Magneto-Oscillations and Anomalous Current States in a Photoexcited Electron Gas on Liquid Helium

Abstract

The paper reviews a novel class of phenomena observed recently in the two-dimensional (2D) electron system formed on the free surface of liquid helium in the presence of a magnetic field directed normally and exposed to microwave radiation. The distinctive feature of these nonequilibrium phenomena is magnetoconductivity oscillations induced by inter-subband (out-of-plane) and intra-subband (in-plane) microwave excitations. The conductivity magneto-oscillations induced by intra-subband excitation are similar to remarkable microwave-induced resistance oscillations (MIRO) reported for semiconductor heterostructures. Investigations of microwave-induced conductivity oscillations (MICO) on liquid helium helped with understanding of the origin of MIRO. Much stronger microwaveinduced conductivity oscillations were observed and well described theoretically for resonant inter-subband microwave excitation. At strong powers, such excitation leads to zero-resistance states (ZRS), the in-plane redistribution of electrons, self-generated audio-frequency oscillations, and incompressible states. These phenomena are caused by unusual current states of the 2D electron system formed under resonant microwave excitation.Keywords Magneto-oscillations · 2D electron gas · Liquid helium 1 Introduction A two-dimensional (2D) electron gas on the free surface of liquid helium is formed by a one-dimensional (1D) potential well V (z) created by a weak polarization attraction of an electron to the liquid medium and by a strong repulsion barrier

Visualization of Oscillatory Electron Dynamics on the Surface of Liquid Helium

“…3(a) is almost independent of V and n e . It may arise from the interaction of surface electrons with gravity waves, also known as ripplons [23,27]: for radially symmetric modes, the lowest resonant frequency is f ≈ 0.609 √ gd/R, where g is the acceleration due to gravity, and d and R are respectively the height and radius of the circular liquid pool [33]. This yields 5.28 Hz, comparable with the observations.…”

“…In conclusion, we have confirmed that the main kHz signals are generated by inter-edge magnetoplasmons [34] modulated by interaction with surface ripplons at about 5 Hz [23,27]. Note that, in attributing the ∼ 5 Hz modulation to the effect of ripplons, we are effectively taking account of vertical motion, i.e.…”

“…2(c) that the basic frequency is varying. Similar behaviour was observed earlier using a single electrode [23,26,27]. The instantaneous frequency of each mode can be determined by ridge extraction [28], which traces in time the loci of the highest amplitude/power.…”

“…3(d) shows how the mean oscillation frequency for C changes with V for different n e . We note that the mean frequency starts to increase at V ∼ 4.19 V when the electron density is low with only a minimal difference between the maxima and minima of the oscillations (red shadow), perhaps due to an accumulation of electrons under the edge electrodes [27] (SM).…”

“…For a circular pool of surface electrons, currents induced at the center of a Corbino electrode geometry above the liquid exhibited nonlinear oscillations in the audiofrequency range. These were attributed to edge magnetoplasmons [15,[23][24][25], but there was also evidence [26,27] of an even lower-frequency modulation.…”