We report ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, with $T_{\mathrm{c}}=35$ K. ${}^{75}\mathrm{As}$ NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies ${\nu }_{\mathrm{Q}}$ of the As(1) and As(2) sites show an opposite temperature $T$ dependence. Nearly $T$ independent behavior of the Knight shifts $K$ is observed in the normal state, and a sudden decrease in $K$ in the superconducting (SC) state suggests spin-singlet Cooper pairs. ${}^{75}\mathrm{As}$ spin-lattice relaxation rates $1/T_1$ show a power-law $T$ dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector $\mathbf{q}=(\pi ,0)$ or $(0,\pi )$ in the single-iron Brillouin zone notation are revealed by $1/T_{1}T$ and $K$ measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the ${}^{75}\mathrm{As}$ quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, $1/T_1$ shows a rapid decrease below $T_{\mathrm{c}}$ without a Hebel-Slichter peak and decreases exponentially at low $T$, consistent with an $s^{\pm }$ nodeless two-gap superconductor.

中文翻译：

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As75NMR研究CaKFe4As4的磁涨落和超导性能

我们报告 ${}^{75}\mathrm{作为}$ 新型铁基超导体的核磁共振（NMR）研究， ${\mathrm{磷酸铁钙}}_4{\mathrm{作为}}_4$， 和 ${Ť}_{\mathrm{C}}=35$ K. ${}^{75}\mathrm{作为}$NMR谱图显示两条分别对应于靠近K和Ca层的As（1）和As（2）位置的两条直线，这表明K和Ca层排列整齐，没有位置反转。我们发现核四极子频率${\nu }_{\mathrm{问}}$ As（1）和As（2）的位置显示相反的温度 $Ť$依赖。几乎$Ť$ 骑士转变的独立行为 $ķ$ 在正常状态下被观察到，并且突然减少 $ķ$ 在超导（SC）状态下，表明存在自旋单重库珀对。 ${}^{75}\mathrm{作为}$ 自旋晶格弛豫率 $\mathrm{1个}/{Ť}_{\mathrm{1个}}$ 出示幂律 $Ť$两个As位点具有不同的指数依赖性。波矢量表征的各向同性反铁磁自旋涨落$\mathbf{q}=（\pi ，0）$ 或者 $（0，\pi ）$ 单铁布里渊区的符号由 $\mathrm{1个}/{Ť}_{\mathrm{1个}}Ť$ 和 $ķ$测量。这种磁性波动对于解释所观察到的温度对温度的依赖性是必要的。${}^{75}\mathrm{作为}$四极杆频率，正如我们的第一性原理计算所证明的那样。在SC状态下，$\mathrm{1个}/{Ť}_{\mathrm{1个}}$ 显示快速下降 ${Ť}_{\mathrm{C}}$ 没有Hebel-Slichter峰，并且在低点呈指数下降 $Ť$，与 $s^{\pm }$ 无节点两间隙超导体。