Broadband illumination of superconducting pair breaking photon detectors

International Nuclear Information System (INIS)

Guruswamy, T; Goldie, D J; Withington, S

2016-01-01

Understanding the detailed behaviour of superconducting pair breaking photon detectors such as Kinetic Inductance Detectors (KIDs) requires knowledge of the nonequilibrium quasiparticle energy distributions. We have previously calculated the steady state distributions resulting from uniform absorption of monochromatic sub gap and above gap frequency radiation by thin films. In this work, we use the same methods to calculate the effect of illumination by broadband sources, such as thermal radiation from astrophysical phenomena or from the readout system. Absorption of photons at multiple above gap frequencies is shown to leave unchanged the structure of the quasiparticle energy distribution close to the superconducting gap. Hence for typical absorbed powers, we find the effects of absorption of broadband pair breaking radiation can simply be considered as the sum of the effects of absorption of many monochromatic sources. Distribution averaged quantities, like quasiparticle generation efficiency η, match exactly a weighted average over the bandwidth of the source of calculations assuming a monochromatic source. For sub gap frequencies, however, distributing the absorbed power across multiple frequencies does change the low energy quasiparticle distribution. For moderate and high absorbed powers, this results in a significantly larger η–a higher number of excess quasiparticles for a broadband source compared to a monochromatic source of equal total absorbed power. Typically in KIDs the microwave power absorbed has a very narrow bandwidth, but in devices with broad resonance characteristics (low quality factors), this increase in η may be measurable. (paper)

Transport and pairing properties of helical edges with proximity induced superconductivity and ferromagnetism

Energy Technology Data Exchange (ETDEWEB)

Keidel, Felix; Burset, Pablo; Trauzettel, Bjoern [Institute of Theoretical Physics and Astrophysics, University of Wuerzburg, 97074 Wuerzburg (Germany); Crepin, Francois [Laboratoire de Physique Theorique de la Matiere Condensee, UPMC, Sorbonne Universites, 75252 Paris (France)

2016-07-01

The scientific interest in Quantum Spin Hall systems is far from declining. While these certainly are fascinating by themselves, there is plenty of new and exciting physics to arise when superconductivity and ferromagnetism are brought into the game. The strong constraint of helicity in the edge states of a two-dimensional topological insulator is responsible for an intimate relation between the allowed scattering processes in a hybrid junction and the parameters of the system, namely the superconducting order parameter and the magnetic field. In our work, we study a helical liquid in proximity to a conventional s-wave superconductor and ferromagnetic insulators by means of a Green's function analysis. The ferromagnet gives rise to sub-gap Andreev/Majorana bound states and non-local crossed Andreev reflection (CAR), both of which decisively affect the pairing and transport properties of the junction. As a result, the simple s-wave symmetry of the superconductor is enriched and unconventional odd-frequency triplet superconductivity emerges. Strikingly, we have identified a setup that favors CAR over electron co-tunneling and may allow for the indirect measurement of the symmetries of the superconducting order parameter.

Probing the unconventional superconducting state of LiFeAs by quasiparticle interference.

Science.gov (United States)

Hänke, Torben; Sykora, Steffen; Schlegel, Ronny; Baumann, Danny; Harnagea, Luminita; Wurmehl, Sabine; Daghofer, Maria; Büchner, Bernd; van den Brink, Jeroen; Hess, Christian

2012-03-23

A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well-suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high T(c)∼18  K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van Hove singularity at the center of the Brillouin zone which is in stark contrast to other pnictide superconductors where nesting is crucial for both scattering and s(±) superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the holelike bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s(++), s(±), and d wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary p wave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g., s+id wave symmetry).

Superconducting state mechanisms and properties

CERN Document Server

Kresin, Vladimir Z; Wolf, Stuart A

2014-01-01

'Superconducting State' provides a very detailed theoretical treatment of the key mechanisms of superconductivity, including the current state of the art (phonons, magnons, and plasmons). A very complete description is given of the electron-phonon mechanism responsible for superconductivity in the majority of superconducting systems, and the history of its development, as well as a detailed description of the key experimental techniques used to study the superconducting state and determine the mechanisms. In addition, there are chapters describing the discovery and properties of the key superconducting compounds that are of the most interest for science, and applications including a special chapter on the cuprate superconductors. It provides detailed treatments of some very novel aspects of superconductivity, including multiple bands (gaps), the "pseudogap" state, novel isotope effects beyond BCS, and induced superconductivity.

Crossover from BCS to composite boson (local pair) superconductivity in quasi-2D systems

International Nuclear Information System (INIS)

Gorbar, E.V.; Loktev, V.M.; Sharapov, S.G.

1995-01-01

The crossover from cooperative Cooper pairing to independent bound state (composite bosons) formation and condensation in quasi-2 D systems is studied. It is shown that at low carrier density the critical superconducting temperature is equal to the temperature of Bose-condensation of ideal quasi-2 D Bose-gas with heavy dynamical mass, meanwhile at high densities the BCS result remains valid. 15 refs

Exchange and spin-fluctuation superconducting pairing in the strong correlation limit of the Hubbard model

International Nuclear Information System (INIS)

Plakida, N. M.; Anton, L.; Adam, S. . Department of Theoretical Physics, Horia Hulubei National Institute for Physics and Nuclear Engineering, PO Box MG-6, RO-76900 Bucharest - Magurele; RO); Adam, Gh. . Department of Theoretical Physics, Horia Hulubei National Institute for Physics and Nuclear Engineering, PO Box MG-6, RO-76900 Bucharest - Magurele; RO)

2001-01-01

A microscopical theory of superconductivity in the two-band singlet-hole Hubbard model, in the strong coupling limit in a paramagnetic state, is developed. The model Hamiltonian is obtained by projecting the p-d model to an asymmetric Hubbard model with the lower Hubbard subband occupied by one-hole Cu d-like states and the upper Hubbard subband occupied by two-hole p-d singlet states. The model requires two microscopical parameters only, the p-d hybridization parameter t and the charge-transfer gap Δ. It was previously shown to secure an appropriate description of the normal state properties of the high -T c cuprates. To treat rigorously the strong correlations, the Hubbard operator technique within the projection method for the Green function is used. The Dyson equation is derived. In the molecular field approximation, d-wave superconducting pairing of conventional hole (electron) pairs in one Hubbard subband is found, which is mediated by the exchange interaction given by the interband hopping, J ij = 4 (t ij ) 2 / Δ. The normal and anomalous components of the self-energy matrix are calculated in the self-consistent Born approximation for the electron-spin-fluctuation scattering mediated by kinematic interaction of the second order of the intraband hopping. The derived numerical and analytical solutions predict the occurrence of singlet d x 2 -y 2 -wave pairing both in the d-hole and singlet Hubbard subbands. The gap functions and T c are calculated for different hole concentrations. The exchange interaction is shown to be the most important pairing interaction in the Hubbard model in the strong correlation limit, while the spin-fluctuation coupling results only in a moderate enhancement of T c . The smaller weight of the latter comes from two specific features: its vanishing inside the Brillouin zone (BZ) along the lines, |k x | + |k y |=π pointing towards the hot spots and the existence of a small energy shell within which the pairing is effective. By

Inhomogeneous superconductivity in a ferromagnet

International Nuclear Information System (INIS)

Kontos, T.; Aprili, M.; Lesueur, J.; Genet, F.; Boursier, R.; Grison, X.

2003-01-01

We have studied a new superconducting state where the condensate wave function resulting from conventional pairing, is modified by an exchange field. Superconductivity is induced into a ferromagnetic thin film (F) by the proximity effect with a superconducting reservoir (S). We observed oscillations of the superconducting order parameter induced in F as a function of the distance from the S/F interface. They originate from the finite momentum transfer provided to Cooper pairs by the splitting of the spin up and down bands. We measured the superconducting density of states in F by tunneling spectroscopy and the Josephson critical current when F is coupled with a superconducting counter-electrode. Negative values of the superconducting order parameter are revealed by capsized tunneling spectra in F and a negative Josephson coupling (π-junction)

Superconducting Analogue of the Parafermion Fractional Quantum Hall States

Directory of Open Access Journals (Sweden)

Abolhassan Vaezi

2014-07-01

Full Text Available Read-Rezayi Z_{k} parafermion wave functions describe ν=2+(k/kM+2 fractional quantum Hall (FQH states. These states support non-Abelian excitations from which protected quantum gates can be designed. However, there is no experimental evidence for these non-Abelian anyons to date. In this paper, we study the ν=2/k FQH-superconductor heterostructure and find the superconducting analogue of the Z_{k} parafermion FQH state. Our main tool is the mapping of the FQH into coupled one-dimensional chains, each with a pair of counterpropagating modes. We show that by inducing intrachain pairing and charge preserving backscattering with identical couplings, the one-dimensional chains flow into gapless Z_{k} parafermions when k<4. By studying the effect of interchain coupling, we show that every parafermion mode becomes massive except for the two outermost ones. Thus, we achieve a fractional topological superconductor whose chiral edge state is described by a Z_{k} parafermion conformal field theory. For instance, we find that a ν=2/3 FQH in proximity to a superconductor produces a Z_{3} parafermion superconducting state. This state is topologically indistinguishable from the non-Abelian part of the ν=12/5 Read-Rezayi state. Both of these systems can host Fibonacci anyons capable of performing universal quantum computation through braiding operations.

Quench Property of Twisted-Pair MgB$_2$ Superconducting Cables in Helium Gas

CERN Document Server

Spurrell, J; Falorio, I; Pelegrin, J; Ballarino, A; Yang, Y

2015-01-01

CERN's twisted-pair superconducting cable is a novel design which offers filament transposition, low cable inductance and is particularly suited for tape conductors such as 2G YBCO coated conductors, Ag-sheathed Bi2223 tapes and Ni/Monel-sheathed MgB2 tapes. A typical design of such twistedpair cables consists of multiple superconducting tapes intercalated with thin copper tapes as additional stabilizers. The copper tapes are typically not soldered to the superconducting tapes so that sufficient flexibility is retained for the twisting of the tape assembly. The electrical and thermal contacts between the copper and superconducting tapes are an important parameter for current sharing, cryogenic stability and quench propagation. Using an MgB2 twisted-pair cable assembly manufactured at CERN, we have carried out minimum quench energy (MQE) and propagation velocity (vp) measurements with point-like heat deposition localized within a tape. Furthermore, different contacts between the copper and superconductor aroun...

Unconventional superconductivity in iron pnictides: Magnon mediated pairing

Science.gov (United States)

kar, Raskesh; Paul, Bikash Chandra; Misra, Anirban

2018-02-01

We study the phenomenon of unconventional superconductivity in iron pnictides on the basis of localized-itinerant model. In this proposed model, superconductivity arises from the itinerant part of electrons, whereas antiferromagnetism arises from the localized part. The itinerant electrons move over the sea of localized electrons in antiferromagnetic alignment and interact with them resulting in excitation of magnons. We find that triplet pairing of itinerant electrons via magnons is possible in checkerboard antiferromagnetic spin configuration of the substances CaFe2As2 and BaFe2As2 in pure form for umklapp scattering with scattering wave vector Q =(1 , 1) , in the unit of π/a where a being one orthorhombic crystal parameter, which is the nesting vector between two Fermi surfaces. The interaction potential figured out in this way, increases with the decrease in nearest neighbour (NN) exchange couplings. Under ambient pressure, with stripe antiferromagnetic spin configuration, a very small value of coupling constant is obtained which does not give rise to superconductivity. The critical temperature of superconductivity of the substances CaFe2As2 and BaFe2As2 in higher pressure checkerboard antiferromagnetic spin configuration are found to be 12.12 K and 29.95 K respectively which are in agreement with the experimental results.

Superconductivity is pair work

International Nuclear Information System (INIS)

Wengenmayr, Roland

2011-01-01

Electric cables that routinely conduct electricity without loss - physicists have been motivated by this idea ever since superconductivity was discovered 100 years ago. Researchers working with Bernhard Keimer at the Max Planck Institute for Solid State Research in Stuttgart and Frank Steglich at the Max Planck Institute for Chemical Physics of Solids in Dresden want to gain a detailed understanding of how unconventional superconductors lose their resistivity. (orig.)

Color symmetrical superconductivity in a schematic nuclear quark model

DEFF Research Database (Denmark)

Bohr, Henrik; Providencia, C.; da Providencia, J.

2010-01-01

In this letter, a novel BCS-type formalism is constructed in the framework of a schematic QCD inspired quark model, having in mind the description of color symmetrical superconducting states. In the usual approach to color superconductivity, the pairing correlations affect only the quasi-particle...... states of two colors, the single-particle states of the third color remaining unaffected by the pairing correlations. In the theory of color symmetrical superconductivity here proposed, the pairing correlations affect symmetrically the quasi-particle states of the three colors and vanishing net color...

Superconductivity pairing mechanism from cobalt impurity doping in FeSe: Spin (s±) or orbital (s++) fluctuation

Science.gov (United States)

Urata, T.; Tanabe, Y.; Huynh, K. K.; Yamakawa, Y.; Kontani, H.; Tanigaki, K.

2016-01-01

In high-superconducting transition temperature (Tc) iron-based superconductors, interband sign reversal (s±) and sign preserving (s++) s -wave superconducting states have been primarily discussed as the plausible superconducting mechanism. We study Co impurity scattering effects on the superconductivity in order to achieve an important clue on the pairing mechanism using single-crystal Fe1 -xCoxSe and depict a phase diagram of a FeSe system. Both superconductivity and structural transition/orbital order are suppressed by the Co replacement on the Fe sites and disappear above x = 0.036. These correlated suppressions represent a common background physics behind these physical phenomena in the multiband Fermi surfaces of FeSe. By comparing experimental data and theories so far proposed, the suppression of Tc against the residual resistivity is shown to be much weaker than that predicted in the case of general sign reversal and full gap s± models. The origin of the superconducting paring in FeSe is discussed in terms of its multiband electronic structure.

Unconventional superconductivity in heavy-fermion compounds

Energy Technology Data Exchange (ETDEWEB)

White, B.D. [Department of Physics, University of California, San Diego, La Jolla, CA 92093 (United States); Center for Advanced Nanoscience, University of California, San Diego, La Jolla, CA 92093 (United States); Thompson, J.D. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Maple, M.B., E-mail: mbmaple@ucsd.edu [Department of Physics, University of California, San Diego, La Jolla, CA 92093 (United States); Center for Advanced Nanoscience, University of California, San Diego, La Jolla, CA 92093 (United States)

2015-07-15

Highlights: • Quasiparticles in heavy-fermion compounds are much heavier than free electrons. • Superconductivity involves pairing of these massive quasiparticles. • Quasiparticle pairing mediated by magnetic or quadrupolar fluctuations. • We review the properties of superconductivity in heavy-fermion compounds. - Abstract: Over the past 35 years, research on unconventional superconductivity in heavy-fermion systems has evolved from the surprising observations of unprecedented superconducting properties in compounds that convention dictated should not superconduct at all to performing explorations of rich phase spaces in which the delicate interplay between competing ground states appears to support emergent superconducting states. In this article, we review the current understanding of superconductivity in heavy-fermion compounds and identify a set of characteristics that is common to their unconventional superconducting states. These core properties are compared with those of other classes of unconventional superconductors such as the cuprates and iron-based superconductors. We conclude by speculating on the prospects for future research in this field and how new advances might contribute towards resolving the long-standing mystery of how unconventional superconductivity works.

Cooper pairs' magnetic moment in MCFL color superconductivity

International Nuclear Information System (INIS)

Feng Bo; Ferrer, Efrain J.; Incera, Vivian de la

2011-01-01

We investigate the effect of the alignment of the magnetic moments of Cooper pairs of charged quarks that form at high density in three-flavor quark matter. The high-density phase of this matter in the presence of a magnetic field is known to be the Magnetic Color-Flavor-Locked (MCFL) phase of color superconductivity. We derive the Fierz identities of the theory and show how the explicit breaking of the rotational symmetry by the uniform magnetic field opens new channels of interactions and allows the formation of a new diquark condensate. The new order parameter is a spin-1 condensate proportional to the component in the field direction of the average magnetic moment of the pairs of charged quarks. The magnitude of the spin-1 condensate becomes comparable to the larger of the two scalar gaps in the region of large fields. The existence of the spin-1 condensate is unavoidable, as in the presence of a magnetic field there is no solution of the gap equations with nonzero scalar gaps and zero magnetic moment condensate. This is consistent with the fact that the extra condensate does not break any symmetry that has not already been broken by the known MCFL gaps. The spin-1 condensate enhances the condensation energy of pairs formed by charged quarks and the magnetization of the system. We discuss the possible consequences of the new order parameter on the issue of the chromomagnetic instability that appears in color superconductivity at moderate density.

Effects of disorder on coexistence and competition between superconducting and insulating states

NARCIS (Netherlands)

Mostovoy, MV; Marchetti, FM; Simons, BD; Littlewood, PB

We study effects of nonmagnetic impurities on the competition between the superconducting and electron-hole pairing. We show that disorder can result in coexistence of these two types of ordering in a uniform state, even when in clean materials they are mutually exclusive.

Theory of exotic superconductivity and normal states of heavy electron and high temperature superconductivity materials. Progress report, February 15, 1994--February 14, 1995

International Nuclear Information System (INIS)

Cox, D.L.

1995-01-01

This is a progress report for the DOE project covering the period 2/15/94 to 2/14/95. The PI had a fruitful sabbatical during this period, and had some important new results, particularly in the area of new phenomenology for heavy fermion superconductivity. Significant new research accomplishments are in the area of odd-in-time-reversal pairing states/staggered superconductivity, the two-channel Kondo lattice, and a general model for Ce impurities which admits one-, two-, and three-channel Kondo effects. Papers submitted touch on these areas: staggered superconductivity - a new phenomenology for UPt 3 ; theory of the two-channel Kondo lattice in infinite dimensions; general model of a Ce 3+ impurity. Other work was done in the areas: Knight shift in heavy fermion alloys and compounds; symmetry analysis of singular pairing correlations for the two-channel Kondo impurity model

Theoretical studies of superconductivity in doped BaCoSO

Science.gov (United States)

Qin, Shengshan; Li, Yinxiang; Zhang, Qiang; Le, Congcong; Hu, Jiangping

2018-06-01

We investigate superconductivity that may exist in the doped BaCoSO, a multi-orbital Mott insulator with a strong antiferromagnetic ground state. The superconductivity is studied in both t-J type and Hubbard type multi-orbital models by mean field approach and random phase approximation (RPA) analysis. Even if there is no C 4 rotational symmetry, it is found that the system still carries a d-wave like pairing symmetry state with gapless nodes and sign changed superconducting order parameters on Fermi surfaces. The results are largely doping insensitive. In this superconducting state, the three {t_{{2_g}}} orbitals have very different superconducting form factors in momentum space. In particular, the intra-orbital pairing of the {d_{{x^2} - {y^2}}} orbital has an s-wave like pairing form factor. The two methods also predict very different pairing strength on different parts of Fermi surfaces. These results suggest that BaCoSO and related materials can be a new ground to test and establish fundamental principles for unconventional high temperature superconductivity.

High-fidelity frequency down-conversion of visible entangled photon pairs with superconducting single-photon detectors

International Nuclear Information System (INIS)

Ikuta, Rikizo; Kato, Hiroshi; Kusaka, Yoshiaki; Yamamoto, Takashi; Imoto, Nobuyuki; Miki, Shigehito; Yamashita, Taro; Terai, Hirotaka; Wang, Zhen; Fujiwara, Mikio; Sasaki, Masahide; Koashi, Masato

2014-01-01

We experimentally demonstrate a high-fidelity visible-to-telecommunicationwavelength conversion of a photon by using a solid-state-based difference frequency generation. In the experiment, one half of a pico-second visible entangled photon pair at 780 nm is converted to a 1522-nm photon. Using superconducting single-photon detectors with low dark count rates and small timing jitters, we observed a fidelity of 0.93±0.04 after the wavelength conversion

Electronically driven short-range lattice instability: Possible role in superconductive pairing

International Nuclear Information System (INIS)

Szasz, A.

1991-01-01

A superconducting pairing mechanism is suggested, mediating by collective and coherent cluster fluctuations in the materials. The model, based on a geometrical frustration, proposes a dynamic effect driven by a special short-range electronic instability. Experimental support for this model is discussed

Large impedances and Majorana bound states in superconducting circuits

International Nuclear Information System (INIS)

Ulrich, Jascha

2017-01-01

Superconducting circuits offer the opportunity to study quantum mechanics on mesoscopic scales unimpeded by dissipation. This fact and the nonlinearity of the Josephson inductance make it possible to use superconducting circuits as artificial atoms whose long-lived states can be selectively addressed and studied. A pronounced nonlinearity of the energy spectrum, however, requires quantum fluctuations of the flux across the Josephson junction which are large on the scale of the superconducting flux quantum Φ Q =h/2e. This implies charge fluctuations below the single Cooper-pair limit via flux-charge duality. The localization of charge leads to a strong susceptibility to interactions with charges in the environment which has motivated the search for schemes to decouple charges from their environment. This thesis is concerned with theoretical challenges arising from two complementary approaches to this problem: the realization of large impedances and the fractionalization of electrons by means of Majorana bound states. In recent years, the decoupling of charges from the environment through reactive large impedances, so-called ''superinductances'' L, has attracted much interest. These inductances feature small parasitic capacitance C such that the characteristic impedance √(L/C) is much larger than the superconducting resistance quantum R Q =h/4e 2 . Superinductances have various applications ranging from qubit designs such as the 0-π qubit or the fluxonium to impedance matching, Bloch oscillations and the stabilization of phase slips in superconducting nanowires. Although there exists a well-established formalism for the quantization of superconducting circuits in terms of node fluxes, this formalism is ill-suited for the description of fast flux transport with localized charges in large-impedance environments. In particular, the nonlinear capacitive behavior of phase slip junctions cannot be modeled in a straightforward way using node fluxes

Effect of superconducting correlation on the localization of quasiparticles in low dimensions

International Nuclear Information System (INIS)

Xiang, T.

1995-01-01

Localization lengths of superconducting quasiparticles λ s are evaluated and compared with the corresponding normal-state values λ n in one-dimensional (1D) and two-dimensional lattices. The effect of superconducting correlation on the localization of quasiparticles is generally stronger in an off-site pairing state than in an on-site pairing state. The modification of superconducting correlation to λ is strongly correlated with the density of states (DOS) of superconducting quasiparticles. λ s drops within the energy gap but is largely enhanced around energies where DOS peaks appear. For a gapless pairing state in 1D or a d-wave pairing state in 2D, λ s /λ n at the Fermi energy E F is of order 1 and determined purely by the value of gap parameter not by the random potential. For the d-wave pairing state, the localization effect is largely weakened compared with the corresponding normal state and quasiparticles with energies close to E F are more strongly localized than other low-energy quasiparticles

Antidiabetic Theory of Superconducting State Transition: Phonons and Strong Electron Correlations the Old Physics and New Aspects

International Nuclear Information System (INIS)

Banacky, P.

2010-01-01

Complex electronic ground state of molecular and solid state system is analyzed on the ab initio level beyond the adiabatic Born-Oppenheimer approximation (BOA). The attention is focused on the band structure fluctuation (BSF) at Fermi level, which is induced by electron-phonon coupling in superconductors, and which is absent in the non-superconducting analogues. The BSF in superconductors results in breakdown of the adiabatic BOA. At these circumstances, chemical potential is substantially reduced and system is stabilized (effect of nuclear dynamics) in the anti adiabatic state at broken symmetry with a gap(s) in one-particle spectrum. Distorted nuclear structure has fluxional character and geometric degeneracy of the anti adiabatic ground state enables formation of mobile bipolarons in real space. It has been shown that an effective attractive e-e interaction (Cooper-pair formation) is in fact correction to electron correlation energy at transition from adiabatic into anti adiabatic ground electronic state. In this respect, Cooper-pair formation is not the primary reason for transition into superconducting state, but it is a consequence of anti adiabatic state formation. It has been shown that thermodynamic properties of system in anti adiabatic state correspond to thermodynamics of superconducting state. Illustrative application of the theory for different types of superconductors is presented.

Introduction to superconductivity

CERN Document Server

Darriulat, Pierre

1998-01-01

The lecture series will address physicists, such as particle and nuclear physicists, familiar with non-relativistic quantum mechanics but not with solid state physics. The aim of this introduction to low temperature superconductivity is to give sufficient bases to the student for him/her to be able to access the scientific literature on this field. The five lectures will cover the following topics : 1. Normal metals, free electron gas, chambers equation. 2. Cooper pairs, the BCS ground state, quasi particle excitations. 3. DC superconductivity, Meissner state, dirty superconductors.4. Self consistent approach, Ginsburg Landau equations, Abrikosov fluxon lattice. 5. Josephson effects, high temperature superconductivity.

Absence of coherent peaks in a Z2 fractionalized BCS superconducting state

Science.gov (United States)

Zhong, Yin; Lu, Han-Tao; Luo, Hong-Gang

2015-01-01

We explore a Z2 fractionalized Bardeen-Cooper-Schrieffer (BCS) superconducting state, which is a minimal extension of usual BCS framework. It is found that this state has similar thermal and transport properties, but its single-particle feature strongly deviates from the coherent quasiparticle behavior of the classic/conventional BCS superconducting state. The fingerprint of such Z2 BCS state is the absence of the BCS coherent peaks and instead a kink in the local density of state occurs, which in principle could be probed by scanning tunneling microscopy or point-contact spectroscopy experiments. The corresponding exactly soluble models that realize the desirable Z2 fractionalized BCS state are presented. In addition, we also study the extended t-U-J model by using Z2 slave-spin representation and find that the Z2 BCS state may exist when the paring structure is fully gapped or has nodes. The prototypical wave-function of such a Z2 BCS state is also proposed, which could be taken as trial wave-function in current numerical techniques. Furthermore, the pairing mechanism of Z2 BCS state is argued from both weak and strong coupling perspective. The present work may be helpful to further study the unconventional superconductivity and its relation to non-Fermi liquids.

On the combination of the Cooper pair and the Ogg pair in the high-Tc oxide superconductor

International Nuclear Information System (INIS)

Zhang Liyuan.

1991-08-01

In this paper it is argued that the superconductivity of the high-T c oxide superconductor (HTOS) can be explained by the combinating mechanism of the Cooper pair and the Ogg pair. The properties of the superconducting state of the HTOS have been calculated under this mechanism, and the theoretical results are overall consistent with the experiment. (author). 37 refs

Scanning tunneling spectroscopy of Co adsorbates on superconducting Pb nanostructures

Energy Technology Data Exchange (ETDEWEB)

Decker, Regis; Caminale, Michael; Oka, Hirofumi; Stepniak, Agnieszka; Leon Vanegas, Augusto A.; Sander, Dirk; Kirschner, Juergen [Max-Planck-Institut fuer Mikrostrukturphysik, Weinberg 2, 06120 Halle (Germany)

2015-07-01

Superconductivity in low-dimensional structures has become an active research area. In order to understand the superconducting pairing, long-standing work has been devoted to the pair breaking effect, where magnetic impurities break Cooper pair singlets. We performed scanning tunneling spectroscopy at low temperature on Co adsorbates on superconducting Pb nanoislands. On the Co adsorbates, we observe spectral features in the superconductor's energy gap, which we attribute to magnetic impurity induced bound states, a hallmark of the pair breaking effect. We discuss the response of the superconducting islands to the presence of Co adsorbates.

Superconducting magnets and cryogenics for the steady state superconducting tokamak SST-1

International Nuclear Information System (INIS)

Saxena, Y.C.

2000-01-01

SST-1 is a steady state superconducting tokamak for studying the physics of the plasma processes in tokamak under steady state conditions and to learn technologies related to the steady state operation of the tokamak. SST-1 will have superconducting magnets made from NbTi based conductors operating at 4.5 K temperature. The design of the superconducting magnets and the cryogenic system of SST-1 tokamak are described. (author)

Superconductivity in doped Dirac semimetals

Science.gov (United States)

Hashimoto, Tatsuki; Kobayashi, Shingo; Tanaka, Yukio; Sato, Masatoshi

2016-07-01

We theoretically study intrinsic superconductivity in doped Dirac semimetals. Dirac semimetals host bulk Dirac points, which are formed by doubly degenerate bands, so the Hamiltonian is described by a 4 ×4 matrix and six types of k -independent pair potentials are allowed by the Fermi-Dirac statistics. We show that the unique spin-orbit coupling leads to characteristic superconducting gap structures and d vectors on the Fermi surface and the electron-electron interaction between intra and interorbitals gives a novel phase diagram of superconductivity. It is found that when the interorbital attraction is dominant, an unconventional superconducting state with point nodes appears. To verify the experimental signature of possible superconducting states, we calculate the temperature dependence of bulk physical properties such as electronic specific heat and spin susceptibility and surface state. In the unconventional superconducting phase, either dispersive or flat Andreev bound states appear between point nodes, which leads to double peaks or a single peak in the surface density of states, respectively. As a result, possible superconducting states can be distinguished by combining bulk and surface measurements.

Crystalline color superconductivity

International Nuclear Information System (INIS)

Alford, Mark; Bowers, Jeffrey A.; Rajagopal, Krishna

2001-01-01

In any context in which color superconductivity arises in nature, it is likely to involve pairing between species of quarks with differing chemical potentials. For suitable values of the differences between chemical potentials, Cooper pairs with nonzero total momentum are favored, as was first realized by Larkin, Ovchinnikov, Fulde, and Ferrell (LOFF). Condensates of this sort spontaneously break translational and rotational invariance, leading to gaps which vary periodically in a crystalline pattern. Unlike the original LOFF state, these crystalline quark matter condensates include both spin-zero and spin-one Cooper pairs. We explore the range of parameters for which crystalline color superconductivity arises in the QCD phase diagram. If in some shell within the quark matter core of a neutron star (or within a strange quark star) the quark number densities are such that crystalline color superconductivity arises, rotational vortices may be pinned in this shell, making it a locus for glitch phenomena

Pair correlations in nuclei

International Nuclear Information System (INIS)

Shimizu, Yoshifumi

2009-01-01

Except for the closed shell nuclei, almost all nuclei are in the superconducting state at their ground states. This well-known pair correlation in nuclei causes various interesting phenomena. It is especially to be noted that the pair correlation becomes weak in the excited states of nuclei with high angular momentum, which leads to the pair phase transition to the normal state in the high spin limit. On the other hand, the pair correlation becomes stronger in the nuclei with lower nucleon density than in those with normal density. In the region of neutron halo or skin state of unstable nuclei, this phenomenon is expected to be further enhanced to be observed compared to the ground state of stable nuclei. An overview of those interesting aspects caused via the pair correlation is presented here in the sections titled 'pair correlations in ground states', pair correlations in high spin states' and 'pair correlations in unstable nuclei' focusing on the high spin state. (S. Funahashi)

Superconductivity

CERN Document Server

Ketterson, John B

2008-01-01

Conceived as the definitive reference in a classic and important field of modern physics, this extensive and comprehensive handbook systematically reviews the basic physics, theory and recent advances in the field of superconductivity. Leading researchers, including Nobel laureates, describe the state-of-the-art in conventional and unconventional superconductors at a particularly opportune time, as new experimental techniques and field-theoretical methods have emerged. In addition to full-coverage of novel materials and underlying mechanisms, the handbook reflects continued intense research into electron-phone based superconductivity. Considerable attention is devoted to high-Tc superconductivity, novel superconductivity, including triplet pairing in the ruthenates, novel superconductors, such as heavy-Fermion metals and organic materials, and also granular superconductors. What’s more, several contributions address superconductors with impurities and nanostructured superconductors. Important new results on...

Voltage-carrying states in superconducting microstrips

International Nuclear Information System (INIS)

Stuivinga, M.E.C.

1983-01-01

When the critical current is exceeded in a superconducting microstrip, voltage-carrying states with a resistance significantly below the normal state resistance can occur. Phase-slip centers (PSC) appear at about the critical temperature. These are successive local voltage units which manifest themselves as strip-like increments in voltage in the I-V characteristic. For temperatures off the critical temperature the PSC regime degenerates into a region of normal material, a so-called hot spot. These two phenomena, PSC and hot spots, form the subject of this thesis. To gain a better understanding of the phase-slip center process, an experiment was designed to measure local values of the quasi-particle and pair potential. The results of local potential and gap measurements at a PSC in aluminium are presented and discussed. Special attention is paid to pair-breaking interactions which can shorten the relaxation time. A non-linear differential equation is derived which describes the development of a PSC into a normal hot spot under the influence of Joule heating. It incorporates the temperature rise due to the dissipative processes occurring in the charge imbalance tails. Numerical solutions are presented for a set of parameters, including those for aluminium and tin. Subsequently, they are compared with experiments. (Auth.)

Mean-field approach to unconventional superconductivity

International Nuclear Information System (INIS)

Sacks, William; Mauger, Alain; Noat, Yves

2014-01-01

Highlights: • A model Hamiltonian for unconventional superconductivity (SC) is proposed. • The pseudogap (PG) state is described in terms of pair fluctuations. • SC coherence is restored by a new pair–pair interaction, which counteracts fluctuations. • Given the temperature dependence of the parameters, the SC to PG transition is examined. • The theory fits the ‘peak–dip–hump’ features of cuprate and pnictide excitation spectra. - Abstract: We propose a model that connects the quasiparticle spectral function of high-T c superconductors to the condensation energy. Given the evidence for pair correlations above T c , we consider a coarse-grain Hamiltonian of fluctuating pairs describing the incoherent pseudogap (PG) state, together with a novel pair–pair interaction term that restores long-range superconducting (SC) coherence below T c . A mean-field solution then leads to a self-consistent gap equation containing the new pair–pair coupling. The corresponding spectral function A(k,E) reveals the characteristic peak–dip–hump features of cuprates, now observed on iron pnictides (LiFeAs). The continuous transition from SC to PG states is discussed

Magnetic Fluctuations in Pair-Density-Wave Superconductors

Science.gov (United States)

Christensen, Morten H.; Jacobsen, Henrik; Maier, Thomas A.; Andersen, Brian M.

2016-04-01

Pair-density-wave superconductivity constitutes a novel electronic condensate proposed to be realized in certain unconventional superconductors. Establishing its potential existence is important for our fundamental understanding of superconductivity in correlated materials. Here we compute the dynamical magnetic susceptibility in the presence of a pair-density-wave ordered state and study its fingerprints on the spin-wave spectrum including the neutron resonance. In contrast to the standard case of d -wave superconductivity, we show that the pair-density-wave phase exhibits neither a spin gap nor a magnetic resonance peak, in agreement with a recent neutron scattering experiment on underdoped La1.905 Ba0.095 CuO4 [Z. Xu et al., Phys. Rev. Lett. 113, 177002 (2014)].

Arrays of Cooper pair boxes coupled to a superconducting reservoir: 'superradiance' and 'revival'

International Nuclear Information System (INIS)

Rodrigues, D A; Gyoerffy, B L; Spiller, T P

2004-01-01

We consider an array of l b Cooper pair boxes, each of which is coupled to a superconducting reservoir by a capacitive tunnel junction. We discuss two effects that probe not just the quantum nature of the islands, but also of the superconducting reservoir coupled to them. These are analogues to the well-known quantum optical effects 'superradiance' and 'revival'. When revival is extended to multiple systems, we find that 'entanglement revival' can also be observed. In order to study the above effects, we utilize a highly simplified model for these systems in which all the single-electron energy eigenvalues are set to be the same (the strong coupling limit), as are the charging energies of the Cooper pair boxes, allowing the whole system to be represented by two coupled quantum spins, one finite, which represents the array of boxes, and one representing the reservoir, which we consider in the limit of infinite size. Although this simplification is drastic, the model retains the main features necessary to capture the phenomena of interest. Given the progress in superconducting box experiments over recent years, it is possible that experiments to investigate both of these interesting quantum coherent phenomena could be performed in the foreseeable future

Theory of novel normal and superconducting states in doped oxide high-Tc superconductors

International Nuclear Information System (INIS)

Dzhumanov, S.

2001-10-01

A consistent and complete theory of the novel normal and superconducting (SC) states of doped high-T c superconductors (HTSC) is developed by combining the continuum model of carrier self-trapping, the tight-binding model and the novel Fermi-Bose-liquid (FBL) model. The ground-state energy of carriers in lightly doped HTSC is calculated within the continuum model and adiabatic approximation using the variational method. The destruction of the long-range antiferromagnetic (AF) order at low doping x≥ x cl ≅0.015, the formation of the in-gap states or bands and novel (bi)polaronic insulating phases at x c2 ≅0.06-0.08, and the new metal- insulator transition at x≅x c2 in HTSC are studied within the continuum model of impurity (defect) centers and large (bi)polarons by using the appropriate tight-binding approximations. It is found that the three-dimensional (3d) large (bi)polarons are formed at ε ∞ /ε 0 ≤0.1 and become itinerant when the (bi)polaronic insulator-to-(bi)polaronic metal transitions occur at x x c2 . We show that the novel pseudogapped metallic and SC states in HTSC are formed at x c2 ≤x≤x p ≅0.20-0.24. We demonstrate that the large polaronic and small BCS-like pairing pseudogaps opening in the excitation spectrum of underdoped (x c2 BCS =0.125), optimally doped (x BCS o ≅0.20) and overdoped (x>x o ) HTSC above T c are unrelated to superconductivity and they are responsible for the observed anomalous optical, transport, magnetic and other properties of these HTSC. We develop the original two-stage FBL model of novel superconductivity describing the combined novel BCS-like pairing scenario of fermions and true superfluid (SF) condensation scenario of composite bosons (i.e. bipolarons and cooperons) in any Fermi-systems, where the SF condensate gap Δ B and the BCS-like pairing pseudogap Δ F have different origins. The pair and single particle condensations of attracting 3d and two- dimensional (2d) composite bosons are responsible for

Magnetic flux periodicities and finite momentum pairing in unconventional superconductors

Energy Technology Data Exchange (ETDEWEB)

Loder, Florian

2009-12-22

This work contains a thorough study of the magnetic flux periodicity of loops of conventional and unconventional, especially d-wave, superconductors. Although already in 1961, several independent works showed that the flux period of a conventional superconducting loop is the superconducting flux quantum hc/2e, this question has never been investigated deeply for unconventional superconductors. And indeed, we show here that d-wave superconducting loops show a basic flux period of the normal flux quantum hc/e, a property originating from the nodal quasi-particle states. This doubling of the flux periodicity is best visible in the persistent current circulating in the loop, and it affects other properties of the superconductor such as the periodicity of d-wave Josephson junctions. In the second part of this work, the theory of electron pairing with finite center-of-mass momentum, necessary for the description of superconducting loops, is extended to systems in zero magnetic field. We show that even in the field free case, an unconventional pairing symmetry can lead to a superconducting ground state with finite-momentum electron pairs. Such a state has an inhomogeneous charge density and therefore is a basis for the description of coexistence of superconductivity and stripe order. (orig.)

Quantum fluctuations in the competition among spin glass, antiferromagnetism and local pairing superconductivity

International Nuclear Information System (INIS)

Magalhaes, S.G.; Zimmer, F.M.; Kipper, C.J.; Calegari, E.J.

2007-01-01

The competition among spin glass (SG), antiferromagnetism (AF) and local pairing superconductivity (PAIR) is studied in a two-sublattice fermionic Ising SG model with a local BCS pairing interaction in the presence of a transverse magnetic field Γ. The spins in different sublattices interact with Gaussian random couplings with an antiferromagnetic mean. The problem is formulated in a Grassmann path integral formalism. The static ansatz and the replica symmetry are used to obtain the half-filling thermodynamic potential. The results are shown in phase diagrams that exhibit a complex transition line separating the PAIR phase from the others. This line is second order at high temperature which ends in a tricritical point. The presence of Γ affects deeply the transition lines

Superconducting proximity in three-dimensional Dirac materials: Odd-frequency, pseudoscalar, pseudovector, and tensor-valued superconducting orders

Science.gov (United States)

Faraei, Zahra; Jafari, S. A.

2017-10-01

We find that a conventional s -wave superconductor in proximity to a three-dimensional Dirac material (3DDM), to all orders of perturbation in tunneling, induces a combination of s - and p -wave pairing only. We show that the Lorentz invariance of the superconducting pairing prevents the formation of Cooper pairs with higher orbital angular momenta in the 3DDM. This no-go theorem acquires stronger form when the probability of tunneling from the conventional superconductor to positive and negative energy states of 3DDM are equal. In this case, all the p -wave contribution except for the lowest order, identically vanish and hence we obtain an exact result for the induced p -wave superconductivity in 3DDM. Fierz decomposing the superconducting matrix we find that the temporal component of the vector superconducting order and the spatial components of the pseudovector order have odd-frequency pairing symmetry. We find that the latter is odd with respect to exchange of position and chirality of the electrons in the Cooper pair and is a spin-triplet, which is necessary for NMR detection of such an exotic pseudovector pairing. Moreover, we show that the tensorial order breaks into a polar vector and an axial vector and both of them have conventional pairing symmetry except for being a spin triplet. According to our study, for gapless 3DDM, the tensorial superconducting order will be the only order that is odd with respect to the chemical potential μ . Therefore we predict that a transverse p -n junction binds Majorana fermions. This effect can be used to control the neutral Majorana fermions with electric fields.

First tests of twisted-pair HTS 1 kA range cables for use in superconducting links

CERN Document Server

Ballarino, A; Hurte, J; Sitko, M; Willering, G

2011-01-01

The requirement at CERN for 1 kA range High Temperature Superconducting (HTS) cables optimized for long electrical transfer has led to the design and assembly of a novel type of cable that can be made from pre-reacted MgB2, Bi-2223 or YBCO tapes. The cable consists of an assembly of twisted pairs, each of which is made from three superconducting tapes with the required copper stabilizer. The twisted pair cable is designed to transfer a DC current of ± 600 A in helium gas environment. The paper reports on the results of the electrical tests performed on twisted-pair cables of identical structure and made from commercially available MgB2, Bi-2223 and YBCO tapes. The twist pitch of the cables is adapted to match the mechanical properties of the different superconductors. Critical current tests were performed at both liquid helium and liquid nitrogen temperature. The electrical performance of several cables made from different conductors is reported and compared.

Quantum Critical Quasiparticle Scattering within the Superconducting State of CeCoIn_{5}.

Science.gov (United States)

Paglione, Johnpierre; Tanatar, M A; Reid, J-Ph; Shakeripour, H; Petrovic, C; Taillefer, Louis

2016-07-01

The thermal conductivity κ of the heavy-fermion metal CeCoIn_{5} was measured in the normal and superconducting states as a function of temperature T and magnetic field H, for a current and field parallel to the [100] direction. Inside the superconducting state, when the field is lower than the upper critical field H_{c2}, κ/T is found to increase as T→0, just as in a metal and in contrast to the behavior of all known superconductors. This is due to unpaired electrons on part of the Fermi surface, which dominate the transport above a certain field. The evolution of κ/T with field reveals that the electron-electron scattering (or transport mass m^{⋆}) of those unpaired electrons diverges as H→H_{c2} from below, in the same way that it does in the normal state as H→H_{c2} from above. This shows that the unpaired electrons sense the proximity of the field-tuned quantum critical point of CeCoIn_{5} at H^{⋆}=H_{c2} even from inside the superconducting state. The fact that the quantum critical scattering of the unpaired electrons is much weaker than the average scattering of all electrons in the normal state reveals a k-space correlation between the strength of pairing and the strength of scattering, pointing to a common mechanism, presumably antiferromagnetic fluctuations.

Similarity in the superconducting properties of chalcogenides, cuprate oxides and fullerides

International Nuclear Information System (INIS)

Tsendin, K.D.; Popov, B.P.; Denisov, D.V.

2004-01-01

The idea of Anderson pairs has been put forward for explanation of many extraordinary properties of chalcogenides glassy semiconductors. Recent decades made obvious that these pairs localized on the centers with negative effective correlation energy (negative-U centers) really exist in chalcogenides. If the concentration of negative-U centers is enough to create the pair band states, this can lead to superconductivity because Anderson pairs are Bose particles. In the present paper we show that several puzzling superconductivity properties of chalcogenides, high-temperature cuprate superconductors and fullerides are similar for these three groups of materials and can be naturally explained in the frame of negative-U centers model of superconductivity

Electronic pairing mechanism due to band modification with increasing pair number

International Nuclear Information System (INIS)

Mizia, J.

1995-01-01

It is shown that a shift of an electron band with electron occupation number n, which is changing during the transition to the superconducting state, can lower the total energy of the system. In fact it will bring a negative contribution to the pairing potential, which is proportional to the product of the electron band shift with occupation number and the charge transfer during the transition to the superconducting state. The shift of the electron band comes from the change of stresses and the change of correlation effects in the CuO 2 plane with n, that in turn is caused by the changing oxygen concentration. This model explains the phenomenological success of Hirsch's model, which gives no explanation how the band shift in energy can give rise to superconductivity. (orig.)

Theories of superconductivity (a few remarks)

International Nuclear Information System (INIS)

Ginzburg, V.L.

1992-01-01

The early history in the development of superconductivity. Idea of pairing, Schafroth and BCS types of theories. Some remarks on present state of the microscopical theory of high-temperature superconductors (HTSC). Mean field macroscopic theory of superconductivity and its specific features in HTSC. About generalized macroscopic theory applicable in critical region. Concluding remarks. (orig.)

Dimensionality Driven Enhancement of Ferromagnetic Superconductivity in URhGe

Science.gov (United States)

Braithwaite, Daniel; Aoki, Dai; Brison, Jean-Pascal; Flouquet, Jacques; Knebel, Georg; Nakamura, Ai; Pourret, Alexandre

2018-01-01

In most unconventional superconductors, like the high-Tc cuprates, iron pnictides, or heavy-fermion systems, superconductivity emerges in the proximity of an electronic instability. Identifying unambiguously the pairing mechanism remains nevertheless an enormous challenge. Among these systems, the orthorhombic uranium ferromagnetic superconductors have a unique position, notably because magnetic fields couple directly to ferromagnetic order, leading to the fascinating discovery of the reemergence of superconductivity in URhGe at a high field. Here we show that uniaxial stress is a remarkable tool allowing the fine-tuning of the pairing strength. With a relatively small stress, the superconducting phase diagram is spectacularly modified, with a merging of the low- and high-field superconducting states and a significant enhancement of the superconductivity. The superconducting critical temperature increases both at zero field and under a field, reaching 1 K, more than twice higher than at ambient pressure. This enhancement of superconductivity is shown to be directly related to a change of the magnetic dimensionality detected from an increase of the transverse magnetic susceptibility: In addition to the Ising-type longitudinal ferromagnetic fluctuations, transverse magnetic fluctuations also play an important role in the superconducting pairing.

Surface Andreev Bound States and Odd-Frequency Pairing in Topological Superconductor Junctions

Science.gov (United States)

Tanaka, Yukio; Tamura, Shun

2018-04-01

In this review, we summarize the achievement of the physics of surface Andreev bound states (SABS) up to now. The route of this activity has started from the physics of SABS of unconventional superconductors where the pair potential has a sign change on the Fermi surface. It has been established that SABS can be regarded as a topological edge state with topological invariant defined in the bulk Hamiltonian. On the other hand, SABS accompanies odd-frequency pairing like spin-triplet s-wave or spin-singlet p-wave. In a spin-triplet superconductor junction, induced odd-frequency pairing can penetrate into a diffusive normal metal (DN) attached to the superconductor. It causes so called anomalous proximity effect where the local density of states of quasiparticle in DN has a zero energy peak. When bulk pairing symmetry is spin-triplet px-wave, the anomalous proximity effect becomes prominent and the zero bias voltage conductance is always quantized independent of the resistance in DN and interface. Finally, we show that the present anomalous proximity effect is realized in an artificial topological superconducting system, where a nanowire with spin-orbit coupling and Zeeman field is put on the conventional spin-singlet s-wave superconductor.

Changes of superconducting interaction in interfaces

International Nuclear Information System (INIS)

Halbritter, J.

1976-01-01

The leakage of conduction electrons from metals into dielectric or semiconducting coatings yields changes in electron phonon coupling and hybridization with localized states in the coating. The changed electron-phonon coupling explains the observed strengthened superconducting interaction with some monolayer thick coating. The hybridization with localized states, i.e. resonance scattering, yields pair weakening and hence a monotonic depression of superconductivity with coating thickness in agreement with experiments. The latter effect explains quantitatively the Tsub(c) and Δ depression (Δ/kTsub(c) approximately equal to const) and a decrease in the Maki-Thompson-fluctuation term observed with thin superconducting films. (author)

Angle-resolved photoemission studies of the superconducting gap symmetry in Fe-based superconductors

Directory of Open Access Journals (Sweden)

Y.-B. Huang

2012-12-01

Full Text Available The superconducting gap is the fundamental parameter that characterizes the superconducting state, and its symmetry is a direct consequence of the mechanism responsible for Cooper pairing. Here we discuss about angle-resolved photoemission spectroscopy measurements of the superconducting gap in the Fe-based high-temperature superconductors. We show that the superconducting gap is Fermi surface dependent and nodeless with small anisotropy, or more precisely, a function of the momentum location in the Brillouin zone. We show that while this observation seems inconsistent with weak coupling approaches for superconductivity in these materials, it is well supported by strong coupling models and global superconducting gaps. We also suggest that a smaller lifetime of the superconducting Cooper pairs induced by the momentum dependent interband scattering inherent to these materials could affect the residual density of states at low energies, which is critical for a proper evaluation of the superconducting gap.

Cooper Pairs in Insulators?

