Anharmonic Model for High- T _c Superconductors

Abstract

A considerable enhancement of the superconducting transition temperature Tc in perovskite-type oxides is explained in the framework of the anharmonic model for superconductors with structurally unstable lattices. It is shown that the interaction of electrons with highly anharmonic oxygen-ion vibrations of a rotational type results in a sufficiently large coupling constant λ. The obtained estimation for Tc is in agreement with experimental data for La(Y)BaCuO systems.

Calculation of the Normal‐State Resistivity for High‐Tc Superconductors

“…The Hamiltonian H, (lc) describes a system of N nonlinear coupled oscillators, where the pseudo-spin representation is the suitable description for this highly anharmonic motion [4,5]. The energy hS1 = E, -E, > 0 is the energy difference of the lowest two levels in the anharmonic double-well potential for a given lattice point (cell): the antisymmetric (y,) and the symmetric (y,) ones.…”

“…Especially, the anharmonic model [4, 51 was introduced as a possible explanation for the occurrence of high-T, superconductivity, where the basic assumption of this model consists in the existence of strong anharmonic vibrations in a local double-well potential (quasilocal excitations). Such strong ionic vibrations can be justified from experimental investigations [3,5,61. The success of this model is due to the fact that the estimates for T, are of the same order as the experimentally observed transition temperatures for the recently discovered copper oxide superconductors.…”

Strong enhancement ofd-wave superconducting state in the three-band Hubbard model coupled to an apical oxygen phonon

“…For a strongly anharmonic motion in the double-well potential, the first excitation energy ∆E = E 1 − E 0 is much smaller than the ones excited to higher energy levels E n≥2 . In this case one can take into account only the lowest quantum states Φ 0 and Φ 1 with energies E 0 and E 1 and model the low-energy motion of apical oxygen by a local two-level system represented by a pseudospin 19,20 degree of freedom. Our main results, obtained by ED and constrainedpath Monte Carlo (CPMC) methods, are presented in Figs.…”

Dielectric Properties and Electron Spectrum of the Müller Model in the High-Temperature Superconductivity Theory

“…[7] argue that the motion of O(4) atoms normal to the CuO2 plane is characterized by strong fluctuations of charge along c-axis and electron (hole) transfer between these planes and chains of CuO. It was shown in [8,9] that the interaction of electrons and local anharmonic vibrations can lead to the superconductivity.…”

“…In order to describe anharmonic vibrations in case of the local potential with two minima separated by a high barrier, the pseudospin formalism can be applied (see [8,9]). Within this approximation the model [15], which allows for the interaction of electrons with local vibrations of ions (these are characterized by pseudospin variables Śi , S; = ±1/2) in the same cell was studied.…”