We report on a novel transport phenomenon realized by optical pumping in surface state electrons on helium subjected to perpendicular magnetic fields. The electron dynamics is governed by the photon-induced excitation and scattering-mediated transitions between electric subbands. In a range of magnetic fields, we observe vanishing longitudinal conductivity, σxx → 0. Our result suggests the existence of radiation-induced zero-resistance states in the nondegenerate 2D electron system. 03.67.Lx, 73.25.+i, 78.70.Gq Electrons exhibit unique transport phenomena when they are confined in two dimensions and subjected to a strong perpendicular magnetic field. In a degenerate 2D electron gas (2DEG), the integer quantum Hall effect is characterized by exponentially small longitudinal resistivity, ρ xx → 0, and quantized Hall resistivity ρ xy [1]. In such a system, the electron transport is determined by the Fermi statistics of the charged carriers and the Landau quantization of their energy spectrum. In the quantum Hall regime, the suppression of scattering also results in vanishing diagonal conductivity, σ xx = ρ xx /(ρ 2 xx + ρ 2 xy ), and an electrical current flowing normal to the applied electric field [2]. Recently, exponentially small photoinduced resistance and conductance of a degenerate 2DEG were discovered in ultrahighmobility GaAs/AlGaAs heterostructures [3-5] causing a surge of theoretical interest in this novel phenomenon [6][7][8][9]. Here we report the occurrence of vanishing σ xx realized by inter-subband excitation in a system of nondegenerate electrons on liquid helium.Electrons on helium provide a unique classical counterpart to quantum Hall systems [10,11]. The impurity-free environment results in an extremely high electron mobility, which, for sufficiently low temperatures, is limited only by the scattering of electrons from the quantized surface vibrations (ripplons) and exceeds 10 8 cm 2 V −1 s −1 . Unlike in semiconductors, electrons on helium retain their free-particle mass and g-factor. For a bulk helium substrate, the instability of the charged surface restricts the areal density of electrons to about 2 × 10 9 cm −2 . The free-electron mass and low densities result in a very low Fermi energy, and at T = 0, the system of interacting electrons favors the classical Wigner solid over the quantum degenerate regime [12]. This makes the observation of the quantum Hall effect impossible.Surface states of electrons are formed owing to the classical image potential, the repulsive barrier that prevents penetration inside the liquid, and the electric field E ⊥ applied perpendicular to the surface. In the resulting confinement potential, the electron dynamics is quantized into discrete electric subbands with energies ǫ n (n = 1, 2, ...). At E ⊥ = 0, the energy spectrum is similar to that of the hydrogen atom, ǫ n = −R/n 2 , where the effective Rydberg constant R is approximately 7.6 and 4.2 K for liquid 4 He and 3 He, respectively. Below 1 K, almost all electrons are frozen into the lowest subband, and t...

We report the observation of novel magnetoresistance oscillations induced by the resonant intersubband absorption in nondegenerate 2D electrons bound to the surface of liquid 3 He. The oscillations are periodic in B −1 and originate from the scattering-mediated transitions of the excited electrons into the Landau levels of the first subband. The structure of the oscillations is affected by the collision broadening of the Landau levels and by many-electron effects.PACS numbers: 03.67.Lx, 73.25.+i, 78.70.Gq The dynamical response of a two-dimensional electron system (2DES) is strongly affected by the magnetic field applied perpendicular to the 2D plane. In particular, the Landau quantization of the electron energy for the inplane motion alters the transport properties of such systems and often results in oscillations of the electron magnetoresistivity. In degenerate 2DESs in semiconductors, the most well known example of such oscillations is the Shubnikov-de Haas oscillations arising from the sequential passing of Landau levels (LLs) through the Fermi level. Other examples include magnetointersubband oscillations in GaAs quantum wells [1] and microwaveinduced resistance oscillations (MIRO) in GaAs/AlGaAs heterostructures [2,3]. A novel type of resistance oscillation induced by resonant inter-subband absorption in nondegenerate 2DES is the subject of this Letter.Low-density classical 2D electrons formed on the surface of liquid helium are the complement of quantum 2DESs in semiconductors [4,5]. The difference in energy between subbands is small and inter-subband transitions can be excited with resonant millimeter-wave radiation [6]. At high temperatures, the electron in-plane transport is determined by the short-range quasi-elastic scattering from helium vapor atoms, which is similar to the scattering due to disorder in semiconductors. At low temperatures (below 0.3 K for 3 He), the scattering only occurs from surface capillary waves (ripplons). Unique correlation properties of 2DES are observed through the Wigner crystallization [7], quantum tunneling phenomena [8] and inter-subband absorption [9].At high temperatures, the magnetotransport of 2DES on helium is well described by an independent-electron theory based on the self-consistent Born approximation (SCBA) [10]. Below 1 K, the many-electron fluctuating electric field originating from the Coulomb interaction affects the electron scattering in both classically strong and quantizing magnetic fields. This was shown by Dykman et al., who presented a theory for both ripplon [11] and vapor atom scattering [12]. Many-electron effects have also been observed in quantum cyclotron resonance [13].Interest in this system also stems from the proposals for quantum computing using electrons on helium [14,15]. These proposals rely on the robust control of electronic quantum states using resonant microwaves. For such applications, microwave-induced inter-subband absorption has been recently studied for electrons on both liquid 4 He and liquid 3 He [16]. It was shown that ...

