Speaker: Herbert Fertig. Indiana University

Title: A Tale of Two Bilayers

Abstract: Modern materials physics has made available true two-dimensional electron systems, in the form of atomic networks bonded only across a single plane.  These van der Waals systems may be formed from a variety of materials, with different electronic properties, which may be combined into bilayer heterostructures with properties not found in either layer individually.  In this talk we will describe quantum coherent states of two such systems, in which nesting plays an important role in determining the ground state phase diagram.  The first of these is a phosphorene – graphene bilayer, for which one finds Fermi surfaces with strong nesting overlaps, leading to spin-density wave ground states for sufficiently strong interactions.  The second involves an idealized bilayer in which each layer supports a particle-hole symmetric band structure, possibly with non-trivial topology.  For half-filling, we find that nesting of the Fermi surfaces on opposite layers leads to different exciton condensate states, separated by a first order transition line which ends in an unusual zero temperature critical endpoint.  We demonstrate that this endpoint is a signature of Lifshitz transitions hosted by the individual layers in the absence of interactions.   In this way their Fermi surface topologies leave an imprint in the interacting phase diagram, in regions where the states themselves are fully gapped and lack Fermi surfaces.