1 High pressure study on LaFeAs(O 1-x F x ) and LaFeAsO δ with different Tc Wei Yi, Chao Zhang, Liling Sun * , Zhi-an Ren, Wei Lu, Xiaoli Dong, Zhengcai Li, Guangcan Che, Jie Yang, Xiaoli Shen, Xi Dai, Zhong Fang, Fang Zhou, Zhongxian Zhao * Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China Abstract We report studies on pressure dependence of superconducting transition temperature (Tc) of LaFeAsO δ (δ<0.3), LaFeAs(O 0.5 F 0.5 ) and LaFeAs(O 0.89 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 the Tc at ambient pressure are different. The onset Tc of LaFeAsO δ 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. Corresponding author: llsun@aphy.iphy.ac.cn zhxzhao@aphy.iphy.ac.cn