Raman study ofR0.5Pr0.5Ba2Cu3O7−δ(R=Y, rare earth)

The polarized Raman scattering of the ${\mathit{R}}_{0.5}$ ${\mathrm{Pr}}_{0.5}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ series (R=La, Pr, Nd, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm, Yb, Lu; δ∼0) which undergoes a superconductor-to-nonsuperconductor transition between R=Gd and Dy was studied. It was found that the appearance of superconductivity correlates with observation of a two-mode behavior of the R-ion-size-sensitive ${\mathit{B}}_{1\mathit{g}}$-like mode of O(2),O(3) out-of-phase vibrations. It was also established that the Ba (xx,${\mathit{A}}_{\mathit{g}}$) phonon line is of asymmetrical (Fano) shape for all R. The origin of the Fano effect usually attributed to interference with an electronic continuum is discussed.