High temperature superconductivity; The spin polaron theory

An outline is given of one of the many models which have been advanced to explain high temperature superconductivity in the quasi-two-dimensional oxides. In these the compensation ensures that there are free positive holes, formed in the oxygen 2p states, which form ‘spin polarons’ by orienting neighbouring spins in Cu 3d⁹ ions. Two spin polarons then combine to form a spinless pair, which obeys Einstein–Bose statistics. It is shown that a degenerate gas of such entities is a superconductor. If this is a correct model, such pairs (bosons) must exist above the critical temperature (T _c). Particular attention is given to the properties of these materials at temperatures above T _c. An unresolved problem is whether all, or only some, of the carriers form bosons.     

New theory of superconductivity

Based on three earlier papers which treat electromagnetic, elastogravitational, and radiant-nonradiant thermal phenomena in terms of six types of electric or nonelectric charges, the authors classify states of matter as hyperefficient, efficient, semiefficient, and hypoefficient in transmitting a particular type of charge, by means of a generalization of Ohm's law to two or three dimensions. Conventional states of matter (solid, liquid, gas, vacuum) are associated with torsional (gravitational) charges. Applications are made to electric superconductivity of crystals at elevated temperatures, and to frequency shift (gravitational red shift, Lamb shift, and Zeeman and Stark effects).     

BCS theory of driven superconductivity

We study the impact of a time-dependent external driving of the lattice phonons in a minimal model of a BCS superconductor. Upon evaluating the driving-induced vertex corrections of the phonon-mediated electron-electron interaction, we show that parametric phonon driving can be used to elevate the critical temperature T_c, while a dipolar phonon drive has no effect. We provide simple analytic expressions for the enhancement factor of T_c. Furthermore, a mean-field analysis of a nonlinear phonon-phonon interaction also shows that phonon anharmonicities further amplify T_c. Our results hold universally for the large class of normal BCS superconductors.     

Phenomenological theory of ferromagnetic superconductivity

It is argued that the pairing symmetry realized in a ferromagnetic superconductor UGe(2) must be a nonunitary triplet pairing. This particular state is free from the Pauli limitation and can survive under a huge internal molecular field. To check our identification we examine its basic properties and several experiments are proposed. In particular, the external field is used to raise T(c) by controlling the internal spontaneous dipole field.     

Functional averages in the theory of superconductivity

The grand canonical partition function of a superconductor described byGorkov's model Hamiltonian is represented as a functional integral with Gaussian measure. The integrand can be regarded as the partition function of a free Fermi system which interacts with a fluctuating external source potential. Perturbation-theoretic techniques are applied to the latter partition function. TheGibbs' potential proves to be stationary with respect to the energy gap parameterΔ. From the stationarity condition an equation forΔ is obtained which is a generalization of the usual Bardeen-Cooper-Schrieffer (BCS) equation. For the evaluation of the functional integral a variational procedure is employed. It leads to an expression for theGibbs' potential which shows a further remarkable stationarity property. As its simplest approximation this expression contains a result that was firstly derived by Thouless in the ladder graph approximation.     

High field superconductivity in thin films

Thin films provide an ideal means for studying the role of spin paramagnetism in the theory of superconductivity. A review is given of the theoretical and experimental work available until now with respect to this problem. It includes a study of the excitation spectrum of thin films in a parallel magnetic field and the experimental evidence of Zeeman splitting of superconducting quasiparticles. The role of spin-orbit interaction is discussed in detail. The application of spin selective tunnelling is shown. Furthermore, it includes a study of the order of phase transition between the normal state and the paramagnetically limited superconducting state. A detailed discussion is given of the generalized Ginzburg-Landau equation including numerical evaluations. The applicability of Tinkham's formula is discussed, which is relevant for the determination of critical fields of arbitrary orientations to the film. Finally, account is given of the work which deals with the influence of spin paramagnetism on superconducting fluctuations. Recent experimental and theoretical work is described and discussed.     

High frequency local modes,superconductivity and anomalous isotope effect in PdH(D) systems

The possible mechanisms of enhanced superconductivity in PdH(D) systems are discussed. It is concluded that quenching of paramagnons due to reduced coulomb pseudopotentialalone cannot account for the highT _c observed in these systems. Possible participation of the high frequency optic phonon band in BCS-type interaction could lead to a highT _c and an opposite isotope effect in these systems. The possibility of the occurrence of some high-q soft modes due to large lattice expansion in these systems to yieldT _c ～9 K requires about 2-fold softing of the effective Pd-Pd force constant in PdH compared to that in metallic Pd.     

Phenomenological theory of superfluidity and superconductivity

Quantum condensation is used here as the basis for a phenomenological theory of superfluidity and superconductivity. It leads to remarkably good calculations of the transition temperaturesT _c of superfluid³He and⁴He, as well as a large number of cuprate, heavy fermion, organic, dichalcogenide, and bismuth oxide superconductors. Although this approach may apply least to the long-coherence-length metallics, reasonably good estimates are made for them and chevral superconductors.T _c for atomic H is estimated.T _c can be calculated as a function of number density or density of states and effective mass of normal carriers; or alternatively with the Fermi energy as the only input parameter. Predictions are made for a total of 26 superconductors and four superfluids. An estimate is also made for coherence lengths.     

On the two-parametric theory of superconductivity

The concept of spin fluctuations in the Hubbard model is used for deriving a closed system of equations for the superconducting order parameter Δ and spin susceptibility. The limiting cases of low temperatures and temperatures close to the superconducting transition temperature are considered. The temperature dependences of the parameter Δ and the Knight shift are obtained. The conditions under which the second-order phase transition turns to the first-order phase transition are established.     

High temperature superconductivity under pressure

Abstract

This review is a summary of the work concerned with the pressure studies of high temperature superconductors (HTSC) held in the LOW Temperature Department of the IHPP since the last Conference in Paderborn (July, 1989).     

High temperature superconductivity in Bi-Sr-Ca-Cu-O oxide system

Stable, reproducible high temperature superconductivity with onset temperatures of 90 K and above is confirmed in the oxide system with nominal metal ion composition BiSrCaCu₂O _x . The onset is nearly the same, but zero-resistance temperatures depend on the heat treatment. Sample quenched from 850 C in air (close to melting) to liquid nitrogen temperature is non-superconducting. BiSr_0.5Ca_1.5Cu₂O _x is also found to be superconducting in which a two-step drop in resistance is observed, one starting around 118 K and the other around 80 K.     

High temperature superconductivity in bismuth-alkaline earth-copper-oxygen system

Electrical resistivity measurements of a series of compounds in the bismuth-alkaline earth-copper-oxygen system have been carried out. Several compositions (BiSrCaCu₂O _x , BiSrCaCu₃O _x , Bi₂Sr₂CaCu₄O _x and Bi₄Sr₃Ca₃Cu₈O _x ) showed superconducting onset temperatures in the range 90–130 K, though none of them showedT _cf(R=0) above 77 K. ExtrapolatedT _cf of three compounds are in the range 72–75 K. All compounds are multiphasic.