International Nuclear Information System (INIS)

Valles, James

2008-01-01

Nearly 50 years elapsed between the discovery of superconductivity and the emergence of the microscopic theory describing this zero resistance state. The explanation required a novel phase of matter in which conduction electrons joined in weakly bound pairs and condensed with other pairs into a single quantum state. Surprisingly, this Cooper pair formation has also been invoked to account for recently uncovered high-resistance or insulating phases of matter. To address this possibility, we have used nanotechnology to create an insulating system that we can probe directly for Cooper pairs. I will present the evidence that Cooper pairs exist and dominate the electrical transport in these insulators and I will discuss how these findings provide new insight into superconductor to insulator quantum phase transitions.

Quasiparticles in the superconducting state of high-Tc metals

International Nuclear Information System (INIS)

Amusia, M.Ya.; Shaginyan, V.R.

2003-01-01

The behavior of quasiparticles in the superconducting state of high-T c metals within the framework of the theory of superconducting state based on the fermion condensation quantum phase transition is considered. It is shown that the behavior coincides with the behavior of Bogoliubov quasiparticles, whereas the maximum value of the superconducting gap and other exotic properties are determined by the presence of the fermion condensate. If at low temperatures the normal state is recovered by the application of a magnetic field suppressing the superconductivity, the induced state can be viewed as Landau-Fermi liquid. These observations are in good agreement with recent experimental facts [ru

Dynamical Cooper pairing in non-equilibrium electron-phonon systems

Energy Technology Data Exchange (ETDEWEB)

Knap, Michael [Technical University of Munich (Germany); Harvard University (United States); Babadi, Mehrtash; Refael, Gil [Caltech (United States); Martin, Ivar [Argonne National Laboratory (United States); Demler, Eugene [Harvard University (United States)

2016-07-01

Ultrafast laser pulses have been used to manipulate complex quantum materials and to induce dynamical phase transitions. One of the most striking examples is the transient enhancement of superconductivity in several classes of materials upon irradiating them with high intensity pulses of terahertz light. Motivated by these experiments we analyze the Cooper pairing instabilities in non-equilibrium electron-phonon systems. We demonstrate that the light induced non-equilibrium state of phonons results in a simultaneous increase of the superconducting coupling constant and the electron scattering. We analyze the competition between these effects and show that in a broad range of parameters the dynamic enhancement of Cooper pair formation dominates over the increase in the scattering rate. This opens the possibility of transient light induced superconductivity at temperatures that are considerably higher than the equilibrium transition temperatures. Our results pave new pathways for engineering high-temperature light-induced superconducting states.

Odd-frequency pairing in superconducting heterostructures .

Science.gov (United States)

Golubov, A. A.; Tanaka, Y.; Yokoyama, T.; Asano, Y.

2007-03-01

We present a general theory of the proximity effect in junctions between unconventional superconductors and diffusive normal metals (DN) or ferromagnets (DF). We consider all possible symmetry classes in a superconductor allowed by the Pauli principle: even-frequency spin-singlet even-parity state, even-frequency spin-triplet odd-parity state, odd-frequency spin-triplet even-parity state and odd-frequency spin-singlet odd-parity state. For each of the above states, symmetry and spectral properties of the induced pair amplitude in the DN (DF) are determined. The cases of junctions with spin-singlet s- and d-wave superconductors and spin-triplet p-wave superconductors are adressed in detail. We discuss the interplay between the proximity effect and midgap Andreev bound states arising at interfaces in unconventional (d- or p-wave) junctions. The most striking property is the odd-frequency symmetry of the pairing amplitude induced in DN (DF) in contacts with p-wave superconductors. This leads to zero-energy singularity in the density of states and to anomalous screening of an external magnetic field. Peculiarities of Josephson effect in d- or p-wave junctions are discussed. Experiments are suggested to detect an order parameter symmetry using heterostructures with unconventional superconductors.

Fulde-Ferrell state in superconducting core/shell nanowires: role of the orbital effect

Science.gov (United States)

Mika, Marek; Wójcik, Paweł

2017-11-01

The orbital effect on the Fulde-Ferrell (FF) phase is investigated in superconducting core/shell nanowires subjected to the axial magnetic field. Confinement in the radial direction results in quantization of the electron motion with energies determined by the radial j and orbital m quantum numbers. In the external magnetic field, the twofold degeneracy with respect to the orbital magnetic quantum number m is lifted which leads to the Fermi wave vector mismatch between the paired electrons, (k, j, m, \\uparrow) ≤ftrightarrow (-k, j, -m, \\downarrow) . This mismatch is transferred to the nonzero total momentum of the Cooper pairs, which results in a formation of the FF phase occurring sequentially with increasing magnetic field. By changing the nanowire radius R and the superconducting shell thickness d, we discuss the role of the orbital effect in the FF phase formation in both the nanowire-like (R/d \\ll 1 ) and nanofilm-like (R/d \\gg 1 ) regime. We have found that the irregular pattern of the FF phase which appears for the case of the nanowire-like regime, for the nanofilm-like geometry evolves towards the regular distribution in which the FF phase stability regions emerge periodically between the BCS states. The transition between these two different phase diagrams is explained as resulting from the orbital effect and the multigap character of superconductivity in the core/shell nanowires.

Superconductivity in LiFeAs probed with quasiparticle interference

Energy Technology Data Exchange (ETDEWEB)

Sun, Zhixiang; Nag, Pranab Kumar; Baumann, Danny; Kappenberger, Rhea [Leibniz Institute for Solid State and Materials Research Dresden, IFW Dresden (Germany); Wurmehl, Sabine [Leibniz Institute for Solid State and Materials Research Dresden, IFW Dresden (Germany); Institute for Solid State Physics, TU Dresden (Germany); Buechner, Bernd [Leibniz Institute for Solid State and Materials Research Dresden, IFW Dresden (Germany); Institute for Solid State Physics, TU Dresden (Germany); Center for Transport and Devices, TU Dresden (Germany); Hess, Christian [Leibniz Institute for Solid State and Materials Research Dresden, IFW Dresden (Germany); Center for Transport and Devices, TU Dresden (Germany)

2016-07-01

In spite of many theoretical and experimental efforts on studying the superconductivity of iron-based high temperature superconductors, the puzzle about LiFeAs's superconducting mechanism and pairing symmetry are still not clear. Here we want to present our low temperature scanning tunneling microscopy results on probing the superconductivity of LiFeAs. By taking conductance spectroscopic maps for both the superconducting state and normal state, we identify the scatterings due to the electron and hole bands close to the Fermi level. We observe a strong indication that the superconducting behavior in the hole bands are important for the formation of superconductivity in LiFeAs. Our results may also shine light on understanding the superconductivity in other iron pnictide superconductors.

Hole superconductivity

International Nuclear Information System (INIS)

Hirsch, J.E.; Marsiglio, F.

1989-01-01

The authors review recent work on a mechanism proposed to explain high T c superconductivity in oxides as well as superconductivity of conventional materials. It is based on pairing of hole carriers through their direct Coulomb interaction, and gives rise to superconductivity because of the momentum dependence of the repulsive interaction in the solid state environment. In the regime of parameters appropriate for high T c oxides this mechanism leads to characteristic signatures that should be experimentally verifiable. In the regime of conventional superconductors most of these signatures become unobservable, but the characteristic dependence of T c on band filling survives. New features discussed her include the demonstration that superconductivity can result from repulsive interactions even if the gap function does not change sign and the inclusion of a self-energy correction to the hole propagator that reduces the range of band filling where T c is not zero

Interplay between superconductivity and magnetism in iron-based superconductors

Energy Technology Data Exchange (ETDEWEB)

Chubukov, Andrey V [University of Wisconsin

2015-06-10

This proposal is for theoretical work on strongly correlated electron systems, which are at the center of experimental and theoretical activities in condensed-matter physics. The interest to this field is driven fascinating variety of observed effects, universality of underlying theoretical ideas, and practical applications. I propose to do research on Iron-based superconductors (FeSCs), which currently attract high attention in the physics community. My goal is to understand superconductivity and magnetism in these materials at various dopings, the interplay between the two, and the physics in the phase in which magnetism and superconductivity co-exist. A related goal is to understand the origin of the observed pseudogap-like behavior in the normal state. My research explores the idea that superconductivity is of electronic origin and is caused by the exchange of spin-fluctuations, enhanced due to close proximity to antiferromagnetism. The multi-orbital/multi-band nature of FeSCs opens routes for qualitatively new superconducting states, particularly the ones which break time-reversal symmetry. By all accounts, the coupling in pnictdes is below the threshold for Mott physics and I intend to analyze these systems within the itinerant approach. My plan is to do research in two stages. I first plan to address several problems within weak-coupling approach. Among them: (i) what sets stripe magnetic order at small doping, (ii) is there a preemptive instability into a spin-nematic state, and how stripe order affects fermions; (iii) is there a co-existence between magnetism and superconductivity and what are the system properties in the co-existence state; (iv) how superconductivity emerges despite strong Coulomb repulsion and can the gap be s-wave but with nodes along electron FSs, (v) are there complex superconducting states, like s+id, which break time reversal symmetry. My second goal is to go beyond weak coupling and derive spin-mediated, dynamic interaction between

Structural change of cooper pairs in color superconductivity. Crossover from weak coupling to strong coupling

Energy Technology Data Exchange (ETDEWEB)

Abuki, Hiroaki; Hatsuda, Tetsuo [Tokyo Univ., Dept. of Physics, Tokyo (Japan); Itakura, Kazunori [Brookhaven National Laboratory, RIKEN BNL Research Center, Upton, NY (United States)

2002-09-01

The two-flavor color superconductivity is studied over a wide range of baryon density with a single model. We pay a special attention to the spatial-momentum dependence of the gap and to the spatial-structure of Cooper pairs. At extremely high baryon density ({approx}O(10{sup 10} {rho}{sub 0}) with {rho}{sub 0} being the normal nuclear matter density), our model becomes equivalent to the usual perturbative QCD treatment and the gap is shown to have a sharp peak near the Fermi surface due to the weak-coupling nature of QCD. On the other hand, the gap is a smooth function of the momentum at lower densities ({approx}O(10{sup 10} {rho}{sub 0})) due to strong color magnetic and electric interactions. To study the structural change of Cooper pairs from high density to lower density, quark correlation in the color superconductor is studied both in the momentum space and in the coordinate space. The size of the Cooper pair is shown to become comparable to the averaged inter-quark distance at low densities. Also, effects of the momentum-dependent running coupling and the antiquark pairing, which are both small at high density, are shown to be non-negligible at low densities. These features are highly contrasted to the standard BCS superconductivity in metals. (author)

Multi-pair states in electron–positron pair creation

Directory of Open Access Journals (Sweden)

Anton Wöllert

2016-09-01

Full Text Available Ultra strong electromagnetic fields can lead to spontaneous creation of single or multiple electron–positron pairs. A quantum field theoretical treatment of the pair creation process combined with numerical methods provides a description of the fermionic quantum field state, from which all observables of the multiple electron–positron pairs can be inferred. This allows to study the complex multi-particle dynamics of electron–positron pair creation in-depth, including multi-pair statistics as well as momentum distributions and spin. To illustrate the potential benefit of this approach, it is applied to the intermediate regime of pair creation between nonperturbative Schwinger pair creation and perturbative multiphoton pair creation where the creation of multi-pair states becomes nonnegligible but cascades do not yet set in. Furthermore, it is demonstrated how spin and helicity of the created electrons and positrons are affected by the polarization of the counterpropagating laser fields, which induce the creation of electron–positron pairs.

Multi-pair states in electron–positron pair creation

Energy Technology Data Exchange (ETDEWEB)

Wöllert, Anton, E-mail: woellert@mpi-hd.mpg.de; Bauke, Heiko, E-mail: heiko.bauke@mpi-hd.mpg.de; Keitel, Christoph H.

2016-09-10

Ultra strong electromagnetic fields can lead to spontaneous creation of single or multiple electron–positron pairs. A quantum field theoretical treatment of the pair creation process combined with numerical methods provides a description of the fermionic quantum field state, from which all observables of the multiple electron–positron pairs can be inferred. This allows to study the complex multi-particle dynamics of electron–positron pair creation in-depth, including multi-pair statistics as well as momentum distributions and spin. To illustrate the potential benefit of this approach, it is applied to the intermediate regime of pair creation between nonperturbative Schwinger pair creation and perturbative multiphoton pair creation where the creation of multi-pair states becomes nonnegligible but cascades do not yet set in. Furthermore, it is demonstrated how spin and helicity of the created electrons and positrons are affected by the polarization of the counterpropagating laser fields, which induce the creation of electron–positron pairs.

Multi-pair states in electron–positron pair creation

International Nuclear Information System (INIS)

Wöllert, Anton; Bauke, Heiko; Keitel, Christoph H.

2016-01-01

Ultra strong electromagnetic fields can lead to spontaneous creation of single or multiple electron–positron pairs. A quantum field theoretical treatment of the pair creation process combined with numerical methods provides a description of the fermionic quantum field state, from which all observables of the multiple electron–positron pairs can be inferred. This allows to study the complex multi-particle dynamics of electron–positron pair creation in-depth, including multi-pair statistics as well as momentum distributions and spin. To illustrate the potential benefit of this approach, it is applied to the intermediate regime of pair creation between nonperturbative Schwinger pair creation and perturbative multiphoton pair creation where the creation of multi-pair states becomes nonnegligible but cascades do not yet set in. Furthermore, it is demonstrated how spin and helicity of the created electrons and positrons are affected by the polarization of the counterpropagating laser fields, which induce the creation of electron–positron pairs.

A common thread in unconventional superconductivity. The functional renormalization group in multi-band systems

International Nuclear Information System (INIS)

Platt, Christian

2012-01-01

The superconducting properties of complex materials like the recently discovered iron-pnictides or strontium-ruthenate are often governed by multi-orbital effects. In order to unravel the superconductivity of those materials, we develop a multi-orbital implementation of the functional renormalization group and study the pairing states of several characteristic material systems. Starting with the iron-pnictides, we find competing spin-fluctuation channels that become attractive if the superconducting gap changes sign between the nested portions of the Fermi surface. Depending on material details like doping or pnictogen height, these spin fluctuations then give rise to s ± -wave pairing with or without gap nodes and, in some cases, also change the symmetry to d-wave. Near the transition from nodal s ± -wave to d-wave pairing, we predict the occurrence of a time-reversal symmetry-broken (s+id)-pairing state which avoids gap nodes and is therefore energetically favored. We further study the electronic instabilities of doped graphene, another fascinating material which has recently become accessible and which can effectively be regarded as multi-orbital system. Here, the hexagonal lattice structure assures the degeneracy of two d-wave pairing channels, and the system then realizes a chiral (d+id)-pairing state in a wide doping range around van-Hove filling. In addition, we also find spin-triplet pairing as well as an exotic spin-density wave phase which both become leading if the long-ranged hopping or interaction parameters are slightly modified, for example, by choosing different substrate materials. Finally, we consider the superconducting state of strontium-ruthenate, a possible candidate for chiral spin-triplet pairing with fascinating properties like the existence of half-quantum vortices obeying non-Abelian statistics. Using a microscopic three orbital description including spin-orbit coupling, we demonstrate that ferromagnetic fluctuations are still

Experimental evidence for s-wave pairing symmetry in superconducting Cu(x)Bi2Se3 single crystals using a scanning tunneling microscope.

Science.gov (United States)

Levy, Niv; Zhang, Tong; Ha, Jeonghoon; Sharifi, Fred; Talin, A Alec; Kuk, Young; Stroscio, Joseph A

2013-03-15

Topological superconductors represent a newly predicted phase of matter that is topologically distinct from conventional superconducting condensates of Cooper pairs. As a manifestation of their topological character, topological superconductors support solid-state realizations of Majorana fermions at their boundaries. The recently discovered superconductor Cu(x)Bi(2)Se(3) has been theoretically proposed as an odd-parity superconductor in the time-reversal-invariant topological superconductor class, and point-contact spectroscopy measurements have reported the observation of zero-bias conductance peaks corresponding to Majorana states in this material. Here we report scanning tunneling microscopy measurements of the superconducting energy gap in Cu(x)Bi(2)Se(3) as a function of spatial position and applied magnetic field. The tunneling spectrum shows that the density of states at the Fermi level is fully gapped without any in-gap states. The spectrum is well described by the Bardeen-Cooper-Schrieffer theory with a momentum independent order parameter, which suggests that Cu(x)Bi(2)Se(3) is a classical s-wave superconductor contrary to previous expectations and measurements.

Drive the Dirac electrons into Cooper pairs in SrxBi2Se3

Science.gov (United States)

Du, Guan; Shao, Jifeng; Yang, Xiong; Du, Zengyi; Fang, Delong; Wang, Jinghui; Ran, Kejing; Wen, Jinsheng; Zhang, Changjin; Yang, Huan; Zhang, Yuheng; Wen, Hai-Hu

2017-01-01

Topological superconductors are a very interesting and frontier topic in condensed matter physics. Despite the tremendous efforts in exploring topological superconductivity, its presence is however still under heavy debate. The Dirac electrons have been proven to exist on the surface of a topological insulator. It remains unclear whether and how the Dirac electrons fall into Cooper pairing in an intrinsic superconductor with the topological surface states. Here we show the systematic study of scanning tunnelling microscope/spectroscopy on the possible topological superconductor SrxBi2Se3. We first demonstrate that only the intercalated Sr atoms can induce superconductivity. Then we show the full superconducting gaps without any in-gap density of states as expected theoretically for a bulk topological superconductor. Finally, we find that the surface Dirac electrons will simultaneously condense into the superconducting state within the superconducting gap. This vividly demonstrates how the surface Dirac electrons are driven into Cooper pairs. PMID:28198378

Superconductivity, Antiferromagnetism, and Kinetic Correlation in Strongly Correlated Electron Systems

Directory of Open Access Journals (Sweden)

Takashi Yanagisawa

2015-01-01

Full Text Available We investigate the ground state of two-dimensional Hubbard model on the basis of the variational Monte Carlo method. We use wave functions that include kinetic correlation and doublon-holon correlation beyond the Gutzwiller ansatz. It is still not clear whether the Hubbard model accounts for high-temperature superconductivity. The antiferromagnetic correlation plays a key role in the study of pairing mechanism because the superconductive phase exists usually close to the antiferromagnetic phase. We investigate the stability of the antiferromagnetic state when holes are doped as a function of the Coulomb repulsion U. We show that the antiferromagnetic correlation is suppressed as U is increased exceeding the bandwidth. High-temperature superconductivity is possible in this region with enhanced antiferromagnetic spin fluctuation and pairing interaction.

Topological phase diagram of superconducting carbon nanotubes

Energy Technology Data Exchange (ETDEWEB)

Milz, Lars; Marganska-Lyzniak, Magdalena; Grifoni, Milena [Institut I - Theoretische Physik Universitaet Regensburg (Germany)

2016-07-01

The topological superconducting phase diagram of superconducting carbon nanotubes is discussed. Under the assumption of a short-ranged pairing potential, there are two spin-singlet states: an s-wave and an exotic p + ip-wave that are possible because of the special structure of the honeycomb lattice. The consequences for the possible presence of Majorana edge states in carbon nanotubes are addressed. In particular, regions in the magnetic field-chemical potential plane possibly hosting localized Majorana modes are discussed.

A Pole Pair Segment of a 2-MW High-Temperature Superconducting Wind Turbine Generator

DEFF Research Database (Denmark)

Song, Xiaowei (Andy); Mijatovic, Nenad; Kellers, Jürgen

2017-01-01

A 2-MW high-temperature superconducting (HTS) generator with 24 pole pairs has been designed for the wind turbine application. In order to identify potential challenges and obtain practical knowledge prior to production, a full-size stationary experimental setup, which is one pole pair segment...... and the setup in terms of the flux density, the operating condition of the HTS winding, and the force-generation capability. Finite element (FE) software MagNet is used to carry out numerical simulations. The findings show that the HTS winding in the setup is a good surrogate for these that would be used...

A Pole Pair Segment of a 2 MW High Temperature Superconducting Wind Turbine Generator

DEFF Research Database (Denmark)

Song, Xiaowei (Andy); Mijatovic, Nenad; Kellers, Jürgen

2016-01-01

A 2 MW high temperature superconducting (HTS) generator with 24 pole pairs has been designed for the wind turbine application. In order to identify potential challenges and obtain practical knowledge prior to production, a fullsize stationary experimental set-up, which is one pole pair segment...... generator and the set-up in terms of the flux density, the operating condition of the HTS winding, and the force-generation capability. Finite element (FE) software MagNet is used to carry out numerical simulations. The findings show that the HTS winding in the set-up is a good surrogate...

Superconductivity in narrow-band systems with local nonretarded attractive interactions

International Nuclear Information System (INIS)

Micnas, R.; Ranninger, J.; Robaszkiewicz, S.

1990-01-01

In narrow-band systems electrons can interact with each other via a short-range nonretarded attractive potential. The origin of such an effective local attraction can be polaronic or it can be due to a coupling between electrons and excitons or plasmons. It can also result from purely chemical (electronic) mechanisms, especially in compounds with elements favoring disproportionation of valent states. These mechanisms are discussed and an exhaustive list of materials in which such local electron pairing occurs is given. The authors review the thermodynamic and electromagnetic properties of such systems in several limiting scenarios: (i) Systems with on-site pairing which can be described by the extended negative-U Hubbard model. The strong-attraction limit of this model, at which it reduces to a system of tightly bound electron pairs (bipolarons) on a lattice, is extensively discussed. These electron pairs behaving as hard-core charged bosons can exhibit a superconducting state analogous to that of superfluid 4 He II. The changeover from weak-attraction BCS-like superconductivity to the superfluidity of charged hard-core bosons is examined. (ii) Systems with intersite pairing described by an extended Hubbard model with U>0 and nearest-neighbor attraction and/or nearest-neighbor spin exchange as well as correlated hopping. (iii) A mixture of local pairs and itinerant electrons interacting via a charge-exchange mechanism giving rise to a mutually induced superconductivity in both subsystems. The authors discuss to what extent the picture of local pairing, and in particular superfluidity of hard-core charged bosons on a lattice, can be an explanation for the superconducting and normal-state properties of the high-T c oxides: doped BaBiO 3 and the cuprates

Pressure effect on the superconducting and the normal state of β -B i2Pd

Science.gov (United States)

Pristáš, G.; Orendáč, Mat.; Gabáni, S.; Kačmarčík, J.; Gažo, E.; Pribulová, Z.; Correa-Orellana, A.; Herrera, E.; Suderow, H.; Samuely, P.

2018-04-01

The pressure effect up to 24.0 kbar on superconducting and normal-state properties of β -B i2Pd single crystal (Tc≈4.98 K at ambient pressure) has been investigated by measurements of the electrical resistivity. In addition, we have performed the heat capacity measurements in the temperature range 0.7-300 K at ambient pressure. The recent calculations of electronic density of states, electron-phonon interaction spectral function, and phonon density of states of β -B i2Pd [Zheng and Margine, Phys. Rev. B 95, 014512 (2017), 10.1103/PhysRevB.95.014512], are used to fit the resistivity and the heat capacity data. In the superconducting state we have focused on the influence of pressure on the superconducting transition temperature Tc and upper critical field Hc 2 and a negative effect with d Tc/d p =-0.025 K /kbar and d Hc 2/d p =-8 mT /kbar is found. A simplified Bloch-Grüneisen model was used to analyze the pressure effect on the temperature dependence of the normal-state resistivity. The obtained results point to a decrease of the electron-phonon coupling parameter λ and to a shift of phonon frequencies to higher values with pressure. Moreover, the temperature dependence of the normal-state resistivity follows a T2 dependence above Tc up to about 25 K. Together with the enhanced value of Sommerfeld coefficient γ =13.23 mJ mo l-1K-2 these results point to a certain role of the electron-electron interaction in the superconducting pairing mechanism in β -B i2Pd .

Pressure controlled transition into a self-induced topological superconducting surface state

KAUST Repository

Zhu, Zhiyong; Cheng, Yingchun; Schwingenschlö gl, Udo

2014-01-01

Ab-initio calculations show a pressure induced trivial-nontrivial-trivial topological phase transition in the normal state of 1T-TiSe2. The pressure range in which the nontrivial phase emerges overlaps with that of the superconducting ground state. Thus, topological superconductivity can be induced in protected surface states by the proximity effect of superconducting bulk states. This kind of self-induced topological surface superconductivity is promising for a realization of Majorana fermions due to the absence of lattice and chemical potential mismatches. For appropriate electron doping, the formation of the topological superconducting surface state in 1T-TiSe 2 becomes accessible to experiments as it can be controlled by pressure.

Pressure controlled transition into a self-induced topological superconducting surface state

KAUST Repository

Zhu, Zhiyong

2014-02-07

Ab-initio calculations show a pressure induced trivial-nontrivial-trivial topological phase transition in the normal state of 1T-TiSe2. The pressure range in which the nontrivial phase emerges overlaps with that of the superconducting ground state. Thus, topological superconductivity can be induced in protected surface states by the proximity effect of superconducting bulk states. This kind of self-induced topological surface superconductivity is promising for a realization of Majorana fermions due to the absence of lattice and chemical potential mismatches. For appropriate electron doping, the formation of the topological superconducting surface state in 1T-TiSe 2 becomes accessible to experiments as it can be controlled by pressure.

Superconducting gap anomaly in heavy fermion systems

International Nuclear Information System (INIS)

Rout, G.C.; Ojha, M.S.; Behera, S.N.

2008-01-01

The heavy fermion system (HFS) is described by the periodic Anderson model (PAM), treating the Coulomb correlation between the f-electrons in the mean-field Hartree-Fock approximation. Superconductivity is introduced by a BCS-type pairing term among the conduction electrons. Within this approximation the equation for the superconducting gap is derived, which depends on the effective position of the energy level of the f-electrons relative to the Fermi level. The latter in turn depends on the occupation probability n f of the f-electrons. The gap equation is solved self-consistently with the equation for n f ; and their temperature dependences are studied for different positions of the bare f-electron energy level, with respect to the Fermi level. The dependence of the superconducting gap on the hybridization leads to a re-entrant behaviour with increasing strength. The induced pairing between the f-electrons and the pairing of mixed conduction and f-electrons due to hybridization are also determined. The temperature dependence of the hybridization parameter, which characterizes the number of electrons with mixed character and represents the number of heavy electrons is studied. This number is shown to be small. The quasi-particle density of states (DOS) shows the existence of a pseudo-gap due to superconductivity and the signature of a hybridization gap at the Fermi level. For the choice of the model parameters, the DOS shows that the HFS is a metal and undergoes a transition to the gap-less superconducting state. (author)

Detecting nonlocal Cooper pair entanglement by optical Bell inequality violation

OpenAIRE

Nigg, Simon E.; Tiwari, Rakesh P.; Walter, Stefan; Schmidt, Thomas L.

2014-01-01

Based on the Bardeen Cooper Schrieffer (BCS) theory of superconductivity, the coherent splitting of Cooper pairs from a superconductor to two spatially separated quantum dots has been predicted to generate nonlocal pairs of entangled electrons. In order to test this hypothesis, we propose a scheme to transfer the spin state of a split Cooper pair onto the polarization state of a pair of optical photons. We show that the produced photon pairs can be used to violate a Bell inequality, unambiguo...

Theory of superconductivity with non-Hermitian and parity-time reversal symmetric Cooper pairing symmetry

Science.gov (United States)

Ghatak, Ananya; Das, Tanmoy

2018-01-01

Recently developed parity (P ) and time-reversal (T ) symmetric non-Hermitian systems govern a rich variety of new and characteristically distinct physical properties, which may or may not have a direct analog in their Hermitian counterparts. We study here a non-Hermitian, PT -symmetric superconducting Hamiltonian that possesses a real quasiparticle spectrum in the PT -unbroken region of the Brillouin zone. Within a single-band mean-field theory, we find that real quasiparticle energies are possible when the superconducting order parameter itself is either Hermitian or anti-Hermitian. Within the corresponding Bardeen-Cooper-Schrieffer (BCS) theory, we find that several properties are characteristically distinct and novel in the non-Hermitian pairing case than its Hermitian counterpart. One of our significant findings is that while a Hermitian superconductor gives a second-order phase transition, the non-Hermitian one produces a robust first-order phase transition. The corresponding thermodynamic properties and the Meissner effect are also modified accordingly. Finally, we discuss how such a PT -symmetric pairing can emerge from an antisymmetric potential, such as the Dzyloshinskii-Moriya interaction, but with an external bath, or complex potential, among others.

Competition between disorder and exchange splitting in superconducting ZrZn2

International Nuclear Information System (INIS)

Powell, B J; Annett, James F; Gyoerffy, B L

2003-01-01

We propose a simple picture for the occurrence of superconductivity and the pressure dependence of the superconducting critical temperature, T SC , in ZrZn 2 . According to our hypothesis the pairing potential is independent of pressure, but the exchange splitting, E xc , leads to a pressure dependence in the (spin dependent) density of states at the Fermi level, D σ (ε F ). Assuming p-wave pairing T SC is dependent on D σ (ε F ) which ensures that, in the absence of non-magnetic impurities, T SC decreases as pressure is applied until it reaches a minimum in the paramagnetic state. Disorder reduces this minimum to zero, this gives the illusion that the superconductivity disappears at the same pressure as ferromagnetism does. (letter to the editor)

Pre-critical phenomena of two-flavor color superconductivity in heated quark matter. Diquark-pair fluctuations and non-Fermi liquid behavior

International Nuclear Information System (INIS)

Kitazawa, Masakiyo; Kunihiro, Teiji; Koide, Tomoi; Nemoto, Yukio

2005-01-01

We investigate the fluctuations of the diquark-pair field and their effects on observables above the critical temperature T c in two-flavor color superconductivity (CSC) at moderate density using a Nambu-Jona-Lasinio-type effective model of QCD. Because of the strong-coupling nature of the dynamics, the fluctuations of the pair field develop a collective mode, which has a prominent strength even well above T c . We show that the collective mode is actually the soft mode of CSC. We examine the effects of the pair fluctuations on the specific heat and the quark spectrum for T above but close to T c . We find that the specific heat exhibits singular behavior because of the pair fluctuations, in accordance with the general theory of second-order phase transitions. The quarks display a typical non-Fermi liquid behavior, owing to the coupling with the soft mode, leading to a pseudo-gap in the density of states of the quarks in the vicinity of the critical point. Some experimental implications of the precursory phenomena are also discussed. (author)

Detecting nonlocal Cooper pair entanglement by optical Bell inequality violation

Energy Technology Data Exchange (ETDEWEB)

Nigg, Simon E.; Tiwari, Rakesh P.; Walter, Stefan; Schmidt, Thomas L. [Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel (Switzerland)

2015-07-01

Based on the Bardeen Cooper Schrieffer (BCS) theory of superconductivity, the coherent splitting of Cooper pairs from a superconductor to two spatially separated quantum dots has been predicted to generate nonlocal pairs of entangled electrons. In order to test this hypothesis, we propose a scheme to transfer the spin state of a split Cooper pair onto the polarization state of a pair of optical photons. We show that the produced photon pairs can be used to violate a Bell inequality, unambiguously demonstrating the entanglement of the split Cooper pairs.

Detecting nonlocal Cooper pair entanglement by optical Bell inequality violation

Science.gov (United States)

Nigg, Simon E.; Tiwari, Rakesh P.; Walter, Stefan; Schmidt, Thomas L.

2015-03-01

Based on the Bardeen-Cooper-Schrieffer theory of superconductivity, the coherent splitting of Cooper pairs from a superconductor to two spatially separated quantum dots has been predicted to generate nonlocal pairs of entangled electrons. In order to test this hypothesis, we propose a scheme to transfer the spin state of a split Cooper pair onto the polarization state of a pair of optical photons. We show that the photon pairs produced can be used to violate a Bell inequality, unambiguously demonstrating the entanglement of the split Cooper pairs.

Unconventional superconductivity and surface pairing symmetry in half-Heusler compounds

Science.gov (United States)

Wang, Qing-Ze; Yu, Jiabin; Liu, Chao-Xing

2018-06-01

Signatures of nodal line/point superconductivity [Kim et al., Sci. Adv. 4, eaao4513 (2018), 10.1126/sciadv.aao4513; Brydon et al., Phys. Rev. Lett. 116, 177001 (2016), 10.1103/PhysRevLett.116.177001] have been observed in half-Heusler compounds, such as LnPtBi (Ln = Y, Lu). Topologically nontrivial band structures, as well as topological surface states, have also been confirmed by angular-resolved photoemission spectroscopy in these compounds [Liu et al., Nat. Commun. 7, 12924 (2016), 10.1038/ncomms12924]. In this paper, we present a systematical classification of possible gap functions of bulk states and surface states in half-Heusler compounds and the corresponding topological properties based on the representations of crystalline symmetry group. Different from all the previous studies based on the four band Luttinger model, our study starts with the six-band Kane model, which involves both four p-orbital type of Γ8 bands and two s-orbital type of Γ6 bands. Although the Γ6 bands are away from the Fermi energy, our results reveal the importance of topological surface states, which originate from the band inversion between Γ6 and Γ8 bands, in determining surface properties of these compounds in the superconducting regime by combining topological bulk state picture and nontrivial surface state picture.

Lighting up superconducting stripes

Science.gov (United States)

Ergeçen, Emre; Gedik, Nuh

2018-02-01

Cuprate superconductors display a plethora of complex phases as a function of temperature and carrier concentration, the understanding of which could provide clues into the mechanism of superconductivity. For example, when about one-eighth of the conduction electrons are removed from the copper oxygen planes in cuprates such as La2‑xBaxCuO4 (LBCO), the doped holes (missing electrons) organize into one-dimensional stripes (1). The bulk superconducting transition temperature (Tc) is greatly reduced, and just above Tc, electrical transport perpendicular to the planes (along the c axis) becomes resistive, but parallel to the copper oxygen planes, resistivity remains zero for a range of temperatures (2). It was proposed a decade ago (3) that this anisotropic behavior is caused by pair density waves (PDWs); superconducting Cooper pairs exist along the stripes within the planes but cannot tunnel to the adjacent layers. On page 575 of this issue, Rajasekaran et al. (4) now report detection of this state in LBCO using nonlinear reflection of high-intensity terahertz (THz) light.

Fidelity study of the superconducting phase diagram in the two-dimensional single-band Hubbard model

Science.gov (United States)

Jia, C. J.; Moritz, B.; Chen, C.-C.; Shastry, B. Sriram; Devereaux, T. P.

2011-09-01

Extensive numerical studies have demonstrated that the two-dimensional single-band Hubbard model contains much of the key physics in cuprate high-temperature superconductors. However, there is no definitive proof that the Hubbard model truly possesses a superconducting ground state or, if it does, of how it depends on model parameters. To answer these longstanding questions, we study an extension of the Hubbard model including an infinite-range d-wave pair field term, which precipitates a superconducting state in the d-wave channel. Using exact diagonalization on 16-site square clusters, we study the evolution of the ground state as a function of the strength of the pairing term. This is achieved by monitoring the fidelity metric of the ground state, as well as determining the ratio between the two largest eigenvalues of the d-wave pair/spin/charge-density matrices. The calculations show a d-wave superconducting ground state in doped clusters bracketed by a strong antiferromagnetic state at half filling controlled by the Coulomb repulsion U and a weak short-range checkerboard charge ordered state at larger hole doping controlled by the next-nearest-neighbor hopping t'. We also demonstrate that negative t' plays an important role in facilitating d-wave superconductivity.

Geometrical resonance effects in thin superconducting films

International Nuclear Information System (INIS)

Nedellec, P.

1977-01-01

Electron tunneling density of states measurements on thick and clear superconducting films (S 1 ) backed by films in the normal or superconducting state (S 2 ) show geometrical resonance effects associated with the spatial variation of Δ(x), the pair potential, near the interface S 1 -S 2 . The present understanding of this so-called 'Tomasch effect' is described. The dispersion relation and the nature of excitations in the superconducting state are introduced. It is shown that the introduction of Green functions give a general description of the superconducting state. The notion of Andreev scattering at the S 1 -S 2 interface is presented and connect the geometrical resonance effects to interference process between excitations. The different physical parameters involved are defined and used in the discussion of some experimental results: the variation of the period in energy with the superconducting thickness is connected to the renormalized group velocity of excitations traveling perpendicular to the film. The role of the barrier potential at the interface on the Tomasch effect is described. The main results discussed are: the decrease of the amplitude of the Tomasch structures with energy is due to the loss of the mixed electron-hole character of the superconducting excitations far away from the Fermi level; the variation of the pair potential at the interface is directly related to the amplitude of the oscillations; the tunneling selectivity is an important parameter as the amplitude as well as the phase of the oscillations are modified depending on the value of the selectivity; the phase of the Tomasch oscillations is different for an abrupt change of Δ at the interface and for a smooth variation. An ambiguity arises due to the interplay between these parameters. Finally, some experiments, which illustrate clearly the predicted effects are described [fr

Robust Concurrent Remote Entanglement Between Two Superconducting Qubits

Directory of Open Access Journals (Sweden)

A. Narla

2016-09-01

Full Text Available Entangling two remote quantum systems that never interact directly is an essential primitive in quantum information science and forms the basis for the modular architecture of quantum computing. When protocols to generate these remote entangled pairs rely on using traveling single-photon states as carriers of quantum information, they can be made robust to photon losses, unlike schemes that rely on continuous variable states. However, efficiently detecting single photons is challenging in the domain of superconducting quantum circuits because of the low energy of microwave quanta. Here, we report the realization of a robust form of concurrent remote entanglement based on a novel microwave photon detector implemented in the superconducting circuit quantum electrodynamics platform of quantum information. Remote entangled pairs with a fidelity of 0.57±0.01 are generated at 200 Hz. Our experiment opens the way for the implementation of the modular architecture of quantum computation with superconducting qubits.

Pairing from strong repulsion in triangular lattice Hubbard model

Science.gov (United States)

Zhang, Shang-Shun; Zhu, Wei; Batista, Cristian D.

2018-04-01

We propose a pairing mechanism between holes in the dilute limit of doped frustrated Mott insulators. Hole pairing arises from a hole-hole-magnon three-body bound state. This pairing mechanism has its roots on single-hole kinetic energy frustration, which favors antiferromagnetic (AFM) correlations around the hole. We demonstrate that the AFM polaron (hole-magnon bound state) produced by a single hole propagating on a field-induced polarized background is strong enough to bind a second hole. The effective interaction between these three-body bound states is repulsive, implying that this pairing mechanism is relevant for superconductivity.

Time evolution of a new superconducting state in long ferromagnetic superconductors

International Nuclear Information System (INIS)

Dharmadurai, G.

1980-01-01

We examine the unique features associated with the onset of a time dependent superconducting state in long reentrant ferromagnetic superconductors due to the self-heating induced breakdown of the ferromagnetic normal state. After solving the relevant one-dimensional heat flow equations in an analytic approximation we estimate the duration of the resulting metastable superconducting state and discuss the qualitative aspects of the temporal behaviour of this new superconducting state. (orig.)

Unconventional superconductivity in heavy fermionic and high-Tc superconductors

International Nuclear Information System (INIS)

Volovik, G.E.

1989-01-01

Splitting of the superconducting transition and glass spectrum in heavy fermion companies and oxide superconductors are discussed. The multicomponent order parameter leads to splitting of transition due to magnetic field, impurities, orthorhombic distortion, etc... Linear specific heat in oxide superconductors may be explained in terms of the Fermi-surface arising in superconducting state if interband is pairing strong enough

Scientific Affairs Division of NATO Advanced Study Institute: abstracts for nonequilibrium superconductivity, phonons and Kapitza boundaries

International Nuclear Information System (INIS)

1980-05-01

Abstracts of papers presented at the meeting are given. Topics covered include: Kapitza resistance; superconducting tunneling; energy gap enhancement in superconductors; instabilities in nonequilibrium superconducting states; exchange of charge between superconducting pairs and quasiparticles; motion of magnetic flux (flux flow); and other new phenomena

Superconducting states and depinning transitions of Josephson ladders

International Nuclear Information System (INIS)

Barahona, M.; Strogatz, S.H.; Orlando, T.P.

1998-01-01

We present analytical and numerical studies of pinned superconducting states of open-ended Josephson ladder arrays, neglecting inductances but taking edge effects into account. Treating the edge effects perturbatively, we find analytical approximations for three of these superconducting states emdash the no-vortex, fully frustrated, and single-vortex states emdash as functions of the dc bias current I and the frustration f. Bifurcation theory is used to derive formulas for the depinning currents and critical frustrations at which the superconducting states disappear or lose dynamical stability as I and f are varied. These results are combined to yield a zero-temperature stability diagram of the system with respect to I and f. To highlight the effects of the edges, we compare this dynamical stability diagram to the thermodynamic phase diagram for the infinite system where edges have been neglected. We briefly indicate how to extend our methods to include self-inductances. copyright 1998 The American Physical Society

Spin-polaron theory of high-Tc superconductivity: I, spin polarons and high-Tc pairing

International Nuclear Information System (INIS)

Wood, R.F.

1993-06-01

The concept of a spin polaron is introduced and contrasted with the more familiar ionic polaron picture. A brief review of aspects of ionic bipolaronic superconductivity is given with particular emphasis on the real-space pairing and true Bose condensation characteristics. The formation energy of spin polarons is then calculated in analogy with ionic polarons. The spin-flip energy of a Cu spin in an antiferromagnetically aligned CuO 2 plane is discussed. It is shown that the introduction of holes into the CuO 2 planes will always lead to the destruction of long-range AF ordering due to the formation of spin polarons. The pairing of two spin polarons can be expected because of the reestablishment of local (short-range) AF ordering; the magnitude of the pairing energy is estimated using a simplified model. The paper closes with a brief discussion of the formal theory of spin polarons

Half-metallic superconducting triplet spin multivalves

Science.gov (United States)

Alidoust, Mohammad; Halterman, Klaus

2018-02-01

We study spin switching effects in finite-size superconducting multivalve structures. We examine F1F2SF3 and F1F2SF3F4 hybrids where a singlet superconductor (S) layer is sandwiched among ferromagnet (F) layers with differing thicknesses and magnetization orientations. Our results reveal a considerable number of experimentally viable spin-valve configurations that lead to on-off switching of the superconducting state. For S widths on the order of the superconducting coherence length ξ0, noncollinear magnetization orientations in adjacent F layers with multiple spin axes leads to a rich variety of triplet spin-valve effects. Motivated by recent experiments, we focus on samples where the magnetizations in the F1 and F4 layers exist in a fully spin-polarized half-metallic phase, and calculate the superconducting transition temperature, spatially and energy resolved density of states, and the spin-singlet and spin-triplet superconducting correlations. Our findings demonstrate that superconductivity in these devices can be completely switched on or off over a wide range of magnetization misalignment angles due to the generation of equal-spin and opposite-spin triplet pairings.

Importance of interlayer pair tunneling: A variational perspective

International Nuclear Information System (INIS)

Medhi, Amal; Basu, Saurabh

2011-01-01

We study the effect of interlayer pair tunneling in a bilayer superconductor where each layer is described by a two dimensional t-J model and the two layers are connected by the Josephson pair tunneling term. We study this model using a grand canonical variational Monte Carlo (GVMC) method, for which we develop a new algorithm to perform Monte Carlo simulation of a system with fluctuating particle number. The variational wavefunction is taken to be the product of two Gutzwiller projected d-wave BCS wavefunctions with variable particle densities, one for each layer. We calculate the energy of the above state as a function of the d-wave superconducting gap parameter, Δ. We find that the interlayer pair tunneling energy, E perpendicular shows interesting variation with Δ. E perpendicular tends to enhance the optimal value of Δ, thereby the superconducting pairing. However the magnitude of the tunneling energy is found to be too small to have any appreciable effect on the physical properties. While the result is supported by early experiments and hence may appear known to the community, the current work presents a new approach to the problem and confirms the diminished role of interlayer pair tunneling by directly calculating its contribution to superconducting condensation energy.

Mesoscopic pairing without superconductivity

Science.gov (United States)

Hofmann, Johannes

2017-12-01

We discuss pairing signatures in mesoscopic nanowires with a variable attractive pairing interaction. Depending on the wire length, density, and interaction strength, these systems realize a simultaneous bulk-to-mesoscopic and BCS-BEC crossover, which we describe in terms of the parity parameter that quantifies the odd-even energy difference and generalizes the bulk Cooper pair binding energy to mesoscopic systems. We show that the parity parameter can be extracted from recent measurements of conductance oscillations in SrTiO3 nanowires by Cheng et al. [Nature (London) 521, 196 (2015), 10.1038/nature14398], where it marks the critical magnetic field that separates pair and single-particle currents. Our results place the experiment in the fluctuation-dominated mesoscopic regime on the BCS side of the crossover.

The state of superconductivity

International Nuclear Information System (INIS)

Clark, T.D.

1981-01-01

The present status of applications based on the phenomena of superconductivity are reviewed. Superconducting materials, large scale applications, the Josephson effect and its applications, and superconductivity in instrumentation, are considered. The influence that superconductivity has had on modern theories of elementary particles, such as gauge symmetry breaking, is discussed. (U.K.)

Superconductivity without inversion symmetry in CePt3Si

International Nuclear Information System (INIS)

Frigeri, P.A.; Agterberg, D.F.; Koga, A.; Sigrist, M.

2005-01-01

Based on symmetry arguments by Anderson, the following conditions are necessary for the formation of Cooper pairs: spin-singlet pairing relies on time-reversal symmetry, while spin-triplet pairing requires parity in addition. The rather general formulation of this rule has led to the common belief that the lack of an inversion center in a material would prevent spin-triplet pairing indiscriminately. In this presentation, we discuss symmetry aspects of superconductivity in a class of systems without inversion symmetry which is connected with spin-orbit coupling. We can show that, not only spin singlet pairing, but also certain spin triplet states remain unaffected by the loss of inversion symmetry. Moreover, the absence of an inversion center reduces the effect of paramagnetic limiting for spin-singlet pairing states in an external magnetic field. Based on this symmetry analysis, we examine the recently discovered heavy Fermion superconductor CePt 3 Si, where a missing inversion plane leads to the well-known Rashba-type of spin-orbit coupling. In particular, the problem of the pairing symmetry will be addressed as well as several properties of the superconducting phase which appears close to a quantum phase transition between a paramagnetic and antiferromagnetic phase. The same kind of analysis will also be done for another example UIr

Theory of itinarant ferromagnetism in superconducting semimetals. Theorie du ferromagnetisme itinerant dans des semimetaux supraconducteurs

Energy Technology Data Exchange (ETDEWEB)

Do Tran, C; Nguyen Van, C [Groupe de Physique Theorique, Inst. National Polytechnique de Hanoi (Viet Nam); Nguyen Manh, D [Groupe de Physique Theorique, Inst. National Polytechnique de Hanoi (Viet Nam) Centre National de la Recherche Scientifique, Lab. d' Etudes des Proprietes Electroniques des Solides, 38 - Grenoble (France)

1991-11-01

A theory of itinerant ferromagnetism in superconducting semimetals is proposed. A nonzero mean magnetisation appears in the superconducting state due to the interaction (interference) of spin density wave (SDW), charge density wave (CDW) and Cooper pair wave. Phase diagram and physical properties of the states considered are investigated analytically and numerically. (orig.).

Concurrence of superconductivity and structure transition in Weyl semimetal TaP under pressure

Energy Technology Data Exchange (ETDEWEB)

Li, Yufeng; Zhou, Yonghui; Guo, Zhaopeng; Han, Fei; Chen, Xuliang; Lu, Pengchao; Wang, Xuefei; An, Chao; Zhou, Ying; Xing, Jie; Du, Guan; Zhu, Xiyu; Yang, Huan; Sun, Jian; Yang, Zhaorong; Yang, Wenge; Mao, Ho-Kwang; Zhang, Yuheng; Wen, Hai-Hu

2017-12-01

Weyl semimetal defines a material with three-dimensional Dirac cones, which appear in pair due to the breaking of spatial inversion or time reversal symmetry. Superconductivity is the state of quantum condensation of paired electrons. Turning a Weyl semimetal into superconducting state is very important in having some unprecedented discoveries. In this work, by doing resistive measurements on a recently recognized Weyl semimetal TaP under pressures up to about 100 GPa, we show the concurrence of superconductivity and a structure transition at about 70 GPa. It is found that the superconductivity becomes more pronounced when decreasing pressure and retains when the pressure is completely released. High-pressure x-ray diffraction measurements also confirm the structure phase transition from I41md to P-6m2 at about 70 GPa. More importantly, ab-initial calculations reveal that the P-6m2 phase is a new Weyl semimetal phase and has only one set of Weyl points at the same energy level. Our discovery of superconductivity in TaP by high pressure will stimulate investigations on superconductivity and Majorana fermions in Weyl semimetals.

Investigations into nuclear pairing

International Nuclear Information System (INIS)

Clark, R.M.

2006-01-01

This paper is divided in two main sections focusing on different aspects of collective nuclear behavior. In the first section, solutions are considered for the collective pairing Hamiltonian. In particular, an approximate solution at the critical point of the pairing transition from harmonic vibration (normal nuclear behavior) to deformed rotation (superconducting behavior) in gauge space is found by analytic solution of the Hamiltonian. The eigenvalues are expressed in terms of the zeros of Bessel functions of integer order. The results are compared to the pairing bands based on the Pb isotopes. The second section focuses on the experimental search for the Giant Pairing Vibration (GPV) in nuclei. After briefly describing the origin of the GPV, and the reasons that the state has remained unidentified, a novel idea for populating this state is presented. A recent experiment has been performed using the LIBERACE+STARS detector system at the 88-Inch Cyclotron of LBNL to test the idea. (Author)

Majorana surface modes of nodal topological pairings in spin-3/2 semimetals

Science.gov (United States)

Yang, Wang; Xiang, Tao; Wu, Congjun

2017-10-01

When solid state systems possess active orbital-band structures subject to spin-orbit coupling, their multicomponent electronic structures are often described in terms of effective large-spin fermion models. Their topological structures of superconductivity are beyond the framework of spin singlet and triplet Cooper pairings for spin-1/2 systems. Examples include the half-Heusler compound series of RPtBi, where R stands for a rare-earth element. Their spin-orbit coupled electronic structures are described by the Luttinger-Kohn model with effective spin-3/2 fermions and are characterized by band inversion. Recent experiments provide evidence to unconventional superconductivity in the YPtBi material with nodal spin-septet pairing. We systematically study topological pairing structures in spin-3/2 systems with the cubic group symmetries and calculate the surface Majorana spectra, which exhibit zero energy flat bands, or, cubic dispersion depending on the specific symmetry of the superconducting gap functions. The signatures of these surface states in the quasiparticle interference patterns of tunneling spectroscopy are studied, which can be tested in future experiments.

