Local lattice anomalies in underdoped YBa2Cu3O7-Σ probed by X-ray absorption fine structure

Temperature-dependent local lattice anomalies in underdoped YBa2Cu3O7-Σ are studied by extended X-ray absorption fine structure (EXAFS) in a fluorescence mode. The in-plane polarized Cu A^-EXAFS is measured on underdoped epitaxial YBa2Cu3O7-Σ films (100-120 nm) prepared onto SrTiO3 substrates using a pulsed laser ablation technique. The Fourier transform (FT) results for a sample withTc = 55K (△T = 4K) shows an in-plane lattice anomaly atT* = 83 K (~ 1.5Tc) below which the in-plane oxygen distribution consists of the two Cu-O peaks separated by 0.2-0.3 A. The contribution of the elongated Cu-O bond increases on loweringT, indicating either the in-plane rhombic distortion of the square-planar CuO4 or the tilting of the CuO5 pyramid. Contrary to the anomalies in spin susceptibility and transport measurements indicating the presence of a spin gap, the onset temperature of the in-plane lattice anomaly (T*) shifts to the lowerTc on decreasing the carrier density. The results suggest that the in-plane local lattice anomaly is anextrinsic lattice effect which is not directly related to the spin-phonon interaction.