 11959 (2009)] to be underestimated by different GGA functionals for exchange and correlation, and corrected by the addition of interatomic pair potentials to describe van der Waals (vdW) interactions. In this contribution we present a DFT-AIMD study of liquid water using several GGA functionals as well as the van der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)]. As expected, we find that the density of water is grossly underestimated by GGA functionals. When a vdW-DF is used, the density improves drastically and the experimental diffusivity is reproduced without the need of thermal corrections. We analyze the origin of the density differences between all the functionals. We show that the vdW-DF increases the population of non-H-bonded interstitial sites, at distances between the first and second coordination shells. However, it excessively weakens the H-bond network, collapsing the second coordination shell. This structural problem is partially associated to the choice of GGA exchange in the vdW-DF. We show that a different choice for the exchange functional is enough to achieve an overall improvement both in structure and diffusivity.
Winther, Kirsten T.; Thygesen, Kristian S.
2017-06-01
The idea of combining different two-dimensional (2D) crystals in van der Waals heterostructures (vdWHs) has led to a new paradigm for band structure engineering with atomic precision. Due to the weak interlayer couplings, the band structures of the individual 2D crystals are largely preserved upon formation of the heterostructure. However, regardless of the details of the interlayer hybridisation, the size of the 2D crystal band gaps are always reduced due to the enhanced dielectric screening provided by the surrounding layers. The effect can be significant (on the order of electron volts) but its precise magnitude is non-trivial to predict because of the non-local nature of the screening in quasi-2D crystals. Moreover, the effect is not captured by effective single-particle methods such as density functional theory. Here we present an efficient and general method for calculating the band gap renormalization of a 2D material embedded in an arbitrary vdWH. The method evaluates the change in the GW self-energy of the 2D material from the change in the screened Coulomb interaction. The latter is obtained using the quantum-electrostatic heterostructure (QEH) model. We benchmark the GΔW method against full first-principles GW calculations and use it to unravel the importance of screening-induced band structure renormalisation in various vdWHs. A main result is the observation that the size of the band gap reduction of a given 2D material when inserted into a heterostructure scales inversely with the polarisability of the 2D material. Our work demonstrates that dielectric engineering via van der Waals heterostructuring represents a promising strategy for tailoring the band structure of 2D materials.
Jing, Lei
Novel two-dimensional materials with weak interlayer Van der Waals interaction are fantastic platforms to study novel physical phenomena. This thesis describes our investigation on two different Van der Waals materials: graphene and bismuth selenide with calcium doping (CaxBi 2-xSe3, x as the doping level) in the topological insulator family. Firstly, we characterize the electrical transport behaviors of high-quality substrate-supported bilayer graphene devices with suspended metal gates. The device exhibits a transport gap induced by external electric field with an on/off ratio of 20,000, which could be explained by variable range hoping between localized states or disordered charge puddles. At large magnetic field, the device presents quantum Hall plateau at fractional values of conductance quantum, which arises from the equilibration of edge states between differentially doped regions. Secondly, we present our study on the electronic transport of CaxBi 2-xSe3 thin films, which are three-dimensional topological insulators and coupled with superconducting leads. In these novel Josephson transistors, we observe different characteristic features by energy dispersion spectrum (EDS) and Raman spectroscopy, and the weak suppression in the critical current Ic. Thirdly, we explore the thermal expansion of suspended graphene. By in-situ scanning electron microscope (SEM), we measure the thickness-dependence of graphene's negative thermal expansion coefficient (TEC). We propose that there is a competitive relation between the intrinsic TEC and the friction from the substrate and the graphene. Lastly, in collaboration with Dr. Nikolai Kalugin from New Mexico Tech., we explore the graphene's application as a quantum Hall effect infrared photodetector. This graphene-based detector can be operated at higher temperature (liquid nitrogen) and wider frequency than the previous implementations of quantum Hall detector.
Jet-Cooled High Resolution Infrared Spectroscopy of Small Van Der Waals SF_6 Clusters
Asselin, Pierre; Boudon, Vincent; Potapov, Alexey; Bruel, Laurent; Gaveau, Marc-André; Mons, Michel
2016-06-01
Using a pulsed slit nozzle multipass absorption spectrometer with a tunable quantum cascade laser we investigated van der Waals clusters involving sulfur hexafluoride in the spectral range near the νb{3} stretching vibration. Different sized homo-complexes were generated in a planar supersonic expansion with typically 0,5 % SF_6 diluted in 6 bar He. Firstly, several rotationally resolved parallel and perpendicular bands of (SF_6)_2, at 934,0 and 956,1 wn (#1 structure) in agreement with Takami et al. but also one band at 933,6 wn (#2 structure) never observed previously, were analyzed in light of a recent theoretical study predicting three nearly isoenergetic isomers of D2d, C2h and C_2 symmetry for the dimer. Furthermore, some broader bands were detected around 938 and 964 wn and assigned to (SF_6)_3 and (SF_6)_4 clusters on the grounds of concentration effects and/or ab initio calculations. Lastly, with 0,5 % rare gas Rg (Rg = Ne, Ar, Kr and Xe) added to the SF_6:He gas mixture, a series of van der Waals (SF_6)_2-Rg hetero-trimers were observed, which display a remarkable linear dependence of the vibrational shift with the polarizability of the rare gas atom provided that the initial SF_6 dimer structure is #2 . In the same time no transitions belonging to the binary complexes SF_6-Rg were found near the νb{3} monomer band. This result suggests a complex thermodynamics within the pulsed supersonic expansion leading to the preponderance of (SF_6)_2-Rg clusters over SF_6-Rg binary systems. R. D. Urban and M. Takami, J. Chem. Phys. 103, 9132 (1995). T. Vazhappily, A. Marjolin and K. D. Jordan, J. Phys. Chem. B, DOI: 10.1021/acs.jpcb.5b09419 (2015).
Antonio Juárez Chicote
2013-10-01
Full Text Available ResumenEl texto pretende asomarse a la mirada interior de Ludwig Mies van der Rohe y tomar conciencia del ejercicio visual y mental que la noción de ‘construcción’ tiene para el arquitecto. A partir de los ejercicios de adiestramiento visual (visual training propuestos en el IIT de Chicago a partir de 1938 propuestos por Mies van der Rohe y desarrollados por Walter Perterhans, y de los collage realizados por Mies para ilustrar su obra de arquitectura, como los realizados para el edificio de oficinas Bacardí en Santiago de Cuba el texto disecciona el universo mental de Mies.El adiestramiento del ojo propuesto por Mies en el IIT supone un adiestramiento en el orden de lo visual de enorme complejidad, pues el orden de la percepción no coincide con el orden de la construcción.Este desfase entre ambos universos plantea problemas casi irresolubles entre los que se debate el arquitecto y que el artículo, tras las lecturas que William Jordy, Robin Evans y Colin Rowe han realizado de la obra miesiana, pretende sintetizar y clarificar.El texto pretende asomarse a la tensión interna que, en la propia mente del arquitecto, alcanzan algunas de sus piezas, en un equilibrio altamente inestable, en el que la estructura superpuesta de la piel del edificio –la ‘representación de la estructura’– no puede sino anunciar con nostalgia el esplendor perdido del primer orden jerárquico estructural, que es de acero recubierto de hormigón, quedando, de esta manera a la vista, su condición inestable que se debate entre ser muro y ser esqueleto.Se proponen algunos conceptos clave extraídos de los cursos de Adiestramiento Visual y de algunos aspectos de la obra europea y americana de Mies a la luz de los cuales se puede hacer una relectura global de su obra: horizonte / obstáculo, textura / estructura, paisaje / material, percepción / construcción, totalidad / fragmento.Palabras claveforma, estructura, textura, percepción, construcci
Versatile van der Waals Density Functional Based on a Meta-Generalized Gradient Approximation
Haowei Peng
2016-10-01
Full Text Available A “best-of-both-worlds” van der Waals (vdW density functional is constructed, seamlessly supplementing the strongly constrained and appropriately normed (SCAN meta-generalized gradient approximation for short- and intermediate-range interactions with the long-range vdW interaction from rVV10, the revised Vydrov–van Voorhis nonlocal correlation functional. The resultant SCAN+rVV10 is the only vdW density functional to date that yields excellent interlayer binding energies and spacings, as well as intralayer lattice constants in 28 layered materials. Its versatility for various kinds of bonding is further demonstrated by its good performance for 22 interactions between molecules; the cohesive energies and lattice constants of 50 solids; the adsorption energy and distance of a benzene molecule on coinage-metal surfaces; the binding energy curves for graphene on Cu(111, Ni(111, and Co(0001 surfaces; and the rare-gas solids. We argue that a good semilocal approximation should (as SCAN does capture the intermediate-range vdW through its exchange term. We have found an effective range of the vdW interaction between 8 and 16 Å for systems considered here, suggesting that this interaction is negligibly small at the larger distances where it reaches its asymptotic power-law decay.
Versatile van der Waals Density Functional Based on a Meta-Generalized Gradient Approximation
Peng, Haowei; Yang, Zeng-Hui; Perdew, John P.; Sun, Jianwei
2016-10-01
A "best-of-both-worlds" van der Waals (vdW) density functional is constructed, seamlessly supplementing the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation for short- and intermediate-range interactions with the long-range vdW interaction from r VV 10 , the revised Vydrov-van Voorhis nonlocal correlation functional. The resultant SCAN +r VV 10 is the only vdW density functional to date that yields excellent interlayer binding energies and spacings, as well as intralayer lattice constants in 28 layered materials. Its versatility for various kinds of bonding is further demonstrated by its good performance for 22 interactions between molecules; the cohesive energies and lattice constants of 50 solids; the adsorption energy and distance of a benzene molecule on coinage-metal surfaces; the binding energy curves for graphene on Cu(111), Ni(111), and Co(0001) surfaces; and the rare-gas solids. We argue that a good semilocal approximation should (as SCAN does) capture the intermediate-range vdW through its exchange term. We have found an effective range of the vdW interaction between 8 and 16 Å for systems considered here, suggesting that this interaction is negligibly small at the larger distances where it reaches its asymptotic power-law decay.
Tjaart van der Walt: slaaf van Christus – δοῦλος Χριστοῦ
E.J. Smit
2011-06-01
Full Text Available Tjaart van der Walt: slave of Christ – δοῦλος Χριστοῦ Tjaart van der Walt has carried an array of titles: from Boy, to Young Man, to Mister, to Reverend, to Doctorandus, to Doctor,to Professor, to Rector, to Vice-Chancellor, to CEO, to Ambassador! But the one title he gladly accepts, is “slave of Christ” – or in the Greek he loves so much: δοῦλος Χριστοῦ (doulos Christou! This has become typical of him and his life: a slave taking his instructions from Jesus Christ as his Owner, his Κύριος (Kurios, being willing to – in each situation and in relationship with any person or organisation – put on his slave clothing, roll up his sleeves, and serve! In this narrative of the life of Tjaart van der Walt we do not want to share lists of successes and failures, but rather we survey the serving life of this “slave of Christ”, as if from an Archimedes vantage point. The present, November 2011, is this Archimedes vantage point.
Feringa, J
2015-01-01
Dit artikel beschrijft de opkomst van de ‘agent’ in de zeventiende eeuw. Meer specifiek komen de agenten aan bod die vanuit de Republiek hun opdrachtgevers bedienden, die zich voornamelijk buiten de Republiek bevonden. Aan de hand van de agency theory betoogt Feringa dat het ontstaan van het fenomee
Paulo Pascoal
1999-01-01
Mr. Jan van der Boon, Leader personnel division (PE) from 01.04.99 to 31.03.02. CERN Council 2000, new position, Mr. Jan van der Boon was nominated Director of Admnistration from 01.01.01 to 31.12.03.
窦晓冬; 刘国杰
2001-01-01
利用实验测得的液体的热压力系数,修正原始van der Waals模型中的排斥体积,得到了一个适用于高密度流体的推广的van der Waals压缩因子。将它与Redlich和Kwong提出的吸引项相结合，建立了一个流体状态方程。用某些有代表性的超临界流体的pVT数据检验，结果表明，其对体积、压力和逸度系数的计算准确度可与CS-RK方程相媲美。
Review of “The kitchen as laboratory” edited by César Vega, Job Ubbink, and Erik van der Linden
Mouritsen Ole G
2012-08-01
Full Text Available Book details Vega C, Ubbink J, van der Linden E The Kitchen as Laboratory. Reflections on the Science of Food and Cooking. New York: Columbia University Press; 2012 312 pages, ISBN 978-0-231-15344-7
Interaction of boron with graphite: A van der Waals density functional study
Liu, Juan; Wang, Chen [Beijing Key Lab of Fine Ceramics, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084 (China); Liang, Tongxiang, E-mail: txliang@tsinghua.edu.cn [State Key Lab of New Ceramic and Fine Processing, Tsinghua University, Beijing 100084 (China); Lai, Wensheng [Advanced Material Laboratory, School of Materials Science & Engineering, Tsinghua University, Beijing, 100084 (China)
2016-08-30
Highlights: • A van der Waals density-functional approach is applied to study the interaction of boron with graphite. • VdW-DF functionals give fair agreement of crystal parameters with experiments. • The π electron approaches boron while adsorbing on graphite surface. • The hole introduced by boron mainly concentrates on boron and the nearest three carbon atoms. • PBE cannot describe the interstitial boron in graphite because of the ignoring binding of graphite sheets. - Abstract: Boron doping has been widely investigated to improve oxidation resistance of graphite. In this work the interaction of boron with graphite is investigated by a van der Waals density-functional approach (vdW-DF). The traditional density-functional theory (DFT) is well accounted for the binding in boron-substituted graphite. However, to investigate the boron atom on graphite surface and the interstitial impurities require use of a description of graphite interlayer binding. Traditional DFT cannot describe the vdW physics, for instance, GGA calculations show no relevant binding between graphite sheets. LDA shows some binding, but they fail to provide an accurate account of vdW forces. In this paper, we compare the calculation results of graphite lattice constant and cohesive energy by several functionals, it shows that vdW-DF such as two optimized functionals optB88-vdW and optB86b-vdW give much improved results than traditional DFT. The vdW-DF approach is then applied to study the interaction of boron with graphite. Boron adsorption, substitution, and intercalation are discussed in terms of structural parameters and electronic structures. When adsorbing on graphite surface, boron behaves as π electron acceptor. The π electron approaches boron atom because of more electropositive of boron than carbon. For substitution situation, the hole introduced by boron mainly concentrates on boron and the nearest three carbon atoms. The B-doped graphite system with the hole has less
Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures
Lin, Yu-Chuan; Li, Jun; de La Barrera, Sergio C.; Eichfeld, Sarah M.; Nie, Yifan; Addou, Rafik; Mende, Patrick C.; Wallace, Robert M.; Cho, Kyeongjae; Feenstra, Randall M.; Robinson, Joshua A.
2016-04-01
Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EGFH) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe2 and graphene and employ a charge transfer model to determine the WSe2 carrier density in both cases. The results indicate that WSe2-EGFH displays ohmic behavior at small biases due to a large hole density in the WSe2, whereas WSe2-EGPH forms a Schottky barrier junction.Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EGFH) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low
Zhang, Chenxi; Gong, Cheng; Nie, Yifan; Min, Kyung-Ah; Liang, Chaoping; Oh, Young Jun; Zhang, Hengji; Wang, Weihua; Hong, Suklyun; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae
2017-03-01
Two-dimensional transition metal dichalcogenides (TMDs) are promising low-dimensional materials which can produce diverse electronic properties and band alignment in van der Waals heterostructures. Systematic density functional theory (DFT) calculations are performed for 24 different TMD monolayers and their bilayer heterostacks. DFT calculations show that monolayer TMDs can behave as semiconducting, metallic or semimetallic depending on their structures; we also calculated the band alignment of the TMDs to predict their alignment in van der Waals heterostacks. We have applied the charge equilibration model (CEM) to obtain a quantitative formula predicting the highest occupied state of any type of bilayer TMD heterostacks (552 pairs for 24 TMDs). The CEM predicted values agree quite well with the selected DFT simulation results. The quantitative prediction of the band alignment in the TMD heterostructures can provide an insightful guidance to the development of TMD-based devices.
Fotsin, Hilaire [Laboratoire d' electronique, Departement de Physique, Faculte des Sciences, Universite de Dschang, BP 067 Dschang (Cameroon); Laboratoire de Mathematiques Appliquees, Departement de Mathematiques et d' lnformatique, Faculte des sciences, Universite de Douala, BP 24157 Douala (Cameroon); E-mail: hbfotsin@yahoo.fr; Bowong, Samuel [CRAN-INPL-ENSEM-2, Avenue de la Foret de Haye-54516, Vandoeuvre-les-Nancy Cedex (France); E-mail: sbowong@uycdc.uninet.cm; Daafouz, Jamal [Laboratoire de Mathematiques Appliquees, Departement de Mathematiques et d' lnformatique, Faculte des sciences, Universite de Douala, BP 24157 Douala (Cameroon)
2005-10-01
In this paper, chaos synchronization between two chaotic systems consisting of modified Van der Pol-Duffing (MVDPD) and Chua oscillators is studied. Such a problem is related to chaos synchronization of different systems. An adaptive feedback synchronization with two controllers is proposed to ensure global chaos synchronization of the modified Van der Pol-Duffing and Chua circuit oscillators. Numerical simulations show the effectiveness of the proposed control method. Moreover, as an application, the proposed scheme is then applied to secure communication whose process consists of a robust filter that recovers the information signal from the scalar transmitted signal. Also, simulation results verify the proposed scheme's success in the communication application.
Pande, Vikram
2016-01-01
Graphite is the most widely used and among the most widely-studied anode materials for lithium-ion batteries. Lithium intercalation into graphite has been extensively studied theoretically using density functional theory (DFT) calculations, complemented by experimental studies through X-ray diffraction, spectroscopy, optical imaging and other techniques. However, previous theoretical studies have not directly included van der Waals (vdW) interactions in their density functional theory calculations and vdW interactions play a crucial role in determining the stable phases. In this work, we present a first principles based model using DFT calculations, employing Bayesian Error Estimation Functional with van der Waals (BEEF-vdW) as the exchange correlation functional, and statistical thermodynamics to determine the phase transformations and subsequently, the thermodynamic intercalation potential diagram. We explore the entire configurational phase space by determining the important interactions and applying clust...
Feng, Zhihong; Chen, Buyun; Qian, Shuangbei; Xu, Linyan; Feng, Liefeng; Yu, Yuanyuan; Zhang, Rui; Chen, Jiancui; Li, Qianqian; Li, Quanning; Sun, Chongling; Zhang, Hao; Liu, Jing; Pang, Wei; Zhang, Daihua
2016-09-01
We report on a new chemical sensor based on black phosphorus/molybdenum diselenide van der Waals hetero-junctions. Due to the atomically thin nature of two-dimensional (2D) materials, surface adsorption of gas molecules can effectively modulate the band alignment at the junction interface, making the device a highly sensitive detector for chemical adsorptions. Compared to sensors made of homogeneous nanomaterials, the hetero-junction demonstrates considerably lower detection limit and higher sensitivity toward nitrogen dioxide. Kelvin probe force microscopy and finite element simulations have provided experimental and theoretical explanations for the enhanced performance, proving that chemical adsorption can induce significant changes in band alignment and carrier transport behaviors. The study demonstrates the potential of van der Waals hetero-junction as a new platform for sensing applications, and provides more insights into the interaction between gaseous molecules and 2D hetero-structures.