Novel superconducting state in ferromagnetic superconductor UCoGe. Microscopic coexistence of ferromagnetism and superconductivity probed by 59Co-NQR measurements

International Nuclear Information System (INIS)

Ishida, Kenji; Hattori, Taisuke; Ihara, Yoshihiko; Nakai, Yusuke; Sato, Noriaki K.; Deguchi, Kazuhiko; Tamura, Nobuyuki; Satoh, Isamu

2010-01-01

We have investigated the relationship between ferromagnetism and superconductivity in ferromagnetic superconductor UCoGe from 59 Co nuclear quadrupole resonance (NQR) measurements. Our experimental results indicate the microscopic coexistence of ferromagnetism and superconductivity in UCoGe, and suggest a 'self-induced vortex state' in its superconducting state. We also review NQR experiments, which play an important role in this study. (author)

Competition between disorder and exchange splitting in superconducting ZrZn sub 2

CERN Document Server

Powell, B J; Györffy, B L

2003-01-01

We propose a simple picture for the occurrence of superconductivity and the pressure dependence of the superconducting critical temperature, T sub S sub C , in ZrZn sub 2. According to our hypothesis the pairing potential is independent of pressure, but the exchange splitting, E sub x sub c , leads to a pressure dependence in the (spin dependent) density of states at the Fermi level, D subsigma (epsilon sub F). Assuming p-wave pairing T sub S sub C is dependent on D subsigma (epsilon sub F) which ensures that, in the absence of non-magnetic impurities, T sub S sub C decreases as pressure is applied until it reaches a minimum in the paramagnetic state. Disorder reduces this minimum to zero, this gives the illusion that the superconductivity disappears at the same pressure as ferromagnetism does. (letter to the editor)

Superconductivity in doped semiconductors

Energy Technology Data Exchange (ETDEWEB)

Bustarret, E., E-mail: Etienne.bustarret@neel.cnrs.fr

2015-07-15

A historical survey of the main normal and superconducting state properties of several semiconductors doped into superconductivity is proposed. This class of materials includes selenides, tellurides, oxides and column-IV semiconductors. Most of the experimental data point to a weak coupling pairing mechanism, probably phonon-mediated in the case of diamond, but probably not in the case of strontium titanate, these being the most intensively studied materials over the last decade. Despite promising theoretical predictions based on a conventional mechanism, the occurrence of critical temperatures significantly higher than 10 K has not been yet verified. However, the class provides an enticing playground for testing theories and devices alike.

d-wave superconductivity in the frustrated two-dimensional periodic Anderson model

Directory of Open Access Journals (Sweden)

Wei Wu

2015-02-01

Full Text Available Superconductivity in heavy-fermion materials can sometimes appear in the incoherent regime and in proximity to an antiferromagnetic quantum critical point. Here, we study these phenomena using large-scale determinant quantum Monte Carlo simulations and the dynamical cluster approximation with various impurity solvers for the periodic Anderson model with frustrated hybridization. We obtain solid evidence for a d_{x^{2}−y^{2}} superconducting phase arising from an incoherent normal state in the vicinity of an antiferromagnetic quantum critical point. There is a coexistence region, and the width of the superconducting dome increases with frustration. Through a study of the pairing dynamics, we find that the retarded spin fluctuations give the main contribution to the pairing glue. These results are relevant for unconventional superconductivity in the Ce-115 family of heavy fermions.

Direct measurement of the quantum state of the electromagnetic field in a superconducting transmission line

International Nuclear Information System (INIS)

Melo, F. de; Aolita, L.; Davidovich, L.; Toscano, F.

2006-01-01

We propose an experimental procedure to directly measure the state of an electromagnetic field inside a resonator, corresponding to a superconducting transmission line, coupled to a Cooper-pair box (CPB). The measurement protocol is based on the use of a dispersive interaction between the field and the CPB, and the coupling to an external classical field that is tuned to resonance with either the field or the CPB. We present a numerical simulation that demonstrates the feasibility of this protocol, which is within reach of present technology

Correlated Dirac particles and superconductivity on the honeycomb lattice

Science.gov (United States)

Wu, Wei; Scherer, Michael M.; Honerkamp, Carsten; Le Hur, Karyn

2013-03-01

We investigate the properties of the nearest-neighbor singlet pairing and the emergence of d-wave superconductivity in the doped honeycomb lattice considering the limit of large interactions and the t-J1-J2 model. First, by applying a renormalized mean-field procedure as well as slave-boson theories which account for the proximity to the Mott-insulating state, we confirm the emergence of d-wave superconductivity, in agreement with earlier works. We show that a small but finite J2 spin coupling between next-nearest neighbors stabilizes d-wave symmetry compared to the extendeds-wave scenario. At small hole doping, to minimize the energy and to gap the whole Fermi surface or all the Dirac points, the superconducting ground state is characterized by a d+id singlet pairing assigned to one valley and a d-id singlet pairing to the other, which then preserves time-reversal symmetry. The slightly doped situation is distinct from the heavily doped case (around 3/8 and 5/8 filling) supporting a pure chiral d+id symmetry and breaking time-reversal symmetry. Then, we apply the functional renormalization group and study in more detail the competition between antiferromagnetism and superconductivity in the vicinity of half filling. We discuss possible applications to strongly correlated compounds with copper hexagonal planes such as In3Cu2VO9. Our findings are also relevant to the understanding of exotic superfluidity with cold atoms.

Probing the superconducting ground state of the rare-earth ternary boride superconductors R RuB2 (R = Lu,Y) using muon-spin rotation and relaxation

Science.gov (United States)

Barker, J. A. T.; Singh, R. P.; Hillier, A. D.; Paul, D. McK.

2018-03-01

The superconductivity in the rare-earth transition-metal ternary borides R RuB2 (where R =Lu and Y) has been investigated using muon-spin rotation and relaxation. Measurements made in zero field suggest that time-reversal symmetry is preserved upon entering the superconducting state in both materials; a small difference in depolarization is observed above and below the superconducting transition in both compounds, however, this has been attributed to quasistatic magnetic fluctuations. Transverse-field measurements of the flux-line lattice indicate that the superconductivity in both materials is fully gapped, with a conventional s -wave pairing symmetry and BCS-like magnitudes for the zero-temperature gap energies. The electronic properties of the charge carriers in the superconducting state have been calculated, with effective masses m*/me=9.8 ±0.1 and 15.0 ±0.1 in the Lu and Y compounds, respectively, with superconducting carrier densities ns=(2.73 ±0.04 ) ×1028m-3 and (2.17 ±0.02 ) ×1028m-3 . The materials have been classified according to the Uemura scheme for superconductivity, with values for Tc/TF of 1 /(414 ±6 ) and 1 /(304 ±3 ) , implying that the superconductivity may not be entirely conventional in nature.

The essential role of vibronic interactions in electron pairing in the micro- and macroscopic sized materials

International Nuclear Information System (INIS)

Kato, Takashi

2010-01-01

Graphical abstract: The electron-phonon interactions destroy the electron pairs formed by Coulomb interactions, and at the same time, form the energy gap by which the electron pairs become stable. - Abstract: In order to discuss how the nondissipative delocalized diamagnetic currents in the microscopic sized materials are closely related to the conventional superconductivity in the macroscopic sized materials, the unified theory, by which various sized superconductivity can be explained, is suggested. It has been believed for a long time that the electron-phonon interactions play an essential role in the attractive electron-electron interactions, as described in the Bardeen-Cooper-Schrieffer (BCS) theory in the conventional superconductivity. However, it is suggested in this paper that the electron-phonon interactions do not play an essential role in the attractive electron-electron interactions but play an essential role in the forming of energy gap by which the electron pairs formed by the attractive Coulomb interactions in the conventional superconducting states become more stable than those in the normal metallic states at low temperatures.

Entangled Coherent States Generation in two Superconducting LC Circuits

International Nuclear Information System (INIS)

Chen Meiyu; Zhang Weimin

2008-01-01

We proposed a novel pure electronic (solid state) device consisting of two superconducting LC circuits coupled to a superconducting flux qubit. The entangled coherent states of the two LC modes is generated through the measurement of the flux qubit states. The interaction of the flux qubit and two LC circuits is controlled by the external microwave control lines. The geometrical structure of the LC circuits is adjustable and makes a strong coupling between them achievable. This entangled coherent state generator can be realized by using the conventional microelectronic fabrication techniques which increases the feasibility of the experiment.

Theory of spin-fluctuation induced superconductivity in iron-based superconductors

International Nuclear Information System (INIS)

Zhang, Junhua

2011-01-01

In this dissertation we focus on the investigation of the pairing mechanism in the recently discovered high-temperature superconductor, iron pnictides. Due to the proximity to magnetic instability of the system, we considered short-range spin fluctuations as the major mediating source to induce superconductivity. Our calculation supports the magnetic fluctuations as a strong candidate that drives Cooper-pair formation in this material. We find the corresponding order parameter to be of the so-called ss-wave type and show its evolution with temperature as well as the capability of supporting high transition temperature up to several tens of Kelvin. On the other hand, our itinerant model calculation shows pronounced spin correlation at the observed antiferromagnetic ordering wave vector, indicating the underlying electronic structure in favor of antiferromagnetic state. Therefore, the electronic degrees of freedom could participate both in the magnetic and in the superconducting properties. Our work shows that the interplay between magnetism and superconductivity plays an important role to the understanding of the rich physics in this material. The magnetic-excitation spectrum carries important information on the nature of magnetism and the characteristics of superconductivity. We analyze the spin excitation spectrum in the normal and superconducting states of iron pnictides in the magnetic scenario. As a consequence of the sign-reversed gap structure obtained in the above, a spin resonance mode appears below the superconducting transition temperature. The calculated resonance energy, scaled with the gap magnitude and the magnetic correlation length, agrees well with the inelastic neutron scattering (INS) measurements. More interestingly, we find a common feature of those short-range spin fluctuations that are capable of inducing a fully gapped ss state is the momentum anisotropy with elongated span along the direction transverse to the antiferromagnetic momentum

Signature of magnetic-dependent gapless odd frequency states at superconductor / ferromagnet interfaces

Energy Technology Data Exchange (ETDEWEB)

Robinson, Jason [Department of Materials Science, 27 Charles Babbage Road, Cambridge, CB30FS (United Kingdom)

2015-07-01

The theory of superconductivity developed by Bardeen, Cooper, and Schrieffer (BCS) explains the stabilisation of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap below which the density of states (DoS) is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. In this talk I will present scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb sub-gap superconducting DoS on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results verify odd frequency spin-triplet superconductivity at superconductor / inhomogeneous magnet interfaces.

Superconductivity and electrical resistivity in alkali metal doped ...

Indian Academy of Sciences (India)

We consider a two-peak model for the phonon density of states to investigate the nature of electron pairing mechanism for superconducting state in fullerides. We first study the intercage interactions between the adjacent C60 cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to ...

Chiral classical states in a rhombus and a rhombi chain of Josephson junctions with two-band superconducting elements

CERN Document Server

Dias, R G; Coutinho, B C; Martins, L P

2014-01-01

We present a study of Josephson junctions arrays with two-band superconducting elements in the highcapacitance limit. We consider two particular geometries for these arrays: a single rhombus and a rhombi chain with two-band superconducting elements at the spinal positions. We show that the rhombus shaped JJ circuit and the rhombi chain can be mapped onto a triangular JJ circuit and a JJ two-leg ladder, respectively, with zero effective magnetic flux, but with Josephson couplings that are magnetic flux dependent. If the two-band superconductors are in a sign-reversed pairing state, one observes transitions to or from chiral phase configurations in the mapped superconducting arrays when magnetic flux or temperature are varied. The phase diagram for these chiral configurations is discussed. When half-flux quantum threads each rhombus plaquette, new phase configurations of the rhombi chain appear that are characterized by the doubling of the periodicity of the energy density along the chain, with every other two-...

Florida State University superconducting linac

International Nuclear Information System (INIS)

Myers, E.G.; Fox, J.D.; Frawley, A.D.; Allen, P.; Faragasso, J.; Smith, D.; Wright, L.

1988-01-01

As early as the fall of 1977 it was decided that the future research needs of their nuclear structure laboratory required an increase in energy capability to at least 8 MeV per nucleon for the lighter ions, and that these needs could be met by the installation of a 17 MV tandem Van de Graaff accelerator. The chief problem with this proposal was the high cost. It became apparent that a far less expensive option was to construct a linear accelerator to boost the energy from their existing 9 MV tandem. The options open to them among linac boosters were well represented by the room temperature linac at Heidelberg and the superconducting Stony Brook and Argonne systems. By the Spring of 1979 it had been decided that both capital cost and electric power requirements favored a superconducting system. As regards the two superconducting resonator technologies - the Argonne niobium-copper or the Caltech-Stony Brook lead plated copper - the Argonne resonators, though more expensive to construct, had the advantages of more boost per resonator, greater durability of the superconducting surface and less stringent beam bunching requirements. In 1980 pilot funding from the State of Florida enabled the construction of a building addition to house the linac and a new target area, and the setting up of a small, three resonator, test booster. Major funding by the NSF for the laboratory upgrade started in 1984. With these funds they purchased their present helium liquefaction and transfer system and constructed three large cryostats, each housing four Argonne beta = 0.105 resonators and two superconducting solenoids. The last large cryostat was completed and installed on-line early this year and the linac was dedicated on March 20. Nuclear physics experiments using the whole linac began in early June. 4 references, 6 figures, 1 table

Superconducting rf activities at Cornell University

International Nuclear Information System (INIS)

Padamsee, H.; Hakimi, M.; Kirchgessner, J.

1988-01-01

Development of rf superconductivity for high energy accelerators has been a robust activity at the Cornell Laboratory of Nuclear Studies (LNS) for many years. In order to realize the potential of rf superconductivity, a two-pronged approach has been followed. On the one hand accelerator applications were selected where the existing state-of-the art of superconducting rf is competitive with alternate technologies, then LNS engaged in a program to design, construct and test suitable superconducting cavities, culminating in a full system test in an operating accelerator. On the second front the discovery and invention of ideas, techniques and materials required to make superconducting rf devices approach the ideal in performance has been aggressively pursued. Starting with the development of superconducting cavities for high energy electron synchrotrons, the technology was extended to high energy e + e - storage rings. The LE5 cavity design has now been adopted for use in the Continuous Electron Beam Accelerator Facility (CEBAF). When completed, this project will be one of the largest applications of SRF technology, using 440 LE5 modules[4]. In the last two years, the cavity design and the technology have been transferred to industry and CEBAF. Cornell has tested the early industrial prototypes and cavity pairs. LNS has developed, in collaboration with CEBAF, designs and procedures for cavity pair and cryomodule assembly and testing. Advanced research for future electron accelerators is badly needed if particle physicists hope to expand the energy frontier. Superconducting cavity technology continues to offer attractive opportunities for further advances in achievable voltage at reasonable cost for future accelerators. For Nb, the full potential implies an order of magnitude increase over current capabilities. 20 references, 11 figures

Intrinsic Paramagnetic Meissner Effect Due to s-Wave Odd-Frequency Superconductivity

Directory of Open Access Journals (Sweden)

A. Di Bernardo

2015-11-01

Full Text Available In 1933, Meissner and Ochsenfeld reported the expulsion of magnetic flux—the diamagnetic Meissner effect—from the interior of superconducting lead. This discovery was crucial in formulating the Bardeen-Cooper-Schrieffer (BCS theory of superconductivity. In exotic superconducting systems BCS theory does not strictly apply. A classical example is a superconductor-magnet hybrid system where magnetic ordering breaks time-reversal symmetry of the superconducting condensate and results in the stabilization of an odd-frequency superconducting state. It has been predicted that under appropriate conditions, odd-frequency superconductivity should manifest in the Meissner state as fluctuations in the sign of the magnetic susceptibility, meaning that the superconductivity can either repel (diamagnetic or attract (paramagnetic external magnetic flux. Here, we report local probe measurements of faint magnetic fields in a Au/Ho/Nb trilayer system using low-energy muons, where antiferromagnetic Ho (4.5 nm breaks time-reversal symmetry of the proximity-induced pair correlations in Au. From depth-resolved measurements below the superconducting transition of Nb, we observe a local enhancement of the magnetic field in Au that exceeds the externally applied field, thus proving the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state.

Deenergizing method of superconducting magnets for Maglev in an emergency

Energy Technology Data Exchange (ETDEWEB)

Kishikawa, Akihiko [Railway Technical Research Inst., Tokyo (Japan); Nemoto, Kaoru [Railway Technical Research Inst., Tokyo (Japan)

1996-12-31

The running stability of the superconducting magnets (SCMs) mounted on the JR Maglev vehicle has been confirmed through many researches and actual running tests. So we could confirm that the high performance of our SCMs during the last few years, but we must bear in mind that the SCM which consists of the superconducting wire has the possibility of changing into normal resistive state from superconducting state. If one of the pair SCMs normalizes, a huge lateral force on one side of a bogie will occur suddenly and push the vehicle toward the sidewall of the guideway. This paper describes the method that reduces this huge force acting on one side of a bogie in an SCM accident. (orig.)

Superconductivity in the background of disordered flux state of spins

International Nuclear Information System (INIS)

Feng Shiping; Guo Rui; Han Fei

1992-01-01

The phase diagram of the copper oxide materials with the antiferromagnetic and the superconducting properties as a function of doping δ is obtained in the framework of the t-J model by using the Schwinger boson-slave fermion theory. The results show that the spiral order of spins competes and coexists with superconductivity for small doping δ. For large doping δ, superconductivity appears, which may be caused by the occurrence of a disordered flux state of spins. The phase diagram suggests a strong relationship between antiferromagnetism and superconductivity. (orig.)

Correlation mediated superconductivity in a Spin Peierls Phase of the Hubbard Model

International Nuclear Information System (INIS)

Long, M.W.

1987-08-01

The author explores the consequences of a mapping of the Hubbard Hamiltonian with a view to finding possible superconducting phases. The transformation pairs up all the sites and is therefore a much more natural starting point for describing a 'Spin Peierls' transition, generating enhanced singlet correlations for this pairing, than it is for describing the 'Resonating Valence Bond' state. It is shown that in the less than half filling case, an effective non-linear hopping Hamiltonian is quite useful in describing half of the electrons. This effective Hamiltonian can show a form of superconducting instability when nearest neighbour hopping is introduced to stabilise it. This superconducting phase seems to be a very unlikely possibility for the standard Hubbard model. (author)

Quasiparticle dynamics in aluminium superconducting microwave resonators

NARCIS (Netherlands)

De Visser, P.J.

2014-01-01

This thesis describes the intrinsic limits of superconducting microresonator detectors. In a superconductor at low temperature, most of the electrons are paired into so called Cooper pairs, which cause the well-known electrical conduction without resistance. Superconducting microwave resonators have

History of the theory of superconductivity

International Nuclear Information System (INIS)

Frohlich, H.

1983-01-01

This chapter points out that in the years from Onnes' discovery that the electric resistivity of mercury completely disappeared at about 4K in 1911 up to 1950, many attempts to develop a theory of superconductivity failed, and the subject became an outstanding problem in physics. Discusses the introduction of field theory into solid state physics, which proved to be the decisive step toward a theory. Describes how Schrieffer's suggestion of a wave function for the multielectron problem in terms of electron pairs led to the theory of superconductivity, or the so called B.C.S. theory

Further test of new pairing scheme used in overhaul of BCS theory

International Nuclear Information System (INIS)

Zheng, X.H.; Walmsley, D.G.

2014-01-01

Highlights: • Explanation of a new pairing scheme to overhaul BCS theory. • Prediction of superconductor properties from normal state resistivity. • Applications to Nb, Pb, Al, Ta, Mo, Ir and W, T c between 9.5 and 0.012 K. • High accuracy compared with measured energy gap of Nb, Pb, Al and Ta. • Prediction of energy gap for Mo, Ir and W (so far not measured). - Abstract: A new electron pairing scheme, rectifying a fundamental flaw of the BCS theory, is tested extensively. It postulates that superconductivity arises solely from residual umklapp scattering when it is not in competition for the same destination electron states with normal scattering. It reconciles a long standing theoretical discrepancy in the strength of the electron–phonon interaction between the normal and superconductive states. The new scheme is exploited to calculate the superconductive electron–phonon spectral density, α 2 F(ν), entirely on the basis of normal state electrical resistivity. This leads to first principles superconductive properties (zero temperature energy gap and tunnelling conductance) in seven metals which turn out to be highly accurate when compared with known data; in other cases experimental verification is invited. The transition temperatures involved vary over almost three orders of magnitude: from 9.5 K for niobium to 0.012 K for tungsten

Ultrasonic attenuation in the superconducting and intermediate states of pure and doped type I superconductors

International Nuclear Information System (INIS)

Chaudhuri, K.D.; Singh, R.

1982-01-01

The attenuation of longitudinal ultrasonic waves has been measured in single crystals of indium (99.999%), indium doped with 0.003 at % of tin, and indium doped with 0.002 at % of bismuth in the intermediate and superconducting states over the frequency range 10--30 MHz. For the bismuth-doped indium specimen, measurements were taken for three different physical states, i.e., for three different dislocation densities, and for the indium and the tin-doped indium specimens, measurements were for one-physical state. For a particular measurement, the same physical state was maintained both in the intermediate and superconducting states. A temperature-dependent oscillatory behavior of the ultrasonic attenuation was observed in the intermediate state in all the three specimens, but in the superconducting state the oscillatory behavior was observed only in the bismuth-doped specimen. Two phases have been identified in the superconducting layers of the intermediate state and there is only one phase in the superconducting state of the bismuth-doped sample. The origin of the two phases in the intermediate state and that of the single phase in the superconducting state of the bismuth-doped sample are discussed. A qualitative explanation is presented for the occurrence of oscillatory attenuation in the intermediate state irrespective of the nature of the dopant and the selective occurrence of oscillatory attenuation in the superconducting state due to the nature of the dopant

Superconductivity and Competing Ordered Phase in RuPn (Pn = As, P)

Science.gov (United States)

Hirai, Daigorou; Takayama, Tomohiro; Hashizume, Daisuke; Yamamoto, Ayako; Takagi, Hidenori

2011-03-01

Unconventional superconductivity likely manifests itself when some competing electronic phases are suppressed down to zero temperature such as cuprates and iron-pnictide superconductors. Therefore, the correlated metallic state neighboring a competing electronic ordering can be a promising playground for unconventional superconductivity. Here we report superconductivity emerging adjacent to electronically ordered phases of RuPn (Pn = As, P). We found that RuAs(P) exhibits phase transitions at 240 (265) K, which is discerned as a drop of magnetic susceptibility or a resistivity upturn. Such anomalies can be suppressed by substituting Rh to the Ru site. Accompanied by the disappearance of the electronic order, superconductivity was found to emerge below 1.8 K and 3.8 K for RuAs and RuP, respectively. The superconductivity in Rh substituted RuPn, which neighbors a competing electronic order, might exhibit an exotic pairing state as seen in the unconventional superconductors known to date.

Anisotropic superconducting state parameters of Tl-2212 superconductors

International Nuclear Information System (INIS)

Khaskalam, Amit K.; Singh, R.K.; Varshney, Dinesh

2001-01-01

We have estimated the superconducting state parameters and their anisotropy in thallium based superconductors (Tl-2212), in the frame work of Fermi liquid approach. Determination of the effective mass of the charge carriers from the Fermi velocity and estimated anisotropic superconducting state parameters, particularly, the magnetic penetration depth along and perpendicular to the conducting plane. The coherence length along and perpendicular to the ab plane is evaluated and appears to be higher. The temperature dependence of penetration depth, their anisotropy and Ginsburg Landau parameter for optimised doped Tl based cuprates shows the power law. The technique permits a consistency with the reported data. (author)

Stability of the Superconducting d-Wave Pairing Toward the Intersite Coulomb Repulsion in CuO_2 Plane

Science.gov (United States)

Val'kov, V. V.; Dzebisashvili, D. M.; Korovushkin, M. M.; Barabanov, A. F.

2018-06-01

Taking into account the real crystalline structure of the CuO_2 plane and the strong spin-fermion coupling, we study the influence of the intersite Coulomb repulsion between holes on the Cooper instability of the spin-polaron quasiparticles in cuprate superconductors. The analysis shows that only the superconducting d-wave pairing is implemented in the whole region of doping, whereas the solutions of the self-consistent equations for the s-wave pairing are absent. It is shown that intersite Coulomb interaction V_1 between the holes located at the nearest oxygen ions does not affect the d-wave pairing, because its Fourier transform V_q vanishes in the kernel of the corresponding integral equation. The intersite Coulomb interaction V_2 of quasiparticles located at the next-nearest oxygen ions does not vanish in the integral equations, however, but it is also shown that the d-wave pairing is robust toward this interaction for physically reasonable values of V_2.

Exotic superconductivity with enhanced energy scales in materials with three band crossings

Science.gov (United States)

Lin, Yu-Ping; Nandkishore, Rahul M.

2018-04-01

Three band crossings can arise in three-dimensional quantum materials with certain space group symmetries. The low energy Hamiltonian supports spin one fermions and a flat band. We study the pairing problem in this setting. We write down a minimal BCS Hamiltonian and decompose it into spin-orbit coupled irreducible pairing channels. We then solve the resulting gap equations in channels with zero total angular momentum. We find that in the s-wave spin singlet channel (and also in an unusual d-wave `spin quintet' channel), superconductivity is enormously enhanced, with a possibility for the critical temperature to be linear in interaction strength. Meanwhile, in the p-wave spin triplet channel, the superconductivity exhibits features of conventional BCS theory due to the absence of flat band pairing. Three band crossings thus represent an exciting new platform for realizing exotic superconducting states with enhanced energy scales. We also discuss the effects of doping, nonzero temperature, and of retaining additional terms in the k .p expansion of the Hamiltonian.

Microtraps for neutral atoms using superconducting structures in the critical state

International Nuclear Information System (INIS)

Emmert, A.; Brune, M.; Raimond, J.-M.; Nogues, G.; Lupascu, A.; Haroche, S.

2009-01-01

Recently demonstrated superconducting atom chips provide a platform for trapping atoms and coupling them to solid-state quantum systems. Controlling these devices requires a full understanding of the supercurrent distribution in the trapping structures. For type-II superconductors, this distribution is hysteretic in the critical state due to the partial penetration of the magnetic field in the thin superconducting film through pinned vortices. We report here an experimental observation of this memory effect. Our results are in good agreement with the predictions of the Bean model of the critical state without adjustable parameters. The memory effect allows to write and store permanent currents in micron-sized superconducting structures and paves the way toward engineered trapping potentials.

Superconductivity in Na{sub 1-x}CoO{sub 2}.yH{sub 2}O thin films

Energy Technology Data Exchange (ETDEWEB)

Hildebrandt, Sandra; Komissinkiy, Philipp; Alff, Lambert [Institute for Materials Science, TU Darmstadt (Germany); Fritsch, Ingo; Habermeier, Hanns-Ulrich [Max-Planck-Institute for Solid State Research, Stuttgart (Germany); Lemmens, Peter [Institute for Condensed Matter Physics, TU Braunschweig (Germany)

2010-07-01

Sodium cobaltate (Na{sub 1-x}CoO{sub 2}) is a novel material with thermoelectric behavior, charge and spin ordered states dependent on the sodium content in the composition. A superconducting phase was found in water intercalated sodium cobaltate (Na{sub 1-x}CoO{sub 2}.yH{sub 2}O) with x=0.65-0.7 and y=0.9-1.3. The pairing state is still under debate, but there are some indications for a spin-triplet or p-wave superconducting pairing state. First films of Na{sub 1-x}CoO{sub 2}.yH{sub 2}O with a superconducting transition temperature near 5 K have been successfully grown. Here we report on thin films of Na{sub 1-x}CoO{sub 2} grown by pulsed laser deposition technique. The deposition parameters, sodium deintercalation and water intercalation conditions are tuned in order to obtain the superconducting phase. The instability of this phase might be an indication for triplet superconductivity, which is known to be affected strongly by impurities and defects.This observation is in agreement with the fact that so far also no superconducting thin films of the most famous triplet superconductor Sr{sub 2}RuO{sub 4} have been reported.

Eliashberg Inversion of superconducting state thermodynamics

International Nuclear Information System (INIS)

Junod, A.

1985-01-01

It is shown that the inversion of the Eliashberg equations currently used in tunneling spectroscopy may be applied to calorimetric data in the superconducting state. This method yields integrals of the spectral function of the electron-phonon interaction α 2 F(ω). Experimental results in pure Nb are presented

Correlated electron motion, flux states and superconductivity

International Nuclear Information System (INIS)

Lederer, P.; Poilblanc, D.; Rice, T.K.

1989-01-01

This paper discusses how, when the on-site correlation is strong, electrons can move by usual hopping only on to empty sites but they can exchange position with their neighbors by a correlated motion. The phase in the former process is fixed and it favors Bloch states. When the concentration of empty sites is small then the latter process dominates and one is free to introduce a phase provided it is chosen to be the same for ↑ and ↓-spin electrons. Since for a partly filled band of non-interacting electrons the introduction of a uniform commensurate flux lowers the energy, the correlated motion can lead to a physical mechanism to generate flux states. These states have a collective gauge variable which is the same for ↑ and ↓-spins and superconducting properties are obtained by expanding around the optimum gauge determined by the usual kinetic energy term. If this latter term has singularities at special fillings then these may affect the superconducting properties

Electron mean free path dependence of the critical currents and the pair-breaking limit in superconducting films

International Nuclear Information System (INIS)

Fedorov, N.; Rinderer, L.

1977-01-01

We have studied the current-induced breakdown of superconductivity in wide (100--980 μm) and thin (0.25--0.98 μm) films of tin. It is shown that the current at which the resistance of the sample begins to rise rapidly in the process of the destruction of superconductivity by a current can be fairly well associated with the theoretical value of the pair-breaking current in the Ginzburg-Landau phenomenological approach (I/sub c//sup G L/). This effect is observed over a rather wide temperature region (up to ΔTapprox.0.7 K), depending on the electron mean free path in the films. The values of the critical currents outside the above-mentioned region correlate qualitatively with those determined by inhomogeneities of the films as proposed by Larkin and Ovchinnikov

Connections between magnetism and superconductivity in UBe13 doped with thorium or boron

International Nuclear Information System (INIS)

Heffner, R.H.; Ott, H.R.; Schenck, A.; Mydosh, J.A.; MacLaughlin, D.E.

1991-06-01

Magnetism and superconductivity appear to be intimately connected in the heavy electron (HE) superconductors. For example, it has been conjectured but not proven that the exchange of antiferromagnetic spin fluctuations are responsible for pairing in HE superconductors. In this paper we review recent results in U 1-x Th x Be 13 , where specific heat, lower critical field and zero-field μSR measurements reveal another second-order phase transition to a state which possesses small-moment magnetic correlations for 0.019 ≤ x ≤ 0.043. We present a new phase diagram for (U,Th)Be 13 which indicates that the superconducting and magnetic order parameters are closely coupled. A discussion of the nature of the lower phase is presented, including the consideration of a possible magnetic superconducting state. When UBe 13 is doped with B (UBe 12.97 B 0.03 ) the Kondo temperature is decreased and the specific heat jump at the superconducting transition temperature is significantly enhanced. However, μSR measurements reveal no magnetic signature in UBe 12.97 B 0.03 , unlike the case for Th doping. The correlation between changes in the Kondo temperature and changes in the superconducting properties induced by B doping provide evidence for the importance of magnetic excitations in the superconducting pairing interaction in UBe 13

Perturbation theory of the periodic Anderson lattice and superconductivity

International Nuclear Information System (INIS)

Geertsuma, W.

1988-01-01

In this paper the author develops a perturbation calculation of the second and fourth order interparticle interaction in band states, based on the Periodic Anderson Lattice. The author shows that 4th order interparticle interactions giving rise to the well known Kondo effect vanish in the superconducting ground state. This term survives in the presence of a magnetic field. Pair excitations can only give rise to an appreciable attractive contribution when the d states are less than half filled and the pair energy is near the Fermi level. The only important attractive interaction comes from the normal fourth order terms

Cooper pair splitters beyond the Coulomb blockade regime

Energy Technology Data Exchange (ETDEWEB)

Amitai, Ehud; Tiwari, Rakesh P.; Nigg, Simon E. [Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel (Switzerland); Walter, Stefan [Institute for Theoretical Physics, University Erlangen Nuernberg, Staudtstrasse 7, 91058 Erlangen (Germany); Schmidt, Thomas L. [Physics and Materials Science Research Unit, University of Luxembourg, L-1511 Luxembourg (Luxembourg)

2016-07-01

We consider the setup of a conventional s-wave Cooper pair splitter. However, we consider the charging energies in the quantum dots to be finite and smaller than the superconducting gap. We find analytically that at low energies the superconductor mediates an inter-dot tunneling term, the spin symmetry of which is influenced by a finite Zeeman field. This effect, together with an electrical tuning scheme of the quantum dot levels, can be used to engineer a non local triplet state on the two quantum dots, thereby extending the non-local state engineering capabilities of the Cooper pair splitter system.

Pairing mechanism in oxide superconductors

International Nuclear Information System (INIS)

Hirsch, J.E.

1988-01-01

A useful way to learn about the pairing mechanism that is responsible for high T c superconductivity is to study properties of model Hamiltonians on small systems. The goal is to find the simplest model that can describe the essential physics of high T c superconductivity. The authors have used Monte Carlo simulation and exact diagonalization techniques to study properties of systems of up to 64 sites. Their results show that spin fluctuations and other spin related mechanisms induced by a Hubbard on-site repulsion U are not likely to give rise to pairing, neither in one nor in multiple band models. In contrast, charge fluctuations in a model with both strong U and V (repulsion between Cu and O) are shown to give rise to pairing and it is suggested that this model provides a plausible mechanism for high T c superconductivity

Topological Crystalline Superconductivity in Locally Noncentrosymmetric Multilayer Superconductors.

Science.gov (United States)

Yoshida, Tomohiro; Sigrist, Manfred; Yanase, Youichi

2015-07-10

Topological crystalline superconductivity in locally noncentrosymmetric multilayer superconductors (SCs) is proposed. We study the odd-parity pair-density wave (PDW) state induced by the spin-singlet pairing interaction through the spin-orbit coupling. It is shown that the PDW state is a topological crystalline SC protected by a mirror symmetry, although it is topologically trivial according to the classification based on the standard topological periodic table. The topological property of the mirror subsectors is intuitively explained by adiabatically changing the Bogoliubov-de Gennes Hamiltonian. A subsector of the bilayer PDW state reduces to the two-dimensional noncentrosymmetric SC, while a subsector of the trilayer PDW state is topologically equivalent to the spinless p-wave SC. Chiral Majorana edge modes in trilayers can be realized without Cooper pairs in the spin-triplet channel and chemical potential tuning.

Pressure-induced electronic phase separation of magnetism and superconductivity in CrAs.

Science.gov (United States)

Khasanov, Rustem; Guguchia, Zurab; Eremin, Ilya; Luetkens, Hubertus; Amato, Alex; Biswas, Pabitra K; Rüegg, Christian; Susner, Michael A; Sefat, Athena S; Zhigadlo, Nikolai D; Morenzoni, Elvezio

2015-09-08

The recent discovery of pressure (p) induced superconductivity in the binary helimagnet CrAs has raised questions on how superconductivity emerges from the magnetic state and on the mechanism of the superconducting pairing. In the present work the suppression of magnetism and the occurrence of superconductivity in CrAs were studied by means of muon spin rotation. The magnetism remains bulk up to p ≃ 3.5 kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at p ≃ 7 kbar. At 3.5 kbar superconductivity abruptly appears with its maximum Tc ≃ 1.2 K which decreases upon increasing the pressure. In the intermediate pressure region (3.5 magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature (Tc) and of the superfluid density (ρs). A scaling of ρs with Tc(3.2) as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs.

Exotic superconducting states in the extended attractive Hubbard model.

Science.gov (United States)

Nayak, Swagatam; Kumar, Sanjeev

2018-04-04

We show that the extended attractive Hubbard model on a square lattice allows for a variety of superconducting phases, including exotic mixed-symmetry phases with [Formula: see text] and [Formula: see text] symmetries, and a novel [Formula: see text] state. The calculations are performed within the Hartree-Fock Bardeen-Cooper-Schrieffer framework. The ground states of the mean-field Hamiltonian are obtained via a minimization scheme that relaxes the symmetry constraints on the superconducting solutions, hence allowing for a mixing of s-, p- and d-wave order parameters. The results are obtained within the assumption of uniform-density states. Our results show that extended attractive Hubbard model can serve as an effective model for investigating properties of exotic superconductors.

Antiferroic electronic structure in the nonmagnetic superconducting state of the iron-based superconductors.

Science.gov (United States)

Shimojima, Takahiro; Malaeb, Walid; Nakamura, Asuka; Kondo, Takeshi; Kihou, Kunihiro; Lee, Chul-Ho; Iyo, Akira; Eisaki, Hiroshi; Ishida, Shigeyuki; Nakajima, Masamichi; Uchida, Shin-Ichi; Ohgushi, Kenya; Ishizaka, Kyoko; Shin, Shik

2017-08-01

A major problem in the field of high-transition temperature ( T c ) superconductivity is the identification of the electronic instabilities near superconductivity. It is known that the iron-based superconductors exhibit antiferromagnetic order, which competes with the superconductivity. However, in the nonmagnetic state, there are many aspects of the electronic instabilities that remain unclarified, as represented by the orbital instability and several in-plane anisotropic physical properties. We report a new aspect of the electronic state of the optimally doped iron-based superconductors by using high-energy resolution angle-resolved photoemission spectroscopy. We find spectral evidence for the folded electronic structure suggestive of an antiferroic electronic instability, coexisting with the superconductivity in the nonmagnetic state of Ba 1- x K x Fe 2 As 2 . We further establish a phase diagram showing that the antiferroic electronic structure persists in a large portion of the nonmagnetic phase covering the superconducting dome. These results motivate consideration of a key unknown electronic instability, which is necessary for the achievement of high- T c superconductivity in the iron-based superconductors.

Probing Andreev bound states in one-atom superconducting contacts

Energy Technology Data Exchange (ETDEWEB)

Pothier, Hugues; Janvier, Camille; Tosi, Leandro; Girit, Caglar; Goffman, Marcelo; Esteve, Daniel; Urbina, Cristian [Quantronics Group, SPEC, CEA-Saclay (France)

2015-07-01

Superconductors are characterized by a dissipationless current. Since the work of Josephson 50 years ago, it is known that a supercurrent can even flow through tunnel junctions between superconductors. This Josephson effect also occurs through any type of ''weak links'' between superconductors: non-superconducting materials, constrictions,.. A unified understanding of the Josephson effect has emerged from a mesoscopic description of weak links. It relies on the existence of doublets of localized states that have energies below the superconducting gap: the Andreev bound states. I will present experiments performed on the simplest conductor possible, a single-atom contact between superconductors, that illustrate these concepts. The most recent work demonstrates time-domain manipulation of quantum superpositions of Andreev bound states.

Microscopic theory of coexistence of superconductivity and antiferromagnetism

International Nuclear Information System (INIS)

Ashkenazi, J.; Kuper, C.G.; Ron, A.

1983-01-01

A theory of the coexistence of superconductivity and antiferromagnetism is presented. We study the role of the ''diagonal'' exchange coupling between magnetic ions and conduction electrons, using Eliashberg's formalism. This coupling generates a spatial displacement of the Cooper-paired states, and thus reduces the pairing strength. The reduction is linear in the exchange integral and the staggered magnetization. The theory agrees well with experiment for Dy/sub 1.2/Mo 6 S 8 and Tb/sub 1.2/Mo 6 S 8

Bell-state generation on remote superconducting qubits with dark photons

Science.gov (United States)

Hua, Ming; Tao, Ming-Jie; Alsaedi, Ahmed; Hayat, Tasawar; Wei, Hai-Rui; Deng, Fu-Guo

2018-06-01

We present a scheme to generate the Bell state deterministically on remote transmon qubits coupled to different 1D superconducting resonators connected by a long superconducting transmission line. Using the coherent evolution of the entire system in the all-resonance regime, the transmission line need not to be populated with microwave photons which can robust against the long transmission line loss. This lets the scheme more applicable to the distributed quantum computing on superconducting quantum circuit. Besides, the influence from the small anharmonicity of the energy levels of the transmon qubits can be ignored safely.

Pairing and low temperature properties of 2 D Fermi-systems with attraction between particles

International Nuclear Information System (INIS)

Gorbar, E.V.; Gusynin, V.P.; Loktev, V.M.

1992-01-01

Proceeding from microscopic model Hamiltonian for the system of Fermi-particles with attraction the effective Lagrangian, admitting the analysis of its superconducting properties at arbitrary fermion concentration, is obtained.Exact solution for gap and chemical potential makes it possible to trace from local pair situation to Cooper pairing. The crucial parameter discriminating between the regions of exotic and normal superconducting behaviour is show to be that of the energy of the bound fermion state, which, however, rapidly disappears with fermion density increasing. The solutions of the equations for the case of finite temperatures are analysed. (author). 42 refs

Even-parity spin-triplet pairing by purely repulsive interactions for orbitally degenerate correlated fermions

International Nuclear Information System (INIS)

Zegrodnik, M; Bünemann, J; Spałek, J

2014-01-01

We demonstrate the stability of the spin-triplet paired s-wave (with an admixture of extended s-wave) state for the limit of purely repulsive interactions in a degenerate two-band Hubbard model of correlated fermions. The repulsive interactions limit represents an essential extension of our previous analysis (2013 New J. Phys. 15 073050), regarded here as I. We also show that near the half-filling the considered type of superconductivity can coexist with antiferromagnetism. The calculations have been carried out with the use of the so-called statistically consistent Gutzwiller approximation (SGA) for the case of a square lattice. We suggest that the electron correlations in conjunction with the Hund's rule exchange play the crucial role in stabilizing the real-space spin-triplet superconducting state. A sizable hybridization of the bands suppresses the homogeneous paired state. (paper)

Competing States in the t-J Model: Uniform d-Wave State versus Stripe State versus Stripe State

NARCIS (Netherlands)

Corboz, P.R.; Rice, T.M.; Troyer, M.

2014-01-01

Variational studies of the t-J model on the square lattice based on infinite projected-entangled pair states confirm an extremely close competition between a uniform d-wave superconducting state and different stripe states. The site-centered stripe with an in-phase d-wave order has an equal or only

Superconducting classes in heavy fermions systems

International Nuclear Information System (INIS)

Volovik, G.E.; Gor'kov, L.P.

1985-01-01

A mathematical method for constructing of the superconductivity classes for nontrivial superconductors is described. All superconducting phases which can arise directly on transition from the normal state for cubic, hexagonal and tetragonal symmetries are enumerated. It is shown that in the triplet case the types of zeros in the energy gap always correspond to points on the Fermi surface, whereas for signlet pairing the whole zero lines are possible. For the phases with zeros on the lines or points, the low-temperature specific heat varies as T 2 on T 3 respectivelty. The superconducting phases which arise from the multydimensional representations may possess a magnetic moment which induces currents on the surface of a monodomain sample even in the absence of an external magnetic field. The specific case of a domain wall is considered and it is shown that large magnetic currents of magnetization are present in the wall

Superconductivity

International Nuclear Information System (INIS)

Taylor, A.W.B.; Noakes, G.R.

1981-01-01

This book is an elementray introduction into superconductivity. The topics are the superconducting state, the magnetic properties of superconductors, type I superconductors, type II superconductors and a chapter on the superconductivity theory. (WL)

Emergent loop-nodal s(±)-wave superconductivity in CeCu(2)Si(2): similarities to the iron-based superconductors.

Science.gov (United States)

Ikeda, Hiroaki; Suzuki, Michi-To; Arita, Ryotaro

2015-04-10

Heavy-fermion superconductors are prime candidates for novel electron-pairing states due to the spin-orbital coupled degrees of freedom and electron correlations. Superconductivity in CeCu_{2}Si_{2} discovered in 1979, which is a prototype of unconventional (non-BCS) superconductors in strongly correlated electron systems, still remains unsolved. Here we provide the first report of superconductivity based on the advanced first-principles theoretical approach. We find that the promising candidate is an s_{±}-wave state with loop-shaped nodes on the Fermi surface, different from the widely expected line-nodal d-wave state. The dominant pairing glue is magnetic but high-rank octupole fluctuations. This system shares the importance of multiorbital degrees of freedom with the iron-based superconductors. Our findings reveal not only the long-standing puzzle in this material, but also urge us to reconsider the pairing states and mechanisms in all heavy-fermion superconductors.

The path integral model of D-pairing for HTSC, heavy fermion superconductors, and superfluids

International Nuclear Information System (INIS)

Brusov, P.N.; Brusova, N.P.

1996-01-01

A model of d-pairing for superconducting and superfluid Fermi-systems has been formulated within the path integration technique. By path integration over open-quote fastclose quotes and open-quotes slowclose quotes Fermi-fields, the action functional (which determines all properties of model system) has been obtained. This functional could be used for the determination of different superconducting (superfluid) states, for calculation of the transition temperatures for these states, and for the calculation of the collective mode spectrum for HTSC, as well as for heavy fermion superconductors

Impurity-induced states in superconducting heterostructures

Science.gov (United States)

Liu, Dong E.; Rossi, Enrico; Lutchyn, Roman M.

2018-04-01

Heterostructures allow the realization of electronic states that are difficult to obtain in isolated uniform systems. Exemplary is the case of quasi-one-dimensional heterostructures formed by a superconductor and a semiconductor with spin-orbit coupling in which Majorana zero-energy modes can be realized. We study the effect of a single impurity on the energy spectrum of superconducting heterostructures. We find that the coupling between the superconductor and the semiconductor can strongly affect the impurity-induced states and may induce additional subgap bound states that are not present in isolated uniform superconductors. For the case of quasi-one-dimensional superconductor/semiconductor heterostructures we obtain the conditions for which the low-energy impurity-induced bound states appear.

Superconducting Fluctuations above T c and pair breaking parameters of two dimensional Niobium Nitride Films

Science.gov (United States)

Shinozaki, B.; Ezaki, S.; Odou, T.; Makise, K.; Asano, T.

2018-03-01

Transport properties have been investigated for the epitaxial superconducting NbN thin films. We analysed the excess conductance σ’ ≡ σ(T) - σN by the sum of the Aslamazov-Larkin (AL) and Maki-Thompson (MT) terms for thermal fluctuations above T c, where the σN ≡1/R sq N is the normal state sheet conductance. We have found that the theoretical expression σ’theo (T) = σ’AL (T) + σ’MT (T,δ) can be well fitted to σ’exp (T) with use of the suitable value of the pair breaking parameter δ in the MT term relating to the inelastic scattering rate 1/τin(T) as δ = πħ/8k B Tτin. The rate 1/τin(T) given by the sum of 1/τfluc(T), 1/τe-e(T) and 1/τe-ph (T) is determined from the analysis of the magneto-conductance Δσ = σ(H) – σ(0) by the sum of AL, MT and the localization terms, where the first, second and third terms correspond to the rate due to the superconducting fluctuation effect, electron-electron and electron-phonon interactions, respectively. The R sq N dependence of δ is expressed by δ = δ0 + αR sq N, where the first term δ0 due to 1/τe-ph (T) and the second term due to the sum of 1/τfluc(T) and 1/τe-e(T). Although we obtained a reasonable value of Debye temperature ΘD ≈630 K from the δ0, the magnitude of the α is about 5 times larger than the theoretical value.

Superconducting proximity effect in topological materials

Science.gov (United States)

Reeg, Christopher R.

In recent years, there has been a renewed interest in the proximity effect due to its role in the realization of topological superconductivity. In this dissertation, we discuss several results that have been obtained in the field of proximity-induced superconductivity and relate the results to the search for Majorana fermions. First, we show that repulsive electron-electron interactions can induce a non-Majorana zero-energy bound state at the interface between a conventional superconductor and a normal metal. We show that this state is very sensitive to disorder, owing to its lack of topological protection. Second, we show that Rashba spin-orbit coupling, which is one of the key ingredients in engineering a topological superconductor, induces triplet pairing in the proximity effect. When the spin-orbit coupling is strong (i.e., when the characteristic energy scale for spin-orbit coupling is comparable to the Fermi energy), the induced singlet and triplet pairing amplitudes can be comparable in magnitude. Finally, we discuss how the size of the proximity-induced gap, which appears in a low-dimensional material coupled to a superconductor, evolves as the thickness of the (quasi-)low-dimensional material is increased. We show that the induced gap can be comparable to the bulk energy gap of the underlying superconductor in materials that are much thicker than the Fermi wavelength, even in the presence of an interfacial barrier and strong Fermi surface mismatch. This result has important experimental consequences for topological superconductivity, as a sizable gap is required to isolate and detect the Majorana modes.

