NMR/NQR and AC-susceptibility Studies in the Weyl Semimetal Superconductor 1T-MoTe${}_2$ under Pressure

Abstract

We performed the Te-nuclear magnetic resonance, the Mo-nuclear quadrupole resonance, and the AC susceptibility in the Weyl semimetal superconductor 1T-MoTe${}_2$ at pressures up to 2.17~GPa. From the temperature and pressure dependence of the AC susceptibility, the superconducting transition temperature $T_{\mathrm{c}}$ and the upper critical field $H_{c2}$ were estimated. The results deviate from the Werthamer-Helfand-Hohenberg model but are well described by $H_{c2}(T)=H_{c2}(0)[1-T/T_{\mathrm{c}}{]}^{\alpha }$. The latter fit yields $H_{c2}(0)=1.50$~T, $T_{\mathrm{c}}=3.81$K, and $\alpha =1.1$ at 2.17GPa, suggesting that the superconductivity lies in a strong-coupling regime. Since the nuclear spin-lattice relaxation rate divided by temperature, $1/T_{1}T$, follows the Korringa relation at ambient pressure, the increase in $1/T_{1}T$ with pressure up to approximately 0.7~GPa indicates an increase in the density of states (DOS), $N(E_{\mathrm{F}})$. This trend mirrors the pressure dependence of $T_{\mathrm{c}}$ in the low-pressure region, consistent with the BCS mechanism. Above 0.7~GPa, however, $N(E_{\mathrm{F}})$ slightly decreases while $T_{\mathrm{c}}$ continues to rise, suggesting an additional pairing contribution beyond the conventional BCS picture. In the 1T${}^{\prime }$ phase at 2.17~GPa, the absence of a coherence peak in $1/T_{1}T$ around $T_{\mathrm{c}}$, accompanied by a two-step decrease just below $T_{\mathrm{c}}$, was observed, which may be a signature of unconventional superconductivity.