Energy gap evolution in the optical spectra of Kondo alloy system Yb1-xLuxB12

We have measured the optical conductivity spectra [sigma(omega)] of Yb1-xLu xB12 (x = 0, 0.25, 0.5, 0,75, 1) single crystals, where the system evolves from a Kondo semiconductor (YbB12, x = 0) to a non-magnetic metal (LuB12, x = 1). For YbB12, sigma(omega) has a clear energy gap at low temperatures, with the onset of sigma(omega) at similar to 20 meV and a 'shoulder' at sim ilar to 40 meV. As x increases, the gap is progressively filled in from the bottom, while the position of the shoulder remains almost the same. This s pectral evolution suggests that a Lu-related band grows within the gap, and that the magnitude of the intrinsic Kondo semiconducting gap is basically unaffected by the Lu substitution. (C) 2000 Elsevier Science B.V. All right s reserved.