The search for Bose–Einstein condensation of excitons in Cu₂O: exciton-Auger recombination versus biexciton formation

Abstract

Excitons in high-purity crystals of Cu 2 O undergo a density-dependent lifetime that opposes Bose-Einstein condensation (BEC). This rapid decay rate of excitons at a density n has generally been attributed to Auger recombination having the form = − n t An d d 2 , where A is an exciton-Auger constant. Various measurements of A, however, have reported values that are orders-of-magnitude larger than the existing theory. In response to this conundrum, recent work has suggested that excitons bind into excitonic molecules, or biexcitons, which are short-lived and expected to be optically inactive. Of particular interest is the case of excitons confined to a parabolic strain well-a method that has recently achieved exciton densities approaching BEC. In this paper we report time-and space-resolved luminescence data that supports the existence of short-lived biexcitons in a strain well, implying an exciton loss rate of the form = − n t Cn d d 2 2 with a biexciton capture coefficient C(T) proportional to T 1 , as predicted by basic thermodynamics. This alternate theory will be considered in relation to recent experiments on the subject. 3 Drift mobilities of excitons in high purity Cu 2 O are 10 7 cm 2 eV −1 s −1 at 1.2 K, corresponding to diffusivities of 1000 cm 2 s −1 , measured by [1]. 4 For theoretical reviews of the spectroscopic properties of forbidden-gap semiconductors, see [2]. 5 BEC of excitons was initially proposed by Blatt et al [5] and reviewed by Moskalenko et al.

Vacancy relaxation in cuprous oxide (Cu2−O1−)

“…Cuprous oxide has a more convenient phase diagram than Ag 2 O [15,16]. Therefore, cuprous oxide is a favored material for the investigation of the fundamentals of Wannier-Mott excitons, including the scattering of excitons with other excitons [17,18], phonons [19], and photons [20], or the formation of Bose-Einstein condensates [21,19]. Using samples the same or similar to those reported here, we previously investigated exciton photoionization [20] and multiphoton excitation of excitons [22].…”

Multicomponent exciton gas in cuprous oxide: cooling behaviour and the role of Auger decay

“…The loss of excitons of the i-th species can then be described by the collision term (17) with constant Auger rates A ij .…”

“…First, the hydrodynamics will be reformulated for a multicomponent exciton gas. Second, the problem of the Auger-like two-body decay will be investigated more in detail, aiming both at a more elaborated calculation of the Auger coefficient as well as at comparing the impact of the Auger process on the hydrodynamic evolution to an alternative mechanism based on a transient biexciton formation [17].…”

Biexciton as a Feshbach resonance and Bose–Einstein condensation of paraexcitons in Cu2O

“…The loss is conventionally attributed to Auger recombination, but its nature has not been elucidated [19,22]. Later, the participation of a biexciton [23,24] and an inelastic collision of paraexcitons [18] have been suggested, but their microscopic mechanisms are still open for making clear. In general, despite the recent experimental progress toward paraexciton BEC [10][11][12][13], a fundamental issue of the problem-the interparticle interaction remains unsolved.…”