15. May. 2014, 16:15 Uhr, Gebäude NW1, Raum H3
Superconductivity in Two-Dimensional Materials
Prof. Dr. Stephan Haas, Univ. of Southern California, L.A., USA
In recent years, it has been shown that graphene displays a plethora of interesting properties. But despite multiple attempts it does not appear to have instabilities such as superconductivity. However, there is a family of structurally related materials, the layered dichalcogenides with a very rich low-temperature phase diagram. In this talk I will focus on the sequence of phases in doped dichalcogenides, such as recently realized in field-gated MoS2. Upon increasing the electron doping level, we observe a succession of semiconducting, metallic, superconducting and charge density wave regimes in this material, i.e. in different order compared to the phase diagram of (semi-)metallic dichalcogenides such as TiSe2. Both instabilities trace back to a softening of phonons which couple the electron populated conduction band minima. The superconducting dome, calculated using Eliashberg theory, is found to fit the experimentally observed phase diagram, obtained from resistivity measurements. The charge density wave phase at higher electron doping concentrations as predicted from instabilities in the phonon modes is further corroborated by detecting the accompanying lattice deformation in density functional based supercell relaxations. Upon charge density wave formation, doped MoS2 remains metallic but undergoes a Lifschitz transition, where the number of Fermi pockets is reduced.