Zhang, Zhilin; Savenije, Hubert H. G.
2017-07-01
The practical value of the surprisingly simple Van der Burgh equation in predicting saline water intrusion in alluvial estuaries is well documented, but the physical foundation of the equation is still weak. In this paper we provide a connection between the empirical equation and the theoretical literature, leading to a theoretical range of Van der Burgh's coefficient of 1/2 residual circulation. This type of mixing is relevant in the wider part of alluvial estuaries where preferential ebb and flood channels appear. Subsequently, this dispersion equation is combined with the salt balance equation to obtain a new predictive analytical equation for the longitudinal salinity distribution. Finally, the new equation was tested and applied to a large database of observations in alluvial estuaries, whereby the calibrated K values appeared to correspond well to the theoretical range.
O'Farrell, E C T; Avsar, A; Tan, J Y; Eda, G; Özyilmaz, B
2015-09-09
Magnetotransport measurements demonstrate that graphene in a van der Waals heterostructure is a sensitive probe of quantum transport in an adjacent WS2 layer via strong Coulomb interactions. We observe a large low-field magnetoresistance (≫ e(2)/h) and a -ln T temperature dependence of the resistance. In-plane magnetic field resistance indicates the origin is orbital and nonclassical. We demonstrate a strong electron-hole asymmetry in the mobility and coherence length of graphene demonstrating the presence of localized Coulomb interactions with ionized donors in the WS2 substrate, which ultimately leads to screening as the Fermi level of graphene is tuned toward the conduction band of WS2. This leads us to conclude that graphene couples to quantum localization processes in WS2 via the Coulomb interaction and results in the observed signatures of quantum transport. Our results show that theoretical descriptions of the van der Waals interface should not ignore localized strong correlations.
Ma Shao-Juan; Xu Wei; Li Wei; Fang Tong
2006-01-01
The Chebyshev polynomial approximation is applied to investigate the stochastic period-doubling bifurcation and chaos problems of a stochastic Duffing-van der Pol system with bounded random parameter of exponential probability density function subjected to a harmonic excitation. Firstly the stochastic system is reduced into its equivalent deterministic one, and then the responses of stochastic system can be obtained by numerical methods. Nonlinear dynamical behaviour related to stochastic period-doubling bifurcation and chaos in the stochastic system is explored. Numerical simulations show that similar to its counterpart in deterministic nonlinear system of stochastic period-doubling bifurcation and chaos may occur in the stochastic Duffing-van der Pol system even for weak intensity of random parameter.Simply increasing the intensity of the random parameter may result in the period-doubling bifurcation which is absent from the deterministic system.
van der Waals forces in density functional theory: a review of the vdW-DF method.
Berland, Kristian; Cooper, Valentino R; Lee, Kyuho; Schröder, Elsebeth; Thonhauser, T; Hyldgaard, Per; Lundqvist, Bengt I
2015-06-01
A density functional theory (DFT) that accounts for van der Waals (vdW) interactions in condensed matter, materials physics, chemistry, and biology is reviewed. The insights that led to the construction of the Rutgers-Chalmers van der Waals density functional (vdW-DF) are presented with the aim of giving a historical perspective, while also emphasizing more recent efforts which have sought to improve its accuracy. In addition to technical details, we discuss a range of recent applications that illustrate the necessity of including dispersion interactions in DFT. This review highlights the value of the vdW-DF method as a general-purpose method, not only for dispersion bound systems, but also in densely packed systems where these types of interactions are traditionally thought to be negligible.
Yu Jun; Zhang Rongbo; Pan Weizhen [Department of Physics, Shaoxing University, Shaoxing 312000 (China); Schimansky-Geier, L [Institute of Physics, Humboldt-University at Berlin, Newton strasse 15, 12489 Berlin (Germany)
2008-08-15
Duffing-Van der Pol equation with the fifth nonlinear-restoring force is investigated. The bifurcation structure and chaotic motion under the periodic perturbation are obtained by numerical simulations. Numerical simulations, including bifurcation diagrams, Lyapunov exponents, phase portraits and Poincare maps, exhibit some new complex dynamical behaviors of the system. Different routes to chaos, such as period doubling and quasi-periodic routes, and various kinds of strange attractors are also demonstrated.
Emelianova, Yu.P., E-mail: yuliaem@gmail.com [Department of Electronics and Instrumentation, Saratov State Technical University, Polytechnicheskaya 77, Saratov 410054 (Russian Federation); Kuznetsov, A.P., E-mail: apkuz@rambler.ru [Kotel' nikov' s Institute of Radio-Engineering and Electronics of RAS, Saratov Branch, Zelyenaya 38, Saratov 410019 (Russian Federation); Turukina, L.V., E-mail: lvtur@rambler.ru [Kotel' nikov' s Institute of Radio-Engineering and Electronics of RAS, Saratov Branch, Zelyenaya 38, Saratov 410019 (Russian Federation); Institute for Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam (Germany)
2014-01-10
The dynamics of the four dissipatively coupled van der Pol oscillators is considered. Lyapunov chart is presented in the parameter plane. Its arrangement is discussed. We discuss the bifurcations of tori in the system at large frequency detuning of the oscillators. Here are quasi-periodic saddle-node, Hopf and Neimark–Sacker bifurcations. The effect of increase of the threshold for the “amplitude death” regime and the possibilities of complete and partial broadband synchronization are revealed.
Guogang LIU; Yi ZHAO
2004-01-01
The one-dimensional linear wave equation with a van der Pol nonlinear boundary condition is one of the simplest models that may cause isotropic or nonisotropic chaotic vibrations.It characterizes the nonisotropic chaotic vibration by means of the total variation theory.Some results are derived on the exponential growth of total variation of the snapshots on the spatial interval in the long-time horizon when the map and the initial condition satisfy some conditions.
Fan, Xiaolin
2017-01-19
This paper presents a componentwise convex splitting scheme for numerical simulation of multicomponent two-phase fluid mixtures in a closed system at constant temperature, which is modeled by a diffuse interface model equipped with the Van der Waals and the Peng-Robinson equations of state (EoS). The Van der Waals EoS has a rigorous foundation in physics, while the Peng-Robinson EoS is more accurate for hydrocarbon mixtures. First, the phase field theory of thermodynamics and variational calculus are applied to a functional minimization problem of the total Helmholtz free energy. Mass conservation constraints are enforced through Lagrange multipliers. A system of chemical equilibrium equations is obtained which is a set of second-order elliptic equations with extremely strong nonlinear source terms. The steady state equations are transformed into a transient system as a numerical strategy on which the scheme is based. The proposed numerical algorithm avoids the indefiniteness of the Hessian matrix arising from the second-order derivative of homogeneous contribution of total Helmholtz free energy; it is also very efficient. This scheme is unconditionally componentwise energy stable and naturally results in unconditional stability for the Van der Waals model. For the Peng-Robinson EoS, it is unconditionally stable through introducing a physics-preserving correction term, which is analogous to the attractive term in the Van der Waals EoS. An efficient numerical algorithm is provided to compute the coefficient in the correction term. Finally, some numerical examples are illustrated to verify the theoretical results and efficiency of the established algorithms. The numerical results match well with laboratory data.
Van der Waals Type Model and Structure in bar{p}p Elastic Scattering at High Energies
Aleem, F.
1983-10-01
The most recent measurements of the angular distribution for bar{p}p elastic scattering at pL=50, 100 and 200 GeV/ c which show a structure near -t=1.4 (GeV/ c)2, with squared four momentum transter -t extended to 5(GeV/ c)2, and the total cross section data for 50 <= pL <= 280GeV/ c have been simultaneously fitted by using the Van der Waals type model.
Yanagisawa, Susumu, E-mail: shou@sci.u-ryukyu.ac.jp [Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213 (Japan); Okuma, Koji; Inaoka, Takeshi [Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213 (Japan); Hamada, Ikutaro, E-mail: Hamada.Ikutaro@nims.go.jp [International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044 (Japan)
2015-10-01
Highlights: • Review of theoretical studies on organic solids with the density-functional methods. • van der Waals (vdW)-inclusive methods to predict cohesive properties of oligoacenes. • A variant of the vdW density functional describes the structures accurately. • The molecular configuration and conformation crucially affects the band dispersion. - Abstract: We review recent studies on electronic properties of the organic solids with the first-principles electronic structure methods, with the emphasis on the roles of the intermolecular van der Waals (vdW) interaction in electronic properties of the organic semiconductors. After a brief summary of the recent vdW inclusive first-principle theoretical methods, we discuss their performance in predicting cohesive properties of oligoacene crystals as examples of organic crystals. We show that a variant of the van der Waals density functional describes structure and energetics of organic crystals accurately. In addition, we review our recent study on the zinc phthalocyanine crystal and discuss the importance of the intermolecular distance and orientational angle in the band dispersion. Finally, we draw some general conclusions and the future perspectives.
Ab initio studies of coherent spin transport in Fe-hBN/graphene van der Waals multilayers
Magnus Ukpong, Aniekan
2017-07-01
This paper presents the results of ab initio studies of the electronic spin inversion and filtering in a ferromagnetic multilayer heterostructure. Spin-polarized electronic structure calculations are performed based on van der Waals density functional theory to give unique insights in to the generation, manipulation and transport of coherent spin conductance. By using an exact theory of the self-consistent ground state of the Fe-hBN/graphene multilayer as a model of the magnetic tunnel junction, hidden asymmetries are unraveled in the spin-resolved charge densities. It is shown that the injection of spin into the graphene/boron nitride tunnel layer from a ferromagnetic contact gives rise to coherent spin current. The projected Fermi surfaces of the up and down spin channels are analyzed to reveal Fermi arc topologies and spin anisotropies. It is also demonstrated that the coherent transport of pure spin-down current in the topological Weyl semimetal phase is robust. The implications of the results on out-of-plane transport of spin polarized conductance in van der Waals multilayer spintronic devices is discussed. The insights derived from this study are expected to open up prospects for further exploration of van der Waals magnetic multilayer heterostructures as a versatile platform for developing materials for Weyltronic applications.
Sheng, Xiao Wei; Mentel, Lukasz; Gritsenko, Oleg V; Baerends, Evert Jan
2011-10-01
This article investigates the errors in supermolecule calculations for the helium dimer. In a full CI calculation, there are two errors. One is the basis set superposition error (BSSE), the other is the basis set convergence error (BSCE). Both of the errors arise from the incompleteness of the basis set. These two errors make opposite contributions to the interaction energies. The BSCE is by far the largest error in the short range and larger than (but much closer to) BSSE around the Van der Waals minimum. Only at the long range, the BSSE becomes the larger error. The BSCE and BSSE largely cancel each other over the Van der Waals well. Accordingly, it may be recommended to not include the BSSE for the calculation of the potential energy curve from short distance till well beyond the Van der Waals minimum, but it may be recommended to include the BSSE correction if an accurate tail behavior is required. Only if the calculation has used a very large basis set, one can refrain from including the counterpoise correction in the full potential range. These results are based on full CI calculations with the aug-cc-pVXZ (X = D, T, Q, 5) basis sets.
Multiple critical points and liquid-liquid equilibria from the van der Waals like equations of state
Artemenko, Sergey; Lozovsky, Taras; Mazur, Victor [Thermodynamics Department, Academy of Refrigeration, 65082 Odessa (Ukraine)
2008-06-18
The principal aim of this work is a comprehensive analysis of the phase diagram of water via the van der Waals like equations of state (EoSs) which are considered as superpositions of repulsive and attractive forces. We test more extensively the modified van der Waals EoS (MVDW) proposed by Skibinski et al (2004 Phys. Rev. E 69 061206) and refine this model by introducing instead of the classical van der Waals repulsive term a very accurate hard sphere EoS over the entire stable and metastable regions (Liu 2006 Preprint cond-mat/0605392). It was detected that the simplest form of MVDW EoS displays a complex phase behavior, including three critical points, and identifies four fluid phases (gas, low density liquid (LDL), high density liquid (HDL), and very high density liquid (VHDL)). Moreover the experimentally observed (Mallamace et al 2007 Proc. Natl Acad. Sci. USA 104 18387) anomalous behavior of the density of water in the deeply supercooled region (a density minimum) is reproduced by the MWDW EoS. An improvement of the repulsive part does not change the topological picture of the phase behavior of water in the wide range of thermodynamic variables. The new parameters set for second and third critical points are recognized by thorough analysis of experimental data for the loci of thermodynamic response function extrema.
Vertical electron transport in van der Waals heterostructures with graphene layers
Ryzhii, V., E-mail: v-ryzhii@riec.tohoku.ac.jp [Research Institute for Electrical Communication, Tohoku University, Sendai 980-8577 (Japan); Center for Photonics and Infrared Engineering, Bauman Moscow State Technical University and Institute of Ultra High Frequency Semiconductor Electronics of RAS, Moscow 111005 (Russian Federation); Otsuji, T. [Research Institute for Electrical Communication, Tohoku University, Sendai 980-8577 (Japan); Ryzhii, M. [Department of Computer Science and Engineering, University of Aizu, Aizu-Wakamatsu 965-8580 (Japan); Aleshkin, V. Ya.; Dubinov, A. A. [Institute for Physics of Microstructures of RAS and Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod 603950 (Russian Federation); Mitin, V. [Research Institute for Electrical Communication, Tohoku University, Sendai 980-8577 (Japan); Department of Electrical Engineering, University at Buffalo, Buffalo, New York 1460-1920 (United States); Shur, M. S. [Department of Electrical, Electronics, and Systems Engineering and Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180 (United States)
2015-04-21
We propose and analyze an analytical model for the self-consistent description of the vertical electron transport in van der Waals graphene-layer (GL) heterostructures with the GLs separated by the barriers layers. The top and bottom GLs serve as the structure emitter and collector. The vertical electron transport in such structures is associated with the propagation of the electrons thermionically emitted from GLs above the inter-GL barriers. The model under consideration describes the processes of the electron thermionic emission from and the electron capture to GLs. It accounts for the nonuniformity of the self-consistent electric field governed by the Poisson equation which accounts for the variation of the electron population in GLs. The model takes also under consideration the cooling of electrons in the emitter layer due to the Peltier effect. We find the spatial distributions of the electric field and potential with the high-electric-field domain near the emitter GL in the GL heterostructures with different numbers of GLs. Using the obtained spatial distributions of the electric field, we calculate the current-voltage characteristics. We demonstrate that the Peltier cooling of the two-dimensional electron gas in the emitter GL can strongly affect the current-voltage characteristics resulting in their saturation. The obtained results can be important for the optimization of the hot-electron bolometric terahertz detectors and different devices based on GL heterostructures.
Non-technical approach to the challenges of ecological architecture: Learning from Van der Laan
María-Jesús González-Díaz
2016-06-01
Full Text Available Up to now, ecology has a strong influence on the development of technical and instrumental aspects of architecture, such as renewable and efficient of resources and energy, CO2 emissions, air quality, water reuse, some social and economical aspects. These concepts define the physical keys and codes of the current ׳sustainable׳ architecture, normally instrumental but rarely and insufficiently theorised. But is not there another way of bringing us to nature? We need a theoretical referent. This is where we place the Van der Laan׳s thoughts: he considers that art completes nature and he builds his theoretical discourse on it, trying to better understand many aspects of architecture. From a conceptual point of view, we find in his works sense of timelessness, universality, special attention on the ׳locus׳ and a strict sense of proportions and use of materials according to nature. Could these concepts complement our current sustainable architecture? How did Laan apply the current codes of ecology in his architecture? His work may help us to get a theoretical interpretation of nature and not only physical. This paper develops this idea through the comparison of thoughts and works of Laan with the current technical approach to ׳sustainable׳ architecture.
Nucleotide insertion initiated by van der Waals interaction during polymerase beta DNA replication
Arulsamy, Andrew Das
2011-01-01
Immortality will remain a fantasy for as long as aging is determined by the erroneous biochemical reactions during a particular DNA replication. The replication and base excision repair mechanism, associated to eukaryotic DNA polymerase-beta enzyme are central to maintaining a healthy cell. Here, we give a series of unambiguous theoretical analyses and prove that the exclusive biochemical reaction involved in a single nucleotide insertion into the DNA primer can be efficiently tracked using the renormalized van der Waals interaction of the stronger type, and the Hermansson blue-shifting hydrogen bond effect. We found that there are two biochemical steps involved to complete the insertion of a single dCTP into the 3' end of a DNA primer. First, the O3' (from a DNA primer) initiates the nucleophilic attack on P_alpha?(from an incoming dCTP), in response, O3_alpha (bonded to P_alpha) retaliates by interacting with H' (bonded to O3'). These interactions are shown to be strongly interdependent and require the form...
Enthalpy, geometric volume and logarithmic correction to entropy for van der Waals black hole
Pradhan, Parthapratim
2016-10-01
If the negative cosmological constant is treated as a dynamical pressure and if the volume is its thermodynamically conjugate variable then the gravitational mass can be expressed as the total gravitational enthalpy rather than the energy. Under these circumtances, a new phenomenon emerges in the context of extended phase space thermodynamics. We examine here these features for the recently discovered van der Waals (VDW) black hole (BH) (Rajagopal A. et al., Phys. Lett. B, 737 (2014) 277) which is analogous to the VDW fluid. We show that the thermodynamic volume is greater than the naive geometric volume. We also show that the Smarr-Gibbs-Duhem relation is satisfied for this BH. Furthermore, by computing the thermal specific heat we find the local thermodynamic stability criterion for this BH. It has been observed that the BH does not possess any kind of second-order phase transition. This is an interesting feature of VDW BH by its own right. Moreover, we also derive the Cosmic-Censorship-Inequality for this class of BH. In addition finally, we compute the logarithmic correction to the entropy of this BH due to the quantum fluctuations around the thermal equilibrium.
Enthalpy, Geometric Volume and Logarithmic correction to Entropy for Van-der-Waals Black Hole
Pradhan, Parthapratim
2016-01-01
If the negative cosmological constant is treated as a dynamical pressure and if the volume be its thermodynamically conjugate variable then the gravitational mass can be expressed as the total gravitational enthalpy rather than the energy. Under these circumstances, a new phenomena emerges in the context of extended phase space thermodynamics. We \\emph{examine} here these features for recently discovered Van-der-Waal (VDW) black hole (BH) \\cite{mann15} which is analogous to the VDW fluid. We show that the thermodynamic volume is \\emph{greater} than the naive geometric volume. We also show that the \\emph{Smarr-Gibbs-Duhem} relation is satisfied for this BH. Furthermore, by computing the thermal specific heat we find the local thermodynamic stability criterion for this BH. It has been observed that the BH does \\emph{not} possess any kind of second order phase transition. This is an interesting feature of VDW BH by its own right. Moreover, we also derive \\emph{Cosmic-Censorship-Inequality} for this class of BH. ...
