Electron-tunneling studies of the hexaboride materials ${\mathrm{SmB}}_6,$ ${\mathrm{EuB}}_6,$ ${\mathrm{CeB}}_6,$ and ${\mathrm{SrB}}_6$

We present a series of tunneling measurements on the hexaboride materials ${\mathrm{SmB}}_6,$ ${\mathrm{EuB}}_6,$ ${\mathrm{CeB}}_6,$ and ${\mathrm{SrB}}_6.$ In this group, ${\mathrm{S}\mathrm{mB}}_6,$ ${\mathrm{EuB}}_6,$ and ${\mathrm{CeB}}_6$ possess magnetic moments, whereas ${\mathrm{SrB}}_6$ 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 $N\left(E\right).$ Our measurements are consistent with a two-band model in which the band overlap is reduced with decreasing temperature.