Towards Bose–Einstein condensation of excitons in potential traps

Abstract

An exciton is an electron-hole bound pair in a semiconductor. In the low-density limit, it is a composite Bose quasi-particle, akin to the hydrogen atom. Just as in dilute atomic gases, reducing the temperature or increasing the exciton density increases the occupation numbers of the low-energy states leading to quantum degeneracy and eventually to Bose-Einstein condensation (BEC). Because the exciton mass is small--even smaller than the free electron mass--exciton BEC should occur at temperatures of about 1 K, many orders of magnitude higher than for atoms. However, it is in practice difficult to reach BEC conditions, as the temperature of excitons can considerably exceed that of the semiconductor lattice. The search for exciton BEC has concentrated on long-lived excitons: the exciton lifetime against electron-hole recombination therefore should exceed the characteristic timescale for the cooling of initially hot photo-generated excitons. Until now, all experiments on atom condensation were performed on atomic gases confined in the potential traps. Inspired by these experiments, and using specially designed semiconductor nanostructures, we have collected quasi-two-dimensional excitons in an in-plane potential trap. Our photoluminescence measurements show that the quasi-two-dimensional excitons indeed condense at the bottom of the traps, giving rise to a statistically degenerate Bose gas.

Observation of long-lived interlayer excitons in monolayer MoSe2–WSe2 heterostructures

“…3a). This is a hallmark of spatially indirect excitons in gallium arsenide (GaAs) coupled quantum wells, which have been intensely studied for exciton Bose-Einstein condensation (BEC) phenomena 29 . The observation of spatially indirect interlayer excitons in a type-II semiconducting 2D HS provides an intriguing platform to explore exciton BEC, where the observed extended lifetimes and repulsive interactions are two key ingredients towards the realization of this exotic state of matter.…”

Critical currents of ideal quantum Hall superfluids

“…For example, it has long [11] been realized that spontaneous coherence at zero magnetic field is a possibility when overlapping or nearby bands have opposite quasiparticle energy vs. wavevector curvatures, the conduction and valence bands in a semiconductor or a semimetal in particular. Recent studies of optically generated electron-hole plasmas in semiconductors, do indeed hint [12] at the expected collective behavior. In a separate materials system, the discovery of weak ferromagnetism in lightly-doped divalent hexaborides [13], which are ferromagnetic despite the absence of partially filled d-or f-orbitals, led Zhitomirsky et al [14] to propose recently that spontaneous coherence between conduction and valence bands could be the mechanism responsible for their ferromagnetism.…”

Excitations in a Nonequilibrium Bose-Einstein Condensate of Exciton Polaritons

“…into (1) and (2). For small values of P , no condensate is present ψ 0 = 0 and the reservoir density is a linear function of the pump intensity n 0 R = P/γ R .…”

“…PACS numbers: 03.75.Kk, 71.36.+c, 42.65.Sf, After a few decades of impressive efforts on a variety of different systems such as bulk cuprous oxide [1] and coupled quantum wells [2], first observations of Bose-Einstein condensation (BEC) of excitons in solid state systems have been recently reported in a gas of exciton-polaritons [3] and immediately confirmed by other groups [4,5]. The system under investigation consists of a semiconductor microcavity containing a few quantum wells with an excitonic transition strongly coupled to the cavity photon mode.…”