RESPONSE OF PARAMETRICALLY EXCITED DUFFING-VAN DER POL OSCILLATOR WITH DELAYED FEEDBACK
LI Xin-ye; CHEN Yu-shu; WU Zhi-qiang; SONG Tao
2006-01-01
The dynamical behaviour of a parametrically excited Duffing-van der Pol oscillator under linear-plus-nonlinear state feedback control with a time delay is concerned.By means of the method of averaging together with truncation of Taylor expansions, two slow-flow equations on the amplitude and phase of response were derived for the case of principal parametric resonance. It is shown that the stability condition for the trivial solution is only associated with the linear terms in the original systems besides the amplitude and frequency of parametric excitation. And the trivial solution can be stabilized by appreciate choice of gains and time delay in feedback control. Different from the case of the trivial solution, the stability condition for nontrivial solutions is also associated with nonlinear terms besides linear terms in the original system. It is demonstrated that nontrivial steady state responses may lose their stability by saddle-node (SN) or Hopf bifurcation (HB) as parameters vary. The simulations, obtained by numerically integrating the original system, are in good agreement with the analytical results.
Stabilization of thin liquid films by repulsive van der waals force
Li, Erqiang
2014-05-13
Using high-speed video recording of bubble rise experiments, we study the stability of thin liquid films trapped between a rising bubble and a surfactant-free liquid-liquid meniscus interface. Using different combinations of nonpolar oils and water that are all immiscible, we investigate the extent to which film stability can be predicted by attractive and repulsive van der Waals (vdW) interactions that are indicated by the relative magnitude of the refractive indices of the liquid combinations, for example, water (refractive index, n = 1.33), perfluorohexane (n = 1.23), and tetradecane (n = 1.43). We show that, when the film-forming phase was oil (perfluorohexane or tetradecane), the stability of the film could always be predicted from the sign of the vdW interaction, with a repulsive vdW force resulting in a stable film and an attractive vdW force resulting in film rupture. However, if aqueous electrolyte is the film-forming bulk phase between the rising air bubble and the upper oil phase, the film always ruptured, even when a repulsive vdW interaction was predicted. We interpret these results as supporting the hypothesis that a short-ranged hydrophobic attraction determines the stability of the thin water film formed between an air phase and a nonpolar oil phase. © 2014 American Chemical Society.
van der Waals Interactions in Hadron Resonance Gas: From Nuclear Matter to Lattice QCD.
Vovchenko, Volodymyr; Gorenstein, Mark I; Stoecker, Horst
2017-05-05
An extension of the ideal hadron resonance gas (HRG) model is constructed which includes the attractive and repulsive van der Waals (VDW) interactions between baryons. This VDW-HRG model yields the nuclear liquid-gas transition at low temperatures and high baryon densities. The VDW parameters a and b are fixed by the ground state properties of nuclear matter, and the temperature dependence of various thermodynamic observables at zero chemical potential are calculated within the VDW-HRG model. Compared to the ideal HRG model, the inclusion of VDW interactions between baryons leads to a qualitatively different behavior of second and higher moments of fluctuations of conserved charges, in particular in the so-called crossover region T∼140-190 MeV. For many observables this behavior resembles closely the results obtained from lattice QCD simulations. This hadronic model also predicts nontrivial behavior of net-baryon fluctuations in the region of phase diagram probed by heavy-ion collision experiments. These results imply that VDW interactions play a crucial role in the thermodynamics of hadron gas. Thus, the commonly performed comparisons of the ideal HRG model with the lattice and heavy-ion data may lead to misconceptions and misleading conclusions.
Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment.
Yan, Rusen; Fathipour, Sara; Han, Yimo; Song, Bo; Xiao, Shudong; Li, Mingda; Ma, Nan; Protasenko, Vladimir; Muller, David A; Jena, Debdeep; Xing, Huili Grace
2015-09-09
van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials.
Global view of Hopf bifurcations of a van der Pol oscillator with delayed state feedback
无
2010-01-01
This paper presents both analytical and numerical studies on the global view of Hopf bifurcations of a van der Pol oscillator with delayed state feedback.Based on a detailed analysis of the stability switches of the trivial equilibrium of the system,the stability charts are given in a parameter space consisting of the time delay and the feedback gains.The center manifold reduc-tion and the normal form method are used to study Hopf bifurcations with respect to the time delay.To gain an insight into the persistence of a Hopf bifurcation as the time delay varies farther away from its critical value,the method of multiple scales is used to obtain the global view of Hopf bifurcations with respect to the time delay.Both the analytical results of Hopf bifurca-tions and global view of those bifurcations are validated via a collocation scheme implemented on DDE-Biftool.The most important discovery in this paper is the well-structured global view of Hopf bifurcations for the system of concern,showing the generality of the persistence of Hopf bifurcations.
Amrillah, Tahta; Bitla, Yugandhar; Shin, Kwangwoo; Yang, Tiannan; Hsieh, Ying-Hui; Chiou, Yu-You; Liu, Heng-Jui; Do, Thi Hien; Su, Dong; Chen, Yi-Chun; Jen, Shien-Uang; Chen, Long-Qing; Kim, Kee Hoon; Juang, Jenh-Yih; Chu, Ying-Hao
2017-06-27
Magnetoelectric nanocomposites have been a topic of intense research due to their profound potential in the applications of electronic devices based on spintronic technology. Nevertheless, in spite of significant progress made in the growth of high-quality nanocomposite thin films, the substrate clamping effect still remains a major hurdle in realizing the ultimate magnetoelectric coupling. To overcome this obstacle, an alternative strategy of fabricating a self-assembled ferroelectric-ferrimagnetic bulk heterojunction on a flexible muscovite via van der Waals epitaxy is adopted. In this study, we investigated the magnetoelectric coupling in a self-assembled BiFeO3 (BFO)-CoFe2O4 (CFO) bulk heterojunction epitaxially grown on a flexible muscovite substrate. The obtained heterojunction is composed of vertically aligned multiferroic BFO nanopillars embedded in a ferrimagnetic CFO matrix. Moreover, due to the weak interaction between the flexible substrate and bulk heterojunction, the interface is incoherent and, hence, the substrate clamping effect is greatly reduced. The phase-field simulation model also complements our results. The magnetic and electrical characterizations highlight the improvement in magnetoelectric coupling of the BFO-CFO bulk heterojunction. A magnetoelectric coupling coefficient of 74 mV/cm·Oe of this bulk heterojunction is larger than the magnetoelectric coefficient reported earlier on flexible substrates. Therefore, this study delivers a viable route of fabricating a remarkable magnetoelectric heterojunction and yet flexible electronic devices that are robust against extreme conditions with optimized performance.
Evolution of Moiré Profiles from van der Waals Superstructures of Boron Nitride Nanosheets
Liao, Yunlong; Cao, Wei; Connell, John W.; Chen, Zhongfang; Lin, Yi
2016-05-01
Two-dimensional (2D) van der Waals (vdW) superstructures, or vdW solids, are formed by the precise restacking of 2D nanosheet lattices, which can lead to unique physical and electronic properties that are not available in the parent nanosheets. Moiré patterns formed by the crystalline mismatch between adjacent nanosheets are the most direct features for vdW superstructures under microscopic imaging. In this article, transmission electron microscopy (TEM) observation of hexagonal Moiré patterns with unusually large micrometer-sized lateral areas (up to ~1 μm2) and periodicities (up to ~50 nm) from restacking of liquid exfoliated hexagonal boron nitride nanosheets (BNNSs) is reported. This observation was attributed to the long range crystallinity and the contaminant-free surfaces of these chemically inert nanosheets. Parallel-line-like Moiré fringes with similarly large periodicities were also observed. The simulations and experiments unambiguously revealed that the hexagonal patterns and the parallel fringes originated from the same rotationally mismatched vdW stacking of BNNSs and can be inter-converted by simply tilting the TEM specimen following designated directions. This finding may pave the way for further structural decoding of other 2D vdW superstructure systems with more complex Moiré images.
Optoelectronic Properties of Van Der Waals Hybrid Structures: Fullerenes on Graphene Nanoribbons
Correa, Julián David; Orellana, Pedro Alejandro; Pacheco, Mónica
2017-01-01
The search for new optical materials capable of absorbing light in the frequency range from visible to near infrared is of great importance for applications in optoelectronic devices. In this paper, we report a theoretical study of the electronic and optical properties of hybrid structures composed of fullerenes adsorbed on graphene and on graphene nanoribbons. The calculations are performed in the framework of the density functional theory including the van der Waals dispersive interactions. We found that the adsorption of the C60 fullerenes on a graphene layer does not modify its low energy states, but it has strong consequences for its optical spectrum, introducing new absorption peaks in the visible energy region. The optical absorption of fullerenes and graphene nanoribbon composites shows a strong dependence on photon polarization and geometrical characteristics of the hybrid systems, covering a broad range of energies. We show that an external electric field across the nanoribbon edges can be used to tune different optical transitions coming from nanoribbon–fullerene hybridized states, which yields a very rich electro-absorption spectrum for longitudinally polarized photons. We have carried out a qualitative analysis on the potential of these hybrids as possible donor-acceptor systems in photovoltaic cells. PMID:28336904
Van der Waals forces and electron-electron interactions in two strained graphene layers
Sharma, Anand; Harnish, Peter; Sylvester, Alexander; Kotov, Valeri N.
2014-03-01
We evaluate the van der Waals (vdW) interaction energy at T=0 between two undoped graphene layers which are separated by a finite distance. Our study is carried out within the Random Phase Approximation and the interaction energy is obtained for variation in the strength of effective Coulomb interaction and anisotropy due to applied uniaxial strain. We consider the following three models for the anisotropic case: a) where one of the two layers is uniaxially strained, b) the two layers are strained in the same direction, and c) one of the layers is strained in the perpendicular direction. We find that for all the three models and any given value of the coupling, the vdW interaction energy increases with increasing anisotropy. The effect is most striking for the case when both the layers are strained in the parallel direction where we observe up to an order of magnitude increase in the strained graphene relative to the unstrained case. We also investigate the effect of intra-layer electron-electron interactions in the region of large separation between the strained graphene layers. We conclude that the many-body contributions to the correlation energy are non-negligible and the vdW interaction energy decreases as a function of increasing distance between the layers. Alexander Sylvester acknowledges financial assistance from the Research Experiences for Undergraduates (REU) Program of the National Science Foundation (NSF) focussing on complex materials.
Arabi, Alya A
2016-11-13
Modelling dispersion interactions with traditional density functional theory (DFT) is a challenge that has been extensively addressed in the past decade. The exchange-dipole moment (XDM), among others, is a non-empirical add-on dispersion correction model in DFT. The functional PW86+PBE+XDM for exchange, correlation and dispersion, respectively, compromises an accurate functional for thermochemistry and for van der Waals (vdW) complexes at equilibrium and non-equilibrium geometries. To use this functional in optimizing vdW complexes, rather than computing single point energies, it is necessary to evaluate accurate forces. The purpose of this paper is to validate that, along the potential energy surface, the distance at which the energy is minimum is commensurate with the distance at which the forces vanish to zero. This test was validated for 10 rare gas diatomic molecules using various integration grids and different convergence criteria. It was found that the use of either convergence criterion, 10(-6) or 10(-8), in Gaussian09, does not affect the accuracy of computed optimal distances and binding energies. An ultra-fine grid needs to be used when computing accurate energies using generalized gradient approximation functionals.This article is part of the themed issue 'Multiscale modelling at the physics-chemistry-biology interface'. © 2016 The Author(s).
Stabilization of thin liquid films by repulsive van der Waals force.
Li, Er Qiang; Vakarelski, Ivan U; Chan, Derek Y C; Thoroddsen, Sigurdur T
2014-05-13
Using high-speed video recording of bubble rise experiments, we study the stability of thin liquid films trapped between a rising bubble and a surfactant-free liquid-liquid meniscus interface. Using different combinations of nonpolar oils and water that are all immiscible, we investigate the extent to which film stability can be predicted by attractive and repulsive van der Waals (vdW) interactions that are indicated by the relative magnitude of the refractive indices of the liquid combinations, for example, water (refractive index, n = 1.33), perfluorohexane (n = 1.23), and tetradecane (n = 1.43). We show that, when the film-forming phase was oil (perfluorohexane or tetradecane), the stability of the film could always be predicted from the sign of the vdW interaction, with a repulsive vdW force resulting in a stable film and an attractive vdW force resulting in film rupture. However, if aqueous electrolyte is the film-forming bulk phase between the rising air bubble and the upper oil phase, the film always ruptured, even when a repulsive vdW interaction was predicted. We interpret these results as supporting the hypothesis that a short-ranged hydrophobic attraction determines the stability of the thin water film formed between an air phase and a nonpolar oil phase.
How van der Waals interactions determine the unique properties of water
Morawietz, Tobias; Dellago, Christoph; Behler, Jörg
2016-01-01
While the interactions between water molecules are dominated by strongly directional hydrogen bonds (HBs), it was recently proposed that relatively weak, isotropic van der Waals (vdW) forces are essential for understanding the properties of liquid water and ice. This insight was derived from ab initio computer simulations, which provide an unbiased description of water at the atomic level and yield information on the underlying molecular forces. However, the high computational cost of such simulations prevents the systematic investigation of the influence of vdW forces on the thermodynamic anomalies of water. Here we develop efficient ab initio-quality neural network potentials and use them to demonstrate that vdW interactions are crucial for the formation of water's density maximum and its negative volume of melting. Both phenomena can be explained by the flexibility of the HB network, which is the result of a delicate balance of weak vdW forces, causing e.g. a pronounced contraction of the second solvation ...
Novel bursting patterns in a Van der pol-Duffing oscillator with slow varying external force
Yu, Yue; Zhao, Min; Zhang, Zhengdi
2017-09-01
In this paper, we investigate the emergence of bursting dynamics with complex waveforms and their relation to periodic behavior in typical Van der pol-Duffing equation with fifth order polynomial stiffness nonlinearity, when the external force changes slowly with the variation of time. We exploit bifurcation characteristics of the fast subsystem using the slowly changing periodic excitation as a bifurcation parameter to show how the bursting oscillations are created in this model. We also identify that some regimes of bursting patterns are related to codimension two bifurcation type over a wide range of parameters. A subsequent two-parameter continuation reveals a transition in the bursting behavior from fold/fold hysteresis cycle to sup-Hopf/sup-Hopf or limit point cycle/sub-Hopf bursting type. Furthermore, the effects of external forcing item on bursting oscillations are investigated. For instance, the time interval between two adjacent spikes of bursting oscillations is dependent on the forcing frequency. Some numerical simulations are included to illustrate the validity of our study.
The influence of van der Waals interactions on a bubble moving in a tube
Hammoud, Naima; Howell, Peter D; Stone, Howard A
2016-01-01
We consider the unsteady thin-film dynamics of a long bubble of negligible viscosity that advances at a uniform speed in a cylindrical capillary tube. The bubble displaces a viscous non-wetting fluid, creating a thin film between its interface and the tube walls. The film is considered thin enough that intermolecular forces in the form of van der Waals attractions are significant and thin-film rupture is possible. In the absence of intermolecular forces, the bubble has a steady-state where a film of uniform thickness is deposited in the annular region between the bubble interface and the tube walls. However, once intermolecular forces are present, the bubble interface is perturbed out of its steady-state and either (i) the perturbation grows sufficiently before reaching the rear meniscus of the bubble such that rupture occurs; or (ii) the perturbation remains small due to weak intermolecular forces until it leaves the bubble interface through the rear meniscus. We obtain, both numerically and asymptotically, ...
Neutron Reflectivity Study of Interdiffusion of Ionomers into Van der Waals Polymer Thin Films
Etampawala, Thusitha; Ratnaweera, Dilru; Wijesinghe, Sidath; Perahia, Dvora; Majewski, Jaroslaw
2012-02-01
The slow dynamic processes in amorphous ionic polymers are affected by physical cross-links resulting from clustering of the ionic groups. Therefore in addition to entanglement barriers, the motion of the polymers is coupled to the dynamics of the ionic clusters where the resulting dynamics is an interplay between the effects of the two types of barriers. Using neutron reflectometry we have probed a model system where interfacial diffusion of a Van der Waals polymer, polystyrene, into its sulfonated analogs. Results controlling the molecular weights that determine the overall number of entanglements as well as the degree of sulfonation which affects the strength and number of the ionic clusters will be presented. Comparison to the diffusion of polystyrene into polystyrene will resolve the effects of the ionic clusters from those of entanglements. The presence of the physical cross-links slows down the dynamics significantly with respect to that of polystyrene and an asymmetric process where the non-ionic blocks migrate into the ionic one is observed. Further rearrangements take place at a later stage.
Arabi, Alya A.
2016-11-01
Modelling dispersion interactions with traditional density functional theory (DFT) is a challenge that has been extensively addressed in the past decade. The exchange-dipole moment (XDM), among others, is a non-empirical add-on dispersion correction model in DFT. The functional PW86+PBE+XDM for exchange, correlation and dispersion, respectively, compromises an accurate functional for thermochemistry and for van der Waals (vdW) complexes at equilibrium and non-equilibrium geometries. To use this functional in optimizing vdW complexes, rather than computing single point energies, it is necessary to evaluate accurate forces. The purpose of this paper is to validate that, along the potential energy surface, the distance at which the energy is minimum is commensurate with the distance at which the forces vanish to zero. This test was validated for 10 rare gas diatomic molecules using various integration grids and different convergence criteria. It was found that the use of either convergence criterion, 10-6 or 10-8, in Gaussian09, does not affect the accuracy of computed optimal distances and binding energies. An ultra-fine grid needs to be used when computing accurate energies using generalized gradient approximation functionals. This article is part of the themed issue 'Multiscale modelling at the physics-chemistry-biology interface'.
Resistive sensitivity functions for van der Pauw astroid and rounded crosses and cloverleafs
Koon, Daniel; Hansen, Ole
2014-03-01
We have calculated the sensitivity of van der Pauw resistances to local resistive variations for circular, square and astroid discs of infinitesimal thickness, as well as for the families of rounded crosses and cloverleafs, as a function of specimen parameters, using the direct formulas of our recent paper (Koon et al. 2013 J. Appl. Phys.114 163710) applied to ``reciprocally dual geometries'' (swapped Dirichlet and Neumann boundary conditions) described by Mareš et al.(2012 Meas. Sci. Technol. 23 045004). These results show that (a) the product of any such sensitivity function times differential area, and thus (b) the ratio of any two sensitivities, is invariant under conformal mapping, allowing for the pointwise determination of the conformal mapping function. The family of rounded crosses, which is bounded in parameter space by the square, the astroid and an ``infinitesimally thin'' cross, seems to represent the best geometry for focusing transport measurements on the center of the specimen while minimizing errors due to edge- or contact-effects. Made possible by an SLU Faculty research grant.
FDE-vdW: A van der Waals Inclusive Subsystem Density-Functional Theory
Kevorkyants, Ruslan; Pavanello, Michele
2014-01-01
We present a formally exact van der Waals inclusive electronic structure theory, called FDE-vdW, based on the Frozen Density Embedding formulation of subsystem Density-Functional Theory. In subsystem DFT, the energy functional is composed of subsystem additive and non-additive terms. We show that an appropriate definition of the long-range correlation energy is given by the value of the non-additive correlation functional. This functional is evaluated using the Fluctuation-Dissipation Theorem aided by a formally exact decomposition of the response functions into subsystem contributions. FDE-vdW is derived in detail and several approximate schemes are proposed, which lead to practical implementations of the method. We show that FDE-vdW is Casimir-Polder consistent, i.e. it reduces to the generalized Casimir-Polder formula for asymptotic inter-subsystems separations. Pilot calculations of binding energies of 13 weakly bound complexes singled out from the S22 set show a dramatic improvement upon semilocal subsys...
