Imaging Topologically Emergent Dirac States of a Kondo Insulator
Heavy fermions have long been fertile grounds for discovery in quantum magnetism, unconventional superconductivity, and quantum criticality. Recent theoretical work predicts that these strongly correlated systems may also generate unique topological phases known as topological Kondo insulators (TKI). Formed from a matrix of localized f-electrons that hybridize with a background Fermi sea, the insulating gap in TKI systems is protected by a bulk topological invariant. Consequently, Dirac surface states with exotic properties are predicted to emerge within the narrow energy window of the bulk gap.
The role of topology in describing phases of quantum matter has gained considerable momentum in the last decade. The topological insulator (TI) mark a significant advance in our understanding and classification of non-trivial topological phases of matter in systems composed of non-interacting Fermions. Strong correlations, however, are predicted to generate more robust forms of ground state entanglement leading to topological order. Furthermore, although topologically protected, TI surface states in and of themselves are topologically trivial metals with ordinary Fermionic excitations. In contrast, predictions for surface states of strongly correlated topological phases include surface topological order, spontaneous breaking of surface time-reversal symmetry leading to gapped Dirac liquids exhibiting anomalous quantum hall physics, fractionalization and non-Abelian statistics. The rich topological structure that can develop at the surface of correlated materials provides a new platform to explore exotic electronic phases of matter impossible to access in the bulk.
In search for these correlated topological states SmB6 has emerged as the most promising candidate. We use Kondo lattice quasiparticle interference imaging and co-tunneling interference spectroscopy to determine the topological state of SmB6. At T* Δ ≈ 35K we first observe the formation of a narrow excitation gap, Δ, generated from hybridization between two narrowly split f-states and the itinerant d-band. The onset and evolution of the gap down to 2K, where it reaches Δ ≈ 10meV, tracks resistivity measurements showing a divergence with decreasing temperature. Next, we directly image two sets of in-gap Dirac surface states in momentum space centered at the Γ and X-points of the 2D Brioullin zone having a common nodalpoint energy of -4±2meV but distinct velocities. Collectively, these discoveries demonstrate the existence of a strongly correlated topological state in the form of a topological Kondo insulator.