Superconductivity pairing induced by two level systems in amorphous metals

From the standard interaction between electrons and two level systems a superconductive pairing is envisaged. The solutions of the Eliashberg equations for the critical temperature as well as the zero temperature gap lead to expressions as exp (?1/√λ₀) instead of exp (?1/λ₀) in the BCS case, which enhances considerably the superconducting properties in the weak coupling case.     

Superconductivity in strongly repulsive fermions: the role of kinetic-energy frustration

We discuss a physical mechanism of a non-BCS nature which can stabilize a superconducting state in a strongly repulsive electronic system. By considering the two-dimensional Hubbard model with spatially modulated electron hoppings, we demonstrate how kinetic-energy frustration can lead to robust d-wave superconductivity at arbitrarily large on-site repulsion. This phenomenon should be observable in experiments using fermionic atoms, e.g. 40K, in specially prepared optical lattices.     

Breached pairing superfluidity: possible realization in QCD

We propose a wide universality class of gapless superfluids, and analyze a limit that might be realized in quark matter at intermediate densities. In the breached pairing color superconducting phase heavy s quarks, with a small Fermi surface, pair with light u or d quarks. The ground state has a superfluid and a normal Fermi component simultaneously. We expect a second-order phase transition, as a function of increasing density, from the breached pairing phase to the conventional color-flavor locked phase.     

Generation of adjustable pure spin currents in negative-U systems

Single-particle sequential tunneling is studied through a negative-U center hybridized with a superconducting, a ferromagnetic, and a normal metal electrodes. In stark contrast to the case of positive U, the single-particle tunneling in attractive charging energy is usually prohibited by ground states with electrons in pairs. We find a microscopic mechanism to induce single-particle sates from pair states. As a consequence, in the nonpolarized metal terminal a remarkable pure spin current with no charge currents survives over a wide range of gate- and bias- voltages, which is rather crucial for experimental observation and design of spintronic devices. In addition, a significant spin-filter effect is presented in certain bias regime.     

Superconductivity in homologous series of cuprates: Deep oscillations of the pairing coulomb potential

Superconducting pairing with a large momentum is characterized by a kinematic constraint, which effectively cuts off the screened Coulomb potential in the momentum transfer at scattering and leads to pairing potential oscillations in real space. Coulomb pairing in cuprates corresponds to a strong interaction between the paired particles lying both in the same cuprate plane and in two neighboring planes. This explains the experimental dependence of the transition temperature on the number of cuprate planes in a unit cell.     

Intralayer and Interlayer Couplings in the Soliton pairing Model of Superconductivity

Explicit expressions and results for T_c(n) of the layer compounds of Bi and T1 are obtained within the generalized soliton pairing framework, in which T_c, is raised by the cooperation l i of the two couplings, intralayer and interlayer. The calculated values are in good agreement I with the observed T_c's. The maximum possible T_c(∞) is predicted at≈ 200 K.     

Superconductivity in the cuprates: Deduction of mechanism for d-wave pairing through analysis of ARPES

In the Eliashberg integral equations for d-wave superconductivity, two different functions (α²F)_n(ω, θ) and (α²F)_p,d(ω) determine, respectively, the “normal” self-energy and the “pairing” self-energy. ω is the frequency of fluctuations scattering the fermions whose momentum is near the Fermi-surface and makes an angle θ to a chosen axis. We present a quantitative analysis of the high-resolution laser based Angle Resolved Photoemission Spectroscopy (ARPES) data on a slightly under doped cuprate compound Bi2212 and use the Eliashberg equations to deduce the ω and θ dependence of (α²F)_n(ω, θ) for T just above T_c and below T_c. Besides its detailed ω dependence, we find the remarkable result that this function is nearly independent of θ between the (π; π)-direction and 25 degrees from it, except for the dependence of the cut-off energy on θ. Assuming that the same fluctuations determine both the normal and the pairing self-energy, we ask what theories give the function (α²F)_p,d(ω) required for the d-wave pairing instability at high temperatures as well as the deduced (α²F)_n(θ, ω). We show that the deduced (α²F)_n(θ, ω) can only be obtained from antiferromagnetic (AFM) fluctuations if their correlation length is smaller than a lattice constant. Using (α²F)_p,d(ω) consistent with such a correlation length and the symmetry of matrix-elements scattering fermions by AFM fluctuations, we calculate T_c and show that AFM fluctuations are excluded as the pairing mechanism for d-wave superconductivity in cuprates. We also consider the quantumcritical fluctuations derived microscopically as the fluctuations of the observed loop–current order discovered in the under-doped cuprates, and which lead to the marginal Fermi–liquid properties in the normal state. We show that their frequency dependence and the momentum dependence of their matrix-elements to scatter fermions are consistent with the θ and ω dependence of the deduced (α²F)_n(ω, θ). The pairing kernel (α²F)_p,d(ω) calculated using the experimental values in the Eliashberg equation gives d-wave instability at T_c comparable to the experiments.     

