Characterization of a split axial-oxygen site in ${\mathrm{TlBa}}_2$${\mathrm{Ca}}_3$${\mathrm{Cu}}_4$${\mathrm{O}}_{11}$ by extended x-ray-absorption fine-structure spectroscopy

The Cu K-edge extended x-ray-absorption fine structure (EXAFS) of ${\mathrm{TlBa}}_2$ ${\mathrm{Ca}}_3$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_{11}$ (${\mathit{T}}_{\mathit{c}}$=118 K) is analyzed in the range 10≤T≤156 K. Nonlinear-least-squares fits to the EXAFS Cu–axial-oxygen contributions show the presence of two equally populated axial-oxygen sites at ${\mathit{R}}_1$=2.57 Å and ${\mathit{R}}_2$=2.74 Å for temperatures above ${\mathit{T}}_{\mathit{c}}$ and at 10 K. The corresponding Tl–axial-oxygen distances are 2.18 and 2.01 Å, which are similar to those found in other thallium superconducting phases. In a fluctuation region below ${\mathit{T}}_{\mathit{c}}$ (T=100, 117 K), the separation between the two sites decreases to less than 0.10 Å. This effect is analogous to the one previously reported in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, suggesting a similar coupling between elastic degrees of freedom and superconductivity. We note, however, that both the separation between the sites and the decrease in this separation within the fluctuation region are greater than in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$.