Nodeless superconductivity and its evolution with pressure in the layered dirac semimetal $2M-WS_2$

Abstract

Recently, the transition metal dichalcogenide (TMD) system 2M-WS<sub>2</sub> has been identified as a Dirac semimetal exhibiting both superconductivity with the highest <i>T</i><sub>c</sub> ~ 8.5 K among all the TMD materials and topological surface states. Here we report on muon spin rotation (<i>μ</i>SR) and density functional theory studies of microscopic SC properties and the electronic structure in 2M-WS<sub>2</sub> at ambient and under hydrostatic pressures (<i>p</i><sub>max</sub> = 1.9 GPa). The SC order parameter in 2M-WS<sub>2</sub> is determined to have single-gap <i>s</i>-wave symmetry. We further show a strong negative pressure effect on <i>T</i><sub><i>c</i></sub> and on the SC gap Δ. This may be partly caused by the pressure induced reduction of the size of the electron pocket around the Γ-point. We also find that the superfluid density <i>n</i><sub><i>s</i></sub> is weakly affected by pressure. The absence of a strong pressure effect on <i>n</i><sub><i>s</i></sub> and the absence of a correlation between <i>n</i><sub><i>s</i></sub> and <i>T</i><sub>c</sub> in 2M-WS<sub>2</sub>, in contrast to the other SC TMDs <i>T</i><sub>d</sub>-MoTe<sub>2</sub> and 2H-NbSe<sub>2</sub>, is explained in terms of its location in the optimal (ambient pressure) and above the optimal (under pressure) superconducting regions of the phase diagram and its large distance to the other possible competing or cooperating orders.