Alexander Zyuzin
Research Fellow, Centre of Excellence in Quantum Technology, QTF, Aalto University, Finland

Superconductivity in flat-band semimetals

Flat-band materials may have relatively high superconducting transition temperature. Several systemswere proposed to host nearly flat bands such as, for example, multilayer graphene with rhombohedral stacking, twisted bilayer graphene, and semimetals with high pseudospin quasiparticles. However, the effect of the flat band on superconductivity can be twofold. Despite favoring Cooper pair formation, its nearly dispersionless nature can be a serious impediment to pair condensation. The Cooper pairs formation and their condensation may occur at different temperatures.

I will talk about the conditions for the emergence of the preformed Cooper pairs in materials hosting flat bands. As a particular example, a semimetal with a pair of three-band crossing points at which a flat band intersects with a Dirac cone will be considered. It will be shown that the flat band promotes local Cooper pair formation so that the system may be modeled as an array of superconducting grains. The delocalized states give rise to the phase-coherent superconductivity at low temperatures. The transition temperature between the preformed Cooper pair state and the phase-coherent state will be discussed.