A S Abhvani

Jahn-Teller effects in alkali halides containing S− and Se− ions

Abstract The EPR data from S− and Se− ions in alkali halide crystals were recently explained in terms of a strong T ⊗ t Jahn-Teller interaction together with spin-orbit coupling. An alternative model is proposed in which the Jahn-Teller interaction is dynamic and of T ⊗ e type with a trigonal crystal field also present.

Comparison of static and dynamic Jahn-Teller models for Cr2+:GaAs

The nature of the approximations involved in moving from a dynamic to a static T(X)e Jahn-Teller model for the 5T2 ground states of a Cr2+ ion occupying a site of tetrahedral symmetry is discussed. It is shown that the spin Hamiltonian formalism is applicable to EPR-type experiments but not to phonon scattering measurements.

Interpretation of the zero-phonon optical absorption lines associated with substitutional Cr2+:GaAs

A dynamic Jahn-Teller model of substitutional Cr2+ in GaAs is used to interpret the fine structure at the 198 THz (0.82 eV) zero-phonon optical absorption line including its temperature dependence. The 5T2 state is treated by a dynamic Jahn-Teller model, with relatively small tunnelling ( gamma <0.001) between equivalent wells, which is consistent with other experiments. A one-parameter model for the excited 5E state corresponding to a dynamic linear Jahn-Teller effect is used.

Dynamic Jahn-Teller theory for substitutional Cr2+ in GaAs

A detailed theoretical model for substitutional (type I) Cr2+ in GaAs based on the multimode dynamic Jahn-Teller effect is described. It is shown that random strain accounts for the observed tetragonal EPR spectra and the low-frequency phonon scattering (<or approximately=5 GHz). The model also explains why the APR spectra observed from n-type crystals in the dark is weak. Mention is made of the problem of type II chromium and a brief discussion is given of the application of the model to Cr2+ in II-VI compounds.

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.

The effects of uniaxial stress on the thermal conductivity of Cr2+:Al2O3

Thermal conductivity measurements on the Cr2+:Al2O3 systems are reported. Results are obtained as an applied uniaxial stress is varied and they are analysed in terms of three resonant scattering frequencies. Two of these frequencies are found to vary with stress but one remains unchanged. The stress-dependent measurements are accounted for by the stress Hamiltonian previously derived with the ion-lattice coupling constant VEb=43000 cm-1.

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.

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+.

Derivation of the Hamiltonian for applied uniaxial stress: application to paramagnetic corundum-type crystals

The Hamiltonian to describe the perturbation for an applied uniaxial stress is derived for Al2O3 using the inner elasticity model of Cousins (1978). The results are applied to the Mn3+:Al2O3 system. Good agreement is found between the computed curves and the previously determined experimental results. The analysis also indicates how more accurate Jahn-Teller calculations could be carried out for trigonal JT systems and why electric-field induced EPR transitions are strong in thermally detected EPR experiments in Al2O3.

Jahn-Teller Model of Cr2+:GaAs

A dynamic Jahn-Teller model of Cr2+ centres in GaAs is developed as a basis for understanding the acoustic paramagnetic resonance (APR) spectrum. It is shown that first and second order spin-orbit coupling together produce two triplets and a singlet as the ground states of the system separate by a few cm-1. APR transitions are allowed between many of the Zeeman-split levels. The possibility of trigonal crystal fields is also considered and is consistent with some of the experimental data. The centres responsible for the APR spectrum appear to be very different from those seen by electron paramagnetic resonance.