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%'e have determined the components of the Raman tensor of single-crystal superconducting YSazCu&07-& for the phouons at -500, 440, 330, and 150 cm '. They all correspond to the totally symmetric modes both in the tetragonal (A~s) and orthorhombic (As) structures except for the 330-cm ' mode which is Bl tetragonal. %e have also performed lattice dynamical calculations with parameters extracted from those of perovslDte and other metallic oxides. The frequencies and eigenvectors obtained agree well with the experimental results. This work settles discrepancies found in the previous literature.Many reports on vibrations of high-T, superconductors have appeared. ' s The disagreement between assignment of phonons by various authors have spurred measurements on single crystals.~s In one cases incorrect experimental selection rules were obtained. Little theoretical work on phonon frequencies and eigenvectors has been published so far. 'e" While in Ref. 10 the semiconducting Os compound was treated, Ref. 11 has adjusted the force constants to fit the experimental data. Here we present theoretical results from parameters not adjusted but taken over from known oxide perovskite and other metallic oxide potentials. We also present an unambiguous experimental determination of the Raman selection rules for single crystals and the details of the Raman tensors. The results agree with the theoretical predictions. This fact enables us to propose an assignment of the ir phonons observed in ceramic samples.Single crystals of YBa2Cus07-s were grown from a nonstoichiometric melt by slow ceoling following Ref. 12.Tetragonal platelets (maximum dimensions 2.5 & 2.5 &0.08 mm) formed and were separated from the Sux; their composition was checked with an EDAX microprobe. With a single~stal diffractometer we found the crystal studied here to be oriented with the c axis perpendicular to the large surface of the platelet. The lattice constants were u~3 .869 A, b 3.872 A. , and c~11. 71 A. , values typical for unannealed crystals. ' These values for u, b, and c indicate 8 values slightly smaller than that of the tetragonal phase (8=0.5). ' By measuring the shielding in a weak magnetic Seld, T, (onset) was found to be 40 K (at 10% of maximum shielding). The complete shielding effect was 77% while the Meissner effect was found to be 6%. The details of the Raman setup are given in Ref.
2.The potentials for the lattice dynamical calculation were taken from least-squares fits to BaT10s, ' SrO"'5 and NiO, '6 with the only modification that the polarizability in the z direction of the Cu-0 bonds was set equal to their longitudinal polarizability.This change is justified by the structural differences between the perovskites and the superconductor, as shown in the b~1 case '0 Let us first consider the case where the wave vector of the incident light kt is parallel to the scattered one k, and to the c axis of the crystal (curves 1-6 of Fig. 1). In curve 1 both the (linear) polarization of the incident light (et) and the orientation of the (linear) analyze...
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