P J King

The low-temperature hypersonic attenuation of some transverse modes in quartz

The low-temperature hypersonic attenuation of transverse modes in quartz is discussed. In particular the attenuation of the slow transverse mode in the X direction and of the degenerate transverse modes in the Z direction is investigated in detail. Careful examination of the energy and pseudo-momentum conserving three-phonon processes in quartz reveals not only interactions of low-frequency transverse modes with longitudinal thermal phonons, but allowed interactions with thermal phonons on transverse acoustic branches. Calculations of both types of process have been performed using a perturbation approach. The anharmonic coupling between the hypersonic mode and the thermal phonons with which it interacts has been computed explicitly using a continuum Hamiltonian and the published second- and third-order elastic constants.

The low temperature anharmonic attenuation of transverse hypersonic waves in anisotropic cubic crystals

The low temperature attenuation of transverse hypersonic waves in cubic media due to the anharmonic interactions with thermal phonons is discussed. The effect of elastic anisotropy on the allowed three phonon processes is treated in detail. Allowed processes where transverse hypersonic waves interact with thermal phonons on the same acoustic branch of the phonon spectrum as themselves are predicted to exist. These processes are expected, however, to be very sensitive to lattice dispersion. This can cause a lower temperature dependence for the hypersonic attenuation than the T4 predicted for higher branch allowed processes in isotropic media by Landau and Rumer (1937).

The role of the Jahn-Teller effect in chromium-doped GaAs

A dynamical Jahn-Teller model for Cr2+ and Cr3+ in GaAs is proposed in order to account for existing and new experimental data. In contrast to previous models, based on a static Jahn-Teller displacement, the tetragonal nature of the EPR spectrum is shown to arise from sites having particular values for the random strain. It is also shown that the data indicate the existence of at least two classes of chromium. In one class, chromium replaces gallium substitutionally and has Td symmetry, while in the second class chromium is at a site having trigonal symmetry.

Acoustic attenuation study of vanadium-doped GaAs

The authors have measured the low-temperature acoustic attenuation in a range of vanadium-doped GaAs samples between 200 MHz and 2 GHz. In n-type material three distinct loss phenomena are found, distinguishable by their symmetry properties. The authors attribute the relation peak associated with the elastic constant C44 to V2+centres. Analysis of the peak shape indicates the presence of levels 34 K above the ground state. The behaviour of a loss component which becomes stronger towards lower temperatures is attributed to the presence of the vanadium-related complex (V+-X), while an intense light-induced low-temperature loss component is attributed to (V2+-X).

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.

Microwave acoustic relaxation losses in MgO due to Fe2

The microwave acoustic attenuation has been measured in samples of single-crystal iron doped MgO, as a function of temperature, and at 0.58 GHz and 1 GHz. The attenuation of (100) propagating compressional waves exhibits a peak at approximately 30K which is attributed to relaxation between the ground state levels of Fe2+ ions on cubic sites. An analysis of the form of the peak enables the relaxation time to be determined as a function of temperature and an excited state at 110+or-2 cm-1 is derived in agreement with infrared spectroscopic measurements. The magnitude of the attenuation of the (100) compressional mode and the absence of a peak in the transverse (100) mode is in fair agreement with a model which supposes that the Fe2+ ions are subject to random static strains, which are predominantly compressional in nature.

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+.

Phonon Spectroscopy of MnF2/ZnF2 Mixed Crystals

Superconducting tunnel junction phonon spectroscopy has shown resonant phonon scattering at magnon-phonon cross-overs in mixed crystals of MnF2/ZnF2. A resonance at 440 GHz and another between 208 GHz and 185 GHz are unexplained.