Raman study ofR2−xCexCuO4(R=Pr,Nd,Sm): Resonance effects

We present resonant Raman scattering results of T’-phase cuprates ${\mathit{R}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ 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 ${\mathit{A}}_{1\mathit{g}}$ is nonresonant in pure compounds and decreases towards the uv in doped ones, the forbidden mode ${\mathit{A}}^{\mathrm{*}}$ resonates towards uv in Nd and Sm samples with x≠0, while for Pr compounds its evolution is similar to that of the ${\mathit{A}}_{1\mathit{g}}$. We assign the ${\mathit{A}}^{\mathrm{*}}$ mode to a locally modified ${\mathit{A}}_{2\mathit{u}}$ mode, due to oxygen relaxation around ${\mathrm{Ce}}^{4+}$. The resonance of this mode is interpreted in terms of electronic-band-structure calculations as involving transitions from O(1)${\mathit{p}}_{\mathit{z}}$+O(2)${\mathit{p}}_{\mathit{z}}$ to empty cerium states, while the ${\mathit{A}}_{1\mathit{g}}$ decrease in the blue is attributed to light absorption due to the metallic character of doped samples.