High-pressure study on LaFeAs(O1−xFx) and LaFeAsOδ with different Tc

We report studies on pressure dependence of superconducting transition temperature (Tc of ${\rm LaFeAsO}_{\delta}\,(\delta < 0.3)$, ${\rm LaFeAs(O}_{0.5}{\rm F}_{0.5})$ and ${\rm LaFeAs}({\rm O}_{0.89}{\rm F}_{0.11})$ samples. In situ resistance measurements under high pressure showed that the Tc of these three compounds increases with pressure initially, reaches a maximum value and then decreases with further increasing pressure, although Tc's at ambient pressure are different. The onset Tc of LaFeAsO$_{\delta}$ is ~ 50 K at 1.5 GPa, which is the highest record in La-based oxypnictide system. The significant change in Tc induced by pressure is attributed to the orbital degeneracy and the electron density of state at the Fermi level.