The Joint Condensed Matter Seminar (JCMS) series is organised by KTH Royal Insitute of Technology, Nordita, and Stockholm University.
On Monday, December 1st, 2025 from 11.00 am to 12.00 we will host a seminar by Lingxian Kong from KTH.
Theoretical developments in the optics and electrics of excitons
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.