J.M. Worlock

Observation of intersubband excitations in a multilayer two dimensional electron gas

Abstract We report the observation, by resonant inelastic light scattering, of intersubband excitations of the multilayer two dimensional electron gas, in modulation doped GaAsAlGaAs heterojunction superlattices. These are the first measurements of these transitions by any technique, and furnish intersubband energies in good agreement with calculated values. The spectral bands are broad, and nearly Lorentzian in shape: the implied relaxation rates scale linearly with band energy and are significantly faster than transport relaxation rates. Finally, the polarized spectra reveal differences between spin-flip and non spin-flip excitations which are unique to multilayer two dimensional electron gases.

Light scattering by two-dimensional electron systems in semiconductors

Abstract We discuss recent work on resonant inelastic light scattering by multilayer and single-layer two-dimensional electron gases in semiconductors. The multilayer systems occur in doped multiple -quantum-well GaAs(AlGa)As heterostructures. The single layers are at doped (GaAs-(AlGa)As heterojunctions; and at InAsMOS interfaces. Light scattering spectra display a wealth of information related to intersubband excitations. We consider in detail the depolarization field effects of collective electron-electron interactions: and correlations between spectral line shapes and transport properties.

Intersubband spectroscopy of two dimensional electron gases: Coulomb interactions

Abstract We have made a direct determination of resonant screening (the depolarization field effect) in the collective intersubband excitations of a dense two dimensional electron gas. The effect was observed, for both odd and even parity transitions, in polarized inelastic light scattering spectra of a modulation-doped GaAs-AlGaAs superlattice. We offer a quantitative interpretation in terms of the Coulomb matrix elements for the transitions. Final state, or exciton-like, many-body effects are considered briefly.

Spectroscopy of Landau transitions of two-dimensional electron gases

Abstract We have employed resonant inelastic light scattering spectroscopy to study electronic transitions in multilayer two-dimensional electron gases in magnetic fields of 4–14 T. From polarized spectra, we have evidence for both single particle Landau transitions (Δl = 1 and Δl = 2) as well as their collective counterparts. We show the variation of intensities with B and the resonant behavior, but are unable to identify a scattering mechanism.

Light scattering from two-dimensional electron systems in strong magnetic fields

Abstract Resonant light scattering has been shown to be a sensitive and versatile technique for the study of two-dimensional electron systems in semiconductors. We have extended the technique by the imposition of strong magnetic fields perpendicular to the two-dimensional planes in modulation doped AlGaAsGaAs heterostructures. At modest magnetic fields, the polarized spectra show a line which we interpret as forbidden cyclotron resonance, and a higher frequency mode probably interpretable as the upper hybrid or magnetoplasma mode. Both modes represent in-plane motion: scattering from in-plane degrees of freedom has not been observed in zero field. No known theory accounts for these observations. We summarize these results, and then discuss the properties of the magnetoplasma mode, suggesting its importance in the study of two-dimensional electron ordering at high fields.

Reconciliation of soft-mode and NQR temperature dependences in KDP-structure ferroelectrics

Abstract The temperature dependences of the dielectric constant, the NQR frequencies, and the frequency of the Raman-active ‘soft-mode’ in KDP-structure ferroelectrics are reconciled. A simple model for the q-dependence of the soft mode shows that the NQR frequency should remain finite at Tc, in contrast to earlier suggestions.

Excitation wavelength dependence of electron-hole drop velocities in Germanium

Abstract We show experimentally that the velocities of electron-hole drops in Germanium are independent of the wavelength of optical excitation. This result implies that the phonons emitted during the thermalization of electron-hole pairs have no effect on droplet motion and also permits us to draw conclusions about the decay of high frequency phonons. This leaves the “phonon wind” as originally proposed by Keldysh as the most likely cause of droplet motion.

Wave vector dependent magneto-optical interband transitions in semiconductors

Abstract The usual interband magneto-optical selection rule, Δn = 0, for transitions between Landau states in s -like and p -like bands, is shown to be easily violated when the optical wave vector perpendicular to the magnetic field is not small compared to the inverse of the magnetic length. This has implications for linear magneto-optical properties as well as for Raman scattering.

Measurement of electron-hole drop size in doped germanium by light scattering

Abstract We report measurements of sizes of electron-hole droplets in germanium with antimony. Between nominally pure crystals and those doped to a level of 3.5 × 1015 Sb cm−3, the droplet radius increases by a factor of two. We believe that this result can only be understood in terms of a comprehensive model in which droplet size and motion are related.

Properties of a giant electron-hole drop in Germanium

Abstract We have observed a giant electron-hole drop in an inhomogeneously stressed germanium crystal. Our measurements show that it is a single giant drop of constant liquid density rather than a cloud of smaller droplets. It has a density of 2 x 10 17 electron-hole pairs/cm 3 with a lifetime of only 30 μsec. These results are discussed in the context of earlier reports giving lower densities and longer lifetimes.