Hydrostatic-pressure-induced phase separation in theYBa2Cu4O8superconductor

Abstract

A micro-Raman study under high hydrostatic pressure (up to $\ensuremath{\sim}7.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$) has been carried out on $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ superconducting single crystals at room and low temperatures down to $77\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Seven strong modes of ${A}_{g}$ symmetry, two of weak ${B}_{2g}$ and ${B}_{3g}$ symmetry, and another mode at $\ensuremath{\sim}431\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ have been investigated. With increasing hydrostatic pressure the first nine phonons shift to higher energies while the latter is independent of pressure. A splitting of the mode attributed to the in-phase vibrations of the plane O oxygen atoms has been observed. Besides, anomalous nonlinear pressure behavior has been observed for the other ${A}_{g}$ phonons, which is correlated with the splitting of the in-phase mode and the pressure dependence of the superconducting transition temperature of this compound. The results indicate a pressure-induced phase separation, which is related with a lattice distortion from the tilting of the $\mathrm{Cu}{\mathrm{O}}_{6}$ octahedra.