We report inelastic light scattering measurements of dispersive spin and charge density excitations in dilute 2D electron systems reaching densities less than ${10}^{10}{\mathrm{cm}}^{-2\mathrm{}}$. In the quantum Hall state at $\nu =2\mathrm{}$, 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 $r_s\sim 3.3$, we find evidence of multiple rotons in the dispersion of the softening spin excitations.

• Received 22 October 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2163