Formation of mid-gap states and ferromagnetism in semiconducting CaB₆

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We present a consistent overall picture of the electronic structure and ferromagnetic interaction in CaB₆, based on our joint transport, optical, and tunneling measurements on high-quality defect-controlled single crystals. Pure CaB₆ single crystals, synthesized with 99.9999 %-pure boron, exhibited fully semiconducting characteristics, such as monotonic resistance for 2--300 K, a tunneling conductance gap, and an optical absorption threshold at 1.0 eV. Boron-related defects formed in CaB₆ single crystals synthesized with 99.9 %-pure boron induced mid-gap states 0.18 eV below the conduction band and extra free charge carriers, with the transport, optical, and tunneling properties substantially modified. Remarkably, no ferromagnetic signals were detected from single crystals made with 99.9999 %-pure boron, regardless of stoichiometry, whereas those made with 99.9 %-boron exhibited ferromagnetism within a finite range of carrier density. The possible surmise between the electronic state and magnetization will be discussed.