Kondo temperature TK’s for Kondo insulator alloys Yb1-xRxB12 (R = Y, Lu, Sc and Zr) show drastic variation as every x increase. It is found that YbB12 in a virtual gap-less state would have a peak at about 25 K in magnetic susceptibility χ(T) from analysis of χ(T) of all these alloys. Up-turns shown in χ(T) below 20 K grow as development of an energy gap ΔE. Those behaviors may be related to the second-gap development within the in-gap state showing metallic behaviors.

YbB12 has been investigated energetically as a typical Kondo insulator. To study detailed formation mechanism of Kondo energy gap, we have investigated transport and magnetic properties of Yb1-xR3+xB12 (R3+ = non-magnetic trivalent ions). Recently, we have recently succeeded in substitution of Ca2+ for Yb site in YbB12 by high-pressure synthesis. In Ca- and Lu-substituted alloys both of magnetic susceptibility and specific heat C/T at low temperatures, remarkably show larger increase than those of powdered YbB12. These increases are due to giant increase of density of state in an in-gap state.

No abstract

It has been recently found that Kondo insulator YbB 12 has the metallic state on the surface by transport properties and photoemission spectra etc. To study such exotic state in detail, we have investigated transport and magnetic properties of Yb 1-x R 3+ x B 12 (R 3+ = non-magnetic trivalent ions). Furthermore, we have succeeded in substitution of Ca 2+ for Yb site in YbB 12 by high-pressure synthesis. As Ca 2+ contents increase, the density of states at Fermi energy D(ε F) is expected to decrease gradually. The activation energies have been found to be almost constant in the range of 0 < x < 0.2. This result suggests that coherence of Kondo clouds in Ca-substituted alloys is maintained up to x = 0.2.

Production of single phase of TmB 6 , which is expected to show valence fluctuation with valence = 2.6 by previous study of Tm x R 1-x B 6 , has not been reported to date. We have succeeded in synthesis of Tm x Ca 1-x B 6 up to x = 0.2 by high-pressure synthesis. The x dependence of lattice parameters in Tm 1-x Ca x B 6 suggests that Tm-valence shows trivalent up to x = 0.2. From the transport and magnetic properties of this alloy, we discuss the possibility that they show non-Fermi liquid behavior.