Role of van der Waals interactions for the intrinsic stability of polyalanine helices
Tkatchenko, Alexandre; Blum, Volker; Ireta, Joel; Scheffler, Matthias
2009-03-01
The helical motif is an ubiquitous conformation adopted by aminoacid residues in a protein structure and helix formation is the simplest example of the protein folding process. How stable is the folded peptide helix in comparison to a random coil structure? What are the interactions responsible for stabilizing the helical conformation? Answering these questions has thus a direct implication for understanding protein folding. In this work we use density functional theory (DFT) augmented with a non-empirical correction for van der Waals (vdW) forces to study the stability of alanine polypeptide helices in vacuo. We find a large stabilization of the native helical forms when vdW correction is used. It amounts to 121%, 157% and 83% on top of the Perdew-Burke-Ernzerhof (PBE) functional in the case of infinite α, π and 310 helices, respectively. Thus, the experimentally observed α helix is significantly stabilized by vdW forces both over the fully extended and the 310 conformations. Our findings also suggest an explanation to the remarkable stability of gas-phase alanine helices up to high temperatures [M. Kohtani et al. JACS 126, 7420 (2004)].
Van der Waals interactions in hadron resonance gas: From nuclear matter to lattice QCD
Vovchenko, Volodymyr; Stoecker, Horst
2016-01-01
An extension of the ideal non-interacting hadron resonance gas (HRG) model is constructed which includes the attractive and repulsive van der Waals (VDW) interactions between baryons. This VDW-HRG model yields the nuclear liquid-gas transition at low temperatures and high baryon densities. The VDW parameters $a$ and $b$ are fixed by the ground state properties of nuclear matter, and the temperature dependence of various thermodynamic observables at zero chemical potential are calculated within VDW-HRG model. Compared to the ideal non-interacting HRG, the inclusion of VDW interactions between baryons leads to a qualitatively different behavior of 2nd and higher moments of fluctuations of conserved charges, in particular in the so-called crossover region $T \\sim 140 \\div 190$ MeV. For many observables this behavior resembles closely the results obtained from lattice QCD simulations. These results imply that VDW interactions play a crucial role in thermodynamics of hadron gas. Thus, the commonly performed compar...
Zhou Shi-Qi
2007-01-01
A universal theoretical approach is proposed which enables all hard sphere density functional approximations(DFAs) applicable to van der Waals fluids. The resultant DFA obtained by combining the universal theoretical approach with any hard sphere DFAs only needs as input a second-order direct correlation function (DCF) of a coexistence bulk fluid, and is applicable in both supercritical and subcritical temperature regions. The associated effective hard sphere density can be specified by a hard wall sum rule. It is indicated that the value of the effective hard sphere density so determined can be universal, i.e. can be applied to any external potentials different from the single hard wall. As an illustrating example, the universal theoretical approach is combined with a hard sphere bridge DFA to predict the density profile of a hard core attractive Yukawa model fluid influenced by diverse external fields; agreement between the present formalism's predictions and the corresponding simulation data is good or at least comparable to several previous DFT approaches. The primary advantage of the present theoretical approach combined with other hard sphere DFAs is discussed.
Chouhan, Rajiv K.; Raghani, Pushpa
2015-09-01
We have investigated the adsorption of Li on graphene oxide using density functional theory. We show a novel and simple approach to achieve a positive lithiation potential on epoxy and hydroxyl functionalized graphene, compared to the negative lithiation potential that has been found on prestine graphene. We included the van der Waals correction into the calculation so as to get a better picture of weak interactions. A positive lithiation potential suggests a favorable adsorption of Li on graphene oxide sheets that can lead to an increase in the specific capacity, which in turn can be used as an anode material in Li-batteries. We find a high specific capacity of ˜860 mAhg-1 by functionalizing the graphene sheet. This capacity is higher than the previously reported capacities that were achieved on graphene with high concentration of Stone-Wales (75%) and divacancy (16%) defects. Creating such high density of defects can make the entire system energetically unstable, whereas graphene oxide is a naturally occurring substance.
Yu, Woo Jong; Vu, Quoc An; Oh, Hyemin; Nam, Hong Gi; Zhou, Hailong; Cha, Soonyoung; Kim, Joo-Youn; Carvalho, Alexandra; Jeong, Munseok; Choi, Hyunyong; Castro Neto, A. H.; Lee, Young Hee; Duan, Xiangfeng
2016-11-01
Two-dimensional layered transition-metal dichalcogenides have attracted considerable interest for their unique layer-number-dependent properties. In particular, vertical integration of these two-dimensional crystals to form van der Waals heterostructures can open up a new dimension for the design of functional electronic and optoelectronic devices. Here we report the layer-number-dependent photocurrent generation in graphene/MoS2/graphene heterostructures by creating a device with two distinct regions containing one-layer and seven-layer MoS2 to exclude other extrinsic factors. Photoresponse studies reveal that photoresponsivity in one-layer MoS2 is surprisingly higher than that in seven-layer MoS2 by seven times. Spectral-dependent studies further show that the internal quantum efficiency in one-layer MoS2 can reach a maximum of 65%, far higher than the 7% in seven-layer MoS2. Our theoretical modelling shows that asymmetric potential barriers in the top and bottom interfaces of the graphene/one-layer MoS2/graphene heterojunction enable asymmetric carrier tunnelling, to generate usually high photoresponsivity in one-layer MoS2 device.
Free-standing electronic character of monolayer MoS2 in van der Waals epitaxy
Kim, HoKwon; Dumcenco, Dumitru; Frégnaux, Mathieu; Benayad, Anass; Chen, Ming-Wei; Kung, Yen-Cheng; Kis, Andras; Renault, Olivier
2016-08-01
We have evaluated as-grown Mo S2 crystals, epitaxially grown on a monocrystalline sapphire by chemical vapor deposition (CVD), with direct electronic band-structure measurements by energy-filtered k -space photoelectron emission microscopy performed with a conventional laboratory vacuum ultraviolet He I light source under off-normal illumination. The valence states of the epitaxial Mo S2 were mapped in momentum space down to 7 eV below the Fermi level. Despite the high nucleation density within the imaged area, the CVD Mo S2 possesses an electronic structure similar to the free-standing monolayer Mo S2 single crystal, and it exhibits hole effective masses of 2.41 ±0.05 m0 , and 0.81 ±0.05 m0 , respectively, at Γ and K high-symmetry points that are consistent with the van der Waals epitaxial growth mechanism. This demonstrates the excellent ability of the Mo S2 CVD on sapphire to yield a highly aligned growth of well-stitched grains through epitaxial registry with a strongly preferred crystallographic orientation.
Topcu, Turker
2015-01-01
We investigate the effect of series perturbation on the second order dipole-dipole interactions between strontium atoms in $5sns({^1}S_0)$ and $5snp({^1}P_1)$ Rydberg states as a means of engineering long-range interactions between atoms in a way that gives an exceptional level of control over the strength and the sign of the interaction by changing $n$. We utilize experimentally available data to estimate the importance of perturber states at low $n$, and find that van der Waals interaction between two strontium atoms in the $5snp({^1}P_1)$ states shows strong peaks outside the usual hydrogenic $n^{11}$ scaling. We identify this to be the result of the perturbation of $5snd({^1}D_2)$ intermediate states by the $4d^2({^1}D_2)$ and $4dn's({^1}D_2)$ states in the $n<20$ range. This demonstrates that divalent atoms in general present a unique advantage for creating substantially stronger or weaker interaction strengths than those can be achieved using alkali metal atoms due to their highly perturbed spectra t...
van der Waals Interactions in Hadron Resonance Gas: From Nuclear Matter to Lattice QCD
Vovchenko, Volodymyr; Gorenstein, Mark I.; Stoecker, Horst
2017-05-01
An extension of the ideal hadron resonance gas (HRG) model is constructed which includes the attractive and repulsive van der Waals (VDW) interactions between baryons. This VDW-HRG model yields the nuclear liquid-gas transition at low temperatures and high baryon densities. The VDW parameters a and b are fixed by the ground state properties of nuclear matter, and the temperature dependence of various thermodynamic observables at zero chemical potential are calculated within the VDW-HRG model. Compared to the ideal HRG model, the inclusion of VDW interactions between baryons leads to a qualitatively different behavior of second and higher moments of fluctuations of conserved charges, in particular in the so-called crossover region T ˜140 - 190 MeV . For many observables this behavior resembles closely the results obtained from lattice QCD simulations. This hadronic model also predicts nontrivial behavior of net-baryon fluctuations in the region of phase diagram probed by heavy-ion collision experiments. These results imply that VDW interactions play a crucial role in the thermodynamics of hadron gas. Thus, the commonly performed comparisons of the ideal HRG model with the lattice and heavy-ion data may lead to misconceptions and misleading conclusions.
Nguyen, Thanh Long; Celistrino Teixeira, Raul; Hermann Avigliano, Carla; Cantat Moltrecht, Tigrane; Raimond, Jean Michel; Haroche, Serge; Gleyzes, Sebastiens; Brune, Michel
2016-05-01
Dipole-dipole interactions between Rydberg atoms are a flourishing tool for quantum information processing and for quantum simulation of complex many-body problems. Microwave spectroscopy of a dense Rydberg gas trapped close to a superconducting atom chip in the strong dipole blockade regime reveals directly the many-body atomic interaction spectrum. We present here a direct measurement of the interaction energy distribution in the strong dipole blockade regime, based on microwave spectroscopy. We first apply this method to the observation of the excitation dynamics of the Rydberg gas, conditioned by dipole-dipole interactions, in either the strong blockade regime or the so-called facilitation regime. We also observe with this method the atomic cloud expansion driven by the repulsive Van der Waals interaction after excitation. This measurement, in good agreement with Monte Carlo simulations of the excitation process and of the cloud dynamics, reveals the limits of the frozen gas approximation. This method can help investigate self-organization and dynamical phase transitions in Rydberg-atom based quantum simulators. This study thus opens a promising route for quantum simulation of many-body systems and quantum information transport in chains of strongly interacting Rydberg atom.
Hamid Moeenfard; Mohammad Taghi Ahmadian
2012-01-01
In this paper,the effect of van der Waals (vdW)force on the pull-in behavior of electrostatically actuated nano/micromirrors is investigated.First,the minimum potential energy principle is utilized to find the equation governing the static behavior of nano/microminror under electrostatic and vdW forces.Then,the stability of static equilibrium points is analyzed using the energy method.It is found that when there exist two equilibrium points,the smaller one is stable and the larger one is unstable.The effects of different design parameters on the mirror's pull-in angle and pull-in voltage are studied and it is found that vdW force can considerably reduce the stability limit of the mirror.At the end,the nonlinear equilibrium equation is solved numerically and analytically using homotopy perturbation method (HPM).It is observed that a sixth order perturbation approximation can precisely model the mirror's behavior.The resuits of this paper can be used for stable operation design and safe fabrication of torsional nano/micro actuators.
Stochastic period-doubling bifurcation analysis of stochastic Bonhoeffer-van der Pol system
Zhang Ying; Xu Wei; Fang Tong; Xu Xu-Lin
2007-01-01
In this paper, the Chebyshev polynomial approximation is applied to the problem of stochastic period-doubling bifurcation of a stochastic Bonhoeffer-van der Pol (BVP for short) system with a bounded random parameter. In the analysis, the stochastic BVP system is transformed by the Chebyshev polynomial approximation into an equivalent deterministic system, whose response can be readily obtained by conventional numerical methods. In this way we have explored plenty of stochastic period-doubling bifurcation phenomena of the stochastic BVP system. The numerical simulations show that the behaviour of the stochastic period-doubling bifurcation in the stochastic BVP system is by and large similar to that in the deterministic mean-parameter BVP system, but there are still some featured differences between them. For example, in the stochastic dynamic system the period-doubling bifurcation point diffuses into a critical interval and the location of the critical interval shifts with the variation of intensity of the random parameter.The obtained results show that Chebyshev polynomial approximation is an effective approach to dynamical problems in some typical nonlinear systems with a bounded random parameter of an arch-like probability density function.
Interface thermal conductance of van der Waals monolayers on amorphous substrates
Correa, Gabriela C.; Foss, Cameron J.; Aksamija, Zlatan
2017-03-01
Heterostructures based on atomic monolayers are emerging as leading materials for future energy efficient and multifunctional electronics. Due to the single atom thickness of monolayers, their properties are strongly affected by interactions with the external environment. We develop a model for interface thermal conductance (ITC) in an atomic monolayer van der Waals bonded to a disordered substrate. Graphene on SiO2 is initially used in our model and contrasted against available experimental data; the model is then applied to monolayer molybdenum disulfide (MoS2) on SiO2 substrate. Our findings show the dominant carrier of heat in both graphene and MoS2 in the cross-plane direction is the flexural (ZA) phonon mode, owing to the large overlap between graphene ZA and substrate vibrational density of states. The rate of phonon transfer across the interface depends quadratically on the substrate coupling constant K a , but this interaction also causes a lifting of the lowest flexural phonon modes. As a result, ITC depends roughly linearly on the strength of the coupling between a monolayer and its substrate. We conclude that, in both graphene and MoS2 on SiO2, substrate adhesion plays a strong role in determining ITC, requiring further study of substrate coupling in TMDCs.
Giesbertz, K J H
2014-01-01
Electron correlations in molecules can be divided in short range dynamical correlations, long range Van der Waals type interactions and near degeneracy static correlations. In this work we analyze how these three types of correlations can be incorporated in a simple wave function of restricted functional form consisting of an orbital product multiplied by a single correlation function $f(r_{12})$ depending on the interelectronic distance $r_{12}$. Since the three types of correlations mentioned lead to different signatures in terms of the natural orbital (NO) amplitudes in two-electron systems we make an analysis of the wave function in terms of the NO amplitudes for a model system of a diatomic molecule. In our numerical implementation we fully optimize the orbitals and the correlation function on a spatial grid without restrictions on their functional form. For the model system we can prove that none of the amplitudes vanishes and moreover that it displays a distinct sign pattern and a series of avoided cro...
Jaegermann, W.; Pettenkofer, C.; Lang, O.; Schlaf, R.; Tiefenbacher, S.; Tomm, Y. [Hahn-Meitner-Inst., Berlin (Germany)
1994-12-31
The preparation of thin films of layered chalcogenide semiconductors as MX and MX{sub 2} (X = S, Se) based on the concept of van der Waals epitaxy (VDWE) is presented for different substrate/overlayer combinations as GaSe, InSe, SnSe{sub 2}, WS{sub 2} on WSe{sub 2}, GaSe, MoTe{sub 2}, graphite and mica. In all cases stoichiometric films are formed either as epitaxial layers or strongly textured films with the c-axis aligned in spite of strong lattice mismatch. The interfaces are non-reactive and atomically abrupt. The electronic properties of the interfaces are mostly ideal showing band offsets according to the electron affinity rule and no operative interface states. However, doping of the films is still a problem which limits the band bending and the attainable surface photovoltage. The perspectives and preconditions for the further development of layered semiconductor VDWE films for solar cells will be critically discussed.
Subharmonic Resonance of Van Der Pol Oscillator with Fractional-Order Derivative
Yongjun Shen
2014-01-01
Full Text Available The subharmonic resonance of van der Pol (VDP oscillator with fractional-order derivative is studied by the averaging method. At first, the first-order approximate solutions are obtained by the averaging method. Then the definitions of equivalent linear damping coefficient (ELDC and equivalent linear stiffness coefficient (ELSC for subharmonic resonance are established, and the effects of the fractional-order parameters on the ELDC, the ELSC, and the dynamical characteristics of system are also analysed. Moreover, the amplitude-frequency equation and phase-frequency equation of steady-state solution for subharmonic resonance are established. The corresponding stability condition is presented based on Lyapunov theory, and the existence condition for subharmonic resonance (ECSR is also obtained. At last, the comparisons of the fractional-order and the traditional integer-order VDP oscillator are fulfilled by the numerical simulation. The effects of the parameters in fractional-order derivative on the steady-state amplitude, the amplitude-frequency curves, and the system stability are also studied.
Infrared Spectroscopy of the H2/HD/D2-O2 Van Der Waals Complexes
Raston, Paul; Bunn, Hayley
2016-06-01
Hydrogen is the most abundant element in the universe and oxygen is the third, so understanding the interaction between the two in their different forms is important to understanding astrochemical processes. The interaction between H2 and O2 has been explored in low energy scattering experiments and by far infrared synchrotron spectroscopy of the van der Waals complex. The far infrared spectra suggest a parallel stacked average structure with seven bound rotationally excited states. Here, we present the far infrared spectrum of HD/D2-O2 and the mid infrared spectrum of H2-O2 at 80 K, recorded at the infrared beamline facility of the Australian Synchrotron. We observed 'sharp' peaks in the mid infrared region, corresponding to the end over end rotation of H2-O2, that are comparatively noisier than analogous peaks in the far infrared where the synchrotron light is brightest. The larger reduced mass of HD and D2 compared to H2 is expected to result in more rotational bound states and narrower bands. The latest results in our ongoing efforts to explore this system will be presented. Y. Kalugina, et al., Phys. Chem. Chem. Phys. 14, 16458 (2012) S. Chefdeville et al. Science 341, 1094 (2013) H. Bunn et al. ApJ 799, 65 (2015)
Non-additivity of molecule-surface van der Waals potentials from force measurements.
Wagner, Christian; Fournier, Norman; Ruiz, Victor G; Li, Chen; Müllen, Klaus; Rohlfing, Michael; Tkatchenko, Alexandre; Temirov, Ruslan; Tautz, F Stefan
2014-11-26
Van der Waals (vdW) forces act ubiquitously in condensed matter. Despite being weak on an atomic level, they substantially influence molecular and biological systems due to their long range and system-size scaling. The difficulty to isolate and measure vdW forces on a single-molecule level causes our present understanding to be strongly theory based. Here we show measurements of the attractive potential between differently sized organic molecules and a metal surface using an atomic force microscope. Our choice of molecules and the large molecule-surface separation cause this attraction to be purely of vdW type. The experiment allows testing the asymptotic vdW force law and its validity range. We find a superlinear growth of the vdW attraction with molecular size, originating from the increased deconfinement of electrons in the molecules. Because such non-additive vdW contributions are not accounted for in most first-principles or empirical calculations, we suggest further development in that direction.
