Pairing and Collective Excitations in Ising Superconductors

Two-dimensional Ising superconductivity formed in NbSe₂, MoS₂, WS₂, etc. transition-metal dichalcogenides is considered. For the superconducting state, the effective low-energy action for phases of the order parameters has been obtained and collective modes in the system have been studied. It has been shown that the system contains not only the Goldstone mode but also the Leggett mode with a mass related to the difference between the singlet and triplet pairing constants. The effect of a low magnetic field parallel to the plane of the system has also been discussed.