Dirac Point Degenerate with Massive Bands at a Topological Quantum Critical Point

The quasilinear bands in the topologically trivial skutterudite insulator CoSb3 are studied under adiabatic, symmetry-conserving displacement of the Sb sublattice. In this cubic, time-reversal and inversion symmetric system, a transition from trivial insulator to topological point Fermi surface system occurs through a critical point in which massless (Dirac) bands appear, and moreover are degenerate with massive bands. Spin-orbit coupling, while small due to the type of band character, coupled with tetragonal strain opens the gap required to give the topological insulator. The mineral skutterudite (CoSb3) is very near the critical point in its natural state.