From pair correlations to the quasi-particle-phonon nuclear model

International Nuclear Information System (INIS)

Solov'ev, V.G.

1986-01-01

Modern state of the nucleus theory is discussed. The main attention is paid to pair correlation theory of superconducting type and quasiparticle - phonon nucleus model. Pair correlation account allowed one to describe in detail a series of nucleus properties which did not fall within the framework of earlier known models as, for example, double-quasi-particle states in even-even deformed nuclei. To describe the wave function low-quasi-particle components at low, mean and high excitation energies, the nucleus quasi-particle-phonon model is formulated. The strength function method is used in the model and fragmentation of mono-quasi-particle, mono-phonon states and quasi-particle phonon state by many nuclear levels is calculated

Unusually large magnetic moments in the normal state and superconducting state of Sn nanoparticles

Energy Technology Data Exchange (ETDEWEB)

Hung, Chi-Hang; Lee, Chi-Hung; Hsu, Chien-Kang; Li, Chi-Yen; Karna, Sunil K.; Wang, Chin-Wei; Wu, Chun-Ming; Li, Wen-Hsien, E-mail: whli@phy.ncu.edu.tw [National Central University, Department of Physics and Center for Neutron Beam Applications (China)

2013-09-15

We report on the observations of spontaneous magnetic moments in the normal as well as in the superconducting states of a 9 nm Sn nanoparticle assembly, through X-ray diffraction, magnetization, ac magnetic susceptibility, and neutron diffraction measurements. The saturation magnetization reaches an unexpectedly large value of 1.04 emu/g at 5 K, with a temperature profile that can be described by Bloch's law with an exponent of b = 1.8. A magnetic moment of Left-Pointing-Angle-Bracket {mu}{sub Z} Right-Pointing-Angle-Bracket = 0.38 {mu}{sub B} develops after cooling from 260 to 4 K. Superconductivity develops below T{sub C} = 3.98 K, which is 7 % higher than the T{sub C} = 3.72 K of bulk Sn. Surprisingly, an addition magnetic moment of Left-Pointing-Angle-Bracket {mu}{sub Z} Right-Pointing-Angle-Bracket = 0.05 {mu}{sub B} develops upon entering the superconducting state.

Axicell MFTF-B superconducting-magnet system

International Nuclear Information System (INIS)

Wang, S.T.; Bulmer, R.; Hanson, C.; Hinkle, R.; Kozman, T.; Shimer, D.; Tatro, R.; VanSant, J.; Wohlwend, J.

1982-01-01

The Axicell MFTF-B magnet system will provide the field environment necessary for tandem mirror plasma physics investigation with thermal barriers. The performance of the device will stimulate DT to achieve energy break-even plasma conditions. Operation will be with deuterium only. There will be 24 superconducting coils consisting of 2 sets of yin-yang pairs, 14 central-cell solenoids, 2 sets of axicell mirror-coil pairs, and 2 transition coils between the axicell mirror coil-pairs and the yin-yang coils. This paper describes the progress in the design and construction of MFTF-B Superconducting-Magnet System

Superconductivity in the Penson-Kolb Model on a Triangular Lattice

Science.gov (United States)

Ptok, A.; Mierzejewski, M.

2008-07-01

We investigate properties of the two-dimensional Penson-Kolb model with repulsive pair hopping interaction. In the case of a bipartite square lattice this interaction may lead to the η-type pairing, when the phase of superconducting order parameter changes from one lattice site to the neighboring one. We show that this interaction may be responsible for the onset of superconductivity also for a triangular lattice. We discuss the spatial dependence of the superconducting order parameter and demonstrate that the total momentum of the paired electrons is determined by the lattice geometry.

Theory of normal and superconducting properties of fullerene-based solids

International Nuclear Information System (INIS)

Cohen, M.L.

1992-10-01

Recent experiments on the normal-state and superconducting properties of fullerene-based solids are used to constrain the proposal theories of the electronic nature of these materials. In general, models of superconductivity based on electron pairing induced by phonons are consistent with electronic band theory. The latter experiments also yield estimates of the parameters characterizing these type H superconductors. It is argued that, at this point, a ''standard model'' of phonons interacting with itinerant electrons may be a good first approximation for explaining the properties of the metallic fullerenes

Paired Hall states

International Nuclear Information System (INIS)

Greiter, M.

1992-01-01

This dissertation contains a collection of individual articles on various topics. Their significance in the corresponding field as well as connections between them are emphasized in a general and comprehensive introduction. In the first article, the author explores the consequences for macroscopic effective Lagrangians of assuming that the momentum density is proportional to the flow of conserved current. The universal corrections obtained for the macroscopic Lagrangian of a superconductor describe the London Hall effect, and provide a fully consistent derivation of it. In the second article, a heuristic principle is proposed for quantized Hall states: the existence and incompressibility of fractionally quantized Hall states is explained by an argument based on an adiabatic localization of magnetic flux, the process of trading uniform flux for an equal amount of fictitious flux attached to the particles. This principle is exactly implemented in the third article. For a certain class of model Hamiltonians, the author obtains Laughlin's Jastrow type wave functions explicitly from a filled Landau level, by smooth extrapolation in quantum statistics. The generalization of this analysis to the torus geometry shows that theorems restricting the possibilities of quantum statistics on closed surfaces are circumvented in the presence of a magnetic field. In the last article, the existence is proposed of a novel incompressible quantum liquid, a paired Hall state, at a half filled Landau level. This state arises adiabatically from free fermions in zero magnetic field, and reduces to a state previously proposed by Halperin in the limit of tightly bound pairs. It supports unusual excitations, including neutral fermions and charge e/4 anyons with statistical parameter θ = π/8

Universal spectral signatures in pnictides and cuprates: the role of quasiparticle-pair coupling.

Science.gov (United States)

Sacks, William; Mauger, Alain; Noat, Yves

2017-11-08

Understanding the physical properties of a large variety of high-T c superconductors (SC), the cuprate family as well as the more recent iron-based superconductors, is still a major challenge. In particular, these materials exhibit the 'peak-dip-hump' structure in the quasiparticle density of states (DOS). The origin of this structure is explained within our pair-pair interaction (PPI) model: The non-superconducting state consists of incoherent pairs, a 'Cooper-pair glass' which, due to the PPI, undergoes a Bose-like condensation below T c to the coherent SC state. We derive the equations of motion for the quasiparticle operators showing that the DOS 'peak-dip-hump' is caused by the coupling between quasiparticles and excited pair states, or 'super-quasiparticles'. The renormalized SC gap function becomes energy-dependent and non retarded, reproducing accurately the experimental spectra of both pnictides and cuprates, despite the large difference in gap value.

Influence of a superconducting lead on orbital entanglement production in chaotic cavities

International Nuclear Information System (INIS)

Rodriguez-Perez, Sergio; Novaes, Marcel

2015-01-01

We study orbital entanglement production in a chaotic cavity connected to four single-channel normal metal leads and one superconducting lead, assuming the presence of time-reversal symmetry and within a random matrix theory approach. The scattered state of two incident electrons is written as the superposition of several two-outgoing quasi-particle components, four of which are orbitally entangled in a left-right bipartition. We calculate numerically the mean value of the squared norm of each entangled component, as functions of the number of channels in the superconducting lead. Its behavior is explained as resulting from the proximity effect. We also study statistically the amount of entanglement carried by each pair of outgoing quasi-particles. When the influence of the superconductor is more intense, the device works as an entangler of electron-hole pairs, and the average entanglement is found to be considerably larger than that obtained without the superconducting lead. (author)

Influence of a superconducting lead on orbital entanglement production in chaotic cavities

Energy Technology Data Exchange (ETDEWEB)

Rodriguez-Perez, Sergio [Universidade Federal do Rio Grande do Norte (UFRN), Natal, RN (Brazil). Escola de Ciencias e Tecnologia; Novaes, Marcel, E-mail: sergio.rodriguez@ect.ufrn.br [Universidade Federal de Uberlandia (UFU), MG (Brazil). Instituto de Fisica

2015-10-15

We study orbital entanglement production in a chaotic cavity connected to four single-channel normal metal leads and one superconducting lead, assuming the presence of time-reversal symmetry and within a random matrix theory approach. The scattered state of two incident electrons is written as the superposition of several two-outgoing quasi-particle components, four of which are orbitally entangled in a left-right bipartition. We calculate numerically the mean value of the squared norm of each entangled component, as functions of the number of channels in the superconducting lead. Its behavior is explained as resulting from the proximity effect. We also study statistically the amount of entanglement carried by each pair of outgoing quasi-particles. When the influence of the superconductor is more intense, the device works as an entangler of electron-hole pairs, and the average entanglement is found to be considerably larger than that obtained without the superconducting lead. (author)

Ruthenates: simple superconducting qubits

International Nuclear Information System (INIS)

Gulian, Armen M.; Wood, Kent S.

2004-01-01

We propose triplet superconductors, such as ruthenates, as a prospective material for qubit construction. The vectorial nature of the order parameter in triplet superconductors makes it conceptually very easy to imagine the performance of the qubits. The Cooper condensate of pairs in triplet superconductors has all the attributes of the Bose-Einstein condensates and should facilitate long decoherence times of these qubits versus other 'vectorial' schemes for qubits, such as small ferromagnets. There are other benefits, which the superconducting state provides for a requirement like entanglement between qubits via the proximity effect

Quantitative measures of entanglement in pair-coherent states

International Nuclear Information System (INIS)

Agarwal, G S; Biswas, Asoka

2005-01-01

The pair-coherent states for a two-mode radiation field are known to belong to a family of states with non-Gaussian wavefunction. The nature of quantum entanglement between the two modes and some features of non-classicality are studied for such states. The existing criterion for inseparability are examined in the context of pair-coherent states

Unconventional superconductivity in Sr{sub 2}RuO{sub 4}

Energy Technology Data Exchange (ETDEWEB)

Liu, Ying [Department of Physics and Materials Research Institute, Pennsylvania State University, University Park, PA 16802 (United States); Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240 (China); Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093 (China); Mao, Zhi-Qiang [Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118 (United States)

2015-07-15

Highlights: • Constraints on and experimental support to unconventional superconductivity in Sr{sub 2}RuO{sub 4}. • Phase-sensitive determination of the pairing symmetry in Sr{sub 2}RuO{sub 4}. • Response of superconductivity to mechanical perturbations. • Superconductivity in non-bulk Sr{sub 2}RuO{sub 4}. • Unresolved issues and outlook in Sr{sub 2}RuO{sub 4} research. - Abstract: Sr{sub 2}RuO{sub 4}, featuring a layered perovskite crystalline and quasi-two-dimensional electronic structure, was first synthesized in 1959. Unconventional, p-wave pairing was predicted for Sr{sub 2}RuO{sub 4} by Rice and Sigrist and Baskaran shortly after superconductivity in this material was discovered in 1994. Experimental evidence for unconventional superconductivity in Sr{sub 2}RuO{sub 4} has been accumulating in the past two decades and reviewed previously. In this article, we will first discuss constraints on the pairing symmetry of superconductivity in Sr{sub 2}RuO{sub 4} and summarize experimental evidence supporting the unconventional pairing symmetry in this material. We will then present several aspects of the experimental determination of the unconventional superconductivity in Sr{sub 2}RuO{sub 4} in some detail. In particular, we will discuss the phase-sensitive measurements that have played an important role in the determination of the pairing symmetry in Sr{sub 2}RuO{sub 4}. The responses of superconductivity to the mechanical perturbations and their implications on the mechanism of superconductivity will be discussed. A brief survey of various non-bulk Sr{sub 2}RuO{sub 4} will also be included to illustrate the many unusual features resulted from the unconventional nature of superconductivity in this material system. Finally, we will discuss some outstanding unresolved issues on Sr{sub 2}RuO{sub 4} and provide an outlook of the future work on Sr{sub 2}RuO{sub 4}.

Probing the superconducting state of CeCoIn{sub 5} by quantum interferometry

Energy Technology Data Exchange (ETDEWEB)

Foyevtsov, Oleksandr; Porrati, Fabrizio; Huth, Michael [Johann Wolfgang Goethe University, Frankfurt am Main (Germany)

2011-07-01

Josephson junction based structures provide a pathway to investigation of the superconducting state of unconventional superconductors. A superconducting quantum interference device (SQUID) structure was fabricated on micro-crystals of the heavy-fermion superconductor CeCoIn{sub 5}. Photo-lithography and ion beam milling/induced deposition were used to prepare the structure on a thin film of CeCoIn{sub 5} grown via molecular beam epitaxy. The interferometer was characterized with regard to the SQUID properties. The unconventional nature of superconducting state in CeCoIn{sub 5}, the implications of the normal-state electronic properties, as well as the weak-link characteristics of the SQUID structure itself lead to a wealth of different features in the I(V) and dI/dV(V) characteristics.

Existence and consequences of Coulomb pairing of electrons in a solid

International Nuclear Information System (INIS)

Mahajan, S.M.; Thyagaraja, A.

1996-11-01

It is shown from first principles that, in the periodic potential of a crystalline solid, short-range (i.e., screened) binary Coulomb interactions can lead to a two-electron bound state. It is further suggested that these composite bosonic states (charge -2e, and typically spin zero) could mediate an effectively attractive interaction between pairs of conduction electrons close to the Fermi level. This necessarily short range attractive interaction, which is crucially dependent on the band structure of the solid, and is complementary to the phonon-mediated one, may provide a source for the existence and properties of short correlation-length electron pairs (analogous to but distinct from Cooper pairs) needed to understand high temperature superconductivity. Several distinctive and observable characteristics of the proposed pairing scheme are discussed

Controllable Quantum States Mesoscopic Superconductivity and Spintronics (MS+S2006)

Science.gov (United States)

Takayanagi, Hideaki; Nitta, Junsaku; Nakano, Hayato

2008-10-01

Mesoscopic effects in superconductors. Tunneling measurements of charge imbalance of non-equilibrium superconductors / R. Yagi. Influence of magnetic impurities on Josephson current in SNS junctions / T. Yokoyama. Nonlinear response and observable signatures of equilibrium entanglement / A. M. Zagoskin. Stimulated Raman adiabatic passage with a Cooper pair box / Giuseppe Falci. Crossed Andreev reflection-induced giant negative magnetoresistance / Francesco Giazotto -- Quantum modulation of superconducting junctions. Adiabatic pumping through a Josephson weak link / Fabio Taddei. Squeezing of superconducting qubits / Kazutomu Shiokawa. Detection of Berrys phases in flux qubits with coherent pulses / D. N. Zheng. Probing entanglement in the system of coupled Josephson qubits / A. S. Kiyko. Josephson junction with tunable damping using quasi-particle injection / Ryuta Yagi. Macroscopic quantum coherence in rf-SQUIDs / Alexey V. Ustinov. Bloch oscillations in a Josephson circuit / D. Esteve. Manipulation of magnetization in nonequilibrium superconducting nanostructures / F. Giazotto -- Superconducting qubits. Decoherence and Rabi oscillations in a qubit coupled to a quantum two-level system / Sahel Ashhab. Phase-coupled flux qubits: CNOT operation, controllable coupling and entanglement / Mun Dae Kim. Characteristics of a switchable superconducting flux transformer with a DC-SQUID / Yoshihiro Shimazu. Characterization of adiabatic noise in charge-based coherent nanodevices / E. Paladino -- Unconventional superconductors. Threshold temperatures of zero-bias conductance peak and zero-bias conductance dip in diffusive normal metal/superconductor junctions / Iduru Shigeta. Tunneling conductance in 2DEG/S junctions in the presence of Rashba spin-orbit coupling / T. Yokoyama. Theory of charge transport in diffusive ferromagnet/p-wave superconductor junctions / T. Yokoyama. Theory of enhanced proximity effect by the exchange field in FS bilayers / T. Yokoyama. Theory of

Mechanisms of conventional and high Tc superconductivity

International Nuclear Information System (INIS)

Kresin, V.L.; Morawitz, H.; Wolf, S.A.

1993-01-01

This book gives a careful and objective review of theories of superconductivity in traditional superconductors, organics, and high Tc cuprates. Of course, the authors do still present their own theories of cuprate superconductivity, but only in the final chapter after other possibilities have been discussed. The book should be especially useful for researchers entering the field of high Tc superconductivity. The reviews of photon mediated pairing and strong coupling theory are very welcome, since much of this material has not been reviewed since the classic 1969 volume edited by Parks. In particular the authors dispel the various myths that phonon mediated pairing leads to upper bounds on Tc. In addition to phonon mediated pairing the book discussed in detail pairing due to exchange of acoustic (demon) plasmons, excitons, or magnetic fluctuations. There have been so many diverse mechanisms based on strong correlation and large U Hubbard models that a book like this can only discuss a limited selection of the main contenders. In particular here the emphasis on Fermi liquid based models no doubt reflects the authors' own point of view. A whole chapter discusses the concepts of induced superconductivity, in the proximity effect, and its application to materials with several different electronic subsystems

Superconductivity

CERN Document Server

Poole, Charles P; Farach, Horacio A

1995-01-01

Superconductivity covers the nature of the phenomenon of superconductivity. The book discusses the fundamental principles of superconductivity; the essential features of the superconducting state-the phenomena of zero resistance and perfect diamagnetism; and the properties of the various classes of superconductors, including the organics, the buckministerfullerenes, and the precursors to the cuprates. The text also describes superconductivity from the viewpoint of thermodynamics and provides expressions for the free energy; the Ginzburg-Landau and BCS theories; and the structures of the high

Superconducting current in a bisoliton superconductivity model

International Nuclear Information System (INIS)

Ermakov, V.N.; Kruchinin, S.P.; Ponezha, E.A.

1991-01-01

It is shown that the transition into a superconducting state with the current which is described by a bisoliton superconductivity model is accompanied by the deformation of the spectrum of one-particle states of the current carriers. The deformation value is proportional to the conducting current force. The residuaby resistance in such state is absent

Tunable hybridization of Majorana bound states at the quantum spin Hall edge

Science.gov (United States)

Keidel, Felix; Burset, Pablo; Trauzettel, Björn

2018-02-01

Confinement at the helical edge of a topological insulator is possible in the presence of proximity-induced magnetic (F) or superconducting (S) order. The interplay of both phenomena leads to the formation of localized Majorana bound states (MBS) or likewise (under certain resonance conditions) the formation of ordinary Andreev bound states (ABS). We investigate the properties of bound states in junctions composed of alternating regions of F or S barriers. Interestingly, the direction of magnetization in F regions and the relative superconducting phase between S regions can be exploited to hybridize MBS or ABS at will. We show that the local properties of MBS translate into a particular nonlocal superconducting pairing amplitude. Remarkably, the symmetry of the pairing amplitude contains information about the nature of the bound state that it stems from. Hence this symmetry can in principle be used to distinguish MBS from ABS, owing to the strong connection between local density of states and nonlocal pairing in our setup.

Effect of localized electron states on superconductivity of ultrathin beryllium films

International Nuclear Information System (INIS)

Tutov, V.I.; Semenenko, E.E.

1988-01-01

A wide spectrum of distortions is induced in ultrathin beryllium films of thickness less than 10 A, which are responsible for the system transition from the strong localization state completely suppressing superconductivity (in this case R □ of the layer reaches 97600 Ohm) to the weak localization stae coexisting with superconductivity at comparatively high T c (5 K). The resistance per square R □ of the films decreases more than by an order of magnitude. The superconductivity with T c =1.7 K occurs at rather strong localization, when R □ of the layer is 34000 Ohm

Shooting quasiparticles from Andreev bound states in a superconducting constriction

Energy Technology Data Exchange (ETDEWEB)

Riwar, R.-P.; Houzet, M.; Meyer, J. S. [University of Grenoble Alpes, INAC-SPSMS (France); Nazarov, Y. V., E-mail: Y.V.Nazarov@tudelft.nl [Delft University of Technology, Kavli Institute of NanoScience (Netherlands)

2014-12-15

A few-channel superconducting constriction provides a set of discrete Andreev bound states that may be populated with quasiparticles. Motivated by recent experimental research, we study the processes in an a.c. driven constriction whereby a quasiparticle is promoted to the delocalized states outside the superconducting gap and flies away. We distinguish two processes of this kind. In the process of ionization, a quasiparticle present in the Andreev bound state is transferred to the delocalized states leaving the constriction. The refill process involves two quasiparticles: one flies away while another one appears in the Andreev bound state. We notice an interesting asymmetry of these processes. The electron-like quasiparticles are predominantly emitted to one side of the constriction while the hole-like ones are emitted to the other side. This produces a charge imbalance of accumulated quasiparticles, that is opposite on opposite sides of the junction. The imbalance may be detected with a tunnel contact to a normal metal lead.

Measurement and Quantum State Transfer in Superconducting Qubits

Science.gov (United States)

Mlinar, Eric

The potential of superconducting qubits as the medium for a scalable quantum computer has motivated the pursuit of improved interactions within this system. Two challenges for the field of superconducting qubits are measurement fidelity, to accurately determine the state of the qubit, and the efficient transfer of quantum states. In measurement, the current state-of-the-art method employs dispersive readout, by coupling the qubit to a cavity and reading the resulting shift in cavity frequency to infer the qubit's state; however, this is vulnerable to Purcell relaxation, as well as being modeled off a simplified two-level abstraction of the qubit. In state transfer, the existing proposal for moving quantum states is mostly untested against non-idealities that will likely be present in an experiment. In this dissertation, we examine three problems within these two areas. We first describe a new scheme for fast and high-fidelity dispersive measurement specifically designed to circumvent the Purcell Effect. To do this, the qubit-resonator interaction is turned on only when the resonator is decoupled from the environment; then, after the resonator state has shifted enough to infer the qubit state, the qubit-resonator interaction is turned off before the resonator and environment are recoupled. We also show that the effectiveness of this "Catch-Disperse-Release'' procedure partly originates from quadrature squeezing of the resonator state induced by the Jaynes-Cummings nonlinearity. The Catch-Disperse-Release measurement scheme treats the qubit as a two-level system, which is a common simplification used in theoretical works. However, the most promising physical candidate for a superconducting qubit, the transmon, is a multi-level system. In the second work, we examine the effects of including the higher energy levels of the transmon. Specifically, we expand the eigenstate picture developed in the first work to encompass multiple qubit levels, and examine the resulting

Superconductivity: A testing ground for models of confinement

International Nuclear Information System (INIS)

Ball, J.S.; Caticha, A.

1988-01-01

The interaction of a magnetic monopole-antimonopole pair in a superconductor is calculated as a function of their separation and the value of the Landau-Ginzburg parameter. This direct numerical result is then compared to the bag approximation to the same interaction in a superconducting medium. The actual potential exhibits the same general features as those obtained in the bag calculation. If the bag pressure is used as a phenomenological parameter, rather than the value fixed by the superconducting energy density, the agreement is excellent. Numerically the actual problem was actually no more difficult than the bag calculation. The interaction between magnetic monopoles and antimonopoles in the superconducting vacuum state is similar to the interaction of heavy colored quarks in a flux-confining physical QCD vacuum state. This means that our results are probably a good indication of the general behavior of the QCD potential and of the reliability of the bag approximation in the calculation of this potential. Our results also show that the bag model is a good approximation to a dual superconductor. This indicates that a dual superconducting picture of QCD would lead to the same heavy-quark potential and perhaps retain more of the physics than the bag model

Superconductivity in all its states

CERN Multimedia

Globe Info

2011-01-01

Temporary exhibition at the Saint-Genis-Pouilly Tourist Office. For the 100th anniversary of its discovery, take a plunge into the amazing world of superconductivity. Some materials, when cooled down to extreme temperatures, acquire a remarkable property -  they become superconducting. Superconductivity is a rare example of a quantum effect that can be witnessed on the macroscopic scale and is today at the heart of much research. In laboratories, researchers try to gain a better understanding of its origins, study new superconducting materials, explore the phenomenon at the nanometric scale and pursue their indefatigable search for new applications. Monday to Friday: 09:00 a.m. to 12:00 and 2:30 p.m. to 6:30 p.m. Saturday: 10:00 a.m. to 12:00 noon » Open to all – Admission free For further information: +33 (0)4 50 42 29 37

Orbitally limited pair-density-wave phase of multilayer superconductors

Science.gov (United States)

Möckli, David; Yanase, Youichi; Sigrist, Manfred

2018-04-01

We investigate the magnetic field dependence of an ideal superconducting vortex lattice in the parity-mixed pair-density-wave phase of multilayer superconductors within a circular cell Ginzburg-Landau approach. In multilayer systems, due to local inversion symmetry breaking, a Rashba spin-orbit coupling is induced at the outer layers. This combined with a perpendicular paramagnetic (Pauli) limiting magnetic field stabilizes a staggered layer dependent pair-density-wave phase in the superconducting singlet channel. The high-field pair-density-wave phase is separated from the low-field BCS phase by a first-order phase transition. The motivating guiding question in this paper is: What is the minimal necessary Maki parameter αM for the appearance of the pair-density-wave phase of a superconducting trilayer system? To address this problem we generalize the circular cell method for the regular flux-line lattice of a type-II superconductor to include paramagnetic depairing effects. Then, we apply the model to the trilayer system, where each of the layers are characterized by Ginzburg-Landau parameter κ0 and a Maki parameter αM. We find that when the spin-orbit Rashba interaction compares to the superconducting condensation energy, the orbitally limited pair-density-wave phase stabilizes for Maki parameters αM>10 .

Junctionless Cooper pair transistor

Energy Technology Data Exchange (ETDEWEB)

Arutyunov, K. Yu., E-mail: konstantin.yu.arutyunov@jyu.fi [National Research University Higher School of Economics , Moscow Institute of Electronics and Mathematics, 101000 Moscow (Russian Federation); P.L. Kapitza Institute for Physical Problems RAS , Moscow 119334 (Russian Federation); Lehtinen, J.S. [VTT Technical Research Centre of Finland Ltd., Centre for Metrology MIKES, P.O. Box 1000, FI-02044 VTT (Finland)

2017-02-15

Highlights: • Junctionless Cooper pair box. • Quantum phase slips. • Coulomb blockade and gate modulation of the Coulomb gap. - Abstract: Quantum phase slip (QPS) is the topological singularity of the complex order parameter of a quasi-one-dimensional superconductor: momentary zeroing of the modulus and simultaneous 'slip' of the phase by ±2π. The QPS event(s) are the dynamic equivalent of tunneling through a conventional Josephson junction containing static in space and time weak link(s). Here we demonstrate the operation of a superconducting single electron transistor (Cooper pair transistor) without any tunnel junctions. Instead a pair of thin superconducting titanium wires in QPS regime was used. The current–voltage characteristics demonstrate the clear Coulomb blockade with magnitude of the Coulomb gap modulated by the gate potential. The Coulomb blockade disappears above the critical temperature, and at low temperatures can be suppressed by strong magnetic field.

Probing spin-polarized edge state superconductivity by Andreev reflection in in-plane magnetic fields

Energy Technology Data Exchange (ETDEWEB)

Reinthaler, Rolf W.; Tkachov, Grigory; Hankiewicz, Ewelina M. [Faculty of Physics and Astrophysics, University of Wuerzburg, Wuerzburg (Germany)

2015-07-01

Finding signatures of unconventional superconductivity in Quantum Spin Hall systems is one of the challenges of solid state physics. Here we induce superconductivity in a 3D topological insulator thin film to cause the formation of helical edge states, which are protected against backscattering even in finite magnetic fields. Above a critical in-plane magnetic field, which is much smaller than the critical field of typical superconductors, the quasi-particle gap closes, giving rise to energy-dependent spin polarization. In this regime the spin-polarized edge state superconductivity can be detected by Andreev reflection. We propose measurement setups to experimentally observe the spin-dependent excess current and dI/dV characteristics.

Topological aspect and the pairing symmetries on spin-triplet chiral p-wave superconductor under strain

Science.gov (United States)

Imai, Yoshiki; Sigrist, Manfred

2018-05-01

Motivated by recent experiments on Sr2RuO4, the effect of uniaxial strain on the chiral p-wave superconductor is discussed. We study particularly the relation between the topological indices and different pairing states in the superconducting phase through the thermal Hall conductivity, which is proportional to temperature and the Chern number in the very low-temperature limit. We show that the temperature-dependence of the thermal Hall conductivity under uniaxial strain depends strongly on the form of the pairing state. The obtained result may provide a possible experimental probe for the pairing structure in Sr2RuO4.

Single-flavor color superconductivity with color-sextet pairing

Czech Academy of Sciences Publication Activity Database

Brauner, Tomáš

2005-01-01

Roč. 55, č. 1 (2005), s. 9-16 ISSN 0011-4626 R&D Projects: GA ČR(CZ) GA202/02/0847 Keywords : color superconductivity * spontaneous symmetry breaking Subject RIV: BE - Theoretical Physics Impact factor: 0.360, year: 2005

Unconventional superconductivity in a two-dimensional repulsive gas of fermions with spin-orbit coupling

Science.gov (United States)

Wang, Luyang; Vafek, Oskar

2014-02-01

We investigate the superconducting instability of a two-dimensional repulsive Fermi gas with Rashba spin-orbit coupling αR. Using renormalization group approach, we find the superconducting transition temperature as a function of the dimensionless ratio Θ=1}/{2}mαR2/EF where EF = 0 when the smaller Fermi surface shrinks to a (Dirac) point. The general trend is that superconductivity is enhanced as Θ increases, but in an intermediate regime Θ ∼ 0.1, a dome-like behavior appears. At a very small value of Θ, the angular momentum channel jz in which superconductivity occurs is quite high. With increasing Θ, jz decreases with a step of 2 down to jz = 6, after which we find the sequence jz = 6, 4, 6, 2, the last value of which continues to Θ → ∞. In an extended range of Θ, the superconducting gap predominantly resides on the large Fermi surface, while Josephson coupling induces a much smaller gap on the small Fermi surface. Below the superconducting transition temperature, we apply mean field theory to derive the self-consistent equations and find the condensation energies. The state with the lowest condensation energy is an unconventional superconducting state which breaks time-reversal symmetry, and in which singlet and triplet pairings are mixed. In general, these states are topologically nontrivial, and the Chern number of the state with total angular momentum jz is C = 2jz.

Exact mapping of the dx2-y2 Cooper-pair wavefunction onto the spin fluctuations in cuprates: the Fermi surface as a driver for 'high Tc' superconductivity

International Nuclear Information System (INIS)

McDonald, Ross D; Harrison, Neil; Singleton, John

2009-01-01

We propose that the extraordinarily high superconducting transition temperatures in the cuprates are driven by an exact mapping of the d x 2 -y 2 Cooper-pair wavefunction onto the incommensurate spin fluctuations observed in neutron-scattering experiments. This is manifested in the direct correspondence between the inverse of the incommensurability factor δ seen in inelastic neutron-scattering experiments and the measured superconducting coherence length ξ 0 . Strikingly, the relationship between ξ 0 and δ is valid for both La 2-x Sr x CuO 4 and YBa 2 Cu 3 O 7-x , suggesting a common mechanism for superconductivity across the entire hole-doped cuprate family. Using data from recent quantum-oscillation experiments in the cuprates, we propose that the fluctuations responsible for superconductivity are driven by a Fermi-surface instability. On the basis of these findings, one can specify the optimal characteristics of a solid that will exhibit 'high T c ' superconductivity. (fast track communication)

Exotic Paired States with Anisotropic Spin-Dependent Fermi Surfaces

International Nuclear Information System (INIS)

Feiguin, Adrian E.; Fisher, Matthew P. A.

2009-01-01

We propose a model for realizing exotic paired states in cold Fermi gases by using a spin-dependent optical lattice to engineer mismatched Fermi surfaces for each hyperfine species. The BCS phase diagram shows a stable paired superfluid state with coexisting pockets of momentum space with gapless unpaired carriers, similar to the Sarma state in polarized mixtures, but in our case the system is unpolarized. We propose the possible existence of an exotic 'Cooper-pair Bose-metal' phase, which has a gap for single fermion excitations but gapless and uncondensed 'Cooper-pair' excitations residing on a 'Bose surface' in momentum space.

On friction of Nb-Nb pair in He1 and He2

International Nuclear Information System (INIS)

Zinenko, S.A.; Karapetyan, S.S.; Silin, A.A.

1990-01-01

Peculiarities of manifestation of the effect of anomalous friction of superconductors (AFS) in He1 and He2 are studied. Helium thermodynamic state effect on the character of friction interaction of Nb-Nb pair velocity and reduction ratio for friction coefficient is studied. The intensity of heat removal released from friction contact region is estimated, the necessary and sufficient conditions for AFC effect manifestation are ascertained using characteristic relaxation time concept. Dependences for Nb-Nb pair friction coefficient in a superconducting state on the time of friction interaction in gaseous helium, He1, He2 are presented

Superconductivity

International Nuclear Information System (INIS)

Palmieri, V.

1990-01-01

This paper reports on superconductivity the absence of electrical resistance has always fascinated the mind of researchers with a promise of applications unachievable by conventional technologies. Since its discovery superconductivity has been posing many questions and challenges to solid state physics, quantum mechanics, chemistry and material science. Simulations arrived to superconductivity from particle physics, astrophysic, electronics, electrical engineering and so on. In seventy-five years the original promises of superconductivity were going to become reality: a microscopical theory gave to superconductivity the cloth of the science and the level of technological advances was getting higher and higher. High field superconducting magnets became commercially available, superconducting electronic devices were invented, high field accelerating gradients were obtained in superconductive cavities and superconducting particle detectors were under study. Other improvements came in a quiet progression when a tornado brought a revolution in the field: new materials had been discovered and superconductivity, from being a phenomenon relegated to the liquid Helium temperatures, became achievable over the liquid Nitrogen temperature. All the physics and the technological implications under superconductivity have to be considered ab initio

Formation of Singlet Fermion Pairs in the Dilute Gas of Boson-Fermion Mixture

Directory of Open Access Journals (Sweden)

Minasyan V.

2010-10-01

Full Text Available We argue the formation of a free neutron spinless pairs in a liquid helium -dilute neutron gas mixture. We show that the term, of the interaction between the excitations of the Bose gas and the density modes of the neutron, meditate an attractive interaction via the neutron modes, which in turn leads to a bound state on a spinless neutron pair. Due to presented theoretical approach, we prove that the electron pairs in superconductivity could be discovered by Frölich earlier then it was made by the Cooper.

Superconductivity mediated by quantum critical antiferromagnetic fluctuations: The rise and fall of hot spots

Science.gov (United States)

Wang, Xiaoyu; Schattner, Yoni; Berg, Erez; Fernandes, Rafael M.

2017-05-01

In several unconventional superconductors, the highest superconducting transition temperature Tc is found in a region of the phase diagram where the antiferromagnetic transition temperature extrapolates to zero, signaling a putative quantum critical point. The elucidation of the interplay between these two phenomena—high-Tc superconductivity and magnetic quantum criticality—remains an important piece of the complex puzzle of unconventional superconductivity. In this paper, we combine sign-problem-free quantum Monte Carlo simulations and field-theoretical analytical calculations to unveil the microscopic mechanism responsible for the superconducting instability of a general low-energy model, called the spin-fermion model. In this approach, low-energy electronic states interact with each other via the exchange of quantum critical magnetic fluctuations. We find that even in the regime of moderately strong interactions, both the superconducting transition temperature and the pairing susceptibility are governed not by the properties of the entire Fermi surface, but instead by the properties of small portions of the Fermi surface called hot spots. Moreover, Tc increases with increasing interaction strength, until it starts to saturate at the crossover from hot-spots-dominated to Fermi-surface-dominated pairing. Our work provides not only invaluable insights into the system parameters that most strongly affect Tc, but also important benchmarks to assess the origin of superconductivity in both microscopic models and actual materials.

Metallization and superconductivity in a multizone doped semiconductor: boron-doped diamond

International Nuclear Information System (INIS)

Loktev, V.M.; Pogorelov, Yu.G.

2005-01-01

Within the framework of Anderson's s - d hybride model, metallization of a semiconductor at collectivization of impurity states is discussed. Taking in mind the description of boron-doped diamond CB x , the model is generalized for the case of the multiband initial spectrum and cluster acceptor states, due to the pairs of the nearest neighbor impurities ('impurity dumbbells'). The parameters of the calculated band of collective impurity states are compared to those observed in metallized and superconducting CB x

Coexistence of spin-triplet superconductivity with magnetism within a single mechanism for orbitally degenerate correlated electrons: statistically consistent Gutzwiller approximation

International Nuclear Information System (INIS)

Zegrodnik, M; Spałek, J; Bünemann, J

2013-01-01

An orbitally degenerate two-band Hubbard model is analyzed with the inclusion of the Hund's rule-induced spin-triplet even-parity paired states and their coexistence with magnetic ordering. The so-called statistically consistent Gutzwiller approximation (SGA) has been applied to the case of a square lattice. The superconducting gaps, the magnetic moment and the free energy are analyzed as a function of the Hund's rule coupling strength and the band filling. Also, the influence of the intersite hybridization on the stability of paired phases is discussed. In order to examine the effect of correlations the results are compared with those calculated earlier within the Hartree–Fock (HF) approximation combined with the Bardeen–Cooper–Schrieffer (BCS) approach. Significant differences between the two methods used (HF + BCS versus SGA + real-space pairing) appear in the stability regions of the considered phases. Our results supplement the analysis of this canonical model used widely in the discussions of pure magnetic phases with the detailed elaboration of the stability of the spin-triplet superconducting states and the coexistent magnetic-superconducting states. At the end, we briefly discuss qualitatively the factors that need to be included for a detailed quantitative comparison with the corresponding experimental results. (paper)

Characterisation of superconducting capillaries for magnetic shielding of twisted-wire pairs in a neutron electric dipole moment experiment

Energy Technology Data Exchange (ETDEWEB)

Henry, S., E-mail: s.henry@physics.ox.ac.uk; Pipe, M.; Cottle, A.; Clarke, C.; Divakar, U.; Lynch, A.

2014-11-01

The cryoEDM neutron electric dipole moment experiment requires a SQUID magnetometry system with pick-up loops inside a magnetically shielded volume connected to SQUID sensors by long (up to 2 m) twisted-wire pairs (TWPs). These wires run outside the main shield, and therefore must run through superconducting capillaries to screen unwanted magnetic pick-up. We show that the average measured transverse magnetic pick-up of a set of lengths of TWPs is equivalent to a loop area of 5.0×10{sup −6} m{sup 2}/m, or 14 twists per metre. From this we set the requirement that the magnetic shielding factor of the superconducting capillaries used in the cryoEDM system must be greater than 8.0×10{sup 4}. The shielding factor—the ratio of the signal picked-up by an unshielded TWP to that induced in a shielded TWP—was measured for a selection of superconducting capillaries made from solder wire. We conclude the transverse shielding factor of a uniform capillary is greater than 10{sup 7}. The measured pick-up was equal to, or less than that due to direct coupling to the SQUID sensor (measured without any TWP attached). We show that discontinuities in the capillaries substantially impair the magnetic shielding, yet if suitably repaired, this can be restored to the shielding factor of an unbroken capillary. We have constructed shielding assemblies for cryoEDM made from lengths of single core and triple core solder capillaries, joined by a shielded Pb cylinder, incorporating a heater to heat the wires above the superconducting transition as required.

Robustness of Topological Superconductivity in Solid State Hybrid Structures

Science.gov (United States)

Sitthison, Piyapong

The non-Abelian statistics of Majorana fermions (MFs) makes them an ideal platform for implementing topological quantum computation. In addition to the fascinating fundamental physics underlying the emergence of MFs, this potential for applications makes the study of these quasiparticles an extremely popular subject in condensed matter physics. The commonly called `Majorana fermions' are zero-energy bound states that emerge near boundaries and defects in topological superconducting phases, which can be engineered, for example, by proximity coupling strong spin-orbit coupling semiconductor nanowires and ordinary s-wave superconductors. The stability of these bound states is determined by the stability of the underlying topological superconducting phase. Hence, understanding their stability (which is critical for quantum computation), involves studying the robustness of the engineered topological superconductors. This work addresses this important problem in the context of two types of hybrid structures that have been proposed for realizing topological superconductivity: topological insulator - superconductor (TI-SC) and semiconductor - superconductor (SM-SC) nanostructures. In both structures, electrostatic effects due to applied external potentials and interface-induced potentials are significant. This work focuses on developing a theoretical framework for understanding these effects, to facilitate the optimization of the nanostructures studied in the laboratory. The approach presented in this thesis is based on describing the low-energy physics of the hybrid structure using effective tight-binding models that explicitly incorporate the proximity effects emerging at interfaces. Generically, as a result of the proximity coupling to the superconductor, an induced gap emerges in the semiconductor (topological insulator) sub-system. The strength of the proximity-induced gap is determined by the transparency of the interface and by the amplitude of the low- energy SM

On the mechanism of high-temperature superconductivity in hydrogen sulfide at 200 GPa: Transition into superconducting anti-adiabatic state in coupling to H-vibrations

Directory of Open Access Journals (Sweden)

Pavol Baňacký

Full Text Available It has been shown that the adiabatic electronic structure of the superconducting phase of sulfur hydride at 200 GPa is unstable toward the vibration motion of H-atoms. A theoretical study indicates that in coupling to H-vibrations, the system undergoes a transition from adiabatic into a stabilized anti-adiabatic multi-gap superconducting state at a temperature that can reach 203 K. Keywords: Superconductivity of sulfur hydride, Electron–phonon coupling in superconductors, Anti-adiabatic theory of superconductivity

Critical current enhancement driven by suppression of superconducting fluctuation in ion-gated ultrathin FeSe

Science.gov (United States)

Harada, T.; Shiogai, J.; Miyakawa, T.; Nojima, T.; Tsukazaki, A.

2018-05-01

The framework of phase transition, such as superconducting transition, occasionally depends on the dimensionality of materials. Superconductivity is often weakened in the experimental conditions of two-dimensional thin films due to the fragile superconducting state against defects and interfacial effects. In contrast to this general trend, superconductivity in the thin limit of FeSe exhibits an opposite trend, such as an increase in critical temperature (T c) and the superconducting gap exceeding the bulk values; however, the dominant mechanism is still under debate. Here, we measured thickness-dependent electrical transport properties of the ion-gated FeSe thin films to evaluate the superconducting critical current (I c) in the ultrathin FeSe. Upon systematically decreasing the FeSe thickness by the electrochemical etching technique in the Hall bar-shaped electric double-layer transistors, we observed a dramatic enhancement of I c reaching about 10 mA and corresponding to about 107 A cm‑2 in the thinnest condition. By analyzing the transition behavior, we clarify that the suppressed superconducting fluctuation is one of the origins of the large I c in the ion-gated ultrathin FeSe films. These results indicate the existence of a robust superconducting state possibly with dense Cooper pairs at the thin limit of FeSe.

Wigner function and tomogram of the pair coherent state

International Nuclear Information System (INIS)

Meng, Xiang-Guo; Wang, Ji-Suo; Fan, Hong-Yi

2007-01-01

Using the entangled state representation of Wigner operator and the technique of integration within an ordered product (IWOP) of operators, the Wigner function of the pair coherent state is derived. The variations of the Wigner function with the parameters α and q in the ρ-γ phase space are discussed. The physical meaning of the Wigner function for the pair coherent state is given by virtue of its marginal distributions. The tomogram of the pair coherent state is calculated with the help of the Radon transform between the Wigner operator and the projection operator of the entangled state |η 1 ,η 2 ,τ 1 ,τ 2 >

Pairing from dynamically screened Coulomb repulsion in bismuth

Science.gov (United States)

Ruhman, Jonathan; Lee, Patrick A.

2017-12-01

Recently, Prakash et al. have discovered bulk superconductivity in single crystals of bismuth, which is a semimetal with extremely low carrier density. At such low density, we argue that conventional electron-phonon coupling is too weak to be responsible for the binding of electrons into Cooper pairs. We study a dynamically screened Coulomb interaction with effective attraction generated on the scale of the collective plasma modes. We model the electronic states in bismuth to include three Dirac pockets with high velocity and one hole pocket with a significantly smaller velocity. We find a weak-coupling instability, which is greatly enhanced by the presence of the hole pocket. Therefore we argue that bismuth is the first material to exhibit superconductivity driven by retardation effects of Coulomb repulsion alone. By using realistic parameters for bismuth we find that the acoustic plasma mode does not play the central role in pairing. We also discuss a matrix element effect, resulting from the Dirac nature of the conduction band, which may affect Tc in the s -wave channel without breaking time-reversal symmetry.

Attenuation in Superconducting Circular Waveguides

Directory of Open Access Journals (Sweden)

K. H. Yeap

2016-09-01

Full Text Available We present an analysis on wave propagation in superconducting circular waveguides. In order to account for the presence of quasiparticles in the intragap states of a superconductor, we employ the characteristic equation derived from the extended Mattis-Bardeen theory to compute the values of the complex conductivity. To calculate the attenuation in a circular waveguide, the tangential fields at the boundary of the wall are first matched with the electrical properties (which includes the complex conductivity of the wall material. The matching of fields with the electrical properties results in a set of transcendental equations which is able to accurately describe the propagation constant of the fields. Our results show that although the attenuation in the superconducting waveguide above cutoff (but below the gap frequency is finite, it is considerably lower than that in a normal waveguide. Above the gap frequency, however, the attenuation in the superconducting waveguide increases sharply. The attenuation eventually surpasses that in a normal waveguide. As frequency increases above the gap frequency, Cooper pairs break into quasiparticles. Hence, we attribute the sharp rise in attenuation to the increase in random collision of the quasiparticles with the lattice structure.

Unconventional superconductivity of the heavy fermion compound UNi2Al3

International Nuclear Information System (INIS)

Zakharov, Andrey

2008-01-01

The heavy fermion compound UNi 2 Al 3 exhibits the coexistence of superconductivity and magnetic order at low temperatures, stimulating speculations about possible exotic Cooper-pairing interaction in this superconductor. However, the preparation of good quality bulk single crystals of UNi 2 Al 3 has proven to be a non-trivial task due to metallurgical problems, which result in the formation of an UAl 2 impurity phase and hence a strongly reduced sample purity. The present work concentrates on the preparation, characterization and electronic properties investigation of UNi 2 Al 3 single crystalline thin film samples. The preparation of thin films was accomplished in a molecular beam epitaxy (MBE) system. (100)-oriented epitaxial thin films of UNi 2 Al 3 were grown on single crystalline YAlO 3 substrates cut in (010)- or (112)-direction. The high crystallographic quality of the samples was proved by several characterisation methods, such as X-ray analysis, RHEED and TEM. To study the magnetic structure of epitaxial thin films resonant magnetic X-ray scattering was employed. The magnetic order of thin the film samples, the formation of magnetic domains with different moment directions, and the magnetic correlation length were discussed. The electronic properties of the UNi 2 Al 3 thin films in the normal and superconducting states were investigated by means of transport measurements. A pronounced anisotropy of the temperature dependent resistivity ρ(T) was observed. Moreover, it was found that the temperature of the resistive superconducting transition depends on the current direction, providing evidence for multiband superconductivity in UNi 2 Al 3 . The initial slope of the upper critical field H' c2 (T) of the thin film samples suggests an unconventional spin-singlet superconducting state, as opposed to bulk single crystal data. To probe the superconducting gap of UNi 2 Al 3 directly by means of tunnelling spectroscopy many planar junctions of different design

Teleportation of Quantum States through Mixed Entangled Pairs

Institute of Scientific and Technical Information of China (English)

ZHENG Shi-Biao

2006-01-01

@@ We describe a protocol for quantum state teleportation via mixed entangled pairs. With the help of an ancilla,near-perfect teleportation might be achieved. For pure entangled pairs, perfect teleportation might be achieved with a certain probability without using an ancilla. The protocol is generalized to teleportation of multiparticle states and quantum secret sharing.

Nonlinearity in superconductivity and Josephson junctions

International Nuclear Information System (INIS)

Lazarides, N.

1995-01-01

Within the framework of the Bardeen, Cooper and Schrieffers (BCS) theory, the influence of anisotropy on superconducting states are investigated. Crystal anisotropy exists in un-conventional low temperature superconductors as e.g. U 1-x Th x Be 13 and in high temperature superconductors. Starting from a phenomenological pairing interaction of the electrons or holes, the BCS approach is used to derive a set of coupled nonlinear algebraic equations for the momentum dependent gap parameter. The emphasis is put on bifurcation phenomena between s-, d-wave and mixed s- and d-wave symmetry and the influence on measurable quantities as the electron specific heat, spin susceptibility and Josephson tunnelling. Pitch-fork and perturbed pitch-fork bifurcations have been found separating s- and d-wave superconducting states from mixed s- and d-wave states. The additional superconducting states give rise to jumps in the electron specific heat below the transition temperature. These jumps are rounded in the case of perturbed pitch-fork bifurcations. An experiment to measure the sign of the interlayer interaction using dc SQUIDS is suggested. The Ambegaokar-Baratoff formalism has been used for calculating the quasiparticle current and the two phase coherent tunnelling currents in a Josephson junction made of anisotropic superconductors. It is shown that anisotropy can lead to a reduction in the product of the normal resistance and the critical current. For low voltages across the junction the usual resistively shunted Josephson model can be used. Finally, bunching in long circular Josephson junctions and suppression of chaos in point junctions have been investigated. (au) 113 refs

Unconventional Cooper pairing results in a pseudogap-like phase in s-wave superconductors

International Nuclear Information System (INIS)

Springer, Daniel; Cheong, Siew Ann

2015-01-01

The impact of disorder on the superconducting (SC) pairing mechanism is the centre of much debate. Some evidence suggests a loss of phase coherence of pairs while others point towards the formation of a competing phase. In our work we show that the two perspectives may be different sides of the same coin. Using an extension of the perturbative renormalization group approach we compare the impact of different disorder-induced interactions on a SC ground state. We find that in the strongly disordered regime an interaction between paired fermions and their respective disordered environment replaces conventional Cooper pairing. For these unconventional Cooper pairs the phase coherence condition, required for the formation of a SC condensate, is not satisfied. (paper)

The role of local repulsion in superconductivity in the Hubbard-Holstein model

Science.gov (United States)

Lin, Chungwei; Wang, Bingnan; Teo, Koon Hoo

2017-01-01

We examine the superconducting solution in the Hubbard-Holstein model using Dynamical Mean Field Theory. The Holstein term introduces the site-independent Boson fields coupling to local electron density, and has two competing influences on superconductivity: The Boson field mediates the effective electron-electron attraction, which is essential for the S-wave electron pairing; the same coupling to the Boson fields also induces the polaron effect, which makes the system less metallic and thus suppresses superconductivity. The Hubbard term introduces an energy penalty U when two electrons occupy the same site, which is expected to suppress superconductivity. By solving the Hubbard-Holstein model using Dynamical Mean Field theory, we find that the Hubbard U can be beneficial to superconductivity under some circumstances. In particular, we demonstrate that when the Boson energy Ω is small, a weak local repulsion actually stabilizesthe S-wave superconducting state. This behavior can be understood as an interplay between superconductivity, the polaron effect, and the on-site repulsion: As the polaron effect is strong and suppresses superconductivity in the small Ω regime, the weak on-site repulsion reduces the polaron effect and effectively enhances superconductivity. Our calculation elucidates the role of local repulsion in the conventional S-wave superconductors.

