Circular-Polarization-Dependent Study of Microwave-Induced Conductivity Oscillations in a Two-Dimensional Electron Gas on Liquid Helium

Abstract

The polarization dependence of photoconductivity response at cyclotron-resonance harmonics in a nondegenerate two-dimensional (2D) electron system formed on the surface of liquid helium is studied using a setup in which a circular polarization of opposite directions can be produced. Contrary to the results of similar investigations reported for semiconductor 2D electron systems, for electrons on liquid helium, a strong dependence of the amplitude of magnetoconductivity oscillations on the direction of circular polarization is observed. This observation is in accordance with theoretical models based on photon-assisted scattering and, therefore, it solves a critical issue in the dispute over the origin of microwave-induced conductivity oscillations.PACS numbers: 73.63.Hs, 78.20.Ls, Studies of microwave (MW) photoconductivity in a two-dimensional (2D) electron gas of semiconductor heterostructures subjected to a perpendicular magnetic field B have revealed remarkable magnetotransport phenomena: giant microwave-induced resistance oscillations (MIRO) [1,2] and associated zero resistance states (ZRS) [3,4]. These discoveries have opened a prominent research area and triggered a large body of theoretical works. The universality of the effect of MIRO was proved by similar observations in hole systems [5] and in a nondegenerate 2D electron system formed on the free surface of liquid helium [6].It is very surprising that by now there is a great body of different theoretical mechanisms explaining MIRO which use quantum and classical effects (for a review, see [7]), but the origin of these oscillations is still under debate. Among these mechanisms there is a large group of models whose description is based on the concept of the photonassisted scattering off disorder which overcomes the selection rules existing for direct photon-induced transitions; direct transitions can be only between adjacent Landau levels: n ′ − n = ±1 . The photon-assisted scattering leads to two important effects. Firstly, it gives a separate contribution to magnetoconductivity σ xx where the displacement of the electron orbit center X ′ − X caused by energy conservation changes its sign when the ratio ω/ω c passes an integer m = n ′ − n = 1, 2, ... (here ω is the MW frequency and ω c is the cyclotron frequency) [8,9]. This mechanism is called the displacement model (DM).Secondly, electron scattering to higher Landau levels (n ′ − n = 2, 3...) changes the distribution function of electrons at these levels f (ε): it acquires an oscillatory form with maxima, and, therefore, a sort of population inversion occurs [10,11]. This mechanism is called the inelastic model (IM) because the inelastic thermalization rate is an important quantity for the description of the effect. The maxima of f (ε) affect the contribution to σ xx caused by usual scattering processes and also lead to a sign-changing correction to σ xx .Both theoretical models (DM and IM) give satisfactory descriptions of MIRO in semiconductor electron systems. Still, there is a critical unresolve...

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

“…23, the open-rhombus symbols show the result of calculations performed for polarization parameters a ± and b ± estimated from the photocurrent response at CR conditions. Thus, the dependence of MICO on the direction of circular polarization of the MW observed [37] is in a good (even numerical) agreement with the theory based on photon-assisted scattering.…”

“…The pool of 2D electrons (not shown) is formed on the surface of liquid helium above electrodes C 1 (central) and C 2 (middle)[36] Color online) σxx of SE at T = 0.2 K and ne = 5.1·10 6 cm −2 vs. B for two directions of circular polarization of the MW field (ω/2π = 35.213 GHz): p = + (red solid) and p = − (blue dashed). Black triangles indicate the values of B such that ω/ωc = m[37]…”

Can Warmer than Room Temperature Electrons Levitate Above a Liquid Helium Surface?

“…Electrons on helium provide a dilute electron system with well-understood disorder potential and weak screening effects thus comparison with theory is simplified compared to GaAs. For example MIRO on electrons on helium strongly depend on the direction of the circular polarisation as expected for non-interacting electrons [21]. In contrast almost no circular polarisation dependence is observed in GaAs [22][23][24], even if a dependence on the orientation of the linear polarization is present [25,26].…”

Strong coupling of a two-dimensional electron ensemble to a single-mode cavity resonator

“…The resonant coupling of electrons to the LHCP component of MW field is possible due to the secondorder processes accompanied by simultaneous scattering of electrons from ripplons, which, for example, give rise to the observed conductivity response of electrons on helium at the harmonics of the cyclotron resonance. 48 However, this contributes only a small fraction of ν/ω c 10 −3 to the electron conductivity, an effect comparable to the counter-rotating terms neglected in (17) under the RWA. Thus, it is unlikely that the counter-rotating component of field can cause effects shown in Fig.…”