GaAs:Cr3+(3d3)—an orthorhombic Jahn-Teller center with a stress-dependent reorientation rate

The ${\mathrm{Cr}}^{3+}\left(3\mathrm{}{d}^3\right)$ EPR center in GaAs has been investigated using controlled uniaxial stress at temperatures from 1.8 to 4.2 K. Stresses up to 1200 kg/${\mathrm{cm}}^2$ were applied along the [001], [111], [110], and [112] axes. The rapidity of stress alignment of the distortions at 1.8 K shows that the orthorhombic symmetry arises from a Jahn-Teller (JT) effect. At high stress levels, the populations of the favored distortions behave as expected for a static JT effect. As the stress is reduced to low levels, however, the signal amplitudes decrease anomalously while the linewidths increase. Simultaneously, the lines develop Lorentzian tails and shift somewhat in position, and even at 1.8 K there is no microwave saturation. This behavior suggests a rapid reorientation between nearly equivalent distortions at low (or zero) applied stress which is quenched at high stress. The single-phonon-induced reorientation rate is shown to be stress dependent and can give rise to line broadening (fast reorientation) at low stress. A model based on this and a Boltzmann distribution over the different distortions gives semiquantitative agreement with the normalized signal amplitudes from the energetically favored distortions. The data show that the center couples strongly to both E and ${\mathrm{T}}_2$ modes and the Jahn-Teller coupling coefficients are determined. The E and ${\mathrm{T}}_2$ couplings are nearly equal in the sense of O'Brien and this is responsible for the fast low-temperature reorientation ($\tau \simeq 0.3$ ns at 1.8 K). For $\overrightarrow{\mathrm{P}}\parallel \mathrm{}\left[001\right]\mathrm{}$ an observed $\frac{P}{T}$ scaling of the data and a splitting of the 4-kG EPR line at low stress remain unexplained.