Systematic Raman study on Tl-based superconductors with Tc-variation due to oxygen deficiency

Using a Raman microprobe technique, we have measured the Raman spectra of single crystallites of Tl₂Ba₂Ca_n-1Cu_nO_2n+4-d (n = 1,2 and 3) with Tc varied by oxygen removal or addition. It is found in Tl₂Ba₂CuO_6-d that the photon modes associated with the vibrations of bridging 02 in Ba-O planes (approximately 485 cm^-1) and 03 in Tl-0 planes (approximately 600 cm^-1) increase in frequency and become broader as the Tc increases from 0 to 70 K with oxygen removal, while the Ba and Tl vibration modes do not change appreciably. The observations are attributed to the valence change of Tl, resulting from a change in the charge transfer between Tl and (Cu-01), and to the distortion and/or disorder in Tl-0 planes due to the oxygen removal. There is a clear correlation between Tc and phonon frequencies in Tl-based superconductors. We suggest that a change in charge distribution in (Cu-O1)-02-(Tl-03) complexes is responsible for the phonon frequency shifts observed in 02 and 03 vibration modes.