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The terahertz photoconductivity of 100 μ m and 20 μ m Hall bars fabricated from narrow AlAs quantum wells (QWs) of different widths is investigated in this paper. The photoresponse is dominated by collective magnetoplasmon excitations within the body of the Hall structure. We observed a radical change of the magnetoplasma spectrum measured precisely for AlAs QWs of widths ranging from 4 nm to 15 nm. We have shown that the observed behavior is a vivid manifestation of valley transition taking place in the two-dimensional electron system. Remarkably, we show that the photoresponse for AlAs QWs with a width of 6 nm features two resonances, indicating simultaneous occupation of strongly anisotropic X x − y valleys and isotropic X z valley in the QW plane. Our results pave the way for realizing valley-selective layered heterostructures, with potential applications in valleytronics. [ABSTRACT FROM AUTHOR]

The phenomenon of spin resonance of two-dimensional electrons confined in a 16 nm AlAs quantum well is studied in the quantum Hall effect regime near filling factor 1. At a temperature of T = 0.5K, spin resonance is not observed directly at filling factor ν = 1, but it is found already with slight deviation from it and at the increase in the temperature of the sample. The dependence of the resonance amplitude on the filling factor has two maxima near filling factors ν ≈ 0.9,1.1 symmetric with respect to ν = 1. The temperature dependences of the spin resonance amplitude near different filling factors are studied. It occurs that the resonance amplitude decreases with decreasing temperature T directly at filling factor 1. With deviation from it, the character of the temperature dependence changes. At ν ≈ 0.9,1.1, the electron paramagnetic resonance amplitude increases with decreasing T. Near filling factor 1, the temperature dependence of the amplitude is with good accuracy proportional to the temperature derivative of the longitudinal magnetoresistance measured at the same. However, quantitative agreement is violated when ν ≠ 1.These experimental observations indicate that the detection of spin resonance at filling factor 1 is based on the sensitivity of the longitudinal magnetoresistance to heating caused by the resonant absorption of microwave radiation, but this mechanism no longer works when ν ≠ 1. [ABSTRACT FROM AUTHOR]

The nuclear spin dynamics in an asymmetrically doped 16-nm AlAs quantum well grown along the [001] direction has been studied experimentally using the time decay of the Overhauser shift of paramagnetic resonance of conduction electrons. The nonzero spin polarization of nuclei causing the initial observed Overhauser shift is due the relaxation of the nonequilibrium spin polarization of electrons into the nuclear subsystem near electron paramagnetic resonance owing to the hyperfine interaction. The measured relaxation time of nuclear spins near the unity filling factor is (530 ± 30) min at the temperature T = 0.5 K. This value exceeds the characteristic spin relaxation times of nuclei in GaAs/AlGaAs heterostructures by more than an order of magnitude. This fact indicates the decrease in the strength of the hyperfine interaction in the AlAs quantum well in comparison with GaAs/AlGaAs heterostructures. [ABSTRACT FROM AUTHOR]

The electron spin resonance (ESR) of two-dimensional electrons confined in a high-quality, 16-nm AlAs quantum well was investigated near the filling factor ν = 2/3 of the fractional quantum Hall effect (FQHE). The spin resonance was robust in the vicinity of the fractional filling ν = 2/3, indicating that the ν = 2/3 state is at least partially spin polarized. The formation of the 2/3 FQHE state did not result in any modifications of the ESR linewidth and, hence, of the electron spin relaxation rate. Yet the nuclear spin-lattice relaxation rate extracted from the time decay of the ESR Overhauser shift demonstrated a strong nonmonotonic dependence on the electron filling factor with a minimum near ν = 2/3. This observation suggests the enhancement of the energy gap in the spin excitation spectrum of two-dimensional electrons at the ν = 2/3 state. [ABSTRACT FROM AUTHOR]

The microwave-induced resistance oscillations were studied in a number of GaAs/AlGaAs quantum wells and heterojunctions of various electron sheet densities n. The effective mass m* extracted from the oscillations period demonstrated strong nonmonotonic dependence on n. At low densities m* was greatly enhanced (heavier than the cyclotron mass) and abruptly decreased with increasing n. Such behavior exhibited by the effective mass at low n is clearly of many-particle origin. The minimal values of m* were observed at moderate densities and were lower than the cyclotron mass in full consistency with earlier publications by other groups. The increase of the effective mass observed at high densities can be ascribed to the nonparabolicity of the conduction band. [ABSTRACT FROM AUTHOR]