Dicke Fellowship candidate seminar: Xiaomeng Liu, Harvard; "Exciton condensation in graphene double-layer heterostructure"

Mon, Jan 8, 2018, 1:30 pm
Jadwin A07 (new location)
Free and open to the public

Excitons are electron-hole pairs bound by the Coulomb attraction, which can form Bose-Einstein condensates at low temperatures. By placing electrons and holes in spatially separated layers, a double-layer electronic system prevents electron-hole recombination and enables exciton condensation in equilibrium. Recent development of 2D material heterostructures provides a great opportunity to create double-layer electronic systems for the study of exciton condensation with negligible tunneling, strong interlayer interaction and large density tunability. In this talk, I will introduce our recent observation of exciton condensation in graphene double-layer heterostructure under strong magnetic fields. Characters of condensation such as interlayer correlation and exciton superfluidity are manifested through quantize Hall drag and vanishing counterflow resistance. In this 2D bosonic superfluid, different pairing regimes can be accessed by tuning the exciton density. When the exciton density is low (electron separation lB >> interlayer distance d), the electrons and holes are spatially paired (BEC condensate), while at high densities (lB∼d), the pairing is in momentum space and caused by the Fermi surface instability (BCS condensate). Furthermore, we observed characters of Berezinskii–Kosterlitz–Thouless (BKT) transition from current (I)-voltage (V) relation in the BCS regime. Last, I will discuss signatures of novel exciton condensate states, such as total filling factor vtot = 0 state and fractional total filling factor states, as well as the conditions for establishing exciton condensate phases.