Studying the Excitonic Phase Transitions

Abstract

Semiconductors are the backbone of the development of modern technology. Every electronic device uses semiconductors in some of its components. Semiconductor heterostructures have allowed physicists to study various quantum mechanical phenomena and use them to develop novel technologies. The coulomb interaction between electrons and holes leads to the formation of a bound state called an exciton. The excitonic energy levels are very similar to that of a hydrogen atom. The ensemble of the excitons shows various interesting optical and electrical properties, especially in semiconductor heterostructures. The collective dynamics of exciton govern many physical, chemical, and biological properties of materials. Excitons have integer spin, and consequently, they exhibit bosonic properties at low densities. Past studies have shown that their ensemble can form Bose-Einstein condensate (BEC) at much higher temperatures than atomic BEC due to their extremely small mass. Theoretically, it has been predicted that they can demonstrate superconducting (BCS state) properties at higher number density. Numerous research studies worldwide are still ongoing to identify the experimental signatures of excitonic superconducting states.

This project focuses on studying many-body physics of excitons at high number densities in the mixed dimensional 0D-2D heterostructure (0D (InAs Quantum dots) and 2D (GaAs 2D electron gas) separated by AlAs double barrier). From past work, it is evident that the photo-capacitance and photocurrent measurements of the sample can reveal the bosonic (BEC) properties of the sample at low number density. The project aimed to study the existence of excitonic BCS state at higher number density. Previous studies suggest that photo-capacitance is a suitable probe to study the dynamics of excitons due to their dipolar nature. The excitonic dipoles contribute to the collective electrical polarisation generated by these excitons, which can be measured using capacitance. The large dipole moments and spatial separation of indirect excitons enable their lifetimes to extend to microseconds or longer. As a result, if excitonic Bose-Einstein condensate (BEC) forms, it is not quickly depleted through radiative recombination. The number of excitons is dependent on the intensity of illumination. In the study, at higher illumination intensities (corresponding to higher density of excitons), splittings in the peaks of oscillations of photo-capacitance are observed. As these splittings appear at higher light intensities, we aim to establish a direct experimental connection between the BCS state and these splittings. In the course of our experiments, we seem to have observed abrupt phase transition as a function of increasing exciton density and temperature. We also investigated the DC quantum conductance of the sample. Interestingly, it showed the properties of fractional quantum conductance. To understand it further, we fitted the curve of DC conductance to estimate its quantized step size, which reaches close to integer steps of G_0=(2e^2)/h at higher photo-excitation intensities. We also conducted time-resolved experiments to investigate the Rabi oscillations in the time domain between the states of the two-component BEC of indirect excitons. By simultaneously conducting these measurements, we were able to explore the rich dynamics of different phases of excitonic populations in our heterostructure sample.