Theory of the density of states of pure type-II superconductors in high magnetic fields

Abstract

This paper deals with the theory of the density of states of pure type-II superconductors in high magnetic fields. An approximate expression of theGreen's function is derived by using the fact that it has the periodicity of the lattice of flux lines in the center-of-mass coordinates. In comparison to the normal stateGreen's function a correction term proportional to the absolute square of the order parameter arises. This is seen to act as a self-energy part depending strongly on the polar angleΘ of the “quasiparticle” momentum relative to the direction of the field. The corresponding density of states, as a function of the excitation energy, is found to vary from the gapless to the BCS type as the parameterΘ changes fromπ/2 to zero (orπ). The averaged state density varies gradually from a uniform to a BCS-like state density as the external field decreases sufficiently far below the upper critical field. The results of the present theory for the averaged density of states agree fairly well with the results due to a conjecture byMaki if the field is very close to the upper critical field.