Laser Raman Evidence for Enhanced Electron-Phonon Coupling in a Y-Ba-Cu- Oxide Superconductor Below T_c: An Application of the Maximum Entropy Method of Spectral Estimation

This report discusses Raman spectra, recorded at various temperatures from 4.2 to 100 K, of the high-temperature oxidic superconducting material YBa_2Cu_3O_7 (T_c = 90 K). By utilizing the maximum entropy method (MEM) the Raman bands were separated from the luminescent background. Our results clearly demonstrate significant enhancement of Raman activity, indicative of strong electron-phonon coupling, below the superconducting transition temperature, together with low-temperature ordering of the copper ions in their lattice sites. Several bands may also be attributed to defects of the oxygen lattice, some of which disappear as the reordering occurs.