Pressure effects on superconductivity in LaFeAsO$_{1-\delta}$

Pressure effects on superconductivity in LaFeAsO$_{1-\delta}$ oxypnictide were studied using piston-cylinder type pressure cell. LaFeAsO$_{1-\delta}$ has been recognized as one of the typical FeAs-based superconductor since the discovery of superconductivity in LaFeAs(O, F). Oxygen deficiency in LaFeAsO leads doping and causes superconductivity. Pressure dependence of the superconducting transition temperature of LaFeAsO$_{0.7}$ was clarified by electrical resistivity measurement up to the pressure of $P=$2.5 GPa. The sample of LaFeAsO$_{0.7}$ was prepared by high pressure synthesis technique. Using a cubic-anvil-type apparatus the sample was sintered at 1250$^{\circ}$C under the pressure of 3 GPa. Oxygen deficiency was estimated from the nominal composition of starting materials. At ambient pressure, LaFeAsO$_{0.7}$ shows superconducting transition at $T_{\mathrm{c}}$ = 21.8 K. With increasing pressure, $T_{\mathrm{c}}$ increased almost linearly with coefficient of d$T_{\mathrm{c}}$/d$P$ = 2.6 K/GPa. The pressure dependence of $T_{\mathrm{c}}$ in LaFeAsO$_{0.7}$ is similar to that of an under doped sample in the LaFeAs(O, F) system.