Resistivity and upper critical field in KFe$_2$As$_2$ single crystals

TL;DR

This study investigates the resistivity and upper critical magnetic field in KFe$_2$As$_2$ single crystals, revealing temperature-dependent anisotropy and signs of strong spin paramagnetic effects in the superconducting state.

Contribution

First detailed resistivity and upper critical field measurements on KFe$_2$As$_2$ single crystals, highlighting anisotropic behavior and spin effects.

Findings

Resistivity shows T^2 dependence below 45 K.

Upper critical fields are B_{c2}^{ab}(0)=4.47 T and B_{c2}^{c}(0)=1.25 T.

Anisotropy parameter increases with temperature, reaching 6.8 at T_c.

Abstract

We report resistivity measurements performed on KFe$_2$As$_2$ single crystals down to $T$ = 0.3 K and in magnetic fields up to 17.5 T. The in-plane resistivity vs. $T$ curve has a convex shape down to $\sim$50 K and shows a $T^2$ dependence below $\sim$45 K. The ratio of the c-axis to in-plane resistivities is $\sim$10 at room temperature and $\sim$40 at 4.2 K. The superconducting upper critical field $B_{c2}(T)$ has been determined from the resistivity data: $B_{c2}^{ab}$(0) = 4.47 T and $B_{c2}^{c}$(0) = 1.25 T. The anisotropy parameter $\Gamma$ = $B_{c2}^{ab}$ / $B_{c2}^{c}$ increases with $T$ and is 6.8 at $T$ = $T_c$. The strong curvature of the $B_{c2}^{ab}(T)$ curve indicates the existence of strong spin paramagnetic effects.