RAMAN-STUDY OF R2-XCEXCUO4 (R=PR,ND,SM) - RESONANCE EFFECTS

We present resonant Raman scattering results of T'-phase cuprates R2-xCexCuO4 with R = Pr,Nd,Sm and x from 0 to 0. 175. The two modes appearing in zz polarization show a different resonant behavior: whereas the allowed A1g is nonresonant in pure compounds and decreases towards the uv in doped ones, the forbidden mode A* resonates towards uv in Nd and Sm samples with x not-equal 0, while for Pr compounds its evolution is similar to that of the A1g. We assign the A* mode to a locally modified A2u mode, due to oxygen relaxation around Ce4+. The resonance of this mode is interpreted in terms of electronic-band-structure calculations as involving transitions from O(1)p(z) + O(2)p(z) to empty cerium states, while the A1g decrease in the blue is attributed to light absorption due to the metallic character of doped samples.