Mode Mixing in Antiferromagnetically Correlated Double Quantum Wells

TL;DR

This paper investigates the stability of an exchange-induced antiferromagnetic phase in double quantum wells, showing it persists under realistic conditions and can be detected via Raman spectroscopy.

Contribution

It demonstrates the robustness of the antiferromagnetic phase using self-consistent Hartree-Fock calculations and explores how asymmetry affects experimental signatures.

Findings

Antiferromagnetic phase survives intra-subband repulsion effects.

Asymmetry induces mode coupling, altering Raman signatures.

Cusp in Raman response signals the phase transition.

Abstract

We examine the robustness of a recently predicted exchange-induced zero-field magnetic phase in semiconductor double quantum wells in which each well is spin-polarized and the polarization vectors are antiparallel. Magnetic instabilities are a general feature of Coulombic double quantum well systems at low densities. We argue that this antiferromagnetic phase is stabilized relative to ferromagnetic ones by an effective superexchange interaction between the wells. Detailed self-consistent Hartree-Fock calculations using a point-contact model for the interaction show that the antiferromagnetic phase survives intra-subband repulsion matrix elements neglected in earlier work in a large portion of the model's parameter space. We also examine the role of asymmetry due to biasing or to differences in the widths of the two quantum wells. The asymmetry creates a mode coupling between the intra- and inter-subband collective spin-density excitations (SDEs) which changes the Raman spectroscopy signature of the phase transition from a complete softening of the inter-subband SDE to a cusp as the density is tuned through the transition. This cusp may be detectable in inelastic light scattering experiments in samples of sufficient quality at low enough temperatures and densities.