Stability of High-Density Two-Dimensional Excitons against a Mott Transition in High Magnetic Fields Probed by Coherent Terahertz Spectroscopy

TL;DR

This study investigates the stability of high-density two-dimensional excitons in GaAs quantum wells under high magnetic fields, revealing enhanced exciton stability and dissociation behavior at elevated temperatures using terahertz spectroscopy.

Contribution

It demonstrates that high magnetic fields significantly increase exciton stability against Mott transition in 2D systems, with detailed spectroscopic evidence.

Findings

Excitons remain stable at high densities under strong magnetic fields.

The 1s-2p- intraexciton transition persists up to high densities at 9 T.

High temperatures cause exciton dissociation despite magnetic stabilization.

Abstract

We have performed time-resolved terahertz absorption measurements on photoexcited electron-hole pairs in undoped GaAs quantum wells in magnetic fields. We probed both unbound- and bound-carrier responses via cyclotron resonance and intraexciton resonance, respectively. The stability of excitons, monitored as the pair density was systematically increased, was found to dramatically increase with increasing magnetic field. Specifically, the 1$s$-2$p_-$ intraexciton transition at 9 T persisted up to the highest density, whereas the 1$s$-2$p$ feature at 0 T was quickly replaced by a free-carrier Drude response. Interestingly, at 9 T, the 1$s$-2$p_-$ peak was replaced by free-hole cyclotron resonance at high temperatures, indicating that 2D magnetoexcitons do dissociate under thermal excitation, even though they are stable against a density-driven Mott transition.