Collective excitations in the dilute 2D electron system

We report inelastic light scattering measurements of dispersive spin and charge density excitations in dilue 2D electron systems reaching densities less than 10 sup (10) cm lsup (-2). In the quantum Hall state at v = 2, roton critical points in the spin inter-Landau level mode show a pronounced softening as r sub s is increased. Instead of a soft mode instability predicted by Hartree-Fock calculations for r sub 8 ~ 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 sub x > over ~ 11.