Intersubband electronic Raman scattering in narrow GaAs single quantum wells dominated by single-particle excitations

Abstract

We measured resonant Raman scattering by intersubband electronic excitations in GaAs/AlAs single quantum wells (QWs) with well widths ranging from 8.5 to 18 nm. In narrow (less than ∼ 10 nm) QWs with sufficiently high electron concentrations, only single-particle excitations (SPEs) were observed in intersubband Raman scattering, which was confirmed by the well-width dependence of Raman spectra. We found characteristic variations in Raman shift and line shape for SPEs with incident photon energy in the narrow QWs.Intersubband electronic excitations in quantum wells (QWs) are strongly affected by many-body Coulomb interactions 1 . In electronic Raman scattering in doped QWs, two types of intersubband collective excitations have been confirmed by many researchers 2 since the first reports by Abstreiter and Ploog 3 and by Pinczuk et al. 4 in 1979: a collective charge-density wave (CDW) appears if the polarizations of incident and scattered lights are parallel ( ), whereas a collective spin-density wave (SDW) appears if they are crossed (⊥).The CDW and SDW excitation energies are given by 1,5based on the local-density functional theory 6,7,8 , where N S is the sheet electron concentration in a QW, E 10 is the intersubband energy separation that includes static many-body corrections, and αN S and βN S are dynamical many-body corrections (called the depolarization shift and excitonic shift) due to the direct and exchangecorrelation intersubband Coulomb interactions, respectively. In the crudest approximation 8,9 , the depolarization shift αN S is proportional to N S d eff and the excitonic shift βN S is proportional to (N S /d eff ) 1/3 , where d eff is the effective well width 1 . Experimentally, αN S can be determined from the measured value of E CD − E SD ; βN S can be estimated only when there is information about E 10 . Later, in 1989, Pinczuk et al. reported 10 that not only CDW and SDW but also single-particle excitations (SPEs) are observed in intersubband electronic Raman scattering and their transition energy is E 10 for modulation-doped 25-nm GaAs/Al 0.3 Ga 0.7 As single QWs with N S = (1.5−3)×10 11 cm −2 . SPEs are observed for both the parallel and crossed polarizations and seem to become stronger 10,11 at higher values of N S . * submitted to Phys. Rev. B.There are many other reports on intersubband electronic Raman scattering for wide (more than ∼ 20 nm) GaAs QWs. It will be interesting to measure Raman scattering also in narrower QWs, considering β/α ∼ (N S d eff 2 ) −2/3 . However, there are only a few reports for narrower GaAs QWs 12,13 because interface roughness makes Raman peaks broader and more difficult to observe. To our knowledge, such an experiment has never been reported for a single QW.In this paper, we report resonant Raman scattering by the lowest (0 → 1) intersubband electronic excitations in a set of modulation-doped GaAs/AlAs single QWs with well widths ranging from 8.5 to 18 nm. We found unexpected results that parallel-and crossed-polarization Raman spectra of narrow QWs had only a si...

Chirality and intrinsic dissipation of spin modes in two-dimensional electron liquids

“…The property (44) is illustrated in Figs. 15a-b, which present the energy and linewidth dispersions of the type of spin wave shown in Fig.…”

“…Intersubband charge and spin plasmons have been experimentally observed [40][41][42][43][44][45] and theoretically investigated [46][47][48][49][50] for three decades.…”

“…The property (44) is illustrated in figures 15(a) and (b), which present the energy and linewidth dispersions of the type of spin wave shown in figure 14 for both directions of the magnetic field, but in a sample meeting the above condition. Since the linewidth of the Raman line yields the damping rate η q , figures 15(a) and (b) shows strikingly that both the spin-wave energy and damping rate exhibit the same chirality: they are invariant under simultaneous inversion of the directions of the magnetic field and of the wavevector.…”

“…Chiral damping dispersions have been observed in Pt/Co/Ni films [142]. However, equations (41) and (42) show a universal linear relation between damping rate and angular frequency of the spin wave, independent of SOC, which reads: Dispersions are shifted by q s from q = 0 with a mirror symmetry when inverting the magnetic field, see equation (44). (c) (•) represents the q s dependence with ϕ, extracted from the measured dispersions.…”

Resonant Raman scattering of electrons in quantum wells: Identification of elementary excitations

“…Sassetti et al 2 considered interband collective electronic excitations in resonant Raman scattering of twosubband quantum wires. Finally, for nanometric quantum wells, an early work was carried out by Pinczuk et al, 3 while another study was done more recently by Unuma et al 4 In all these works, the collective electronic excitations were analyzed. It may also be noted that resonant Raman scattering is relevant to the realization of intersubband Raman lasers, involving the possibility of reaching the far-infrared region in an operation at room temperature.…”