Raman study of phonons in superconducting epitaxial (Tl,Bi)Sr_1.6Ba_0.4Ca₂Cu₃O_{9 - δ} films

We have investigated Raman spectra from (Tl,Bi)Sr_1.6Ba_0.4Ca₂Cu₃O_{9 - δ} films deposited by laser ablation on the (100)-surface of LaAlO₃ substrates. The films had a thickness of 1 μm and were annealed after deposition. The as-deposited samples (before annealing) were amorphous and their Raman spectrum consists of a broad density-of-state type feature centered around 560 cm^-1. After annealing the Raman spectrum contains sharp features characteristic of crystalline materials. In some films remnants of the broad peak associated with the amorphous phase persisted. The most prominent peak occurs at 552 cm^-1. Comparison with the results from McCarty et al. [3] strongly suggest that this A_1g mode is associated with the motion along the c-axis of the O3 atoms, i.e. bridging oxygen atoms coordinated to thallium, modified by the partial substitution of Tl by Bi and Ba by Sr. The 552 cm^-1 line is allowed when the incident and scattered light polarization planes are parallel; it becomes substantially weaker when they are crossed. This strongly indicates that the constituent crystals are well aligned in the a-b plane.