Structural transition in Bi-Sr-Ca-Cu-O revealed by Fermi level control and the mechanism of high-Tc superconductivity

Abstract

A technique is developed for control of the Fermi level in HTS materials by the oxygen dosage in the sample using a solid-state electrochemical cell. The dependences of the oxygen content in the sample n(EF) and the electron density of states at the Fermi level D(EF) on the Fermi level have been studied. At a separation of several tens of mV above the Fermi level of the HTS state, an n(EF) step and D(EF) peak are revealed which are related to the structural phase transition. The character of the dependences obtained allows the authors to relate high-temperature superconductivity to the interaction between the virtual structural phase transition and the transition to the conventional superconducting state. In the phenomenological model proposed this interaction is described by two order parameters: Delta the superconductive gap width, and in g the energy separating the superconducting state from the virtual structural phase transition. In the microscopic model under consideration this interaction results in the appearance of an electron-phonon gap above the Fermi level. The position of the gap increases the electron density of states in the region of BCS interaction with phonon, thus excluding dielectrization of the electronic spectrum.