Plasma modes in low-dimensional electron system over surface of liquid helium

The collective plasma modes in a quasi-two-dimensional (Q2D) electron system located over the free surface of liquid helium are studied theoretically within many-body dielectric formalism. The dispersion of modes is considered both over bulk liquid and over helium film where the essential modification of interelectron interaction occurs due to screening effects in the substrate with a large value of dielectric constant. It is shown that the plasma spectrum consists of longitudinal and transverse branches which dispersion laws depend on the values of the dielectric constant of helium and the film thickness. For the helium film covering metal, the longitudinal mode is acoustic differing of that for the surface electron (SE) system over bulk helium.     

Weak localization effects in a quasi-one-dimensional electron system over liquid helium

Magnetoresistance ρ_xx measurements are performed for a quasi-one-dimensional electron system over liquid helium in the gas-scattering region (the temperature range 1.3–2.0 K). The measurements show that, as the magnetic field increases, the magnetoresistance ρ_xx first decreases and then passes through a minimum and increases according to the law ρ_xx～B ². It is suggested that the negative magnetoresistance observed in the experiment is caused by the weak localization effects. The results of the experiment are in qualitative agreement with the theoretical model describing the weak localization effects in a one-dimensional nondegenerate electron system.     

Spectroscopic studies of electron surface states on liquid helium

Field-ion microscopy has been previously used [1–4] to determine the surface energy anisotropy of tungsten and iridium. Drechsler and Nicholas [5] have used field emission patterns of equilibrium shapes of emitters to determine the surface energy anisotropies for a number of fcc and bcc metals including α-iron. The purpose of the present paper is to report some observations by field-ion microscopy on the vacuum faceting of α-iron and the surface energy anisotropy at about half the absolute melting point. Qualitative results on the effect of hydrogen on the surface energy anisotropy are also presented.     

Photogalvanic current in a parabolic well

We study the in-plane stationary photocurrent in a parabolic potential well. The well has vertical asymmetry due to inhomogeneous distribution of scatterers. The electric field of light has both vertical and in-plane components. The photogalvanic effect originates from the periodic oscillation of electrons in a vertical direction caused by the normal component of the alternating electric field with simultaneous in-plane acceleration/deceleration by the in-plane component of electric field. The problem is considered in classical approximation assuming inhomogeneously-distributed friction. Photocurrent has a resonance character. Resonance occurs at light frequencies close to a characteristic well frequency. The effect of in-plane magnetic field is also studied.     

Theory of electron escape from the surface of liquid helium

We have studied the properties of an electron bubble close to the surface of liquid³ He, by using a Density Functional approach. We find that up to an electron-surface distanced ₀ 23 Åthe bubble is stable, while at smaller distances it becomes unstable and bursts. A potential energy barrier /K _B 38°K for the thermal emission of electrons is obtained from our results, in agreement with experiments. Even when the electron-surface distance is larger thand ₀, however, tunneling through the surface layer dominates the electron escape probability. Large deviations of the electron potential energy from its ideal value are found close to the surface. These deviations have a profound effect on the calculated decay rates of the tunneling curent, which are much smaller than those obtained previously and in semi-quantitative agreement with experiments.     

Photogalvanic current in a double quantum well

A double quantum well affected by external alternating electric field with in- and out-of-plane components is studied. This field causes transitions between near-degenerate states located in different wells. The phototransitions are accompanied by the in-plane momentum nonconservation caused by the impurity scattering. We study the in-plane stationary current due to the lack of the in-plane symmetry of these indirect phototransitions. It is shown that the value and direction of the current are determined by the polarization of light. The linear and circular photogalvanic coefficients are found. When the photon energy approaches the distance between subbands these coefficients have their symmetric and antisymmetric resonance behaviors, respectively.     

Bound states of an electron and a macroscopic cluster at a liquid helium surface

In a strong electric field, there are bound states of an electron at the surface of liquid helium, interacting with a large cluster of atoms in the bulk of liquid. This phenomenon is related to long-range interaction between the electron and the dipole moment of the cluster. The electron, holding the cluster under the liquid surface, is localized at this surface. One electron is capable of binding a cluster of up to 10⁶ atoms. The value of the binding energy may reach up to several kelvins.     

Image forces and electron spectrum at the surface of liquid helium

On the basis of the longitudinal field Green function, in a three-layer non-symmetrical system of media with spatial dispersion the image-force potential is calculated for surfaces of the semi-infinite helium bath and a liquid helium film, and also for a surface of the phase-separated ³He⁴He mixture. The dependence of the surface electron spectrum on the film thickness and on the pressing electric field strength is analyzed.     

