High pressure study on LaFeAsO with different Tc
  We report studies on pressure dependence of superconducting transition temperature (Tc) of LaFeAsO, LaFeAs(O0.5F0.5) and LaFeAs(O0.89F0.11) 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 is ~50 K at 1.5 GPa, which is the highest record in La-based oxypnicited 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. 
Wei Yi, Chao Zhang, Liling Sun, Zhi-an Ren, Wei Lu, Xiaoli Dong,   Zhengcai Li, Guangcan Che, Jie Yang, Xiaoli Shen, Xi Dai,...