Magnetic Raman Scattering in Rare-Earth Cuprates.
Abstract
The Raman-scattering spectra of single crystals of La_2CuO_4, Nd_2CuO_4 and Pr_2CuO_4 have been measured in the high Raman-shift region. These materials are antiferromagnetic insulators, and the main peak observed in the spectra has been assigned to a two -magnon process. We focus on the lineshape of the main peak observed in the B_{1g} scattering geometry, for which no quantitative theory exists. We have determined experimentally that a shoulder systematically appears in high-quality spectra of La _2CuO_4, and that different but well defined shapes also exist for the spectra of Nd_2CuO_4 and Pr_2CuO_4. We discuss the interpretation of the spectra, reviewing the ideas in the literature and proposing a new model which accounts for the peculiar shapes and widths of the spectra of the three compounds. The model includes further-neighbor interactions which had been hitherto neglected, and uses the Green function theory to obtain a quantitative prediction for the lineshape. A good fit to the data is obtained when 15-20% further-neighbor interactions are included. The non-negligible Raman intensities observed in symmetry-forbidden A_{1g} and B_{2g} geometries are shown to be natural consequences of the new interactions, while the resonance effect is easily accounted for in the proposed effective Raman Hamiltonian of the model. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).
- Publication:
-
Ph.D. Thesis
- Pub Date:
- January 1992
- Bibcode:
- 1992PhDT.......111M
- Keywords:
-
- Physics: Condensed Matter