Pressure-induced lattice instabilities and superconductivity in YBa2Cu4O8 and optimally doped YBa2Cu3O7-δ

Abstract

Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate the pressure-induced lattice instabilities that are accompanied by T${}_{\mathrm{c}}$ anomalies in YBa${}_2$Cu${}_4$O${}_8$, in comparison with the optimally doped YBa${}_2$Cu${}_3$O${}_{7-\delta }$ and the non-superconducting PrBa${}_2$Cu${}_3$O${}_{6.92}$. In the first two superconducting systems there is a clear anomaly in the evolution of the lattice parameters and an increase of lattice disorder with pressure, that starts at $\approx 3.7GPa$ as well as irreversibility that induces a hysteresis. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. In complete agreement with the structural data, the micro-Raman data of the superconducting compounds show that the energy and width of the A${}_{\mathrm{g}}$ phonons show anomalies at the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O${}_{pl}$ bond length exhibits a strong contraction and correlate with the non-linear pressure dependence of T${}_{\mathrm{c}}$. This is not the case for the non superconducting Pr sample, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.

6 pages, 7 figures