Band Calculations on YbB12, SmB6and CeNiSn

Abstract

Many rare earth and actinide compounds are well known as heavy fermion systems with a large specific heat coefficient or as valence fluctuation systems with an energy-gap of activation type. Band calculation is a powerful method for investigat­ ing the electronic structures of these materials, although many-body effects are not sufficiently contained. If hybridization between /-electrons and conduction electrons is large, the Fermi surfaces are .explained as a result of a conventional band calcula­ tion using the local density approximation (LDA). If /-electrons are well localized and have a large magnetic moment, the Fermi surfaces are expected to be similar to those of non-I reference materials. Therefore investigations for the electronic struc­ tures of /-electron systems, provided the heavy masses cannot be expected, are important to know the role of /-electrons in their ground states. For the compounds with an energy-gap, results of band calculations using LDA are needed to answer the question whether hybridization causes a gap or not. Both of SmB6 1 > and YbB1z 2 >' 3 > are known as valence fluctuation compounds with clear gap. CeNiSn is known as a dense Kondo compound with a formation of an energy-gap. 4 >' 5 > The existence of the gap is suggested in the experiments. The resistivity of YbB12 and SmB6 rise in low temperatures in order of magnitude, while the resistivity in the c-direction of CeNiSn becomes about only 4 times larger below 5 K at most in three directiQns of the orthorhombic axes. Therefore, the energy gap in CeNiSn may have different charac­ ter. The compound SmB6 has a CaB6 type crystal structure with the space 'group of Pm3m. Trivalent compounds with this structure such as LaB6, PrB6, NdB6 and YB6 are good metals with one conduction electron per metallic ion, while divalent com­ pounds such as CaB6 and SrB6 are insulator. The band calculation for CaB6 calcu­ lated with LDA gives a small direct gap between the valence and conduction bands at the X points in the simple cubic Brillouin zone. 6 > The magnitude of the energy gap is 0.3 eV, in reasonable agreement with the experimental value, 0.4 eV suggested by