Boundary Effects in Superconductors
TL;DR: In this paper, the properties of layered structures of superconducting (and non-superconducting) materials are studied, with emphasis on the related boundary value problems, and theoretical interpretations are proposed for them.
Abstract: The properties of layered structures of superconducting (and non- superconducting) materials are studied, with emphasis on the related boundary- value problems. The experimental results are summarized, and theoretical interpretations are proposed for them. (T.F.H.)
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TL;DR: In this paper, the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations, are discussed.
Abstract: This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.
20,824 citations
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...The ele troni spe trum of the model an also be al ulated in a magneti eld (de Gennes,1964; Nakao, 1976), and the results are also onsistentwith STM on graphite surfa es (Kobayashi et al., 2005;Li and Andrei, 2007; Matsui et al., 2005; Niimi et al.,2006), epitaxially grown graphene sta ks (Mallet et…...
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TL;DR: In this article, the triplet component of the condensate generated by superconductor-ferromagnet structures is considered, which is odd in frequency and even in momentum, making it insensitive to nonmagnetic impurities.
Abstract: This review considers unusual effects in superconductor-ferromagnet structures, in particular, the triplet component of the condensate generated in those systems. This component is odd in frequency and even in momentum, which makes it insensitive to nonmagnetic impurities. If the exchange field is not homogeneous in the system, the triplet component is not destroyed even by a strong exchange field and can penetrate the ferromagnet over long distances. Some other effects considered here and caused by the proximity effect are enhancement of the Josephson current due to the presence of the ferromagnet, induction of a magnetic moment in superconductors resulting in a screening of the magnetic moment, and formation of periodic magnetic structures due to the influence of the superconductor. Finally, theoretical predictions are compared with existing experiments.
1,144 citations
Cites background from "Boundary Effects in Superconductors..."
...…a superconductor (S) and a normal metal (N) in simpler types of S/N structures (for example, S/N bilayers) has been under study for a long time and the main physics of this so called proximity effect is well described in the review articles by de Gennes (1964) and Deutscher and de Gennes (1969)....
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...Study of the proximity effect goes back to the beginning of the 1960’s and was reviewed in many publications (see, e.g. de Gennes (1964) and Deutscher and de Gennes (1969))....
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TL;DR: In this article, the authors provide a theoretical basis for understanding the current phase relation (CPhiR) for the stationary Josephson effect in various types of superconducting junctions.
Abstract: This review provides a theoretical basis for understanding the current-phase relation (CPhiR) for the stationary (dc) Josephson effect in various types of superconducting junctions The authors summarize recent theoretical developments with an emphasis on the fundamental physical mechanisms of the deviations of the CPhiR from the standard sinusoidal form A new experimental tool for measuring the CPhiR is described and its practical applications are discussed The method allows one to measure the electrical currents in Josephson junctions with a small coupling energy as compared to the thermal energy A number of examples illustrate the importance of the CPhiR measurements for both fundamental physics and applications
1,084 citations
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...There is therefore a crossover from the linear temperature dependence ICRN}Tc2T , predicted by the AslamazovLarkin theory, to the quadratic dependence ICRN}(Tc 2T)2, first suggested by de Gennes (1964)....
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...(74) An exponential decrease in ICRN as d/j* →` is typical for SNS weak links (see de Gennes, 1964; Aslamazov and Larkin, 1969; Clarke, 1969), but the exact values of the prefactor V* and the effective decay length j* in Eq....
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TL;DR: Light is shed on the special role of time reversal symmetry in graphene, and phase coherent electronic transport at the Dirac point is demonstrated, finding that not only the normal state conductance of graphene is finite, but also a finite supercurrent can flow at zero charge density.
Abstract: Graphene--a recently discovered form of graphite only one atomic layer thick--constitutes a new model system in condensed matter physics, because it is the first material in which charge carriers behave as massless chiral relativistic particles. The anomalous quantization of the Hall conductance, which is now understood theoretically, is one of the experimental signatures of the peculiar transport properties of relativistic electrons in graphene. Other unusual phenomena, like the finite conductivity of order 4e(2)/h (where e is the electron charge and h is Planck's constant) at the charge neutrality (or Dirac) point, have come as a surprise and remain to be explained. Here we experimentally study the Josephson effect in mesoscopic junctions consisting of a graphene layer contacted by two closely spaced superconducting electrodes. The charge density in the graphene layer can be controlled by means of a gate electrode. We observe a supercurrent that, depending on the gate voltage, is carried by either electrons in the conduction band or by holes in the valence band. More importantly, we find that not only the normal state conductance of graphene is finite, but also a finite supercurrent can flow at zero charge density. Our observations shed light on the special role of time reversal symmetry in graphene, and demonstrate phase coherent electronic transport at the Dirac point.
1,081 citations
TL;DR: In this paper, it is clarified theoretically that the observed ZBCPs reflect the formation of zero-energy states on the surface due to the?-phase shift of internal phase in the d-wave pairing symmetry.
Abstract: Recent studies on high-Tc superconductors have aroused new interest in tunnelling effects in unconventional superconductors. Unlike in conventional s-wave superconductors, the d-wave pairing state in these materials has an internal phase of the pair potential. The internal phase as a function of the wavevector of the Cooper pairs has a large influence on the electric properties of tunnelling junctions. Important effects of the internal phase on the Josephson current were first predicted theoretically. The idea has been established through several experiments using high-Tc Josephson junctions, which detect ?-phase shift between the a- and b-axis directions and fractional flux quanta. These results give convincing evidence for d-wave symmetry in high-Tc superconductors. In addition, the existence of new interference effects in the quasiparticle states near surfaces and boundaries has been suggested through theoretical predictions. Experimentally, a large number of tunnelling spectroscopy data showed zero-bias conductance peaks (ZBCPs), the origin of which cannot be explained in terms of the classical concept that a tunnelling conductance spectrum is a phase-insensitive probe of the electronic states. It is clarified theoretically that the observed ZBCPs reflect the formation of zero-energy states on the surface due to the ?-phase shift of internal phase in the d-wave pairing symmetry. The formulation developed for tunnelling spectroscopy suggests that tunnelling spectroscopy is essentially phase sensitive. In addition, the formation of the bound states has been shown to have a serious influence on the electrical properties of Josephson junctions. Several anomalous properties including strong enhancement of the Josephson current in the low-temperature region have been predicted theoretically. In this report, recent developments in tunnelling effects on surface bound states in unconventional superconductors are reviewed.
800 citations