Effective Ginzburg-Landau free energy functional for multi-band isotropic superconductors

TL;DR

This paper demonstrates that interband gradient coupling (drag effect) in isotropic multi-band superconductors leads to a single coherence length and suppresses type-1.5 behavior, allowing a reduction to an effective single-band Ginzburg-Landau functional.

Contribution

It introduces an approximate method to reduce the multi-band Ginzburg-Landau functional to an effective single-band form considering the drag effect.

Findings

Interband gradient coupling results in a single coherence length.

Type-1.5 superconductivity is shown to be impossible under these conditions.

The free-energy functional simplifies to an effective single-band form.

Abstract

It has been shown that interband mixing of gradients of two order parameters (drag effect) in an isotropic bulk two-band superconductor plays important role - such a quantity of the intergradients coupling exists that the two-band superconductor is characterized with a single coherence length and a single Ginzburg-Landau (GL) parameter. Other quantities or neglecting of the drag effect lead to existence of two coherence lengths and dynamical instability due to violation of the phase relations between the order parameters. Thus so-called type-1.5 superconductors are impossible. An approximate method for solving of set of GL equations for a multi-band superconductor has been developed: using the result about the drag effect it has been shown that the free-energy functional for a multi-band superconductor can be reduced to the GL functional for an effective single-band superconductor.