Multi-Valley Superconductivity in Ion-Gated MoS₂ Layers

Abstract

Layers of transition metal dichalcogenides (TMDs) combine the enhanced effects of correlations associated with the two-dimensional limit with electrostatic control over their phase transitions by means of an electric field. Several semiconducting TMDs, such as MoS₂, develop superconductivity (SC) at their surface when doped with an electrostatic field, but the mechanism is still debated. It is often assumed that Cooper pairs reside only in the two electron pockets at the K/K' points of the Brillouin Zone. However, experimental and theoretical results suggest that a multivalley Fermi surface (FS) is associated with the SC state, involving six electron pockets at Q/Q'. Here, we perform low-temperature transport measurements in ion-gated MoS₂ flakes. We show that a fully multivalley FS is associated with the SC onset. The Q/Q' valleys fill for doping ≳ 2 × 10¹³ cm^-2, and the SC transition does not appear until the Fermi level crosses both spin-orbit split sub-bands Q ₁ and Q ₂. The SC state is associated with the FS connectivity and promoted by a Lifshitz transition due to the simultaneous population of multiple electron pockets. This FS topology will serve as a guideline in the quest for new superconductors.