S. S. Sokolov

Carrier transport and localization in a one-dimensional electronic system over liquid helium

The carrier mobility in a nearly one-dimensional electronic system over liquid helium is measured. One-dimensional conducting channels are created by using the curvature of the surface of liquid helium covering a profiled dielectric substrate and applying a clamping electric field, which holds the electrons on the bottom of the liquid troughs. Measurements are made in a temperature interval of 0.5–1.6 K at linear densities in the range (0.5–2.5)×104 cm−1 at a generator voltage of 2–200 mV. It is shown that for a clean substrate the mobility of the electrons is governed by their interaction with helium atoms in the vapor and with ripplons; the results of the measurements are in satisfactory agreement with a theoretical calculation that assumes no localization. It is found that for substrates carrying a charge or having defects on the surface, the electron mobility decreases in comparison with the value for a clean substrate, and at temperatures T<1 K is either practically independent of temperature or decr...

Decay of excited surface electron states in liquid helium and related relaxation phenomena induced by short-wavelength ripplons

Decay rates of excited surface electron states on liquid helium are theoretically studied for different electron confinement potentials and in the presence of quantizing magnetic field. Contributions of both one-ripplon and two-ripplon scattering processes are analyzed. Regarding the decay rate of the first excited surface level (l=2), two-ripplon emission of short wavelength capillary waves is shown to dominate the conventional one-ripplon scattering in two distinct cases: the ambient temperature is low enough or the surface state excitation energy Δ2−Δ1 does not match an excitation energy of the in-plane motion quantized under a strong magnetic field or in a quantum dot. In these cases magnetic field and confinement cannot essentially reduce the decay rate which is of order of 106s−1 and does not depend on temperature. The importance of these findings for a microwave resonance experiment is discussed.

Electron transport in a quasi-one-dimensional channel on suspended helium films

Quasi-one-dimensional electron systems have been created using a suspended helium film on a structured substrate. The electron mobility along the channel is calculated by taking into account the essential scattering processes of electrons by helium atoms in the vapor phase, ripplons, and surface defects of the film substrate. It is shown that the last scattering mechanism may dominate the electron mobility in the low-temperature limit changing drastically the temperature dependence of the mobility in comparison with that controlled by the electron-ripplon scattering.

Plasma dispersion of multisubband electron systems over liquid helium

Density-density response functions are evaluated for nondegenerate multisubband electron systems in the random-phase approximation for arbitrary wavenumber and subband index. We consider both quasi-two-dimensional and quasi-one-dimensional systems for electrons confined to the surface of liquid helium. The dispersion relations of longitudinal intrasubband and transverse intersubband modes are calculated at low temperatures and for long wavelengths. We discuss the effects of screening and two-subband occupancy on the plasmon spectrum. The characteristic absorption edge of the intersubband modes is shifted relatively to the single-particle intersubband separation and the depolarization shift correction can be significant at high electron densities.

Coupled phonon-ripplon modes in a single wire of electrons on the liquid-helium surface

The coupled phonon-ripplon modes of the quasi-one-dimensional electron chain on the liquid helium surface are studied. It is shown that the electron-ripplon coupling leads to the splitting of the collective modes of the wire with the appearance of low-frequency modes and high-frequency optical modes starting from threshold frequencies. The effective masses of an electron plus the associated dimple for low-frequency modes are estimated and the values of the threshold frequencies are calculated. The results obtained can be used in experimental attempts to observe the phase transition of the electron wire into a quasiordered phase.

Effective mass of surface electrons in helium under non-equilibrium conditions

Abstract The mass shift of surface electrons in helium has been investigated theoretically caused by electron-ripplon interaction as a function of electromagnetic field power absorbed by a system. It is shown that the mass shift is negative at weak powers of external field but it increases as the power increases and changes its sign. The results of calculation and comparison with the experimental data testify to the favourable role of two-ripplon scattering processes in energy relaxation of surface electrons above helium.

Dissipation of the kinetic energy of a tuning fork immersed in superfluid helium at different oscillation frequencies

The drag coefficient characterizing the dissipation of the energy of oscillating tuning forks immersed in liquid helium is studied experimentally. The experiments are done at temperatures from 0.1 to 3.5 K, a range that covers both hydrodynamic flow and the ballistic transport of thermal excitations in superfluid helium below 0.6 K. It is found that a frequency dependence of the drag coefficient exists in the hydrodynamic limit, where the main dissipation mechanism is viscous friction of the liquid against the surface of the oscillating object at temperatures above 0.7 K. In this case, the drag coefficient is proportional to the square root of the oscillation frequency and its temperature dependence in He II is determined by the corresponding relationships between the density of the normal component and the viscosity of the liquid. At lower temperatures, there is no frequency dependence of the drag coefficient and the magnitude of the dissipative losses is determined only by the temperature dependence of ...

Decay rate of the excited surface electron states on liquid helium

The low temperature bound of the decay rate of the excited surface electron states on liquid helium is theoretically studied. It is shown that the lifetime and dephasing time of the surface electron states are strongly limited by spontaneous emission of couples of short-wavelength capillary wave quanta (ripplons). These two-ripplon scattering processes are of the second order in the nonlinear interaction Hamiltonian. In contrast to the usual one-ripplon scattering contribution, the decay rate found here cannot be substantially reduced, neither by lowering of the temperature nor by external magnetic field, which is important for a recently discussed implementation of quantum bits in such a system.

Transport properties of surface electrons in helium on a structured substrate

A zero-dimensional electron system is proposed and realized on superfluid helium in cylindrical macropores of a structured substrate made of silicon which undergoes a transition to an insulating state at liquid helium temperatures. It is shown that in the presence of holding electric field the depth of the potential well for an electron over a spherically concave surface of helium depends strongly on the radius of the liquid surface, which makes it possible to vary widely the system parameters. The conductivity of surface electrons on a structured substrate was measured. The experiments were carried out in the temperature range T = 0.5–1.6 K for the electron density from 2.6·106 to 8·108 cm−2 for the holding electric fields up to 10 V/cm. It is established that the character of the charge transfer on helium is highly dependent on both the carrier concentration and the curvature radius of liquid filling the substrate macropores. For a large curvature radius and thus a relatively large thickness of the heli...

Features of the transport of quasi-one-dimensional surface electrons in dense gaseous helium

The results of an experimental investigation of the mobility of surface electrons localized in quasi-two-dimensional conducting channels in liquid helium on a profiled substrate are presented. The electron mobility is measured in a temperature region corresponding to scattering of particles on helium atoms in the vapor phase. It is shown that with increasing temperature the mobility decreases monotonically in proportion to the change in helium vapor density, then suffers a sharp drop that the authors attribute to the possible formation of a surface anion—electron polaron formation in the dense helium vapor. The results are in agreement with the results on the mobility of two-dimensional and three-dimensional electron systems in helium vapor.