Micro-Raman study of the role of pressure in mercury-based superconductors.

Understanding the role of pressure in increasing ${\mathit{T}}_{\mathit{c}}$ is a crucial issue in mercury-based superconductors. We report on the effect of pressure on the Raman frequency (\ensuremath{\omega}) of the apical oxygen (${\mathrm{O}}_{\mathit{A}}$) in Hg-${\mathrm{O}}_{\mathit{A}}$-Cu bonds of ${\mathrm{HgBa}}_{2}$${\mathrm{Ca}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+2+\mathrm{\ensuremath{\delta}}}$ superconductors. For both Hg-1201 and Hg-1223, \ensuremath{\Delta}(${\mathrm{\ensuremath{\omega}}}^{2}$/${\mathrm{\ensuremath{\omega}}}_{0}^{2}$)/\ensuremath{\Delta}P is found to be strongly correlated with \ensuremath{\Delta}${\mathit{T}}_{\mathit{c}}$/\ensuremath{\Delta}P, where ${\mathrm{\ensuremath{\omega}}}_{0}$ is the frequency at ambient pressure. Together with our local-density approximation total-energy phonon study of the ${\mathrm{O}}_{\mathit{A}}$-${\mathit{A}}_{1\mathit{g}}$ Raman mode, it is concluded that the change of the ${\mathrm{O}}_{\mathit{A}}$ Raman frequency is due to the change in Hg-${\mathrm{O}}_{\mathit{A}}$ bond strength, which may be directly related to the charge transfer between the ${\mathrm{CuO}}_{2}$ planes and the Hg-O layers.