Angular dependent x-ray absorption fine structure (XAFS) measurements of all La, Ba, and Cu sites determined the origin and nature of the intrinsic disorder present in the low temperature tetragonal (LTT) structural ground state of ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ with $x=0.125,0.15\mathrm{}$ at $T=10\mathrm{}\hspace{0.5em}\mathrm{K}.$ Ba doping induces major local distortions that extend as far as 5 Å from its lattice site. Despite the low Ba content the large distortions significantly affect the XAFS of the majority La and Cu sites. The distribution of the local LTT tilt angle of ${\mathrm{CuO}}_6$ octahedra was directly determined from single and multiple scattering contributions to the XAFS of La and Cu sites. This angle, which regulates the strength of electron-lattice and spin-spin coupling in this material, was found to exhibit large, random fluctuations with rms $\left[〈\right(\theta -〈\theta 〉{)}^2〉{]}^{1/2}\approx 2〈\theta 〉,$ $〈\theta 〉$ being the average tilt angle determined by diffraction techniques. These fluctuations originate in the large, random, static disorder introduced with Ba doping. Their presence has implications for the plausibility of a mobility gap causing $T_c$ suppression at $x=0.125\mathrm{}$ in only the LTT phase, and for the correlation length of the postulated charge and spin stripes.

• Received 23 July 1999

DOI:https://doi.org/10.1103/PhysRevB.61.7055