Pressure induced insulator-to-metal transition at 170 GPa of Kondo semiconductor YbB12

The electrical resistivity in the Kondo semiconductor shows a metallic behavior at higher temperature region than the Kondo temperature TK and becomes semiconducting one in the low temperature region due to open the temperature-dependent energy gap [1]. YbB12, which crystalizes in the cubic NaCl type crystal structure, has two energy gaps whose sizes were estimated as E1 = 28 K and E2 = 78 K from the transport measurement [2]. SmB6 also forms cubic crystal structure of CsCl type [3] and is called as Kondo semiconductor [4]. YbB12 and SmB6 show similar transport behaviors [5]. A previous electrical resistivity measurement up to 14 GPa by Derr et al. shows that SmB6 was metalized at 10.5 GPa and a magnetic order appeared at about 10 K [4]. In case of YbB12, the electrical resistivity had been measured up to 8 and 20 GPa [2,6], however, has shown a semiconducting behavior. The pressure dependence of the activation energy estimated from the electrical resistivity suggests that the semiconductor to metal transition occurs at 100 GPa in YbB12. We are interested in metallic ground state of YbB12 that may exist above 100 GPa. In this paper, we report the results of electrical resistance measurement under ultra high pressures of up to 195 GPa with the diamond-anvil cell.