Advanced control scenario of high-performance steady-state operation for JT-60 superconducting tokamak

International Nuclear Information System (INIS)

Tamai, H.; Kurita, G.; Matsukawa, M.; Urata, K.; Sakurai, S.; Tsuchiya, K.; Morioka, A.; Miura, Y.M.; Kizu, K.; Kamada, Y.; Sakasai, A.; Ishida, S.

2004-01-01

Plasma control on high-β N steady-state operation for JT-60 superconducting modification is discussed. Accessibility to high-β N exceeding the free-boundary limit is investigated with the stabilising wall of reduced-activated ferritic steel and the active feedback control of the in-vessel non-axisymmetric field coils. Taking the merit of superconducting magnet, advanced plasma control for steady-state high performance operation could be expected. (authors)

Pair q-coherent states and their antibunching effects

International Nuclear Information System (INIS)

Wang Zhongqing; Li Junhong; An Guanglei; Chongqing Univ. of Posts and Telecommunications, Chongqing

2005-01-01

Using the properties of the q-deformed boson creation and annihilation operators and their inverse operators, two kind of q-deformed pair coherent states are introduced. Antibunching effects and correlation properties between two modes in the states are investigated. It is shown that q-deformed pair coherent states exhibit antibunching effects and the photons of the two modes are correlated. These nonclassical effects are influenced by the parameter q. These effects increase when |lnq| increases. (authors)

Characterization of the superconducting state in hafnium hydride under high pressure

Science.gov (United States)

Duda, A. M.; Szewczyk, K. A.; Jarosik, M. W.; Szcześniak, K. M.; Sowińska, M. A.; Szcześniak, D.

2018-05-01

The hydrogen-rich compounds at high pressure may exhibit notably high superconducting transition temperatures. In the paper, we have calculated the basic thermodynamic parameters of the superconducting state in two selected phases of HfH2 hydride under high-pressure respectively at 180 GPa for Cmma and 260 GPa for P21 / m . Calculations has been conducted in the framework of the Eliashberg formalism. In particular, we have determined the values of the critical temperature (TC) to be equal to 8 K and 13 K for the Cmma and P21 / m phases, respectively. Moreover, we have estimated other thermodynamic properties such as the order parameter (Δ (T)) , the thermodynamic critical field (HC (T)) , and the specific heat for the normal (CN) and superconducting (CS) state. Finally, we have shown that the characteristic ratios: RΔ = 2 Δ (0) /kBTC and RC = ΔC (TC) /CN (TC) , which are related to the above thermodynamic functions, slightly differ from the predictions of the Bardeen-Cooper-Schrieffer theory due to the strong-coupling and retardation effects.

Color superconductivity in dense quark matter

International Nuclear Information System (INIS)

Alford, Mark G.; Schmitt, Andreas; Rajagopal, Krishna; Schaefer, Thomas

2008-01-01

Matter at high density and low temperature is expected to be a color superconductor, which is a degenerate Fermi gas of quarks with a condensate of Cooper pairs near the Fermi surface that induces color Meissner effects. At the highest densities, where the QCD coupling is weak, rigorous calculations are possible, and the ground state is a particularly symmetric state, the color-flavor locked (CFL) phase. The CFL phase is a superfluid, an electromagnetic insulator, and breaks chiral symmetry. The effective theory of the low-energy excitations in the CFL phase is known and can be used, even at more moderate densities, to describe its physical properties. At lower densities the CFL phase may be disfavored by stresses that seek to separate the Fermi surfaces of the different flavors, and comparison with the competing alternative phases, which may break translation and/or rotation invariance, is done using phenomenological models. We review the calculations that underlie these results and then discuss transport properties of several color-superconducting phases and their consequences for signatures of color superconductivity in neutron stars.

Mechanism of Superconductivity in Quasi-Two-Dimensional Organic Conductor β-(BDA-TTP) Salts

Science.gov (United States)

Nonoyama, Yoshito; Maekawa, Yukiko; Kobayashi, Akito; Suzumura, Yoshikazu; Ito, Hiroshi

2008-09-01

We investigate theoretically the superconductivity of two-dimensional organic conductors, β-(BDA-TTP)2SbF6 and β-(BDA-TTP)2AsF6, to understand the role of the spin and charge fluctuations. The transition temperature is estimated by applying random phase approximation to an extended Hubbard model wherein realistic transfer energies are estimated by extended Hückel calculation. We find a gapless superconducting state with a dxy-like symmetry, which is consistent with the experimental results obtained by specific heat and scanning tunneling microscope. In the present model with an effectively half-filled triangular lattice, spin fluctuation competes with charge fluctuation as a mechanism of pairing interaction since both fluctuations have the same characteristic momentum q=(π,0) for V being smaller than U. This is in contrast to a model with a quarter-filled square lattice, wherein both fluctuations contribute cooperatively to pairing interaction due to fluctuations having different characteristic momenta. The resultant difference in the superconductivity of these two materials is also discussed.

Non-separable pairing interaction kernels applied to superconducting cuprates

International Nuclear Information System (INIS)

Haley, Stephen B.; Fink, Herman J.

2014-01-01

Highlights: • Non-separable interaction kernels with weak interactions produces HTS. • A probabilistic approach is used in filling the electronic states in the unit cell. • A set of coupled equations is derived which describes the energy gap. • SC properties of separable with non-separable interactions are compared. • There is agreement with measured properties of the SC and normal states. - Abstract: A pairing Hamiltonian H(Γ) with a non-separable interaction kernel Γ produces HTS for relatively weak interactions. The doping and temperature dependence of Γ(x,T) and the chemical potential μ(x) is determined by a probabilistic filling of the electronic states in the cuprate unit cell. A diverse set of HTS and normal state properties is examined, including the SC phase transition boundary T C (x), SC gap Δ(x,T), entropy S(x,T), specific heat C(x,T), and spin susceptibility χ s (x,T). Detailed x,T agreement with cuprate experiment is obtained for all properties

Wide applicability of high-Tc pairing originating from coexisting wide and incipient narrow bands in quasi-one-dimensional systems

Science.gov (United States)

Matsumoto, Karin; Ogura, Daisuke; Kuroki, Kazuhiko

2018-01-01

We study superconductivity in the Hubbard model on various quasi-one-dimensional lattices with coexisting wide and narrow bands originating from multiple sites within a unit cell, where each site corresponds to a single orbital. The systems studied are the two-leg and three-leg ladders, the diamond chain, and the crisscross ladder. These one-dimensional lattices are weakly coupled to form two-dimensional (quasi-one-dimensional) ones, and the fluctuation exchange approximation is adopted to study spin-fluctuation-mediated superconductivity. When one of the bands is perfectly flat and the Fermi level intersecting the wide band is placed in the vicinity of, but not within, the flat band, superconductivity arising from the interband scattering processes is found to be strongly enhanced owing to the combination of the light electron mass of the wide band and the strong pairing interaction due to the large density of states of the flat band. Even when the narrow band has finite bandwidth, the pairing mechanism still works since the edge of the narrow band, due to its large density of states, plays the role of the flat band. The results indicate the wide applicability of the high-Tc pairing mechanism due to coexisting wide and "incipient" narrow bands in quasi-one-dimensional systems.

On the superconducting state in Ba0.6K0.4BiO3 perovskite oxide

Science.gov (United States)

Szcześniak, D.; Kaczmarek, A. Z.; Drzazga, E. A.; Szewczyk, K. A.; Szcześniak, R.

2018-05-01

We report study on the superconducting state in Ba0.6K0.4BiO3 (BKBO) perovskite oxide, motivated by the inconclusive results on the pairing mechanism in this compound. Our investigations are conducted within the Migdal-Eliashberg formalism, to account for the phonon-mediated superconducting phase. The considered doping level of the discussed material corresponds to the highest critical temperature in this compound, and allows simultaneous analysis of the oxygen isotope effect, for the O16 and O18 isotopes, respectively. We found that such effect is particularly visible for the critical values of the Coulomb pseudopotential (μC⋆) , which equals to 0.18 for the O16 and 0.16 for the O18 isotope in BKBO. Moreover, we determine the size of the superconducting energy band gap (Δg) and note that obtained values (9.68 meV and 9.55 meV for the O16 and O18, respectively) are in good agreement with the experimental predictions which give Δg ∼ 8.68 meV . Finally, we calculate the characteristic dimensionless parameters, such as the zero-temperature energy gap to the critical temperature, the ratio for the specific heat, as well as the ratio associated with the zero-temperature thermodynamic critical field, which suggest occurrence of the strong-coupling and retardation effects within the phonon-mediated scenario in the analyzed material. Where possible the dimensionless ratios are compared to the experimental estimates, and agrees with these which account for the strong-coupling character of the BKBO superconductor.

Interplay of magnetism and superconductivity

International Nuclear Information System (INIS)

Akhavan, M.

2006-01-01

After about two decades of intense research since the discovery of high-temperature superconductivity (HTSC) in cuprates, although many aspects of the physics and chemistry of these cuprate superconductors are now well understood, the underlying pairing mechanism remains elusive. Magnetism and superconductivity are usually thought as incompatible, but in number of special materials including HTSCs these two mutually excluding mechanisms are found to coexist. The presence in a system of superconductivity and magnetism, gives rise to a large number of interesting phenomenon. This article provides perspective on recent developments and their implications for our understanding of the interplay between magnetism and superconductivity in new materials. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (Abstract Copyright [2006], Wiley Periodicals, Inc.)

Superconducting and normal state properties of carbon doped and neutron irradiated MgB2

International Nuclear Information System (INIS)

Wilke, R.H.T.; Samuely, P.; Szabo, P.; Holanova, Z.; Bud'ko, S.L.; Canfield, P.C.; Finnemore, D.K.

2007-01-01

Current research in MgB 2 focuses on the effects various types of perturbations have on the superconducting properties of this novel two-gap superconductor. In this article we summarize the effects of carbon doping and neutron irradiation in bulk MgB 2 . Low levels of carbon doping and light neutron irradiation result in significant enhancements in H c2 . At high fluences, where superconductivity is nearly fully suppressed, superconductivity can be restored through post exposure annealing. However, this results in a change in the interdependencies of the normal state and superconducting properties (ρ 0 , T c , H c2 ), with little or no enhancement in H c2

The possibility of superconductivity in twisted bilayer graphene

International Nuclear Information System (INIS)

Manaf, Muhamad Nasruddin; Santoso, Iman; Hermanto, Arief

2015-01-01

We discuss the possibility of superconductivity in Twisted Bilayer Graphene (TBG). In this study we use TBG model with commensurate rotation θ=1.16° in which the van-Hove singularities (VHS) arise at 6 meV from the Fermi level. We use BCS standard formula that include Density of States (DOS) to calculate the critical temperature (T C ). Based on our calculation we predict that superconductivity will not arise in Pristine TBG because pairing potential has infinity value. In this situation, Dirac Fermions do not interact with each other since they do not form the bound states. Superconductvity may arise when the Fermi level is shifted towards the VHS. Based on this calculation, we predict that T C has value between 0.04 K and 0.12 K. The low value of T C is due to highly energetic of in plane phonon vibration which reduce the effective electron-phonon coupling. We conclude that doped TBG is candidate for Dirac Fermion superconductor

Doping dependence of low-energy quasiparticle excitations in superconducting Bi2212.

Science.gov (United States)

Ino, Akihiro; Anzai, Hiroaki; Arita, Masashi; Namatame, Hirofumi; Taniguchi, Masaki; Ishikado, Motoyuki; Fujita, Kazuhiro; Ishida, Shigeyuki; Uchida, Shinichi

2013-12-05

: The doping-dependent evolution of the d-wave superconducting state is studied from the perspective of the angle-resolved photoemission spectra of a high-Tc cuprate, Bi2Sr2CaCu2 O8+δ (Bi2212). The anisotropic evolution of the energy gap for Bogoliubov quasiparticles is parametrized by critical temperature and superfluid density. The renormalization of nodal quasiparticles is evaluated in terms of mass enhancement spectra. These quantities shed light on the strong coupling nature of electron pairing and the impact of forward elastic or inelastic scatterings. We suggest that the quasiparticle excitations in the superconducting cuprates are profoundly affected by doping-dependent screening.

Fermiology and Superconductivity of Topological Surface States in PdTe2

Science.gov (United States)

Clark, O. J.; Neat, M. J.; Okawa, K.; Bawden, L.; Marković, I.; Mazzola, F.; Feng, J.; Sunko, V.; Riley, J. M.; Meevasana, W.; Fujii, J.; Vobornik, I.; Kim, T. K.; Hoesch, M.; Sasagawa, T.; Wahl, P.; Bahramy, M. S.; King, P. D. C.

2018-04-01

We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor PdTe2 by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe2 with its sister compound PtSe2 , we demonstrate how enhanced interlayer hopping in the Te-based material drives a band inversion within the antibonding p -orbital manifold well above the Fermi level. We show how this mediates spin-polarized topological surface states which form rich multivalley Fermi surfaces with complex spin textures. Scanning tunneling spectroscopy reveals type-II superconductivity at the surface, and moreover shows no evidence for an unconventional component of its superconducting order parameter, despite the presence of topological surface states.

On anyon superconductivity--

International Nuclear Information System (INIS)

Chen, Y.-H.; Wilczek, F.; Witten, E.; Halperin, B.I.

1989-01-01

We investigate the statistical mechanics of a gas of fractional statistics particles in 2 + 1 dimensions. In the case of statistics very close to Fermi statistics (statistical parameter θ = π(1 - 1/n), for large n), the effect of the statistics is a weak attraction. Building upon earlier RPA calculation for the case n = 2, the authors argue that for large n perturbation theory is reliable and exhibits superfluidity (or superconductivity after coupling to electromagnetism). They describe the order parameter for this superconductng phase in terms of spontaneous breaking of commutativity of translations as opposed to the usual pairing order parameters. The vortices of the superconducting anyon gas are charged, and superconducting order parameters of the usual type vanish. They investigate the characteristic P and T violating phenomenology

Transition from Sign-Reversed to Sign-Preserved Cooper-Pairing Symmetry in Sulfur-Doped Iron Selenide Superconductors.

Science.gov (United States)

Wang, Qisi; Park, J T; Feng, Yu; Shen, Yao; Hao, Yiqing; Pan, Bingying; Lynn, J W; Ivanov, A; Chi, Songxue; Matsuda, M; Cao, Huibo; Birgeneau, R J; Efremov, D V; Zhao, Jun

2016-05-13

An essential step toward elucidating the mechanism of superconductivity is to determine the sign or phase of the superconducting order parameter, as it is closely related to the pairing interaction. In conventional superconductors, the electron-phonon interaction induces attraction between electrons near the Fermi energy and results in a sign-preserved s-wave pairing. For high-temperature superconductors, including cuprates and iron-based superconductors, prevalent weak coupling theories suggest that the electron pairing is mediated by spin fluctuations which lead to repulsive interactions, and therefore that a sign-reversed pairing with an s_{±} or d-wave symmetry is favored. Here, by using magnetic neutron scattering, a phase sensitive probe of the superconducting gap, we report the observation of a transition from the sign-reversed to sign-preserved Cooper-pairing symmetry with insignificant changes in T_{c} in the S-doped iron selenide superconductors K_{x}Fe_{2-y}(Se_{1-z}S_{z})_{2}. We show that a rather sharp magnetic resonant mode well below the superconducting gap (2Δ) in the undoped sample (z=0) is replaced by a broad hump structure above 2Δ under 50% S doping. These results cannot be readily explained by simple spin fluctuation-exchange pairing theories and, therefore, multiple pairing channels are required to describe superconductivity in this system. Our findings may also yield a simple explanation for the sometimes contradictory data on the sign of the superconducting order parameter in iron-based materials.

Quantitative Raman Measurement of the Evolution of the Cooper-pair Density with Doping in Bi2Sr2CaCu2O8+δ Superconductors

International Nuclear Information System (INIS)

Blanc, S.; Gu, G.; Gallais, Y.; Sacuto, A.; Cazayous, M.; Measson, M.A.; Wen, J.S.; Xu, Z.J.

2009-01-01

We report Raman measurements on Bi 2 Sr 2 CaCu 2 O 8+δ single crystals that allow us to quantitatively evaluate the doping dependence of the density of Cooper pairs in the superconducting state. We show that the drastic loss of Cooper pairs in the antinodal region as the doping level is reduced is concomitant with a deep alteration of the quasiparticles dynamic above T c and consistent with a pseudogap that competes with superconductivity. Our data also reveal that the overall density of Cooper pairs evolves with doping, distinctly from the superfluid density above the doping level p c = 0.2.

The origins of macroscopic quantum coherence in high temperature superconductivity

International Nuclear Information System (INIS)

Turner, Philip; Nottale, Laurent

2015-01-01

Highlights: • We propose a new theoretical approach to superconductivity in p-type cuprates. • Electron pairing mechanisms in the superconducting and pseudogap phases are proposed. • A scale free network of dopants is key to macroscopic quantum coherence. - Abstract: A new, theoretical approach to macroscopic quantum coherence and superconductivity in the p-type (hole doped) cuprates is proposed. The theory includes mechanisms to account for e-pair coupling in the superconducting and pseudogap phases and their inter relations observed in these materials. Electron pair coupling in the superconducting phase is facilitated by local quantum potentials created by static dopants in a mechanism which explains experimentally observed optimal doping levels and the associated peak in critical temperature. By contrast, evidence suggests that electrons contributing to the pseudogap are predominantly coupled by fractal spin waves (fractons) induced by the fractal arrangement of dopants. On another level, the theory offers new insights into the emergence of a macroscopic quantum potential generated by a fractal distribution of dopants. This, in turn, leads to the emergence of coherent, macroscopic spin waves and a second associated macroscopic quantum potential, possibly supported by charge order. These quantum potentials play two key roles. The first involves the transition of an expected diffusive process (normally associated with Anderson localization) in fractal networks, into e-pair coherence. The second involves the facilitation of tunnelling between localized e-pairs. These combined effects lead to the merger of the super conducting and pseudo gap phases into a single coherent condensate at optimal doping. The underlying theory relating to the diffusion to quantum transition is supported by Coherent Random Lasing, which can be explained using an analogous approach. As a final step, an experimental program is outlined to validate the theory and suggests a new

Superatom representation of high-TC superconductivity

International Nuclear Information System (INIS)

Panas, Itai

2012-01-01

A “super-atom” conceptual interface between chemistry and physics is proposed in order to assist in the search for higher T C superconductors. The plaquettes generating the checkerboard superstructure in the cuprates, the C 60 molecules in K 3 C 60 , and the Mo 6 S 8 2- clusters in Chevrel phase materials offer such candidate super-atoms. Thus, in the present study high-T C superconductivity HTSC is articulated as the entanglement of two disjoint electronic manifolds in the vicinity of a common Fermi energy. The resulting HTSC ground state couples near-degenerate protected local super-atom states to virtual magnons in an antiferromagnetic AFM embedding. The composite Cooper pairs emerge as the interaction particles for virtual magnons mediated “self-coherent entanglement” of super-atom states. A Hückel type resonating valence bond RVB formalism is employed in order to illustrate the real-space Cooper pairs as well as their delocalization and Bose Einstein condensation BEC on a ring of super-atoms. The chemical potential μ BEC for Cooper pairs joining the condensate is formulated in terms of the super-exchange interaction, and consequently the T C in terms of the Neél temperature. A rationale for the robustness of the HTSC ground state is proposed: achieving local maximum “electron correlation entropy” at the expense of non-local phase rigidity.

Pairing correlations in nuclei

International Nuclear Information System (INIS)

Baba, C.V.K.

1988-01-01

There are many similarities between the properties of nucleons in nuclei and electrons in metals. In addition to the properties explainable in terms of independent particle motion, there are many important co-operative effects suggesting correlated motion. Pairing correlation which leads to superconductivity in metals and several important properties in nuclei , is an exmple of such correlations. An attempt has been made to review the effects of pairing correlations in nuclei. Recent indications of reduction in pairing correlations at high angular momenta is discussed. A comparision between pairing correlations in the cases of nuclei and electrons in metals is attempted. (author). 20 refs., 10 figs

Gauge Model of High-Tc Superconductivity

International Nuclear Information System (INIS)

Ng, Sze Kui

2012-01-01

A simple gauge model of superconductivity is presented. The seagull vertex term of this gauge model gives an attractive potential between electrons for the forming of Cooper pairs of superconductivity. This gauge model gives a unified description of superconductivity and magnetism including antiferromagnetism, pseudogap phenomenon, stripes phenomenon, paramagnetic Meissner effect, Type I and Type II supeconductivity and high-T c superconductivity. The doping mechanism of superconductivity is found. It is shown that the critical temperature T c is related to the ionization energies of elements and can be computed by a formula of T c . For the high-T c superconductors such as La 2-x Sr x CuO 4 , Y Ba 2 Cu 3 O 7 , and MgB 2 , the computational results of T c agree with the experimental results.

Deformed nuclear state as a quasiparticle-pair

International Nuclear Information System (INIS)

Dobaczewski, J.; Skalski, J.

1988-01-01

The deformed nuclear states, obtained in terms of the Hartree-Fock plus BCS method with the Skyrme SIII interaction, are approximated by condensates of the low-angular-momentum quasiparticle and particle pairs. The optimal pairs are determined by the variation after truncation method. The influence of the truncation on the deformation energy and the importance of the core-polarization effects are investigated

The role of local repulsion in superconductivity in the Hubbard–Holstein model

Energy Technology Data Exchange (ETDEWEB)

Lin, Chungwei, E-mail: clin@merl.com; Wang, Bingnan; Teo, Koon Hoo

2017-01-15

Highlights: • There exists an optimal Boson energy for superconductivity in Hubbard–Holstein model. • The electron-Boson coupling is essential for superconductivity, but the same coupling can lead to polaron insulator, which is against superconductivity. • The local Coulomb repulsion can sometimes enhance superconductivity. - Abstract: We examine the superconducting solution in the Hubbard–Holstein model using Dynamical Mean Field Theory. The Holstein term introduces the site-independent Boson fields coupling to local electron density, and has two competing influences on superconductivity: The Boson field mediates the effective electron-electron attraction, which is essential for the S-wave electron pairing; the same coupling to the Boson fields also induces the polaron effect, which makes the system less metallic and thus suppresses superconductivity. The Hubbard term introduces an energy penalty U when two electrons occupy the same site, which is expected to suppress superconductivity. By solving the Hubbard–Holstein model using Dynamical Mean Field theory, we find that the Hubbard U can be beneficial to superconductivity under some circumstances. In particular, we demonstrate that when the Boson energy Ω is small, a weak local repulsion actually stabilizes the S-wave superconducting state. This behavior can be understood as an interplay between superconductivity, the polaron effect, and the on-site repulsion: As the polaron effect is strong and suppresses superconductivity in the small Ω regime, the weak on-site repulsion reduces the polaron effect and effectively enhances superconductivity. Our calculation elucidates the role of local repulsion in the conventional S-wave superconductors.

Unconventional superconductivity of the heavy fermion compound UNi{sub 2}Al{sub 3}

Energy Technology Data Exchange (ETDEWEB)

Zakharov, Andrey

2008-07-01

The heavy fermion compound UNi{sub 2}Al{sub 3} exhibits the coexistence of superconductivity and magnetic order at low temperatures, stimulating speculations about possible exotic Cooper-pairing interaction in this superconductor. However, the preparation of good quality bulk single crystals of UNi{sub 2}Al{sub 3} has proven to be a non-trivial task due to metallurgical problems, which result in the formation of an UAl{sub 2} impurity phase and hence a strongly reduced sample purity. The present work concentrates on the preparation, characterization and electronic properties investigation of UNi{sub 2}Al{sub 3} single crystalline thin film samples. The preparation of thin films was accomplished in a molecular beam epitaxy (MBE) system. (100)-oriented epitaxial thin films of UNi{sub 2}Al{sub 3} were grown on single crystalline YAlO{sub 3} substrates cut in (010)- or (112)-direction. The high crystallographic quality of the samples was proved by several characterisation methods, such as X-ray analysis, RHEED and TEM. To study the magnetic structure of epitaxial thin films resonant magnetic X-ray scattering was employed. The magnetic order of thin the film samples, the formation of magnetic domains with different moment directions, and the magnetic correlation length were discussed. The electronic properties of the UNi{sub 2}Al{sub 3} thin films in the normal and superconducting states were investigated by means of transport measurements. A pronounced anisotropy of the temperature dependent resistivity {rho}(T) was observed. Moreover, it was found that the temperature of the resistive superconducting transition depends on the current direction, providing evidence for multiband superconductivity in UNi{sub 2}Al{sub 3}. The initial slope of the upper critical field H'{sub c2}(T) of the thin film samples suggests an unconventional spin-singlet superconducting state, as opposed to bulk single crystal data. To probe the superconducting gap of UNi{sub 2}Al{sub 3

Superconducting fault current-limiter with variable shunt impedance

Science.gov (United States)

Llambes, Juan Carlos H; Xiong, Xuming

2013-11-19

A superconducting fault current-limiter is provided, including a superconducting element configured to resistively or inductively limit a fault current, and one or more variable-impedance shunts electrically coupled in parallel with the superconducting element. The variable-impedance shunt(s) is configured to present a first impedance during a superconducting state of the superconducting element and a second impedance during a normal resistive state of the superconducting element. The superconducting element transitions from the superconducting state to the normal resistive state responsive to the fault current, and responsive thereto, the variable-impedance shunt(s) transitions from the first to the second impedance. The second impedance of the variable-impedance shunt(s) is a lower impedance than the first impedance, which facilitates current flow through the variable-impedance shunt(s) during a recovery transition of the superconducting element from the normal resistive state to the superconducting state, and thus, facilitates recovery of the superconducting element under load.

On magnon mediated Cooper pair formation in ferromagnetic superconductors

Directory of Open Access Journals (Sweden)

Rakesh Kar

2014-08-01

Full Text Available Identification of pairing mechanism leading to ferromagnetic superconductivity is one of the most challenging issues in condensed matter physics. Although different models have been proposed to explain this phenomenon, a quantitative understanding about this pairing is yet to be achieved. Using the localized-itinerant model, we find that in ferromagnetic superconducting materials both triplet pairing and singlet pairing of electrons are possible through magnon exchange depending upon whether the Debye cut off frequency of magnons is greater or lesser than the Hund's coupling (J multiplied by average spin (S per site. Taking into account the repulsive interaction due to the existence of paramagnons, we also find an expression for effective interaction potential between a pair of electrons with opposite spins. We apply the developed formalism in case of UGe2 and URhGe. The condition of singlet pairing is found to be fulfilled in these cases, as was previously envisaged by Suhl [Suhl, Phys. Rev. Lett. 87, 167007 (2001]. We compute the critical temperatures of URhGe at ambient pressure and of UGe2 under different pressures for the first time through BCS equation. Thus, this work outlines a very simple way to evaluate critical temperature in case of a superconducting system. A close match with the available experimental results strongly supports our theoretical treatment.

The superconducting magnet system for the Tokamak Physics Experiment

International Nuclear Information System (INIS)

Lang, D.D.; Bulmer, R.J.; Chaplin, M.R.; O'Connor, T.G.; Slack, D.S.; Wong, R.L.; Zbasnik, J.P.; Schultz, J.H.; Diatchenko, N.; Montgomery, D.B.

1994-01-01

The superconducting magnet system for the Tokamak Physics eXperiment (TPX) will be the first all superconducting magnet system for a Tokamak, where the poloidal field coils, in addition to the toroidal field coils are superconducting. The magnet system is designed to operate in a steady state mode, and to initiate the plasma discharge ohmically. The toroidal field system provides a peak field of 4.0 Tesla on the plasma axis at a plasma major radius of 2.25 m. The peak field on the niobium 3-tin, cable-in-conduit (CIC) conductor is 8.4 Tesla for the 16 toroidal field coils. The toroidal field coils must absorb approximately 5 kW due to nuclear heating, eddy currents, and other sources. The poloidal field system provides a total of 18 volt seconds to initiate the plasma and drive a plasma current up to 2 MA. The poloidal field system consists of 14 individual coils which are arranged symmetrically above and below the horizontal mid plane. Four pairs of coils make up the central solenoid, and three pairs of poloidal ring coils complete the system. The poloidal field coils all use a cable-in-conduit conductor, using either niobium 3-tin (Nb 3 Sn) or niobium titanium (NbTi) superconducting strands depending on the operating conditions for that coil. All of the coils are cooled by flowing supercritical helium, with inlet and outlet connections made on each double pancake. The superconducting magnet system has gone through a conceptual design review, and is in preliminary design started by the LLNL/MIT/PPPL collaboration. A number of changes have been made in the design since the conceptual design review, and are described in this paper. The majority of the design and all fabrication of the superconducting magnet system will be ,accomplished by industry, which will shortly be taking over the preliminary design. The magnet system is expected to be completed in early 2000

Superconductive analogue of spin glasses

International Nuclear Information System (INIS)

Feigel'man, M.; Ioffe, L.; Vinokur, V.; Larkin, A.

1987-07-01

The properties of granular superconductors in magnetic fields, namely the existence of a new superconductive state analogue of the low-temperature superconductive state in spin glasses are discussed in the frame of the infinite-range model and the finite-range models. Experiments for elucidation of spin-glass superconductive state in real systems are suggested. 30 refs

Luminescence and squeezing of a superconducting light-emitting diode

Science.gov (United States)

Hlobil, Patrik; Orth, Peter P.

2015-05-01

We investigate a semiconductor p -n junction in contact with superconducting leads that is operated under forward bias as a light-emitting diode. The presence of superconductivity results in a significant increase of the electroluminescence in a sharp frequency window. We demonstrate that the tunneling of Cooper pairs induces an additional luminescence peak on resonance. There is a transfer of superconducting to photonic coherence that results in the emission of entangled photon pairs and squeezing of the fluctuations in the quadrature amplitudes of the emitted light. We show that the squeezing angle can be electrically manipulated by changing the relative phase of the order parameters in the superconductors. We finally derive the conditions for lasing in the system and show that the laser threshold is reduced due to superconductivity. This reveals how the macroscopic coherence of a superconductor can be used to control the properties of light.

First-Order 0-π Quantum Phase Transition in the Kondo Regime of a Superconducting Carbon-Nanotube Quantum Dot

Directory of Open Access Journals (Sweden)

Romain Maurand

2012-02-01

Full Text Available We study a carbon-nanotube quantum dot embedded in a superconducting-quantum-interference-device loop in order to investigate the competition of strong electron correlations with a proximity effect. Depending on whether local pairing or local magnetism prevails, a superconducting quantum dot will exhibit a positive or a negative supercurrent, referred to as a 0 or π Josephson junction, respectively. In the regime of a strong Coulomb blockade, the 0-to-π transition is typically controlled by a change in the discrete charge state of the dot, from even to odd. In contrast, at a larger tunneling amplitude, the Kondo effect develops for an odd-charge (magnetic dot in the normal state, and quenches magnetism. In this situation, we find that a first-order 0-to-π quantum phase transition can be triggered at a fixed valence when superconductivity is brought in, due to the competition of the superconducting gap and the Kondo temperature. The superconducting-quantum-interference-device geometry together with the tunability of our device allows the exploration of the associated phase diagram predicted by recent theories. We also report on the observation of anharmonic behavior of the current-phase relation in the transition regime, which we associate with the two accessible superconducting states. Our results finally demonstrate that the spin-singlet nature of the Kondo state helps to enhance the stability of the 0 phase far from the mixed-valence regime in odd-charge superconducting quantum dots.

A modified BCS theory of heavy fermion superconductivity

International Nuclear Information System (INIS)

Baral, P.C.; Rout, G.C.

2012-01-01

In this paper we derive an expression for the superconducting gap equation for U and Ce based heavy fermion (HF) systems within a modified weak coupling theory of superconductivity. The calculated gap equation presents a mixture of pairing amplitudes of two different quasi-particle bands α and β. These two gap equations are solved numerically and self-consistently within the cut-off energy which arises due to the Kondo energy. It is found that the energy dependence of the enhanced density of states for the HF systems clearly manifests itself in the theory and the Kondo energy naturally takes the role of cut-off energy (ω c ), as long as the effective cut-off energy is large in comparison with the Kondo energy. The numerical analysis confirms this result and shows that superconducting transition temperature is independent of effective cut-off energy employed within this approach. The temperature dependence of gap equations are studied by varying the model parameters like positions of f-level, hybridization and coupling constants of the HF systems. (author)

Spin-rotation symmetry breaking and triplet superconducting state in doped topological insulator CuxBi2Se3

Science.gov (United States)

Zheng, Guo-Qing

Spontaneous symmetry breaking is an important concept for understanding physics ranging from the elementary particles to states of matter. For example, the superconducting state breaks global gauge symmetry, and unconventional superconductors can break additional symmetries. In particular, spin rotational symmetry is expected to be broken in spin-triplet superconductors. However, experimental evidence for such symmetry breaking has not been obtained so far in any candidate compounds. We report 77Se nuclear magnetic resonance measurements which showed that spin rotation symmetry is spontaneously broken in the hexagonal plane of the electron-doped topological insulator Cu0.3Bi2Se3 below the superconducting transition temperature Tc =3.4 K. Our results not only establish spin-triplet (odd parity) superconductivity in this compound, but also serve to lay a foundation for the research of topological superconductivity (Ref.). We will also report the doping mechanism and superconductivity in Sn1-xInxTe.

Enhancement of superconducting state in the system 2H-NbSe2 - hydrogen

International Nuclear Information System (INIS)

Obolenskij, M.A.; Beletskij, V.I.; Chashka, Kh.B.; Basteev, A.V.

1984-01-01

The enhancement of the upper critical field and superconducting temperature of Hsub(x)NbSesub(2) system (x<=0.01) was experimentally observed. This phenomenon is observed after cycling influence by the external magnetic field at temperatures lower than the critical temperature of superconducting transition Tsub(c). The authors think that this effect is connected with hydrogen ordering in the field of moving vortex lattice in dynamically mixed state

Geneva University - Superconducting flux quantum bits: fabricated quantum objects

CERN Multimedia

2007-01-01

Ecole de physique Département de physique nucléaire et corspusculaire 24, Quai Ernest-Ansermet 1211 GENEVE 4 Tél: (022) 379 62 73 Fax: (022) 379 69 92 Lundi 29 janvier 2007 COLLOQUE DE LA SECTION DE PHYSIQUE 17 heures - Auditoire Stueckelberg Superconducting flux quantum bits: fabricated quantum objects Prof. Hans Mooij / Kavli Institute of Nanoscience, Delft University of Technology The quantum conjugate variables of a superconductor are the charge or number of Cooper pairs, and the phase of the order parameter. In circuits that contain small Josephson junctions, these quantum properties can be brought forward. In Delft we study so-called flux qubits, superconducting rings that contain three small Josephson junctions. When a magnetic flux of half a flux quantum is applied to the loop, there are two states with opposite circulating current. For suitable junction parameters, a quantum superposition of those macroscopic states is possible. Transitions can be driven with resonant microwaves. These quantum ...

Cooper Pair Writing at the LaAlO3/ SrTiO 3 Interface

Science.gov (United States)

Cen, Cheng; Bogorin, Daniela F.; Bark, Chung Wung; Folkman, Chad M.; Eom, Chang-Beom; Levy, Jeremy

2011-03-01

Superconducting semiconductors offer unique ways to exert electrostatic control over macroscopic quantum phases. The recently demonstrated nanoscale control over conductivity at the LaAl O3 / SrTi O3 interface raises the question of whether nanoscale control over superconducting phases can be realized. Here we report low-temperature magnetotransport experiments on structures defined with nanoscale precision at the LaAl O3 / SrTi O3 interface. A quantum phase transition is observed that is associated with the formation of Cooper pairs, but a finite resistance is observed at the lowest temperatures. Higher mobility interfaces exhibit larger Ginsburg-Landau coherence lengths, a stronger suppression of pairing by magnetic field as well as Shubnikov-de Haas oscillations. Cooper pair localization, spin-orbit coupling, and finite-size effects may factor into an explanation for some of the unusual properties observed. The work is supported by Department of Energy and State of Florida, NSF (DMR-0906443 and DMR-0704022), DOE (DE-FG02-06ER46327) and the Fine Foundation.

Approximating local observables on projected entangled pair states

Science.gov (United States)

Schwarz, M.; Buerschaper, O.; Eisert, J.

2017-06-01

Tensor network states are for good reasons believed to capture ground states of gapped local Hamiltonians arising in the condensed matter context, states which are in turn expected to satisfy an entanglement area law. However, the computational hardness of contracting projected entangled pair states in two- and higher-dimensional systems is often seen as a significant obstacle when devising higher-dimensional variants of the density-matrix renormalization group method. In this work, we show that for those projected entangled pair states that are expected to provide good approximations of such ground states of local Hamiltonians, one can compute local expectation values in quasipolynomial time. We therefore provide a complexity-theoretic justification of why state-of-the-art numerical tools work so well in practice. We finally turn to the computation of local expectation values on quantum computers, providing a meaningful application for a small-scale quantum computer.

Imaging the Statics and Dynamics of Superconducting Vortices and Antivortices Induced by Magnetic Microdisks

Directory of Open Access Journals (Sweden)

R. B. G. Kramer

2011-10-01

Full Text Available If a magnet of microscopic dimensions is brought in close proximity to a superconductor, the quantized nature of their interaction due to the creation of flux quanta in the superconducting system becomes noticeable. Herein, we directly image, via scanning Hall microscopy, the vortex-antivortex pairs in a superconducting film created by micromagnets. The number of antivortices at equilibrium conditions can be changed either by tuning the magnetic moment of the magnets or by annihilation with externally induced vortices. We demonstrate that small ac field excitations shake the antivortices sitting next to the micromagnets whereas no sizable motion is observed for the vortices sitting on top of the magnets, clearly revealing the different mobility of these two vortex species. A metastable state, which is obtained by applying a field after the system has been cooled down below the superconducting transition, shows a complex graded distribution of coexisting vortices and antivortices forming an intertwined critical state.

Superconductivity and spin fluctuations

International Nuclear Information System (INIS)

Scalapino, D.J.

1999-01-01

The organizers of the Memorial Session for Herman Rietschel asked that the author review some of the history of the interplay of superconductivity and spin fluctuations. Initially, Berk and Schrieffer showed how paramagnon spin fluctuations could suppress superconductivity in nearly-ferromagnetic materials. Following this, Rietschel and various co-workers wrote a number of papers in which they investigated the role of spin fluctuations in reducing the Tc of various electron-phonon superconductors. Paramagnon spin fluctuations are also believed to provide the p-wave pairing mechanism responsible for the superfluid phases of 3 He. More recently, antiferromagnetic spin fluctuations have been proposed as the mechanism for d-wave pairing in the heavy-fermion superconductors and in some organic materials as well as possibly the high-Tc cuprates. Here the author will review some of this early history and discuss some of the things he has learned more recently from numerical simulations

Topological superfluids with finite-momentum pairing and Majorana fermions.

Science.gov (United States)

Qu, Chunlei; Zheng, Zhen; Gong, Ming; Xu, Yong; Mao, Li; Zou, Xubo; Guo, Guangcan; Zhang, Chuanwei

2013-01-01

Majorana fermions (MFs), quantum particles that are their own antiparticles, are not only of fundamental importance in elementary particle physics and dark matter, but also building blocks for fault-tolerant quantum computation. Recently MFs have been intensively studied in solid state and cold atomic systems. These studies are generally based on superconducting pairing with zero total momentum. On the other hand, finite total momentum Cooper pairings, known as Fulde-Ferrell (FF) Larkin-Ovchinnikov (LO) states, were widely studied in many branches of physics. However, whether FF and LO superconductors can support MFs has not been explored. Here we show that MFs can exist in certain types of gapped FF states, yielding a new quantum matter: topological FF superfluids/superconductors. We demonstrate the existence of such topological FF superfluids and the associated MFs using spin-orbit-coupled degenerate Fermi gases and derive their parameter regions. The implementation of topological FF superconductors in semiconductor/superconductor heterostructures is also discussed.

Magnetism and superconductivity driven by identical 4f states in a heavy-fermion metal

Energy Technology Data Exchange (ETDEWEB)

Thompson, Joe E [Los Alamos National Laboratory; Nair, S [MAX PLANCK INST.; Stockert, O [MAX PLANCK INST.; Witte, U [INST. FUR FESTKORPERPHYSIK; Nicklas, M [MAX PLANCK INST.; Schedler, R [HELMHOLTZ - ZENTRUM; Bianchi, A [UC, IRVINE; Fisk, Z [UC, IRVINE; Wirth, S [MAX PLANCK INST.; Steglich, K [HELMHOLTZ - ZENTRUM

2009-01-01

The apparently inimical relationship between magnetism and superconductivity has come under increasing scrutiny in a wide range of material classes, where the free energy landscape conspires to bring them in close proximity to each other. Particularly enigmatic is the case when these phases microscopically interpenetrate, though the manner in which this can be accomplished remains to be fully comprehended. Here, we present combined measurements of elastic neutron scattering, magnetotransport, and heat capacity on a prototypical heavy fermion system, in which antiferromagnetism and superconductivity are observed. Monitoring the response of these states to the presence of the other, as well as to external thermal and magnetic perturbations, points to the possibility that they emerge from different parts of the Fermi surface. Therefore, a single 4f state could be both localized and itinerant, thus accounting for the coexistence of magnetism and superconductivity.

Superconducting Ferromagnetic Nanodiamond.

Science.gov (United States)

Zhang, Gufei; Samuely, Tomas; Xu, Zheng; Jochum, Johanna K; Volodin, Alexander; Zhou, Shengqiang; May, Paul W; Onufriienko, Oleksandr; Kačmarčík, Jozef; Steele, Julian A; Li, Jun; Vanacken, Johan; Vacík, Jiri; Szabó, Pavol; Yuan, Haifeng; Roeffaers, Maarten B J; Cerbu, Dorin; Samuely, Peter; Hofkens, Johan; Moshchalkov, Victor V

2017-06-27

Superconductivity and ferromagnetism are two mutually antagonistic states in condensed matter. Research on the interplay between these two competing orderings sheds light not only on the cause of various quantum phenomena in strongly correlated systems but also on the general mechanism of superconductivity. Here we report on the observation of the electronic entanglement between superconducting and ferromagnetic states in hydrogenated boron-doped nanodiamond films, which have a superconducting transition temperature T c ∼ 3 K and a Curie temperature T Curie > 400 K. In spite of the high T Curie , our nanodiamond films demonstrate a decrease in the temperature dependence of magnetization below 100 K, in correspondence to an increase in the temperature dependence of resistivity. These anomalous magnetic and electrical transport properties reveal the presence of an intriguing precursor phase, in which spin fluctuations intervene as a result of the interplay between the two antagonistic states. Furthermore, the observations of high-temperature ferromagnetism, giant positive magnetoresistance, and anomalous Hall effect bring attention to the potential applications of our superconducting ferromagnetic nanodiamond films in magnetoelectronics, spintronics, and magnetic field sensing.

Organic superconductivity

International Nuclear Information System (INIS)

Jerome, D.

1980-01-01

We present the experimental evidences for the existence of a superconducting state in the Quasi One Dimensional organic conductor (TMTSF) 2 PF 6 . Superconductivity occuring at 1 K under 12 kbar is characterized by a zero resistance diamagnetic state. The anistropy of the upper critical field of this type II superconductor is consistent with the band structure anistropy. We present evidences for the existence of large superconducting precursor effects giving rise to a dominant paraconductive contribution below 40 K. We also discuss the anomalously large pressure dependence of T sb(s), which drops to 0.19 K under 24 kbar in terms of the current theories. (author)

Local destruction of superconductivity by non-magnetic impurities in mesoscopic iron-based superconductors.

Science.gov (United States)

Li, Jun; Ji, Min; Schwarz, Tobias; Ke, Xiaoxing; Van Tendeloo, Gustaaf; Yuan, Jie; Pereira, Paulo J; Huang, Ya; Zhang, Gufei; Feng, Hai-Luke; Yuan, Ya-Hua; Hatano, Takeshi; Kleiner, Reinhold; Koelle, Dieter; Chibotaru, Liviu F; Yamaura, Kazunari; Wang, Hua-Bing; Wu, Pei-Heng; Takayama-Muromachi, Eiji; Vanacken, Johan; Moshchalkov, Victor V

2015-07-03

The determination of the pairing symmetry is one of the most crucial issues for the iron-based superconductors, for which various scenarios are discussed controversially. Non-magnetic impurity substitution is one of the most promising approaches to address the issue, because the pair-breaking mechanism from the non-magnetic impurities should be different for various models. Previous substitution experiments demonstrated that the non-magnetic zinc can suppress the superconductivity of various iron-based superconductors. Here we demonstrate the local destruction of superconductivity by non-magnetic zinc impurities in Ba0.5K0.5Fe2As2 by exploring phase-slip phenomena in a mesoscopic structure with 119 × 102 nm(2) cross-section. The impurities suppress superconductivity in a three-dimensional 'Swiss cheese'-like pattern with in-plane and out-of-plane characteristic lengths slightly below ∼1.34 nm. This causes the superconducting order parameter to vary along abundant narrow channels with effective cross-section of a few square nanometres. The local destruction of superconductivity can be related to Cooper pair breaking by non-magnetic impurities.

Superconducting spin valves based on epitaxial Fe/V-hybrid thin film heterostructures

Energy Technology Data Exchange (ETDEWEB)

Nowak, Gregor

2010-12-10

screening layers to suppress the stray field effect, the concentration of the ferromagnetic component in the alloy should be high enough to avoid the generation of non-magnetic/paramagnetic layers. Paramagnetic layers induce pair breaking, thus reducing the quantum-mechanical transparency T{sub F} for the Cooper pair wave function at the FM/SC interface. 3. Ferromagnetic alloys should be chosen as to achieve a better band structure match between the ferromagnetic component and the superconductor. Thereby, T{sub F} will be increased, augmenting the SSV effect. 4. The distance between FM1 and FM2, i.e. the thickness of the superconductor d{sub S}, should be of the order of the Cooper pair coherence length ξ{sub S} ∼ d{sub S}. Ideally, (ξ{sub S})/(d{sub S}) ∼ 1 or larger (see 125), since then the Cooper pair wave function is most effectively influenced by both magnetic layers FM1 and FM2. In the AFM/FM/SC/FM heterostructures, which we prepared according to these guidelines, we could observe satisfying switching characteristics, and SSV effects up to ΔT{sub S} = 20 mK. In this study we have verified the magnetic and superconducting properties of the unusual sample containing a [Fe(2ML)/V(180ML)] superlattice by means of conventional SQUID magnetometry as well as polarized neutron scattering (PNR) method. In the PNR we found a magnetic splitting in the PNR curves for the normal and superconducting state of the sample. Further we found weak indications for a reduction of the magnetic splitting beyond the error bars in the superconducting state compared to the normal conducting state of the sample, what can be considered as a weak indication of the Inverse Proximity effect. However, the measured spin asymmetry difference of the magnetic splitting between the superconducting and normal conducting state of the sample is not clear enough to give an unambiguous answer on the presence or absence of the inverse proximity effect.

Conceptual design of the superconducting magnet for the 250 MeV proton cyclotron.

Science.gov (United States)

Ren, Yong; Liu, Xiaogang; Gao, Xiang

2016-01-01

The superconducting cyclotron is of great importance to treat cancer parts of the body. To reduce the operation costs, a superconducting magnet system for the 250 MeV proton cyclotron was designed to confirm the feasibility of the superconducting cyclotron. The superconducting magnet system consists of a pair of split coils, the cryostat and a pair of binary high temperature superconductor current leads. The superconducting magnet can reach a central magnetic field of about 1.155 T at 160 A. The three GM cryocooler with cooling capacities of 1.5 W at 4.5 K and 35 W at 50 K and one GM cryocooler of 100 W at 50 K were adopted to cool the superconducting magnet system through the thermosiphon technology. The four GM cryocoolers were used to cool the superconducting magnet to realize zero evaporation of the liquid helium.