Dielectric theory of metal-insulator transitions in doped systems

Model dielectric functions for doped systems show how the dopant excitations change from single-particle-like to collective in character as the concentration is increased from a dilute insulating limit to a more concentrated metallic regime.     

Superconductivity caused by the pairing of plutonium 5f Eelectrons in PuCoGa5

On the basis of electronic structure calculations we identify the superconductivity in the novel, high-temperature superconductor PuCoGa5 to be caused by the pairing of Pu 5f electrons. Assuming delocalized Pu 5f states, we compute theoretical crystallographic constants very near to the experimental ones, and the calculated specific heat coefficient compares reasonably to the measured coefficient. The theoretical Fermi surface is quasi-two-dimensional and the material appears to be close to a magnetic phase instability.     

Critical properties of the metal-insulator transition in anisotropic systems

We study the three-dimensional Anderson model of localization with anisotropic hopping, i.e., weakly coupled chains and weakly coupled planes. In our extensive numerical study we identify and characterize the metal-insulator transition by means of the transfer-matrix method. The values of the critical disorder obtained are consistent with results of previous studies, including multifractal analysis of the wave functions and energy-level statistics. decreases from its isotropic value with a power law as a function of anisotropy. Using high accuracy data for large system sizes we estimate the critical exponent as . This is in agreement with its value in the isotropic case and in other models of the orthogonal universality class. Received 25 October 1999     

Theory of the metal-insulator transition in two-dimensional electron systems

The metal-insulator transition in a two-dimensional disordered electron system (as the carrier concentration decreases) is considered in terms of a percolation theory. The fact that this is a strong-coupling system is substantially taken into account. In our model, the initial structure is taken to be the skeleton of an infinite cluster. Percolation paths are assumed to have regions where two phases having similar energies, namely, liquid (conducting) and solid (nonconducting) phases, can compete with each other. The ratio of these phases changes as a function of the system parameters and temperature. This behavior generates a change in the infinite cluster and results in the conductor-insulator transition. The obtained temperature dependences of resistivity agree qualitatively with experiment. A quantitative comparison of the calculated results with experimental data allows the system parameters to be estimated in each specific case. The temperature dependences of resistivity are mainly determined by the sign of the difference (and also the scatter of) in the initial energies of the phases, and they have a metallic, dielectric, or intermediate (nonmonotonic temperature dependence with a maximum) character. A separatrix can occur only in the case of a sufficiently small scatter of the phase energies.     

Manifestation of the superconducting pairing of repulsive particles with a large total momentum in Andreev reflection

Features of Andreev reflection at the normal metal-superconductor interface in the presence of repulsive-interaction-induced pairing with a large total momentum K have been analyzed. When the direction of the motion of a hole arising upon the formation of a pair with K ≠ 0 by an incident electron corresponds to transmission, the intensity of Andreev reflection decreases compared to the case K = 0. Another cause of the decrease in the intensity is that, owing to the repulsive interaction, the superconducting order parameter has a zero line, and the quasiparticle energy minimum, which determines the turning point, does not coincide with the Fermi contour on which the quasiparticle charge changes its sign (charge asymmetry).     

The moment problem in statistical mechanics: Correlation functions of purely dissipative systems

The question of convergence of the Mori formalism is investigated in the frame of the classical theory of moments. The autocorrelation functions (acfs) of purely dissipative systems admit of both moment and continued fraction expansions (like that obtained by Mori). New physical conditions for the convergence of the latter ones are related to the analytical behaviour of the corresponding spectral moment sequences. From discussion of the analytical properties of such continued fractions it is also proved that the knowledge of the spectral moments does not suffices for determining infinite acf eigenvalue spectra. Mori's continued fraction always converges at zero frequency, thus reproducing the value of the acf relaxation time correctly. Finally, our predictions are verified by comparison with the analytical results for the Verhulst model published recently by Jung and Risken.