# Condensation to a strongly correlated dark fluid of two dimensional dipolar excitons

## TL;DR

This paper reports on the experimental observation of a dense, dark, strongly correlated phase of dipolar excitons in a 2D system, characterized by a phase transition with specific optical and spatial properties, and explores the underlying physical mechanisms.

## Contribution

It provides a detailed analysis of the phase transition in dipolar excitons, identifying key properties and proposing a possible physical mechanism for the observed darkening and phase behavior.

## Key findings

- Sharp increase in non-emitting dipoles during phase transition
- Contraction of the exciton fluid into the trap bottom
- Spectral narrowing indicating condensation

## Abstract

Recently we reported on the condensation of cold, electrostatically trapped dipolar excitons in GaAs bilayer heterostructure into a new, dense and dark collective phase. Here we analyze and discuss in detail the experimental findings and the emerging evident properties of this collective liquid-like phase. We show that the phase transition is characterized by a sharp increase of the number of non-emitting dipoles, by a clear contraction of the fluid spatial extent into the bottom of the parabolic-like trap, and by spectral narrowing. We extract the total density of the condensed phase which we find to be consistent with the expected density regime of a quantum liquid. We show that there are clear critical temperature and excitation power onsets for the phase transition and that as the power further increases above the critical power, the strong darkening is reduced down until no clear darkening is observed. At this point another transition appears which we interpret as a transition to a strongly repulsive yet correlated $e$-$h$ plasma. Based on the experimental findings, we suggest that the physical mechanism that may be responsible for the transition is a dynamical final-state stimulation of the dipolar excitons to their dark spin states, which have a long lifetime and thus support the observed sharp increase in density. Further experiments and modeling will hopefully be able to unambiguously identify the physical mechanism behind these recent observations.