Effects of heating on the Raman spectra of YBa2Cu3O7??

Abstract

We report effects of heating to the intensities of Raman modes in the spectra of single-crystals embedded in ceramic YBa2Cu3O7−δ pellets, and compare the results with those from YBa2Cu3O7−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)\(\bar z\) polarization, we find that the relative intensity of the B1g mode of the plane oxygens to that of the A1g mode of apical oxygens decreases strongly as the samples are heated. For a crystal in near x(zz)\(\bar x\) 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.