Hydrostatic-pressure-induced phase separation in the $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ superconductor

A micro-Raman study under high hydrostatic pressure (up to $\sim 7.5\mathrm{GPa}$) 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 $77\mathrm{K}$. Seven strong modes of $A_g$ symmetry, two of weak $B_{2g}$ and $B_{3g}$ symmetry, and another mode at $\sim 431{\mathrm{cm}}^{-1}$ 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.

Figure 1

Typical room temperature Raman spectra of the $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ compound for selected hydrostatic pressures. The almost complete absence of the $B_{1g}$-like mode of the ${\mathrm{O}}_{\mathit{pl}}$ atoms denotes the nearly pure $zz$ scattering geometry (Ref. 12). The splitting of the in-phase ${\mathrm{O}}_{\mathit{pl}}$ mode is obvious at high pressures.

Figure 2

Room temperature Raman spectra from the same crystallite shown in Fig. 1, at $\sim 6.56\mathrm{GPa}$. When the splitting of the in-phase mode takes place, the relative intensity of the two in-phase components is modified by slightly rotating the sample orientation for the approximate $zz$ polarization.

Figure 3

Typical low temperature Raman spectra from a crystallite of the $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ sample at $\sim 6.2\mathrm{GPa}$, where the splitting of the two ${\mathrm{A}}_g$ phonons of the $\mathrm{Cu}{\mathrm{O}}_2$ plane is observed. No significant changes in the Raman spectra occur below and above $T_c$, as expected.

Figure 4

Typical room temperature Raman spectra of the $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ compound for selected hydrostatic pressures. The almost complete absence of the in-phase ${\mathrm{A}}_g$-symmetry mode of the ${\mathrm{O}}_{\mathit{pl}}$ atoms and the reduced intensity of the ${\mathrm{Cu}}_{\mathit{pl}}$ atoms ${\mathrm{A}}_g$ mode denote the nearly pure $xx∕yy$ scattering configuration (Ref. 12). However, the splitting of the latter can be clearly seen in the low energy part of the spectra, as well as the appearance of an unknown mode at $\sim 431{\mathrm{cm}}^{-1}$, which does not seem to be affected by the application of hydrostatic pressure.

Figure 5

(Color online) Hydrostatic pressure dependence of the (a) energies and (b) widths for the ${\mathrm{A}}_g$ Raman modes appearing in the $zz$ polarization spectra of the $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ compound, at RT. The open symbols represent the measurements and the crystal orientations where, due to the crystal orientation inside the DAC, no splitting of the in-phase mode has been observed, while the filled symbols represent the measurements where the splitting of this phonon was obvious. For the width of the in-phase mode, only the data where no splitting was apparent were plotted.

Figure 6

(Color online) Hydrostatic pressure dependence of the energy for the $B_{1g}$-like Raman mode of the ${\mathrm{O}}_{\mathit{pl}}$ atoms appearing in the $xx$ polarization spectra (a), the mode at $\sim 431{\mathrm{cm}}^{-1}$ (b), the $B_{3g}$ mode of the ${\mathrm{Cu}}_{\mathit{ch}}$ atoms, appearing in the parallel polarization spectra (c), and the $B_{2g}$ mode of the ${\mathrm{O}}_{\mathit{ch}}$ atoms (d).

Figure 7

(Color online) The variation of the intensities of the in-phase vibrations of the ${\mathrm{O}}_{\mathit{pl}}$ and ${\mathrm{Cu}}_{\mathit{pl}}$ ${\mathrm{A}}_g$ modes over the intensities of the ${\mathrm{O}}_{\mathit{ap}}$, ${\mathrm{O}}_{\mathit{ch}}$, and ${\mathrm{Cu}}_{\mathit{ch}}$ phonons as a function of hydrostatic pressure at RT (a)–(e). (f) The relative intensity of the ${\mathrm{O}}_{\mathit{pl}}$ to ${\mathrm{Cu}}_{\mathit{pl}}$ modes. Total intensity of the bands has been taken into account.