This study of EuVO4 is the first application of hole-burning techniques to the investigation of defect sites in a rare-earth compound. Laser excitation of the Eu3+ absorption spectrum in the region of the 7F0-5D0 forbidden transition ( approximately=580 nm) reveals some 30 absorption lines. Each corresponds to a distinct crystallographic site in EuVO4, as there are no crystal-field splittings in a 0-0 transition. Hole-burning of several of these lines indicates a wide range of quadrupole splitting parameters P and eta , as would be expected for different crystallographic sites. More detailed study of two of these hole-burning spectra shows that, once the parameters P and eta are determined, Zeeman spectra of selected holes directly determine the orientation of the quadrupole tensor relative to the crystallographic axes. In such a simple crystal structure as EuVO4 it is not at all obvious that there could be as many as 30 different type of defect site, and further studies will be needed to explain this.
The authors report the observation of surface Brillouin spectra from seven layered transition metal dichalcogenide metals and semimetals. Spectra of all materials clearly exhibit narrow peaks corresponding to thermally excited surface Rayleigh waves which scatter light via the recently proposed surface ripple mechanism. Materials of lower opacity exhibit additional spectral features attributable to scattering from bulk transverse and longitudinal phonons via the elasto-optic mechanism. Peak frequencies, intensities and scattering selection rules are compared with theoretical predictions.
Elastic constants C66 and C44 have been measured for TmPO4 from 4.2 to 300K. C66 shows evidence of strong Jahn-Teller coupling of Tm3+ electronic states to lattice strain but there is no cooperative phase transition. Theory previously developed to describe TbVO4 gives a very good description of the data for TmPO4.
We have explored the dependence of electron spin relaxation in undoped GaAs/AlGaAs quantum wells on well width (confinement energy) at 300 K. For wide wells, the relaxation rate tends to the intrinsic bulk value due to the D’yakonov–Perel (DP) mechanism with momentum scattering by phonons. In narrower wells, there is a strong dependence of relaxation rate on well width, as expected for the DP mechanism, but also considerable variation between samples from different sources, which we attribute to differences in sample interface morphology.
Recent and ongoing optical experiments on mechanisms and methods for control and gating of spin relaxation in semiconductor quantum wells are reviewed. We discuss work on high-mobility two-dimensional electron gases in (001)-grown GaAs/AlGaAs wells which reveals two new aspects of D'yakonov, Perel' and Kachorovskii (DPK) spin dynamics, namely oscillatory spin evolution in a quasi-collision-free regime at low temperatures and strong deviation from the standard expectation that spin-relaxation rate will be proportional to electron mobility at higher temperatures. The latter may indicate that electron–electron scattering, neglected hitherto, is important for spin relaxation. Experiments on (011)-grown GaAs/AlGaAs quantum wells confirm that this orientation leads to extension of electron spin memory by as much as two orders of magnitude at room temperature due to suppression of the major contribution to DPK spin relaxation. Results for a sample with built-in electric field give a strong indication that, for this growth orientation, room temperature spin memory may be gated by external applied voltage.
PrAlO3 has the cubic perovskite structure at high temperatures but is trigonally distorted at room temperature. At about 200 K it undergoes a first-order phase transition to an orthogonal phase, and there is a further phase transition to a triclinic phase at about 150 K. The authors have studied this system by Raman scattering, and optical fluorescence spectroscopy, and by observing the EPR of Gd3+ impurities. A further transition to a tetragonal phase at 99+or-5 K has been established. From the observed behaviour of the electronic energy levels of the Pr3+ ion, it is found that the main features of the phase transitions can be explained by consideration of the coupling of the Pr3+ ion to the soft Gamma 25 R-point phonon of the high-temperature cubic phase. A molecular-field model for this coupling which qualitatively accounts for the existence of the observed phases has been developed.
We have performed magnetophotoluminescence and magnetophotoluminescence-excitation spectroscopy at 1.8 K on a GaAs/${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structure in which the carrier density in a single 50 \AA{} quantum well can be varied from zero up to about 2\ifmmode\times\else\texttimes\fi{}${10}^{11}$ carriers ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ by use of a Schottky gate. This results from transfer of either electrons or holes photoexcited in thicker GaAs layers in the structure and thereby allows the investigation of carrier-density-dependent effects in a single sample. The data for the empty well are consistent with previous studies of magneto-optics of atomic excitons. With a Fermi sea of heavy holes or electrons present, the spectra show evidence of band-gap renormalization and phase-space filling. Also the lowest inter-Landau-level transition was observed to follow a linear field dependence for fields lower than filling factor \ensuremath{\nu}=2 but to have a particularly weak magnetic field dependence at higher fields. This is consistent with crossover from free carrier to excitonic behavior at \ensuremath{\nu}=2 and is compared to recent theoretical calculations.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
