On wetting transition in superconductivity

The existence of two solutions for proximity-induced superconductivity described by the Ginzburg-Landau theory is established in the general case. A first-order phase transition can occur between these states, resulting in superconductive wetting.     

Resurgence in quasiclassical scattering

In quasiclassical spectral theory, "resurgence" means that long periodic orbits can be expressed by short ones in such a way that the spectral determinant is real. The question has thus long been posed whether long scattering orbits can be expressed by short orbits in such a way as to make the quasiclassical scattering matrix unitary. We here find a resurgent and manifestly Hermitean expression for Wigner's R matrix, implying a unitary scattering matrix. The result is particularly important if the average resonance width is comparable with the average resonance spacing.     

On the s-type superconductivity in heavy-fermion compounds

The amplitude of scattering of f electrons has been calculated for the periodic Anderson model in the strong-correlation limit (U = ∞) in the Cooper channel. From the condition of the existence of a pole of this amplitude, an equation is derived for determining the critical temperature (T _c) of the transition to the superconducting phase with the s symmetry of the order parameter. The temperature T _c as a function of the electron density and hybridization parameter has been calculated by self-consistently solving the system of equations. The region of the existence of the superconducting phase is found to adjoin the region of the existence of the unsaturated ferromagnetic state and does not overlap it. The results can be used to describe the transition to the superconducting phase with the s symmetry of the order parameter in heavy-fermion skutterudite LaFe₄P₁₂. In this case, the inclusion of the scattering of fermions by spin fluctuations turns out to be substantial enough to obtain T _c values close to the experimental data.     

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.     

On the boson-fermion model of superconductivity

Summary The existence of pair excitations in a Fermi gas interacting via a short-range attractive potential is investigated. Within the ladder approximation to the Bethe-Salpeter equation for the effective two-particle interaction, evidence of pair excitation is found at energies slightly larger than the chemical potential. The link between those excitations and a boson-fermion model of superconductivity (Phys. Lett. A,196 (1995) 359) is discussed. In particular it is shown that the charge carrier density dependence of the pair excitation (boson) energy, assumed phenomenologically in the boson-fermion model, is consistent with the properties of the interacting Fermi gas studied. These results give support to the microscopic origin of the phenomenological boson-fermion model of superconductivity. Within the polaron-bipolaron theory the assumption that bosons and fermions can be described as mobile particles has found support in ref.[6] where the small bipolaron (boson) delocalization is discussed and in ref.[7] where the large polaron delocalization is analysed.     

On the superconductivity of graphite interfaces

We propose an explanation for the appearance of superconductivity at the interfaces of graphite with Bernal stacking order. A network of line defects with flat bands appears at the interfaces between two slightly twisted graphite structures. Due to the flat band the probability to find high temperature superconductivity at these quasi one-dimensional corridors is strongly enhanced. When the network of superconducting lines is dense it becomes effectively two-dimensional. The model provides an explanation for several reports on the observation of superconductivity up to room temperature in different oriented graphite samples, graphite powders as well as graphite-composite samples published in the past.     

On the possibility of superconductivity in electron-hole liquids

It is shown that an electron-hole liquid under suitable conditions can become superconducting. This conclusion is reached by using an effective electron-electron interaction which includes vertex corrections and multiple electron-hole scattering. The mechanism for superconductivity is novel. The intermediate bosons responsible for the superconducting pairing are not acoustic plasmons but correlated pair excitations from the Fermi sea of the holes. Some materials are proposed for an experimental verification of the ideas presented here.     

On the dimensionality of superconductivity in cuprate superconductors

The question of the dimensionality of superconductivity is considered within the framework of a model of superconductivity via asymmetric, delocalized “crystalline” π orbitals (analogous to the corresponding molecular orbitals) extending along chains of covalently bonded copper and oxygen ions. It is shown that superconductivity is preceded by a separation of the bonds in the CuO₂ layer into covalent and ionic bonds with ordering of the covalent bonds into chains. Such an ordering facilitates the formation of a crystalline π orbital lowering the crystal energy by the resonance energy of the π bond and is therefore favored. The superconducting current is created by non-dissipative motion of π-electron pairs along the asymmetric, “crystalline” π orbitals extending along chains of covalently bonded copper and oxygen ions, in the presence of an ionic bond between neighboring chains extending through the easily polarizable O²⁻ ions. This ionic bond correlates the motion of the electron pairs along all the π orbitals and stabilizes the superconducting state. Only in this sense is the apparent “onedimensionality” of superconductivity in cuprate superconductors to be understood. Zh. Tekh. Fiz. 68, 82–84 (November 1998)     

On superconductivity of high-spin transition metal compounds

The possibility of Cooper instability in transition metal compounds is established based of the concept of the strong interaction in the same unit cell. The multicomponent scattering amplitude of excitations is calculated. The superconductivity equations are derived for compounds of 3d transition metals. It is shown that in the pole approximation, the superconductivity equations can be reduced to the multicomponent superconductivity equations with preset BCS constants. A method is developed for calculating one-orbital constants and constants with different orbitals as functions of the total spin. The concentration ranges of superconducting ordering are obtained for one-orbital equations.     

