Excitonic instability in optically-pumped three-dimensional Dirac materials

TL;DR

This paper explores the possibility of transient excitonic condensates in optically-pumped three-dimensional Dirac materials, revealing coexistence of phases, effects of screening, and experimental signatures, extending prior 2D findings to 3D systems.

Contribution

It demonstrates that in 3D Dirac semimetals, excitonic phases can form under optical pumping with weak coupling, and analyzes their phase diagram and experimental detection methods.

Findings

Coexistence of excitonic insulator and charge density wave phases.

Critical coupling for excitonic instability vanishes in pumped 3D DMs.

Screening effects can be weaker in 3D DMs compared to 2D DMs.

Abstract

Recently it was suggested that transient excitonic instability can be realized in optically-pumped two-dimensional (2D) Dirac materials (DMs), such as graphene and topological insulator surface states. Here we discuss the possibility of achieving a transient excitonic condensate in optically-pumped three-dimensional (3D) DMs, such as Dirac and Weyl semimetals, described by non-equilibrium chemical potentials for photoexcited electrons and holes. Similar to the equilibrium case with long-range interactions, we find that for pumped 3D DMs with screened Coulomb potential two possible excitonic phases exist, an excitonic insulator phase and the charge density wave phase originating from intranodal and internodal interactions, respectively. In the pumped case, the critical coupling for excitonic instability vanishes; therefore, the two phases coexist for arbitrarily weak coupling strengths. The excitonic gap in the charge density wave phase is always the largest one. The competition between screening effects and the increase of the density of states with optical pumping results in a reach phase diagram for the transient excitonic condensate. Based on the static theory of screening, we find that under certain conditions for the value of the dimensionless coupling constant screening in 3D DMs can be weaker than in 2D DMs. Furthermore, we identify the signatures of the transient excitonic condensate that could be probed by scanning tunneling spectroscopy, photoemission and optical conductivity measurements. Finally, we provide estimates of critical temperatures and excitonic gaps for existing and hypothetical 3D DMs.