Pressure-induced Enhancement of Superconductivity in BiS$_2$-layered LaO$_{1-x}$F$_x$BiS$_2$

The newly discovered BiS$_2$-based LaO$_{1-x}$F$_{x}$BiS$_2$ ($x$=0.5) becomes superconductive at $T_c$=2.5 K. Electrical resistivity and magnetization measurements are performed under pressure to determine the pressure dependence of the superconducting transition temperature $T_c$. We observe that $T_c$ abruptly increases from 2.5 K to 10.7 K at a pressure of 0.7 GPa. According to high-pressure X-ray diffraction measurements, a structural phase transition from a tetragonal phase ($P$4/$nmm$) to a monoclinic phase ($P$2$_1/m$) also occurs at around $sim$ 1 GPa. We consider that a pressure-induced enhancement of superconductivity is caused by the structural phase transition.