Electromagnetic responses of Excitonic Insulators

In this seminar, I will first introduce the main concepts of bilayer exciton insulator, a new type of charge neutral quantum liquid recently realized in 2D materials. Then I will mostly focus on the electromagnetic responses of bilayer excitonic insulators (EIs) and identify two distinct collective modes: (1) Two gapped plasmon modes couple to the layer symmetric gauge field. The transverse mode is nearly dispersionless in the long wavelength limit, while the longitudinal mode, accounting for total charge fluctuations, has a linear dispersion with velocity proportional to 2D electrical polarizability. (2) A gapless phase (Goldstone) mode and a gapped amplitude mode, associated with the fluctuations of EI order parameter, couple to the layer antisymmetric gauge field. In the long wavelength and low frequency limit, the phase mode behaves like an acoustic phonon with speed inversely proportional to the square root of exciton compressibility. Significantly, its linear dispersion yields a cubic frequency dependence of the real admittance in microwave impedance microscopy (MIM), providing a method to detect the Goldstone mode directly.