Upper critical field of high quality single crystals of KFe$_2$As$_2$

TL;DR

This study measures the upper critical field and its anisotropy in high-quality KFe$_2$As$_2$ single crystals, revealing an unusual linear scaling with $T_c$ that challenges conventional orbital limiting theories.

Contribution

It provides new high-quality measurements of $H_{c2}$ in KFe$_2$As$_2$, showing a linear $H_{c2}(T_c)$ scaling not predicted by standard models.

Findings

Increased $H_{c2}$ correlates with higher $T_c$ in high-quality crystals.

$H_{c2}(T)$ exhibits an unusual linear scaling with $T_c$.

Anisotropy of $H_{c2}$ was characterized in the crystals.

Abstract

Measurements of temperature-dependent in-plane resistivity, $\rho(T)$, were used to determine the upper critical field and its anisotropy in high quality single crystals of stoichiometric iron arsenide superconductor KFe$_2$As$_2$. The crystals were characterized by residual resistivity ratio, $\rho(300K)/\rho(0)$ up to 3000 and resistive transition midpoint temperature, $T_c$=3.8 K, significantly higher than in previous studies on the same material. We find increased $H_{c2}(T)$ for both directions of the magnetic field, which scale with the increased $T_c$. This unusual linear $H_{c2}(T_c)$ scaling is not expected for orbital limiting mechanism of the upper critical field in clean materials.