Formation of midgap states and ferromagnetism in semiconductingCaB6

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 defect-controlled single crystals. Pure CaB 6 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 CaB 6 single crystals synthesized with 99.9% pure boron induced midgap 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.

Metal Hexaborides with Sc, Ti or Mn

“…Valence state also influences these properties as divalent hexaborides are commonly insulators or semiconductors and trivalent hexaborides are commonly metallic [5,33,37]. As noted earlier, KB 6 is identified as not a "normal" metal [12] and ThB 6 is a paramagnetic metal [13].…”

“…For example, in CaB 6 the presence of boron-related defects (the difference between 6N and 3N purity or << 1%) are closely associated with carrier doping and the formation of mid-gap states 0.18 eV below the conduction band [5]. Small amounts of La substitution for Ca are also associated with magnetic effects [44].…”

“…In this work, we compile and compare these structure determinations of end-member metal hexaborides with other hexaborides that may be formed at stoichiometric, or near stoichiometric composition given the development of facile and chemically precise synthesis methods. While electronic structure calculations have been undertaken for a range of compositions [5,12,21], theoretical studies of Sc, Ti or Mn hexaborides are not extant in the literature and are provided in this work.…”

“…M = La, Gd, Dy) [3,4] and semi-conductors (e.g. M = Ca, Sr and Yb) [5,6] while others display intermediate or unusual behavior (e.g. M = Sm, Ce, Nd) [7,8] or have notable thermal stability and hardness [9,10].…”

Electronic structure of calcium hexaborides

“…Although it now seems quite conclusive that the ferromagnetism is originated from the external impurities on the surface of the samples [6,7] rather than from an intrinsic property of the system, the electronic band structure of the bulk systems is still under debate. While Fermi surface measurements [8] imply that CaB 6 is a semimetal, angle-resolved photoemission spectroscopy (ARPES), momentum-resolved inelastic x-ray scattering [9], and transport [10] measurements indicate that CaB 6 is a semiconductor with a gap larger than 1.0 eV. Theoretically, the local density approximation (LDA) has predicted a semimetallic band structure for CaB 6 with a small overlap of boron-bands at the X point of the Brillouin zone [11][12][13][14], but it is well known that LDA calculations often underestimate the eigenenergy gap.…”