Effects of heating on the Raman spectra of YBa₂Cu₃O_7-δ

We report effects of heating to the intensities of Raman modes in the spectra of single-crystals embedded in ceramic YBa₂Cu₃O_7-δ pellets, and compare the results with those from YBa₂Cu₃O_7-δ thin-films. Heating was done by either increasing the laser-beam power or resistively heating in a heating cell. From the measurements on a single-crystal in z(xx) {Mathematical expression} polarization, we find that the relative intensity of the B_{1 g} mode of the plane oxygens to that of the A_{1 g} mode of apical oxygens decreases strongly as the samples are heated. For a crystal in near x(zz) {Mathematical expression} polarization, the relative intensity of the two modes increases up to certain value. In thin-films, however, no such change was observed even when heated to higher temperatures than the crystals were. Therefore, we interpret that the change in the relative intensity of the two modes of single-crystals is not due to electronic or structural changes nor thermal decomposition in the heated samples. We propose that a single-crystal, when heated, is fractured into many "micro-crystals" and these micro-crystals thermally fluctuate in orientations. For thin-films, such fluctuation in orientations might be hindered because there are only a few layers of micro-crystals on top of the substrate. Scanning electron micrographs (SEM) of the heated part of the samples seem to provide the evidence of the micro-crystals.