Optical conductivity of Yb1-xLuxB12:: Energy gap and mid-infrared peak in diluted Kondo semiconductors

We have measured the optical conductivity sigma(omega) of Yb1-xLuxB12 (0 less than or equal tox less than or equal to1), where the system evolves from a Kondo semiconductor at x=0 to a nonmagnetic metal at x=1; For x=0, sigma(omega) exhibits a clear energy gap with an onset at similar to 20 meV and a shoulder at 38 meV. As. x increases, the gap:is rapidly filled in from the bottom, while the shoulder remains at similar to 40 meV up to x = 1/2. These results suggest that Lu substitution into YbB12 produces an in-gap band, but the characteristic energy for the gap remains unchanged in a wide range of x. Spectral evolutions of a characteristic mid-infrared peak, which results from Yb 4f-derived states near the Fermi level, suggest a strong coupling of Yb 4f electrons with conduction electrons.