Heisenberg-limited interferometry with pair coherent states and parity measurements

International Nuclear Information System (INIS)

Gerry, Christopher C.; Mimih, Jihane

2010-01-01

After reviewing parity-measurement-based interferometry with twin Fock states, which allows for supersensitivity (Heisenberg limited) and super-resolution, we consider interferometry with two different superpositions of twin Fock states, namely, two-mode squeezed vacuum states and pair coherent states. This study is motivated by the experimental challenge of producing twin Fock states on opposite sides of a beam splitter. We find that input two-mode squeezed states, while allowing for Heisenberg-limited sensitivity, do not yield super-resolutions, whereas both are possible with input pair coherent states.

Unsynchronized resonance of covalent bonds in the superconducting state

International Nuclear Information System (INIS)

Costa, Marconi B.S.; Bastos, Cristiano C.; Pavao, Antonio C.

2012-01-01

Daft calculations performed on different cluster models of cuprates (LaBa 2 Cu 3 O 6.7 , La 1.85 Sr 0.15 CuO 4 , YBa 2 Cu 3 O 7 , TlBa 2 Ca 2 Cu 3 O 8.78 , HgBa 2 Ca 2 Cu 3 O 8.27 ), metallic systems (Nb 3 Ge, MgB 2 ) and the pnictide LaO 0.92 F 0.08 FeAs made evident the occurrence of un synchronized resonance of covalent bonds in the superconducting state, as predicted by Paling's resonating valence bond Rb) theory. For cuprates, the un synchronized resonance involves electron transfer between Cu atoms accompanied by a decrease in the charge of the La, Sr, Y and Ca atoms. For MgB 2 , electron transfer occurs in the Mg layer, while the B layer behaves as charge reservoir. For Nb 3 Ge, unsynchronized resonance occurs among the Ge atoms, which should be responsible for charge transfer. For LaO 0.92 F 0.08 FeAs, the results suggest that both La-O and Fe-As layers are involved in the mechanism of superconductivity. The identification of unsynchronized resonances in these systems provides evidence which supports RVB as a suitable theory for high-temperature superconductivity (high-TC). (author)

Superconductivity in compensated and uncompensated semiconductors

Directory of Open Access Journals (Sweden)

Youichi Yanase and Naoyuki Yorozu

2008-01-01

Full Text Available We investigate the localization and superconductivity in heavily doped semiconductors. The crossover from the superconductivity in the host band to that in the impurity band is described on the basis of the disordered three-dimensional attractive Hubbard model for binary alloys. The microscopic inhomogeneity and the thermal superconducting fluctuation are taken into account using the self-consistent 1-loop order theory. The superconductor-insulator transition accompanies the crossover from the host band to the impurity band. We point out an enhancement of the critical temperature Tc around the crossover. Further localization of electron wave functions leads to the localization of Cooper pairs and induces the pseudogap. We find that both the doping compensation by additional donors and the carrier increase by additional acceptors suppress the superconductivity. A theoretical interpretation is proposed for the superconductivity in the boron-doped diamond, SiC, and Si.

Mirror nesting and repulsion-induced superconductivity

International Nuclear Information System (INIS)

Belyavsky, Vladimir I.; Kapaev, Vladimir V.; Kopaev, Yurii V.

2004-01-01

Mirror nesting condition that is a rise of pair Fermi contour due to matching of some pieces of the Fermi contour and an isoline of the pair-relative-motion kinetic energy may be satisfied, at definite total pair momenta, due to special features of electron dispersion. Perfect mirror nesting results in a rise of the possibility of superconducting ordering up to arbitrary small pairing repulsive interaction strength. Due to kinematical constraints, the order parameter exists only inside some definite domain of the momentum space and changes its sign on a line belonging to this domain

Microscopic Superconductivity and Room Temperature Electronics of High-Tc Cuprates

International Nuclear Information System (INIS)

Liu Fusui; Chen Wanfang

2008-01-01

This paper points out that the Landau criterion for macroscopic superfluidity of He II is only a criterion for microscopic superfluidity of 4 He, extends the Landau criterion to microscopic superconductivity in fermions (electron and hole) system and system with Cooper pairs without long-range phase coherence. This paper gives another three non-superconductive systems that are of microscopic superconductivity. This paper demonstrates that one application of microscopic superconductivity is to establish room temperature electronics of the high-T c cuprates

Coherent Cooper pair tunneling in systems of Josephson junctions: effects of quasiparticle tunneling and of the electromagnetic environment

NARCIS (Netherlands)

Maassen van den Brink, A.; Odintsov, A.A.; Bobbert, P.A.; Schön, G.

1991-01-01

Small capacitance tunnel junctions show single electron effects and, in the superconducting state, the coherent tunneling of Cooper pairs. We study these effects in a system of two Josephson junctions, driven by a voltage source with a finite impedance. Novel features show up in theI–V

The possibility of superconductivity in twisted bilayer graphene

Energy Technology Data Exchange (ETDEWEB)

Manaf, Muhamad Nasruddin, E-mail: muhamad.nasruddin.manaf@mail.ugm.ac.id; Santoso, Iman, E-mail: iman.santoso@ugm.ac.id; Hermanto, Arief [Jurusan Fisika, Fakultas Matematika dan Ilmu Pengetahuan Alam, Universitas Gadjah Mada, Bulaksumur 55281, Yogyakarta (Indonesia); Yayasan Hikmah Teknosains, Jl. Kaliurang Km 5,3 Gg. Pamungkas No. 16 A, Yogyakarta (Indonesia)

2015-09-30

We discuss the possibility of superconductivity in Twisted Bilayer Graphene (TBG). In this study we use TBG model with commensurate rotation θ=1.16° in which the van-Hove singularities (VHS) arise at 6 meV from the Fermi level. We use BCS standard formula that include Density of States (DOS) to calculate the critical temperature (T{sub C}). Based on our calculation we predict that superconductivity will not arise in Pristine TBG because pairing potential has infinity value. In this situation, Dirac Fermions do not interact with each other since they do not form the bound states. Superconductvity may arise when the Fermi level is shifted towards the VHS. Based on this calculation, we predict that T{sub C} has value between 0.04 K and 0.12 K. The low value of T{sub C} is due to highly energetic of in plane phonon vibration which reduce the effective electron-phonon coupling. We conclude that doped TBG is candidate for Dirac Fermion superconductor.

Superconductivity and fast proton transport in nanoconfined water

Science.gov (United States)

Johnson, K. H.

2018-04-01

A real-space molecular-orbital density-wave description of Cooper pairing in conjunction with the dynamic Jahn-Teller mechanism for high-Tc superconductivity predicts that electron-doped water confined to the nanoscale environment of a carbon nanotube or biological macromolecule should superconduct below and exhibit fast proton transport above the transition temperature, Tc ≅ 230 K (-43 °C).

10-Qubit Entanglement and Parallel Logic Operations with a Superconducting Circuit

Science.gov (United States)

Song, Chao; Xu, Kai; Liu, Wuxin; Yang, Chui-ping; Zheng, Shi-Biao; Deng, Hui; Xie, Qiwei; Huang, Keqiang; Guo, Qiujiang; Zhang, Libo; Zhang, Pengfei; Xu, Da; Zheng, Dongning; Zhu, Xiaobo; Wang, H.; Chen, Y.-A.; Lu, C.-Y.; Han, Siyuan; Pan, Jian-Wei

2017-11-01

Here we report on the production and tomography of genuinely entangled Greenberger-Horne-Zeilinger states with up to ten qubits connecting to a bus resonator in a superconducting circuit, where the resonator-mediated qubit-qubit interactions are used to controllably entangle multiple qubits and to operate on different pairs of qubits in parallel. The resulting 10-qubit density matrix is probed by quantum state tomography, with a fidelity of 0.668 ±0.025 . Our results demonstrate the largest entanglement created so far in solid-state architectures and pave the way to large-scale quantum computation.

The role of local tunneling states in superconductivity at disordered interfaces

International Nuclear Information System (INIS)

Nguyen, B.D.; Simanek, E.

1982-01-01

The origin of the enhancement of the superconducting transition temperature caused by the presence of disordered interfaces is studied. An enhancement mechanism involving the tunneling of the conduction electrons from metal into the local state coupled to the ''two-level state systems'' in the interfaces region, is considered. In this model, the reduction of the tunneling matrix elements by orthogonality blocking can be avoided. (author)

Superconducting gap anisotropy and d-wave pairing in YBa2Cu3O7-δ

Science.gov (United States)

Verma, Sanjeev K.; Gupta, Anushri; Kumari, Anita; Indu, B. D.

2018-02-01

Considering Born-Mayer-Huggins potential as a most suitable potential to study the dynamical properties of high-temperature superconductors (HTS), the many-body quantum dynamics to obtain phonon Green’s functions has been developed via a Hamiltonian that incorporates the contributions of harmonic electron and phonon fields, phonon field anharmonicities, defects and electron-phonon interactions without considering BCS structure. This enables one to develop the quasiparticle renormalized frequency dispersion in the representative high-temperature cuprate superconductor YBa2Cu3O7-δ. The superconducting gap shows substantial changes with increased doping. The in-plane gap study revealed a v-shape gap with a nodal point along kx = ±ky direction for optimum doping (δ = 0.16) and the nodal point vanished in underdoped and overdoped regimes. The dx2-y2 pairing symmetry is observed at optimum doping with the presence of s or dxy components ( < 3%) in underdoped and overdoped regimes.

Possible antipolar pairing mechanism in high-temperature superconductors

International Nuclear Information System (INIS)

Cardwell, D.A.; Shorrocks, N.M.

1989-01-01

An antipolar pairing mechanism for free charge carriers in high-T c superconducting compounds is proposed qualitatively. This involves the establishment of a two-dimensional (2D) array of effective charge-coupling centers within the superconducting lattice via a specific phonon distortion of cation species along a nonmajor crystallographic direction. A fundamental requirement of this model is that the density of such coupling centers decreases with decreasing temperature. In the case of Y-Ba-Cu-O, it is asserted that charge carriers in a 2D oxygen band adjacent to the phonon-containing plane become localized by the resulting (local) field distortion. Cooper pairs may then form when the charge-coupling-center density falls below the charge-carrier density. Such a mechanism could be mediated by a longitudinal phonon which softens at low temperatures to produce an antipolar state, such as that incipient to an antiferroelectric distortion of the lattice (i.e., in the zero-frequency limiting case). This model, which may be investigated experimentally by low-temperature Raman spectroscopy, isotopic substitution, and x-ray diffractometry at 4.2 K, can be applied to all p-type high-T c systems. In addition, it may account for the observed lattice anisotropy and short coherence length characteristic of these materials

Volcanic materials superconductivity in desert areas of the states of Sonora and Baja California

International Nuclear Information System (INIS)

Holguín, Aldo

2017-01-01

Research was conducted to find materials in their natural state at room temperature and exhibit the effects of superconductivity in the volcanic region of deserts Altar in Sonora and Baja California Norte. 100 were collected at random samples of materials from different parts of the region and underwent tests to determine their electromagnetic parameters of electrical resistance, magnetism, temperature and conductivity. Only it has been found that the effects of superconductivity in them is only present at very low temperatures corroborating what has been done in other investigations, however no indication that there is a material or combination of materials that can produce the effects of superconductivity other temperatures so it is suggested to continue the search for such materials and / or develop a technique at room temperature to allow mimic the behavior of atoms when superconductivity occurs at. (paper)

The interplay of superconducting quantum circuits and propagating microwave states

International Nuclear Information System (INIS)

Goetz, Jan

2017-01-01

Superconducting circuit quantum electrodynamics (QED) has developed into a powerful platform for studying the interaction between matter and different states of light. In this context, superconducting quantum bits (qubits) act as artificial atoms interacting with quantized modes of the electromagnetic field. The field can be trapped in superconducting microwave resonators or propagating in transmission lines. In this thesis, we particularly study circuit QED systems where microwave fields are coupled with superconducting flux and transmon qubits. We optimize the coherence properties of the resonators, by analyzing loss mechanisms at excitation powers of approximately one photon on average. We find that two-level fluctuators associated with oxide layers at substrate and metal surfaces and metal-metal interfaces represent the predominant loss channel. Furthermore, we show how broadband thermal photon fields influence the relaxation and dephasing properties of a superconducting transmon qubit. To this end, we study several second-order loss channels of the transmon qubit and find that the broadband fields introduce a larger decay rate than expected from the Purcell filter defined by the resonator. Additionally, we show that qubit dephasing at the flux-insensitive point as well as low-frequency parameter fluctuations can be enhanced by thermal fields. Finally, we study how artificial atoms react to changes in inherent properties of the light fields. We perform a detailed analysis of the photon statistics of thermal fields using their relation to the qubits coherence properties. We quantitatively recover the expected n 2 + n-law for the photon number variance and confirm this result by direct correlation measurements. We then show a novel technique for the in-situ conversion of the interaction parity in light-matter interaction. To this end, we couple spatially controlled microwave fields to a flux qubit with two degrees of freedom.

The interplay of superconducting quantum circuits and propagating microwave states

Energy Technology Data Exchange (ETDEWEB)

Goetz, Jan

2017-06-26

Superconducting circuit quantum electrodynamics (QED) has developed into a powerful platform for studying the interaction between matter and different states of light. In this context, superconducting quantum bits (qubits) act as artificial atoms interacting with quantized modes of the electromagnetic field. The field can be trapped in superconducting microwave resonators or propagating in transmission lines. In this thesis, we particularly study circuit QED systems where microwave fields are coupled with superconducting flux and transmon qubits. We optimize the coherence properties of the resonators, by analyzing loss mechanisms at excitation powers of approximately one photon on average. We find that two-level fluctuators associated with oxide layers at substrate and metal surfaces and metal-metal interfaces represent the predominant loss channel. Furthermore, we show how broadband thermal photon fields influence the relaxation and dephasing properties of a superconducting transmon qubit. To this end, we study several second-order loss channels of the transmon qubit and find that the broadband fields introduce a larger decay rate than expected from the Purcell filter defined by the resonator. Additionally, we show that qubit dephasing at the flux-insensitive point as well as low-frequency parameter fluctuations can be enhanced by thermal fields. Finally, we study how artificial atoms react to changes in inherent properties of the light fields. We perform a detailed analysis of the photon statistics of thermal fields using their relation to the qubits coherence properties. We quantitatively recover the expected n{sup 2} + n-law for the photon number variance and confirm this result by direct correlation measurements. We then show a novel technique for the in-situ conversion of the interaction parity in light-matter interaction. To this end, we couple spatially controlled microwave fields to a flux qubit with two degrees of freedom.

Atomic-scale Visualization of Electronic Nematicity and Cooper Pairing in Iron-based Superconductors

Science.gov (United States)

Allan, Milan P.

2013-03-01

The mechanism of high-temperature superconductivity in the relatively novel iron-based high-Tc superconductors is unresolved, both in terms of how the phases evolve with doping, and in terms of the actual Cooper pairing process. To explore these issues, we used spectroscopic-imaging scanning tunneling microscopy to study the electronic structure of CaFe2As2 in the antiferromagnetic-orthorhombic `parent' state from which the superconductivity emerges. We discovered and visualized the now widely studied electronic `nematicity' of this phase, whose suppression is associated with the emergence of superconductivity (Science 327, 181, 2010). As subsequent transport experiments discovered a related anisotropic conductance which increases with dopant concentration, the interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom, and the transport nematicity has become a pivotal focus of research. We find that substituting Co for Fe atoms in underdoped Ca(Fe1-xCox)2As2 generates a dense population of identical and strongly anisotropic impurity states that are distributed randomly but aligned with the antiferromagnetic a-axis. We also demonstrate, by imaging their surrounding interference patterns, that these impurity states scatter quasiparticles and thus influence transport in a highly anisotropic manner (M.P. Allan et al., 2013). Next, we studied the momentum dependence of the energy gaps of iron-based superconductivity, now focusing on LiFeAs. If strong electron-electron interactions mediate the Cooper pairing, then momentum-space anisotropic superconducting energy gaps Δi (k) were predicted by multiple techniques to appear on the different electronic bands i. We introduced intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for the determination of anisotropic energy gaps to test these hypotheses and discovered the anisotropy, magnitude, and relative orientations of the energy gaps on multiple

Tunneling processes into localized subgap states in superconductors

Energy Technology Data Exchange (ETDEWEB)

Ruby, Michael; Heinrich, Benjamin W.; Franke, Katharina J. [Freie Universitaet Berlin, Arnimallee 14, 14195 Berlin (Germany); Pientka, Falko; Peng, Yang; Oppen, Felix von [Freie Universitaet Berlin, Arnimallee 14, 14195 Berlin (Germany); Dahlem Center for Complex Quantum Systems, Freie Universitaet Berlin, 14195 Berlin (Germany)

2016-07-01

The Yu-Shiba-Rusinov states bound by magnetic impurities in conventional s-wave superconductors are a simple model system for probing the competition between superconducting and magnetic correlations. Shiba states can be observed in scanning tunneling spectroscopy (STS) as a pair of resonances at positive and negative bias voltages in the superconducting gap. These resonances have been interpreted in terms of single-electron tunneling into the localized sub-gap states. This requires relaxation mechanisms that depopulate the state after an initial tunneling event. Recently, theory suggests that the current can also be carried by Andreev processes which resonantly transfer a Cooper pair into the superconductor. We performed high-resolution STS experiments on single adatom Shiba states on the superconductor Pb, and provide evidence for the existence of two transport regimes. The single-electron processes dominate at large tip-sample distances and small tunneling currents, whereas Andreev processes become important at stronger tunneling. Our conclusions are based on a careful comparison of experiment and theory.

Quantized levitation states of superconducting multiple-ring systems

International Nuclear Information System (INIS)

Haley, S.B.; Fink, H.J.

1996-01-01

The quantized levitation, trapped, and suspension states of a magnetic microsphere held in equilibrium by two fixed superconducting (SC) microrings are calculated by minimizing the free energy of the system. Each state is a discrete function of two independent fluxoid quantum numbers of the rings. When the radii of the SC rings are of the same order as the Ginzburg-Landau coherence length ξ(T), the system exhibits a small set of gravity and temperature-dependent levels. The levels of a weakly magnetized particle are sensitive functions of the gravitational field, indicating potential application as an accelerometer, and for trapping small magnetic particles in outer space or on Earth. The equilibrium states of a SC ring levitated by another SC ring are also calculated. copyright 1996 The American Physical Society

Thermal microwave states acting on a superconducting qubit

Energy Technology Data Exchange (ETDEWEB)

Goetz, Jan; Mueting, Miriam; Haeberlein, Max; Wulschner, Friedrich; Fischer, Michael; Deppe, Frank; Fedorov, Kirill; Huebl, Hans [Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching (Germany); Physik-Department, TU Muenchen, 85748 Garching (Germany); Xie, Edwar; Eder, Peter; Deppe, Frank; Gross, Rudolf [Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching (Germany); Physik-Department, TU Muenchen, 85748 Garching (Germany); Nanosystems Initiative Munich (NIM), Schellingstrasse 4, 80799 Muenchen (Germany); Marx, Achim [Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching (Germany)

2016-07-01

We analyze the influence of broadband thermal states in the microwave regime on the coherence properties of a superconducting (transmon) qubit coupled to a transmission line resonator. We generate the thermal states inside the resonator by heating a 30 dB attenuator to emit blackbody radiation into a transmission line. In the absence of thermal fluctuations, the qubit coherence time is limited by relaxation. We find that the relaxation rate is almost unaffected by the presence of a thermal field inside the resonator. However, such states induce significant dephasing which increases quadratically with the number of thermal photons, whereas for a coherent population of the resonator, the increase shows a linear behavior. These results confirm the different photon statistics, being Poissonian for a coherent population and super-Poissonian for a thermal population of the resonator.

[A quest for a new superconducting state

International Nuclear Information System (INIS)

Collman, J.P.; Little, W.A.

1993-01-01

The authors report on progress of work for the past twelve months, and the work which is planned for the coming year. They have been working on the theory of the long range proximity effect discovered experimentally on Bi-Ag-Tl(2223) sandwiches during the period of the previous grant. This has led to the prediction of a novel effect which they refer to as a ''pair echo'' which occurs in these sandwiches and is analogous to the familiar ''spin echo'' seen in certain NMR experiments. Experimentally, they have extended the measurements of the proximity effect to lower temperatures and to other superconductors. They have completed high resolution studies of the thermal difference optical reflectance of the high T c superconductor Tl(2223), for temperatures both above and below the transition temperature, over a photon energy range from 0.3 eV to 5.3 eV. A striking anomaly in this thermal difference spectrum has been found near 1.6 eV, which appears below T c and scales in magnitude with temperature like Δ(T) 2 where Δ(T) is a BCS gap. This is precisely the kind of effect which they had hoped to find, based on a Holstein-like mechanism in these materials, and which they had predicted in a paper on ''Gap Modulation''. The proper interpretation of the effect observed should reveal the mechanism responsible for the superconductivity of the cuprates. They have completed a search for the, so-called Zhang-modes in the above high T c superconductor, Tl(2223) over the same energy range, 0.3 eV to 5.3 eV. These modes are predicted to exist if the proper description of the ground state of the system is that of a Hubbard model

Controlling superconductivity by tunable quantum critical points.

Science.gov (United States)

Seo, S; Park, E; Bauer, E D; Ronning, F; Kim, J N; Shim, J-H; Thompson, J D; Park, Tuson

2015-03-04

The heavy fermion compound CeRhIn5 is a rare example where a quantum critical point, hidden by a dome of superconductivity, has been explicitly revealed and found to have a local nature. The lack of additional examples of local types of quantum critical points associated with superconductivity, however, has made it difficult to unravel the role of quantum fluctuations in forming Cooper pairs. Here, we show the precise control of superconductivity by tunable quantum critical points in CeRhIn5. Slight tin-substitution for indium in CeRhIn5 shifts its antiferromagnetic quantum critical point from 2.3 GPa to 1.3 GPa and induces a residual impurity scattering 300 times larger than that of pure CeRhIn5, which should be sufficient to preclude superconductivity. Nevertheless, superconductivity occurs at the quantum critical point of the tin-doped metal. These results underline that fluctuations from the antiferromagnetic quantum criticality promote unconventional superconductivity in CeRhIn5.

Superconductivity in graphite intercalation compounds

International Nuclear Information System (INIS)

Smith, Robert P.; Weller, Thomas E.; Howard, Christopher A.; Dean, Mark P.M.; Rahnejat, Kaveh C.; Saxena, Siddharth S.; Ellerby, Mark

2015-01-01

Highlights: • Historical background of graphite intercalates. • Superconductivity in graphite intercalates and its place in the field of superconductivity. • Recent developments. • Relevant modeling of superconductivity in graphite intercalates. • Interpretations that pertain and questions that remain. - Abstract: The field of superconductivity in the class of materials known as graphite intercalation compounds has a history dating back to the 1960s (Dresselhaus and Dresselhaus, 1981; Enoki et al., 2003). This paper recontextualizes the field in light of the discovery of superconductivity in CaC 6 and YbC 6 in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how these relate to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity, and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition

Superconductivity in graphite intercalation compounds

Energy Technology Data Exchange (ETDEWEB)

Smith, Robert P. [Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE (United Kingdom); Weller, Thomas E.; Howard, Christopher A. [Department of Physics & Astronomy, University College of London, Gower Street, London WCIE 6BT (United Kingdom); Dean, Mark P.M. [Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, NY 11973 (United States); Rahnejat, Kaveh C. [Department of Physics & Astronomy, University College of London, Gower Street, London WCIE 6BT (United Kingdom); Saxena, Siddharth S. [Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE (United Kingdom); Ellerby, Mark, E-mail: mark.ellerby@ucl.ac.uk [Department of Physics & Astronomy, University College of London, Gower Street, London WCIE 6BT (United Kingdom)

2015-07-15

Highlights: • Historical background of graphite intercalates. • Superconductivity in graphite intercalates and its place in the field of superconductivity. • Recent developments. • Relevant modeling of superconductivity in graphite intercalates. • Interpretations that pertain and questions that remain. - Abstract: The field of superconductivity in the class of materials known as graphite intercalation compounds has a history dating back to the 1960s (Dresselhaus and Dresselhaus, 1981; Enoki et al., 2003). This paper recontextualizes the field in light of the discovery of superconductivity in CaC{sub 6} and YbC{sub 6} in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how these relate to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity, and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition.

Spin dynamics in the pseudo-gap state of a high-temperature superconductor

Energy Technology Data Exchange (ETDEWEB)

Hinkov, V; Lin, C T; Chen, D P; Keimer, B [Max Planck Inst Solid State Res, D-70569 Stuttgart, (Germany); Bourges, P; Pailhes, S; Sidis, Y [CEA, CNRS, CE Saclay, Lab Leon Brillouin, F-91191 Gif Sur Yvette, (France); Ivanov, A [Inst Max Von Laue Paul Langevin, F-38042 Grenoble, (France); Frost, C D; Perring, T G [Rutherford Appleton Lab, ISIS Facil, Didcot OX11 0QX, Oxon, (United Kingdom)

2007-07-01

The pseudo-gap is one of the most pervasive phenomena of high-temperature superconductors. It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature, Tc, or to a hidden order parameter competing with superconductivity. Here, we use inelastic neutron scattering from under-doped YBa{sub 2}Cu{sub 3}O{sub 6.6} to show that the dispersion relations of spin excitations in the superconducting and pseudo-gap states are qualitatively different. Specifically, the extensively studied 'hour glass' shape of the magnetic dispersions in the superconducting state is no longer discernible in the pseudo-gap state and we observe an unusual 'vertical' dispersion with pronounced in-plane anisotropy. The differences between superconducting and pseudo-gap states are thus more profound than generally believed, suggesting a competition between these two states. Whereas the high-energy excitations are common to both states and obey the symmetry of the copper oxide square lattice, the low-energy excitations in the pseudo-gap state may be indicative of collective fluctuations towards a state with broken orientational symmetry predicted in theoretical work. (authors)

Topological superconductivity in the extended Kitaev-Heisenberg model

Science.gov (United States)

Schmidt, Johann; Scherer, Daniel D.; Black-Schaffer, Annica M.

2018-01-01

We study superconducting pairing in the doped Kitaev-Heisenberg model by taking into account the recently proposed symmetric off-diagonal exchange Γ . By performing a mean-field analysis, we classify all possible superconducting phases in terms of symmetry, explicitly taking into account effects of spin-orbit coupling. Solving the resulting gap equations self-consistently, we map out a phase diagram that involves several topologically nontrivial states. For Γ breaking chiral phase with Chern number ±1 and a time-reversal symmetric nematic phase that breaks the rotational symmetry of the lattice. On the other hand, for Γ ≥0 we find a time-reversal symmetric phase that preserves all the lattice symmetries, thus yielding clearly distinguishable experimental signatures for all superconducting phases. Both of the time-reversal symmetric phases display a transition to a Z2 nontrivial phase at high doping levels. Finally, we also include a symmetry-allowed spin-orbit coupling kinetic energy and show that it destroys a tentative symmetry-protected topological order at lower doping levels. However, it can be used to tune the time-reversal symmetric phases into a Z2 nontrivial phase even at lower doping.

Emergent Higgsless Superconductivity

Directory of Open Access Journals (Sweden)

Cristina Diamantini M.

2017-01-01

Full Text Available We present a new Higgsless model of superconductivity, inspired from anyon superconductivity but P- and T-invariant and generalizable to any dimension. While the original anyon superconductivity mechanism was based on incompressible quantum Hall fluids as average field states, our mechanism involves topological insulators as average field states. In D space dimensions it involves a (D-1-form fictitious pseudovector gauge field which originates from the condensation of topological defects in compact lowenergy effective BF theories. There is no massive Higgs scalar as there is no local order parameter. When electromagnetism is switched on, the photon acquires mass by the topological BF mechanism. Although the charge of the gapless mode (2 and the topological order (4 are the same as those of the standard Higgs model, the two models of superconductivity are clearly different since the origins of the gap, reflected in the high-energy sectors are totally different. In 2D thi! s type of superconductivity is explicitly realized as global superconductivity in Josephson junction arrays. In 3D this model predicts a possible phase transition from topological insulators to Higgsless superconductors.

Ferromagnetic pairing states on two-coupled chains

International Nuclear Information System (INIS)

Tanaka, Akinori

2008-01-01

We propose a concrete model which exhibits ferromagnetism and electron-pair condensation simultaneously. The model is defined on two chains and consists of the electron hopping term, the on-site Coulomb repulsion and a ferromagnetic interaction which describes ferromagnetic coupling between two electrons, one on a bond in a chain and the other on a site in the other chain. It is rigorously shown that the model has fully-polarized ferromagnetic pairing ground states. The higher dimensional version of the model is also presented

Asymmetric d-wave superconducting topological insulator in proximity with a magnetic order

Science.gov (United States)

Khezerlou, M.; Goudarzi, H.; Asgarifar, S.

2018-02-01

In the framework of the Dirac-Bogoliubov-de Gennes formalism, we investigate the transport properties in the surface of a 3-dimensional topological insulator-based hybrid structure, where the ferromagnetic and superconducting orders are simultaneously induced to the surface states via the proximity effect. The superconductor gap is taken to be spin-singlet d-wave symmetry. The asymmetric role of this gap respect to the electron-hole exchange, in one hand, affects the topological insulator superconducting binding excitations and, on the other hand, gives rise to forming distinct Majorana bound states at the ferromagnet/superconductor interface. We propose a topological insulator N/F/FS junction and proceed to clarify the role of d-wave asymmetry pairing in the resulting subgap and overgap tunneling conductance. The perpendicular component of magnetizations in F and FS regions can be at the parallel and antiparallel configurations leading to capture the experimentally important magnetoresistance (MR) of junction. It is found that the zero-bias conductance is strongly sensitive to the magnitude of magnetization in FS region mzfs and orbital rotated angle α of superconductor gap. The negative MR only occurs in zero orbital rotated angle. This result can pave the way to distinguish the unconventional superconducting state in the relating topological insulator hybrid structures.

Weak antilocalization effect and noncentrosymmetric superconductivity in a topologically nontrivial semimetal LuPdBi

KAUST Repository

Xu, Guizhou; Wang, Wenhong; Zhang, Xiaoming; Du, Yin; Liu, Enke; Wang, Shouguo; Wu, Guangheng; Liu, Zhongyuan; Zhang, Xixiang

2014-01-01

A large number of half-Heusler compounds have been recently proposed as three-dimensional (3D) topological insulators (TIs) with tunable physical properties. However, no transport measurements associated with the topological surface states have been observed in these half-Heusler candidates due to the dominating contribution from bulk electrical conductance. Here we show that, by reducing the mobility of bulk carriers, a two-dimensional (2D) weak antilocalization (WAL) effect, one of the hallmarks of topological surface states, was experimentally revealed from the tilted magnetic field dependence of magnetoconductance in a topologically nontrivial semimetal LuPdBi. Besides the observation of a 2D WAL effect, a superconducting transition was revealed at T c ∼ 1.7â.K in the same bulk LuPdBi. Quantitative analysis within the framework of a generalized BCS theory leads to the conclusion that the noncentrosymmetric superconductivity of LuPdBi is fully gapped with a possibly unconventional pairing character. The co-existence of superconductivity and the transport signature of topological surface states in the same bulk alloy suggests that LuPdBi represents a very promising candidate as a topological superconductor.

Weak antilocalization effect and noncentrosymmetric superconductivity in a topologically nontrivial semimetal LuPdBi

KAUST Repository

Xu, Guizhou

2014-07-21

A large number of half-Heusler compounds have been recently proposed as three-dimensional (3D) topological insulators (TIs) with tunable physical properties. However, no transport measurements associated with the topological surface states have been observed in these half-Heusler candidates due to the dominating contribution from bulk electrical conductance. Here we show that, by reducing the mobility of bulk carriers, a two-dimensional (2D) weak antilocalization (WAL) effect, one of the hallmarks of topological surface states, was experimentally revealed from the tilted magnetic field dependence of magnetoconductance in a topologically nontrivial semimetal LuPdBi. Besides the observation of a 2D WAL effect, a superconducting transition was revealed at T c ∼ 1.7â.K in the same bulk LuPdBi. Quantitative analysis within the framework of a generalized BCS theory leads to the conclusion that the noncentrosymmetric superconductivity of LuPdBi is fully gapped with a possibly unconventional pairing character. The co-existence of superconductivity and the transport signature of topological surface states in the same bulk alloy suggests that LuPdBi represents a very promising candidate as a topological superconductor.

Quasiparticle dynamics in superconducting tunnel junctions

International Nuclear Information System (INIS)

Kozorezov, A.G.; Brammertz, G.; Hijmering, R.A.; Wigmore, J.K.; Peacock, A.; Martin, D.; Verhoeve, P.; Golubov, A.A.; Rogalla, H.

2006-01-01

Superconducting Tunnel Junctions (STJs) used as single photon detectors possess extreme sensitivity and excellent resolving power. However, like many other cryogenic detectors they operate under extremely non-equilibrium conditions. In order to understand the physics of the non-equilibrium, non-stationary state, to interpret experimental data adequately, and to optimize the STJs unique performance, it is necessary to use a fully kinetic approach. We have developed the detailed theory of interactions between quasiparticles (qps) and the two types of phonons, sub-gap and pair-breaking, in STJ photon detectors. We discuss the results of extensive sets of experiments to study the non-equilibrium state in Al-based STJs. For the first time we are capable of explaining all available data systematically using a single set of parameters determined from STJ diagnostics and independent experiments

Pairing States of Spin-3/2 Fermions: Symmetry-Enforced Topological Gap Functions

Science.gov (United States)

Venderbos, Jörn W. F.; Savary, Lucile; Ruhman, Jonathan; Lee, Patrick A.; Fu, Liang

2018-01-01

We study the topological properties of superconductors with paired j =3/2 quasiparticles. Higher spin Fermi surfaces can arise, for instance, in strongly spin-orbit coupled band-inverted semimetals. Examples include the Bi-based half-Heusler materials, which have recently been established as low-temperature and low-carrier density superconductors. Motivated by this experimental observation, we obtain a comprehensive symmetry-based classification of topological pairing states in systems with higher angular momentum Cooper pairing. Our study consists of two main parts. First, we develop the phenomenological theory of multicomponent (i.e., higher angular momentum) pairing by classifying the stationary points of the free energy within a Ginzburg-Landau framework. Based on the symmetry classification of stationary pairing states, we then derive the symmetry-imposed constraints on their gap structures. We find that, depending on the symmetry quantum numbers of the Cooper pairs, different types of topological pairing states can occur: fully gapped topological superconductors in class DIII, Dirac superconductors, and superconductors hosting Majorana fermions. Notably, we find a series of nematic fully gapped topological superconductors, as well as double- and triple-Dirac superconductors, with quadratic and cubic dispersion, respectively. Our approach, applied here to the case of j =3/2 Cooper pairing, is rooted in the symmetry properties of pairing states, and can therefore also be applied to other systems with higher angular momentum and high-spin pairing. We conclude by relating our results to experimentally accessible signatures in thermodynamic and dynamic probes.

Superconducting state in (W, Ta)5SiB2

Science.gov (United States)

Fukuma, M.; Kawashima, K.; Akimitsu, J.

We characterize the superconducting state in a boro-silicide (W, Ta)5SiB2, with Tc of 6.5 K by means of magnetization, electrical resistivity, and specific heat measurements. As x increased, the transition temperature Tc abruptly enhances from 5.8 to 6.5 K. The magnetization versus magnetic field (M-H) curve indicated that (W, Ta)5SiB2 was a conventional type-II superconductor. The estimated lower critical field Hc1(0) and upper critical field Hc2(T) are about 121 Oe and 14.7 kOe, respectively. The penetration depth λ(0) and coherence length ξ(0) are calculated to be approximately 369 and 14.9 nm, respectively, using Ginzburg-Landau (GL) equations. Specific heat data shows the superconductivity in W4.5Ta0.5SiB2 belongs to a week-coupling BCS superconductor. Finally, we discuss the increasing of Tc in of (W, Ta)5SiB2 system.

Progress of plasma experiments and superconducting technology in LHD

International Nuclear Information System (INIS)

Motojima, O.; Sakakibara, S.; Imagawa, S.; Sagara, A.; Seki, T.; Mutoh, T.; Morisaki, T.; Komori, A.; Ohyabu, N.; Yamada, H.

2006-01-01

The large helical device is a heliotron device with L = 2 and M = 10 continuous helical coils and three pairs of poloidal coils, and all of coils are superconductive. Since the experiments started in 1998, the development of engineering technologies and the demonstration of large-superconducting-machine operations have greatly contributed to an understanding of physics in currentless plasmas and a verification of the capability of fully steady-state operation. In recent plasma experiments, the steady state and high-beta experiments, which are the most important subjects for the realization of attractive fusion reactors, have progressed remarkably and produced two world-record parameters, i.e. the highest average beta of 4.5% in helical devices and the highest total input energy of 1.6 GJ in all magnetic confinement devices. No degradation has been observed in the coil performance, and stable cryogenic operational schemes at 4.4 K have been established. The physics and engineering results from the LHD experiment directly contribute to the design study for a D-T fusion demo reactor FFHR with a LHD-type heliotron configuration

Controllable manipulation of superconductivity using magnetic vortices

International Nuclear Information System (INIS)

Villegas, J E; Schuller, Ivan K

2011-01-01

The magneto-transport of a superconducting/ferromagnetic hybrid structure, consisting of a superconducting thin film in contact with an array of magnetic nanodots in the so-called 'magnetic vortex state', exhibits interesting properties. For certain magnetic states, the stray magnetic field from the vortex array is intense enough to drive the superconducting film into the normal state. In this fashion, the normal-to-superconducting phase transition can be controlled by the magnetic history. The strong coupling between superconducting and magnetic subsystems allows characteristically ferromagnetic properties, such as hysteresis and remanence, to be dramatically transferred into the transport properties of the superconductor.

An introduction to superconductivity. The Meissner effect and the derivation of phenomenological models

Energy Technology Data Exchange (ETDEWEB)

Habeney, Lucas

2016-09-23

The purpose of this work was to give the reader insight into the topic of conventional superconductors. It started out with defining the superconductive state itself as a state of ideal conductivity and ideal diamagnetism. An important phenomenon to keep in mind in this regard is the Meissner-Ochsenfeld effect. It then went on to attempt to understand those properties on a macroscopic level. This was achieved in the framework of the two major macroscopic theories, the London theory and the Ginzburg-Landau theory. While the London theory focused on the electrodynamic qualities of the superconductive state, the Ginzburg-Landau theory dealt with events close to the superconducting phase transition in a thermodynamic scope. The highlight of this section was the investigation of the Abrikosov lattice, the geometric disposition of the flux tubes in the intermediate Shubnikov phase. We closed with the BCS theory as the premier microscopic theory of superconductivity. Main subjects of this section were the concept of Cooper pairs and the calculation of various energy gap equations. We also looked at real properties of superconductors such as the specific heat to test our rather abstract calculations and came to outstanding agreements. The principles presented in this document should serve as a foundation to work on more advanced problems in superconductivity. Especially the large field of unconventional superconductivity is of huge interest in current research, as most of the high T{sub c} superconductors fall in that category. As unconventional superconductors can not be explained with BCS theory, the search for a uniform theory to describe them is still on-going. Unconventional superconductors include but are not limited to cuprates (T{sub c}

Projected entangled pair states: status and prospects

Energy Technology Data Exchange (ETDEWEB)

Verstraete, Frank [Universitaet Wien (Austria)

2008-07-01

We report on the progress made to extend the density matrix renormalization group to higher dimensions, discuss the underlying theory of projected entangled pair states (PEPS) and illustrate its potential on the hand of a few examples.

Superconductivity in Bismuth. A New Look at an Old Problem.

Science.gov (United States)

Mata-Pinzón, Zaahel; Valladares, Ariel A; Valladares, Renela M; Valladares, Alexander

2016-01-01

To investigate the relationship between atomic topology, vibrational and electronic properties and superconductivity of bismuth, a 216-atom amorphous structure (a-Bi216) was computer-generated using our undermelt-quench approach. Its pair distribution function compares well with experiment. The calculated electronic and vibrational densities of states (eDOS and vDOS, respectively) show that the amorphous eDOS is about 4 times the crystalline at the Fermi energy, whereas for the vDOS the energy range of the amorphous is roughly the same as the crystalline but the shapes are quite different. A simple BCS estimate of the possible crystalline superconducting transition temperature gives an upper limit of 1.3 mK. The e-ph coupling is more preponderant in a-Bi than in crystalline bismuth (x-Bi) as indicated by the λ obtained via McMillan's formula, λc = 0.24 and experiment λa = 2.46. Therefore with respect to x-Bi, superconductivity in a-Bi is enhanced by the higher values of λ and of eDOS at the Fermi energy.

Fluctuations and dark count rates in superconducting NbN single-photon detectors

International Nuclear Information System (INIS)

Engel, Andreas; Semenov, Alexei; Huebers, Heinz-Wilhelm; Il'in, Kostya; Siegel, Michael

2005-01-01

We measured the temperature- and current-dependence of dark count rates of a superconducting singlephoton detector. The detector's key element is a 84 nm wide meander strip line fabricated from a 5 nm thick NbN film. Due to its reduced dimensions various types of fluctuations can cause temporal and localized transitions into a resistive state leading to dark count events. Adopting a recent refinement of the hotspot model we achieve a satisfying description of the experimental dark count rates taking into account fluctuations of the Cooper-pair density and current-assisted unbinding of vortex-antivortex pairs. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Superconducting instabilities and quasipartical interference in the LiFeAs and Co-doped NaFeAs iron-based superconductors

Energy Technology Data Exchange (ETDEWEB)

Altenfeld, Dustin; Ahn, Felix; Eremin, Ilya [Institut fuer Theoretische Physik III, Ruhr-Universitaet Bochum, D-44801 Bochum (Germany); Borisenko, Sergey [Leibniz-Institute for Solid State Research, IFW-Dresden, D-01171 Dresden (Germany)

2015-07-01

We analyze and compare the structure of the pairing interaction and superconducting gaps in LiFeAs and Co-doped NaFeAs by using the ten-orbital tight-binding model, derived from ab initio LDA calculations with hopping parameters extracted from the fit to ARPES experiments. We discuss the phase diagram and experimental probes to determine the structure of the superconducting gap in these systems with special emphasis on the quasiparticle interference, computed using the T-matrix approximation. In particular, we analyze how the superconducting state with opposite sign of the gaps on the two inner hole pockets in LiFeAs evolve upon changing the parameters towards NaFeAs compound.

Quantitative determination of high-Tc superconductivity from the lattice polarization, structure effect and approach to 300 K Tc

International Nuclear Information System (INIS)

Green, B.A.

1997-01-01

The quasiparticle state defined by the low temperature polar mobility is a large, quantized positive energy shift that is identical to the constant pairing energy (pseudogap and 2Λ) in the multi-plane high-T c superconductors, showing the lattice polarization produces the single-particle shift and pairing which then transfer to the superconducting state. The coupling-independence and state transfer follow from the nonlocality and action length of the polarization interaction, and the multiple plane requirement from the location of the polar mode. 2Δ is thereby obtained directly, without scaling, from the strong interaction on the plane. The interaction accordingly produces a dynamic, highly stable state, in contrast to BCS superconductors, and the zero-order state is the quantized energy shift. (orig.)

Amperean Pairing and the Pseudogap Phase of Cuprate Superconductors

Science.gov (United States)

Lee, Patrick A.

2014-07-01

The enigmatic pseudogap phase in underdoped cuprate high-Tc superconductors has long been recognized as a central puzzle of the Tc problem. Recent data show that the pseudogap is likely a distinct phase, characterized by a medium range and quasistatic charge ordering. However, the origin of the ordering wave vector and the mechanism of the charge order is unknown. At the same time, earlier data show that precursive superconducting fluctuations are also associated with this phase. We propose that the pseudogap phase is a novel pairing state where electrons on the same side of the Fermi surface are paired, in strong contrast with conventional Bardeen-Cooper-Schrieffer theory which pairs electrons on opposite sides of the Fermi surface. In this state the Cooper pair carries a net momentum and belongs to a general class called pair density wave. The microscopic pairing mechanism comes from a gauge theory formulation of the resonating valence bond (RVB) picture, where spinons traveling in the same direction feel an attractive force in analogy with Ampere's effects in electromagnetism. We call this Amperean pairing. Charge order automatically appears as a subsidiary order parameter even when long-range pair order is destroyed by phase fluctuations. Our theory gives a prediction of the ordering wave vector which is in good agreement with experiment. Furthermore, the quasiparticle spectrum from our model explains many of the unusual features reported in photoemission experiments. The Fermi arc, the unusual way the tip of the arc terminates, and the relation of the spanning vector of the arc tips to the charge ordering wave vector also come out naturally. Finally, we propose an experiment that can directly test the notion of Amperean pairing.

Superconducting characteristics of the Penson-Kolb model

International Nuclear Information System (INIS)

Czart, W.R.; Robaszkiewicz, S.

2000-01-01

We study superconducting properties of the Penson-Kolb model, i. e. the tight-binding model with the pair-hopping (intersite charge exchange) interaction J. The evolution of the critical fields, the coherence length, the Ginzburg ratio, and London penetration depth with particle concentration n and pairing strength are determined. The results are compared with those found earlier for the attractive Hubbard model. (author)

Interband superconductivity: Contrasts between Bardeen-Cooper-Schrieffer and Eliashberg theories

NARCIS (Netherlands)

Dolgov, Oleg V.; Mazin, Igor I.; Parker, David; Golubov, Alexandre Avraamovitch

2009-01-01

recently discovered iron pnictide superconductors apparently present an unusual case of interband-channel pairing superconductivity. Here we show that in the limit where the pairing occurs within the interband channel, several surprising effects occur quite naturally and generally: different density

Strong temperature effect on the sizes of the Cooper pairs in a two-band superconductor

Science.gov (United States)

Örd, Teet; Rägo, Küllike; Vargunin, Artjom; Litak, Grzegorz

2018-01-01

We study the temperature dependencies of the mean sizes of the Cooper pairs in a two-band BCS-type s-wave superconductivity model with coupling cut-off in the momentum space. It is found that, in contrast to single-band systems, the size of Cooper pairs in the weaker superconductivity band can significantly decrease with a temperature increase due to an interband proximity effect. The relevant spatial behaviour of the wave functions of the Cooper pairs is analyzed. The results also indicate a possibility that the size of Cooper pairs in two-band systems may increase with an increase in temperature.

Thermodynamic Green functions in theory of superconductivity

Directory of Open Access Journals (Sweden)

N.M.Plakida

2006-01-01

Full Text Available A general theory of superconductivity is formulated within the thermodynamic Green function method for various types of pairing mediated by phonons, spin fluctuations, and strong Coulomb correlations in the Hubbard and t-J models. A rigorous Dyson equation for matrix Green functions is derived in terms of a self-energy as a many-particle Green function. By applying the noncrossing approximation for the self-energy, a closed self-consistent system of equations is obtained, similar to the conventional Eliashberg equations. A brief discussion of superconductivity mediated by kinematic interaction with an estimation of a superconducting transition temperature in the Hubbard model is given.

Renormalization group approach to a p-wave superconducting model

International Nuclear Information System (INIS)

Continentino, Mucio A.; Deus, Fernanda; Caldas, Heron

2014-01-01

We present in this work an exact renormalization group (RG) treatment of a one-dimensional p-wave superconductor. The model proposed by Kitaev consists of a chain of spinless fermions with a p-wave gap. It is a paradigmatic model of great actual interest since it presents a weak pairing superconducting phase that has Majorana fermions at the ends of the chain. Those are predicted to be useful for quantum computation. The RG allows to obtain the phase diagram of the model and to study the quantum phase transition from the weak to the strong pairing phase. It yields the attractors of these phases and the critical exponents of the weak to strong pairing transition. We show that the weak pairing phase of the model is governed by a chaotic attractor being non-trivial from both its topological and RG properties. In the strong pairing phase the RG flow is towards a conventional strong coupling fixed point. Finally, we propose an alternative way for obtaining p-wave superconductivity in a one-dimensional system without spin–orbit interaction.

Synthesis, structure and superconductivity in Ba1-xKxBiO3

International Nuclear Information System (INIS)

Hinks, D.G.

1989-01-01

Ba 1-x K x BiO 3 (with x = 0.4) has the highest T c (30 K) of any copperless compound. The superconducting transition temperature of this material is expected to be at the limit of conventional electron-phonon coupling. Since this material is much simpler than the copper containing high-T c superconductors (it is cubic in its superconducting state and only sp electrons are involved in the transport properties), it should be much easier to unravel the nature of the superconducting pairing mechanism in this system. Understanding this system may help explain superconductivity in the more complex copper-oxide materials. In this paper, the authors report on the development of a synthesis method which allows the preparation of stoichiometric, single-phase materials with x between 0.0 and 0.5. The structural phase diagram was determined using powder neutron diffraction as a function of both composition and temperature. Superconductivity only occurs in the cubic perovskite phase which is stable for x larger than 0.3. At a x = 0.3 composition the material undergoes a semiconductor to metal transition with a maximum value for T c . As the K content is further increased, T c is reduced

The Zeeman-split superconductivity with Rashba and Dresselhaus spin-orbit coupling

Science.gov (United States)

Zhao, Jingxiang; Yan, Xu; Gu, Qiang

2017-10-01

The superconductivity with Rashba and Dressehlaus spin-orbit coupling and Zeeman effect is investigated. The energy gaps of quasi-particles are carefully calculated. It is shown that the coexistence of two spin-orbit coupling might suppress superconductivity. Moreover, the Zeeman effect favors spin-triplet Cooper pairs.

