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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 MoS2flakes. We show that a fully multivalley FS is associated with the SC onset. The Q/Q′ valleys fill for doping ≳ 2 × 1013cm–2, and the SC transition does not appear until the Fermi level crosses both spin–orbit split sub-bands Q 1and 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.

We review recent results of point-contact spectroscopy (PCS) in doped (with Al, C or Mn) and neutron-irradiated samples of the two-band superconductor MgB2. The aim of these measurements was to clarify the role of inter- and intra-band scattering, and to test the theoretical expectations about the effects of substitutions and disorder on two-band superconductivity. The gap amplitudes, Δπ and Δσ, were obtained as a function of the critical temperature of the junctions (or of the doping content) by fitting the conductance curves with the two-band BTK model. The analysis of the resulting curves within the Eliashberg theory allowed us to extract information on the partial DOSs, as well as on the scattering rates within and between the two bands. The results are discussed and compared to theoretical expectations.

We review recent results of point-contact spectroscopy (PCS) in doped (with Al, C or Mn) and neutron-irradiated samples of the two-band superconductor MgB2. The aim of these measurements was to clarify the role of inter- and intra-band scattering, and to test the theoretical expectations about the effects of substitutions and disorder on two-band superconductivity. The gap amplitudes, Δπ and Δσ, were obtained as a function of the critical temperature of the junctions (or of the doping content) by fitting the conductance curves with the two-band BTK model. The analysis of the resulting curves within the Eliashberg theory allowed us to extract information on the partial DOSs, as well as on the scattering rates within and between the two bands. The results are discussed and compared to theoretical expectations.