Pressure dependence of T_c in LnFeAsO_1-y (Ln = La, Ce, Nd, Tb)

We measured the temperature dependence of electrical resistivity of non-fluorine- substituted, oxygen-deficient LnFeAsO_1-y (Ln = La, Ce, Nd, Tb) under hydrostatic high pressure up to '8 GPa in order to observe pressure dependence of superconducting transition temperature T_c. In LaFeAsO_0.65, onset T_c initially enhances below 4 GPa, then decreases by applying further pressure. However, the zero-resistivity temperature does not show enhancement by applying pressure. In the case of NdFeAsO_1-y and TbFeAsO_1-y, T_c shows monotonic decrease as increasing pressure. T_c is much more likely to go down faster if Ln with small ionic size was taken. Therefore, the bulk superconductivity is suppressed finally at ~7 GPa in TbFeAsO_0.7.