Electron-tunneling studies of the hexaboride materials SmB6, EuB6, CeB6, and SrB6

We present a series of tunneling measurements on the hexaboride materials SmB6, EuB6, CeB6, and SrB6. In this group, SmB6, EuB6, and CeB6 possess magnetic moments, whereas SrB6 is nonmagnetic. Our tunneling measurements show an asymmetric conductance, exhibiting a background that grows with energy in all four materials. We find evidence of a partial gap, but with only a small depletion in N(E). Our measurements are consistent with a two-band model in which the band overlap is reduced with decreasing temperature. [S0163-1829(98)03415-8].