Superconductivity from magnetic elements under high pressure

International Nuclear Information System (INIS)

Shimizu, Katsuya; Amaya, Kiichi; Suzuki, Naoshi; Onuki, Yoshichika

2006-01-01

Can we expect the appearance of superconductivity from magnetic elements? In general, superconductivity occurs in nonmagnetic metal at low temperature and magnetic impurities destroy superconductivity; magnetism and superconductivity are as incompatible as oil and water. Here, we present our experimental example of superconducting elements, iron and oxygen. They are magnetic at ambient pressure, however, they become nonmagnetic under high pressure, then superconductor at low temperature. What is the driving force of the superconductivity? Our understanding in the early stages was a simple scenario that the superconductive state was obtained as a consequence of an emergence of the nonmagnetic states. In both cases, we may consider another scenario for the appearance of superconductivity; the magnetic fluctuation mechanism in the same way as unconventional superconductors

About stability of levitating states of superconducting myxini of plasma traps-galateas

International Nuclear Information System (INIS)

Bishaev, A.M.; Bush, A.A.; Denis'uk, A.I.; D'yakonitsa, O.Y.; Kamentsev, K.Y.; Kozintseva, M.V.; Kolesnikova, T.G.; Shapovalov, M.M.; Voronchenko, S.A.; Gavrikov, M.B.; Savelyev, V.V.; Smirnov, P.G.

2015-01-01

To develop a plasma trap with levitating superconducting magnetic coils it is necessary to carry out the search of their stable levitating states. With this purpose, based upon the superconductor property to conserve the trapped magnetic flux, in the uniform gravitational field the analytical dependence of the potential energy of one or two superconducting rings, having trapped the given magnetic fluxes, in the field of the fixed ring with the constant current from the coordinates of the free rings and the deflection angle of their axes from the common axis of the magnetic system has been obtained in the thin ring approximation. Under magnetic fluxes of the same polarity in coils the existence of the found from the calculations equilibrium levitating states for the manufactured HTSC rings stable relative to the vertical shifts of levitating rings and to the deflection angle of their axes from the vertical has been confirmed experimentally

Possible universal cause of high-Tc superconductivity in different metals

International Nuclear Information System (INIS)

Amusia, M.Ya.; Shaginyan, V.R.

2002-01-01

Using the theory of the high temperature superconductivity based on the idea of the fermion condensation quantum phase transition (FCQPT) it is shown that neither the d-wave pairing symmetry, nor the pseudogap phenomenon, nor the presence of the Cu-O 2 planes are of decisive importance for the existence of the high-T c superconductivity. The analysis of recent experimental data on this type of superconductivity in different materials is carried out. It is shown that these facts can be understood within the theory of superconductivity based on the FCQPT. The main features of a room-temperature superconductor are discussed [ru

The pairing theory of polarons in real- and impulse spaces

International Nuclear Information System (INIS)

Dzhumanov, S.; Abboudy, S.; Baratov, A.A.

1995-07-01

A consistent pairing theory of carriers in real- and impulse spaces is developed. The pairing of different free (F), delocalized (D) and self-trapped (S) carriers in real-space, leading to the formation of various bipolaronic states are considered within the continuum model and adiabatic approximation taking into account the combined effect of the short- and long-range components of electron-lattice interaction with and without electron correlation. The formation possibility of D- and S-bipolarons as a function of ε ∞ /ε 0 are shown. The pairing scenarios of carriers in k-space leading to the formation of different bipolarons (including also Cooper pairs dynamic bipolarons) are considered within the generalized BCS-like model taking into account the combined phonon and polaron-bag mediated processes. It is shown that the pure BCS pairing picture is the particular case of the general BCS-like one. The possible relevance of the obtained results to high-T c superconductors is discussed in details in the framework of a novel two-stage Fermi-Bose-liquid scenarios of superconductivity which is caused by single particle and pair condensation of an attracting bipolarons. (author). 51 refs, 6 figs

WORKSHOP: Radiofrequency superconductivity

Energy Technology Data Exchange (ETDEWEB)

Anon.

1984-10-15

The Second Workshop on Radiofrequency Superconductivity was held at CERN from 23-27 July, four years after the first, organized at Karlsruhe. 35 invited talks were presented to the about 80 participants from Australia, Brazil, Europe, Japan and the United States. For the first time, ten Laboratories operating or planning superconducting accelerators for heavy ions participated and shared their experience with the community proposing the use of superconducting accelerating sections for electron accelerators.

WORKSHOP: Radiofrequency superconductivity

International Nuclear Information System (INIS)

Anon.

1984-01-01

The Second Workshop on Radiofrequency Superconductivity was held at CERN from 23-27 July, four years after the first, organized at Karlsruhe. 35 invited talks were presented to the about 80 participants from Australia, Brazil, Europe, Japan and the United States. For the first time, ten Laboratories operating or planning superconducting accelerators for heavy ions participated and shared their experience with the community proposing the use of superconducting accelerating sections for electron accelerators

Pb induces superconductivity in Bi2Se3 analyzed by point contact spectroscopy

OpenAIRE

Arevalo-López, P.; López-Romero, R. E.; Escudero, R.

2015-01-01

Some topological insulators become superconducting when doped with Cu and Pd. Superconductivity in a non-superconductor may be induced by proximity effect: i.e. Contacting a non-superconductor with a superconductor. The superconducting macroscopic wave function will induce electronic pairing into the normal compound. In the simplest topological insulator, Bi$_2$Se$_3$, superconductivity may be induced with Pb. We studied with point contact junctions formed by contacting Bi$_2$Se$_3$ crystals ...

Possibility of Cooper-pair formation controlled by multi-terminal spin injection

Science.gov (United States)

Ohnishi, K.; Sakamoto, M.; Ishitaki, M.; Kimura, T.

2018-03-01

A multi-terminal lateral spin valve consisting of three ferromagnetic nanopillars on a Cu/Nb bilayer has been fabricated. We investigated the influence of the spin injection on the superconducting properties at the Cu/Nb interface. The non-local spin valve signal exhibits a clear spin insulation signature due to the superconducting gap of the Nb. The magnitude of the spin signal is found to show the probe configuration dependence. From the careful analysis of the bias current dependence, we found the suppression of the superconductivity due to the exchange interaction between the Cooper pair and accumulated spin plays an important role in the multi-terminal spin injections. We also discuss about the possibility of the Cooper-pair formation due to the spin injection from the two injectors with the anti-parallel alignment.

Generating stationary entangled states in superconducting qubits

International Nuclear Information System (INIS)

Zhang Jing; Liu Yuxi; Li Chunwen; Tarn, T.-J.; Nori, Franco

2009-01-01

When a two-qubit system is initially maximally entangled, two independent decoherence channels, one per qubit, would greatly reduce the entanglement of the two-qubit system when it reaches its stationary state. We propose a method on how to minimize such a loss of entanglement in open quantum systems. We find that the quantum entanglement of general two-qubit systems with controllable parameters can be controlled by tuning both the single-qubit parameters and the two-qubit coupling strengths. Indeed, the maximum fidelity F max between the stationary entangled state, ρ ∞ , and the maximally entangled state, ρ m , can be about 2/3≅max(tr(ρ ∞ ρ m ))=F max , corresponding to a maximum stationary concurrence, C max , of about 1/3≅C(ρ ∞ )=C max . This is significant because the quantum entanglement of the two-qubit system can be produced and kept, even for a long time. We apply our proposal to several types of two-qubit superconducting circuits and show how the entanglement of these two-qubit circuits can be optimized by varying experimentally controllable parameters.

Color-symmetric superconductivity in a phenomenological QCD model

DEFF Research Database (Denmark)

Bohr, Henrik; Providencia, C.; Providencia, J. da

2009-01-01

In this paper, we construct a theory of the NJL type where superconductivity is present, and yet the superconducting state remains, in the average, color symmetric. This shows that the present approach to color superconductivity is consistent with color singletness. Indeed, quarks are free...... in the deconfined phase, but the deconfined phase itself is believed to be a color singlet. The usual description of the color superconducting state violates color singletness. On the other hand, the color superconducting state here proposed is color symmetric in the sense that an arbitrary color rotation leads...

Ab initio molecular-orbital study on electron correlation effects in CuO6 clusters relating to high-Tc superconductivity

International Nuclear Information System (INIS)

Yamamoto, S.; Yamaguchi, K.; Nasu, K.

1990-01-01

Ab initio molecular-orbital calculations for CuO 6 clusters have been performed to elucidate the electronic structures of undoped and doped copper oxides, which are of current interest in relation to high-T c superconductivity. The electron correlation effects for these species are thoroughly investigated by the full-valence configuration-interaction method and the complete-active-space self-consistent-field method. The electron correlation effect is relatively simple for the A g state (σ hole), whereas pair excitations and spin-flip excitations give sizable contributions to the configuration-interaction wave function for the B state (in-plane π hole). Implications of these results are discussed in relation to the mechanisms of the high-T c superconductivity

Selection rules for Cooper pairing in two-dimensional interfaces and sheets

Science.gov (United States)

Scheurer, Mathias S.; Agterberg, Daniel F.; Schmalian, Jörg

2017-12-01

Thin sheets deposited on a substrate and interfaces of correlated materials offer a plethora of routes towards the realization of exotic phases of matter. In these systems, inversion symmetry is broken which strongly affects the properties of possible instabilities—in particular in the superconducting channel. By combining symmetry and energetic arguments, we derive general and experimentally accessible selection rules for Cooper instabilities in noncentrosymmetric systems, which yield necessary and sufficient conditions for spontaneous time-reversal-symmetry breaking at the superconducting transition and constrain the orientation of the triplet vector. We discuss in detail the implications for various different materials. For instance, we conclude that the pairing state in thin layers of Sr2RuO4 must, as opposed to its bulk superconducting state, preserve time-reversal symmetry with its triplet vector being parallel to the plane of the system. All triplet states of this system allowed by the selection rules are predicted to display topological Majorana modes at dislocations or at the edge of the system. Applying our results to the LaAlO3/SrTiO3 heterostructures, we find that while the condensates of the (001) and (110) oriented interfaces must be time-reversal symmetric, spontaneous time-reversal-symmetry breaking can only occur for the less studied (111) interface. We also discuss the consequences for thin layers of URu2Si2 and UPt3 as well as for single-layer FeSe. On a more general level, our considerations might serve as a design principle in the search for time-reversal-symmetry-breaking superconductivity in the absence of external magnetic fields.

State-of-the-art superconducting accelerator magnets

CERN Document Server

Rossi, L

2002-01-01

With the LHC the technology of NbTi-based accelerator magnets has been pushed to the limit. By operating in superfluid helium, magnetic fields in excess of 10 T have been reached in various one meter-long model magnets while full scale magnets, 15 meter-long dipoles, have demonstrated possibility of safe operation in the 8.3-9 tesla range, with the necessary, very tight, field accuracy. The paper reviews the key points of the technology that has permitted the construction of the largest existing superconducting installations (Fermilab, Desy and Brookhaven), highlighting the novelties of the design of the LHC dipoles, quadrupoles and other superconducting magnets. All together the LHC project will need more than 5000 km of fine filament superconducting cables capable of 14 kA @ 10 T, 1.9 K. (13 refs).

Interface superconductivity

Energy Technology Data Exchange (ETDEWEB)

Gariglio, S., E-mail: stefano.gariglio@unige.ch [DQMP, Université de Genève, 24 Quai E.-Ansermet, CH-1211 Genève (Switzerland); Gabay, M. [Laboratoire de Physique des Solides, Bat 510, Université Paris-Sud 11, Centre d’Orsay, 91405 Orsay Cedex (France); Mannhart, J. [Max Planck Institute for Solid State Research, 70569 Stuttgart (Germany); Triscone, J.-M. [DQMP, Université de Genève, 24 Quai E.-Ansermet, CH-1211 Genève (Switzerland)

2015-07-15

Highlights: • We discuss interfacial superconductivity, a field boosted by the discovery of the superconducting interface between LaAlO. • This system allows the electric field control and the on/off switching of the superconducting state. • We compare superconductivity at the interface and in bulk doped SrTiO. • We discuss the role of the interfacially induced Rashba type spin–orbit. • We briefly discuss superconductivity in cuprates, in electrical double layer transistor field effect experiments. • Recent observations of a high T{sub c} in a monolayer of FeSe deposited on SrTiO{sub 3} are presented. - Abstract: Low dimensional superconducting systems have been the subject of numerous studies for many years. In this article, we focus our attention on interfacial superconductivity, a field that has been boosted by the discovery of superconductivity at the interface between the two band insulators LaAlO{sub 3} and SrTiO{sub 3}. We explore the properties of this amazing system that allows the electric field control and on/off switching of superconductivity. We discuss the similarities and differences between bulk doped SrTiO{sub 3} and the interface system and the possible role of the interfacially induced Rashba type spin–orbit. We also, more briefly, discuss interface superconductivity in cuprates, in electrical double layer transistor field effect experiments, and the recent observation of a high T{sub c} in a monolayer of FeSe deposited on SrTiO{sub 3}.

Superatom representation of high-T{sub C} superconductivity

Energy Technology Data Exchange (ETDEWEB)

Panas, Itai, E-mail: itai@chalmers.se [Environmental Inorganic Chemistry, Division of Energy and Materials, Department of Chemistry and Biotechnology, Chalmers University of Technology, S-412 96 Gothenburg (Sweden)

2012-10-15

A 'super-atom' conceptual interface between chemistry and physics is proposed in order to assist in the search for higher T{sub C} superconductors. The plaquettes generating the checkerboard superstructure in the cuprates, the C{sub 60} molecules in K{sub 3}C{sub 60}, and the Mo{sub 6}S{sub 8}{sup 2-} clusters in Chevrel phase materials offer such candidate super-atoms. Thus, in the present study high-T{sub C} superconductivity HTSC is articulated as the entanglement of two disjoint electronic manifolds in the vicinity of a common Fermi energy. The resulting HTSC ground state couples near-degenerate protected local super-atom states to virtual magnons in an antiferromagnetic AFM embedding. The composite Cooper pairs emerge as the interaction particles for virtual magnons mediated 'self-coherent entanglement' of super-atom states. A Hueckel type resonating valence bond RVB formalism is employed in order to illustrate the real-space Cooper pairs as well as their delocalization and Bose Einstein condensation BEC on a ring of super-atoms. The chemical potential {mu}{sub BEC} for Cooper pairs joining the condensate is formulated in terms of the super-exchange interaction, and consequently the T{sub C} in terms of the Neel temperature. A rationale for the robustness of the HTSC ground state is proposed: achieving local maximum 'electron correlation entropy' at the expense of non-local phase rigidity.

Valence skipping driven superconductivity and charge Kondo effect

International Nuclear Information System (INIS)

Yanagisawa, Takashi; Hase, Izumi

2013-01-01

Highlights: •Valence skipping in metallic compounds can give rise to an unconventional superconductivity. •Several elements in the periodic table show valence skipping (or valence missing), for example, Bi forms the compounds in valence states +3 and +5. •The doping of valence skipping elements will induce superconductivity and this will lead to a possibility of high temperature superconductivity. •We consider the Wolf model with negative-U impurities, and show a phase diagram including superconducting phase. •There is a high temperature region near the boundary. -- Abstract: Valence skipping in metallic compounds can give rise to an unconventional superconductivity. Several elements in the periodic table show valence skipping (or valence missing), for example, Bi forms the compounds in valence states +3 and +5. The doping of valence skipping elements will induce superconductivity and this will lead to a possibility of high temperature superconductivity. We consider the Wolf model with negative-U impurities, and show a phase diagram including superconducting phase. The superconducting state is changed into a metallic state with a local singlet as the attractive interaction |U| increases. There is a high temperature region near the boundary

Superconducting elliptical cavities

CERN Document Server

Sekutowicz, J K

2011-01-01

We give a brief overview of the history, state of the art, and future for elliptical superconducting cavities. Principles of the cell shape optimization, criteria for multi-cell structures design, HOM damping schemes and other features are discussed along with examples of superconducting structures for various applications.

Topological Superconductivity on the Surface of Fe-Based Superconductors.

Science.gov (United States)

Xu, Gang; Lian, Biao; Tang, Peizhe; Qi, Xiao-Liang; Zhang, Shou-Cheng

2016-07-22

As one of the simplest systems for realizing Majorana fermions, the topological superconductor plays an important role in both condensed matter physics and quantum computations. Based on ab initio calculations and the analysis of an effective 8-band model with superconducting pairing, we demonstrate that the three-dimensional extended s-wave Fe-based superconductors such as Fe_{1+y}Se_{0.5}Te_{0.5} have a metallic topologically nontrivial band structure, and exhibit a normal-topological-normal superconductivity phase transition on the (001) surface by tuning the bulk carrier doping level. In the topological superconductivity (TSC) phase, a Majorana zero mode is trapped at the end of a magnetic vortex line. We further show that the surface TSC phase only exists up to a certain bulk pairing gap, and there is a normal-topological phase transition driven by the temperature, which has not been discussed before. These results pave an effective way to realize the TSC and Majorana fermions in a large class of superconductors.

Impacts of Co-doping on the superconductivity and the orbital ordering state in Fe1-xCoxSe single crystal studied by the electrical transport.

Science.gov (United States)

Urata, Takahiro; Tanabe, Yoichi; Heguri, Satoshi; Tanigaki, Katsumi

2015-03-01

In the FeSe with the simplest crystal structure in the Fe-based superconductor families, although both the superconductivity and the orbital ordering states are investigated, the relation between them is still unclear. Here, we report Co doping effects on the superconductivity and the orbital ordering state in Fe1-xCoxSe single crystals. The electrical transport measurements demonstrated that the superconductivity vanishes at 4 % Co doping while the orbital ordering state may be robust against Co doping. Present results suggest that the orbital ordering state is not related to the emergence of the superconductivity in FeSe.

Pair breaking and density of states in disordered superconductors

International Nuclear Information System (INIS)

Weinkauf, A.; Zittartz, J.

1975-01-01

It is shown that pair breaking occurs in a disordered superconductor due to spatial variations of the order parameter, although the system is time reversal invariant. The pair breaking effect is reflected by the occurence of some interesting fine structure in the one particle density of states. Discrete bound states and split-off impurity bands show up in the single impurity case and for very dilute alloys, respectively. For finite alloy concentrations the calculations are done within the CPA. Although principally important, the fine structure is concentrated in an energy range too narrow to be detected experimentally. (orig.) [de

Sample of superconducting wiring (Niobium Titanium)

CERN Multimedia

About NbTi cable: The cable consists of 36 strands of superconducting wire, each strand has a diameter of 0.825 mm and houses 6300 superconducting filaments of niobium-titanium (Nb-Ti, a superconducting alloy). Each filament has a diameter of about 0.006 mm, i.e. 10 times smaller than a typical human hair. The filaments are embedded in a high-purity copper matrix. Copper is a normal conducting material. The filaments are in the superconductive state when the temperature is below about -263ºC (10.15 K). When the filaments leave the superconductive state, the copper acts as conductor transports the electrical current. Each strand of The NbTi cable (at superconducting state) has a current density of up to above 2000 A/mm2 at 9 T and -271ºC (2.15 K). A cable transport a current of about 13000 A at 10 T and -271ºC (2.15 K). About LHC superconducting wiring: The high magnetic fields needed for the LHC can only be reached using superconductors. At very low temperatures, superconductors have no electrical resistan...

Sample of superconducting wiring (Niobium Titanium)

CERN Multimedia

About NbTi cable: The cable consists of 36 strands of superconducting wire, each strand has a diameter of 0.825 mm and houses 6300 superconducting filaments of niobium-titanium (Nb-Ti, a superconducting alloy). Each filament has a diameter of about 0.006 mm, i.e. 10 times smaller than a typical human hair. The filaments are embedded in a high-purity copper matrix. Copper is a normal conducting material. The filaments are in the superconductive state when the temperature is below about -263ºC (10.15 K). When the filaments leave the superconductive state, the copper acts as conductor transports the electrical current. Each strand of The NbTi cable (at superconducting state) has a current density of up to above 2000 A/mm2 at 9 T and -271ºC (2.15 K). A cable transport a current of about 13000 A at 10 T and -271ºC (2.15 K). About LHC superconducting wiring: The high magnetic fields needed for the LHC can only be reached using superconductors. At very low temperatures, superconductors have no electrical resista...

Superconducting state parameters of monovalent and polyvalent amorphous

Energy Technology Data Exchange (ETDEWEB)

Sonvane, Y. A., E-mail: yas@ashd.svnit.ac.in [Department of Applied Physics, S. V. National Institute of Technology, Surat 395 007, Gujarat (India); Patel, H. P., E-mail: patel.harshal2@gmail.com; Thakor, P. B., E-mail: pbthakor@rediffmail.com [Department of Physics, Veer Narmad South Gujarat University, Surat 395 007, Gujarat (India)

2015-08-28

In the present study deals, we have calculated superconducting state parameter (SSP) like electron-phonon coupling strength λ, coulomb pseudo potential, μ*, transition temperature Tc, isotope effect exponent α and effective interaction strength N{sub 0}V of monovalent (Li), divalent (Zn), trivalent (In) and tetravalent (Pb) amorphous. To carry out this work we have used our newly constructed model pseudo potential to describe electron ion interaction along with three different local field correction functions like Hartree, Taylor and Sarkar et al. The present results are found in good agreement with other available theoretical as well as experimental data.

Effect of single interstitial impurity in iron-based superconductors with sign-changed s-wave pairing symmetry

Energy Technology Data Exchange (ETDEWEB)

Yu, Xiang-Long, E-mail: xlyu@theory.issp.ac.cn [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031 (China); Liu, Da-Yong; Quan, Ya-Min; Zheng, Xiao-Jun [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031 (China); Zou, Liang-Jian, E-mail: zou@theory.issp.ac.cn [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031 (China); Department of Physics, University of Science and Technology of China, Hefei 230026 (China)

2015-12-15

Highlights: • Effects of single interstitial impurity are studied in iron-based superconductors. • Bound states within the superconducting gap can be induced. • The interstitial impurity can induce a π phase shift of pairing order parameter. • For strong magnetic scattering the bound-state peak can appear at the Fermi level. - Abstract: We employ the self-consistent Bogoliubov-de Gennes (BdG) formulation to investigate the effect of single interstitial nonmagnetic/magnetic impurity in iron-based superconductors with s ± -wave pairing symmetry. We find that both the nonmagnetic and magnetic impurities can induce bound states within the superconducting (SC) gap and a π phase shift of SC order parameter at the impurity site. However, different from the interstitial-nonmagnetic-impurity case characterized by two symmetric peaks with respect to zero energy, the interstitial magnetic one only induces single bound-state peak. In the strong scattering regime this peak can appear at the Fermi level, which has been observed in the recent scanning tunneling microscope (STM) experiment of Fe(Te,Se) superconductor with interstitial Fe impurities (Yin et al. 2015 [44]). This novel single in-gap peak feature also distinguishes the interstitial case from the substitutional one with two peaks. These results provide important information for comparing the different impurity effects in the iron-based superconductors.

Generation of three-qubit Greenberger-Horne-Zeilinger states of superconducting qubits by using dressed states

Science.gov (United States)

Zhang, Xu; Chen, Ye-Hong; Shi, Zhi-Cheng; Shan, Wu-Jiang; Song, Jie; Xia, Yan

2017-12-01

Combining the advantages of the dressed states and superconducting quantum interference device (SQUID) qubits, we propose an efficient scheme to generate Greenberger-Horne-Zeilinger (GHZ) states for three SQUID qubits. Firstly, we elaborate how to generate GHZ states of three SQUID qubits by choosing a set of dressed states suitably. Then, we compare the scheme by using dressed states with that via the adiabatic passage. Lastly, the influence of various decoherence factors, such as cavity decay, spontaneous emission and dephasing, is analyzed numerically. All of the results show that the GHZ state can be obtained fast and with high fidelity and that the present scheme is robust against the cavity decay and spontaneous emission. In addition, our scheme is more stable against the dephasing than the adiabatic scheme.

Bosonic excitations and electron pairing in an electron-doped cuprate superconductor

Science.gov (United States)

Wang, M. C.; Yu, H. S.; Xiong, J.; Yang, Y.-F.; Luo, S. N.; Jin, K.; Qi, J.

2018-04-01

By applying ultrafast optical spectroscopy to electron-doped La1.9Ce0.1CuO4 ±δ , we discern a bosonic mode of electronic origin and provide the evolution of its coupling with the charge carriers as a function of temperature. Our results show that it has the strongest coupling strength near Tc and can fully account for the superconducting pairing. This mode can be associated with the two-dimensional antiferromagnetic spin correlations emerging below a critical temperature T† larger than Tc. Our work may help to establish a quantitative relation between bosonic excitations and superconducting pairing in electron-doped cuprates.

Superconductivity in technology

International Nuclear Information System (INIS)

Komarek, P.

1976-01-01

Physics, especially high energy physics and solid state physics was the first area in which superconducting magnets were used but in the long run, the most extensive application of superconductivity will probably be in energy technology. Superconducting power transmission cables, magnets for energy conversion in superconducting electrical machines, MHD-generators and fusion reactors and magnets for energy storage are being investigated. Magnets for fusion reactors will have particularly large physical dimensions, which means that much development effort is still needed, for there is no economic alternative. Superconducting surfaces in radio frequency cavities can give Q-values up to a factor of 10 6 higher than those of conventional resonators. Particle accelerators are the important application. And for telecommunication, simple coaxial superconducting radio frequency cables seem promising. The tunnel effect in superconducting junctions is now being developed commercially for sensitive magnetometers and may soon possibly feature in the memory cells of computer devices. Hence superconductivity can play an important role in the technological world, solving physical and technological problems and showing economic advantages as compared with possible conventional techniques, bearing also in mind the importance of reliability and safety. (author)

Cooper-pair size and binding energy for unconventional superconducting systems

Science.gov (United States)

Dinóla Neto, F.; Neto, Minos A.; Salmon, Octavio D. Rodriguez

2018-06-01

The main proposal of this paper is to analyze the size of the Cooper pairs composed by unbalanced mass fermions from different electronic bands along the BCS-BEC crossover and study the binding energy of the pairs. We are considering an interaction between fermions with different masses leading to an inter-band pairing. In addiction to the attractive interaction we have an hybridization term to couple both bands, which in general acts unfavorable for the pairing between the electrons. We get first order phase transitions as the hybridization breaks the Cooper pairs for the s-wave symmetry of the gap amplitude. The results show the dependence of the Cooper-pair size as a function of the hybridization for T = 0 . We also propose the structure of the binding energy of the inter-band system as a function of the two-bands quasi-particle energies.

A large channel count multi client data acquisition system for superconducting magnet system of SST-1

International Nuclear Information System (INIS)

Doshi, K.; Pradhan, S.; Masand, H.; Khristi, Y.; Dhongde, J.; Sharma, A.; Parghi, B.; Varmora, P.; Prasad, U.; Patel, D.

2012-01-01

The magnet system of the Steady-state Superconducting Tokamak-1 at the Institute for Plasma Research, Gandhinagar, India, consists of sixteen Toroidal field and nine Poloidal field Superconducting coils together with a pair of resistive PF coils, an air core ohmic transformer and a pair of vertical field coils. These coils are instrumented with various cryogenic grade sensors and voltage taps to monitor its operating status and health during different operational scenarios. A VME based data acquisition system with remote system architecture is implemented for data acquisition and control of the complete magnet operation. Client-Server based architecture is implemented with remote hardware configuration and continuous online/offline monitoring. A JAVA based platform independent client application is developed for data analysis and data plotting. The server has multiple data pipeline architecture to send data to storage database, online plotting application, numerical display screen, and run time calculation. This paper describes software architecture, design and implementation of the data acquisition system. (author)

Dependence of the quasiparticle recombination rate on the superconducting gap and TC

Science.gov (United States)

Carr, G. L.; Xi, Xiaoxiang; Hwang, J.; Tashiro, H.; Reitze, D. H.; Tanner, D. B.

2010-03-01

The relaxation of excess quasiparticles in a BCS superconductor is known to depend on quantities such as the quasiparticle & phonon density of states, and their coupling (Kaplan et al, Phys. Rev. B 14 4854, 1976). Disorder or an applied field can disrupt superconductivity, as evidenced by a reduced TC. We consider some simple modifications to the quasiparticle density of states consistent with a suppressed energy gap and TC, leading to changes in the intrinsic and effective (measured) rates for excess quasiparticles to recombine into pairs. We review some results for disordered MoGe and discuss the magnetic-field dependence of the recombination process.

Electride and superconductivity behaviors in Mn5Si3-type intermetallics

Science.gov (United States)

Zhang, Yaoqing; Wang, Bosen; Xiao, Zewen; Lu, Yangfan; Kamiya, Toshio; Uwatoko, Yoshiya; Kageyama, Hiroshi; Hosono, Hideo

2017-08-01

Electrides are unique in the sense that they contain localized anionic electrons in the interstitial regions. Yet they exist with a diversity of chemical compositions, especially under extreme conditions, implying generalized underlying principles for their existence. What is rarely observed is the combination of electride state and superconductivity within the same material, but such behavior would open up a new category of superconductors. Here, we report a hexagonal Nb5Ir3 phase of Mn5Si3-type structure that falls into this category and extends the electride concept into intermetallics. The confined electrons in the one-dimensional cavities are reflected by the characteristic channel bands in the electronic structure. Filling these free spaces with foreign oxygen atoms serves to engineer the band topology and increase the superconducting transition temperature to 10.5 K in Nb5Ir3O. Specific heat analysis indicates the appearance of low-lying phonons and two-gap s-wave superconductivity. Strong electron-phonon coupling is revealed to be the pairing glue with an anomalously large ratio between the superconducting gap Δ0 and Tc, 2Δ0/kBTc = 6.12. The general rule governing the formation of electrides concerns the structural stability against the cation filling/extraction in the channel site.

Creation of Spin-Triplet Cooper Pairs in the Absence of Magnetic Ordering

Science.gov (United States)

Breunig, Daniel; Burset, Pablo; Trauzettel, Björn

2018-01-01

In superconducting spintronics, it is essential to generate spin-triplet Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid structures in which a superconductor (SC) is combined with a magnetically ordered material (or an external magnetic field). We, instead, identify a novel way to create and isolate spin-triplet Cooper pairs in the absence of any magnetic ordering. This achievement is only possible because we drive a system with strong spin-orbit interaction—the Dirac surface states of a strong topological insulator (TI)-out of equilibrium. In particular, we consider a bipolar TI-SC-TI junction, where the electrochemical potentials in the outer leads differ in their overall sign. As a result, we find that nonlocal singlet pairing across the junction is completely suppressed for any excitation energy. Hence, this junction acts as a perfect spin-triplet filter across the SC, generating equal-spin Cooper pairs via crossed Andreev reflection.

Normal-state conductance used to probe superconducting tunnel junctions for quantum computing

Energy Technology Data Exchange (ETDEWEB)

Chaparro, Carlos; Bavier, Richard; Kim, Yong-Seung; Kim, Eunyoung; Oh, Seongshik [Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854 (United States); Kline, Jeffrey S; Pappas, David P, E-mail: carlosch@physics.rutgers.ed, E-mail: ohsean@physics.rutgers.ed [National Institute of Standards and Technology, Boulder, CO 80305 (United States)

2010-04-15

Here we report normal-state conductance measurements of three different types of superconducting tunnel junctions that are being used or proposed for quantum computing applications: p-Al/a-AlO/p-Al, e-Re/e-AlO/p-Al, and e-V/e-MgO/p-V, where p stands for polycrystalline, e for epitaxial, and a for amorphous. All three junctions exhibited significant deviations from the parabolic behavior predicted by the WKB approximation models. In the p-Al/a-AlO/p-Al junction, we observed enhancement of tunneling conductances at voltages matching harmonics of Al-O stretching modes. On the other hand, such Al-O vibration modes were missing in the epitaxial e-Re/e-AlO/p-Al junction. This suggests that absence or existence of the Al-O stretching mode might be related to the crystallinity of the AlO tunnel barrier and the interface between the electrode and the barrier. In the e-V/e-MgO/p-V junction, which is one of the candidate systems for future superconducting qubits, we observed suppression of the density of states at zero bias. This implies that the interface is electronically disordered, presumably due to oxidation of the vanadium surface underneath the MgO barrier, even if the interface was structurally well ordered, suggesting that the e-V/e-MgO/p-V junction will not be suitable for qubit applications in its present form. This also demonstrates that the normal-state conductance measurement can be effectively used to screen out low quality samples in the search for better superconducting tunnel junctions.

Superconductivity and its application

International Nuclear Information System (INIS)

Spadoni, M.

1988-01-01

This paper, after a short introduction to superconductivity and to multifilamentary superconducting composites is aiming to review the state of the art and the future perspective of some of the applications of the superconducting materials. The main interest is focussed to large scale applications like, for istance, magnets for accelerators or fusion reactors, superconducting system for NMR thomography, etc. A short paragraph is dedicated to applications for high sensitivity instrumentation. The paper is then concluded by some considerations about the potentialities of the newly discovered high critical temperature materials

Superconductivity in doped two-leg ladder cuprates

International Nuclear Information System (INIS)

Qin Jihong; Yuan Feng; Feng Shiping

2006-01-01

Within the t-J ladder model, superconductivity with a modified d-wave symmetry in doped two-leg ladder cuprates is investigated based on the kinetic energy driven superconducting mechanism. It is shown that the spin-liquid ground-state at the half-filling evolves into the superconducting ground-state upon doping. In analogy to the doping dependence of the superconducting transition temperature in the planar cuprate superconductors, the superconducting transition temperature in doped two-leg ladder cuprates increases with increasing doping in the underdoped regime, and reaches a maximum in the optimal doping, then decreases in the overdoped regime

Theory of superconductivity. II. Excited Cooper pairs. Why does sodium remain normal down to 0 K?

International Nuclear Information System (INIS)

Fujita, S.

1992-01-01

Based on a generalized BCS Hamiltonian in which the interaction strengths (V 11 , V 22 , V 12 ) among and between electron (12) and hole (2) Cooper pairs are differentiated, the thermodynamic properties of a type-I superconductor below the critical temperature T c are investigated. An expression for the ground-state energy, W - W 0 , relative to the unperturbed Block system is obtained. The usual BCS formulas are obtained in the limits: (all) V jl = V 0 , N 1 (0) = N 2 (0). Any excitations generated through the BCS interaction Hamiltonian containing V jl must involve Cooper pairs of antiparallel spins and nearly opposite momenta. The nonzero momentum or excited Cooper pairs below T c are shown to have an excitation energy band minimum lower than the quasi-electrons, which were regarded as the elementary excitations in the original BCS theory. The energy gap var-epsilon g (T) defined relative to excited and zero-momentum Copper pairs (when V jl > 0) decreases from var-epsilon g (0) to 0 as the temperature T is raised from 0 to T c . If electrons only are available as in a monovalent metal like sodium (V 12 = 0), the energy constant Δ 1 is finite but the energy gap vanishes identically for all T. In agreement with the BCS theory, the present theory predicts that a pure nonmagnetic metal in any dimensions should have a Cooper-pair ground state whose energy is lower than that of the Bloch ground state. Additionally it predicts that a monovalent metal should remain normal down to 0 K, and that there should be no strictly one-dimensional superconductor

Zero-bias conductance quantization in a normal / superconducting junction of nano wire

International Nuclear Information System (INIS)

Asano, Yasuhiro; Tanaka, Yukio

2012-01-01

We discuss a strong relationship between Majorana fermions and odd-frequency Cooper pairs which appear at a disordered normal nano wire attached to a topologically nontrivial superconducting one. The zero-bias differential conductance in a normal / superconducting nano wire junctions is quantized at 2e 2 /h irrespective of degree of disorder, length of disordered segment, and random realization of disordered potential. Such behaviors are exactly the same as those in the anomalous proximity effect of p x -wave spin-triplet superconductors. We show that odd-frequency Cooper pairs assist the unusual transport properties.

p -wave superconductivity in weakly repulsive 2D Hubbard model with Zeeman splitting and weak Rashba spin-orbit coupling

Science.gov (United States)

Hugdal, Henning G.; Sudbø, Asle

2018-01-01

We study the superconducting order in a two-dimensional square lattice Hubbard model with weak repulsive interactions, subject to a Zeeman field and weak Rashba spin-orbit interactions. Diagonalizing the noninteracting Hamiltonian leads to two separate bands, and by deriving an effective low-energy interaction we find the mean field gap equations for the superconducting order parameter on the bands. Solving the gap equations just below the critical temperature, we find that superconductivity is caused by Kohn-Luttinger-type interaction, while the pairing symmetry of the bands is indirectly affected by the spin-orbit coupling. The dominating attractive momentum channel of the Kohn-Luttinger term depends on the filling fraction n of the system, and it is therefore possible to change the momentum dependence of the order parameter by tuning n . Moreover, n also determines which band has the highest critical temperature. Rotating the magnetic field changes the momentum dependence from states that for small momenta reduce to a chiral px±i py type state for out-of-plane fields, to a nodal p -wave-type state for purely in-plane fields.

Possible coexistence of antiferromagnetism and superconductivity in the Hubbard model

International Nuclear Information System (INIS)

Su Zhaobin; Dong Jinming; Yu Lu; Shen Juelian

1988-01-01

The Hubbard model in the nearly half-filled case was studied in the mean field approximation using the effective Hamiltonian approach. Both antiferromagnetic order parameter and condensation of singlet pairs were considered. In certain parameter ranges the coexistence of antiferromagnetism and superconductivity is energetically favourable. Relevance to the high temperature superconductivity and other theoretical approaches is also discussed. (author). 10 refs, 3 figs

Coupled superconducting resonant cavities for a heavy ion linac

Energy Technology Data Exchange (ETDEWEB)

Shepard, K W [Argonne National Lab., IL (United States); Roy, A [Nuclear Science Center, New Delhi (India)

1992-11-01

A design for a superconducting niobium slow-wave accelerating structure has been explored that may have performance and cost advantages over existing technology. The option considered is an array of pairs of quarter-wave coaxial-line resonant cavities, the two elements of each pair strongly coupled through a short superconducting transmission line. In the linac formed by such an array, each paired structure is independently phased. A disadvantage of two-gap slow wave structures is that each cavity is relatively short, so that a large number of independently-phased elements is required for a linac. Increasing the number of drift tubes per cavity reduces the number of independently-phased elements but at the cost of reducing the range of useful velocity acceptance for each element. Coupling two cavities splits the accelerating rf eigenmode into two resonant modes each of which covers a portion of the full velocity acceptance range of the original, single cavity mode. Using both of these resonant modes makes feasible the use of coupled cavity pairs for a linac with little loss in velocity acceptance. (Author) 2 figs., 8 refs.

Coupled superconducting resonant cavities for a heavy ion linac

International Nuclear Information System (INIS)

Shepard, K.W.; Roy, A.

1992-01-01

A design for a superconducting niobium slow-wave accelerating structure has been explored that may have performance and cost advantages over existing technology. The option considered is an array of pairs of quarter-wave coaxial-line resonant cavities, the two elements of each pair strongly coupled through a short superconducting transmission line. In the linac formed by such an array, each paired structure is independently phased. A disadvantage of two-gap slow wave structures is that each cavity is relatively short, so that a large number of independently-phased elements is required for a linac. Increasing the number of drift tubes per cavity reduces the number of independently-phased elements but at the cost of reducing the range of useful velocity acceptance for each element. Coupling two cavities splits the accelerating rf eigenmode into two resonant modes each of which covers a portion of the full velocity acceptance range of the original, single cavity mode. Using both of these resonant modes makes feasible the use of coupled cavity pairs for a linac with little loss in velocity acceptance. (Author) 2 figs., 8 refs

Origin of photovoltaic effect in superconducting YBa2Cu3O6.96 ceramics

Science.gov (United States)

Yang, F.; Han, M. Y.; Chang, F. G.

2015-01-01

We report remarkable photovoltaic effect in YBa2Cu3O6.96 (YBCO) ceramic between 50 and 300 K induced by blue-laser illumination, which is directly related to the superconductivity of YBCO and the YBCO-metallic electrode interface. There is a polarity reversal for the open circuit voltage Voc and short circuit current Isc when YBCO undergoes a transition from superconducting to resistive state. We show that there exists an electrical potential across the superconductor-normal metal interface, which provides the separation force for the photo-induced electron-hole pairs. This interface potential directs from YBCO to the metal electrode when YBCO is superconducting and switches to the opposite direction when YBCO becomes nonsuperconducting. The origin of the potential may be readily associated with the proximity effect at metal-superconductor interface when YBCO is superconducting and its value is estimated to be ~10–8 mV at 50 K with a laser intensity of 502 mW/cm2. Combination of a p-type material YBCO at normal state with an n-type material Ag-paste forms a quasi-pn junction which is responsible for the photovoltaic behavior of YBCO ceramics at high temperatures. Our findings may pave the way to new applications of photon-electronic devices and shed further light on the proximity effect at the superconductor-metal interface. PMID:26099727

Concealed d-wave pairs in the s± condensate of iron-based superconductors.

Science.gov (United States)

Ong, Tzen; Coleman, Piers; Schmalian, Jörg

2016-05-17

A central question in iron-based superconductivity is the mechanism by which the paired electrons minimize their strong mutual Coulomb repulsion. In most unconventional superconductors, Coulomb repulsion is minimized through the formation of higher angular momentum Cooper pairs, with Fermi surface nodes in the pair wavefunction. The apparent absence of such nodes in the iron-based superconductors has led to a belief they form an s-wave ([Formula: see text]) singlet state, which changes sign between the electron and hole pockets. However, the multiorbital nature of these systems opens an alternative possibility. Here, we propose a new class of [Formula: see text] state containing a condensate of d-wave Cooper pairs, concealed by their entanglement with the iron orbitals. By combining the d-wave ([Formula: see text]) motion of the pairs with the internal angular momenta [Formula: see text] of the iron orbitals to make a singlet ([Formula: see text]), an [Formula: see text] superconductor with a nontrivial topology is formed. This scenario allows us to understand the development of octet nodes in potassium-doped Ba1-x KXFe2As2 as a reconfiguration of the orbital and internal angular momentum into a high spin ([Formula: see text]) state; the reverse transition under pressure into a fully gapped state can then be interpreted as a return to the low-spin singlet. The formation of orbitally entangled pairs is predicted to give rise to a shift in the orbital content at the Fermi surface, which can be tested via laser-based angle-resolved photoemission spectroscopy.

Two-fluid model of the superconductivity in the BCS's theory

International Nuclear Information System (INIS)

Rangelov, J.

1977-01-01

The coefficients of Bogolubov-Valatin's transformation are chosen in accordance with the two-fluid model of superconductivity. The energy spectrum of superconducting quasi-particles is obtained as a solution of the linearized equation of motion of interacting particles. The energy distribution of the superconducting and normal quasi-particles is discussed from a new view-point. The correlation between the quasi-particles forming the Cooper's pair is discussed in accordance with the proposed ideas. The tunnelling of the normal quasi-particles in systems M-I-S and S 1 -I-S 2 is investigated qualitatively

Superconducting and Normal State Properties of OsB2*

Science.gov (United States)

Singh, Yogesh; Niazi, A.; Zong, X.; Suh, B. J.; Vannette, M. W.; Prozorov, R.; Johnston, D. C.

2007-03-01

OsB2 is a layered superhard metallic material that was found to superconduct below Tc= 2.1 K.^1 We report the first detailed measurements of the static and dynamic magnetic susceptibilities χ, electrical resistivity, heat capacity Cp, penetration depth, and ^11B NMR on OsB2 to characterize its superconducting and normal state properties. The results confirm that OsB2 is a bulk superconductor below Tc= 2.1 K@. Its properties can be described by a close to weak-coupling s-wave BCS model with an electron-phonon coupling constant λ= 0.4--0.5, δ(0)/(kBTc) 1.9, a small Ginzburg-Landau parameter κ of order 5 or less, and a small zero-temperature critical magnetic field of roughly 500 Oe. The ^11B NMR measurements in the normal state show a nuclear spin-lattice relaxation time T1= 2.1 s at room temperature and a Korringa law with T1T = 610 s.K at lower T, and a correspondingly small T-independent Knight shift. These results indicate a small s character of the conduction electron wave function at the B site at the Fermi level. Our results will be compared to corresponding data for MgB2.1. J. K. Vandenberg et al., Mater. Res. Bull. 10, 889 (1975).^*Supported by the USDOE under Contract No. W-7405-Eng-82. Permanent address: Dept. Phys., The Catholic Univ. Korea.

Half-integer flux quantum effect in cuprate superconductors - a probe of pairing symmetry

International Nuclear Information System (INIS)

Tsuei, C.C.; Kirtley, J.R.; Gupta, A.; Sun, J.Z.; Moler, K.A.; Wang, J.H.

1996-01-01

Based on macroscopic quantum coherence effects arising from pair tunneling and flux quantization, a series of tricrystal experiments have been designed and carried out to test the order parameter symmetry in high-T c cuprate superconductors. By using a scanning SQUID microscope, we have directly and non-invasively observed the spontaneously generated half-integer flux quantum effect in controlled-orientation tricrystal cuprate superconducting systems. The presence or absence of the half-integer flux quantum effect as a function of the tricrystal geometry allows us to prove that the order parameter symmetry in the YBCO and Tl2201 systems is consistent with that of the d x 2 -y 2 pair state. (orig.)

Kinetics-Driven Superconducting Gap in Underdoped Cuprate Superconductors Within the Strong-Coupling Limit

Directory of Open Access Journals (Sweden)

Yucel Yildirim

2011-09-01

Full Text Available A generic theory of the quasiparticle superconducting gap in underdoped cuprates is derived in the strong-coupling limit, and found to describe the experimental “second gap” in absolute scale. In drastic contrast to the standard pairing gap associated with Bogoliubov quasiparticle excitations, the quasiparticle gap is shown to originate from anomalous kinetic (scattering processes, with a size unrelated to the pairing strength. Consequently, the k dependence of the gap deviates significantly from the pure d_{x^{2}-y^{2}} wave of the order parameter. Our study reveals a new paradigm for the nature of the superconducting gap, and is expected to reconcile numerous apparent contradictions among existing experiments and point toward a more coherent understanding of high-temperature superconductivity.

Superconducting rotating machines

International Nuclear Information System (INIS)

Smith, J.L. Jr.; Kirtley, J.L. Jr.; Thullen, P.

1975-01-01

The opportunities and limitations of the applications of superconductors in rotating electric machines are given. The relevant properties of superconductors and the fundamental requirements for rotating electric machines are discussed. The current state-of-the-art of superconducting machines is reviewed. Key problems, future developments and the long range potential of superconducting machines are assessed

Electron-tunneling observation of localized excited states in superconducting manganese-doped lead

International Nuclear Information System (INIS)

Tsang, J.; Ginsberg, D.M.

1980-01-01

We have made electron-tunneling measurements on a dilute, superconducting lead-manganese alloy. A well-defined structure was observed in the ac-conductance--voltage curves, indicating excited states within the BCS energy gap. These states were partially accounted for by Shiba theory when spin-dependent s-, p-, and d-wave scattering were included. The phase shifts used in doing that were the results of band calculations. The experimental data also show the existence of a broad background density of states in the energy gap, which cannot be accounted for by the theory

Anomalous enhancement of the lower critical field deep in the superconducting state of LaRu4As12

Science.gov (United States)

Juraszek, J.; Bochenek, Ł.; Wawryk, R.; Henkie, Z.; Konczykowski, M.; Cichorek, T.

2018-05-01

LaRu4As12 with the critical temperature Tc = 10.4 K displays several features which point at a non-singlet superconducting order parameter, although the bcc crystal structure of the filled skutterudites does not favour the emergence of multiple energy gaps. LaRu4As12 displays an unexpected enhancement of the lower critical field deep in superconducting state which can be attributed to the existence of two superconducting gaps. At T = 0.4 K, the local magnetization measurements were performed utilizing miniaturized Hall sensors.

Recent advances in the 5f-relevant electronic states and unconventional superconductivity of actinide compounds

International Nuclear Information System (INIS)

Haga, Yoshinori; Sakai, Hironori; Kambe, Shinsaku

2007-01-01

Recent advances in the understanding of the 5f-relevant electronic states and unconventional superconducting properties are reviewed in actinide compounds of UPd 2 Al 3 . UPt 3 , URu 2 Si 2 , UGe 2 , and PuRhGa 5 . These are based on the experimental results carried out on high-quality single crystal samples, including transuranium compounds, which were grown by using combined techniques. The paring state and the gap structure of these superconductors are discussed, especially for the corresponding Fermi surfaces which were clarified by the de Haas-van Alphen experiment and the energy band calculations. A detailed systematic study using the NQR/NMR spectroscopy reveals the d-wave superconductivity in PuRhGa 5 and the difference of magnetic excitations due to the difference of ground states in U-, Np-, and Pu-based AnTGa 5 (T: transition metal) compounds. (author)

Magnetism and Pressure-Induced Superconductivity of Checkerboard-Type Charge-Ordered Molecular Conductor β-(meso-DMBEDT-TTF2X (X = PF6 and AsF6

Directory of Open Access Journals (Sweden)

Yutaka Nishio

2012-11-01

Full Text Available The metallic state of the molecular conductor β-(meso-DMBEDT-TTF2X (DMBEDT-TTF = 2-(5,6-dihydro-1,3-dithiolo[4,5-b][1,4]dithiin-2-ylidene-5,6-dihydro-5,6-dimethyl-1,3-dithiolo[4,5-b][1,4]dithiin, X = PF6, AsF6 is transformed into the checkerboard-type charge-ordered state at around 75–80 K with accompanying metal-insulator (MI transition on the anisotropic triangular lattice. With lowering temperatures, the magnetic susceptibility decreases gradually and reveals a sudden drop at the MI transition. By applying pressure, the charge-ordered state is suppressed and superconductivity appears in β-(meso-DMBEDT-TTF2AsF6 as well as in the reported β-(meso-DMBEDT-TTF2PF6. The charge-ordered spin-gapped state and the pressure-induced superconducting state are discussed through the paired-electron crystal (PEC model, where the spin-bonded electron pairs stay and become mobile in the crystal, namely the valence-bond solid (VBS and the resonant valence bonded (RVB state in the quarter-filled band structure.

