Anomalous raman scattering in the normal state of uederdoped crystals of YBA2Cu3O7-x
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The integrated intensity of the electronic background between 200 and 700 cm-1 observed in the Raman spectra of the YBa2Cu3O7-x 1-2-3 system exciting at 1.16 eV shows three changes with temperature. This electronic background is due to the scattering between quasi-degenerate bands at the Fermi level with apical oxygen character which have neither CuO2 nor chain character. One of these transitions coincides with the appearance of superconductivity in the system. This indicates that even if the opening of the gap in the apical oxygen electronic levels is not clearly observed, the interband transition between these two levels is coupled to the superconducting order parameter. The other two changes of the background intensity occur at temperatures greater thanT c . The one at lower temperature is proportional toT c and coincides probably with the appearance of self-organized electronic nanostructures already observed by EXAFS measurements in other HTc material systems (La-Sr-Cu-O, Ba-Sr-Ca-Cu-O). The latter transition is observed at TD>- Tc (where TD decreases as Tc increases). This temperature coincides with the opening of a pseudogap that has been detected by several experimental techniques in underdoped High-T,. systems. The possibility to observe the opening of a pseudogap in the density of states by means of Raman scattering is analyzed in terms of different theoretical models that have been postulated to explain superconductivity in cuprates.
Key WordsRaman scattering apical oxygen pseudogap
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