Signature of multigap nodeless superconductivity in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$

A newly discovered family of high-$T_{\mathrm{c}}$ Fe-based superconductors, $A_{e}A{\mathrm{Fe}}_4{\mathrm{As}}_4$ ($A_e=\text{Ca}$, Sr, Eu and $A=\text{K}$, Rb, Cs), offers further opportunities to understand unconventional superconductivity in these materials. In this Rapid Communication, we report on the superconducting and magnetic properties of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, studied using muon spectroscopy. Zero-field muon spin relaxation studies carried out on the ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ superconductor do not show any detectable magnetic anomaly at $T_{\mathrm{c}}$ or below, implying that time-reversal symmetry is preserved in the superconducting ground state. The temperature dependence of the superfluid density of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ is found to be compatible with a two-gap $s+s$-wave model with gap values of 8.6(4) and 2.5(3) meV, similar to the other Fe-based superconductors. The presence of two superconducting energy gaps is consistent with theoretical and other experimental studies on this material. The value of the penetration depth at $T=0$ K has been determined as $289\left(22\right)$ nm.

Figure 1

(a) Zero-field ($T=1.65$ and $T=35$ K) and longitudinal-field ($T=50$ K and ${\mu }_{0}H=5$ mT) $\mu \mathrm{SR}$ time spectra for ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. The solid lines are the fits to the data using Eq. (1), described in the text. (b) Temperature dependence of the muon spin relaxation rate $\mathrm{\Lambda }$, extracted from the ZF-$\mu \mathrm{SR}$ time spectra.

Figure 2

TF-$\mu \mathrm{SR}$ time spectra of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, collected at 1.6 and 35 K in a transverse field of 40 mT. The solid lines are the fits to the data using Eq. (2).

Figure 3

(a) Temperature dependence of the muon spin relaxation rate $\sigma$ of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. (b) Temperature dependence of the internal field strength of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ for an applied field of 40 mT.

Figure 4

Temperature dependence of ${\lambda }^{-2}$ for ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. The solid curves are the fits to the data using a single-gap $s$-wave and two-gap $s+s$-wave models.