Collective Excitations in the Dilute 2D Electron System

We report inelastic light scattering measurements of dispersive spin and charge density excitations in dilute 2D electron systems reaching densities less than 10¹⁰cm^-2. In the quantum Hall state at ν0ex0ex=0ex0ex2, roton critical points in the spin inter-Landau level mode show a pronounced softening as rₛ is increased. Instead of a soft mode instability predicted by Hartree-Fock calculations for rₛ~3.3, we find evidence of multiple rotons in the dispersion of the softening spin excitations.