Superconductivity in a Molecular Field. I

Abstract

The Fulde-Ferrel model, in which a molecular field is acting on the spins of the conduction electrons responsible for superconductivity, is investigated. The superconducting state, that is described by the condensate wave function oscillating in space and gives rise to a gapless energy spectrum, is shown to be the ground or equilibrium state for some values of the pairing interaction and the molecular field. Thermodynamics properties of such gapless superconducting states are investigated extensively. Special attention is paid on the properties of the phase transitions from such superconducting states to the other states.