Charge transport and phase transition in exciton rings

TL;DR

This paper explains the formation of macroscopic exciton rings in coupled quantum wells through experiments and theory, revealing a phase transition to an ordered exciton state at very low temperatures.

Contribution

It introduces a new understanding of exciton ring formation and identifies a second order phase transition to an ordered state at sub-Kelvin temperatures.

Findings

Observation of exciton rings as cold exciton sources

Identification of a second order phase transition at low temperatures

Ring fragmentation into a periodic array of exciton aggregates

Abstract

The macroscopic exciton rings observed in the photoluminescence (PL) patterns of excitons in coupled quantum wells (CQWs) are explained by a series of experiments and a theory based on the idea of carrier imbalance, transport and recombination. The rings are found to be a source of cold excitons with temperature close to that of the lattice. We explored states of excitons in the ring over a range of temperatures down to 380 mK. These studies reveal a sharp, albeit continuous, second order phase transition to a low-temperature ordered exciton state, characterized by ring fragmentation into a periodic array of aggregates. An instability at the onset of degeneracy in the cold exciton system, due to stimulated exciton formation, is proposed as the transition mechanism.