Stress Dependence of the g-Factors of Yb3+ in ThO2 and Ho2+ in CaF2

Abstract

The changes of the spin‐Hamiltonian parameters g and A have been measured for Yb³⁺ in ThO₂ and Ho²⁺ in CaF₂ (both in cubic sites) as a function of applied stress. The stress‐induced crystalline field is calculated in point‐charge approximation and the g‐shifts are obtained from the second‐order perturbation calculations of this crystalline field and the Zeeman energy. In the case of CaF₂:Ho²⁺ stressed along [001] and in the case of ThO₂:Yb³⁺ stressed along [111], the contributions of the second‐order crystalline field coefficients to the g‐shift are dominant, and very good agreements with experiment are obtained without any shielding of the crystalline field. In the case of CaF₂:Ho²⁺ stressed along [111], the correct sign of Δg is obtained, but the absolute magnitude is four times larger than the experimental value. To get agreement with experiment, the contribution of the fourth‐order crystalline field coefficient, which is dominant in this case, should be multiplied by 3. The oxygen polarization contribution to the second‐order crystalline field coefficient in the case of ThO₂:Yb³⁺ is six times larger than the point‐charge contribution, and thus the calculated g‐shift becomes five times larger than the observed one. The discrepancy is attributed to the inadequacy of the usually used oxygen polarizability.