Modification of structural disorder by hydrostatic pressure in the superconducting cuprate ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.73}$

Compelling efforts to improve the critical temperature $\left(T_c\right)$ of superconductors have been made through high-pressure application. Understanding the underlying mechanism behind such improvements is critically important; however, much remains unclear. Here we studied ortho-III ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.73}$ (YBCO) using x-ray scattering under hydrostatic pressure (HP) up to $\sim 6.0\mathrm{GPa}$. We found the reinforced oxygen order of YBCO under HP, revealing an oxygen rearrangement in the Cu-O layer, which evidently shows the charge-transfer phenomenon between the $\mathrm{Cu}{\mathrm{O}}_2$ plane and Cu-O layer. Concurrently, we also observed no disorder-pinned charge-density-wave signature in $\mathrm{Cu}{\mathrm{O}}_2$ plane under HP. This indicates that the oxygen rearrangement modifies the quenched disorder state in the $\mathrm{Cu}{\mathrm{O}}_2$ plane. Using these results, we appropriately explain why pressure condition can achieve higher $T_c$ compared with the optimal $T_c$ under ambient pressure in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. As an implication of these results, finally we have discussed that the change in disorder could make it easier for ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ to undergo a transition to the nematic order under an external magnetic field.

Figure 1

(a) $T_c$ phase diagram of YBCO with/without the high pressure. The arrows indicate the optimal $T_c$ values. The black circles denote ambient pressure data [27]. Red-colored filled (opened) circles are at 15 GPa (2 GPa) [12, 17]. (b) The crystal structure of YBCO (left) and extended drawings of the Cu-O chain layer with the oxygen order and $\mathrm{Cu}{\mathrm{O}}_2$ plane with CDW. (c), (d) RSXS data––ortho-III oxygen order and CDW––measured at 933.7 and 930.8 eV, respectively. Dashed lines indicate peak positions of the oxygen order (at $h=-0.33$ r.l.u.) and CDW (at $h=-0.31$ r.l.u.). Solid lines are curves fitted to the data.

Figure 2

(a) Transmission geometry for x-ray diffraction at hydrostatic high pressure. Images show 2D diffraction patterns obtained at 1.0 and 5.8 GPa. Black lines on diffraction patterns are the shadows of the beamstop. (b) Zoomed images of lattice peaks −(2, 0, 0) and (0, 2, 0). Dashed and dotted lines denote positions of $2{\theta }_{\parallel }$ at 1.5 and 5.8 GPa, respectively. (c) Pressure-dependent $a$- and b-axis lattice parameters (upper panel) and $b/a$ ratio, i.e., orthorhombicity (lower). Note that data at $P=0$ (closed circles) are obtained from the RSXS. Error bars correspond to 1 standard deviation.

Figure 3

(a) Experimental patterns of the oxygen order at $\left(h,k\right)=(3-q,0)$ with $q=0.33$ with 1.0 and 5.8 GPa. (b) Summary of fitting results on five oxygen-order peaks with varying pressure. The value of height is vertically shifted for comparison. Inset shows the full width at half maximum along the $k$ direction. Note that the red-colored shading and data point at $P=0$ (closed symbol) are estimated from the RSXS.

Figure 4

(a) Modeled ortho-III structures with $50\%$ (left) and $0\%$ (right) randomness. (b) Simulated diffraction patterns of the ortho-III structures with $50\%$ and $25\%$ randomness. (c) Summary of the height of simulated oxygen-order peaks as a function of randomness.

Figure 5

Schematic drawing of charge transfer between Cu-O chain layer and $\mathrm{Cu}{\mathrm{O}}_2$ plane upon HP. Curved black arrow represents the oxygen movement. The semitransparent arrows schematically represent the charge (electron) transfer from $\mathrm{Cu}{\mathrm{O}}_2$ plane to Cu-O layer (i.e., hole doping of $\mathrm{Cu}{\mathrm{O}}_2$ planes).

Figure 6

(a) $hk$-diffraction pattern by 30-keV x ray under $P=1.0\mathrm{GPa}$. The bottom panel is extracted by subtracting 150-K data (top) from 70-K data (middle). White dashed lines denote the CDW $q$ position. (b) Schematic CDW phase diagram (more details in Ref. [39]). The green arrow along the CDW strength (Λ) axis and the orange arrow along the disorder strength $\left(\sigma \right)$ axis indicate the directions of pressure effect. Blue and red (dotted) arrows denote total-pressure and magnetic-field effects, respectively. Note that state I and state II denote the ambient and compressed YBCO states, respectively.