Soft-Mode-Phonon-Mediated Unconventional Superconductivity in Monolayer 1 T'-WTe2

Recent experiments have tuned the monolayer 1T'-WTe2 to be superconducting by electrostatic gating. Here, we theoretically study the phonon-mediated superconductivity in monolayer 1T'-WTe2 via charge doping. We reveal that the emergence of soft-mode phonons with specific momentum is crucial to give rise to the superconductivity in the electron-doping regime, whereas no such soft-mode phonons and no superconductivity emerge in the hole-doping regime. We also find a superconducting dome, which can be attributed to the change of Fermi surface nesting conditions with electron doping. By taking into account the experimentally established strong anisotropy of temperature-dependent upper critical field Hc2 between the in-plane and out-of-plane directions, we show that the superconducting state probably has the unconventional equal-spin-triplet pairing in the Au channel of the C2h point group. Our studies provide a promising understanding to the doping dependent superconductivity and strong anisotropy of Hc2 in monolayer 1T'-WTe2, and can be extended to understand the superconductivity in other gated transition metal dichalcogenides.