Topological insulators in filled skutterudites

We propose new topological insulators in cerium-filled skutterudite (FS) compounds based on ab initio calculations. We find that two compounds, CeOs${}_4$As${}_{12}$ and CeOs${}_4$Sb${}_{12}$, are zero gap materials with band inversions between Os-$d$ and Ce-$f$ orbitals, similar to HgTe. Both compounds are predicted to become topological Kondo insulators at low temperatures, which are Kondo insulators in the bulk but with robust Dirac surface states on the boundary. Furthermore, this family of topological insulators has more unique features. Due to similar lattice parameters there will be a good proximity effect with other superconducting FS compounds, which may realize Majorana fermions. Additionally, the experimentally observed antiferromagnetic phase of CeOs${}_4$Sb${}_{12}$ at very low temperature provides a way to realize the massive Dirac fermion with topological magnetoelectric effects.

Figure 1

(a) Crystal structures of the filled skutterudites $R{T}_4{X}_{12}$ ($R=\text{rare}\text{earth}$; $T=\text{Fe}$, Ru, or Os; and $X$$=$ P, As, or Sb) in a bcc lattice and (b) the corresponding first Brillouin zone, in which $\Gamma$, $N$, and $H$ are time-reversal invariant momenta.

Figure 2

Band structures of (a) CeOs${}_4$P${}_{12}$, (b)CeOs${}_4$As${}_{12}$, (c) CeOs${}_4$Sb${}_{12}$, and (d) CeOs${}_4$As${}_{12}$ under $2\%$ uniaxial strain. Red/gray dots stand for the components of Ce-$f$ states, black dots stand for Os-$d$ states, and green/light gray stand for $p$ states. The size of the dots represents the relative amplitude of corresponding components. The Fermi energy is set to zero.

Figure 3

Fermi surface in the first Brillouin zone for CeOs${}_4$Sb${}_{12}$ at $E_F=$ (a) 0.0 and (b) $-0.1$ eV. The top and side views are shown in the upper and lower panels, respectively.