Coulomb repulsion-induced hyperbolic pairing as a mechanism of high-T c superconductivity

The hyperbolic metric of the dispersion law (the effective mass tensor components of carriers are opposite in sign) in the vicinity of the Fermi contour in high-T _c superconducting cuprates in the case of repulsive interaction gives rise to a superconducting state characterized by the condensate of pairs with a large total momentum (hyperbolic pairing). The gain in the energy of the superconducting state over the normal state is due to the fact that a change in the kinetic energy of pairs (because of the negative light component of the effective mass) dominates over the change in the potential energy (corresponding to energy loss). The shift of the chemical potential upon the transition to the superconducting phase is substantial in this case. With increasing repulsive interaction, the superconducting gap δ_K increases and the resulting gain in energy changes to an energy loss at a certain critical value of the repulsive potential. The low temperature T _c of the superconducting transition and the large value of δ _K in this region of potential values are the reasons for the high value of the 2δ_K/T _c ratio and for the developed quantum fluctuations that are observed in underdoped cuprate superconductors.     

On the pairing interaction in highT c superconductors

It is argued, from the dependence of the critical current density and ofT _c on applied magnetic field, that the pairing interaction in highT _c superconductors must be strongly field-dependent.     

Generalization of the Cooper pairing mechanism for spin-triplet in superconductors

A generalization of the Cooper pairing mechanism is proposed which allows for a triplet state of lower energy. This is achieved by incorporating spin into the canonical commutation relations and by modifying the δ potential contact interaction. The gap equation contain as solutions both singlet and triplet states. It is shown that the triplet state is lower in energy than the singlet state which may explain the spin-triplet superconductivity observed.     

Electronic inhomogeneities,electron-lattice and pairing interactions in high-T c superconductors

The presence of electronic inhomogeneities strongly reduces the screening of the electron-ion interaction in high-temperature superconductors. This implies the existence of an non-totally screened long-range contribution to the electron-lattice coupling and opens an additional channel for the formation of copper pairs. We calculate the superconducting order parameter taking into account a) the longrange and the short-range parts of the electron-lattice interaction and b) the Coulomb repulsion between charge-carriers. We show that whereas the long-range electron-lattice coupling determines the anisotropy of the order parameter, the Coulomb repulsion and the short-range interactions determine its symmetry. Thus, different high-T _c superconductors may have s- or d-wave symmetry, depending on the relative strength of the interactions involved in the pairing.     

Incipient nodal pairing in planar d-wave superconductors

The possibility of a second pairing transition d --> d + is ( d + id') in planar d-wave superconductors which occurs in the absence of external magnetic field, magnetic impurities, or boundaries is established in the framework of the nonperturbative phenomenon of dynamical chiral symmetry breaking in the system of (2+1)-dimensional Dirac-like nodal quasiparticles. We determine the critical exponents and quasiparticle spectral functions that characterize the corresponding quantum-critical behavior and discuss some of its potentially observable spectral and transport features.     

On the optical conductivity of high-T c superconductors

We have solved the self-consistent equation for self-energy of a hole in a quantum antiferromagnet. The optical conductivity is estimated. The results are in good agreement with experiments and numerical simulations.     

Magnetization of high-T c superconductors

We have tried to understand the field dependence of magnetization of high temperature superconductors in the light of phenomenological theory. Especially, the field dependence of dM/d lnB of polycrystalline Bi(2212) is understood by incorporating the overlap of vortices in the London theory.     

Flux diffusion in high-T c superconductors

In type-II superconductors in the flux flow (J J _c ), flux creep (J _c J _c ), and thermally activated flux flow (TAFF) (J J _c ) regimes the inductionB(r,t), averaged over several penetration depths , in general follows from a nonlinear equation of motion into which enter the nonlinear resistivities (B, J ,T) caused by flux motion and (B, J ,T) caused by other dissipative processes.J andJ are the current densities perpendicular and parallel toB,B=|B|, andT is the temperature. For flux flow and TAFF in isotropic superconductors with weak relative spatial variation ofB, this equation reduces to the diffusion equation plus a correction term which vanishes whenJ =0 (this means B××B=0) or when – = 0 (isotropic normal conductor). When this diffusion equation holds the material anisotropy may be accounted for by a tensorial . The response of a superconductor to an applied current or to a change of the applied magnetic field is considered for various geometries. Such perturbations affect only a surface layer of thickness where a shielding current flows which pulls at the flux lines; the resulting deformation of the vortex lattice diffuses into the interior until a new equilibrium or a new stationary state is reached. The a.c. response, in particular the frequency with maximum damping, depends thus on the geometry and size of the superconductor.     

