Microwave absorption saturation and decay heating of surface electrons on liquid helium

Abstract

The microwave (MW) resonance absorption and decay heating of surface electrons (SEs) on liquid 4 He are theoretically studied for the vapor atom scattering regime. The decay heating is shown to be an essential occurrence of a MW resonance experiment appearing even at low excitation rates. It strongly affects the occupancies of surface levels and the broadening of resonance lines long before the absorption suturation condition is reached. Contrary to the model of cold SEs usually used for description of the MW resonance, the new theory leads to MW absorption saturation when only a very small fraction of electrons (less than 10%) is left on the ground and the first excited levels. The microwave (MW) resonance absorption experiment [1] had given the first direct observation of image-potential-induced Rydberg levels outside liquid helium. For weak holding fields E^, positions of surface levels were shown to be well described by the simple for-/ 2 , where l = 1 2 , , ... , and D R is the corresponding Rydberg energy which is about 7.6 K for liquid 4 He. Because the dielectric constant of liquid helium e is close to unity (e -1 0 057; . ), the effective Bohr radius of these Rydberg levels,, is large (about 76 C). Therefore, surface electrons (SEs) can move freely along the interface, forming a remarkable two-dimensional (2D) electron liquid, useful for studying many-body effects in a highly correlated 2D electron gas (for a review see [2]).The recent interest in the MW resonance is evoked by experimental development of the idea of using SEs as electronic qubits (see [3] and references there in). These qubits would be controlled by the MW field whose frequency w is close to w 0 2 1tem is tuned to resonance by varying E^). Since the electron potential is anharmonic, it is assumed that coupling to outer levels (l > 2) is negligible, and a simple two-level model is an excellent approximation. As an important achievement in this field, observation of MW absorption saturation of Rydberg states of SEs was reported [3]. The electron decay from the excited surface level to the ground level due to scattering by vapor atoms and ripplons is a negative factor for SE qubits. MW absorption saturation appears when the stimulated absorption (emission) rate r is larger than the decay rate of the first excited level n t 2 1 1 ® º / . The dimensionless parameter rt could be considered as a measure of electron excitation by MW. In the simple model of cold SEs [3], for low excitation rt << 1, the fractional occupancy of the first excited level n 2 is much smaller than that of the ground level n 1 . In this case, power absorbed by electrons from the filed P A increases linear with r, or with the in-put power P in .

Can Warmer than Room Temperature Electrons Levitate Above a Liquid Helium Surface?

“…More generally, the study of non equilibrium phenomena in electrons on helium is strongly connected with the prospect of using Rydberg states [32,33], created by the interaction of electrons with their image charge inside liquid helium, for quantum computing [34]. So the relaxation times for excited states and the absorption lineshapes have all been carefully investigated [35][36][37][38][39][40]. From this perspective the possibility of creating overheated electrons by excitation of cyclotron resonance that was reported in [41] is highly interesting since it challenges the view that energy relaxation rates are all relatively fast in the microsecond range [42] due to two riplon emission processes [43] which should prevent an overheating of the electrons.…”