Structural disorder and the origin of high-T_c suppression in La_1.875Ba_0.125CuO₄

Polarized x-ray absorption fine structure (XAFS) measurements at all La, Ba and Cu sites were used to study local structural features that accompany the strong T_c suppression at x ∼ 1/8 in La_2-xBa_xCuO₄. Two critical parameters regulating the strength of hole-lattice coupling in this material were determined, i.e., the direction and magnitude of CuO₆ octahedra local tilts. Our findings include the existence of large disorder in the magnitude of the tilt angle about its average value 〈θ〉, with an rms √〈δθ₂〉 ≈ 2〈θ〉. We suggest that this large disorder strongly scatters a large fraction of states at the Fermi surface for x ∼ 1/8 leading to the observed T_c suppression.