Structural and vibrational properties of NaFe$_0.925$Co$_0.075$As under pressure

We perform high-pressure Raman scattering and synchrotron X-ray diffraction measurements on an overdoped NaFe$_{0.925}$Co$_{0.075}$As single crystal up to 20 GPa at room temperature. Both phonon spectra and structural parameters exhibit abnormal behaviors at a critical pressure around 3.0GPa. The superconducting transition temperature was observed to have a maximum at the same critical pressure. Pressure-induced modification of the Fermi surface topology is suggested to account for the observed behaviors. These results offer a better understanding on the superconductivity in this system far away the complex of structural and spin density wave phase transitions in the underdoped regime.