Observation of Collective Excitations of the Dilute 2D Electron System

TL;DR

This study uses inelastic light scattering to explore collective excitations in dilute 2D electron systems, revealing multiple rotons and potential instabilities at high interaction strengths.

Contribution

First observation of dispersive spin and charge excitations in dilute 2D electron systems at very low densities, challenging existing theoretical predictions.

Findings

Detection of roton softening in spin excitations at nu=2

Evidence of multiple rotons in the dispersion relation

Potential instability at high r_s values (~11)

Abstract

We report inelastic light scattering measurements of dispersive spin and charge density excitations in dilute 2D electron systems reaching densities less than 10^{10} cm^{-2}. In the quantum Hall state at nu=2, roton critical points in the spin inter--Landau level mode show a pronounced softening as r_s is increased. Instead of a soft mode instability predicted by Hartree--Fock calculations for r_s ~ 3.3, we find evidence of multiple rotons in the dispersion of the softening spin excitations. Extrapolation of the data indicates the possibility of an instability for r_s >~ 11.