Research Activities I. THEORY OF STRONGLY CORRELATED ELECTRON SYSTEMS

1. High-temperature ferromagnetism in borides Some borides are attracting increasing interest: MgB2 by its superconductiv-ity, and CaB6 by its high-temperature ferromagnetism. We explain [1] the present status of the research in these borides as a section for “Review of Physical Science in 2001”. As a joint work between theory and experiment, we report a high Curie-temperature ferromagnet, CaB2C2 [2]. Although the compound has neither tran-sition metal nor rare earth ions, the ferromagnetic transition temperature Tc is about 770 K. Despite this high Tc, the magnitude of the ordered moment at room temperatures is on the order of 10−4 Bohr magneton per formula unit. These properties are rather similar to those of doped divalent hexaborides, such as Ca1−xLaxB6. The calculated electronic states also show similarity near the Fermi level between CaB2C2 and divalent hexaborides. However, there is an im-portant difference: CaB2C2 crystallizes in a tetragonal structure, and there are no equivalent pockets in the energy bands for electrons and holes in contrast wit