Strong-coupling effects in high-T c superconductors

It is conceivable that the high-T _c superconducting perovskites are conventional electronphonon superconductors. In this case one expects significant strong-coupling effects because of the unusually high ratiok _B T _c / of the order 0.1 and greater. We use a set of reasonable models for the Eliashberg function ² F() (which takes into account available information on the phonon spectra and which fit the measuredT _c 's) and calculate strong-coupling effects in the specific heatc _s (T)/T _c , the ratio ₀/k _B T _c , the critical fieldsH _c (T) andH _c2 (T) including Pauli limiting, and other measurable quantities. Strongcoupling corrections turn out to be in the range of 0 to about 100%, depending on the quantity of interest. We discuss the perspectives of using strong-coupling effects as indicators for conventional electron-phonon superconductivity in the new materials.     

Magnetostriction of several high-T c superconductors

We report on magnetostriction measurements of superconducting and nonsuperconducting YBa₂Cu₃O_7– and of two Bi samples with the nominal compositions Bi₂CaSr₂Cu₂O _x and Bi_0.8Pb_0.2Sr_0.8CaCu_1.5O _x . Both types of high temperature superconductors show a nearly identical strongly hysteretic field dependence of the magnetostriction (MS) (l/l+(2–6)*10^–6 at 5 T and 1.5 K). In both cases the MS is a linear function of field in the region 1–5 T, which we explain by striction due to surface currents. Between 1.5 and 35 K the hysteresis of the MS decreases strongly with increasing temperature, which implies a decrease of the pinning force. The MS of quenched nonsuperconducting YBa₂Cu₃O_7– is at least two orders of magnitude smaller than that of the superconductor.     

Cooper pairs with broken parity and spin-rotational symmetries in d-wave superconductors

Paramagnetic effects are shown to result in the appearance of a triplet component of order parameter in a vortex phase of a -wave superconductor in the absence of impurities. This component, which breaks parity and spin-rotational symmetries of Cooper pairs, is expected to be of the order of unity in a number of modern superconductors such as organic, high Tc, and some others. A generic phase diagram of such type-IV superconductors, which are singlet ones at H=0 and in the Meissner phase, and characterized by singlet-triplet mixed Copper pairs Deltas+iDeltat with broken symmetries in a vortex phase, is discussed.     

Phase-sensitive evidence for d-wave pairing symmetry in electron-doped cuprate superconductors

We present phase-sensitive evidence that the electron-doped cuprates Nd(1.85)Ce(0.15)CuO(4-y) (NCCO) and Pr(1.85)Ce(0.15)CuO(4-y) (PCCO) have d-wave pairing symmetry. This evidence was obtained by observing the half-flux quantum effect, using a scanning SQUID microscope, in c-axis-oriented films of NCCO or PCCO epitaxially grown on tricrystal [100] SrTiO3 substrates designed to be frustrated for a d(x(2)-y(2)) order parameter. Samples with two other configurations, designed to be unfrustrated for a d-wave superconductor, do not show the half-flux quantum effect.     

Possible double-pairing effect in high-T c superconductors

Based on a reexamination of experimental facts a simple model called Double-Pairing Superconductivity is set up to describe the high temperature layered superconductors (HTLS). A mean-field scenario is carried out to investigate the model Hamiltonian. Within our model some of the exotic properties of HTLS may be understood, at least qualitatively.     

Thermoelectric effect in high-T c superconductors: The role of density-of-states fluctuations

We study the effect of the density-of-states (DOS) fluctuations on the thermoelectric coefficient of a highly anisotropic superconductor above the critical temperature. It is shown that it is the DOS contribution which gives rise to the leading correction to the thermoelectric coefficient, in spite of previous results where only the Aslamazov-Larkin term was taken into account. This conclusion is valid for an arbitrary impurity concentration. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 182–187 (25 January 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Spin and charge order around vortices and impurities in high-T(c) superconductors

A comparative study is made for the spin and charge structure around superconducting vortices and unitary impurities, by solving self-consistently an effective Hamiltonian including interactions for both antiferromagnetic spin-density wave (SDW) and d-wave superconducting orderings. Around vortices, we show the induction of an SDW two-dimensionally modulated with a period of eight lattice constants (8a(0)) and an associated charge-density wave (CDW) with a period of 4a(0), which explains very well recent experimental observations. In the case of unitary impurities, an SDW modulation with identical periodicity, but without an associated CDW, is also predicted.     

Critical temperature of low-dimensional high-T c superconductors

We generalized the Coherent Potential Approximation (CPA) method for the calculation of the critical temperature of superconductors of lower dimensionality. Two dimensional (2d) and one-dimensional (1d) models have been considered. The critical temperature is calculated as function of the electronic concentration and of the position of the Fermi level. The critical temperature can be expressed with either of these two parameters. We also calculate the dependence ofT _c on an additional parameter which is the measure of the interplane coupling.