Raman spectra, superconductivity, and structure of Co-substituted ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

A systematic Raman spectroscopic investigation has been performed on ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{3\mathrm{-}\mathrm{x}}$ ${\mathrm{Co}}_{\mathrm{x}}$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ ceramic samples of different Co content x in the interval 0≤x≤0.6. The structure and the superconducting properties of the samples have been carefully characterized by x-ray diffraction, magnetic susceptibility, and resistivity measurements. It is found that the frequency of the axial stretching vibration of O(4) at 502 ${\mathrm{cm}}^{\mathrm{-}1}$ decreases, the frequency of the Cu(2)-O(2),O(3) in-phase bending vibration at 440 ${\mathrm{cm}}^{\mathrm{-}1}$ increases, and the frequency of the Cu(2)-O(2),O(3) out-of-phase bending vibration at 335 ${\mathrm{cm}}^{\mathrm{-}1}$ remains constant for increasing Co concentration. The dependencies of the frequencies as well as of the intensities of these vibrational modes on cobalt substitution are reminiscent of the behavior of the corresponding modes for pure ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ of increasing oxygen deficiency. It is proposed that the frequency of this mode is directly dependent on the free-charge carrier concentration and is sensitive to the charge balance between the chains and the planes. Moreover, the resultant changes of the vibrational modes on Co substitution provide additional evidence for the mode assignments of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$