Raman-scattering results from Y1-xCaxSr2Cu2GaO7.

Abstract

We present a Raman-scattering study of ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{GaO}}_{7}$ for both the x=0 parent compound and doped compositions with x=0.25 and x=0.40. Extrapolation from ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{d}}$ and other cuprates allows us to assign many of the Raman-active phonon modes in the x=0 material, as well as identify a two-magnon scattering peak, a second-order phonon scattering peak, and a Raman continuum out to 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Despite compositional inhomogeneities, the doped superconducting samples show some of the same low-energy phonon features as the x=0 material. There is, however, a doping-dependent shift in the positions of features in the 500--700 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range, possibly due to Ca locating on Sr sites instead of Y sites. The relative intensities of the phonon peaks in the doped material are also changed from the insulator (x=0), suggesting that a resonant Raman phenomenon is occurring. The temperature-dependent spectra show what appears to be a superconducting dip in the background intensity, but the low superconducting fractions in these samples make this difficult to verify. The doped material also has a Raman continuum out to 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ just as in the x=0 samples, but with the two-magnon and second-order phonon scattering peaks significantly reduced in intensity. This may be a result of changes in the long-range ordering, or another manifestation of the same resonance phenomena occurring at lower energies. Single-crystal samples of the doped material ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{GaO}}_{7}$ are necessary for a more conclusive Raman study.