Photo-induced enhancement of excitonic order

Recently, the nonequilibrium dynamics of the excitonic insulator, which is a pair condensation of electrons and holes, starts to attract interest. In this talk, we discuss the dynamical properties of excitonic insulators coupled to phonons [1]. Without phonon couplings, the linear response is given by the damped amplitude oscillations of the order parameter with frequency equal to the minimum band gap. A phonon coupling to the interband transfer integral induces two types of long-lived collective oscillations of the amplitude, one originating from the phonon dynamics and the other from the phase mode, which becomes massive. We show that even for small phonon coupling, a photo-induced enhancement of the exciton condensation and the gap can be realized. Using the Anderson pseudo-spin picture, we argue that the origin of the enhancement is a cooperative effect of the massive phase mode and the Hartree shift induced by the photo excitation. We also discuss how the enhancement of the order and the collective modes can be observed with time-resolved photo-emission spectroscopy. In addition, we also show the possible dynamical phase transition in the excitonic insulator.
[1]Yuta Murakami, Denis Golez, Martin Eckstein, Philipp Werner, arXiv:1707.07706.