We observed an ultra-strong photovoltaic effect induced by resonant intersubband absorption of microwaves in a two-dimensional electrons system on the surface of liquid helium. The effect emerges in the regime of microwave-induced vanishing of dissipative conductance, σxx → 0, reported previously [D. Konstantinov and K. Kono: Phys. Rev. Lett. 105 (2010) 226801)] and is characterized by a nonequilibrium spatial distribution of electrons in the confining electrostatic potential. The electrostatic energy acquired by an electron exceeds other relevant energies by several orders of magnitude.PACS numbers: 73.20. At, 72.20.My, 78.70.Gq A nondegenerate two-dimensional electron system can be formed on the surface of liquid helium [1,2]. Here, surface state subbands with energies ǫ n (n = 1, 2, ..) appear owing to the attractive image force, the repulsive surface barrier, and an electric field E ⊥ applied perpendicular to the surface. Below 1 K, almost all electrons are frozen into the lowest subband forming an equipotential 2D charge layer confined on helium surface. It is a strongly-correlated system of particles interacting via unscreened Coulomb interaction. A recent work reported an effect of vanishing conductance in this system under resonant excitation by microwaves [3]. In the experiment, the inter-subband n = 1 → 2 transition is excited using radiation with angular frequency ω such that hω ≈ ǫ 2 − ǫ 1 . Under such conditions, the longitudinal conductivity σ xx , measured as a time-averaged response using a Corbino disc, oscillates upon varying the perpendicular magnetic field B, showing a sequence of minima shifted to lower values with respect to B satisfying the relation ω/ω c = l, where ω c = eB/m is the cyclotron frequency and l = 4, 5, .. (the lowest l was limited by B < 0.85 T employed in the experiment). At low l, the conductivity drops abruptly to zero. In accordance with the standard tensor relation, the vanishing conductivity σ xx → 0 at the minima corresponds to vanishing resistivity ρ xx suggesting connection with radiationinduced zero-resistance states (ZRS) of degenerate 2D electron gas in high mobility GaAs/AlGaAs heterostructures [4,5].In this work, we study a transient response of the electron system upon irradiation in the regime of vanishing conductance. We show that the radiation causes strongly nonequilibrium distribution of electrons in the confining electrostatic potential produced by the surrounding metallic electrodes. This corresponds to an increase of the electrostatic potential energy of an electron of the order electron volt, which exceeds any relevant energies, such as for example the average kinetic energy of the electrons, by orders of magnitude. The effect must have strong connection with the zero-resistance states. Figure 1 shows a schematic diagram of the experimental apparatus. Liquid 3 He is condensed into a cell having flat cylindrical shape, which contains a sintered silver heat exchanger to ensure sufficient heat contact between the liquid and the cell body. The liquid ...

We show that electrons on liquid helium display intrinsic bistability of resonant inter-subband absorption. The bistability occurs for comparatively weak microwave power. The underlying giant nonlinearity of the many-electron response results from the interplay of the strong short-range electron correlations, the long relaxation time, and the multi-subband character of the electron energy spectrum. 73.63.Hs, 03.67.Lx, 42.65.Pc Electrons on liquid helium provide a unique tool for studying correlation effects in two-dimensions (2D). The ratio of the characteristic Coulomb energy to the in-plane kinetic energy, the plasma parameter Γ = e 2 (πn s ) 1/2 /k B T (n s is the electron surface density), can vary by orders of magnitude, from Γ 1 where the electron system is a weakly-interacting gas to Γ > 130 where it is a Wigner crystal [1,2]. In the broad range 1 < Γ < 130 the electron system is a correlated 2D electron liquid, with unusual and sometimes counter-intuitive properties of classical and quantum 2D magneto-transport [3] and activated and tunneling escape from the surface [4].The effect of correlations in an electron liquid was also seen in weak-field spectroscopy of transitions to excited subbands of motion along the helium surface [5]. In a sense, it is a counter-part, for a strongly correlated system, of the depolarization effect in inter-subband absorption in semiconductor heterostructures with high electron densities [6]. For semiconductors, of much interest are both correlation transport effects [7] and nonlinear optical effects related to radiation-induced population of excited subbands [8]. Long sought has been optical bistability in intersubband absorption [9].For electrons on helium, optical nonlinearity should be strong, since the electron relaxation time is unusually long, reaching ∼ 10 −7 s for T > ∼ 0.1 K, and saturation of resonant inter-subband absorption has been indeed seen [10]. The interplay of the long relaxation time and strong spatial correlations provides a qualitatively new nonlinearity mechanism and should lead to new resonant effects. Such effects are indeed found in this paper.Our central result is the first, to the best of our knowledge, direct experimental observation and a theory of the bistability of resonant inter-subband absorption in a correlated electron system. The bistability is due to the correlation-induced strong dependence of the intersubband transition frequencies of an electron on the state of other electrons. An additional interest in this dependence comes from the proposals of quantum computing with electrons on helium [11], as it provides a mechanism of two-qubit gate operations.We study electrons on liquid 3 He. The experimental setup is similar to that previously described [12]. Electrons are confined on a helium surface between two circular parallel electrodes in an asymmetric potential formed by the barrier on the surface, the image force, and an electric field E ⊥ normal to the surface from the voltage on the electrodes [13]. The quantized energies of 1D ...