Electronic Structure and Origin of Ferromagnetism in CaB$_6$

TL;DR

Quantum Monte Carlo calculations reveal CaB$_6$ is a narrow-gap semiconductor, clarifying its electronic nature and suggesting ferromagnetism arises from magnetic impurities rather than intrinsic properties.

Contribution

First conclusive QMC evidence showing CaB$_6$ as a narrow-gap semiconductor, resolving debates on its electronic classification and the origin of ferromagnetism.

Findings

CaB$_6$ has a ~1.3 eV X-point gap, confirming it as a semiconductor.

Ferromagnetism can be induced by magnetic impurities like Fe.

La impurities lead to a metallic state with a small Fermi surface.

Abstract

Electronic structure calculations using quantum Monte Carlo (QMC) methods conclusively show that pure CaB$_6$ is a narrow-gap semiconductor with an $X$-point gap $\sim 1.3$ eV. This should put to rest controversies as to whether the compound is a semimetal or a semiconductor, as also theories suggesting it to be an excitonic insulator. Our extensive Hartree-Fock (HF) and density functional theory (DFT) calculations support the view that the hexaboride ferromagnetism can be induced by contamination with magnetic element atoms like Fe and that presence of La is not essential for this. La impurity, however, gives rise to a metallic state with a small electron-like Fermi surface as seen in ARPES experiments.