Superfluidity of "dirty" indirect excitons and magnetoexcitons in two-dimensional trap

TL;DR

This paper investigates how disorder, external potential, and magnetic fields influence the superfluid phase transition of indirect excitons in two-dimensional systems, revealing conditions for superfluidity and its suppression.

Contribution

It develops a theoretical framework to analyze superfluidity of excitons in disordered 2D traps, incorporating local density approximation and magnetic field effects, which is novel.

Findings

Superfluidity exists near the trap center and shrinks with increasing temperature.

Disorder raises the minimum exciton number needed for superfluidity and can destroy it.

Strong magnetic fields reduce superfluid component by increasing exciton effective mass.

Abstract

The superfluid phase transition of bosons in a two-dimensional (2D) system with disorder and an external parabolic potential is studied. The theory is applied to experiments on indirect excitons in coupled quantum wells. The random field is allowed to be large compared to the dipole-dipole repulsion between excitons. The slope of the external parabolic trap is assumed to change slowly enough to apply the local density approximation (LDA) for the superfluid density, which allows us to calculate the Kosterlitz-Thouless temperature $T_{c}(n(r))$ at each local point $r$ of the trap. The superfluid phase occurs around the center of the trap ($\mathbf{r}=0$) with the normal phase outside this area. As temperature increases, the superfluid area shrinks and disappears at temperature $T_{c}(n(r=0))$. Disorder acts to deplete the condensate; the minimal total number of excitons for which superfluidity exists increases with disorder at fixed temperature. If the disorder is large enough, it can destroy the superfluid entirely. The effect of magnetic field is also calculated for the case of indirect excitons. In a strong magnetic field $H$, the superfluid component decreases, primarily due to the change of the exciton effective mass.