Effects of uniaxial stress and temperature variation on theCr2+center in GaAs

The effects both of applied uniaxial stress and of temperature variation on the EPR spectrum of ${\mathrm{Cr}}^{2+}$ in GaAs have been studied. The rapid stress-induced alignment of the ${\mathrm{Cr}}^{2+}$ centers at 4.2 K shows that the observed tetragonal symmetry is due to the Jahn-Teller effect as was previously suggested. The Jahn-Teller coefficient $V_E$ for an $E$-symmetry distortion mode is 0.85±0.09 eV/Å and the Jahn-Teller splitting $3E_{\mathrm{JT}}$ of the ground ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ manifold is estimated. Stress has only a small effect on the spin-Hamiltonian parameters in agreement with the large $E_{\mathrm{JT}}$ found. When taken together with previous optical work the results imply a significant Jahn-Teller splitting in the ${}_{}{}^5{}_{}{}^{}E$ state also. The EPR lines broaden in the 9—20-K temperature range and the temperature-dependent linewidth parameter $T_2^{-1\mathrm{}}$ is deduced from both the observed signal amplitudes and linewidths of the inhomogeneously broadened lines. The relation $T_2^{-1\mathrm{}}={\left(T_2^{-1\mathrm{}}\right)}_0\exp (-\frac{\Delta E}{\mathrm{kT}})$ is obeyed with $\Delta E=67\mathrm{}\pm 5$ ${\mathrm{cm}}^{-1\mathrm{}}$. Comparison with recent ultrasonic attenuation studies in GaAs: Cr shows that $T_2^{-1\mathrm{}}\simeq {\tau }^{-1\mathrm{}}$, the ${\mathrm{Cr}}^{2+}$ reorientation rate. The dependence of $T_2^{-1\mathrm{}}$ on temperature is probably due to motional broadening.