Nodeless superconducting gap in the topological superconductor candidate 2M–WS2

Recently, a new transition-metal dichalcogenide (TMD) material $2M$-phase ${\mathrm{WS}}₂$ was synthesized. The material $2M\text{\ensuremath{-}}{\mathrm{WS}}₂$ not only exhibits superconductivity, with the highest ${T}_{\text{c}}$ being 8.8 K at ambient pressure among TMDs, but also hosts a topological surface state. Here we report the low-temperature thermal conductivity measurements on $2M\text{\ensuremath{-}}{\mathrm{WS}}₂$ single crystals to investigate its superconducting gap structure. A negligible residual linear term ${\ensuremath{\kappa}}₀/T$ in zero field shows that $2M\text{\ensuremath{-}}{\mathrm{WS}}₂$ has a fully superconducting gap with all electrons paired. The field dependence of ${\ensuremath{\kappa}}₀/T$ suggests an anisotropic superconducting gap or multiple nodeless superconducting gaps in $2M\text{\ensuremath{-}}{\mathrm{WS}}₂$. Such a fully gapped superconducting state is compatible with $2M\text{\ensuremath{-}}{\mathrm{WS}}₂$ being a topological superconductor candidate.