Conductance spectroscopy of spin-triplet superconductors

We propose a novel experiment to identify the symmetry of superconductivity on the basis of theoretical results for differential conductance of a normal metal connected to a superconductor. The proximity effect from the superconductor modifies the conductance of the remote current depending remarkably on the pairing symmetry: spin singlet or spin triplet. The clear-cut difference in the conductance is explained by symmetry of Cooper pairs in a normal metal with respect to frequency. In the spin-triplet case, the anomalous transport is realized due to an odd-frequency symmetry of Cooper pairs.     

Diamagnetic susceptibility of spin-triplet ferromagnetic superconductors

We calculate the diamagnetic susceptibility in zero external magnetic field above the phase transition from ferromagnetic phase to phase of coexistence of ferromagnetic order and unconventional superconductivity. For this aim we use generalized Ginzburg-Landau free energy of unconventional ferromagnetic superconductor with spin-triplet electron pairing. A possible application of the result to some intermetallic compounds is briefly discussed.     

Spontaneous flux lattices in ferromagnetic spin-triplet superconductors

A theory is developed for the spontaneous vortex lattice that is expected to occur in the ferromagnetic superconductors ZrZn2, UGe2, and URhGe, where the superconductivity is likely of the spin-triplet nature. The long-wavelength fluctuations of this spontaneous flux lattice are predicted to be huge compared to those of a conventional flux lattice, and to be the same as those for spin-singlet ferromagnetic superconductors. It is shown that these fluctuations lead to unambiguous experimental signatures which may provide the easiest way to observe the spontaneous flux lattice.     

低维超导的实验进展

超导自发现以来, 已成为凝聚态物理领域最重要的方向之一. 近年来, 低维材料制备技术的进步使得一维或二维的超导特性实验研究成为可能. 本文在简要介绍超导现象的基础上, 重点回顾了近些年二维超导薄膜和一维超导纳米线的制备和电输运研究, 以及在低维超导体中发现的相移、近邻效应、铁磁超导相互作用和高温超导等新奇的现象, 并对该领域的进一步发展做出了展望.     

Generalization of the Cooper pairing mechanism for spin-triplet in superconductors

A generalization of the Cooper pairing mechanism is proposed which allows for a triplet state of lower energy. This is achieved by incorporating spin into the canonical commutation relations and by modifying the δ potential contact interaction. The gap equation contain as solutions both singlet and triplet states. It is shown that the triplet state is lower in energy than the singlet state which may explain the spin-triplet superconductivity observed.     

Common phase diagram for low-dimensional superconductors

A phenomenological phase diagram which has been derived for high-temperature superconductors from NMR Knight-shift measurements of the pseudogap is compared to the phase diagram that is obtained for organic superconductors and spin-ladder superconductors, both low-dimensional systems. This is contrasted to the phase diagram of some Heavy Fermion superconductors, i.e. superconductors not constrained to a low dimensionality.     

High-resolution photoemission in low-dimensional conductors and superconductors

We review some recent spectroscopic results on low-dimensional systems, including high-T_c superconductors, and charge density wave compounds. We briefly discuss the reasons for the present interest in these materials, and the relevance of band mapping experiments by angle-resolved photoemission spectroscopy. The main focus of the paper is on high-energy resolution, which can be exploited to probe the quasiparticle excitations, and to study the characteristic instabilities of the Fermi surface. Gap spectroscopy in the ordered phases is a prominent part of this research, namely in the cuprates. The normal state properties, however, are often as interesting. This is particularly true in 1D, where the photoemission results indicate strong deviations from a Fermi liquid scenario.     

Critical temperature of low-dimensional high-T c superconductors

We generalized the Coherent Potential Approximation (CPA) method for the calculation of the critical temperature of superconductors of lower dimensionality. Two dimensional (2d) and one-dimensional (1d) models have been considered. The critical temperature is calculated as function of the electronic concentration and of the position of the Fermi level. The critical temperature can be expressed with either of these two parameters. We also calculate the dependence ofT _c on an additional parameter which is the measure of the interplane coupling.     

Parity violation in a single domain of spin-triplet Sr2RuO4 superconductors

We observed an unconventional parity-violating vortex in single domain Sr₂RuO₄ single crystals using a transport measurement. The current–voltage characteristics of submicron Sr₂RuO₄ show that the induced voltage has anomalous components which are even functions of the bias current. The results may suggest that the vortex itself has a helical internal structure characterized by a Hopf invariant (a topological invariant). We also discuss that the hydrodynamics of such a helical vortex causes the parity violation to retain the topological invariant.     

Quantum tunneling of flux lines in superconductors

We discuss the different regimes which can be expected for quantum creep of flux lines by analysing the experimental results obtained for the relaxation rate as T→0 for Y₁Ba₂Cu₃O₇, Bi₂Sr₂Ca₁Cu₂O₈ and (ET)₂Cu(NCN)₂Br single crystals. We address the dimensionality of the pinned flux line, the possible bundle regime at high fields for the organic superconductor and the relative invariability of the quantum rate upon introduction of columnar defects.     

Electronic and electron-phonon phenomena in low-dimensional BEDT-TTF-based organic conductors and superconductors

Two related groups of k-phase ion-radical salts (BEDT-TTF) with different electrical properties, namely, superconductors with different transition temperatures and conductors, which transfer to insulating state with decreasing temperature, have been studied by micro-optic spectroscopy. Polarized reflectance spectra of microcrystals have been measured for the three principal crystallographic directions within the 700–40000 cm⁻¹ region, and the corresponding spectra of the optical functions obtained. The anisotropy of the electronic system in the crystals has been established as two-dimensional. The spectra obtained were quantitatively analyzed, the key parameters of the electronic structure and the vibronic coupling constants determined. It is concluded that the conductors have smaller vibronic coupling constants, more narrow allowed electronic bands, and stronger electron-electron interaction compared to those of the superconductors, and that vibronic coupling is the necessary condition for the onset of superconductivity in the superconductors studied. Fiz. Tverd. Tela (St. Petersburg) 41, 897–899 (May 1999)     

A generalized critical-state model for low-dimensional superconductors with an edge barrier

A generalized model of the critical state in low-dimensional superconductors is proposed, which includes the effects of bulk and edge pinning of the magnetic flux. The conditions are determined for the penetration of Pearl-Abrikosov vortices into the superconductor, and metastable flux structures in these systems are described. A diagram of different vortex states is constructed for a complete cycle of variations of the external magnetic field.     

A novel device layout for tunneling spectroscopy of low-dimensional electron systems

We report tunneling spectroscopic investigations on a barrier separated double-electron-layer system using a novel selective depletion scheme based on self-aligned side gates. We show that the side gate technique is also suitable to impose a tunable lateral confinement on a narrow channel. From the study of tunneling processes on a single-wire device we conclude that the energetic spacing of the one-dimensional subbands can be varied from effectively zero to up to 6 meV. A simple electrostatic model confirms the order of magnitude of the measured one-dimensional level spacing.     

Zero bias tunneling conductance for high field superconductors

The zero bias tunneling conductance is investigated theoretically and shown to be a promising experimental tool for detecting first order phase transitions in high field superconductors. We calculate and evaluate numerically the zero bias tunneling conductance for arbitrary spin orbital mean free path. In doing the calculations we distinguish between thin films in a parallel field and high?-value materials in a perpendicular field. In the former case anomalous properties appear for sufficient long spin orbital mean free path. Finally we determine the minimum spin orbital mean free path which is compatible with a first order phase transition and show that it is much smaller than commonly believed.