CaKFe $_4$ As $_4$单晶的各向异性磁电阻和高达63 T的上临界场
Anisotropic magnetoresistance and upper critical fields up to 63 T in CaKFe$_4$As$_4$ single crystals
摘要：
本文报道了CaKFe单晶$_4$的上临界场$H_{c\text{2}}^{\text{c}}(T)$与c轴平行和$H_{c\text{2}}^{\text{ab}}(T)$与ab面平行的温度关系。我们发现，所观察到的各向异性参数$γ(T)=H_{c\text{2}}^{\text{ab}}/H_{c\text{2}}^{\text{c}}$从$\simeq 2.5$在$T_c$ $\simeq 1.5$在25 K的减少可以解释的顺磁对破缺和轨道效应的多带理论的$H_{c2}$的相互作用。$dH_{c\text{2}}^{\text{c}}/dT\simeq-4.4$ T/K和$dH_{c\text{2}}^{\text{ab}}/dT \simeq-10.9$ T/K在$T_c$处的斜率产生$m_{ab}/m_c\simeq 1/6$的电子质量各向异性和短相干长度$ξ_c\simeq 5.8\,\textÅ$和$ξ_{ab}\simeq 14.3\,\textÅ$。$H_{c\text{2}}(T)$的行为与最佳掺杂的（Ba，K）Fe $_2$ As $_2$的行为相似，$H_{c\text{2}}^{\text{ab}}(0)$外推到$\simeq 92$ T，远高于BCS顺磁极限。
英文摘要：
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 $γ(T)=H_{c\text{2}}^{\text{ab}}/H_{c\text{2}}^{\text{c}}$ 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 $ξ_c\simeq 5.8\,\textÅ$ and $ξ_{ab}\simeq 14.3\,\textÅ$. 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.
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