Interband coupling and transport interband scattering in $s_{\pm}$ superconductors

TL;DR

This paper investigates how interband coupling and scattering affect the properties of $s_{}$ superconductors, revealing conditions for gapless states and changes in electromagnetic response.

Contribution

It provides a detailed analysis of the effects of interband scattering on the energy gaps, critical temperature, and penetration depth in $s_{}$ superconductors, highlighting the transition to gapless behavior.

Findings

Energy gaps become equal due to scattering.

Superconductors can become gapless at high interband scattering.

Penetration depth transitions from exponential to linear behavior.

Abstract

A two-band model with repulsive interband coupling and interband {\it transport} (potential) scattering is considered to elucidate their effects on material properties. In agreement with previous work, we find that the bands order parameters $\Delta_{1,2}$ differ and the large is at the band with a smaller normal density of states (DOS), $N_{n2}<N_{n1}$. However, the bands energy gaps, as determined by the energy dependence of the DOS, are equal due to scattering. For each temperature, the gaps turn zero at a certain critical interband scattering rate, i.e. for strong enough scattering the model material becomes gappless. In the gapless state, the DOS at the band 2 is close to the normal state value, whereas at the band 1 it has a V-shape with non-zero minimum. When the normal bands DOS' are mismatched, $N_{n1}\ne N_{n2}$, the critical temperature $T_c$ is suppressed even in the absence of interband scattering, $T_c(N_{n1})$ has a dome-like shape. With increasing interband scattering, the London penetration depth at low temperatures evolves from being exponentially flat to the power-law and even to near linear behavior in the gapless state, the latter being easily misinterpreted as caused by order parameter nodes.