基于镜像电荷法探测液氦表面电子的自旋态

文献[Phys. Rev. A, 2006, 74:052338]的理论研究表明液氦上电子自旋具有长达100秒的相干时间,因此在实施量子信息处理方面具有很大的应用前景.然而,这一理论还未得到实验的证实.液氦上电子的自旋-轨道耦合可以为电子自旋探测提供一种可选的方法.理论上,电子自旋-轨道耦合可以由液氦膜下方的微电极电流产生.在微波驱动下,电子发生电偶极跃迁,导致电子距离液氦表面的平均高度发生改变.利用镜像电荷法[Phys. Rev. Lett.,2019,123:086801],电子的这种轨道运动可以被实验测量.建立基于液氦上电子-自旋轨道耦合的密度矩阵方程,并数值求解镜像电荷的振动行为.结果表明不同的自旋态将会引起不同的感应电流,这种差异提供了一种可选的电子自旋探测方法.     

Bound states of an electron in an impurity potential on the surface of liquid helium

The energies and widths of the levels of an electron on impurity centers on the surface of liquid helium are calculated with allowance for the deformation of the surface. The level shift associated with the deformation effects is small and decreases very slowly with increasing level number. However, even a small shift of the energy levels relative to one another affects ripplon scattering, which makes the main contribution to the level width at low temperatures. It is predicted theoretically that this width depends very strongly on the external parameters and on the level number and that a maximum obtains at a clamping field E _⊥=51500 V/cm. The width of the levels of an electron in a bound state is found to be less than for free electrons. This makes it possible to perform a beautiful spectroscopic experiment. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 599–604 (10 November 1997)     

Anomalous behavior of conductivity of the confined Q1D electron system over liquid helium

Conductivity of electrons in a quasi-one-dimensional (Q1D) system over liquid helium in narrow channels with the parabolic profile of the potential well has been investigated at temperature T, from 0.4 to 1.8 K, for different driving electric fields and radius of channel curvature. The interval of linear electron densities varied from 2.18×10³ up to 1.7×10⁶ cm⁻¹.

The inverse mobility (1/μ_eff) in the electron-ripplon scattering region at the high linear densities of charges in the channel increases with temperature decreasing. This anomalous behavior of the electron transport in the low-temperature region has been explained by either the electron ordering or the polaronic effects in confined conducting channels. The nonlinear behavior of the electron velocity as a function of a driving electric field is supposed to be due to Breg–Cherenkov radiation of the ripplons. The radiation occurred if the velocity of electrons in the channel approaches to the critical value.     

A possible new electron state above liquid helium

Excited states of helium atoms have been observed in the bulk liquid. We discuss the properties of such states if created at the surface. For low quantum numbers the surface is a weak perturbation compared to the atomic excitation energy. At high quantum numbers (weakly bound states) the electron wave function is forced out of the interior of the fluid and the energy spectrum is Rydberg-like.     

New state of a 2D electron crystal above liquid helium

A transition to a new, presumably unstable, state of a two-dimensional (2D) electron crystal above liquid helium has been discovered at temperatures well below the melting point. The transition is manifested as an abrupt increase in the active component of the inverse conductivity of the crystal with a decrease in the potential pressing of the electrons to the surface. The state can be destroyed by the in-plane electric field of a sufficiently high amplitude. The new state is supposed to be a 2D electron glass.     

Properties of surface state electrons on liquid helium

Electrons on the surface of liquid helium form a two-dimensional system that is of great interest for its own unique properties as well as being a probe of the helium surface. The spectroscopic evidence for the hydrogenic nature of the surface state is compared with predictions. Measurements of the electron mobility parallel to the surface in low and high electric fields are compared with theory. The lifetime in the surface state is discussed as well as the effect of the electron on the liquid surface. The possibility that the electrons crystallize to form a two-dimensional lattice and the properties of this lattice are also analyzed.     

On the stability of a weakly charged surface of liquid helium

The traditional approach to the development of instability of a weakly charged helium surface needs correction. It is shown that the well-known electrostatically “equipotential” Frenkel-Tonks scenario should be transformed to a more general sequence of events that would remain reasonable when the 2D charge density tends to zero. Under these conditions, the priorities change and the instability development through nucleation (with the formation of separate multicharged dimples) becomes preferable. The experiment qualitatively confirms the predictions of the theory.     

Turbulence over arrays of obstacles in low temperature helium gas

Turbulence produced in low temperature helium gas flowing over arrays of rectangular- and triangular-shaped blunt obstacles is investigated experimentally. The set-up allows both low fluctuation rates (down to 8%), and high microscale Reynolds numbers, (up to 1 150). The forced Kolmogorov equation is found to apply accurately. Similar to another flow configuration (counter rotating flow case [1]), the analysis of the flatness factor evolution with the Reynolds number reveals a transitional behavior around 650. Received 26 August 1999 and Received in final form 28 August 2000