What is the highest Tc for phonon-mediated superconductivity?

We suggest that the high-temperature superconductivity can be attributed to the director-roles of the van Hove singularity between an electron-electron interaction and an electron-phonon interaction. The difference between the critical temperature and the pairing temperature is presented, and the Fermi arc, the d-wave symmetry and the poor conductivity, etc., are discussed. In particular, the non-s-wave symmetry is predicted to have the highest T _c for superconductors.      

What is the highest T c for phonon-mediated superconductivity?

We suggest that the high-temperature superconductivity can be attributed to the director-roles of the van Hove singularity between an electron-electron interaction and an electron-phonon interaction. The difference between the critical temperature and the pairing temperature is presented, and the Fermi arc, the d-wave symmetry and the poor conductivity, etc., are discussed. In particular, the non-s-wave symmetry is predicted to have the highest T _c for superconductors.     

Mechanism of superconductivity in metal-oxide compounds

A novel mechanism of superconductivity is suggested in materials with variable valence. The pairing of the conduction electrons is caused by their interaction with the virtual excitations of the dynamical short-range order, excitations describing the state of the ion subsystem, which exhibit a dual valence. An evaluation of the coupling constant of the electrons with these excitations _con and of the transition temperature to the superconducting state T_c showed that _con 100, T_c 100 K.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 33–35, December, 1987.     

Magnetism and superconductivity in intermetallic uranium compounds

Superconductivity, spin-fluctuation effects and magnetic order are reviewed for intermetallic compounds of uranium with d-transition elements. Large values for the effective mass of the pairing electrons are deduced from critical field studies on the superconducting compounds. Magnetism in the 5f-electron compounds is strongly anisotropic even for paramagnetic materials. Magnetic order does not frequently occur in these intermetallics, spin fluctuations are often observed. A unique combination of spin-fluctuation phenomena and superconductivity is met in UPt₃.     

Excitation spectrum and superconductivity in diamond-type compounds

The occurrence of superconductivity in a diamond-type lattice is investigated. For each integer interval of electron concentration n _p , narrow concentration regions are discovered in which the superconducting transition temperature has the highest possible value. Analysis is carried out under the assumption that the energy of strong electron-electron correlations is the largest energy parameter. The results are in qualitative agreement with experimental data.     

CrAs——第一个Cr 基化合物超导体的发现

非常规超导体自发现以来便引起了凝聚态物理学家的广泛兴趣,诸如重费米子超导体,铜基高温超导体和铁基高温超导体等。这类超导体的一个基本特征是,当长程反铁磁序被抑制时会出现超导现象。而要破坏长程磁有序,除了掺杂不同元素以引入电荷载流子或者化学压力外,物理加压也是一种有效的调控手段。文章将介绍第一个Cr基化合物超导体——CrAs单晶通过物理加压首次实现超导电性的过程。当压力为8 kbar 时,超导临界温度出现在2 K,这时发生在常压下265 K的一级反铁磁相变被完全压制。在CrAs体系中,超导与反铁磁序之间的竞争关系说明CrAs是非常规超导体。CrAs的超导也为发现新型超导体打开了一扇大门。     

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.     

Criterion for superconductivity in highT c oxidic compounds

A criterion for superconductivity in highT _c oxidic compounds is proposed based on a short range cooperative electron-phonon coupling and the Darwin interaction for Cooper pairing. It is shown that the small polaron stabilization energy (ε _p) should be less than about 0.04 eV and the system should have a metal ion for which Darwin interaction energy is large.     

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 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 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.     

A new aspect of superconductivity in A-15 compounds

We present a new aspect of superconductivity in A-15 compounds which is able to explain their exceptional role among the highT _c superconductors. The basic idea is that a strong energy dependence of the electronic density of states near the Fermi level may greatly reduce the repulsive part of the frequency dependent electron-phonon interaction. This leads to a large enhancement ofT _c which is a maximum when the Fermi energy is comparable to a typical phonon energy. Our findings are based on numerical solutions of the Eliashberg equations where both the retardation of the electron-phonon coupling and the energy dependence of the electronic density of states have been included. For the electronic density of states we use the models of Labbé und Friedel and of Cohen et al., while the shape of the Eliashberg function ² F() is taken from the tunneling results of Shen. We compare our theory to experimental results for ternary A-15 compounds.     

A phenomenological study of superconductivity in rare earth compounds

The effect of critical magnetic scattering is studied in paramagnetic phase of rare earth compounds showing superconductivity and magnetic order. If the exchange coupling between the conduction electrons and the magnetic ions is small the system becomes superconducting between two transition temperatures. The lower one is slightly higher than the magnetic ordering temperature and this is due to the critical magnetic scattering of the conduction electrons. The influence of the lifetime of one electron on the phase transition is briefly discussed.     

A two-band electron-phonon model for superconductivity in graphite intercalation compounds

A two-band electron-phonon model for superconductivity in graphite intercalation compounds is developed. A new mechanism for the relaxation times due to interband scattering of intraband pairs is proposed for a superconductor with a two-component order parameter. Two distinct relaxation times τ₁ and τ₂ of the order parameters are predicted for C₆K and C₈K.     

The phonon-mediated intervalley electron-electron interaction in silicon

A tight-binding formulation is used to estimate both the electron-phonon matrix element and the intervalley phonon-mediated exchange interaction in bulk silicon. The resulting interactions are about an order of magnitude too small to be able to account, of their own, for the broken symmetry states observed in inversion layers at the Si/SiO₂ interface, where the selection rules are relaxed and other modes are available to enhance the interactions.     

Unconventional high temperature superconductivity in cubic zinc-blende transition metal compounds

We consider possible high temperature superconductivity(high-T_c)in transition metal compounds with a cubic zinc-blende lattice structure.When the electron filling configuration in the d-shell is close to d~7,all three t_(2g) orbitals are near half filling with strong nearest neighbor antiferromagnetic(AFM)superexchange interactions.We argue that upon doping,this electronic environment can be one of"genes"to host unconventional high T_cwith a time reversal symmetry broken d_(2x~2-x~2-y~2)±id_(x~2-y~2)pairing symmetry.With gapless nodal points along the diagonal directions,this state is a direct three-dimensional analogue to the two-dimensional B_(1g)d-wave state in cuprates.We suggest that such a case may be realized in electron doped CoN,such as CoN_(1-x)O_x and(H,Li)_(1-x)CoN.     

Phonon-free mechanism of superconductivity in compounds containing quasi-two-dimensional NiB complexes

A study is made of some characteristics of phonon-free pairing of hybridized p and d electrons in planar NiB complexes in the presence of strong short-range Hubbard repulsion. A generalized Hubbard model is used to calculate the superconductivity phase diagram as a function of the degree of underfilling of the 2p ⁶ and 3d ¹⁰ shells in NiB complexes. The phase region of states having the highest superconducting transition temperatures is established. Fiz. Tverd. Tela (St. Petersburg) 40, 198–201 (February 1998)     

Gossamer superconductivity near antiferromagnetic Mott insulator in layered organic conductors

Layered organic superconductors are on the verge of the Mott insulator. We use the Gutzwiller variational method to study a two-dimensional Hubbard model including a spin exchange coupling term as a minimal model for the compounds. The ground state is found to be a Gossamer superconductor at small on-site Coulomb repulsion U and an antiferromagnetic Mott insulator at large U, separated by a first order phase transition. Our theory is qualitatively consistent with major experiments reported in organic superconductors.