Arbitrary-coupling electron-ripplon system on liquid-helium films

Two-dimensional electrons interacting with the ripplon field on the surface of a thin film of liquid helium is investigated by introducing a modified Lee-Low-Pines transformation that depends explicitly on the coupling constant. The theory yields correct results in the weak- and strong-coupling limits. Ground-state properties of the ripplonic polaron are calculated as functions of the coupling constant. It is found that the ground-state energy varies smoothly with the coupling constant throughout the whole range, and there is no indication of any phase-transition-like abrupt change. However, there exists a transition region when a quasi-free-polaron changes to the self-trapped state.