Effect of fermion indistinguishability on optical absorption of doped two-dimensional semiconductors

Abstract

We study the optical absorption spectrum of a doped two-dimensional semiconductor in the spinvalley polarized limit. In this configuration, the carriers in the Fermi sea are indistinguishable from one of the two carriers forming the exciton. Most notably, this indistinguishability requires the three-body trion state to have p-wave symmetry. To explore the consequences of this, we evaluate the system's optical properties within a polaron description, which can interpolate from the low density limit-where the relevant excitations are few-body bound states-to higher density manybody states. In the parameter regime where the trion is bound, we demonstrate that the spectrum is characterized by an attractive quasiparticle branch, a repulsive branch, and a many-body continuum, and we evaluate the doping dependence of the corresponding energies and spectral weights. In particular, at low doping we find that the oscillator strength of the attractive branch scales with the square of the Fermi energy as a result of the trion's p-wave symmetry. Upon increasing density, we find that both the repulsive and attractive branches blue-shift, and that the orbital character of the states associated with these branches interchanges. We compare our results with previous investigations of the scenario where the Fermi sea involves carriers distinguishable from those in the exciton, for which the trion ground state is s-wave.

Diffusion of Excitons in a Two-Dimensional Fermi Sea of Free Charges

“…Coexistence of optical and electrical excitations in semiconductors and their nanostructures has a long history of research. , It typically involves simultaneous presence of Coulomb-bound electron–hole pairs, known as excitons, and either free electrons or free holes . The emerging many-particle interactions result in the formation of new electronic states of the mixed system: ionic three-particle states labeled as trions, − attractive and repulsive Fermi polarons, − and complexes with a higher number of constituting particles. − Conceptually similar to related phenomena in the ultracold atom gases, , exciton–carrier mixtures were shown to strongly influence the properties of excited semiconductors and their response to external fields. ,,,, They offered fertile ground to merge the realms of optics and transport, motivated by the availability of excitations that are both electrically tunable and couple strongly to light. − This sparked intense research activities, including demonstrations of trion diffusion, ,, electrically driven, optically detected currents, ,,,, observations of magneto-transport, and efficient acceleration of polaron–polaritons in optical microcavities . Despite this remarkable progress, however, the comparatively weak Coulomb interaction in quantum wells limited the accessible range of free carrier densities due to the screening and ionization of excitons at the Mott transition .…”

“…14−19 Conceptually similar to related phenomena in the ultracold atom gases, 20,21 exciton−carrier mixtures were shown to strongly influence the properties of excited semiconductors and their response to external fields. 5,19,22,23,24 They offered fertile ground to merge the realms of optics and transport, motivated by the availability of excitations that are both electrically tunable and couple strongly to light. 25−33 This sparked intense research activities, including demonstrations of trion diffusion, 25,30,34 electrically driven, optically detected currents, 26,28,29,32,33 observations of magneto-transport, 27 and efficient acceleration of polaron−polaritons in optical microcavities.…”