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

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 $d{H}_{c\text{2}}^{\text{c}}/dT\simeq -4.4$ T/K and $d{H}_{c\text{2}}^{\text{ab}}/dT\simeq -10.9$ T/K at $T_c$ yield an electron mass anisotropy of $m_{ab}/m_c\simeq 1/6$ and short coherence lengths ${\xi }_c\simeq 5.8\text{A˚}$ and ${\xi }_{ab}\simeq 14.3\text{A˚}$. 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.

arXiv comment5 pages, 3 figures