The study of nuclear magnetic resonance and nuclear spin-lattice relaxation was conducted in an asymmetrically doped to n ∼ 1.8 × 10 11 cm −2 16 nm AlAs quantum well grown in the [001]-direction. Dynamic polarization of nuclear spins due to the hyperfine interaction resulted in the socalled Overhauser shift of the two-dimensional conduction electron spin resonance. The maximum shifts achieved in the experiments are several orders of magnitude smaller than in GaAs-based heterostructures indicating that hyperfine interaction is weak. The nuclear spin-lattice relaxation time extracted from the decay of Overhauser shift over time turned out to depend on the filling factor of the two-dimensional electron system. This observation indicates that nuclear spin-lattice relaxation is mostly due to the interaction between electron and nuclear spins. Overhauser shift diminishes resonantly when the RF-radiation of certain frequencies was applied to the sample. This effect served as an indirect, yet powerful method for nuclear magnetic resonance detection: NMR quadrupole splitting of 75 As nuclei was clearly resolved. Theoretical calculations performed describe well these experimental findings.Extensive studies of nuclear spin physics in various semiconductor heterostructures have been performed in the past several decades [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Such keen interest was brought about in view of both applied and fundamental significance of the topic. Nuclear spins may be utilized to store information [10,20] in terms of spinbased electronics [21][22][23][24][25], as non-equilibrium spin polarization of lattice nuclei may have extremely long lifetime [2]. On the other hand, fundamental properties of the two-dimensional conduction electrons and nuclear spins are interconnected. The effect of huge longitudinal resistance near certain fractional fillings [8,26] may be mentioned as one of the brightest examples. Moreover, the ground state spin polarization of the twodimensional electron system (2DES) can be extracted from the Knight shift [27] of the nuclear magnetic resonance (NMR) [3,4,9,12,14]. This offers the approach to investigate various 2DES exotic states including Wigner crystal [18] and ν = 5/2 state [15][16][17].One of the most fruitful approaches is to access nuclear spins experimentally through the spins of conduction electrons coupled to them by the hyperfine interaction. Spin properties of the electrons, in turn, can be effectively studied with the aid of electron spin resonance (ESR). One of the earliest adaptations of this principle for the experiments on GaAs-based heterostructures can be found in the papers [1,2]. Let us address the idea of studying nuclear spins through ESR in more details. The actual magnetic field position of ESR turned out to be dependent on the spin polarization of the nuclear system. Indeed the spin part of the Hamiltonian for the single electron in a [001] quantum well can be expressed as:Here g * is the bare electron g-factor, which does not take...
Spectra of magnetoplasma excitations have been investigated in a two-dimensional electron systems in AlAs quantum wells (QWs) of different widths. The magnetoplasma spectrum have been found to change profoundly when the quantum well width became thinner than 5.5 nm, indicating a drastic change in the conduction electron energy spectrum. The transformation can be interpreted in terms of transition from the in-plane strongly anisotropic Xx − Xy valley occupation to the outof-plane isotropic Xz valley in the QW plane. Strong enhancement of the cyclotron effective mass over the band value in narrow AlAs QWs is reported.
Spin resonance of a two-dimensional electron system confined in a GaN/AlGaN heterostructure grown by molecular beam epitaxy was resistively detected over a wide range of magnetic field and microwave frequency. Although the spin-orbit interaction is strong in this type of heterostructure at zero magnetic field, surprisingly the width of the detected spin resonance line was very narrow—down to 6.5 mT at 13.3 T. The spin depolarization time extracted from the resonance linewidth was estimated to be 2 ns. The electron g-factor was measured with high accuracy, resembling a value close to the free-electron value and its dependence on the magnetic field was studied.
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