Spin Effects of Low-dimensional Electron Gases Studied by Far-infrared Photoconductivity Experiments
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Spin Effects of Low-dimensional Electron Gases Studied by Far-infrared Photoconductivity Experiments C. -M. Hu Institut für Angewandte Physik und Zentrum für Mikrostrukturforschung, Universität Hamburg, Jungiusstraße 11, D-20355 Hamburg, Germany Email: [email protected] ABSTRACT We review our recent work on spin effects in low-dimensional electron gases studied using far-infrared photoconductivity technique. We measure the spin-orbit coupling parameter α via spectroscopy by detecting the combined resonance. Detailed filling-factor dependent study shows the collective nature of this excitation, in accordance to theoretical predictions that both Kohn and Larmor theorem are broken for long-wavelength excitations that changes both the Landau and spin quantum numbers. We find that the long spin-relaxation time of a two-dimensional electron gas results in a novel bolometric spin effect, which gives rise to a substantial photo resistance change by reversing the spin polarization of electrons at the Fermi-level.
INTRODUCTION In the classical picture, conduction electrons of a semiconductor placed in a magnetic field B feel a Lorenz force that drives the electron moving in the cyclotron orbit, while the magnetic moment of the spin feels a torque that causes the spin to precess. These motions resonantly interact with the electromagnetic radiation, with the cyclotron resonance (CR) [1] frequency
ω c = eB m ∗
(1)
and electron-spin-resonance (ESR) [2] frequency
ω Z = −ν 0ω c
(2)
being typically in the THz and the GHz regime, respectively (ν 0 = gm ∗ 2me < 0 for most semiconductors). They provide textbook examples [1,2] of accurate determination of the electron effective mass m* and the Landé g factor. In the quantum mechanical picture, CR is the interLandau-level electric dipole transition with ∆N = 1 and ∆S = 0 , and ESR is the inter-Zeemanlevel magnetic dipole transition with ∆N = 0 and ∆S = −1 , where N and S are the Landau and spin quantum numbers, respectively. These are simplified pictures that neglect the nonparabolicity in narrow gap semiconductors and spin-orbit coupling in semiconductors lacking an inversion center. In both cases, coupling between the orbital and the spin motion of the electrons breaks the simple selection rules described above. In InGaAs/InAlAs heterojunctions where the structure inversion asymmetry dominates the spin-orbit coupling over the bulk inversion asymmetry [3], a combined resonance (CBR) with both the Landau and spin quantum numbers changed [4] can be excited by either the
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electric (E) or the magnetic (H) component of the radiation, typically with the THz frequency given by [3]
ω CBR =
(ω c + ω z )2 + (2∆ R h )2
,
(3)
which approaches ω c + ω z only at high B fields when hω c >> 2∆ R (1 − ν 0 ) . Here the matrix element ∆ R = αk F depends on the spin-orbit parameter α and the Fermi wave vector kF , which both can be controlled via a front gate [5]. The potential significance of manipulating spin via the gate is best illustrated in the classic paper of Da
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