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Formation of mid-gap states and ferromagnetism in semiconducting CaB$_6$

Jong-Soo Rhyee·B. H. Oh·B. K. Cho·M. H. Jung·H. C. Kim·Y. K. Yoon·Jae Hoon Kim·T. Ekino
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Abstract

We present a consistent overall picture of the electronic structure and ferromagnetic interaction in CaB$_6$, based on our joint transport, optical, and tunneling measurements on high-quality {\em defect-controlled} single crystals. Pure CaB$_6$ single crystals, synthesized with 99.9999 %-pure boron, exhibited fully {\em semiconducting} characteristics, such as monotonic resistance for 2--300 K, a tunneling conductance gap, and an optical absorption threshold at 1.0 eV. {\em Boron-related defects} formed in CaB$_6$ single crystals synthesized with 99.9 %-pure boron induced {\em 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.

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Jong-Soo Rhyee, B. H. Oh, B. K. Cho, M. H. Jung, H. C. Kim, Y. K. Yoon, Jae Hoon Kim, T. Ekino. 2003-10-03. Formation of mid-gap states and ferromagnetism in semiconducting CaB$_6$. https://arxiv.org/abs/cond-mat/0310068

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