Microwave-resonance-induced magnetooscillations and vanishing resistance states in multisubband two-dimensional electron systems

The dc magnetoconductivity of the multisubband two-dimensional electron system formed on the liquid helium surface in the presence of resonant microwave irradiation is described, and a new mechanism of the negative linear response conductivity is studied using the self-consistent Born approximation. Two kinds of scatterers (vapor atoms and capillary wave quanta) are considered. Besides a conductivity modulation expected near the points, where the excitation frequency for inter-subband transitions is commensurate with the cyclotron frequency, a sign-changing correction to the linear conductivity is shown to appear for usual quasi-elastic inter-subband scattering, if the collision broadening of Landau levels is much smaller than thermal energy. The decay heating of the electron system near the commensurability points leads to magnetooscillations of electron temperature, which are shown to increase the importance of the sign-changing correction. The line-shape of magnetoconductivity oscillations calculated for wide ranges of temperature and magnetic field is in a good accordance with experimental observations

The fine structure of microwave-induced magneto-oscillations in photoconductivity of the two-dimensional electron system formed on a liquid-helium surface

The influence of the inelastic nature of electron scattering by surface excitations of liquid helium (ripplons) on the shape of magnetoconductivity oscillations induced by resonance microwave (MW) excitation is theoretically studied. The MW field provides a substantial filling of the first excited surface subband which sparks off inter-subband electron scattering by ripplons. This scattering is the origin of magneto-oscillations in the momentum relaxation rate. The inelastic effect becomes important when the energy of a ripplon involved compares with the collision broadening of Landau levels. Usually, such a condition is realized only at sufficiently high magnetic fields. On the contrary, the inelastic nature of inter-subband scattering is shown to be more important in a lower magnetic field range because of the new enhancement factor: the ratio of the inter-subband transition frequency to the cyclotron frequency. This inelastic effect affects strongly the shape of conductivity oscillations which acquires an additional wavy feature (a mixture of splitting and inversion) in the vicinity of the level-matching points where the above noted ratio is close to an integer

Polarization dependence of microwave-induced magnetoconductivity oscillations in a two-dimensional electron gas on liquid helium

The dependence of radiation-induced dc magnetoconductivity oscillations on the microwave polarization is theoretically studied for a two-dimensional system of strongly interacting electrons formed on the surface of liquid helium. Two different theoretical mechanisms of magnetooscillations (the displacement and inelastic models) are investigated. We found that both models are similarly sensitive to a change of circular polarization, but they respond differently to a change of linear polarization. Theoretical results are compared with the recent observation of a photoconductivity response at cyclotron-resonance harmonics

Photon-assisted scattering and magnetoconductivity oscillations in a strongly correlated 2D electron system formed on the surface of liquid helium

The influence of strong internal forces on photon-assisted scattering and on the displacement mechanism of magnetoconductivity oscillations in a two-dimensional (2D) electron gas is theoretically studied. The theory is applied to the highly correlated system of surface electrons on liquid helium under conditions that the microwave frequency is substantially different from inter-subband resonance frequencies. A strong dependence of the amplitude of magnetoconductivity oscillations on the electron density is established. The possibility of experimental observation of such oscillations caused by photon-assisted scattering is discussed

Density domains of a photo-excited electron gas on liquid helium

The Coulombic effect on the stability range of the photo-excited electron gas on liquid helium is shown to favor formation of domains of different densities. Domains appear to eliminate or greatly reduce regions with negative conductivity. An analysis of the density domain structure allows explaining remarkable observations re-ported recently for the photo-excited electron gas

Microwave-induced magnetooscillations and absolute negative conductivity in the multisubband twodimensional electron system on liquid helium

It is shown that a nonequilibrium filling of an upper surface subband induced by the microwave resonance can be the origin of the absolute negative conductivity and zero-resistance states for the two-dimensional electron system on liquid helium under magnetic field applied normally. Contrary to the similar effect reported for semiconductor systems, an oscillating sign-changing correction to the dc-magnetoconductivity appears due to quasi-elastic inter-subband scattering which does not involve photons. The analysis given explains remarkable magnetooscillations and zero-resistance states recently observed for electrons on liquid helium

Temperature bistability in a 2D electron system on liquid helium induced by Coulomb interaction under cyclotronresonance excitation

The energy balance of strongly interacting surface electrons on liquid helium under cyclotron-resonance excitation is theoretically studied. The Coulomb interaction is shown to induce temperature bistability of the electron system, if the magnetic field and electron density are high enough. Surprisingly, bistability appears already for quite low average kinetic energies, when nearly all electrons occupy the ground surface subband. The electron temperature Te, as the function of the magnetic field B, exhibits hysteresis and bistability jumps in a certain range of the microwave power. Above the threshold microwave field, the line shape Te(B) is shown to be sensitive to details of the ripplon dispersion at large wave numbers

Coulombic effects on magnetoconductivity oscillations induced by microwave excitation in multisubband two-dimensional electron systems

We develop a theory of magneto–oscillations in photoconductivity of multisubband two-dimensional electron systems which takes into account strong Coulomb interaction between electrons. In the presence of a magnetic field oriented perpendicular, internal electric fields of fluctuational origin cause fast drift velocities of electron orbit centers which affect probabilities of inter-subband scattering and the photoconductivity. For the electron system formed on the liquid helium surface, internal forces are shown to suppress the amplitude of magnetooscillations, and change positions of magnetoconductivity minima which evolve in zero-resistance states for high radiation power

Low-frequency conductivity of a two-dimensional Wigner solid coupled to surface excitations of liquid helium

The line-shape and broadening of coupled phonon-ripplon resonances of Wigner-solid conductivity are studied using the memory function formalism. The analytic properties of the memory function permit coordinating the approximations of the secular equation for the coupled phonon-ripplon modes and the line-broadening of these resonances. Special attention is paid to the description of the strong-coupling regime realized for surface electrons on superfluid helium. For this case it is shown that the line-broadening is much smaller than and the line-shape is different from those found previously using the weak-coupling theory. Different theoretical approaches are compared with available experimental data

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 temperature nor by external magnetic field, which is important for recently discussed implementation of quantum bits in such a system