Mechanism of Cooper-pairing in layered high temperature superconductors

Abstract

In this study, the pairing mechanism for layered HTS materials based on attraction between electrons from adjacent layers is proposed. Initially, each layer has expanded Fermi sphere owing to ridged geometry. When the two layers are close enough for tunneling, it becomes energetically advantageous to form correlated quantum states (CQS), reducing the Fermi sphere volume. Cooper pairs, comprising inter-tunneling electrons, occupy the CQS. The image force is responsible for the electron-electron attraction. Pair-binding energy and the corresponding effective mass vary in a wide range. At T>0, some heavy pairs do not condense. Such pairs are responsible for pseudogap. Light pairs get Bose condensed and are responsible for superconductivity. The proposed mechanism provides clarification of superconductivity in cuprates, iron based superconductors and LSCO/LCO interfaces. It provides explanation of two energy gaps and two characteristic temperatures in layered superconducting materials. It also provides clarification on the Fermi surface pockets, anisotropy of charge transport in pseudogap state, and other properties of HTS materials. The pseudogap, estimated within the model, fits the experimental values for the two-layer cuprates, such as YBCO, Bi2212, Tl2212, and Hg1212.