Pressure-induced lattice instabilities and superconductivity in ${\text{YBa}}_2{\text{Cu}}_4{\text{O}}_8$ and optimally doped ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$

Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate pressure-induced lattice instabilities that are accompanied by superconducting $T_{\text{c}}$ anomalies in ${\text{YBa}}_2{\text{Cu}}_4{\text{O}}_8$ and optimally doped ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$, in comparison with the nonsuperconducting ${\text{PrBa}}_2{\text{Cu}}_3{\text{O}}_{6.92}$. In the first two superconducting systems there is a clear anomaly and hysteresis in the evolution of the lattice parameters and increasing lattice disorder with pressure, which starts at $\approx 3.7\text{GPa}$. On the contrary, in the Pr compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. The micro-Raman data of the superconducting compounds show that the energy and width of the ${\text{A}}_{\text{g}}$ phonons exhibit anomalies over the same pressure range where the lattice parameters deviate from the EOS and the average $\text{Cu}2{\text{-O}}_{\text{pl}}$ bond length exhibits a strong contraction that correlates with the nonlinear pressure dependence of $T_{\text{c}}$. The anomalous Raman behavior is not observed for the nonsuperconducting Pr compound, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.

Figure 1

(Color online) (a) Rietveld refinement of synchrotron powder-diffraction patterns for Pr123 at $p=4.7\text{GPa}$ and (b) LeBail refinement for Y124 at $p=4.7\text{GPa}$. Experimental (circles), calculated (continuous line) intensities, their difference (bottom line), and corresponding positions of Bragg peaks (bars). The part of the pattern near $2\theta =20°$, where the lines from the gasket appear, has been excluded. $(\ast )$ shows the peaks of impurity ${\text{BaCuO}}_2$ in Pr123 and (◇) the weak peaks appearing for $p\ge 3.7\text{GPa}$ in Y124.

Figure 2

(Color online) Pressure dependence of the $a$ and $b$ axes for the (a) Pr123, (b) Y123 (Ref. 6), and (c) Y124 compounds. Full symbols correspond to increasing pressure and open symbols to pressure release. Full triangles correspond to the increase in pressure for second time. Solid lines are fits to the Murnaghan EOS.

Figure 3

(Color online) Pressure dependence of the $c$ axis for the (a) Pr123, (b) Y123 (from Ref. 6), and (c) Y124 compounds. Evolution of apparent disorder $\epsilon$ with applied pressure for the (d) Pr123, (e) Y123, and (f) Y124 compounds. Full symbols correspond to increasing pressure and open symbols to pressure release. Full triangles correspond to the increase in pressure for second time. Solid lines [and dashed line in (c)] are fits to the Murnaghan EOS as described in the text. Dotted lines in (d)–(f) are guide to the eye to emphasize the regions with different increase in disorder upon pressure.

Figure 4

Typical Raman spectra of the Pr123 compound at selected pressures in the (a) xx and (b) zz scattering configurations.

Figure 5

(Color online) The variation in the energy of the (a) ${\text{Cu}}_{\text{pl}}$, (b) ${\text{O}}_{\text{ap}}$, (c) Ba, and (d) ${\text{O}}_{\text{pl}}$ phonon upon pressure for different cuprates; ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6.5}$ (Y1236.5), ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7$ (Y123), and Pr123.

Figure 6

(Color online) The variation in the width of the (a) ${\text{Cu}}_{\text{pl}}$, (b) ${\text{O}}_{\text{ap}}$, (c) Ba, and (d) ${\text{O}}_{\text{pl}}$ phonon upon pressure for the cuprates of Fig. 5.

Figure 7

The pressure dependence of the $\text{Cu}2{\text{-O}}_{\text{pl}}$ bond length for the (a) Pr123 in comparison to that of (b) Y123 (Ref. 6). Dashed lines are guide to the eye.