Giant magnetoresistance oscillations caused by cyclotron resonance harmonics

For high-mobility two-dimensional electrons at a GaAs/AlGaAs heterojunction, we have studied, both experimentally and theoretically, the recently discovered giant magnetoresistance oscillations with nearly zero resistance in the oscillation minima which appear under microwave radiation. We have proposed a model based on nonequilibrium occupation of Landau levels caused by radiation which describes the oscillation picture. PACS numbers: 73.40.-c, 73.43.-f Recent observations, in high-quality twodimensional electron systems, of microwavestimulated giant magnetoresistance oscillations (MSGMO) [1, 2] with positions corresponding to subharmonics of the cyclotron resonance [3] and especially the discover of zero-resistance states in the MSGMO minima [5, 6] attract a great attention to this spectacular phenomenon [7, 8, 9, 10, 11, 12, 13, 14, 15]. Observation of zero-resistance states was a reason to assume [5, 6] their collective origin, like photon-stimulated superconductivity [5]. Alternative approach to explanation of zero-resistance states is based on the peculiarities of electron motion in crossed electric and magnetic fields, when electron drift along electric field can occur only as a result of scattering events. In recent preprint [8], it has been demonstrated that MSGMO with negative magnetoresistance in minima can result from transitions between broadened Landau levels caused by photon absorption and accompanied by elastic scattering on short-range scatterers. Similar effect was shown rather long ago [16] to give a sequence of photocurrent peaks of different signs for unbroadened Landau levels and nonlinear conditions with respect to electric field. For bulk semiconductors and quantising magnetic fields, photocurrent oscillations with negative conductivity in minima were predicted in Ref. [17] for δ -type photoelectron energy distribution function. A link between states with negative dissipative conductivity and the zero-resistance states was proposed in Ref [9] (see also Ref. [12]). It implies that negative dissipative resistivity gives rise to instability in a system which finally breaks its symmetry and produces inhomogeneous states with nearly zero average resistance. This result allows to associate theoretical states with negative dissipative resistivity and the experimental zero-resistance states. The inhomogeneous states are characterized by high local current density even at zero net current through a sample. One more approach to explanation of MSGMO based on edge magnetoplasma instability was considered in preprint [15]. In this paper, we reproduce previous experimental observations [1, 2, 5, 6] of MSGMO with some additional data and compare them with our calculations based on results of self-consistent Born approximation (SCBA) which are capable of explaining the main features of MSGMO in terms of nonequilibrium occupation of broadened Landau levels under microwave radiation. Additionally, our model predicts appearance of the second harmonic in the Shubnikovde Haas oscillations under appropriate choice of mi-