MOLECULAR SPIN-ORBIT INTERACTION FOR D5-IONS IN COVALENT CRYSTALS - SPIN-LATTICE COUPLING-COEFFICIENTS OF MN-2+ IN II-VI COMPOUNDS

A very detailed ligand-field (LF) model is developed to account for the increase of one or two orders of magnitude of the spin-lattice coupling coefficients (SLCC) of Mn2+ in the common cation series ZnS, ZnSe, ZnTe, and CdTe. First, an overall LF model shows that the SLCC's are correctly given for all studied compounds by a second-order perturbation scheme involving twice the molecular spin-orbit interaction acting between the fundamental state 6A1 and the three excited states 4T1 at lower energy. This model gives the contributions to the SLCC's of the strain-induced variations of the electrostatic field of the crystal, of the ligand-ligand and metal-ligand group overlaps, and of the molecular spin-orbit interaction. Second, a new analysis of the LF model gives the strain-induced variations of the splitting of the states 4T1, of the monoelectronic molecular wave functions, and of the orbital operator of the molecular spin-orbit interaction. Finally, by expressing the SLCC's as a linear combination of quadratic terms in the spin-orbit constants zeta(M) of the electrons d of the metal and zeta(L) of the electrons p of the ligands and bilinear terms in zeta(M)zeta(L), it is shown that the terms in zeta(M)2 are approximately identical for all compounds, while the terms in zeta(M)zeta(L) which are partly compensated by the terms in zeta(L)2 become preponderant when passing from ZnS to ZnSe, ZnTe, and CdTe. These results account for the observed roughly linear dependence of the SLCC's on zeta(L) for the common cation series ZnS, ZnSe, and ZnTe and show that molecular spin-orbit interaction is essential to analyze spin-orbit-dependent spectroscopic constants when the ratio zeta(L)/zeta(M) is greater than unity.