V. E. Sivokon

2D Wigner Solid over Liquid Helium: Resonant and Non–Linear Phenomena

Results are presented of an experimental study of coupled phonon–ripplon resonances in the Wigner crystal over a liquid helium surface in high driving fields. The resonant frequencies are shown to be in a good agreement with existing theoretical results in the limit of low exciting signal. Novel additional resonances appear at low frequencies for high driving fields. The mobility of the Wigner crystal at fixed frequency, measured as a function of driving field, is essentially nonmonotonic showing a mobility jump in high enough driving fields.

Nonlinear surface electron transport over liquid helium

We present experimental data and a theoretical analysis of the nonequilibrium mobility of surface electrons in liquid helium. The experiments are carried out in the temperature range where electron mobility is limited by electron scattering on surface excitations of the liquid helium (ripplons). Holding and driving electric fields of wide ranges are used in the measurements. Special attention is paid to the condition of strong holding fields, under which hot electrons are confined to the ground surface level. Depending on the relation between the momentum relaxation rate and electron-electron collision frequency, different theoretical approaches are used to describe the nonlinear mobility of surface electrons. The results obtained allow one to estimate the range of physical parameters in which experimental data can be described by the theory of nonlinear electron transport within the ground surface level.

Conductivity of a two-dimensional electronic crystal over 3He–4He liquid solutions

Experimental data on and a theoretical analysis of the conductivity of a two-dimensional Wigner crystal (WC) on the surface of weak solutions of quantum liquids 3He–4He (x<0.25% 3He) are presented. The conductivity is determined using resonance excitation of low-frequency coupled longitudinal phonon-ripplon modes of a WC. A strong temperature dependence of the resistance of a WC, differing substantially from that found previously for a nondegenerate electron gas, is found. At the same time the experimental dependence of the resistance on the impurity concentration is found to be quite weak, which shows that the scattering of transverse phonons by ripplons plays the dominant role in the resistance of a two-dimensional WC for x<0.25%.

Non-linear mobility of two-dimensional electron layer over liquid helium

The dependence of the mobility on driving electric field for electrons localized in two-dimensional electron layer over liquid helium surface is studied experimentally at temperatures below 1 K in the region of electron-ripplon scattering. The measurements are carried out at electron concentrations lower than those leading to a screening the holding electric field by the field of electron layer (unsaturated case). The electron mobility in ground surface level is shown to be monotonically increasing function of driving field. The results of the investigation agree with theoretical dependences obtained under the supposition of coinciding the short-wave region of ripplon spectrum with long-wave asymptotics.

Effect of a guiding electric field on the melting of a 2D electron crystal above liquid helium

The conductivity of a two-dimensional electron system with surface density 8⋅108cm−2 and 12⋅108cm−2 above a liquid-helium surface is investigated near a phase transition into the crystalline state (Wigner crystal). The measurements are performed in the frequency range 3–6MHz for different amplitudes of a guiding field. The phase transition was detected according to a sharp change of the amplitude and phase of the response signal. The temperature dependences of the real and imaginary components of the conductivity of two-dimensional system of electrons and the melting temperature are calculated. It is found that the temperature dependences of the imaginary part of the reciprocal of the conductivity, which reflect the inertial properties of the electron system, match well for different frequencies of the exciting signal, while the real component, which characterizes dissipation, increases with increasing frequency at the transition into an ordered state. It is found that at the transition into an ordered st...

Nonlinear phenomena in phonon–ripplon oscillations in a two-dimensional electron crystal over liquid helium

The characteristics of the spectra of phonon–ripplon oscillations of a two-dimensional electron crystal over liquid helium are investigated at temperatures of 70–500 mK, electron surface densities of (3–12)×108 cm−2, and clamping electric fields of 500–2300 V/cm. Experiments are done in a frequency range of 1–20 MHz. It is shown that at a low level of the exciting signal the characteristics of the spectra are well described by the self-consistent theory of phonon–ripplon oscillations in a crystal. As the exciting signal is increased there is a decrease in the frequencies of the fundamental harmonics of the oscillations, apparently because of a nonlinearity in the electron displacements in the depressions on the liquid helium surface. As the level of the exciting signal is increased further, new modes of phonon–ripplon oscillations begin to be excited in the crystal. These modes are either the theoretically predicted division resonances arising at large electron displacements or the asymmetric modes caused...

Non-linear behaviour of an electron crystal over liquid helium

Abstract Coupled phonon–ripplon oscillations in a two-dimensional electron crystal on liquid helium have been studied experimentally in a wide interval of driving electric fields E. The results obtained for low values of E are in good agreement with theory. Increasing driving electric fields leads to the excitation of non-axisymmetric modes of coupled phonon–ripplon oscillations. At high values of E phonon–ripplon resonances disappear. This effect is suggested to be explained by plastic deformation and destruction of an electron crystal.

The study of phonon-ripplon resonances in Wigner crystal over liquid helium

The positions and line widths of the phonon-ripplon resonances have been measured in Wigner crystal localized over superfluid helium surface at electron density 1.3·109 cm−2 in temperature range 0.08–0.6 K. The positions of the resonances agree with theoretical models available at temperatures not far from melting point. The line width decreases linearly with lowering temperature.