LOCAL LATTICE ANOMALIES IN UNDERDOPED YBA2CU3O7-DELTA PROBED BY X-RAY-ABSORPTION FINE-STRUCTURE

Temperature-depend ent local lattice anomalies in underdoped YBa2Cu3O7 -delta are studied by extended X-ray absorption fine structure (EXAFS) in a fluorescence mode. The in-plane polarized Cu K-EXAFS is measured on underdoped epitaxial YBa2Cu3O7-delta films (100-120 nm) prepared o nto SrTiO3 substrates using a pulsed laser ablation technique. The Fou rier transform (FT) results for a sample with T-c=55 K (Delta T=4 K) s hows an in-plane lattice anomaly at T = 83 K; (similar to 1.5T(c)) be low which the in-plane oxygen distribution consists of the two Cu-O pe aks separated by 0.2-0.3 Angstrom. The contribution of the elongated C u-O bond increases on lowering T, indicating either the in-plane rhomb ic distortion of the square-planar CuO4 or the tilting of the CuO5 pyr amid. Contrary to the anomalies in spin susceptibility and transport m easurements indicating the presence of a spin gap, the onset temperatu re of the in-plane lattice anomaly (T) shifts to the lower T on decre asing the carrier density. The results suggest that the in-plane local lattice anomaly is an extrinsic lattice effect which is not directly related to the spin-phonon interaction.