Imaging emergent heavy Dirac fermions of a topological Kondo insulator

Abstract

Kondo insulators are primary candidates in the search for strongly correlated topological quantum phases, which may host topological order, fractionalization, and non-Abelian statistics. Within some Kondo insulators, the hybridization gap is predicted to protect a nontrivial topological invariant and to harbor emergent heavy Dirac fermion surface modes. We use high-energy-resolution spectroscopic imaging in real and momentum space on the Kondo insulator, SmB6. On cooling through T * ∆ ≈ 35 K we observe the opening of an insulating gap that expands to ∆ ≈ 10 meV at 2 K. Within the gap, we image the formation of linearly dispersing surface states with effective masses reaching m * = (410 ± 20)me. We thus demonstrate existence of a strongly correlated topological Kondo insulator phase hosting the heaviest known Dirac fermions.

Comparative Scanning Tunneling Microscopy Study on Hexaborides

“…There are subtle inhomogeneities in the background at T ≈ 6 K; we can only speculate that they result from a not fully developed conducting surface state because we so far did not observe such inhomogeneities at 1.7 K (see also discussion of Figure 7 below). We note that very similar inhomogeneities are reported on atomically flat surfaces, Figure S1B, in the study by Pirie et al [ 30 ]…”

“…Upon searching, a surface reconstruction can usually be found. [ 28,30,39 ] The reconstruction was also observed by low‐energy electron diffraction, [ 36,40,41 ] as well as by STM on LaB 6 . [ 42 ] Clearly, if we assume that the reconstruction results from each second row of Sm atoms missing on top of an otherwise B‐terminated surface, such a reconstruction is energetically favorable compared with an unreconstructed polar surface.…”

“…This is corroborated by a change in height on and between these rows of atoms of the order 40–50 pm; yet, an order of magnitude of smaller height oscillations for the (2 × 1) reconstructed surface was also reported. [ 30,39 ] The reconstruction is likely formed during the cleaving process or subsequently upon some additional diffusion of surface atoms. In both cases, one may expect domains of (2 × 1) and (1 × 2) reconstructed areas and dislocations between the Sm rows by one lattice constant, both of which can easily be recognized in the topography of Figure 1.…”

“…The exact number, however, may depend on details of the tip, i.e., how well it may penetrate between the rows of atoms, and may even be much smaller. [ 30,39 ]…”

“…[ 26 ] Because of the cubic structure of SmB 6 , in situ‐cleaved surfaces usually required for surface‐sensitive techniques like ARPES or scanning tunneling microscopy/spectroscopy (STM/S) are often atomically rough or reconstructed. [ 27–30 ] But even in case of atomically flat surfaces the interpretation of the surface termination in STM is controversial. [ 31,32 ] In an effort to make progress in this complex situation, we here compare topographies obtained by STM on SmB 6 and EuB 6 .…”