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)     

Spin-triplet superconductivity in Sr2RuO4 probed by andreev reflection

The superconducting gap function of Sr2RuO4 was investigated by means of quasiparticle reflection and transmission at the normal conductor-superconductor interface of Sr2RuO4-Pt point contacts. We found two distinctly different types of dV/dI vs V spectra either with a double-minimum structure or with a zero-bias conductance anomaly. Both types of spectra are expected in the limit of high and low transparency, respectively, of the interface barrier between a normal metal and a spin-triplet superconductor. Together with the temperature dependence of the spectra this result strongly supports a spin-triplet superconducting order parameter for Sr2RuO4.     

Triplet superconductivity in Sr2RuO4 in terms of the t-J-I-model

A generalized t-J-I-model is proposed for Sr₂RuO₄ that takes the strong intra-atomic correlations of the d electrons and the features of the electronic structure of Sr₂RuO₄ into account. It is shown that, in the limit of strong correlations, there are no singlet s-type solutions for the superconducting state, but triplet solutions exist because of ferromagnetic spin correlations. For typical values of the model parameters, T _c ∼1 K, consistent with the value of T _c for Sr₂RuO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 1936–1938 (November 1999)     

Spin-triplet superconductivity due to antiferromagnetic spin-fluctuation in Sr2RuO4

A mechanism leading to the spin-triplet superconductivity is proposed based on the antiferromagnetic spin-fluctuation. The effects of anisotropy in spin-fluctuation on the Cooper pairing and on the direction of d vector are examined in the one-band Hubbard model with random-phase approximation. The gap equations for the anisotropic case are derived and applied to Sr2RuO4. It is found that a nesting property of the Fermi surface together with the anisotropy leads to the triplet superconductivity with the d = &zcirc;(sink(x)+/-isink(y)), which is consistent with experiments.     

Thermodynamic properties of the superconductivity in quasi-two-dimensional Dirac electronic systems

The thermodynamic properties of superconducting Dirac electronic systems is analyzed in the vicinity of quantum critical point. The system is characterized by a quantum critical point at zero doping, such that the critical temperature vanishes below some finite value of interaction strength. It is found that the specific heat jump of the system largely deviates from the conventional BCS theory value in the vicinity of quantum critical point. We investigated the region of applicability of the mean-field theory using the Ginzburg-Landau functional.     

Nodeless superconductivity in a quasi-two-dimensional superconductor AuTe_2Se_(4/3)

We performed ultra-low temperature thermal conductivity measurements on the single crystal of a new gold-based quasi-two-dimensional superconductor Au Te_2Se_(4/3), which has a superconducting transition temperature Tc = 2.70 K. A negligible residual linear term κ0/T in zero magnetic field is observed, which suggests fully gapped superconducting state.Furthermore, the field dependence of κ0/T is similar to that of the multi-band s-wave superconductor Ba Fe1.9 Ni0.1 As2 at low field. These results reveal multiple nodeless superconducting gaps in this interesting quasi-two-dimensional superconductor with Berezinsky–Kosterlitz–Thouless topological transition.     

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.     

Theory of the spin-triplet superconductivity in Sr2RuO4 based on perturbation approach to three-band Hubbard model

We investigate a mechanism of the spin-triplet superconductivity in Sr₂RuO₄ by solving Eliashberg equation for three-band Hubbard model, where the effective pairing interaction is expanded perturbatively up to the third order with respect to the Coulomb integrals and the transition temperature is estimated numerically. The most significant momentum dependence of the effective interaction for the spin-triplet p-wave superconductivity does not originate from that of the spin susceptibility, but is brought by the third order vertex corrections. The results show that the p-wave pairing state is stabilized for not so strong inter-orbit couplings.     

Possible superconductivity mechanism in high-temperature superconductors

The formation temperature (T*~ 135 K) is determined in the Shubin-Vonsovski approximation for local electron pairs in the CuO₂ planes of YBa₂Cu₃O₇ crystal. This estimate is used to obtain the Coulomb pseudopotential µ*≈?0.15. In the presence of strong electron-phonon coupling (λ ~0.5) and electron correlation in the electron pairing, the estimate of critical temperature T _c ≈99 K agrees, by the order of magnitude, with its experimental value. The calculated ratio 2Δ/kT _c ≈4.13 confirms the presence of strong electron pairing.     

Small-q phonon-mediated superconductivity in organic kappa-BEDT-TTF compounds

We propose a new picture for superconductivity in kappa-(BEDT-TTF)2X salts arguing that small- q electron-phonon scattering dominates the pairing. We reproduce the distinct X-shaped d-wave gap reported recently by magneto-optic measurements and we argue that the softness of the momentum structure of the gap and the near degeneracy of s- and d-wave gap states may be at the origin of the experimental controversy about the gap symmetry. We show that a magnetic field applied parallel to the planes may induce extended gapless regions on the Fermi surface accounting for the experimental signatures of a Fulde-Ferrel-Larkin-Ovchinikov state and it may induce gap symmetry transitions as well.     

Frozen magnetic states in quasi-two-dimensional Cr1/3−x NixTaS2 compounds with competing exchange interactions

A study of the dynamic and static magnetic susceptibility of quasi-two-dimensional, doubly intercalated compounds Cr_1/3−x Ni_xTaS₂ (0.08⩽x⩽0.23) is reported. It is shown that cooling from paramagnetic phase in this concentration range produces a spin-glass-type frozen magnetic state. In contrast to the classical 3D spin glasses, however, this state sets in within a broad temperature range rather than at a phase transition. A magnetic phase diagram of the studied compounds has been constructed. Fiz. Tverd. Tela (St. Petersburg) 39, 1801–1805 (October 1997)     

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.     

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.     

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.     

Antiferromagnetic exchange mechanism of superconductivity in cuprates

The theory of superconducting pairing due to antiferromagnetic exchange is considered. The strong dependence of the superconducting transition temperature T _c on the lattice constant a observed recently in mercury superconductors is explained within the framework of this theory. Calculations have been performed based on the two-band p-d Hubbard model in the strong correlation limit. The large excitation energy Δ_pd for the antiferromagnetic exchange of two particles from different Hubbard subbands results in the suppression of the retardation effects and in the pairing of all the particles in the conduction subband with the Fermi energy E _F ?Δ_pd:T _c ?E _F exp(?1/λ), where λ∝J. The dependence T _c (a) and the isotope effect are explained by the dependence of the exchange interaction J on a and on zero-point vibrations of oxygen ions.     

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.