A dark incompressible dipolar liquid of excitons

TL;DR

This paper reports the discovery of a dark, incompressible dipolar exciton liquid in two dimensions, formed below a critical temperature, exhibiting spontaneous condensation and complex phase behavior influenced by spin interactions.

Contribution

It introduces the first observation of a dark dipolar exciton liquid in two dimensions, highlighting the role of spin degrees of freedom and quantum fluctuations in its formation.

Findings

Formation of a dark exciton liquid below 4.8K

Observation of phase transition to a bright plasma at higher power

Evidence of attractive interactions beyond dipole repulsion

Abstract

The possible phases and the nano-scale particle correlations of two-dimensional interacting dipolar particles is a long-sought problem in many-body physics. Here we observe a spontaneous condensation of trapped two-dimensional dipolar excitons with internal spin degrees of freedom from an interacting gas into a high density, closely packed liquid state made mostly of dark dipoles. Another phase transition, into a bright, highly repulsive plasma is observed at even higher excitation powers. The dark liquid state is formed below a critical temperature $T_c \approx 4.8K$, and it is manifested by a clear spontaneous spatial condensation to a smaller and denser cloud, suggesting an attractive part to the interaction which goes beyond the purely repulsive dipole-dipole forces. Contributions from quantum mechanical fluctuations are expected to be significant in this strongly correlated, long living dark liquid. This is a new example of a two-dimensional atomic-like interacting dipolar quantum liquid, but where the coupling of light to its internal spin degrees of freedom plays a crucial role in the dynamical formation and the nature of resulting ground state.