Exciton–Mott Physics in Two-Dimensional Electron–Hole Systems: Phase Diagram and Single-Particle Spectra

Abstract

Exciton Mott physics in two-dimensional electron-hole (e-h) systems is studied in the quasiequilibrium, which is the crossovers or phase transitions between the insulating exciton gas and the metallic e-h plasma. By developing a self-consistent screened T -matrix approximation, we succeed in obtaining the "global" phase diagram on the plane of the e-h density and the temperature as a contour plot of the exciton ionization ratio. The detailed features of the exciton-Mott crossover at high temperature are figured out beyond the conventionally used concept of the Mott density. At low temperature, we find not only the region unstable toward the inhomogeneity but the pure Mott transition point characterized by the discontinuity in the ionization ratio. The single particle spectra also exhibit interesting features reflecting the excitonic correlations.

Theory of two-dimensional spatially indirect equilibrium exciton condensates

“…It is well known however that at high electron and hole densities a first-order Mott transition occurs [33][34][35][36][37] from the gapped exciton condensate phase to an ungapped electron-hole plasma state. The electron-hole plasma state is preferred energetically because it can achieve better correlations between likecharge particles, reducing the probability that they are close together, while maintaining good correlations between oppositely-charged particles.…”

Rate Equation Analysis of the Dynamics of First-order Exciton Mott Transition

“…There is a long-standing argument whether EMT is a crossover or a first-order phase transition accompanied by a bistability at sufficiently low temperatures. [16][17][18][19][20][21][22] In general, bistability appears in diverse systems such as electronic circuits, optoelectronic systems, magnetic systems, molecular and biological systems, etc. Figure 1(a) depicts the schematic view of bistability in the parameter space, where the function y(x) exhibits a bistable region as a function of the control parameter x.…”

“…Such a positive feedback can induce a hysteresis of the ionization ratio of excitons as defined by  = nf/ntotal (ntotal = nf + nex, with nf and nex the densities of free e-h pairs and excitons, respectively) with respect to the total pair e-h density, invoking the possibility that EMT becomes a first-order phase transition. [16][17][18][19][20][21][22] Experimentally, the problem of EMT has long been studied using e.g. photoluminescence spectroscopy.…”

“…2(c) as a function of the e-h pair density. Although the precise description on the reduction of exciton binding energy requires more sophisticated theoretical treatment, 21) we can capture the overall behavior of the system qualitatively with such an empirical model. First, we discuss the case without a positive feedback by assuming that both excitons and unbound e-h pairs contribute to the screening equally.…”

Microscopic theory of equilibrium polariton condensates

“…[21][22][23][24] As emphasized in earlier work, 8,10,11,18 because the coupling to the photon field is independent of momentum in the k · p theory we use, which is accurate for all systems of interest, it yields electron-hole pairs that are more tightly bound than they would be if only electron-electron interactions were present. For purely excitonic condensates meanfield theory is known to fail as the Mott-transition [25][26][27][28] is approached, by overestimating the tendency toward coherence. This deficiency can be partly remedied by adding screening effects [29][30][31] in an ad-hoc manner.…”