Photoproduction of Long-Lived Holes and Electronic Processes in Intrinsic Electric Fields Seen through Photoinduced Absorption and Dichroism in Ca_3Ga_{2-x}Mn_xGe_3O_{12} Garnets

Long-lived photoinduced absorption and dichroism in the Ca_3Ga_{2-x}Mn_xGe_3O_{12} garnets with x < 0.06 were examined versus temperature and pumping intensity. Unusual features of the kinetics of photoinduced phenomena are indicative of the underlying electronic processes. The comparison with the case of Ca_3Mn_2Ge_3O_{12}, explored earlier by the authors, permits one to finally establish the main common mechanisms of photoinduced absorption and dichroism caused by random electric fields of photoproduced charges (hole polarons). The rate of their diffusion and relaxation through recombination is strongly influenced by the same fields, whose large statistical straggling is responsible for a broad continuous set of relaxation components (observed in the relaxation time range from 1 to about 1000 min). For Ca_3Ga_{2-x}Mn_xGe_3O_{12}, the time and temperature dependences of photoinduced absorption and dichroism bear a strong imprint of structure imperfection increasing with x.Comment: 20 pages, 10 figure

Influence of magnetic ordering on electronic structure of Tb 3+ ion in TbFe 3(BO 3) 4 crystal

Optical absorption spectra of trigonal crystal TbFe 3(BO 3) 4 have been studied in the region of 7 F6 → 5 D4 transition in Tb 3+  ion depending on temperature (2–220 K) and on magnetic field (0–60 kOe). Splitting of the Tb 3+ excited states, both under the influence of the external magnetic field and effective exchange field of the Fe-sublattice, have been determined. Landé factors of the excited states have been found. Stepwise splitting of one of the absorption lines has been discovered in the region of the Fe-sublattice magnetic ordering temperature. This is shown to be due to the abrupt change of equilibrium geometry of the local Tb 3+  ion environment only in the excited state of the Tb 3+  ion. In general, the magnetic ordering is accompanied by temperature variations of the Tb 3+ local environment in the excited states. The crystal field splitting components have been identified. In particular, it has been shown that the ground state (in D 3 symmetry approximation) consists of two close singlet states of A 1 and A 2 type, which are split and magnetized by effective exchange field of the Fe-sublattice. Orientations of magnetic moments of the excited electronic states relative to that of the ground state have been experimentally determined in the magnetically ordered state of the crystal. A pronounced shift of one of absorption lines has been observed in the vicinity of the TbFe 3(BO 3) 4 structural phase transition. The temperature interval of coexistence of the phases is about 3 K. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2011