Arbelo-Gonzalez, W., E-mail: wilmer@instec.cu [Institut des Sciences Moleculaires, Universite Bordeaux 1, 351 Cours de la Liberation, 33405 Talence Cedex (France); Departamento de Fisica General, Instituto Superior de Tecnologias y Ciencias Aplicadas, Habana 10600 (Cuba); Bonnet, L., E-mail: l.bonnet@ism.u-bordeaux1.fr [Institut des Sciences Moleculaires, Universite Bordeaux 1, 351 Cours de la Liberation, 33405 Talence Cedex (France); Larregaray, P.; Rayez, J.-C. [Institut des Sciences Moleculaires, Universite Bordeaux 1, 351 Cours de la Liberation, 33405 Talence Cedex (France); Rubayo-Soneira, J. [Departamento de Fisica General, Instituto Superior de Tecnologias y Ciencias Aplicadas, Habana 10600 (Cuba)
2012-05-03
Graphical abstract: A recent classical description of photodissociation dynamics in a quantum spirit is applied for the first time to a realistic process, the fragmentation of NeBr{sub 2}. Highlights: Black-Right-Pointing-Pointer The photo-dissociation of NeBr{sub 2} is studied by means of two approaches. Black-Right-Pointing-Pointer The first is the standard classical one with Gaussian binning. Black-Right-Pointing-Pointer The second is a new method applied for the first time to a realistic system. Black-Right-Pointing-Pointer The new method leads to exactly the same results as the standard one. Black-Right-Pointing-Pointer However, it requires about 10 times less trajectories in the present case. - Abstract: The recent classical dynamical approach of photodissociations with Bohr quantization [L. Bonnet, J. Chem. Phys. 133 (2010) 174108] is applied for the first time to a realistic process, the photofragmentation of the van der Waals cluster NeBr{sub 2}. We illustrate the fact that this approach, formally equivalent to the standard one, may be numerically much more efficient.
Ferri, Nicola; Distasio, Robert A., Jr.; Ambrosetti, Alberto; Car, Roberto; Scheffler, Matthias; Tkatchenko, Alexandre
2015-03-01
Ubiquitous long-range van der Waals (vdW) interactions play a fundamental role in the structure and stability of a wide range of systems. Within the DFT framework, the vdW energy represents a crucial, but tiny part of the total energy, hence its influence on the electronic density, n (r) , and electronic properties is typically assumed to be rather small. Here, we address this question via a fully self-consistent (SC) implementation of the interatomic Tkatchenko-Scheffler vdW functional and its extension to surfaces. Self-consistency leads to large changes in the binding energies and electrostatic moments of highly polarizable alkali metal dimers. For some metal surfaces, vdW interactions increase dipole moments and induce non-trivial charge rearrangements, leading to visible changes in the metal workfunctions. Similar behavior is observed for molecules adsorbed on metals. Our study reveals a non-trivial connection between electrostatics and long-range electron correlation effects.
Self-Consistent Density Functional Including Long-Range van der Waals Interactions
Ferri, Nicola; Distasio, Robert A., Jr.; Car, Roberto; Scheffler, Matthias; Tkatchenko, Alexandre
2013-03-01
Van der Waals (vdW) interactions are significant for a wide variety of systems, from noble-gas dimers to organic/inorganic interfaces. The long-range vdW energy is a tiny fraction (0.001%) of the total energy, hence it is typically assumed not to change electronic properties. Although the vdW-DF functional includes the effect of vdW energy on electronic structure, the influence of ``true'' long-range vdW interactions is difficult to assess since a significant part of vdW-DF energy arises from short distances. Here, we present a self-consistent (SC) implementation of the long-range Tkatchenko-Scheffler (TS) functional, including its extension to surfaces. The analysis of self-consistency for rare-gas dimers allows us to reconcile two different views on vdW interactions: (i) Feynman's view that claims changes in the electron density and (ii) atoms separated by infinite barrier. In agreement with previous work, we find negligible contribution from self-consistency in the structure and stability of vdW-bound complexes. However, a closer look at organic/inorganic interfaces reveals notable modification of energy levels when using the SC-TS vdW density functional.
Efficient Oscillator-Based Approach for Polarizability and van der Waals Interactions
Gobre, Vivekanand; Distasio, Robert A., Jr.; Car, Roberto; Scheffler, Matthias; Tkatchenko, Alexandre
2013-03-01
The dynamic polarizability measures the response to an applied time-dependent electric field, and its accurate determination is crucial for van der Waals (vdW) interactions and other response properties. First-principles calculations of polarizabilities in principle require a computationally expensive explicit treatment of many-electron excitations, and are only applicable in practice to systems with less than about 100 atoms. In this work, we present an efficient parameter-free approach for calculating accurate frequency dependent polarizabilities for molecules with thousands of atoms, as well as periodic materials. This is achieved by the synergistic coupling of the Tkatchenko-Scheffler method, which accurately treats short-range hybridization effects, with the self-consistent screening equation from classical electrodynamics. Using only the electron density and free atom reference, we obtain an accuracy of 7% for both static polarizabilities and vdW coefficients for an extensive database of gas-phase molecules and crystals. We analyze the interplay of hybridization and long-range electrostatic screening effects on the polarizability.
Ferri, Nicola; Distasio, Robert A., Jr.; Car, Roberto; Tkatchenko, Alexandre; Scheffler, Matthias
2014-03-01
The long-range van der Waals (vdW) energy is only a small part of the total energy, hence it is typically assumed to have a minor influence on the electronic properties. Here, we address this question through a fully self-consistent (SC) implementation of the Tkatchenko-Scheffler (TS) density functional. The analysis of TS-vdWSC effects on electron density differences for atomic and molecular dimers reveals quantitative agreement with correlated densities obtained from ``gold standard'' coupled-cluster quantum-chemical calculations. In agreement with previous work, we find a very small overall contribution from self-consistency in the structure and stability of vdW-bound molecular complexes. However, TS-vdWSC (coupled with PBE functional) significantly affects electronic properties of coinage metal (111) surfaces, leading to an increase of up to 0.3 eV in the workfunction in agreement with experiments. Furthermore, vdW interactions visibly influence workfunctions in hybrid organic/metal interfaces, changing Pauli push-back and charge transfer contributions.
Thin Film Evaporation Model with Retarded Van Der Waals Interaction (Postprint)
2013-11-01
large heat transfer rates within evaporators and condensers. At the contact line present between a wetting liquid and a solid, there exists a thin...silicon. In contrast, a non- wetting thin film becomes thinner until it ruptures to produce discrete droplets on the substrate. The net attractive...Interline Heat-Transfer Coefficient of an Evaporating Wetting Film, Int. J. Heat Mass Transfer, 19 (1976) 487-492 [5] Hallinan K.P. & Chebaro H.C
Thermal electron attachment to oxygen and van der Waals molecules containing oxygen
Shimamori, H.; Fessenden, R.W.
1981-01-01
Thermal electron attachment to O/sub 2/ has been studied for pure O/sub 2/ (/sup 16/O/sub 2/ and /sup 18/O/sub 2/), O/sub 2/--N/sub 2/, O/sub 2/--CO, and O/sub 2/--n-C/sub 4/H/sub 10/ (/sup 16/O/sub 2/ and /sup 18/O/sub 2/) systems at temperatures from approx.330 down to 78/sup 0/ K using pulse radiolysis and microwave conductivity. For pure O/sub 2/, O/sub 2/--N/sub 2/, and O/sub 2/--CO mixtures, the electron attachment rates showed three-body pressure dependences at all temperatures over the pressure range studied (P/sub O2/<10Torr, P/sub N2/<60 Torr, P/sub CO/<40 Torr). The three-body rate constant of /sup 16/O/sub 2/ decreases from approx.2.4 x 10/sup -30/ cm/sup 6/ molecule/sup -2/ sec/sup -1/ at 330 /sup 0/K to about 0.9 x 10/sup -30/ cm/sup 6/ molecule/sup -2/ sec/sup -1/ at approx. 140 /sup 0/K but unexpectedly increases again to about 1.7 x 10/sup -30/ cm/sup 6/ molecule/sup -2/ sec/sup -1/ at 79 /sup 0/K. Similarly, the three-body rate constant of /sup 18/O/sub 2/ decreases from 5.1 x 10/sup -30/ cm/sup 6/ molecule/sup -2/ sec/sup -1/ at 300 /sup 0/K to 1.8 x 10/sup -30/ cm/sup 6/ molecule/sup -2/ sec/sup -1/ at approx.110 /sup 0/K but increases to 2.3 x 10/sup -30/ cm/sup 6/ molecule/sup -2/ sec/sup -1/ at 80 /sup 0/K. The three-body rate constant of N/sub 2/ shows a more dramatic monotonic increase from 0.9 x 10/sup -31/ cm/sup 6/ molecule/sup -2/ sec/sup -1/ at 300 /sup 0/K to 9.4 x 10/sup -31/ cm/sup 6/ molecule/sup -2/ sec/sup -1/ at 78 /sup 0/K. In the case of CO, the three-body rate constant appears to have a very shallow minimum around 170 /sup 0/K and again increases with further decrease of temperature. Since theory predicts a simple decrease in rate constant with reduced temperature, an extra contribution to the rate constant which increases with lowered temperature is evident. Electron attachment to the van der Waals molecules (O/sub 2/)/sub 2/, (O/sub 2/xN/sub 2/), and (O/sub 2/xCO) is proposed to account for this behavior. It has been found
H.L. de Jonge
2005-01-01
Full Text Available Might, machinations and museums. Jan van der Hoeven, Hermann Schlegel and their battle over the National Museum for Natural History in LeidenMuseums had a central place in nineteenth-century natural history. Aside from being important places for carrying out research into nature, they were also powerful vehicles for transmitting and communicating scientific knowledge. That is why, in the nineteenth century, natural history museums were frequently battlegrounds for rival scientists. Often, clashes over scientific matters were at the same time conflicts about power and space. After all, the naturalist who could get control over a museum’s collection and decide how the specimens should be classified and exhibited ultimately had the power to dictate which scientific knowledge was communicated; which story was told by a collection of specimens. This article is about the power struggle over the directorate of the National Museum of Natural History in Leiden that broke out between Jan van der Hoeven, Professor of Natural History and Comparative Anatomy at Leiden and Herman Schlegel, the museum’s chief curator. Both were rooted in very different natural historical traditions and had completely different views on the role of the museum and how the collection should be arranged and exhibited. In this article, we follow the two scientists in their attempts to gain support for their nominations. In this way, we also get a picture of nineteenth century Dutch political culture and the role scientists could play in it.
Vibrations of double-nanotube systems with mislocation via a newly developed van der Waals model
Kiani, Keivan
2015-06-01
This study deals with transverse vibrations of two adjacent-parallel-mislocated single-walled carbon nanotubes (SWCNTs) under various end conditions. These tubes interact with each other and their surrounding medium through the intertube van der Waals (vdW) forces, and existing bonds between their atoms and those of the elastic medium. The elastic energy of such forces due to the deflections of nanotubes is appropriately modeled by defining a vdW force density function. In the previous works, vdW forces between two identical tubes were idealized by a uniform form of this function. The newly introduced function enables us to investigate the influences of both intertube free distance and longitudinal mislocation on the natural transverse frequencies of the nanosystem which consists of two dissimilar tubes. Such crucial issues have not been addressed yet, even for simply supported tubes. Using nonlocal Timoshenko and higher-order beam theories as well as Hamilton's principle, the strong form of the equations of motion is established. Seeking for an explicit solution to these integro-partial differential equations is a very problematic task. Thereby, an energy-based method in conjunction with an efficient meshfree method is proposed and the nonlocal frequencies of the elastically embedded nanosystem are determined. For simply supported nanosystems, the predicted first five frequencies of the proposed model are checked with those of assumed mode method, and a reasonably good agreement is achieved. Through various studies, the roles of the tube's length ratio, intertube free space, mislocation, small-scale effect, slenderness ratio, radius of SWCNTs, and elastic constants of the elastic matrix on the natural frequencies of the nanosystem with various end conditions are explained. The limitations of the nonlocal Timoshenko beam theory are also addressed. This work can be considered as a vital step towards better realizing of a more complex system that consists of
Rajter, Rick; French, Roger H; Podgornik, Rudi; Ching, W Y; Parsegian, V Adrian
2008-09-01
Recognition of spatially varying optical properties is a necessity when studying the van der Waals-London dispersion (vdW-Ld) interactions of carbon nanotubes (CNTs) that have surfactant coatings, tubes within tubes, andor substantial core sizes. The ideal way to address these radially dependent optical properties would be to have an analytical add-a-layer solution in cylindrical coordinates similar to the one readily available for the plane-plane geometry. However, such a formulation does not exist nor does it appear trivial to be obtained exactly. The best and most pragmatic alternative for end-users is to take the optical spectra of the many components and to use a spectral mixing formulation so as to create effective solid-cylinder spectra for use in the far-limit regime. The near-limit regime at "contact" is dominated by the optical properties of the outermost layer, and thus no spectral mixing is required. Specifically we use a combination of a parallel capacitor in the axial direction and the Bruggeman effective medium in the radial direction. We then analyze the impact of using this mixing formulation upon the effective vdW-Ld spectra and the resulting Hamaker coefficients for small and large diameter single walled CNTs (SWCNTs) in both the near- and far-limit regions. We also test the spectra of a [16,0,s+7,0,s] multiwalled CNT (MWCNT) with an effective MWCNT spectrum created by mixing its [16,0,s] and [7,0,s] SWCNT components to demonstrate nonlinear coupling effects that exist between neighboring layers. Although this paper is primarily on nanotubes, the strategies, implementation, and analysis presented are applicable and likely necessary to any system where one needs to resolve spatially varying optical properties in a particular Lifshitz formulation.
Thermal transport across carbon nanotube-graphene covalent and van der Waals junctions
Shi, Jingjing; Dong, Yalin; Fisher, Timothy; Ruan, Xiulin
2015-07-01
Carbon nanotubes and graphene are promising materials for thermal management applications due to their high thermal conductivities. However, their thermal properties are anisotropic, and the radial or cross-plane direction thermal conductivity is low. A 3D Carbon nanotube (CNT)-graphene structure has previously been proposed to address this limitation, and direct molecular dynamics simulations have been used to predict the associated thermal conductivity. In this work, by recognizing that thermal resistance primarily comes from CNT-graphene junctions, a simple network model of thermal transport in pillared graphene structure is developed. Using non-equilibrium molecular dynamics, the resistance across an individual CNT-graphene junction with sp2 covalent bonds is found to be around 6 ×10-11 m2K/W, which is significantly lower than typical values reported for planar interfaces between dissimilar materials. In contrast, the resistance across a van der Waals junction is about 4 ×10-8 m2K/W. Interestingly, when the CNT pillar length is small, the interfacial resistance of the sp2 covalent junction is found to decrease as the CNT pillar length decreases, suggesting the presence of coherence effects. To explain this intriguing trend, the junction thermal resistance is decomposed into interfacial region and boundary components, and it is found that while the boundary resistance has little dependence on the pillar length, the interfacial region resistance decreases as the pillar length decreases. This is explained by calculating the local phonon density of states (LDOS) of different regions near the boundary. The LDOS overlap between the interfacial region and the center region of CNT increases as the pillar length decreases, leading to the decrease of interfacial region resistance. The junction resistance Rj is eventually used in the network model to estimate the effective thermal conductivity, and the results agree well with direct MD simulation data, demonstrating the
Tao, Jianmin; Yang, Jing; Rappe, Andrew M. [Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 (United States)
2015-04-28
Sublimation energy is one of the most important properties of molecular crystals, but it is difficult to study, because the attractive long-range van der Waals (vdW) interaction plays an important role. Here, we apply efficient semilocal density functional theory (DFT), corrected with the dynamically screened vdW interaction (DFT + vdW), the Rutgers-Chalmers nonlocal vdW-DF, and the pairwise-based dispersion-corrected DFT-D2 developed by Grimme and co-workers, to study the sublimation of fullerenes. We find that the short-range part, which accounts for the interaction due to the orbital overlap between fullerenes, is negligibly small. Our calculation shows that there exists a strong screening effect on the vdW interaction arising from the valence electrons of fullerenes. On the other hand, higher-order contributions can be as important as the leading-order term. The reasons are that (i) the surface of fullerene molecules is metallic and thus highly polarizable, (ii) the band gap of fullerene solids is small (less than 2 eV), and (iii) fullerene molecules in the solid phase are so densely packed, yielding the high valence electron density and small equilibrium intermolecular distances (the first nearest neighbor distance is only about 10 Å for C{sub 60}). However, these two effects make opposite contributions, leading to significant error cancellation between these two contributions. We demonstrate that, by considering higher-order contributions and the dynamical screening, the DFT + vdW method can yield sublimation energies of fullerenes in good agreement with reference values, followed by vdW-DF and DFT-D2. The insights from this study are important for a better understanding of the long-range nature of vdW interactions in nanostructured solids.
Tao, Jianmin; Yang, Jing; Rappe, Andrew M
2015-04-28
Sublimation energy is one of the most important properties of molecular crystals, but it is difficult to study, because the attractive long-range van der Waals (vdW) interaction plays an important role. Here, we apply efficient semilocal density functional theory (DFT), corrected with the dynamically screened vdW interaction (DFT + vdW), the Rutgers-Chalmers nonlocal vdW-DF, and the pairwise-based dispersion-corrected DFT-D2 developed by Grimme and co-workers, to study the sublimation of fullerenes. We find that the short-range part, which accounts for the interaction due to the orbital overlap between fullerenes, is negligibly small. Our calculation shows that there exists a strong screening effect on the vdW interaction arising from the valence electrons of fullerenes. On the other hand, higher-order contributions can be as important as the leading-order term. The reasons are that (i) the surface of fullerene molecules is metallic and thus highly polarizable, (ii) the band gap of fullerene solids is small (less than 2 eV), and (iii) fullerene molecules in the solid phase are so densely packed, yielding the high valence electron density and small equilibrium intermolecular distances (the first nearest neighbor distance is only about 10 Å for C60). However, these two effects make opposite contributions, leading to significant error cancellation between these two contributions. We demonstrate that, by considering higher-order contributions and the dynamical screening, the DFT + vdW method can yield sublimation energies of fullerenes in good agreement with reference values, followed by vdW-DF and DFT-D2. The insights from this study are important for a better understanding of the long-range nature of vdW interactions in nanostructured solids.
van der Waals epitaxy of CdTe thin film on graphene
Mohanty, Dibyajyoti; Xie, Weiyu; Wang, Yiping; Lu, Zonghuan; Shi, Jian; Zhang, Shengbai; Wang, Gwo-Ching; Lu, Toh-Ming; Bhat, Ishwara B.
2016-10-01
van der Waals epitaxy (vdWE) facilitates the epitaxial growth of materials having a large lattice mismatch with the substrate. Although vdWE of two-dimensional (2D) materials on 2D materials have been extensively studied, the vdWE for three-dimensional (3D) materials on 2D substrates remains a challenge. It is perceived that a 2D substrate passes little information to dictate the 3D growth. In this article, we demonstrated the vdWE growth of the CdTe(111) thin film on a graphene buffered SiO2/Si substrate using metalorganic chemical vapor deposition technique, despite a 46% large lattice mismatch between CdTe and graphene and a symmetry change from cubic to hexagonal. Our CdTe films produce a very narrow X-ray rocking curve, and the X-ray pole figure analysis showed 12 CdTe (111) peaks at a chi angle of 70°. This was attributed to two sets of parallel epitaxy of CdTe on graphene with a 30° relative orientation giving rise to a 12-fold symmetry in the pole figure. First-principles calculations reveal that, despite the relatively small energy differences, the graphene buffer layer does pass epitaxial information to CdTe as the parallel epitaxy, obtained in the experiment, is energetically favored. The work paves a way for the growth of high quality CdTe film on a large area as well as on the amorphous substrates.
