PHOTOINDUCED CHANGES IN RAMAN-SPECTRA OF YBA2CU3O6.4 FILMS

The evolution of Raman spectra with illumination has been studied in YBa2Cu3O6.4 films at temperatures between 5-300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm-1, O(2)-O(3) at 338 cm-1, and O(4) at 488 cm-1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequencies omega, which is in agreement with the enhancement of the static conductivity sigma(omega --> 0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.