Evidence of a first-order quantum phase transition of excitons in electron double layers

TL;DR

This paper provides experimental evidence that the transition between excitonic insulator and Fermi-metal phases in electron double layers is a first-order quantum phase transition, influenced by electron-electron interactions and measured via inelastic light scattering.

Contribution

It demonstrates the first direct experimental evidence of a first-order quantum phase transition between excitonic and metallic phases in coupled electron layers.

Findings

Transition is discontinuous (first-order) as shown by order parameter measurements.

Low-lying excitations soften near the phase boundary.

Competing orders influence quantum fluctuations and phase stability.

Abstract

Complexity in many-particle systems occurs through processes of qualitative differentiation. These are described by concepts such as emerging states with specific symmetries that are linked to order parameters. In quantum Hall phases of electrons in semiconductor double layers with large inter-layer electron correlation there is an emergent many body exciton phase with an order parameter that measures the condensate fraction of excitons across the tunneling gap. As the inter-layer coupling is reduced by application of an in-plane magnetic field, this excitonic insulating state is brought in competition with a Fermi-metal phase of composite fermions (loosely, electrons with two magnetic flux quanta attached) stabilized by intra-layer electron correlation. Here we show that the quantum phase transformation between metallic and excitonic insulating states in the coupled bilayers becomes discontinuous (first-order) by impacts of different terms of the electron-electron interactions that prevail on weak residual disorder. The evidence is based on precise determinations of the excitonic order parameter by inelastic light scattering measurements close to the phase boundary. While there is marked softening of low-lying excitations, our experiments underpin the roles of competing orders linked to quasi-particle correlations in removing the divergence of quantum fluctuations.