Effect of equatorial line nodes on upper critical field and London penetration depth

TL;DR

This paper models how equatorial line nodes in the Fermi surface of uniaxial superconductors influence the upper critical field and penetration depth anisotropy, explaining behaviors observed in Fe-based superconductors.

Contribution

It demonstrates that equatorial line nodes and Fermi surface anisotropy can account for specific temperature-dependent features of $H_{c2}$ and penetration depth anisotropy in uniaxial superconductors.

Findings

Nearly linear $H_{c2}(T)$ over broad temperature range.

Anisotropy $eta_H(T)$ can change sign with temperature.

Penetration depth anisotropy $eta_ ext{lambda}(T)$ decreases and matches $eta_H$ at $T_c$.

Abstract

The upper critical field $H_{c2}$ and its anisotropy are calculated for order parameters with line nodes at equators, $k_z=0$, of the Fermi surface of uniaxial superconductors. It is shown that characteristic features found in Fe-based materials -- a nearly linear $H_{c2}(T)$ in a broad $T$ domain, a low and increasing on warming anisotropy $\gamma_H= H_{c2,ab}/ H_{c2,c}\, $ -- can be caused by competing effects of the equatorial nodes and of the Fermi surface anisotropy. For certain material parameters, $\gamma_H(T)-1$ may change sign on warming in agreement with recorded behavior of FeTeS system. It is also shown that the anisotropy of the penetration depth $\gamma_\lambda= \lambda_c/\lambda_{ab} $ decreases on warming to reach $\gamma_H$ at $T_c$ in agreement with data available. For some materials $\gamma_\lambda(T)$ may change on warming from $\gamma_\lambda>1$ at low $T$s to $\gamma_\lambda<1$ at high $T$s.