An investigation of the effect of uniaxial stress on the EPR spectrum of U_+ in CaF_
Wunsch, Paul Kenneth
Donoho, Paul L.
Master of Arts
The effect of a uniaxial compressive stress on the electron-paramagnetic-resonance spectrum of an estimated 0.03 molar percent U to the 4+ in CaF2 has been investigated at liquid helium temperatures. The trigonal crystalline field symmetry at the impurity site leaves a non-Kramers doublet in the ground state which can be handled theoretically by means of an effective spin-1 formalism. Strain effects are attributed to terms in the spin-Hamiltonian of the form DijSiSj where the tensor elements DDij are related linearly to the conventional strain components e by the expression Dij = E Gijkl ekl. The Gijkl are elements of a fourth rank tensor called the magneto-elastic coupling tensor. Four lines, broadened asymmetrically by internal strain, appear in the spectrum, one for each site of trigonal symmetry. The application of stress has the effect of greatly weakening the lines without noticeably altering their positions. Experimental observations indicate that resonant transitions in the microwave spectrum are primarily electric-field induced. This indicates that the symmetry group at the impurity site lacks the inversion property for a reason not yet understood. A lineshape calculation was performed employing Gaussian distributions for "both the homogeneous broadening and the strain broadening. The rf electric-field interaction used in the calculation connects the levels of the doublet directly and is of the form Hi = iR[Eyrf(Sy2-Sx2)-Exrf(SxSy+SySx)]. The stressed and unstressed behavior of the experimentally observed lineshape was compared with that of the theoretical model, and a correlation was drawn which fixed certain adjustable parameters in the model. This permitted the determination of the magnitude of the spin-lattice coupling constant and the combination G11-G12. The resulting values which are (in units of cm”^/unit strain) |G14| = 50 and |G11-G12! = 643 rank among the largest found to date. These figures are estimated to be good to +/- 20%.