V W Rampton

The acoustic paramagnetic resonance spectrum of chromous ions in magnesium oxide

The acoustic paramagnetic resonance spectrum of the chromous ion as an impurity in magnesium oxide has tetragonal symmetry. There are three sets of chromous ions each giving a spectrum with its fourfold axis of symmetry along a direction in the magnesium oxide. The three sets of ions can be aligned by the application of external uniaxial compression. The results are consistent with the chromous ion undergoing a dynamic Jahn-Teller effect and there being a distortion from cubic symmetry at each chromous ion site due to random internal strains in the crystal.

The acoustic paramagnetic resonance of vanadium doped magnesium oxide

The acoustic paramagnetic resonance spectrum of V3+ in MgO has been found to have axial symmetry about a axis of the MgO and can be described by an effective spin of one with gparallel = 069, D = 102 cm-1 and the hyperfine structure constant|A| = 00042 cm-1. The ion is strongly coupled to the lattice with|G11-G12| >or= 1000 cm-1 but|G66| <or= 30 cm-1.

Acoustic paramagnetic resonance of vanadium in semi-insulating GaAs

The authors report strong acoustic paramagnetic resonances (APR) from LEC-grown semi-insulating GaAs:V. The results obtained are consistent with observations made on similar samples by other workers using thermally detected electron paramagnetic resonance (TDEPR). The results can be interpreted as being due to a V2+-X centre producing phonon scattering where X is some other lattice defect or impurity associated with the vanadium. In one sample the APR is only observed after illumination of the sample while another sample that contained fewer impurities apart from vanadium showed the APR in the dark and enhancement after illumination. They suggest that illumination converts V3+-X centres into V2+-X by capturing an excited electron in the former case whereas in the second case illumination converts V+-X centres into V2+-X centres and the electron released is trapped at another site with a very long life at low temperature.

Further studies of substitutional Cr2+ in GaAs: the APR spectrum and its interpretation

APR experiments on weak n-type samples of chromium-doped GaAs produce a weak set of absorptions consisting of sharp peaks on a broad background when the sample is kept in the dark. A model proposed earlier for substitutional Cr2+ ions at Ga sites in which the dynamic Jahn-Teller effect and random strains play a crucial role, has been examined to see whether it can account for the EPR and for this weak APR absorption. Revised electronic and Jahn-Teller parameters are deduced which satisfy the requirements of data from conventional and thermally detected EPR experiments and data deduced from the structure of the zero-phonon line and phonon scattering experiments, while at the same time the model then satisfactorily fits most, but not all, of the APR data. The first-order Ham factor gamma is found to be about 10-4 and the implications of this value in understanding the properties of Cr2+ are also discussed.

Investigations of Cr in GaAs Using Phonons

The paper reviews published spectroscopic work on Cr in GaAs using a variety of phonon techniques from 0.1 to 1000 GHz. All this work was done on semi-insulating material. It then describes recent Nottingham work on n-type, p-type and semi-insulating samples before and after photoexcitation using acoustic paramagnetic resonance, relaxation measurements, thermal conductivity and magnetothermal conductivity (frequency crossing). The spectra observed are thought to be largely due to Cr2+ but cannot be explained by the model with a static Jahn-Teller effect, which describes the EPR data at two very different frequencies. Possible explanations are advanced.

An investigation of V3+ ion pairs in Al2O3 by phonon spectroscopy

The phonon transmission spectra of single crystals of Al2O3 containing V3+ ions have been measured with a superconducting tunnel-junction phonon spectrometer over the range 3-24 cm-1. In more concentrated samples it was found that the single-ion absorption at 8.3 cm-1 has three satellite lines and these are interpreted as arising from phonon absorption by V3+ pairs. The positions of these lines can be fitted by an isotropic exchange Hamiltonian with J=3.46 and 1.86 cm-1.

The Cr2+:CaO Jahn-Teller system

Acoustic paramagnetic resonance from Cr2+ ions in CaO has been observed at frequencies close to 9 GHz and detailed results have been obtained with the magnetic field H directed along the (111) direction. With this direction for H, the peaks in the APR spectrum are expected to arise from those Cr2+ ions occupying zero-strain sites as the Hamiltonian displays C3i symmetry. An analysis of the data obtained is given and it is shown that the results can be fitted to a multimode Jahn-Teller model. Values for the basic crystal-field parameters and an energy-level diagram for zero-strain sites are obtained.

The properties of (3d)4 ions in corundum. II. Effects of electric fields on the APR spectra and lineshape analysis

For pt. I, see ibid., vol.5, 3397 (1972). The shape of APR lines from (3d)4 ions in Al2O3 has been investigated when the direction of the applied magnetic field and constant electric fields up to 70 kV cm-1 are varied. It is found that the spectrum is much more sensitive to an electric field applied along the a axis than the same field along the b axis. A detailed semiquantitative discussion of the intensities of the edges and peaks in the spectrum is given. From the results, information is obtained on the E type components of the random internal strains and electric field distributions experienced by the Cr2+ and Mn3+ ion impurities in Al2O3.

The acoustic paramagnetic resonance of x-irradiated ruby

The results of an acoustic paramagnetic resonance study of x-irradiated ruby using longitudinal ultrasonic waves at 9.1-9.6 ghz are presented. Most resonances are attributed to the cr2+ ion which is very strongly coupled to the lattice and is subject to a dynamic jahn-teller effect. Four principal resonances are observed with the magnetic field along the c axis at approximately 6.3 kg, 7.7 kg, 11.4 kg and 15.8 kg. These split up into several components as the field is turned away from the c axis. The results are in broad agreement with the theory of bates and dixon (1969). Lines due to other centres are observed.

The Acoustic Paramagnetic Resonance of Cr2+ in n-Type GaAs

We have compared the acoustic paramagnetic resonance spectra found from unilluminated n-type chromium-doped gallium arsenide with the predictions of a dynamic Jahn-Teller model for Cr2+.