Optical gap in diluted Kondo semiconductors Yb1−xLuxB12: lattice and single-site effects

Abstract We have studied the evolution of energy gap in the optical conductivity [ σ ( ω )] of Yb 1− x Lu x B 12 as functions of temperature ( T ) and non-magnetic Lu substitution ( x ). YbB 12 has a well-developed energy gap in σ ( ω ) below 20 K, with the onset of σ ( ω ) at ∼20 meV and a shoulder at ∼40 meV. With increasing T or x , the energy gap is rapidly filled in, which indicates the importance of coherence among the Yb 4f orbitals at different sites. The shoulder, on the other hand, remains nearly unshifted in wide ranges of T (to 70 K @) and x (0–1/2), which shows that the characteristic energy for the gap is given by some single-site energy scale of Yb 3+ . Namely, both lattice and single-site effects play important roles in the gap formation of YbB 12 .