Collective Phenomena in Excitonic Quantum Matter

Abstract

Electron-hole pairs, known as excitons, are fundamental excitations of solids, and are expected to realise a rich phase diagram of quantum phases in- and out-of-equilibrium: a hypothesized pairing instability leads to the macroscopic condensation of “excitonic insulators” in small-bandgap materials, with quantum coherence and possible super-transport persisting up to room temperature. Alternatively, in large-bandgap semiconductors, the Bose-Einstein condensation of optically active metastable excitons opens the door to nonlinear optical processes, coherent light generation, and exciton lasing. Though first predicted in the 1960s, these excitonic quantum phases have proven frustratingly difficult to implement and identify. In the last decade, novel materials and experimental methods have revitalized this pursuit. This signals an urgent need for theory to propose experimental fingerprints that will distinguish between quantum-coherent excitonic states, uncondensed excitons, and normal host materials. I address this need by focusing on a key common feature: a spontaneously broken U(1) symmetry and the resulting gapless Goldstone collective excitations, which generate unique distinguishing signatures. I first develop a theory for disordered excitonic insulators, which are classified by the symmetries of the introduced impurities. I demonstrate that the Goldstone modes are robust to disorder scattering, and thus dominate low-energy long-range dynamics. This is corroborated by ballistic transport measurements in candidate material Ta₂NiSe₅. I subsequently consider photopumped exciton BECs in twisted heterobilayers of transition metal dichalcogenides such as MoSe₂/WSe₂. I show that despite a momentum-indirect bandgap, strong interactions enable emission from the condensate via a spontaneous production of collective excitations. I predict this “leaky emission” dominates at low temperatures, with a unique spectrum and density dependence.