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2009 TWO-DIMENSIONAL ELECTRON GASMicrowave photoconductivity in multilayer systems: triple quantum wellsTwo-dimensional (2D) systems exposed to microwave(MW) irradiation in the presence of a weak perpendicularmagnetic field have attracted much experimental andtheoretical interest over the past years. In systems withone occupied subband, the magnetoresistance exhibitsmicrowave-induced resistance oscillations (MIROs), whichare governed by the ratio of the radiaton frequency ω tothe cyclotron frequency ω c and evolve into “zero-resistancestates” for elevated MW intensity and ultrahigh mobility.MIROs have also been investigated in systems with two occupiedsubbands (double quantum wells, DQWs), wherethe magneto-intersubband (MIS) oscillations are observedin the absence of microwave irradition. The MIS resonancesoccur when different Landau levels of the two subbandsare sequentially aligned by the magnetic field. Themost interesting aspect of two-subband systems is the interferencebetween MIROs and MIS oscillations, which leadsto a peculiar magnetoresistance pattern [Wiedmann et al.,Phys. Rev. B 78, 121301(R) (2008)].We present in this contribution the interference of MIS andMIROs which is theoretically studied in multilayer electronsystems and experimentally confirmed in high-densitytriple quantum wells (TQWs). The dissipative resistivity inthe presence of MW irradiation is given byA ω = P ω(2πω/ω c )sin(2πω/ω c )1 + P ω sin 2 . (8)(πω/ω c )In figure 49 we compare symmetrically doped GaAs TQWswith a total electron sheet density of n s = 9 × 10 11 cm −2 , amobility of 5×10 5 cm 2 /V s with a central well width of 230Å and side wells widths of 100 Å to the theoretical model inequation (7). The barrier thickness is d b = 14 Å. The MWelectric field is slightly varied around 3.2 V/cm.The MIS oscillation picture in TQWs is more complicatedbecause of the presence of three subbands with energiesε j ( j = 1,2,3). The periodicity of the MIS oscillationsis determined by several subband separation energies∆ j j ′ = |ε j −ε j ′|. Therefore, if such a TQW is exposed to microwaveirradiation, MIS oscillations also change its oscillationpicture, correlated with the radiation frequency. Thetheoretical model generalized to the multi-subband case isin agreement with experimental results for N=3.ρ MWd = ρ d + ρ in + ρ di , (6)where ρ d is the dark resistivity. ρ in and ρ di are themicrowave-induced contributions due to inelastic [Dmitrievet al., Phys. Rev. B 71, 115316 (2005)] and displacementmechanism with the dominance of the inelastic contributionat low temperature because of τ in ∝ T −2 . In a simplifiedapproach and for the case of three layers (N=3), we assumethat the partial densities n j , transport scattering rates ν trj j ′ ,and Dingle factors d j = e −π/ω cτ j , where τ j is the quantumlifetime for subband j, are equal to each other (d j = d).Then, neglecting the SdH oscillations, one can write themagnetoresistance in the form:ρ d (B)ρ d (0) = 1 + 2 [3 (1 − A ω)d 2 1 + 2 ( )3 cos 2π∆12ω c+ 2 ( )3 cos 2π∆13+ 2 ( )]ω c 3 cos 2π∆23, (7)ω cwhich describes both the contribution of MIS oscillationsgoverned by the intersubband energy gaps ∆ j j ′ and modificationof the quantum part of the resistivity by the MWirradiation, given by a dimensionless factorFigure 49: Measured (upper panel) and calculated (lower panel)photoresistance of a TQW with a barrier thickness of 14 Å exposedto MW irradiation at a lattice temperature of 1.4 K. Thecuvres for 70, 110 and 170 GHz are shifted up for clarity.S. Wiedmann, J.C. PortalN.C. Mamani, G.M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), O.E. Raichev (Institute ofSemiconductor Physics, NAS of Ukraine, Kiev, Ukraine), A.K. Bakarov (Institute of Semiconductor Physics, Novosibirsk,Russia)37