Studying the Coherence Properties of Excitonic Bose-Einstein Condensates

Abstract

An exciton is a bound pair of negatively charged electron and positively charged hole, which is formed by mutual coulomb attraction. In the simplest of approximations, it can be likened to a modified hydrogen atom. Usually, it is expected to be a composite boson. This analogy facilitates the exploration of intriguing phase transitions such as Bose-Einstein Condensation (BEC), which is anticipated at temperatures (few to few 100 Kelvin) much higher than those typically observed for bosonic atom BECs which is typically in the nano-Kelvin to micro-Kelvin range. Despite numerous efforts to investigate excitonic BEC, results have often been inconsistent or have necessitated extremely low temperatures in the millikelvin range. This challenge stems from the tendency of excitons to undergo radiative and non-radiative recombination, hindering the realization of pure excitonic BEC. Thus, the pursuit of pure excitonic BEC remains ongoing. In this project, we investigated the presence of Excitonic Bose-Einstein Condensate (BEC) in a 0D-2D Mixed Dimensional III-IV semiconductor. Our approach involved measuring the 2D transverse spatial coherence function and the 2D momentum space distribution of light emitted from exciton recombination, as BEC excitons are expected to exhibit long-range spatial order and occupy a narrow momentum space. To achieve this, we developed a modified Michelson Interferometer capable of measuring the 2D spatial coherence function. Utilizing the Wiener-Khintchine Theorem, we calculated the 2D momentum space distribution of the light. This setup offers advantages over previous methods for measuring spatial coherence, as it employs simple components that are easy to control and provides significant control over the additional phase introduced into the light, which was lacking in many previous attempts. Additionally, we conducted photo-luminescence spectroscopy and photo-capacitance experiments to gain further insight into the physics of our sample. We also explored the feasibility of measuring spatial coherence through photo-capacitance measurements. Such a study can be likened to investigating the matter wave interference of two populations of BEC excitons. Through our various measurements, it became apparent that each method probes different populations of excitons. By simultaneously conducting these measurements, we were able to explore the rich dynamics of different excitonic populations in our sample.