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TWO-DIMENSIONAL ELECTRON GAS 2009Crossover between distinct mechanisms of microwave photoconductivity indouble quantum wellsDouble quantum wells (DQWs) or bilayer electron systemsexposed to microwave (MW) irradiation exhibit a peculiarphotoresistance which can be explained by an interferencebetween magneto-intersubband (MIS) oscillationsand microwave induced resistance oscillations (MIROs).The microscopic mechanism describing the MIROs at lowtemperatures T is associated with a MW-generated nonequilibriumoscillatory component of the electron distributionfunction. The corresponding contribution to magnetoresistanceis proportional to the inelastic scattering timeτ in due to electron-electron scattering and decreases withincreasing temperature according to τ in ∝ T −2 [Wiedmannet al., Phys. Rev. B 78, 121301(R) (2008)].For temperature dependent measurements we have choosenthe frequency range between 55 and 140 GHz and we focushere on measurements with a fixed frequency (85 GHz) at aconstant MW electric field E ω , see figure 40. For these temperatures,Shubnikov-de Haas (SdH) oscillations are completelysuppressed. The amplitude of enhanced MIS peaksis not saturated yet for this electric field to ensure that weare far away from the saturated regime. The estimated electricfield is E ω ≃ 2.0 V/cm. Figure 40 presents the analysisof experimental data for different temperatures up to8 K. The temperature-dependent quantum lifetime τ q andtransport time τ tr are extracted from dark magnetoresistancemeasurements if T e ≃ T . The comparison of experimentalmagnetoresistance, see model in [Wiedmann et al.,Phys. Rev. B 78, 121301(R) (2008)], at constant ω andE ω shows that the theoretical temperature dependence ofinelastic scattering time τ in ∝ Te−2 fits well up to 3.5 K. Assumingthat τ in = ε F /λ in Te 2 , where λ is a numerical constantof order unity, we can find λ in ≃ 1. Note that theelectron temperature T e is at least 2.8 K (for this choosenMW electric field) because of heating of electrons due tomicrowave irradiation (see also figure 41), but for the latticetemperatures T > 3 K the heating effect is negligible,T e ≃ T . By comparing experiment and theory, e.g. figure40(c) at T = 6.0 K for 0.1 T < B < 0.2 T, we see thatthe inverted MIS oscillations are not resolved in the theoreticalplot and we have to introduce an enhanced inelasticscattering time τ ∗ in with τ∗ in = 3.5τ in to fit the experimentalresults.The deviation starts at T c = 4 K, and for T c > 4 K we obtaina nearly constant τ ∗ in ≃ 190 ps [figure 41]. Similar saturationof τ in with increasing temperature at T ≃ 2 K has beenrecently observed in a one-subband system and attributed toa crossover to the displacement mechanism of photoresistance[Hatke et al., Phys. Rev. Lett. 102, 066804 (2009)].This explanation is not applicable to our samples becausethe quantum lifetime, according to our estimates, is stillmuch smaller than τ in at T e ∼ 4 K. Our theoretical model,based on inelastic mechanism of MW photoresistance, failsto describe the obtained oscillation picture for T e > 4 K andfurther theoretical studies of MW photoresistance, possiblyinvolving new mechanisms, are necessary.Figure 40: Temperature dependence at 85 GHz up to 8.0 K.Experimental traces (bottom, black) are fitted to the theoreticalmodel (top, red). With increasing temperature, an enhanced inelasticscattering time τ ∗ in (middle, blue) is necessary to fit experimentaltraces. SdH oscillations are suppressed for these temperatures.Figure 41: Inelastic scattering time τ ∗ in as a function of electrontemperature T e . Deviation from inelastic mechanism occurs forT e > 4 K, where T -dependence of τ in is saturated.S. Wiedmann, J.C. PortalG.M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), O.E. Raichev (Institute of SemiconductorPhysics, NAS of Ukraine, Kiev, Ukraine), A.K. Bakarov (Institute of Semiconductor Physics, Novosibirsk, Russia)32