A New Mechanism for High-Tc: Electron Scattering from Interacting Tunneling Units

Abstract

A theoretical model for high-T c superconductivity is presented based on electron pairing due to their interaction with two-level or multi-level tunneling units (TU's) present in high-T c materials. TU's were found experimentally in YBa2Cu3O 7-δ, Bi2CaSr2Cu2O8 , Tl2CaBa2Cu2O8 and in Tl2CaBa2CuO6 . The TU's have specific directions of orientation with respect to the crystal axis and hence cause a strongly anisotropic scattering of the conduction electrons and a strongly anisotropic gap function Δ(k) with wave vector k. For the weak coupling case we obtain, (i) an analytically derived Δ(k) which has a linear combination of s-wave and dx2-y2-wave symmetry with nodes in Δ(k), (ii) a high-T c determined by the elastic interaction potential between the TU's, (iii) an isotope effect consistent with experiment, (iv) a large uniaxial pressure p dependence of T c for YBa2Cu3O 7-δ close to optimal oxygen doping and (dT c /dpa) ≈ -(dT c /dpb) where Pa and Pb are the pressures in the a and b directions, (v) a quantitative agreement with experiment for dT c /dPi (i = a, b) when we use the elastic constant. The recently observed d ± s wave pairing in twinned YBa2Cu3O 7-δ, with a reversal of the sign of s-wave component across the twin boundary, is also in agreement with our model.