Das, Saunak; Herrmann-Westendorf, Felix; Schacher, Felix H; Täuscher, Eric; Ritter, Uwe; Dietzek, Benjamin; Presselt, Martin
2016-08-24
Morphologies crucially determine the optoelectronic properties of organic semiconductors. Therefore, hierarchical and supramolecular approaches have been developed for targeted design of supramolecular ensembles of organic semiconducting molecules and performance improvement of, e.g., organic solar cells (OSCs), organic light emitting diodes (OLEDs), and organic field-effect transistors (OFETs). We demonstrate how the photonic properties of fullerenes change with the formation of van der Waals aggregates. We identified supramolecular structures with broadly tunable absorption in the visible spectral range and demonstrated how to form aggregates with targeted visible (vis) absorption. To control supramolecular structure formation, we functionalized the C60-backbone with polar (bis-polyethylene glycol malonate-MPEG) tails, thus yielding an amphiphilic fullerene derivative that self-assembles at interfaces. Aggregates of systematically tuned size were obtained from concentrating MPEGC60 in stearic acid matrices, while different supramolecular geometries were provoked via different thin film preparation methods, namely spin-casting and Langmuir-Blodgett (LB) deposition from an air-water interface. We demonstrated that differences in molecular orientation in LB films (C2v type point group aggregates) and spin-casting (stochastic aggregates) lead to huge changes in electronic absorption spectra due to symmetry and orientation reasons. These differences in the supramolecular structures, causing the different photonic properties of spin-cast and LB films, could be identified by means of quantum chemical calculations. Employing supramolecular assembly, we propounded that molecular symmetry in fullerene aggregates is extremely important in controlling vis absorption to harvest photons efficiently, when mixed with a donor molecule, thus improving active layer design and performance of OSCs.
Interaction of boron with graphite: A van der Waals density functional study
Liu, Juan; Wang, Chen; Liang, Tongxiang; Lai, Wensheng
2016-08-01
Boron doping has been widely investigated to improve oxidation resistance of graphite. In this work the interaction of boron with graphite is investigated by a van der Waals density-functional approach (vdW-DF). The traditional density-functional theory (DFT) is well accounted for the binding in boron-substituted graphite. However, to investigate the boron atom on graphite surface and the interstitial impurities require use of a description of graphite interlayer binding. Traditional DFT cannot describe the vdW physics, for instance, GGA calculations show no relevant binding between graphite sheets. LDA shows some binding, but they fail to provide an accurate account of vdW forces. In this paper, we compare the calculation results of graphite lattice constant and cohesive energy by several functionals, it shows that vdW-DF such as two optimized functionals optB88-vdW and optB86b-vdW give much improved results than traditional DFT. The vdW-DF approach is then applied to study the interaction of boron with graphite. Boron adsorption, substitution, and intercalation are discussed in terms of structural parameters and electronic structures. When adsorbing on graphite surface, boron behaves as π electron acceptor. The π electron approaches boron atom because of more electropositive of boron than carbon. For substitution situation, the hole introduced by boron mainly concentrates on boron and the nearest three carbon atoms. The B-doped graphite system with the hole has less ability to offer electrons to oxygen, ultimately resulted in the inhibition of carbon oxidation. For interstitial doping, vdW-DFs show more accurate formation energy than LDA. PBE functional cannot describe the interstitial boron in graphite reasonably because of the ignoring binding of graphite sheets. The investigation of electron structures of boron doped graphite will play an important role in understanding the oxidation mechanism in further study.
Comparative study of van der Waals corrections to the bulk properties of graphite.
Rêgo, Celso R C; Oliveira, Luiz N; Tereshchuk, Polina; Da Silva, Juarez L F
2015-10-21
Graphite is a stack of honeycomb (graphene) layers bound together by nonlocal, long-range van der Waals (vdW) forces, which are poorly described by density functional theory (DFT) within local or semilocal exchange-correlation functionals. Several approximations have been proposed to add a vdW correction to the DFT total energies (Stefan Grimme (D2 and D3) with different damping functions (D3-BJ), Tkatchenko-Scheffler (TS) without and with self-consistent screening (TS + SCS) effects). Those corrections have remarkly improved the agreement between our results and experiment for the interlayer distance (from 3.9 to 0.6%) [corrected] and high-level random-phase approximation (RPA) calculations for interlayer binding energy (from 69.5 to 1.5%). [corrected]. We report a systematic investigation of various structural, energetic and electron properties with the aforementioned vdW corrections followed by comparison with experimental and theoretical RPA data. Comparison between the resulting relative errors shows that the TS + SCS correction provides the best results; the other corrections yield significantly larger errors for at least one of the studied properties. If considerations of computational costs or convergence problems rule out the TS + SCS approach, we recommend the D3-BJ correction. Comparison between the computed π(z)Γ-splitting and experimental results shows disagreements of 10% or more with all vdW corrections. Even the computationally more expensive hybrid PBE0 has proved unable to improve the agreement with the measured splitting. Our results indicate that improvements of the exchange-correlation functionals beyond the vdW corrections are necessary to accurately describe the band structure of graphite.
PENELITIAN SPESIES SIBLING NYAMUK ANOPHELES BARBIROSTRIS VAN DER WULP DI INDONESIA
S. Sukowati
2012-11-01
Full Text Available Malaria continues to be a public health problem of high priority in the majority of malaria endemic countries of the South East Asia region, such as Indonesia. Vector control remain the most effective measure to prevent malaria transmission. The study of mosquito species complexes and the understanding of the biology, evolution as well as epidemiology of malaria transmission and the possibly of development of efficient control measures against malaria have been subjects of greatly increased interest in recent years. Natural populations of Anopheles barbirostris Van der Wulp were sampled from 5 geographically isolated populations in Indonesia: (1 Ambarawa, Central Java. (2 Tara-Tara, North Sulawesi, (3 Boru-Boru, (4 Konga and (5 Tanjung Bunga, the last three in Flores, East Nusa Tenggara. Analysis of larval mitotic chromosomes had been made from F I progeny of isofemale lines derived from these 5 populations. Specimens from these populations appear to share a similar mitotic karyotype (2n=6 consisting of two pairs of autosomes and one pair of sex chromosomes. The Giemsa staining of mitotic chromosomes of the An. barbirostris complex showed considerable variation in the amount and distribution of constitutive heterochromatin in sex chromosomes. These cytological differences have led to the recognation of 4 distinct cytological forms within the taxon An. barbirostris, viz., form A (X1,X2,X3,Y1, form B (X1,X2,X3,Y2 form C (X2,X3,Y3 and form D (X2,X3,Y4. Form A is widely distributed in Indonesia, while, form Band D have been found in sympatric association with form A in Tara-tara 2, North Sulawesi; Konga, Flores and Tanjung Bunga, Form C, however, has been found only in Boru-Boru, and sympatric with forms A and B. Keywords: Anopheles barbirostris, malaria, mitotic chromosome, sibling species
Luna Hernández, David Ricardo
2010-01-01
El método de Sharp–Van der Heijde es utilizado para evaluar y medir la progresión del daño articular y establecer los efectos del tratamiento en radiografías de manos y pies de pacientes con artritis reumatoidea. El objetivo de este estudio fue evaluar la confiabilidad de la escala de SVdH determinando la concordancia intra e interobservador entre radiólogos utilizando radiografías de pacientes colombianos con artritis reumatoidea. Se incluyeron 102 radiografías digitalizadas (51 de manos (en...
Tremps, E.; Enrique, J. L.; Moron, C.; Garcia, A.; Gomez, A.
2013-07-01
In this paper we show a system based on the resistivity measurement of samples of gray cast iron by the Van der Pauw method to calculate the silicon content in the samples. Twenty five trials have been carried out, studying resistive and metallographic characteristics of the samples. This has demonstrated that it is possible to obtain, by this method, the silicon content in molten flat with low content of alloying elements, also the content of chromium in series smelters where the rate of silicon remains constant. (Author)
Nelson H. T. Lemes
2010-01-01
Full Text Available Analytical solutions of a cubic equation with real coefficients are established using the Cardano method. The method is first applied to simple third order equation. Calculation of volume in the van der Waals equation of state is afterwards established. These results are exemplified to calculate the volumes below and above critical temperatures. Analytical and numerical values for the compressibility factor are presented as a function of the pressure. As a final example, coexistence volumes in the liquid-vapor equilibrium are calculated. The Cardano approach is very simple to apply, requiring only elementary operations, indicating an attractive method to be used in teaching elementary thermodynamics.
Robertson, Joyce; McGilly, Katherine
2009-11-01
The importance of James Robertson's work in changing the hospital conditions for young children is widely acknowledged but disputed by van der Horst and van der Veer (2009), although they presented evidence supporting his influence, they also presented considerable evidence showing that others were ignored, including writers in The Lancet and the British Medical Journal. van der Horst and van der Veer presented a muddled picture by failing to understand how the work of James Robertson influenced a wide range of people, opened up the debate and, through the Platt Report, gave parents the authority and confidence to challenge hospitals and stay with their young children. James Robertson's 1952 film "a Two-year-old Goes to Hospital" provided the 'visual communication to pierce defences as the spoken word cannot do'. This film continues to be widely used today in teaching about young children.
Feng, Ya; Zhu, Jie; Tang, Dawei
2014-12-01
Molecular dynamics simulations are performed to evaluate the effect of van der Waals forces among single-wall carbon nanotubes (SWNTs) on the interfacial thermal conductance between a SWNT array and silicon substrate. First, samples of SWNTs vertically aligned on silicon substrate are simulated, where both the number and arrangement of SWNTs are varied. Results reveal that the interfacial thermal conductance of a SWNT array/Si with van der Waals forces present is higher than when they are absent. To better understand how van der Waals forces affect heat transfer through the interface between SWNTs and silicon, further constructs of one SWNT surrounded by different numbers of other ones are studied, and the results show that the interfacial thermal conductance of the central SWNT increases with increasing van der Waals forces. Through analysis of the covalent bonds and vibrational density of states at the interface, we find that heat transfer across the interface is enhanced with a greater number of chemical bonds and that improved vibrational coupling of the two sides of the interface results in higher interfacial thermal conductance. Van der Waals forces stimulate heat transfer at the interface.
Ya Feng
2014-12-01
Full Text Available Molecular dynamics simulations are performed to evaluate the effect of van der Waals forces among single-wall carbon nanotubes (SWNTs on the interfacial thermal conductance between a SWNT array and silicon substrate. First, samples of SWNTs vertically aligned on silicon substrate are simulated, where both the number and arrangement of SWNTs are varied. Results reveal that the interfacial thermal conductance of a SWNT array/Si with van der Waals forces present is higher than when they are absent. To better understand how van der Waals forces affect heat transfer through the interface between SWNTs and silicon, further constructs of one SWNT surrounded by different numbers of other ones are studied, and the results show that the interfacial thermal conductance of the central SWNT increases with increasing van der Waals forces. Through analysis of the covalent bonds and vibrational density of states at the interface, we find that heat transfer across the interface is enhanced with a greater number of chemical bonds and that improved vibrational coupling of the two sides of the interface results in higher interfacial thermal conductance. Van der Waals forces stimulate heat transfer at the interface.
Donaire, M.
2016-05-01
I revisit the problem of the interaction between two dissimilar atoms with one atom in an excited state, recently addressed by Berman [Phys. Rev. A 91, 042127 (2015), 10.1103/PhysRevA.91.042127], Donaire et al. [Phys. Rev. Lett. 115, 033201 (2015), 10.1103/PhysRevLett.115.033201], and Milonni and Rafsanjani [Phys. Rev. A 92, 062711 (2015), 10.1103/PhysRevA.92.062711], for which precedent approaches have given conflicting results. In the first place, I discuss to what extent these works provide equivalent results. I show that the phase-shift rate of the two-atom wave function computed by Berman, the van der Waals potential of the excited atom by Donaire et al., and the level shift of the excited atom by Milonni and Rafsanjani possess equivalent expressions in the quasistationary approximation. In addition, I show that the level shift of the ground-state atom computed by Milonni and Rafsanjani is equivalent to its van der Waals potential. A diagrammatic representation of all those quantities is provided. The equivalences among them are, however, not generic. In particular, it is found that for the case of the interaction between two identical atoms excited, the phase-shift rate and the van der Waals potentials differ. Concerning the conflicting results of previous approaches in regards to the spatial oscillation of the interactions, I conclude, in agreement with Berman and with Milonni and Rafsanjani, that they refer to different physical quantities. The impacts of free-space dissipation and finite excitation rates on the dynamics of the potentials are analyzed. In contrast with Milonni and Rafsanjani, the oscillatory versus monotonic spatial forms of the potentials of each atom are found not to be related to the reversible versus irreversible nature of the excitation transfer involved.
Zhang, Zibin; Yu, Guocan; Han, Chengyou; Liu, Jiyong; Ding, Xia; Yu, Yihua; Huang, Feihe
2011-09-16
Two new copillar[5]arenes were prepared. They are arranged in two completely different motifs, a cyclic dimer containing two monomers with two different conformations that are mirror images of each other and linear supramolecular polymers in the solid state. Not only has it been shown that to form this kind of dimer is a unique feature associated with pillar[5]arene macrocycles but also it was demonstrated that weak van der Waals forces can be used to control the self-organization of monomers during their supramolecular polymerization process.
Fodjouong, G J [Laboratory of Nonlinear Modelling and Simulation in Engineering and Biological Physics, Faculty of Science, University of Yaounde I, Box 812, Yaounde (Cameroon); Fotsin, H B [Laboratoire d' Electronique, Departement de physique, Faculte des Sciences, Universite de Dschang, B P 67 Dschang, Cameroun (Germany); Woafo, P [Laboratory of Nonlinear Modelling and Simulation in Engineering and Biological Physics, Faculty of Science, University of Yaounde I, Box 812, Yaounde (Cameroon)
2007-05-15
This study addresses the adaptive synchronization of the modified van der Pol-Duffing (MVDPD) oscillator with offset terms. From our investigations of the system dynamics, we obtain that the system presents a chaotic behaviour at weak values of the offset parameters. Routh-Hurwitz criteria are used to study the asymptotic stability of the steady states. An adaptive observer design method is applied to achieve synchronization of two identical MVDPD oscillators with offset. Numerical simulations are given to validate the proposed synchronization approach. Moreover, as an application, the proposed scheme is applied to secure communication. Also, simulation results verify the proposed scheme's success in the communication application.
Gray, S.K.
1994-03-01
Vibrational predissociation of XI{sub 2} and X{sub 2}I{sub 2} van der Waals complexes, with X = He and Ne, is studied with wave packets. Three-dimensional calculations are carried out on the three-atom systems. Suitable X{center_dot}{center_dot}I potential interactions are determined, and product distributions are predicted. Reduced dimension models of X{sub 2}I{sub 2}(v{prime}) {yields} 2X + I{sub 2}(v < v{prime}) are investigated. Comparison is made with available experimental results. Mechanistic issues, including the role of intramolecular vibrational relaxation resonances, are addressed.
陈予恕; 徐鉴
1996-01-01
The bifurcation of the second-order approximate solutions of nonlinear parametrically excited systems possessing generalized van der Pol’s dampings and quintic Duffing’s nonlinearities subjected to a primary parametric resonance is investigated. Using singularity theory with Z2-symmetry, bifurcations of the solutions are universally classified in a topologically equivalent sense for Z2-codimension>3. The question of whether the approximate solutions from the classical perturbation methods can be topologically equivalent in describing the periodic responses and the bifurcations of the original systems is made clear. The numerical results indicate that the vibration characteristic may suddenly disappear in the range of Z2-codimension>4.
Ratnawati Ratnawati
2012-01-01
A solid-supercritical fluid system is highly asymmetric in terms of the size and energy differences of the components. The key point in extending a cubic equation of state to such system is on the choice of proper mixing rules. New mixing rules for the Redlich-Kwong equation of state are developed. The developement is based on the statistical-mechanical theory of the van der Waals mixing rules. The Redlich Kwong equation of state with the proposed mixing rules along with the original o...
Semenova, Nadezhda I; Anishchenko, Vadim S
2015-07-01
Statistics of Poincaré recurrences is studied in the stroboscopic section of trajectories of a nonautonomous van der Pol oscillator in the framework of the global approach. It is shown that when the oscillator frequency and the frequency of the external force are irrationally related, the set obtained stroboscopically is equivalent to the circle map. For small values of the external amplitude, the Fibonacci stairs is constructed for the golden and silver ratios and its universal properties are confirmed. It is established that the Afraimovich-Pesin dimension for the map in the stroboscopic section is αc = 1 for Diophantine irrational rotation numbers.
A Note on the Solutions of the Van der Pol and Duffing Equations Using a Linearisation Method
Sandile S. Motsa
2012-01-01
Full Text Available We present a novel application of the successive linearisation method to the classical Van der Pol and Duffing oscillator equations. By recasting the governing equations as nonlinear eigenvalue problems we obtain accurate values of the frequency and amplitude. We demonstrate that the proposed method can be used to obtain the limit cycle and bifurcation diagrams of the governing equations. Comparison with exact and other results in the literature shows that the method is accurate and effective in finding solutions of nonlinear equations with oscillatory solutions, nonlinear eigenvalue problems, and other nonlinear problems with bifurcations.
Lu, N.; Guo, H. Y.; L. Wang; Wu, X. J.; Zeng, X. C.
2014-01-01
We perform a comprehensive first-principles study of the electronic properties of van der Waals (vdW) trilayers via intercalating a two-dimensional (2D) monolayer (ML = BN, MoSe2, WS2, or WSe2) between MoS2 bilayer to form various MoS2/ML/MoS2 sandwich trilayers. We find that the BN monolayer is the most effective sheet to decouple the interlayer vdW coupling of the MoS2 bilayer, and the resulting sandwich trilayer can recover the electronic structures of the MoS2 monolayer, particularly the ...
Hybrid van der Waals p-n Heterojunctions based on SnO and 2D MoS2
Wang, Zhenwei
2016-08-30
A p-type oxide/2D hybrid van der Waals p-n heterojunction is demonstrated for the first time between SnO (tin monoxide) (the p-type oxide) and 2D MoS2 (molybdenum disulfide), showing an ideality factor of 2 and rectification ratio up to 10(4) . The reported heterojunction is gate-tunable with typical anti-ambipolar transfer characteristics. Surface potential mapping is performed and a current model for such a heterojunction is proposed.
Amila P. Sumanapala
2016-11-01
Full Text Available Lyriothemis defonsekai van der Poorten, 2009 is a nationally Critically Endangered odonate species in Sri Lanka. It is endemic to the country and was known only from the type locality, Kudawa, Sinharaja Forest Reserve and its vicinity thus it was considered to be a point endemic. We report the first ever record of the species outside Sinharaja extending the known range of the species. The present observations were recorded from Yagirala Forest Reserve where an immature male and one or two mature females of the species were observed. We also discuss the observations on its habitat and distribution range.
Van der Waals corrected DFT study of adsorption of groups VA and VIA hydrides on graphene monoxide
Notash, M. Yaghoobi; Ebrahimzadeh, A. Rastkar
2016-06-01
Adsorption properties of H2O, H2S, NH3 and PH3 on graphene monoxide (GMO) nano flack are investigated using density functional theory (DFT). Calculations were carried out by van der Waals correction and general gradient approximation. The adsorption energies and charge transfer between species are obtained and discussed for the considered positions of adsorbate molecules. Charge transfer analysis show that the gas molecules act as an electron acceptor in all cases. The analysis of the adsorption energies suggest GMO can be a good candidate for the adsorption of these molecules.
