High-pressure study on LaFeAs(O1-xFx) and LaFeAsO delta with different T-c

We report studies on pressure dependence of superconducting transition temperature (T-c) of LaFeAsO delta (delta < 0.3), 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 T-c's at ambient pressure are different. The onset T-c of LaFeAsO delta is similar to 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. Copyright (C) EPLA, 2008