Yb4As3 and Yb4(As0.71Sb0.29)3, two ways to heavy-fermion character

After the observation of heavy-fermion characteristics in the charge-ordered phase of Yb4As3, it is surprising that Yb-4(As0.71Sb0.29)(3) shows heavy-fermion behavior, though the charge ordering is suppressed. To shed more light onto this enigma, we measured the optical reflectivity of Yb-4(As0.71Sb0.29)(3) at several temperatures down to 15 K in the photon energy range from 1 meV to 5.5 eV. The comparison of the optical functions of Yb4As3 and Yb-4(As0.71Sb0.29)(3) revealed decisive differences in the far-IR region. For the latter material, we found an hybridization gap of 3.3 meV, absent in Yb4As3. The effective mass of the conduction carriers takes the temperature-independent value of 2 m(e) in Yb4As3, while m* shows a logarithmic temperature dependency and reaches 80 m(e) at 15 K in Yb-4(As0.71Sb0.29)(3). This proves that two different mechanisms lead to a large gamma value in Yb4As3 and Yb-4(As0.71Sb0.29)(3). (C) 1998 Elsevier Science B.V. All rights reserved.