Large exchange interactions in the electron gas of GaAs quantum wells

Abstract

Inelastic-light-scattering measurements show that exchange Coulomb interactions in the twodimensional electron gas of GaAs microstructures are more important than previously anticipated. Small-wave-vector spectra from modulation-doped quantum wells exhibit unexpected single-particle intersubband transitions in addition to collective spin-density and charge-density modes. From the measured spectral energies the direct and exchange intersubband Coulomb interactions are determined and found to be of comparable strengths. PACS numbers: 71.45.Gm, 73.20.Dx, 73.20.Mf, 78.30.Fs Free electrons in semiconductor microstructures reveal new behaviors that arise from fundamental electronelectron interactions and the reduced dimensionality. 1 Strong correlations in the electron gas, as in the fractional quantum Hall effect, are revealed in magnetotransport 2 and also in magneto-optics. 3,4 More generally, Coulomb interactions and reduced dimensionality have strong manifestations in the elementary excitation spectrum of the free electrons. The energies and character of these excitations are directly studied in optical experiments, such as infrared absorption 5 and inelastic light scattering. 6The light-scattering method is especially powerful because both spin-density and charge-density excitations can be measured. 6 ' 7 At small wave vectors the energies of spin-density modes are shifted from single-particle transition energies by the exchange Coulomb interaction. 1 ' 8 Charge-density modes have energy shifts due to direct as well as exchange terms. 16 " 9 However, since exchange interactions were expected to be small in GaAs, 8 spin-density excitations were previously interpreted as the energy spacings of the quantum-well states and referred to as single-particle excitations. 7,10 " 12 Similarly, the shift of charge-density excitations from singleparticle transition energies was considered in terms of direct Coulomb interactions and coupling to polar optical phonons. 6 ' 7,10 This Letter presents new results showing that such widely used interpretations of inelastic-light-scattering experiments require important revisions. In spectra of small-wave-vector intersubband excitations we find unexpected single-particle transitions in addition to the peaks of collective spin-density and charge-density modes. The significant shifts of the spin-density excitations from intersubband transition energies reveal large exchange interactions. With increasing wave vector, the collective spin-density and charge-density modes display enhanced broadening when their energies overlap the continua of single-particle transition (Landau damping). Analysis of the energies of the three excitations yields quantitative determinations of Coulomb interactions in

Giant Collective Spin-Orbit Field in a Quantum Well: Fine Structure of Spin Plasmons

“…These so-called intersubband (ISB) spin plasmons, which arise from Coulomb interactions, are energetically well separated from the continuum of ISB single-particle excitations [16,17]. In the absence of a transferred momentum q and external magnetic field B ext , time reversal symmetry, together with the [001]-axis symmetry of the quantum well, average out the k-dependent B SO .…”

“…2(a), inset) [20]. Standard selection rules [16] allow us to address the various types of intersubband excitations individually. As shown in the spectra of Fig.…”

“…ILS [16,21] is a powerful tool to study spin excitations at a given transferred momentum q ( Fig. 2(a), inset) [20].…”

Theory of inter-Landau-level magnetoexcitons in bilayer graphene

“…In semiconducting samples this technique has been successfully applied, e.g., to the study of spinconserving and spin-flipping excitations at integers, 42 of the long wave-length behavior of the low-lying 43 and higher 44 excitations of fractional quantum Hall states, and of magnetoroton minuma at ν = 2 45 and at fractions. 46 In graphene, the magnetophonon resonance was observed by this method.…”

“…Some of these modes couple to circularly polarized light, 41 while others may be observable in inelastic light scattering experiments. 11,[42][43][44][45][46][47][48] For monolayer graphene Yang et al 49 studied the intra-Landau level excitations of quantum Hall ferromagnetic states, and Iyengar et al, 50 Bychkov and Martinez, 51 Roldán et al, 52 and Lozovik and Sokolik 53 discussed the inter-Landau level excitations in detail both for the IQHE and QHF states. In particular, the linear dispersion of electrons and holes in graphene makes Kohn's theorem 54 inapplicable.…”