Z. H. Wang; H. Y. Hu
2008-01-01
In this paper, a modified averaging scheme is presented for a class of time-delayed vibration systems with slow variables. The new scheme is a combination of the averaging techniques proposed by Hale and by Lehman and Weibel, respectively. The averaged equation obtained from the modified scheme is simple enough but it retains the required information for the local nonlinear dynamics around an equilibrium. As an application of the present method, the delay value for which a secondary Hopf bifurcation occurs is successfully located for a delayed van der Pol oscillator.
Parshin, D. V.; Ufimtseva, I. V.; Cherevko, A. A.; Khe, A. K.; Orlov, K. Yu; Krivoshapkin, A. L.; Chupakhin, A. P.
2016-06-01
The present paper discusses the method of identification (diseased/healthy) human cerebral vessels by using of mathematical model. Human cerebral circulation as a single tuned circuit, which consists of blood flow, elastic vessels and elastic brain gel tissue is under consideration. Non linear Van der Pol-Duffing equation is assumed as mathematical model of cerebrovascular circulation. Hypothesis of vascular pathology existence in some position of blood vessel, based on mathematical model properties for this position is formulated. Good reliability of hypothesis is proved statistically for 7 patients with arterial aneurysms.
基于van der Waals状态方程的剩余自由能模型%An Excess Free Energy Model from van der Waals Equation of State
高军; 朱兆友; 李留定
2003-01-01
结合Wong-Sandler的混合规则,即在无穷大压力下剩余Helmholtz自由能与无穷小剩余Gibbs自由能近似相等的关系,给出了一个基于van der Waals状态方程的剩余自由能模型,由此得到可以描述汽液相平衡的活度系数模型.通过对35个体系的考核表明,该模型可以较好的关联实验数据,包括极性与极性物系,其结果好于Wilson、Margules、NRTL与Uniquac等模型的计算结果.同时,因为该模型基于Wong-Sandler的混合规则,因此可以实现在理想溶液中,由Wong-Sandler 混合规则到van der Waals混合规则平滑过渡.
Redox levels in aqueous solution: Effect of van der Waals interactions and hybrid functionals
Ambrosio, Francesco; Miceli, Giacomo; Pasquarello, Alfredo
2015-12-01
We investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H+/H2 level defining the standard hydrogen electrode, the OH-/OH∗ level corresponding to the oxidation of the hydroxyl ion, and the H2O/OH∗ level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction α of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with α. For α ≃ 0.40, we achieve a band gap, band-edge positions, and redox levels in overall
Van der Woude syndrome: A review of 11 cases seen at the Lagos University Teaching Hospital
Olutayo James
2014-01-01
Full Text Available Background: Van der Woude syndrome (VWS, an autosomal dominant condition associated with clefts of the lip and/or palate and lower lip pits and is caused by mutations in interferon regulatory factor six gene. It is reported to be the most common syndromic cleft world-wide. Non-penetrance for the lip pit phenotype is found in at least 10% of affected individuals and those without the pits are phenocopies for non-syndromic clefting. The aim of this study is to present the phenotypic characteristic of VWS seen at the Lagos University Teaching Hospital (LUTH cleft clinic. Materials and Methods: A review of cases of patients with VWS that attended the cleft lip and palate clinic at the LUTH Idi-Araba, Lagos, from January 2007 to December 2012 was conducted. Data analyses included sex of affected patients, types of cleft, presence of lower lip pits and history of lower lip pits/cleft in the family. Results: A total of 11 cases were seen during the period (male = 4; female = 7. Age at presentation ranged between 1 week and 12 years, with majority (n = 10 less than 2 years of age. Bilateral cleft lip and palate (BCLP was seen in six patients, isolated soft palatal cleft (n = 3 and unilateral cleft lip and alveolus (n = 1 and cleft of hard and soft palate (n = 1. Bilateral lower lips were presented in 10 out of the 11 cases. The mother of the only patient without lip pits presented with bilateral lower lip pits. No family history of cleft/lip pits was elicited in 10 other cases. Conclusion: Most of the cases of VWS presented with BCLP and lower lip pits. Non-penetrance for the lip pits was seen in one out of 11 cases. Our study emphasizes the need to screen family members in all cleft cases, especially clinically diagnosed non-syndromic cases who may be VWS with no lip pits. Future studies are required to investigate the genetic causes of this syndrome in our population.
Redox levels in aqueous solution: Effect of van der Waals interactions and hybrid functionals
Ambrosio, Francesco, E-mail: Francesco.Ambrosio@epfl.ch; Miceli, Giacomo; Pasquarello, Alfredo [Chaire de Simulation à l’Echelle Atomique (CSEA), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne (Switzerland)
2015-12-28
We investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H{sup +}/H{sub 2} level defining the standard hydrogen electrode, the OH{sup −}/OH{sup ∗} level corresponding to the oxidation of the hydroxyl ion, and the H{sub 2}O/OH{sup ∗} level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction α of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with α. For α ≃ 0.40, we achieve a band gap, band
Band engineering in a van der Waals heterostructure using a 2D polar material and a capping layer
Cho, Sung Beom; Chung, Yong-Chae
2016-06-01
Van der Waals (vdW) heterostructures are expected to play a key role in next-generation electronic and optoelectronic devices. In this study, the band alignment of a vdW heterostructure with 2D polar materials was studied using first-principles calculations. As a model case study, single-sided fluorographene (a 2D polar material) on insulating (h-BN) and metallic (graphite) substrates was investigated to understand the band alignment behavior of polar materials. Single-sided fluorographene was found to have a potential difference along the out-of-plane direction. This potential difference provided as built-in potential at the interface, which shift the band alignment between h-BN and graphite. The interface characteristics were highly dependent on the interface terminations because of this built-in potential. Interestingly, this band alignment can be modified with a capping layer of graphene or BN because the capping layer triggered electronic reconstruction near the interface. This is because the bonding nature is not covalent, but van der Waals, which made it possible to avoid Fermi-level pinning at the interface. The results of this study showed that diverse types of band alignment can be achieved using polar materials and an appropriate capping layer.
A van der Waals DFT study of PtH{sub 2} systems absorbed on pristine and defective graphene
López-Corral, Ignacio [Instituto de Química del Sur (INQUISUR), UNS-CONICET, Av. Alem 1253, B8000 Bahía Blanca (Argentina); Departamento de Química, Universidad Nacional del Sur, Av. Alem 1253, B8000 Bahía Blanca (Argentina); Piriz, Sebastián; Faccio, Ricardo [Centro NanoMat/DETEMA, Facultad de Química, Universidad de la República, Montevideo (Uruguay); Centro Interdisciplinario de Nanotecnología, Química y Física de Materiales (CINQUIFIMA), Facultad de Química, Universidad de la República, Montevideo (Uruguay); Juan, Alfredo, E-mail: cajuan@uns.edu.ar [Instituto de Física del Sur (IFISUR), UNS-CONICET, Av. Alem 1253, B8000 Bahía Blanca (Argentina); Departamento de Física, Universidad Nacional del Sur, Av. Alem 1253, B8000 Bahía Blanca (Argentina); Avena, Marcelo [Instituto de Química del Sur (INQUISUR), UNS-CONICET, Av. Alem 1253, B8000 Bahía Blanca (Argentina); Departamento de Química, Universidad Nacional del Sur, Av. Alem 1253, B8000 Bahía Blanca (Argentina)
2016-09-30
Highlights: • We performed DFT calculations including van der Waals interactions. • Kubas-type Pt-H2 complex is stable on defective graphene. • Carbon vacancy decreases the reactivity of the metal decoration. • The interaction between σ-H and π-C states favors the Kubas-type complex. - Abstract: We used a density functional that incorporates van der Waals interactions to study hydrogen adsorption onto Pt atoms attached to carbon-vacancies on graphene layers, considering molecular and dissociated hydrogen-platinum coordination structures. PtH{sub 2} complexes adsorbed on several sites of pristine graphene were also studied for comparison. Our results indicate that both a Kubas-type dihydrogen complex and a classic hydride without H−H bond are the preferential PtH{sub 2} systems on the vacancy site of graphene. In contrast, the Kubas complex is unstable onto pristine graphene and the hydride is obtained at all adsorption sites. Our simulations suggest that the C-vacancy decreases the reactivity of the metal decoration, allowing a non-dissociative hydrogen adsorption. The H{sub 2} molecule is oriented almost perpendicular to the outermost C−Pt bond, interacting also with the graphene surface through σ-H and π-C states. This stabilization of the Kubas-type complex could play a very important role for hydrogen storage in Pt-decorated carbon adsorbents with vacancies.
Wang, Shunfeng; Wang, Junyong; Zhao, Weijie; Giustiniano, Francesco; Chu, Leiqiang; Verzhbitskiy, Ivan; Zhou Yong, Justin; Eda, Goki
2017-08-09
We report on efficient carrier-to-exciton conversion and planar electroluminescence from tunnel diodes based on a metal-insulator-semiconductor (MIS) van der Waals heterostack consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS2). These devices exhibit excitonic electroluminescence with extremely low threshold current density of a few pA·μm(-2), which is several orders of magnitude lower compared to the previously reported values for the best planar EL devices. Using a reference dye, we estimate the EL quantum efficiency to be ∼1% at low current density limit, which is of the same order of magnitude as photoluminescence quantum yield at the equivalent excitation rate. Our observations reveal that the efficiency of our devices is not limited by carrier-to-exciton conversion efficiency but by the inherent exciton-to-photon yield of the material. The device characteristics indicate that the light emission is triggered by injection of hot minority carriers (holes) to n-doped WS2 by Fowler-Nordheim tunneling and that hBN serves as an efficient hole-transport and electron-blocking layer. Our findings offer insight into the intelligent design of van der Waals heterostructures and avenues for realizing efficient excitonic devices.
Brennan, M. J.; Tang, B.; Carranza, J. C.
2016-09-01
The van der Pol oscillator is an archetypal nonlinear oscillator that has been studied for many years. It is a self-sustaining oscillator that vibrates in a limit cycle, and has the characteristic that it generates energy in the part of the cycle when the displacement is small and dissipates energy in the part of the cycle when the displacement is large. Almost all analyses for this type of oscillator have been conducted in a strict mathematical framework using the displacement and velocity of the mass to describe the motion in the phase plane. Physical insight into the behaviour is then generally only possible for very small or for very large damping nonlinearity. In this paper a fresh approach is taken. The internal forces of the Rayleigh oscillator are studied rather than van der Pol's equation as the key damping force is a function of only velocity. Simulations are presented which show how the stiffness and damping forces vary when the system is vibrating in a steady-state limit cycle.
Decoupling interface effect on the phase stability of CdS thin films by van der Waals heteroepitaxy
Sun, Xin; Wang, Yiping; Seewald, Lucas J.; Chen, Zhizhong; Shi, Jian; Washington, Morris A.; Lu, Toh-Ming
2017-01-01
Wurtzite (W) and zinc-blende (ZB) polytypism has long been observed in epitaxial CdS thin films. The present work, based on van der Waals epitaxial CdS thin films, is an attempt to explain which crystal modification, W or ZB, is favored under different growth conditions. In this van der Waals epitaxy system where the substrate influence is considered weak, it is found that the substrate temperature plays a crucial role in determining the crystal modification of CdS, that is, W and ZB CdS are more stable at low and high ends of substrate temperature, respectively. We attribute this temperature effect to the entropy difference (SW benefit in terms of high density of bonding contributed by the substrate-film interface is believed to be too overwhelming for the intrinsic entropy difference to overcome. Furthermore, the deposition rate is found to affect the crystalline quality and strain level in CdS films but not the crystal modification of the CdS films. Last, Raman and photoluminescence spectroscopies reveal the strain behaviors in the films. The phase change from W to ZB CdS is well-correlated with the observed peak shifts in Raman and photoluminescence spectroscopies.
Samad, Leith; Bladow, Sage M; Ding, Qi; Zhuo, Junqiao; Jacobberger, Robert M; Arnold, Michael S; Jin, Song
2016-07-26
The fascinating semiconducting and optical properties of monolayer and few-layer transition metal dichalcogenides, as exemplified by MoS2, have made them promising candidates for optoelectronic applications. Controllable growth of heterostructures based on these layered materials is critical for their successful device applications. Here, we report a direct low temperature chemical vapor deposition (CVD) synthesis of MoS2 monolayer/multilayer vertical heterostructures with layer-controlled growth on a variety of layered materials (SnS2, TaS2, and graphene) via van der Waals epitaxy. Through precise control of the partial pressures of the MoCl5 and elemental sulfur precursors, reaction temperatures, and careful tracking of the ambient humidity, we have successfully and reproducibly grown MoS2 vertical heterostructures from 1 to 6 layers over a large area. The monolayer MoS2 heterostructure was verified using cross-sectional high resolution transmission electron microscopy (HRTEM) while Raman and photoluminescence spectroscopy confirmed the layer-controlled MoS2 growth and heterostructure electronic interactions. Raman, photoluminescence, and energy dispersive X-ray spectroscopy (EDS) mappings verified the uniform coverage of the MoS2 layers. This reaction provides an ideal method for the scalable layer-controlled growth of transition metal dichalcogenide heterostructures via van der Waals epitaxy for a variety of optoelectronic applications.
Sleiman, Chantal; González, Sergio; Klippenstein, Stephen J; Talbi, Dahbia; El Dib, Gisèle; Canosa, André
2016-06-01
The gas phase reaction between the CN radical and acetonitrile CH3CN was investigated experimentally, at low temperatures, with the CRESU apparatus and a slow flow reactor to explore the temperature dependence of its rate coefficient from 354 K down to 23 K. Whereas a standard Arrhenius behavior was found at T > 200 K, indicating the presence of an activation barrier, a dramatic increase in the rate coefficient by a factor of 130 was observed when the temperature was decreased from 168 to 123 K. The reaction was found to be pressure independent at 297 K unlike the experiments carried out at 52 and 132 K. The work was complemented by ab initio transition state theory based master equation calculations using reaction pathways investigated with highly accurate thermochemical protocols. The role of collisional stabilization of a CNCH3CN van der Waals complex and of tunneling induced H atom abstractions were also considered. The experimental pressure dependence at 52 and 132 K is well reproduced by the theoretical calculations provided that an anharmonic state density is considered for the van der Waals complex CH3CNCN and its Lennard-Jones radius is adjusted. Furthermore, these calculations indicate that the experimental observations correspond to the fall-off regime and that tunneling remains small in the low-pressure regime. Hence, the studied reaction is essentially an association process at very low temperature. Implications for the chemistry of interstellar clouds and Titan are discussed.
Optimizing Protein-Protein van der Waals Interactions for the AMBER ff9x/ff12 Force Field.
Chapman, Dail E; Steck, Jonathan K; Nerenberg, Paul S
2014-01-14
The quality of molecular dynamics (MD) simulations relies heavily on the accuracy of the underlying force field. In recent years, considerable effort has been put into developing more accurate dihedral angle potentials for MD force fields, but relatively little work has focused on the nonbonded parameters, many of which are two decades old. In this work, we assess the accuracy of protein-protein van der Waals interactions in the AMBER ff9x/ff12 force field. Across a test set of 44 neat organic liquids containing the moieties present in proteins, we find root-mean-square (RMS) errors of 1.26 kcal/mol in enthalpy of vaporization and 0.36 g/cm(3) in liquid densities. We then optimize the van der Waals radii and well depths for all of the relevant atom types using these observables, which lowers the RMS errors in enthalpy of vaporization and liquid density of our validation set to 0.59 kcal/mol (53% reduction) and 0.019 g/cm(3) (46% reduction), respectively. Limitations in our parameter optimization were evident for certain atom types, however, and we discuss the implications of these observations for future force field development.
Microwave spectrum and structure of the 3,5-difluoropyridine⋯CO2 van der Waals complex
Dewberry, Christopher T.; Cornelius, Ryan D.; Mackenzie, Rebecca B.; Smith, C. J.; Dvorak, Michael A.; Leopold, Kenneth R.
2016-10-01
The rotational spectrum of the weakly bound complex 3,5-difluoropyridine⋯CO2 has been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for the parent and 13CO2 isotopologues. The data indicate a planar structure in which the nitrogen approaches the carbon of the CO2 with either a C2v or effectively C2v geometry in the ground vibrational state. The N⋯C van der Waals bond distance is 2.8245(16) Å and the oxygen⋯ortho-hydrogen distance is 3.091(2) Å. The N⋯C van der Waals bond length is 0.027(8) Å longer than that previously determined for pyridine-CO2, but is still considerably shorter than the 2.998 Å distance in HCN⋯CO2. M06-2X/6-311++G(3df,3pd) calculations place the binding energy of the complex at 4.3 kcal/mol (4.1 kcal/mol with counterpoise correction). The calculations further indicate that a secondary interaction between the ortho-hydrogens of the ring and the CO2 oxygens account for ∼50% of the total binding energy.
Henck, Hugo; Pierucci, Debora; Chaste, Julien; Naylor, Carl H.; Avila, Jose; Balan, Adrian; Silly, Mathieu G.; Asensio, Maria C.; Sirotti, Fausto; Johnson, A. T. Charlie; Lhuillier, Emmanuel; Ouerghi, Abdelkarim
2016-09-01
Van der Waals (vdW) heterostructures obtained by stacking 2D materials offer a promising route for next generation devices by combining different unique properties in completely new artificial materials. In particular, the vdW heterostructures combine high mobility and optical properties that can be exploited for optoelectronic devices. Since the p-n junction is one of the most fundamental units of optoelectronics, we propose an approach for its fabrication based on the intrinsic n doped MoS2 and the p doped bilayer graphene hybrid interfaces. We demonstrate the control of the photoconduction properties using electrolytic gating which ensures a low bias operation. We show that by finely choosing the doping value of each layer, the photoconductive properties of the hybrid system can be engineered to achieve magnitude and sign control of the photocurrent. Finally, we provide a simple phase diagram relating the photoconductive behavior with the chosen doping, which we believe can be very useful for the future design of the van der Waals based photodetectors.
Kudrynskyi, Z. R.; Bakhtinov, A. P.; Vodopyanov, V. N.; Kovalyuk, Z. D.; Tovarnitskii, M. V.; Lytvyn, O. S.
2015-11-01
The growth morphology, composition and structure of PbSe nanostructures grown on the atomically smooth, clean, nanoporous and oxidized van der Waals (0001) surfaces of GaSe layered crystals were studied by means of atomic force microscopy, x-ray diffractometry, photoelectron spectroscopy and Raman spectroscopy. Semiconductor heterostructures were grown by the hot-wall technique in vacuum. Nanoporous GaSe substrates were fabricated by the thermal annealing of layered crystals in a molecular hydrogen atmosphere. The irradiation of the GaSe(0001) surface by UV radiation was used to fabricate thin Ga2O3 layers with thickness clusters with a square or rectangular symmetry on the clean low-energy (0001) GaSe surface, and (001)-oriented growth of PbSe thin films takes place on this surface. Using this growth technique it is possible to grow PbSe nanostructures with different morphologies: continuous epitaxial layers with thickness quantum dots with a high lateral density (more than 1011 cm-2) on the oxidized van der Waals (0001) surfaces and faceted square pillar-like nanostructures with a low lateral density (˜108 cm-2) on the nanoporous GaSe substrates. We exploit the ‘vapor-liquid-solid’ growth with low-melting metal (Ga) catalyst of PbSe crystalline branched nanostructures via a surface-defect-assisted mechanism.
