High-pressure ground state of SmB6: electronic conduction and long range magnetic order.

High-pressure 149Sm nuclear forward scattering of synchrotron radiation and specific heat measurements have been performed on the intermediate valent Kondo insulator SmB6. The results show that at a critical pressure p(c) approximately = 6 GPa, where the charge gap closes, a first order transition occurs to a magnetically ordered state, which shows typical features of trivalent samarium compounds. The similarity with SmS stresses the role of local correlations and gives important insight into the debate on the local or itinerant character of the f electrons in heavy fermion systems.