Signatures of the topological s<sup>+−</sup> superconducting order parameter in the type-II Weyl semimetal T<sub>d</sub>-MoTe<sub>2</sub>

Abstract

In its orthorhombic <i>T</i><sub>d</sub> polymorph, MoTe<sub>2</sub> is a type-II Weyl semimetal, where the Weyl fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance and superconductivity were also observed in <i>T</i><sub>d</sub>-MoTe<sub>2</sub>. Understanding the superconductivity in <i>T</i><sub>d</sub>-MoTe<sub>2</sub>, which was proposed to be topologically non-trivial, is of eminent interest. Here, we report high-pressure muon-spin rotation experiments probing the temperature-dependent magnetic penetration depth in <i>T</i><sub>d</sub>-MoTe<sub>2</sub>. A substantial increase of the superfluid density and a linear scaling with the superconducting critical temperature <i>T</i><sub>c</sub> is observed under pressure. Moreover, the superconducting order parameter in <i>T</i><sub>d</sub>-MoTe<sub>2</sub> is determined to have 2-gap <i>s</i>-wave symmetry. We also exclude time-reversal symmetry breaking in the superconducting state with zero-field μSR experiments. Considering the strong suppression of <i>T</i><sub>c</sub> in MoTe<sub>2</sub> by disorder, we suggest that topologically non-trivial <i>s</i><sup>+−</sup> state is more likely to be realized in MoTe<sub>2</sub> than the topologically trivial <i>s</i><sup>++</sup> state.