Local lattice anomalies in underdoped YBa₂Cu₃O_7-Σ probed by X-ray absorption fine structure

Abstract

Temperature-dependent local lattice anomalies in underdoped YBa₂Cu₃O_7-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 YBa₂Cu₃O_7-Σ films (100-120 nm) prepared onto SrTiO₃ substrates using a pulsed laser ablation technique. The Fourier transform (FT) results for a sample withT _c = 55K (△T = 4K) shows an in-plane lattice anomaly atT* = 83 K (~ 1.5T _c ) 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 lowerT _c 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.