On the nonphonon superconductivity of silver-oxygen chains

It is established on the basis of a concept of strong coupling in a single unit cell that the Cooper instability can occur in a system with hopping between the cations and anions of transition and main-group elements. A phase diagram is constructed for the ferromagnetic ordering in the coordinates (h _d , h _p ) of the degrees of underfilling of, respectively, the 4d ¹⁰ and 2p ⁶ shells of transition and main-group elements.     

On the superconductivity of the tin-hydrogen system

Conductivity and superconductivity studies of amorphous [Sn_1–y Cu _y–]_1–x H _x samples in connection with¹¹⁹Sn Mössbauer effect experiments on¹¹⁹Sn_1–x H _x give strong evidence that the observed increase of the superconducting transition temperatureT _c in the Sn–H-system is caused by the stabilization of an amorphous structure. Thus the Sn–H-system is very similar to the Sn–Cu-system and no H-specific effect is needed to explain the increase ofT _c.     

On the superconductivity of planar carbon compounds

The possibility of the existence of Cooper instability in a system with hops between carbon anions that form a honeycomb lattice has been established based on an idea about the strong interaction in one atom of the unit cell. The phase diagram of the existence of the superconducting ordering and the BCS constant has been calculated for various filling factors of the π or σ shell.     

On the problem of high temperature superconductivity

This article is a brief review of the present situation in theoretical investigations of high temperature superconductivity. The main subject is a discussion of the formula for a critical temperature T_c for a homogeneous and isotropic substance with arbitrary permeability ?(q, ω). The problem of the maximum possible value of T_c is discussed. It is concluded that at present there are no grounds to consider that values of T_c ? 100°K to be impossible. The paper ends with some remarks on the exciton mechanism of superconductivity for the case of superconductors with a plane geometry.     

On the superconductivity of a Wigner liquid

The disorder effect on the interaction of quasiparticles between each other is discussed. The occurrence of a soft mode is taken as the basic assumption. The interaction through the soft mode results in attraction between Fermi quasiparticles (this is apart from the repulsion that has remained from the initial Coulomb interaction between particles). This attraction (in the vicinity of the Fermi surface) is strengthened with increasing concentration of the scattering centers. Therefore, even if the pure system exhibits no superconductivity, superconductivity could appear in the impurity system.     

On the theory of superconductivity in Kondo lattice systems

An attempt is made to explain the occurrence of superconductivity in Kondo lattice systems with special reference to CeCu₂Si₂. Starting point is the Fermi liquid approach. It is generalized from a Kondo impurity to the Kondo lattice by means of the Korringa-Kohn-Rostocker method. From it a hybridization model is derived and discussed in detail. Two electron-phonon mechanisms are investigated which appear in Kondo lattices. One results from the additional phase shifts caused by the Kondo ions while the other is responsible for the so-called Kondo volume collapse. It is shown that the latter is sufficiently strong in order to explain why CeCu₂Si₂ is a superconductor while LaCu₂Si₂ is not. An estimate for the superconducting transition temperatureT _c produces the right order of magnitude.     

On the superconductivity in β-beryllium and related phases

Two high temperature beryllium allotropes have been stabilized at room temperature through the addition of transition elements. Superconductivity has been observed in almost all of them. Beryllium films reported superconducting in the past appear in all probability to require the presence of transition element impurities, in order to show the phenomenon.     

On the superconductivity of high-spin transition-metal compounds

The possibility of the existence of Cooper instability in transition-metal compounds has been established on the basis of an idea about the strong interaction in a single unit cell. The phase diagram of the existence of the superconducting ordering depending on the degree of underfilling (h _d ) of the 3d shells has been obtained. The nonphonon BCS constant has been calculated.     

On the coexistence of superconductivity and ferromagnetism

The magnetic scattering of electrons is studied, paying special attention to the problem of coexistence of superconductivity and ferromagnetism. The utilized model consists of two electronic bands, one of which can become superconducting, and a system of localized spins. The calculated transition temperatures of the superconduction and the magnetic subsystems show regions of coexistence for not too large exchange interactions. Generally speaking, coexistence is favoured due to spin-orbit scattering as well as due to the interaction of the superconducting band with the normal band in certain cases.     

Surface superconductivity and twinning-plane superconductivity in aluminum

The critical supercooling field H _sc is measured in aluminum single crystals and twinned bicrystals in a temperature range slightly below T _c0 (T _c0 ? 0.055 K < T < T _c0), where T _c0 is the critical superconducting transition temperature. It is found that, even in this small temperature range, the H _sc(H _c) dependence, which is considered to be identical to the H _c3(H _c) dependence for single crystals, is substantially nonlinear. The H _sc(H _c) dependences of the twinned bicrystals and single crystals are shown to be significantly different. The qualitative features of the phase diagram of the twinned aluminum bicrystals coincide with those of the phase diagram of twinning-plane superconductivity obtained earlier for tin in [1]. These findings allow the conclusion that the phenomenon of twinning-plane superconductivity also exists in face-centered cubic crystal lattices.