Elemental superconductivity at high pressure*

Most of superconducting materials show a negative pressure effect in the superconducting critical temperature, $T_{c}$, however, some of simple elements show the positive effect. It has been already revealed that not a few elements that are not the superconductor at ambient pressure became superconductive under combination of low temperature and high pressure. Not only for searching higher $T_{c}$ but also for understanding the fundamental mechanism of ``superconductivity'' systematically, we have worked on pressure effect as well as pressure-induced superconductivity especially in simple elements. Here we report two characteristic results of the high-pressure phenomena including superconductivity in calcium (Ca) and lithium (Li). The $T_{c}$ of Ca increases with pressure and reaches 29 K, the highest $T_{c}$ in elements, at very high pressure above 200 GPa. The lightest metal element of Li exhibits relatively high $T_{c}$ at high pressure, however suddenly becomes semiconductor at 80 GPa. Recently we discovered the reentrance of the superconductivity in Li at around 120 GPa.