Anisotropic magnetoresistance and upper critical fields up to 63 T in CaKFe$_4$As$_4$ single crystals

TL;DR

This study investigates the upper critical magnetic fields and anisotropic properties of CaKFe$_4$As$_4$ single crystals up to 63 T, revealing multiband effects, anisotropy reduction with temperature, and high extrapolated critical fields exceeding BCS limits.

Contribution

It provides the first detailed analysis of high-field anisotropic upper critical fields in CaKFe$_4$As$_4$, highlighting multiband effects and the interplay of paramagnetic and orbital pairbreaking mechanisms.

Findings

Anisotropy parameter decreases from 2.5 to 1.5 between $T_c$ and 25 K.

Upper critical field extrapolates to about 92 T, above the BCS paramagnetic limit.

Short coherence lengths and moderate mass anisotropy are determined.

Abstract

We report the temperature dependencies of the upper critical fields $H_{c\text{2}}^{\text{c}}(T)$ parallel to the c-axis and $H_{c\text{2}}^{\text{ab}}(T)$ parallel to the ab-plane of single crystalline CaKFe$_4$As$_4$ inferred from the measurements of the temperature-dependent resistance in static magnetic fields up to 14 T and magnetoresistance in pulsed fields up to 63 T. We show that the observed decrease of the anisotropy parameter $\gamma(T)=H_{c\text{2}}^{\text{ab}}/H_{c\text{2}}^{\text{c}}$ from $\simeq 2.5$ at $T_c$ to $\simeq 1.5$ at 25 K can be explained by interplay of paramagnetic pairbreaking and orbital effects in a multiband theory of $H_{c2}$. The slopes of $dH_{c\text{2}}^{\text{c}}/dT\simeq-4.4$ T/K and $dH_{c\text{2}}^{\text{ab}}/dT \simeq-10.9$ T/K at $T_c$ yield an electron mass anisotropy of $m_{ab}/m_c\simeq 1/6$ and short coherence lengths $\xi_c\simeq 5.8\,\text{\AA}$ and $\xi_{ab}\simeq 14.3\,\text{\AA}$. The behavior of $H_{c\text{2}}(T)$ turns out to be similar to that of the optimal doped (Ba,K)Fe$_2$As$_2$, with $H_{c\text{2}}^{\text{ab}}(0)$ extrapolating to $\simeq 92$ T, well above the BCS paramagnetic limit.