恒元物理学讲座（第102期）： Excitons are Super!

讲座内容：Long-range Coulomb interaction plays a significant rolein low dimensional systems. In this talk, we will present a few examples howCoulomb interaction can modify, even induce new phases of matter. First exampleis spatially indirect exciton condensates(SIEXC) in the case of atwo-dimensional semiconductor bilayer formed by two transition metaldichalcogenides (TMD) that are separated and surrounded by an insulator, forexample hexagonal boron nitride (hBN). Because of the spin and valley degrees offreedom, there are two distinct condensate phases with different number ofcondensate flavors as layer separation and exciton density vary. Second exampleis exciton-polariton condensate in multiple quantum wells system which can bedescribed by a microscopic equilibrium polariton condensate theory. We treatexciton conduction and valence band states explicitly providing a mechanism forelectrical manipulation of polariton condensates. Third example is newinteraction-induced time-reversal symmetry broken nematic insulator(NTRB) phasenear normal insulator/ quantum spin Hall insulator phase transitions. Wedemonstrate that this NTRB is inevitable because of the large energy differencebetween s-wave and p-wave excitons. Last example is Weyl semimetal instabilityagainst long-range Coulomb interaction. Though strong enough interaction canopen the gap by internode interband pairing, finite band Chern number jumps areretained indicating the robustness of the topological non-trivial features.