Chenoweth, D R
1983-06-01
An ideal-gas, quasi-steady, duct-flow model previously formulated for small scale gas-transfer problems is extended to real gases via the van der Waals equation of state as well as general virial expansions. The model is applicable for an arbitrary series of ducting components where each is described empirically by total pressure and total temperature change correlations. The adequacy of the van der Waals model for gas-transfer calculations is verified by comparisons with: (1) real gas PVT data; (2) the magnitudes of the controlling effects; and (3) approximate limiting case solutions with numerical results using more accurate real-gas modeling. 25 figures.
LIU Nan; BAI Yi-Long; XIA Meng-Fen; KE Fu-Jiu
2005-01-01
@@ Aiming at understanding how a liquid film on a substrate affects the atomic force microscopic image in experiments, we present an analytical representation of the shape of liquid surface under van der Waals interaction induced by a non-contact probe tip. The analytical expression shows good consistence with the corresponding numerical results. According to the expression, we find that the vertical scale of the liquid dome is mainly gov erned by a combination of van der Waals force, surface tension and probe tip radius, and is weekly related to gravity. However, its horizontal extension is determined by the capillary length.
Arkesteijn, M.; Wurff, van der A.W.G.; Noort, van F.R.
2014-01-01
Dat planten ziek kunnen worden door stress is bekend. Bij een verminderde weerbaarheid, zoals in de stekfase, de overgang van vegetatief naar generatief of gewoon door de klimaatomstandigheden, krijgen schimmels en bacteriën eerder de kans met uitval of beschadiging als gevolg. Door ‘plantversterker
Beyond van der Pauw: Novel methods for four-point magnetotransport characterization
Zhou, Wang
In this thesis, the conventional four-point measurement technique and the van der Pauw (vdP) method are systematically investigated in the presence of non-ideal conditions, namely, non-uniform carrier density distribution and absence of ohmic contacts, which are nonetheless commonly encountered in semiconductor characterizations. Upon understanding the challenges in the conventional methods, novel characterization techniques are developed to address these challenges. A longitudinal magnetoresistance asymmetry method was developed to study the carrier density non-uniformity in two-dimensional samples. By analyzing the asymmetric longitudinal magnetoresistance under positive and negative B-fields, an analytical model based on a linear density gradient across the sample was deduced to quantitatively describe the asymmetry. Based on the theoretical model, a practical method was described which enabled one to experimentally measure the density gradient within a single sample. The method requires only measurements of longitudinal resistances R xx and Ryy under both positive and negative B-fields, and equations have been provided to extract both the angle and the magnitude of density gradients from the measured resistances. The method was demonstrated in a GaAs quantum well wafer at cryogenic temperatures and n-GaAs bulk-doped wafer at room temperature. In both systems, the density gradient vectors extracted with our method matched well with the interpolated density gradient vectors estimated from actual density distribution maps as a base comparison set, suggesting that our method can be a universal extension of the vdP method to extract density gradients in various systems. The method also allows one to uncover the true local longitudinal resistivity rhoxx at the center of the sample, which the conventional vdP method cannot describe in the presence of non-uniform densities. The ability to find rhoxx makes it possible to study interesting physics in semiconductors such
Ideaalbeeld freesia op de kop (interview met Frank van der Helm)
Sleegers, J.; Helm, van der F.P.M.
2012-01-01
Een onderzoek naar bodemisolatie in freesia heeft niet het bedoelde effect opgeleverd. Maar het heeft wel het ideaalbeeld van de plant op zijn kop gezet. De schraalste planten leverden de mooiste takken op.
Ruimteclub NASA doet spectaculaire vondst (interview met Prof. Dr. John van der Oost)
Vreede, de J.; Oost, van der J.
2010-01-01
De Amerikaanse ruimtevaartorganisatie NASA heeft een bijzondere bacterie ontdekt, die gedijt op het giftige arsenicum, oftewel rattengif. De ontdekking vergroot de kans dat op andere planeten vormen van primitief leven voorkomt.
Ruimteclub NASA doet spectaculaire vondst (interview met Prof. Dr. John van der Oost)
Vreede, de J.; Oost, van der J.
2010-01-01
De Amerikaanse ruimtevaartorganisatie NASA heeft een bijzondere bacterie ontdekt, die gedijt op het giftige arsenicum, oftewel rattengif. De ontdekking vergroot de kans dat op andere planeten vormen van primitief leven voorkomt.
Weseman, J.M.; Ruig, de W.G.
1982-01-01
Doel van dit onderzoek is: Het verkrijgen van meer inzicht in de dunnelaagchromatografie en de werkwijze voor het aantonen van anabolica in urine en vlees, in het bijzonder omtrent het beoordelen van dunnelaagplaatjes en de interpretatie van de verkregen resultaten. Er is daadwerkelijk ervaring opge
Andersen, Jonas; Heimdal, J.; Wallin Mahler Andersen, Denise
2014-01-01
Terahertz absorption spectra have been recorded for the weakly bound CO2–H2O complex embedded in cryogenic neon matrices at 2.8 K. The three high-frequency van der Waals vibrational transitions associated with out-of-plane wagging, in-plane rocking, and torsional motion of the isotopic H2O subunit...
Lynn, Richard
2010-01-01
Wicherts, Dolan, Carlson & van der Maas (WDCM) (2010) contend that the average IQ in sub-Saharan Africa is about 76 in relation to a British mean of 100 and sd of 15. This result is achieved by including many studies of unrepresentative elite samples. Studies of acceptably representative samples indicate a sub-Saharan Africa IQ of…
Youniss, James
1994-01-01
Briefly summarizes Vygotsky's life, the appeal and subsequent abandonment of his ideas in the 1960s, and renewal of interest in the 1970s and 1980s (often at the expense of Piaget). Praises van der Veer and Valsinger's book as a realistic picture of Vygotsky's background, life, and work, of the scientific and political context in Russia and of his…
Lee, Kyuho; Kelkkanen, Kari André; Berland, Kristian
2011-01-01
Detailed experimental data for physisorption potential-energy curves of H2 on low-indexed faces of Cu challenge theory. Recently, density-functional theory has been developed to also account for nonlocal correlation effects, including van der Waals forces. We show that one functional, denoted vd...
Ding, Wenjun; Zhu, Jianbao; Wang, Zhe; Gao, Yanfei; Xiao, Di; Gu, Yi; Zhang, Zhenyu; Zhu, Wenguang
2017-04-01
Interest in two-dimensional (2D) van der Waals materials has grown rapidly across multiple scientific and engineering disciplines in recent years. However, ferroelectricity, the presence of a spontaneous electric polarization, which is important in many practical applications, has rarely been reported in such materials so far. Here we employ first-principles calculations to discover a branch of the 2D materials family, based on In2Se3 and other III2-VI3 van der Waals materials, that exhibits room-temperature ferroelectricity with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations. The device potential of these 2D ferroelectric materials is further demonstrated using the examples of van der Waals heterostructures of In2Se3/graphene, exhibiting a tunable Schottky barrier, and In2Se3/WSe2, showing a significant band gap reduction in the combined system. These findings promise to substantially broaden the tunability of van der Waals heterostructures for a wide range of applications.
Laura Lizondo Sevilla, José Santatecla Fayos, Nuria Salvador Luján, Ignacio Bosch Reig
2013-05-01
Full Text Available RESUMEN El artículo analiza la muestra Die Wohnung unserer Zeit (La Vivienda de Nuestro Tiempo, perteneciente a la Deutsche Bauausstellung (Exposición de la Construcción Alemana de 1931, e indaga en las posibilidades de experimentación arquitectónica que desempeñaron este tipo de arquitecturas en el desarrollo profesional de Mies van der Rohe. El discurso constata el potencial de esta exposición para actuar como campo de experimentación efímero, capaz de dar forma construida a una idea o concepto; una arquitectura temporal que rompió con los convencionalismos expositivos del momento, convirtiéndose en un evento de relevancia a nivel cultural, histórico y arquitectónico. La muestra materializó, físicamente y a escala 1.1, la idea que un grupo de arquitectos del Movimiento Moderno tenían sobre la vivienda del futuro; una vivienda pensada y diseñada con ideas basadas en el hipotético individuo de la nueva época.SUMMARY The article analyses the exhibition Die Wohnung unserer Zeit (The House of Our Time, pertaining to the Deutsche Bauausstellung (1931 German Construction Exhibition, and explores the possibilities of architectural experimentation that such architecture played in the professional development of Mies van der Rohe. It discusses the potential of this exhibition to act as an ephemeral testing ground, capable of giving constructed form to an idea or concept; an architecture that broke with the expository conventionalisms of the time, becoming an important event in the cultural, historical and architectural sense. The exhibition presented, physically, and on a scale of 1 to 1, the idea that a group of architects of the Modern Movement had about the house of the future, a house conceived and designed with ideas based on the hypothetical individual of the new age.
Zhang, Kenan; Zhang, Tianning; Cheng, Guanghui; Li, Tianxin; Wang, Shuxia; Wei, Wei; Zhou, Xiaohao; Yu, Weiwei; Sun, Yan; Wang, Peng; Zhang, Dong; Zeng, Changgan; Wang, Xingjun; Hu, Weida; Fan, Hong Jin; Shen, Guozhen; Chen, Xin; Duan, Xiangfeng; Chang, Kai; Dai, Ning
2016-03-22
We demonstrate the type-II staggered band alignment in MoTe2/MoS2 van der Waals (vdW) heterostructures and an interlayer optical transition at ∼1.55 μm. The photoinduced charge separation between the MoTe2/MoS2 vdW heterostructure is verified by Kelvin probe force microscopy (KPFM) under illumination, density function theory (DFT) simulations and photoluminescence (PL) spectroscopy. Photoelectrical measurements of MoTe2/MoS2 vdW heterostructures show a distinct photocurrent response in the infrared regime (1550 nm). The creation of type-II vdW heterostructures with strong interlayer coupling could improve our fundamental understanding of the essential physics behind vdW heterostructures and help the design of next-generation infrared optoelectronics.
Lee, Young Tack, E-mail: 023273@kist.re.kr, E-mail: stunalren@gmail.com [Center of Opto-Electronic Materials and Devices, Post-Silicon Semiconductor Institute Korea Institute of Science and Technology (KIST), Seoul 02792 (Korea, Republic of); Choi, Won Kook [Department of Nanomaterials and Nano Science, Korea University of Science and Technology (KUST), Daejeon 34113 (Korea, Republic of); Materials and Life Science Research Division, Korea Institute of Science and Technology (KIST), Seoul 02792 (Korea, Republic of); Hwang, Do Kyung, E-mail: dkhwang@kist.re.kr [Center of Opto-Electronic Materials and Devices, Post-Silicon Semiconductor Institute Korea Institute of Science and Technology (KIST), Seoul 02792 (Korea, Republic of); Department of Nanomaterials and Nano Science, Korea University of Science and Technology (KUST), Daejeon 34113 (Korea, Republic of)
2016-06-20
We report on a chemical free one-off imprinting method to fabricate two dimensional (2D) van der Waals (vdWs) materials based transistors. Such one-off imprinting technique is the simplest and effective way to prevent unintentional chemical reaction or damage of 2D vdWs active channel during device fabrication process. 2D MoS{sub 2} nanosheets based transistors with a hexagonal-boron-nitride (h-BN) passivation layer, prepared by one-off imprinting, show negligible variations of transfer characteristics after chemical vapor deposition process. In addition, this method enables the fabrication of all 2D MoS{sub 2} transistors consisting of h-BN gate insulator, and graphene source/drain and gate electrodes without any chemical damage.
Xu, Fei; Luo, WEi; Lu, Yan-qing
2014-01-01
The ultra-short range force, van der Waals force (VWF), will rise rapidly when one nanoscale waveguide is close to another one, and be stronger than the external transverse gradient force (TGF). We theoretically investigate the giant influence of the VWF on the device performance in a typical optomechanical system consisting of a suspended silicon waveguide and a silica substrate including waveguide deformation stiction and failure mechanism. The device shows unique optically-activated plastic/elastic behaviors and stiction due to the VWF. When the input optical power is above the critical power, the waveguide is sticking to the substrate and the deformation is plastic and unrecoverable, even though the total force is less than the yield strength of the waveguide material. This is important and helpful for the design and applications of optomechanical devices.
Kleis, Jesper; Schröder, Elsebeth; Hyldgaard, Per
2008-01-01
The dispersive interaction between nanotubes is investigated through ab initio theory calculations and in an analytical approximation. A van der Waals density functional (vdW-DF) [M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)] is used to determine and compare the binding of a pair of nanotubes...... for the nanotube-crystal binding energy can be approximated by a sum of nanotube-pair interactions when these are calculated in vdW-DR This observation suggests a framework for an efficient implementation of quantum-physical modeling of the carbon nanotube bundling in more general nanotube bundles, including...... as well as in a nanotube crystal. To analyze the interaction and determine the importance of morphology, we further compare results of our ab initio calculations to a simple analytical result,that we obtain for a pair of well-separated nanotubes. In contrast to traditional density functional theory...
Zhang, Hui; Sun, Cheng H; Li, Feng; Li, Hong X; Cheng, Hui M
2006-05-18
The potential energies of van der Waals interactions between two multiwalled carbon nanotubes (MWNTs) as well as two carbon nanoparticles (CNPs) were calculated and compared on the basis of the continuum Lennard-Jones model. The well depth of the potential is 1 order of magnitude higher for MWNTs than for CNPs, indicating that MWNTs and CNPs can be separated from each other through polymer-induced steric stabilization. On the basis of this prediction, a novel method for the purification of MWNTs was proposed. The method involves a high-temperature annealing (2600 degrees C, 1 h) followed by an extraction treatment with a selected dispersing agent. While the annealing process evaporates the metal particles, the extraction treatment removes CNPs. The quality of the nanotubes obtained after purification was examined by laser Raman, thermogravimetric analysis, and electron microscopy observations.
Zhang, Xue; Zhou, Wu-Xing; Chen, Xue-Kun; Liu, Yue-Yang; Chen, Ke-Qiu
2016-05-01
The thermal transport properties of multi-walled carbon nanotubes (MWCNTs) were investigated by using non-equilibrium molecular dynamics simulation. The results show that the thermal conductivity of MWCNTs decreases significantly comparing to that of single-walled carbon nanotubes (SWCNTs) due to the inter-wall van der Waals interactions. The more interesting is a fact that the thermal conductance of MWCNTs is significantly greater than the thermal conductance summation of each SWCNTs. This is because the thermal conductance of a carbon nanotube protected by an outer tube is much larger than that of one that is not protected. Moreover, we also studied the thermal flux distribution of MWCNTs, and found that the outer tube plays a dominant role in heat energy transfer.
Milton, Kimball A; Wagner, Jef
2008-01-01
In earlier papers we have applied multiple scattering techniques to calculate Casimir forces due to scalar fields between different bodies described by delta function potentials. When the coupling to the potentials became weak, closed-form results were obtained. We simplify this weak-coupling technique and apply it to the case of tenuous dielectric bodies, in which case the method involves the summation of van der Waals (Casimir-Polder) interactions. Once again exact results for finite bodies can be obtained. We present closed formulas describing the interaction between spheres and between cylinders, and between an infinite plate and a retangular slab of finite size. For such a slab, we consider the torque acting on it, and find non-trivial equilibrium points can occur.
Venkatesan, A
1997-01-01
The behaviour of a driven double well Duffing-van der Pol (DVP) oscillator for a specific parametric choice ($\\mid \\alpha \\mid =\\beta$) is studied. The existence of different attractors in the system parameters ($f-\\omega$) domain is examined and a detailed account of various steady states for fixed damping is presented. Transition from quasiperiodic to periodic motion through chaotic oscillations is reported. The intervening chaotic regime is further shown to possess islands of phase-locked states and periodic windows (including period doubling regions), boundary crisis, all the three classes of intermittencies, and transient chaos. We also observe the existence of local-global bifurcation of intermittent catastrophe type and global bifurcation of blue-sky catastrophe type during transition from quasiperiodic to periodic solutions. Using a perturbative periodic solution, an investigation of the various forms of instablities allows one to predict Neimark instablity in the $(f-\\omega)$ plane and eventually res...
Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform
Cui, Xu; Lee, Gwan-Hyoung; Kim, Young Duck
2015-01-01
Atomically thin two-dimensional semiconductors such as MoS2 hold great promise for electrical, optical and mechanical devices and display novel physical phenomena. However, the electron mobility of mono-and few-layer MoS2 has so far been substantially below theoretically predicted limits, which has...... in performance, including a record-high Hall mobility reaching 34,000 cm2 V-1 s-1 for six-layer MoS2 at low temperature, confirming that low-temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS2. We also observed Shubnikov-de Haas...... scattering, we have developed here a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Magneto-transport measurements show dramatic improvements...
Van der Waals materials for the passivation of monolayer closed-packed films of CdSe quantum dots
Wang, Dennis Zi-Ren; Zhang, Datong; Creswell, Richard; Lu, Chenguang; Hu, Jiayang; Herman, Irving P.
2015-03-01
Van der Waals (vdW) materials are shown to protect CdSe quantum dots (QDs) from oxidization. Few-layer vdW materials, e.g. graphene and MoS2, were transferred onto a monolayer closed-packed CdSe quantum dots and were examined by photoluminescence (PL) after different time periods. By comparing the PL of CdSe QDs in uncovered areas and those covered by different numbers of layers of graphene and MoS2, we saw that vdW encapsulation slows down the aging of CdSe QDs dramatically. PL mapping results clearly showed better protection of the CdSe QDs under the central part of the vdW material compared to that at the edge; this can be explained by the diffusion of oxygen and water vapor from the edge of the vdW materials.
Crystallographic features related to a van der Waals coupling in the layered chalcogenide FePS3
Murayama, Chisato; Okabe, Momoko; Urushihara, Daisuke; Asaka, Toru; Fukuda, Koichiro; Isobe, Masahiko; Yamamoto, Kazuo; Matsushita, Yoshitaka
2016-10-01
We investigated the crystallographic structure of FePS3 with a layered structure using transmission electron microscopy and powder X-ray diffraction. We found that FePS3 forms a rotational twin structure with the common axis along the c*-axis. The high-resolution transmission electron microscopy images revealed that the twin boundaries were positioned at the van der Waals gaps between the layers. The narrow bands of dark contrast were observed in the bright-field transmission electron microscopy images below the antiferromagnetic transition temperature, TN ≈ 120 K. Low-temperature X-ray diffraction showed a lattice distortion; the a- and b-axes shortened and lengthened, respectively, as the temperature decreased below TN. We propose that the narrow bands of dark contrast observed in the bright-field transmission electron microscopy images are caused by the directional lattice distortion with respect to each micro-twin variant in the antiferromagnetic phase.