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

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.

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Cited by in corpus (1)

• Time reversal invariant single gap superconductivity with upper critical field larger than Pauli limit in NbIr${}_2$B${}_2$