Fluctuations in two-band superconductors in a strong magnetic field

TL;DR

This paper analyzes how fluctuations in specific heat and conductivity behave near the upper critical field in two-band superconductors, highlighting the effects of multiple bands with different coherence lengths on fluctuation phenomena.

Contribution

It provides a detailed theoretical analysis of fluctuation effects in two-band superconductors under strong magnetic fields, emphasizing the role of band differences and nonlocality.

Findings

Multiple-band effects are significant when bands have different coherence lengths.

The upper critical field exhibits upward curvature influenced by large in-plane coherence length.

Fluctuation amplitude and width increase at specific field scales, affecting transport and thermodynamic transitions.

Abstract

We consider the behaviour of the fluctuating specific heat and conductivity in the vicinity of the upper critical field line for a two-band superconductor. Multiple-band effects are pronounced when the bands have very different coherence lengths. The transition to superconductive state is mainly determined by the properties of the rigid condensate of the "strong" band, while the "weak" band with a large coherence length of the Cooper pairs causes the nonlocality in fluctuation behaviour and break down of the simple Ginzburg-Landau picture. As expected, the multiple-band electronic structure does not change the functional forms of dominating divergencies of the fluctuating corrections when the magnetic field approaches the upper critical field. The temperature dependence of the coefficients, however, is modified. The large in-plane coherence length sets the field scale at which the upper critical field has upward curvature. The amplitude of fluctuations and fluctuation width enhances at this field scale due to reduction of the effective z-axis coherence length. We also observe that the apparent transport transition displaces to lower temperatures with respect to the thermodynamic transition. Even though this effect exists already in a single-band case at sufficiently high fields, it may be strongly enhanced in multiband materials.