JCMS: Theoretical developments in the optics and electrics of excitons

by Lingxiang Kong (KTH)

Monday 1 Dec 2025, 11:00 → 12:00 Europe/Stockholm

Albano 3: 6228 - Mega (22 seats) (Albano Building 3)

Excitons are two-body bound states formed by electrons and holes, playing a crucial role in the optical spectra of semiconductors. Moreover, excitons may organize into condensates, breaking a U(1) symmetry and leading to observable phenomenon such as the super-counterflow. In the first part of this seminar, we extend the concept of excitons from semiconductors to semimetals with a negative energy gap, investigating collective modes in interband excitations. I will show that these excitations generically consist of excitons and “anti-excitons” carrying opposite U(1) charges, directly analogous to particle and antiparticle in an effective Klein–Gordon theory. In the second part, I will discuss exciton condensation in double-layer flat-band systems, exemplified by double-layer quantum Hall systems, where the condensate is naturally described as a layer-pseudospin ferromagnet. I will demonstrate that applying an interlayer bias voltage drives the system into non-equilibrium steady states exhibiting a range of experimentally accessible signatures, including both AC and DC interlayer Josephson currents as well as electromagnetic radiation.