Phonon and crystal field excitations in geometrically frustrated rare earth titanates
Abstract
The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of Gd2Ti2O7 , Tb2Ti2O7 , Dy2Ti2O7 , and Ho2Ti2O7 are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations in the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated (295-4 K). The resulting full phononic characterization of the R2Ti2O7 pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb2Ti2O7 in Raman spectroscopy, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent Tb3+ sites in the low-temperature structure. Furthermore, the crystal field level at approximately 13cm-1 is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field-phonon coupling in Tb2Ti2O7 . The additional crystal field information on Tb2Ti2O7 adds to the recent discussion on the low temperature symmetry of this system and may serve to improve its theoretical understanding.
- Publication:
-
Physical Review B
- Pub Date:
- June 2008
- DOI:
- 10.1103/PhysRevB.77.214310
- arXiv:
- arXiv:0804.2796
- Bibcode:
- 2008PhRvB..77u4310L
- Keywords:
-
- 75.50.Lk;
- 63.20.dd;
- 71.70.Ch;
- 75.30.Cr;
- Spin glasses and other random magnets;
- Measurements;
- Crystal and ligand fields;
- Saturation moments and magnetic susceptibilities;
- Condensed Matter - Materials Science
- E-Print:
- 13 pages, 7 figures