Plasma Resonance of the Wigner Solid on the Free Surface of Normal and Superfluid ³He

The Wigner solid on liquid ³He is used as a sensitive probe for the study of the surface dynamics. We have experimentally investigated the behavior of optical coupled plasmon-ripplon (CPR) resonances for temperatures down to 0.4 mK. We have used a linewidth of CPR resonance Δf for getting τ = (2πΔf)^-1, the collision time in the scattering of surface electrons from the surface roughness. The collision time is in good agreement with single-electron-ripplon scattering theory down to 30 mK, while below 30 mK the collision time deviates from the theory, and at 1 mK experimental value of τ is one order of magnitude longer than the theoretically predicted one.