Superconductivity in Layered Organic Metals

Directory of Open Access Journals (Sweden)

Jochen Wosnitza

2012-04-01

Full Text Available In this short review, I will give an overview on the current understanding of the superconductivity in quasi-two-dimensional organic metals. Thereby, I will focus on charge-transfer salts based on bis(ethylenedithiotetrathiafulvalene (BEDT-TTF or ET for short. In these materials, strong electronic correlations are clearly evident, resulting in unique phase diagrams. The layered crystallographic structure leads to highly anisotropic electronic as well as superconducting properties. The corresponding very high orbital critical field for in-plane magnetic-field alignment allows for the occurrence of the Fulde–Ferrell– Larkin–Ovchinnikov state as evidenced by thermodynamic measurements. The experimental picture on the nature of the superconducting state is still controversial with evidence both for unconventional as well as for BCS-like superconductivity.

Entanglement and Teleportation of Pair Cat States in Amplitude Decoherence Channel

International Nuclear Information System (INIS)

Xu Hangshi; Xu Jingbo

2009-01-01

The dynamic behavior of the entanglement for the pair cat states in the amplitude decoherence channel is studied by adopting the entanglement of formation determined by the concurrence. Then, we consider the teleportation by using joint measurements of the photon-number sum and phase difference with the pair cat states as an entangle resource and discuss the influence of amplitude decoherence on the mean fidelity of the teleportation.

Formation energies of local pairs in fullerene isomer mixtures

International Nuclear Information System (INIS)

Solecki, J.

1996-01-01

It is well-known that the alkali metal-doped fullerides are superconductors of type II. There were so far many trials to explain the pairing mechanism in the superconducting fullerides. A description of the superconducting mechanism in terms of the so-called local pair model has been proposed in this note. A purely electronic interaction was also considered within the resonating valence bond model (RVB). In fact, other models were not able to explain exactly why superconductivity appears for the stoichiometry of A 3 C 60 in the alkali metal-doped fullerides. An exception of this rule is, e.g., Ca 5 C 60 which is a superconductor with T c = 8.4 K. However, measurements show that electronic structures near the Fermi level of the A 3 C 60 (A = K, Rb) as well as the Ca 5 C 60 superconductors are remarkably similar although their charge carriers form energy bands of different character. Therefore, the results obtained for the stoichiometry A 3 C 60 can roughly refer to the Ca 5 C 60 case as well. (orig.)

The Kohn-Luttinger mechanism and phase diagram of the superconducting state in the Shubin-Vonsovsky model

International Nuclear Information System (INIS)

Kagan, M. Yu.; Val’kov, V. V.; Mitskan, V. A.; Korovuskin, M. M.

2013-01-01

Using the Shubin-Vonsovsky model in the weak-coupling regime W > U > V (W is the bandwidth, U is the Hubbard onsite repulsion, and V is the Coulomb interaction at neighboring sites) based on the Kohn-Luttinger mechanism, we determined the regions of the existence of the superconducting phases with the d xy , p, s, and d x 2 -y 2 symmetry types of the order parameter. It is shown that the effective interaction in the Cooper channel considerably depends not only on single-site but also on intersite Coulomb correlations. This is demonstrated by the example of the qualitative change and complication of the phase diagram of the superconducting state. The superconducting (SC) phase induction mechanism is determined taking into account polarization contributions in the second-order perturbation theory in the Coulomb interaction. The results obtained for the angular dependence of the superconducting gap in different channels are compared with angule-resolved photoemission spectroscopy (ARPES) results. The influence of long-range hops in the phase diagram and critical superconducting transition temperature in different channels is analyzed. The conditions for the appearance of the Kohn-Luttinger superconductivity with the d x 2 -y 2 symmetry and high critical temperatures T c ∼ 100 K near the half-filling are determined

Single-particle spectra and magnetic field effects within precursor superconductivity

International Nuclear Information System (INIS)

Pieri, P.; Pisani, L.; Strinati, G.C.; Perali, A.

2004-01-01

We study the single-particle spectra below the superconducting critical temperature from weak to strong coupling within a precursor superconductivity scenario. The spectral-weight function is obtained from a self-energy that includes pairing-fluctuations within a continuum model representing the hot spots of the Brillouin zone. The effects of strong magnetic fields on the pseudogap temperature are also discussed within the same scenario

Bec Model of HIGH-Tc Superconductivity in Layered Cuprates

Science.gov (United States)

Lomnitz, M.; Villarreal, C.; de Llano, M.

2013-11-01

High-Tc superconductivity in layered cuprates is described in a BCS-BEC formalism with linearly-dispersive s- and d-wave Cooper pairs moving in quasi-2D finite-width layers around the CuO2 planes. This yields a closed formula for Tc involving the layer width, the Debye frequency, the pairing energy and the in-plane penetration depth. The new formula has no free parameters and reasonably reproduces empirical values of superconducting Tcs for 11 different layered superconductors over a wide doping regime including YBCO itself as well as other compounds like LSCO, BSCCO and TBCCO. In agreement with the London formalism, the formula also yields a fair description of the Tc dependence of the lower critical magnetic field in highly underdoped YBCO.

Superconducting spin valve effect in Fe/In based heterostructures

Energy Technology Data Exchange (ETDEWEB)

Leksin, Pavel; Schumann, Joachim; Kataev, Vladislav; Schmidt, Oliver; Buechner, Bernd [Leibniz Institute for Solid State and Materials Research IFW Dresden (Germany); Garifyanov, Nadir; Garifullin, Ilgiz [Zavoisky Physical-Technical Institute, Kazan Scientific Center, Russian Academy of Sciences (Russian Federation)

2015-07-01

We report on magnetic and superconducting properties of the spin-valve multilayer system CoOx/Fe1/Cu/Fe2/In. The Superconducting Spin Valve Effect (SSVE) assumes the T{sub c} difference between parallel (P) and antiparallel (AP) orientations of the Fe1 and Fe2 layers' magnetizations. The SSVE value oscillates and changes its sign when the Fe2 layer thickness d{sub Fe2} is varied from 0 to 5 nm. The SSVE value is positive, as expected, in the range 0.4 nm ≤ d{sub Fe2} ≤ 0.8 nm. For a rather broad range of thicknesses 1 nm ≤ d{sub Fe2} ≤ 2.6 nm the SSVE has negative sign assuming the inverse SSVE. Moreover, the magnitude of the inverse effect is larger than that of the positive direct effect. We attribute these oscillations to a quantum interference of the cooper pair wave functions in the magnetic part of the system. For most of the spin-valve samples from this set we experimentally realized the full switching between normal and superconducting states due to direct and inverse SSVE. The analysis of the experimental data has enabled the determination of all microscopic parameters of the studied system.

Visualizing pair formation on the atomic scale in high-Tc superconductors

International Nuclear Information System (INIS)

Pasupathy, A.

2008-01-01

Full text: Unlike traditional superconductors, the density of states (DOS) of the high-T c superconductor Bi-2212 shows large nanoscale variations that have been detected using scanning tunneling microscopy (STM). Such variations are seen in the low temperature superconducting gap and in features associated with the coupling of pairs to boson modes. In order to understand these variations in the spectra, we perform atomic resolution STM measurements of Bi-2212 as a function of temperature. Using newly developed experimental techniques, we measure the evolution of the DOS from low temperature (T c ) to temperatures where all gaps in the spectrum have disappeared (T>T*). Such measurements show that the pairing gap nucleates in nanoscale regions at temperatures between T c and T*. By normalizing the low temperature DOS (T c ) to the DOS at high temperature, we are able to fit the superconducting DOS to the d-wave BCS form. We find that the experimental spectrum deviations from a simple d-wave fit indicating a strong coupling between electrons and bosonic modes. We will discuss the temperature evolution of these as well as other features in the DOS and correlate such measurements with the inhomogeneity seen in the gap magnitude at low temperature

Mirror nesting of the Fermi contour and enhanced diamagnetism of the pseudogap state in cuprates

Energy Technology Data Exchange (ETDEWEB)

Kapaev, V.V.; Belyavsky, V.I. [P.N. Lebedev Physical Institute of Russian Academy of Sciences, Moscow 119991 (Russian Federation); Kopaev, Yu.V. [P.N. Lebedev Physical Institute of Russian Academy of Sciences, Moscow 119991 (Russian Federation)], E-mail: kopaev@sci.lebedev.ru; Smirnov, M.Yu. [State Pedagogical University, Voronezh 394043 (Russian Federation)

2007-09-01

Since the insulating gap in parent spin antiferromagnet survives under a hole underdoping, it might result in a rise of a metal state with a pocket-like Fermi contour with both conventional and mirror nesting corresponding to the same momentum K = ({pi}, {pi}). The nesting leads to a possibility of singlet orbital antiferromagnetic order whereas the mirror nesting promotes the superconducting pairing with the momentum K. We assume screened Coulomb repulsion to be the dominating pairing interaction in the cuprates resulting in the two-component superconducting order parameter. The relative phase of the parameter can be related to orbital current circulations as it follows from the Ginzburg-Landau phenomenology. The orbital antiferromagnetic state with the insulating gap on the Fermi contour is related to the pseudogap state with enhanced diamagnetic response.

Mirror nesting of the Fermi contour and enhanced diamagnetism of the pseudogap state in cuprates

International Nuclear Information System (INIS)

Kapaev, V.V.; Belyavsky, V.I.; Kopaev, Yu.V.; Smirnov, M.Yu.

2007-01-01

Since the insulating gap in parent spin antiferromagnet survives under a hole underdoping, it might result in a rise of a metal state with a pocket-like Fermi contour with both conventional and mirror nesting corresponding to the same momentum K = (π, π). The nesting leads to a possibility of singlet orbital antiferromagnetic order whereas the mirror nesting promotes the superconducting pairing with the momentum K. We assume screened Coulomb repulsion to be the dominating pairing interaction in the cuprates resulting in the two-component superconducting order parameter. The relative phase of the parameter can be related to orbital current circulations as it follows from the Ginzburg-Landau phenomenology. The orbital antiferromagnetic state with the insulating gap on the Fermi contour is related to the pseudogap state with enhanced diamagnetic response

Low-velocity superconducting accelerating structures

International Nuclear Information System (INIS)

Delayen, J.R.

1990-01-01

The present paper reviews the status of RF superconductivity as applied to low-velocity accelerating properties. Heavy-ion accelerators must accelerate efficiently particles which travel at a velocity much smaller than that of light particles, whose velocity changes along accelerator, and also different particles which have different velocity profiles. Heavy-ion superconducting accelerators operate at frequencies which are lower than high-energy superconducting accelerators. The present paper first discusses the basic features of heavy-ion superconducting structures and linacs. Design choices are then addressed focusing on structure geometry, materials, frequency, phase control, and focusing. The report also gives an outline of the status of superconducting booster projects currently under way at the Argonne National Laboratory, SUNY Stony Brook, Weizmann Institute, University of Washington, Florida State, Saclay, Kansas State, Daresbury, Japanese Atomic Energy Research Institute, Legnaro, Bombay, Sao Paulo, ANU (Canberra), and Munich. Recent developments and future prospects are also described. (N.K.) 68 refs

Effective theory of exotic superconductivity in LaAlO3/SrTiO3 interfaces

Science.gov (United States)

Esmailzadeh, Haniyeh; Moghaddam, Ali G.

2018-05-01

Motivated by experimental and theoretical works about superconductivity at the oxide interfaces, we provide a simple model for possible unconventional pairings inside the exotic two-dimensional electron gas formed in heterostructures of SrTiO3 and LaAlO3. At the low energy limit, the electron gas at the interfaces is usually modeled with an effective three band model considering of 3d t2g orbitals which are slightly coupled by atomic spin-orbit couplings (SOC). Considering direct superconducting pairing in two higher delocalized bands and by exploiting a perturbative scheme based on canonical transformation, we derive the effective pairing amplitudes with possibly exotic nature inside the localized dxy band as well as various inter-band pairing components. In particular we show that equal-spin triplet pairings are possible between the band dxy and any of other dxz and dyz bands. In addition weaker effective pairings take place inside the localized band itself and between delocalized dxz and dyz bands with singlet and opposite-spin triplet characters. These unconventional effective pairings are indeed mediated by SOC-induced higher order virtual transitions between the bands and particularly into the localized band. Our model suggest that unconventional effective superconductivity is possible at oxide interfaces, simply, due to the special band structure and important role of atomic SOC and perhaps other magnetic effects present at these heterostructures.

Unconventional superconductivity in cuprates, cobaltates and graphene. What is universal and what is material-dependent in strongly versus weakly correlated materials?

International Nuclear Information System (INIS)

Kiesel, Maximilian Ludwig

2013-01-01

A general theory for all classes of unconventional superconductors is still one of the unsolved key issues in condensed-matter physics. Actually, it is not yet fully settled if there is a common underlying pairing mechanism. Instead, it might be possible that several distinct sources for unconventional (not phonon-mediated) superconductivity have to be considered, or an electron-phonon interaction is not negligible. The focus of this thesis is on the most probable mechanism for the formation of Cooper pairs in unconventional superconductors, namely a strictly electronic one where spin fluctuations are the mediators. Studying different superconductors in this thesis, the emphasis is put on material-independent features of the pairing mechanism. In addition, the investigation of the phase diagrams enables a view on the vicinity of superconductivity. Thus, it is possible to clarify which competing quantum fluctuations enhance or weaken the propensity for a superconducting state. The broad range of superconducting materials requires the use of more than one numerical technique to study an appropriate microscopic description. This is not a problem but a big advantage because this facilitates the approach-independent description of common underlying physics. For this evaluation, the strongly correlated cuprates are simulated with the variational cluster approach. Especially the question of a pairing glue is taken into consideration. Furthermore, it is possible to distinguish between retarded and non-retarded contributions to the gap function. The cuprates are confronted with the cobaltate Na x CoO 2 and graphene. These weakly correlated materials are investigated with the functional renormalization group (fRG) and reveal a comprehensive phase diagram, including a d+id-wave superconductivity, which breaks time-reversal symmetry. The corresponding gap function is nodeless, but for NaCoO, it features a doping-dependent anisotropy. In addition, some general considerations on

Unconventional superconductivity in cuprates, cobaltates and graphene. What is universal and what is material-dependent in strongly versus weakly correlated materials?

Energy Technology Data Exchange (ETDEWEB)

Kiesel, Maximilian Ludwig

2013-02-08

A general theory for all classes of unconventional superconductors is still one of the unsolved key issues in condensed-matter physics. Actually, it is not yet fully settled if there is a common underlying pairing mechanism. Instead, it might be possible that several distinct sources for unconventional (not phonon-mediated) superconductivity have to be considered, or an electron-phonon interaction is not negligible. The focus of this thesis is on the most probable mechanism for the formation of Cooper pairs in unconventional superconductors, namely a strictly electronic one where spin fluctuations are the mediators. Studying different superconductors in this thesis, the emphasis is put on material-independent features of the pairing mechanism. In addition, the investigation of the phase diagrams enables a view on the vicinity of superconductivity. Thus, it is possible to clarify which competing quantum fluctuations enhance or weaken the propensity for a superconducting state. The broad range of superconducting materials requires the use of more than one numerical technique to study an appropriate microscopic description. This is not a problem but a big advantage because this facilitates the approach-independent description of common underlying physics. For this evaluation, the strongly correlated cuprates are simulated with the variational cluster approach. Especially the question of a pairing glue is taken into consideration. Furthermore, it is possible to distinguish between retarded and non-retarded contributions to the gap function. The cuprates are confronted with the cobaltate Na{sub x}CoO{sub 2} and graphene. These weakly correlated materials are investigated with the functional renormalization group (fRG) and reveal a comprehensive phase diagram, including a d+id-wave superconductivity, which breaks time-reversal symmetry. The corresponding gap function is nodeless, but for NaCoO, it features a doping-dependent anisotropy. In addition, some general

Can magnetism and superconductivity coexist

International Nuclear Information System (INIS)

Ishikawa, M.

1982-01-01

Recent syntheses of rare earth (RE) ternary superconductors such as (RE)Mo 6 X 8 (X=S or Se) and (RE)Rh 4 B 4 have provided the first opportunity to explore the interaction between magnetism and superconductivity in detail owing to their particular crystal structure. The regular sublattice of the rare-earth ions in these new ternary compounds undergoes a ferro- or antiferromagnetic phase transition in the superconducting state. If the transition is antiferromagnetic, the superconductivity is preserved so that true coexistence results. If it is ferromagnetic, on the other hand, the superconductivity eventually gives way to uniform ferromagnetism at low temperatures. However, recent theories predict several possible states of coexistence even in ferromagnetic superconductors. This article reviews aspects of these new phase transitions in ternary superconductors. (author)

Supercurrent Spectroscopy of Andreev States

Directory of Open Access Journals (Sweden)

L. Bretheau

2013-12-01

Full Text Available We measure the excitation spectrum of a superconducting atomic contact. In addition to the usual continuum above the superconducting gap, the single-particle excitation spectrum contains discrete, spin-degenerate Andreev levels inside the gap. Quasiparticle excitations are induced by a broadband on-chip microwave source and detected by measuring changes in the supercurrent flowing through the atomic contact. Since microwave photons excite quasiparticles in pairs, two types of transitions are observed: Andreev transitions, which consist of putting two quasiparticles in an Andreev level, and transitions to odd states with a single quasiparticle in an Andreev level and the other one in the continuum. In contrast to absorption spectroscopy, supercurrent spectroscopy allows detection of long-lived odd states.

Behaviour of superconductivity energetic characteristics in electron-doped cuprates. A simple model

International Nuclear Information System (INIS)

Kristoffel, N.; Rubin, P.

2008-01-01

A simple model to describe the energetic phase diagram of electron-doped cuprate superconductor is developed. Interband pairing operates between the UHB and the defect states created by doping and supplied by both extincting HB-s. Two defect subbands correspond to the (π,0) and (π/2,π/2) momentum regions. Extended doping quenches the bare normal state gaps (pseudogaps). Maximal transition temperature corresponds to overlapping bands ensemble intersected by the chemical potential. Illustrative results for T c , pseudo- and superconducting gaps are calculated on the whole doping scale. Major characteristic features on the phase diagram are reproduced. Anticipated manifestation of gaps doping dynamics is discussed

The Rashba spin-orbit coupling for superconductivity in oxide interfaces

Energy Technology Data Exchange (ETDEWEB)

Beyl, Stefan; Orth, Peter P.; Schmalian, Joerg [Institut fuer Theorie der Kondensierten Materie, Karlsruher Institut fuer Technologie, Karlsruhe (Germany)

2014-07-01

We investigate the role of the Rashba spin-orbit coupling on the superconducting order parameter and the phase stiffness at the interface of LaAlO{sub 3} and SrTiO{sub 3}. In particular, we analyze the gate controlled crossover between BCS superconductivity and Bose-Einstein condensation of Cooper pairs, amplified by the Rashba coupling and the possibility of a phase fluctuation induced quantum critical point.

Evidence for unconventional d-wave superconducting state in CeCu{sub 2}Si{sub 2}

Energy Technology Data Exchange (ETDEWEB)

Vieyra, Hugo A.; Geibel, Christoph; Steglich, Frank; Oeschler, Niels [Max-Planck-Institute for Chemical Physics of Solids, Dresden 01187 (Germany); Parker, David [US Naval Research Laboratory, Washington, DC 20375 (United States); Jeevan, Hirale S. [I. Physik. Institut, Georg-August-Universitaet Goettingen, Goettingen 37077 (Germany)

2010-07-01

The heavy-fermion CeCu{sub 2}Si{sub 2} represents a prime system to study unconventional superconductivity in the vicinity of a magnetic instability. Within the homogeneity range of pure CeCu{sub 2}Si{sub 2} different ground states can be obtained. S-type crystals exhibit a superconducting transition at T{sub c}=0.6 K, whereas A/S-type show in addition antiferromagnetic order at T{sub N}=0.8 K. In recent years, the synthesis techniques have been optimized in order to obtain large high-quality single crystals with well defined ground state properties. This allows the systematic study of the superconducting order parameter and its variation at the border with magnetic order. In this work, we present angular dependent resistivity measurements on high-quality S- and A/S-type single-crystalline CeCu{sub 2}Si{sub 2} samples. The experimental results for the angular dependence of the upper critical field B{sub c2} as well as theoretical calculations taking into account effects like the strong Pauli paramagnetism, hint towards an unconventional d-wave symmetry of the order parameter in CeCu{sub 2}Si{sub 2}.

Time-resolved X-ray diffraction study on superconducting YBa{sub 2}Cu{sub 3}O{sub 7} epitaxially grown on SrTiO{sub 3}

Energy Technology Data Exchange (ETDEWEB)

Luebcke, A.

2007-07-01

In this PhD thesis time-resolved X-ray diffraction in optical pump - X-ray probe scheme was applied for the first time to a High-Temperature Superconductor in the superconducting state. The aim was to study the possible lattice response to optical Cooper pair breaking. As sample a thin YBa{sub 2}Cu{sub 3}O{sub 7} film with a superconducting transition temperature of T{sub c}=90 K, epitaxially grown on a SrTiO{sub 3} single crystal was used. (orig.)

The evidence of unconventional pairing in heavy fermion superconductors and high-Tc superconductors

International Nuclear Information System (INIS)

Tien, C.; Wur, C.S.; Jiang, I.M.

1989-01-01

Recently there has been a great deal of interest in two classes of superconductors, heavy fermion superconductors and high T c copper oxide superconductors. The behavior and nature of superconductivity in these two classes of materials are very similar. The temperature dependences of spin-lattice relaxation time (T 1 ) and spin-spin relaxation time (T 2 ) of 9 Be in UBe 13 are quite similar to those of 63 Cu and 89 Y in YBa 2 Cu 3 O 7-δ . The Knight shift of UBe 13 is unchanged during the superconducting phase transition. The Knight shift of YBa 2 Cu 3 O 7-δ changes from the value in the normal state K n /K s = 1 at T ≥ T c to K n /K s = 0.5 at T = 6 K. Both do not approach zero as expected in BCS theory. The acoustic attenuation is enhanced just below T c instead of rapid drop near T c for these two superconducting system. Neither the enhancement, the temperature variation, nor any other anomalous behaviors appear to be mirrored in EPR data for heavy Fermion superconductors and high T c superconductors. This strongly suggests that the unconventional pairing mechanism which induces superconductivity in heavy fermion materials might also involve in high T c superconductors

Superconductivity

CERN Document Server

Poole, Charles P; Creswick, Richard J; Prozorov, Ruslan

2014-01-01

Superconductivity, Third Edition is an encyclopedic treatment of all aspects of the subject, from classic materials to fullerenes. Emphasis is on balanced coverage, with a comprehensive reference list and significant graphics from all areas of the published literature. Widely used theoretical approaches are explained in detail. Topics of special interest include high temperature superconductors, spectroscopy, critical states, transport properties, and tunneling. This book covers the whole field of superconductivity from both the theoretical and the experimental point of view. This third edition features extensive revisions throughout, and new chapters on second critical field and iron based superconductors.

The happy marriage between electron-phonon superconductivity and Mott physics in Cs3C60: A first-principle phase diagram

Science.gov (United States)

Capone, Massimo; Nomura, Yusuke; Sakai, Shiro; Giovannetti, Gianluca; Arita, Ryotaro

The phase diagram of doped fullerides like Cs3C60 as a function of the spacing between fullerene molecules is characterized by a first-order transition between a Mott insulator and an s-wave superconductor with a dome-shaped behavior of the critical temperature. By means of an ab-initio modeling of the bandstructure, the electron-phonon interaction and the interaction parameter and a Dynamical Mean-Field Theory solution, we reproduce the phase diagram and demonstrate that phonon superconductivity benefits from strong correlations confirming earlier model predictions. The role of correlations is manifest also in infrared measurements carried out by L. Baldassarre. The superconducting phase shares many similarities with ''exotic'' superconductors with electronic pairing, suggesting that the anomalies in the ''normal'' state, rather than the pairing glue, can be the real common element unifying a wide family of strongly correlated superconductors including cuprates and iron superconductors

The pairing theory - its physical basis and its consequences

International Nuclear Information System (INIS)

Schrieffer, J.R.

1992-01-01

The key developments which set the scene for the microscopic theory of superconductivity are discussed and the physical reasoning which lead to the pairing theory is presented. Consequences of the BCS theory are reviewed. (orig.)

Frontiers in Superconducting Materials

CERN Document Server

Narlikar, Anant V

2005-01-01

Frontiers in Superconducting Materials gives a state-of-the-art report of the most important topics of the current research in superconductive materials and related phenomena. It comprises 30 chapters written by renowned international experts in the field. It is of central interest to researchers and specialists in Physics and Materials Science, both in academic and industrial research, as well as advanced students. It also addresses electronic and electrical engineers. Even non-specialists interested in superconductivity might find some useful answers.

The Kohn-Luttinger mechanism and phase diagram of the superconducting state in the Shubin-Vonsovsky model

Energy Technology Data Exchange (ETDEWEB)

Kagan, M. Yu., E-mail: kagan@kapitza.ras.ru [Russian Academy of Sciences, Kapitza Institute for Physical Problems (Russian Federation); Val' kov, V. V.; Mitskan, V. A.; Korovuskin, M. M. [Russian Academy of Sciences, Kirenskii Physics Institute, Siberian Branch (Russian Federation)

2013-10-15

Using the Shubin-Vonsovsky model in the weak-coupling regime W > U > V (W is the bandwidth, U is the Hubbard onsite repulsion, and V is the Coulomb interaction at neighboring sites) based on the Kohn-Luttinger mechanism, we determined the regions of the existence of the superconducting phases with the d{sub xy}, p, s, and d{sub x{sup 2}-y{sup 2}} symmetry types of the order parameter. It is shown that the effective interaction in the Cooper channel considerably depends not only on single-site but also on intersite Coulomb correlations. This is demonstrated by the example of the qualitative change and complication of the phase diagram of the superconducting state. The superconducting (SC) phase induction mechanism is determined taking into account polarization contributions in the second-order perturbation theory in the Coulomb interaction. The results obtained for the angular dependence of the superconducting gap in different channels are compared with angule-resolved photoemission spectroscopy (ARPES) results. The influence of long-range hops in the phase diagram and critical superconducting transition temperature in different channels is analyzed. The conditions for the appearance of the Kohn-Luttinger superconductivity with the d{sub x{sup 2}-y{sup 2}} symmetry and high critical temperatures T{sub c} {approx} 100 K near the half-filling are determined.

Theory of high temperature superconductivity

International Nuclear Information System (INIS)

Srivastava, C.M.

1989-01-01

This paper develops a semi-empirical electronic band structure for a high T c superconductor like YBa 2 Cu 3 O 6 - δ . The author accounts for the electrical transport properties on the model based on the correlated electron transfer arising from the electron-phonon interaction. The momentum pairing leading to the superconducting phase amongst the mobile charge carriers is shown

Microscopic model of quasiparticle wave packets in superfluids, superconductors, and paired Hall states.

Science.gov (United States)

Parameswaran, S A; Kivelson, S A; Shankar, R; Sondhi, S L; Spivak, B Z

2012-12-07

We study the structure of Bogoliubov quasiparticles, bogolons, the fermionic excitations of paired superfluids that arise from fermion (BCS) pairing, including neutral superfluids, superconductors, and paired quantum Hall states. The naive construction of a stationary quasiparticle in which the deformation of the pair field is neglected leads to a contradiction: it carries a net electrical current even though it does not move. However, treating the pair field self-consistently resolves this problem: in a neutral superfluid, a dipolar current pattern is associated with the quasiparticle for which the total current vanishes. When Maxwell electrodynamics is included, as appropriate to a superconductor, this pattern is confined over a penetration depth. For paired quantum Hall states of composite fermions, the Maxwell term is replaced by a Chern-Simons term, which leads to a dipolar charge distribution and consequently to a dipolar current pattern.

Correlation effects in superconducting quantum dot systems

Science.gov (United States)

Pokorný, Vladislav; Žonda, Martin

2018-05-01

We study the effect of electron correlations on a system consisting of a single-level quantum dot with local Coulomb interaction attached to two superconducting leads. We use the single-impurity Anderson model with BCS superconducting baths to study the interplay between the proximity induced electron pairing and the local Coulomb interaction. We show how to solve the model using the continuous-time hybridization-expansion quantum Monte Carlo method. The results obtained for experimentally relevant parameters are compared with results of self-consistent second order perturbation theory as well as with the numerical renormalization group method.

Properties of a tunnel point contact between aluminum and the superconducting amorphous alloy NiZr2

International Nuclear Information System (INIS)

Gantmakher, V.F.; Golubov, A.A.; Osherov, M.V.

1989-01-01

The I-V characteristics of a tunnel point contact between aluminum and an amorphous ribbon resembling NiZr 2 in composition differ qualitatively from those of ordinary tunnel junctions between a superconductor and a normal metal. It is demonstrated that the observed curve shapes cannot be explained by accounting solely for one-particle tunneling in conditions of the proximity effect. This is followed by a qualitative discussion of a model in which the current rise at the potential eV equal to the gap width Δ (T) is generated by two-particle electron tunneling through the intermediate state which is manifested as a Cooper pair on the Fermi-level in the thin superconducting layer produced in the aluminum by the proximity effect. The current flowing through the contact will cause a breakdown of superconductivity in the vicinity of the contact at voltages exceeding the gap width. Smallness of the junction plays a decisive role both in inducing superconductivity for eV Δ

{sup 119}Sn NMR investigations on superconducting Ca{sub 3}Ir{sub 4}Sn{sub 13}

Energy Technology Data Exchange (ETDEWEB)

Sarkar, Rajib; Brueckner, Felix; Guenther, Marco; Klauss, Hans-Henning [IFP, TU Dresden (Germany); Petrovic, Cedomir; Wang, Kefeng [CMPMS, BNL, Upton, NY (United States); Luetkens, Hubertus; Biswas, Pabitra; Morenzoni, Elvezio; Amato, Alex [PSI, Villigen (Switzerland)

2014-07-01

Ca{sub 3}Ir{sub 4}Sn{sub 13} was found to exhibit superconducting transition with T{sub c} ∼ 7 K. It received considerable attention due to the possible coexistence of superconductivity and ferromagnetic spin fluctuation as well as the three-dimensional charge density wave (CDW) from the superlattice transition. While thermal, transport, and thermodynamic characterization of Ca{sub 3}Ir{sub 4}Sn{sub 13} single crystals suggest that it is a weakly correlated nodeless superconductor, recent μSR investigation reveals that the electron-phonon pairing interaction is in the strong-coupling limit. Here we present {sup 119}Sn NMR investigations on Ca{sub 3}Ir{sub 4}Sn{sub 13} polycrystalline samples and discuss the symmetry of the superconducting order parameter together with the normal state properties. Our preliminary results of spin-lattice relaxation rate (1/T{sub 1}) indicate that this is a BCS superconductor with weak-coupling limit.

Two-dimensional superconducting state of monolayer Pb films grown on GaAs(110) in a strong parallel magnetic field.

Science.gov (United States)

Sekihara, Takayuki; Masutomi, Ryuichi; Okamoto, Tohru

2013-08-02

Two-dimensional (2D) superconductivity was studied by magnetotransport measurements on single-atomic-layer Pb films on a cleaved GaAs(110) surface. The superconducting transition temperature shows only a weak dependence on the parallel magnetic field up to 14T, which is higher than the Pauli paramagnetic limit. Furthermore, the perpendicular-magnetic-field dependence of the sheet resistance is almost independent of the presence of the parallel field component. These results are explained in terms of an inhomogeneous superconducting state predicted for 2D metals with a large Rashba spin splitting.

Steady-state heat transfer in He II through porous superconducting cable insulation

International Nuclear Information System (INIS)

Baudouy, B.J.P.; Juster, F.P.; Meuris, C.; Vieillard, L.

1996-01-01

The LHC program includes the study of thermal behavior of the superconducting cables wound in the dipole magnet cooled by superfluid helium (He II). Insulation of these superconducting cables forms the major thermal shield hindering the He II cooling. This is particularly a problem in magnets which are subjected to thermal loads. To investigate He II heat transfer processes an experimental model has been realized which creates a one-dimensional heat transfer in such media. Insulation is generally realized by wrapping around the superconducting cable a combination of different kind of Kapton reg-sign tapes, fiber-glass impregnated by epoxy resin or Kevlar reg-sign fiber tapes. Steady-state heat transfer in He II through these multi-layer porous slabs has been analyzed. Experimental results for a range of heat flux show the existence of different thermal regimes related to He II. It is shown that the parameters of importance are a global geometrical factor which could be considered as an equivalent open-quotes permeabilityclose quotes related to He II heat transfer, the transfer function f(T) of He II and the thermal conductivity of the slab. The authors present and analyze results for different insulations as a function of the temperature

Entanglement between total intensity and polarization for pairs of coherent states

Science.gov (United States)

Sanchidrián-Vaca, Carlos; Luis, Alfredo

2018-04-01

We examine entanglement between number and polarization, or number and relative phase, in pair coherent states and two-mode squeezed vacuum via linear entropy and covariance criteria. We consider the embedding of the two-mode Hilbert space in a larger space to get a well-defined factorization of the number-phase variables. This can be regarded as a kind of protoentanglement that can be extracted and converted into real particle entanglement via feasible experimental procedures. In particular this reveals interesting entanglement properties of pairs of coherent states.

Superconducting and normal-state properties of the layered boride OsB2

Science.gov (United States)

Singh, Yogesh; Niazi, A.; Vannette, M. D.; Prozorov, R.; Johnston, D. C.

2007-12-01

OsB2 crystallizes in an orthorhombic structure (Pmmn) which contains alternate boron and osmium layers stacked along the c axis. The boron layers consist of puckered hexagons as opposed to the flat graphite-like boron layers in MgB2 . OsB2 is reported to become superconducting below 2.1K . We report results of the dynamic and static magnetic susceptibilities, electrical resistivity, Hall effect, heat capacity, and penetration depth measurements on arc-melted polycrystalline samples of OsB2 to characterize its superconducting and normal-state properties. These measurements confirmed that OsB2 becomes a bulk superconductor below Tc=2.1K . Our results indicate that OsB2 is a moderate-coupling type-II superconductor with an electron-phonon coupling constant λep≈0.4-0.5 , a small Ginzburg-Landau parameter κ˜1-2 , and an upper critical magnetic field Hc2(0.5K)˜420Oe for an unannealed sample and Hc2(1K)˜330Oe for an annealed sample. The temperature dependence of the superfluid density ns(T) for the unannealed sample is consistent with an s -wave superconductor with a slightly enhanced zero temperature gap Δ(0)=1.9kBTc and a zero temperature London penetration depth λ(0)=0.38(2)μm . The ns(T) data for the annealed sample show deviations from the predictions of the single-band s -wave BCS model. The magnetic, transport, and thermal properties in the normal state of isostructural and isoelectronic RuB2 , which is reported to become superconducting below 1.6K , are also reported.

Normal and superconducting metals at microwave frequencies-classic experiments

International Nuclear Information System (INIS)

Dheer, P.N.

1999-01-01

A brief review of experimental and theoretical work on the behaviour of normal and superconducting materials at microwave frequencies before the publication of Bardeen, Cooper and Schrieffer's theory of superconductivity is given. The work discussed is mostly that of Pippard and his coworkers. It is shown that these investigations lead not only to a better understanding of the electrodynamics of normal and superconducting state but also of the nature of the superconducting state itself. (author)

Topological confinement and superconductivity

Energy Technology Data Exchange (ETDEWEB)

Al-hassanieh, Dhaled A [Los Alamos National Laboratory; Batista, Cristian D [Los Alamos National Laboratory

2008-01-01

We derive a Kondo Lattice model with a correlated conduction band from a two-band Hubbard Hamiltonian. This mapping allows us to describe the emergence of a robust pairing mechanism in a model that only contains repulsive interactions. The mechanism is due to topological confinement and results from the interplay between antiferromagnetism and delocalization. By using Density-Matrix-Renormalization-Group (DMRG) we demonstrate that this mechanism leads to dominant superconducting correlations in aID-system.

Relativistic BCS-BEC crossover at finite temperature and its application to color superconductivity

International Nuclear Information System (INIS)

He Lianyi; Zhuang Pengfei

2007-01-01

The nonrelativistic G 0 G formalism of BCS-BEC crossover at finite temperature is extended to relativistic fermion systems. The uncondensed pairs contribute a pseudogap to the fermion excitations. The theory recovers the BCS mean field approximation at zero temperature and the nonrelativistic results in a proper limit. For massive fermions, when the coupling strength increases, there exist two crossovers from the weak coupling BCS superfluid to the nonrelativistic BEC state and then to the relativistic BEC state. For color superconductivity at moderate baryon density, the matter is in the BCS-BEC crossover region, and the behavior of the pseudogap is quite similar to that found in high temperature superconductors

Magnetic oscillations and quasiparticle band structure in the mixed state of type-II superconductors

International Nuclear Information System (INIS)

Norman, M.R.; MacDonald, A.H.; Akera, H.

1995-01-01

We consider magnetic oscillations due to Landau quantization in the mixed state of type-II superconductors. Our work is based on a previously developed formalism which allows the mean-field gap equations of the Abrikosov state to be conveniently solved in a Landau-level representation. We find that the quasiparticle band structure changes qualitatively when the pairing self-energy becomes comparable to the Landau-level separation. For small pairing self-energies, Landau-level mixing due to the superconducting order is weak and magnetic oscillations survive in the superconducting state although they are damped. We find that the width of the quasiparticle Landau levels in this regime varies approximately as Δ 0 n μ -1/4 where Δ 0 is proportional to the magnitude of the order parameter and n μ is the Landau-level index at the Fermi energy. For larger pairing self-energies, the lowest energy quasiparticle bands occur in pairs which are nearly equally spaced from each other and evolve with weakening magnetic field toward the bound states of an isolated vortex core. These bands have a weak magnetic field dependence and magnetic oscillations vanish rapidly in this regime. We discuss recent observations of the de Haas--van Alphen effect in the mixed state of several type-II superconductors in light of our results

Predissociation of high-lying Rydberg states of molecular iodine via ion-pair states

Energy Technology Data Exchange (ETDEWEB)

Bogomolov, Alexandr S. [Institute of Chemical Kinetics and Combustion, Institutskaya Str. 3, Novosibirsk 630090 (Russian Federation); Grüner, Barbara; Mudrich, Marcel [Physikalisches Institut, Universität Freiburg, D-79104 Freiburg (Germany); Kochubei, Sergei A. [Institute of Semiconductor Physics, ac. Lavrent' yev ave., 13, Novosibirsk 630090 (Russian Federation); Baklanov, Alexey V. [Institute of Chemical Kinetics and Combustion, Institutskaya Str. 3, Novosibirsk 630090 (Russian Federation); Novosibirsk State University, Pirogova Str. 2, Novosibirsk 630090 (Russian Federation)

2014-03-28

Velocity map imaging of the photofragments arising from two-photon photoexcitation of molecular iodine in the energy range 73 500–74 500 cm{sup −1} covering the bands of high-lying gerade Rydberg states [{sup 2}Π{sub 1/2}]{sub c}6d;0{sub g}{sup +} and [{sup 2}Π{sub 1/2}]{sub c}6d;2{sub g} has been applied. The ion signal was dominated by the atomic fragment ion I{sup +}. Up to 5 dissociation channels yielding I{sup +} ions with different kinetic energies were observed when the I{sub 2} molecule was excited within discrete peaks of Rydberg states and their satellites in this region. One of these channels gives rise to images of I{sup +} and I{sup −} ions with equal kinetic energy indicating predissociation of I{sub 2} via ion-pair states. The contribution of this channel was up to about 50% of the total I{sup +} signal. The four other channels correspond to predissociation via lower lying Rydberg states giving rise to excited iodine atoms providing I{sup +} ions by subsequent one-photon ionization by the same laser pulse. The ratio of these channels varied from peak to peak in the spectrum but their total ionic signal was always much higher than the signal of (2 + 1) resonance enhanced multi-photon ionization of I{sub 2}, which was previously considered to be the origin of ionic signal in this spectral range. The first-tier E0{sub g}{sup +} and D{sup ′}2{sub g} ion-pair states are concluded to be responsible for predissociation of Rydberg states [{sup 2}Π{sub 1/2}]{sub c}6d;0{sub g}{sup +} and [{sup 2}Π{sub 1/2}]{sub c}6d;2{sub g}, respectively. Further predissociation of these ion-pair states via lower lying Rydberg states gives rise to excited I(5s{sup 2}5p{sup 4}6s{sup 1}) atoms responsible for major part of ion signal. The isotropic angular distribution of the photofragment recoil directions observed for all channels indicates that the studied Rydberg states are long-lived compared with the rotational period of the I{sub 2} molecule.

Competition between superconductivity and magnetism in ferromagnet/superconductor heterostructures

International Nuclear Information System (INIS)

Izyumov, Yurii A; Proshin, Yurii N; Khusainov, Mensur G

2002-01-01

The mutual influence of superconductivity and magnetism in F/S systems, i.e. systems of alternating ferromagnetic (F) and superconducting (S) layers, is comprehensively reviewed. For systems with ferromagnetic metal (FM) layers, a theory of the proximity effect in the dirty limit is constructed based on the Usadel equations. For an FM/S bilayer and an FM/S superlattice, a boundary-value problem involving finite FM/S boundary transparency and the diffusion and wave modes of quasi-particle motion is formulated; and the critical temperature T c is calculated as a function of FM- and S-layer thicknesses. A detailed analysis of a large amount of experimental data amply confirms the proposed theory. It is shown that the superconducting state of an FM/S system is a superposition of two pairing mechanisms, Bardin - Cooper - Schrieffer's in S layers and Larkin - Ovchinnikov - Fulde - Ferrell's in FM ones. The competition between ferromagnetic and antiferromagnetic spontaneous moment orientations in FM layers is explored for the 0- and π-phase superconductivity in FM/S systems. For FI/S structures, where FI is a ferromagnetic insulator, a model for exchange interactions is proposed, which, along with direct exchange inside FI layers, includes indirect Ruderman - Kittel - Kasuya - Yosida exchange between localized spins via S-layer conduction electrons. Within this framework, possible mutual accommodation scenarios for superconducting and magnetic order parameters are found, the corresponding phase diagrams are plotted, and experimental results are explained. The results of the theory of the Josephson effect for S/F/S junctions are presented and the application of the theory of spin-dependent transport to F/S/F junctions is discussed. Application aspects of the subject are examined. (reviews of topical problems)

Nucleation of Quark Matter in Neutron Stars:. Role of Color Superconductivity

Science.gov (United States)

Bombaci, Ignazio; Lugones, Germán; Vidaña, Isaac

2008-02-01

Pure hadronic compact stars ("neutron stars") above a critical mass Mcr are metastable1,2 for the conversion to quark stars (hybrid or strange stars). This conversion process liberates an enormous amount of energy (Econv ~ 1053 ergs), which could power some of the observed gamma ray bursts.1-3 In cold deleptonized hadronic stars, the conversion process is triggered by the quantum nucleation of a quark matter drop in the stellar center. These drops can be made up of normal (i.e. unpaired) quark matter, or color superconducting quark matter, depending on the details of the equation of state of quark and hadronic matter.4 In this talk, we present the results of recent calculations5 of the effects of color superconductivity on the conversion of hadronic stars to quark stars. In particular, we study the dependence of the critical mass Mcr and conversion energy Econv on the quark-quark pairing gap Δ, the bag constant B, and the surface tension σ of the quark-hadron interface.

Pairing phase transition and thermodynamical quantities in 148,149Sm

International Nuclear Information System (INIS)

Razavi, R.; Behkami, A.N.; Dehghani, V.

2014-01-01

The nuclear level densities and entropies in 148,149 Sm have been calculated in the framework of the superconducting theory that includes modified nuclear pairing gap. For modified pairing gap parameter the smooth transition from the BCS to the Fermi type distributions is used. By applying modified pairing gap, the extracted S-shaped heat capacity as a function of nuclear temperature exhibits a physical and smoother behavior instead of the singular behavior predicted by the BCS equations at critical temperature

Novel Approach to Linear Accelerator Superconducting Magnet System

International Nuclear Information System (INIS)

Kashikhin, Vladimir

2011-01-01

Superconducting Linear Accelerators include a superconducting magnet system for particle beam transportation that provides the beam focusing and steering. This system consists of a large number of quadrupole magnets and dipole correctors mounted inside or between cryomodules with SCRF cavities. Each magnet has current leads and powered from its own power supply. The paper proposes a novel approach to magnet powering based on using superconducting persistent current switches. A group of magnets is powered from the same power supply through the common, for the group of cryomodules, electrical bus and pair of current leads. Superconducting switches direct the current to the chosen magnet and close the circuit providing the magnet operation in a persistent current mode. Two persistent current switches were fabricated and tested. In the paper also presented the results of magnetic field simulations, decay time constants analysis, and a way of improving quadrupole magnetic center stability. Such approach substantially reduces the magnet system cost and increases the reliability.

Pairing correction of particle-hole state densities for two kinds of Fermions

International Nuclear Information System (INIS)

Fu, C.Y.

1985-01-01

Pairing corrections in particle-hole (exciton) state-density formulas used in precompound nuclear reaction theories are, strictly speaking, dependent on the nuclear excitation energy U and the exciton number n. A general formula for (U,n)-dependent pairing corrections has been derived in an earlier paper for exciton state-density formulas for one kind of Fermion. In the present paper, a similar derivation is made for two kinds of Fermions. It is shown that the constant-pairing-energy correction used in standard level-density formulas, such as U 0 in Gilbert and Cameron, is a limiting case of the present general (U,n)-dependent results

Effect of Fibonacci modulation on superconductivity

International Nuclear Information System (INIS)

Gupta, Sanjay; Sil, Shreekantha; Bhattacharyya, Bibhas

2006-01-01

We have studied finite-sized single band models with short-range pairing interactions between electrons in the presence of diagonal Fibonacci modulation in one dimension. Two models, namely the attractive Hubbard model and the Penson-Kolb model, have been investigated at half-filling at zero temperature by solving the Bogoliubov-de Gennes equations in real space within a mean-field approximation. The competition between 'disorder' and the pairing interaction leads to a suppression of superconductivity (of usual pairs with zero centre-of-mass momenta) in the strong-coupling limit while an enhancement of the pairing correlation is observed in the weak-coupling regime for both models. However, the dissimilarity of the pairing mechanisms in these two models brings about notable differences in the results. The extent to which the bond-ordered wave and the η-paired (of pairs with centre-of-mass momenta = π) phases of the Penson-Kolb model are affected by the disorder has also been studied in the present calculation. Some finite size effects are also identified

Majorana edge States in atomic wires coupled by pair hopping.

Science.gov (United States)

Kraus, Christina V; Dalmonte, Marcello; Baranov, Mikhail A; Läuchli, Andreas M; Zoller, P

2013-10-25

We present evidence for Majorana edge states in a number conserving theory describing a system of spinless fermions on two wires that are coupled by pair hopping. Our analysis is based on a combination of a qualitative low energy approach and numerical techniques using the density matrix renormalization group. In addition, we discuss an experimental realization of pair-hopping interactions in cold atom gases confined in optical lattices.

Dirac-fermions in graphene d-wave superconducting heterojunction with the spin orbit interaction

Science.gov (United States)

Wang, Juntao; Wang, Andong; Zhang, Rui; Sun, Deng; Yang, Yanling

2017-09-01

In this study, based on the Dirac-Bogoliubov-de Gennes equation, we theoretically investigate the interaction effect between the anisotropic d-wave pairing symmetry and the spin orbit interaction (the Rashba spin orbit interaction (RSOI) and the Dresselhaus spin orbit interaction (DSOI)) in a graphene superconducting heterojunction. We find that the spin orbit interaction (SOI) plays a critical role on the tunneling conductance in the pristine case, but minimally affecting the tunneling conductance in the heavily doped case. As for the zero bias state, in contrast to the keep intact feature in the heavily doped case, it exhibits a distinct dependence on the RSOI and the DSOI in the pristine case. In particular, the damage of the zero bias state with a slight DSOI results in the disappearance of the zero bias conductance peak. Moreover, the tunneling conductances also show a qualitative difference with respect to the RSOI when both the RSOI and the DSOI are finite. These remarkable results suggest that the SOI and the anisotropic superconducting gap can be regarded as a key tool for diagnosing the specular Andreev reflection.

Pairing vibrational and isospin rotational states in a particle number and isospin projected generator coordinate method

International Nuclear Information System (INIS)

Chen, H.T.; Muether, H.; Faessler, A.

1978-01-01

Pairing vibrational and isospin rotational states are described in different approximations based on particle number and isospin projected, proton-proton, neutron-neutron and proton-neutron pairing wave functions and on the generator coordinate method (GCM). The investigations are performed in models for which an exact group theoretical solution exists. It turns out that a particle number and isospin projection is essential to yield a good approximation to the ground state or isospin yrast state energies. For strong pairing correlations (pairing force constant equal to the single-particle level distance) isospin cranking (-ωTsub(x)) yields with particle number projected pairing wave function also good agreement with the exact energies. GCM wave functions generated by particle number and isospin projected BCS functions with different amounts of pairing correlations yield for the lowest T=0 and T=2 states energies which are practically indistinguishable from the exact solutions. But even the second and third lowest energies of charge-symmetric states are still very reliable. Thus it is concluded that also in realistic cases isospin rotational and pairing vibrational states may be described in the framework of the GCM method with isospin and particle number projected generating wave functions. (Auth.)