Defect-Driven Superfluid Crossover for Two-Dimensional Dipolar Excitons Trapped at Thermodynamic Equilibrium

TL;DR

This study demonstrates the formation of a superfluid in a two-dimensional dipolar exciton system at thermodynamic equilibrium, revealing a defect-driven crossover consistent with BKT physics, based on the first equation of state measurement.

Contribution

It provides the first measurement of the exciton equation of state at equilibrium and links superfluidity onset to defect concentration, suggesting a BKT transition in 2D dipolar excitons.

Findings

Superfluid forms below 1 Kelvin in the trap center.

Defect concentration drops sharply at superfluid transition.

Spatial density profile correlates with superfluid and normal regions.

Abstract

We study ultra-cold dipolar excitons confined in a 10$\mu$m trap of a double GaAs quantum well. Based on the local density approximation, we unveil for the first time the equation of state of excitons at pure thermodynamic equilibrium. In this regime we show that, below a critical temperature of about $1$ Kelvin, a superfluid forms in the inner region of the trap at a local exciton density $n \sim 2-3 \, 10^{10} \text{cm}^{-2}$, encircled by a more dilute and normal component in the outer rim of the trap. Remarkably, this spatial arrangement correlates directly with the concentration of defects in the exciton density which exhibits a sudden decrease at the onset of superfluidity, thus pointing towards an underlying Berezinskii-Kosterlitz-Thouless mechanism.