Florent Perez

Spin-polarized two-dimensional electron gas embedded in a semimagnetic quantum well : Ground state, spin responses, spin excitations, and Raman spectrum

We present theoretical aspects of spin-polarized two-dimensional electron gas (SP2DEG) which can be achieved in doped semimagnetic quantum wells. This original model system has been recently studied by magneto-Raman-scattering experiments, which has given access to spin-resolved excitations and spectrum of the SP2DEG. Starting from the diluted magnetic semiconductor (DMS) Hamiltonian in the presence of the Coulomb interaction between conduction electrons, we define the conditions to reach such a SP2DEG. The equilibrium state is studied at low temperature; in particular, a theory for the degree of spin polarization is derived. Dynamical spin susceptibilities are further calculated in the framework of a spin-density-functional formalism already developed in the past. We then derive spin-conserving and spin-flip excitation dispersions using a recent determination of the SP2DEG correlation energy corrected from the thickness of the well. The SP2DEG presents two key features: the spin-flip wave, whose existence is a direct consequence of the Coulomb interaction between the spin-polarized electrons with a dispersion and energy range typical of the SP2DEG obtained in DMS, and the spin-density fluctuations exhibiting a specific collective behavior when the spin polarization is increased. The dissipation spectrum through these excitations is studied in detail. Particular attention is given to the spectrum determined by resonant Raman scattering. We show, indeed, that the latter gives unique access to the spin-fluctuation spectrum of the SP2DEG.

Measuring the spin polarization and Zeeman energy of a spin-polarized electron gas: Comparison between Raman scattering and photoluminescence

We compare resonant electronic Raman scattering and photoluminescence measurements for the characterization of a spin-polarized two-dimensional electron gas embedded in Cd(1-x)Mn(x)Te single quantum wells. From Raman scattering by single-particle excitations in a zero magnetic field, we measure the Fermi velocity and then obtain the Fermi energy (as well as the electron density), which is comparable to that extracted from photoluminescence for moderate electron densities, assuming a bare band-edge mass. At large electron densities, the Fermi energies derived from Raman scattering and photoluminescence differ. For an applied in-plane magnetic field and zero wave vector transferred to the electron gas, Raman scattering spectra show peaks at both the Zeeman energy Z, resulting from collective excitations of the spin-polarized electron gas, and the one electron spin-flip energy Z(*). Magnetophotoluminescence spectra show conduction band splitting that are equivalent to Z, suggesting that collective effects are present in the photoluminescence spectra. Assuming an uncorrected band-edge mass, the degree of spin polarization zeta determined from the magnetophotoluminescence line shape is found to differ from that derived from the magnetic field dependent Raman scattering measurements for large electron densities. We attribute the discrepancy in measuring zeta and the Fermi energy to the renormalized mass resulting from many-body electron-electron interactions.

Lateral electron confinement in narrow deep etched wires

Narrow, dry, deep-etched GaAs/GaAlAs quantum wires with modulation doping are investigated by electronic Raman scattering (RS) and far-infrared magnetotransmission (FIR). Laterally-confined plasmon and magnetoplasmon dispersions and intensities lead to the determination of the confinement potential and the free and trapped electron densities. Free electrons are observed down to a critical width wc, significantly smaller under strong illumination (RS, wc=50 nm) than in dark conditions (FIR, wc=130 nm). The induced changes of the external confining potential and lateral electron distribution are analyzed in terms of a semiclassical electrostatic approach.

Dispersive and localized one-dimensional plasmons in very narrow quantum wires

Abstract We report on the Raman scattering determination of the dispersion of electronic excitations of 1D electron gases in deep etched GaAs wires, based on high mobility modulation-doped single-quantum wells. From the observation of well-defined 1D plasmons and of uncorrelated electron–hole pair excitations (or spin density waves, depending on the intepretation scheme), we deduce the electron density and lateral confining potential. We demonstrate the fabrication of high-quality 1D systems in the quantum limit, where a single 1D subband is occupied. We quantitatively analyse the measured dispersions and we discuss the possible roles of exchange-correlation corrections and of image charge effects. We critically discuss the interpretation of our results in terms of the Fermi versus Tomonoga–Luttinger liquid descriptions.

Spin waves in magnetic quantum wells with Coulomb interaction andsdexchange coupling

We theoretically describe the spin excitation spectrum of a two dimensional electron gas embedded in a quantum well with localized magnetic impurities. Compared to the previous work, we introduce equations that allow to consider the interplay between the Coulomb interaction of delocalized electrons and the

Spin precession and spin waves in a chiral electron gas: Beyond Larmor's theorem

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Intrinsic normal Zeeman effect for spin plasmons in semiconductor quantum wells

exchange coupling between electrons and magnetic impurities. Strong qualitative changes are found : mixed waves propagate below the single particle continuum, an anticrossing gap is open at a specific wavevector and the kinetic damping due to the electron motion strongly influences the coupling strength between electrons and impurities spins.

Propagation length of spin waves in a conducting system

Larmors theorem holds for magnetic systems that are invariant under spin rotation. In the presence of spin-orbit coupling this invariance is lost and Larmors theorem is broken: for systems of interacting electrons, this gives rise to a subtle interplay between the spin-orbit coupling acting on individual single-particle states and Coulomb many-body effects. We consider a quasi-two-dimensional, partially spin-polarized electron gas in a semiconductor quantum well in the presence of Rashba and Dresselhaus spin-orbit coupling. Using a linear-response approach based on time-dependent density-functional theory, we calculate the dispersions of spin-flip waves. We obtain analytic results for small wave vectors and up to second order in the Rashba and Dresselhaus coupling strengths

Spin-helix Larmor mode

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Anomalously large spin susceptibility enhancement in n-doped CdMnTe quantum wells

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