Local structure, stripe pinning, and superconductivity in La${}_{1.875}$Ba${}_{0.125}$CuO${}_4$ at high pressure

The interplay between stripe correlations, local structure, and superconductivity in La${}_{1.875}$Ba${}_{0.125}$CuO${}_4$ is studied with concomitant polarized x-ray absorption fine structure (XAFS) and x-ray diffraction measurements at high pressure. Long-range order of the CuO${}_6$ octahedral tilt pattern that pins charge-stripe order vanishes at a pressure-induced structural transition ($P=1.8$ GPa at $T=5$ K). Diffraction shows that static charge stripe and associated octahedral tilt correlations which survive in the high-pressure phase are effectively suppressed above 3.5 GPa. In contrast, XAFS analysis shows that instantaneous local correlations of the characteristic octahedral tilt pattern remain robust to at least 5 GPa. The decreasing local tilt angle is well correlated with a gradual increase in the superconducting transition temperature, suggesting that orientational pinning of charge correlations can survive the loss of static stripe order.

Figure 1

(a) HTT structure of LBCO${}_{1/8}$ (Ref. 13) showing inequivalent oxygen sites. (b), (c) Rotation of CuO${}_6$ octahedra along a Cu-Cu in-plane direction leading to the LTO structure, and along a Cu-O in-plane direction leading to the LTT structure. (d) In-plane, La-O(2) distances for HTT, LTO, and LTT phases. The LTO and LTT phases display a very different distribution of La-O(2) distances (three vs two unique bond lengths, respectively).

Figure 2

(a) Experimental setup. The sample surface contains both $a$ and $c$ axes. (b) Diamond anvil cell showing the sample (near center), Ag foil used for in situ pressure calibration, and two ruby balls used for pressure calibration during sample loading. (c) XAFS, $\chi \left(k\right)$, data for the two polarizations at $P=0.3$ GPa and $T=5$ K.

Figure 3

(a), (c) Magnitude of FT-XAFS data for both polarizations taken at ambient pressure ($T=5$ K) together with a fit to an LTT model. (b), (d) Magnitude of FT-XAFS data for both polarizations taken at $P=2.7$ GPa, $T=5$ K (well within the macroscopic HTT phase). (e) Back FT of $\widehat{e}\parallel a$ XAFS data in the $r=(1.7–2.5)$ Å region containing the La-O distances. (f) Fitted rms disorder in La-O bond length using LTT and HTT models.

Figure 4

(a) Pressure dependence of in-plane La-O(2) distances (black squares and red circles). The average of these local distances (solid blue triangles) is in good agreement with the average distance measured by diffraction (open blue circles). (b) Pressure dependence of the La-O(2) splitting (black squares) and T${}_c$ (red circles) (Ref. 15). (c) Pressure dependence of LTT-(100) peak intensity together with results from Ref. 15. Both data were normalized to unity at ambient pressure for clarity.

Figure 5

(a) Pressure dependence of $(1.5,1.5,2)$ $(h,k)$ scans at $T=5$ K. A significant drop in intensity is observed across the LTT-HTT transition at $p_c\sim 1.8$ GPa, but diffuse scattering from $\sim$80 Å distorted domains remains present in the HTT phase. (b) Above 1.7 GPa the $(1.5,1.5,2)$ peak has the same width as that of the $(2-2\delta ,0,0.5)$ CO peak ($h$ scan), indicating the coexistence of CO and distorted structural domains. (c) Integrated intensity of $(1.5,1.5,2)$ ($hk$ scan) and $(2-2\delta ,0,0.5)$ ($h$ scan) peaks as a function of pressure.