Planar tunneling spectroscopy of the topological Kondo insulator SmB6

Abstract

Several technical issues and challenges are identified and investigated for the planar tunneling spectroscopy of the topological Kondo insulator SmB6. Contrasting behaviors of the tunnel junctions prepared in two different ways are analyzed and explained in detail. The conventional approach based on an AlOx tunnel barrier results in unsatisfactory results due to the inter-diffusion between SmB6 and deposited Al. On the contrary, plasma oxidation of SmB6 crystals produces high-quality tunnel barriers on both (001) and (011) surfaces. Resultant conductance spectra are highly reproducible with clear signatures for the predicted surface Dirac fermions and the bulk hybridization gap as well. The surface states are identified to reside on two or one distinguishable Dirac cone(s) on the (001) and (011) surface, respectively, in good agreement with the recent literature. However, their topological protection is found to be limited within the low energy region due to their inevitable interaction with the bulk excitations, called spin excitons, consistent with a recent theoretical prediction. Implications of our findings on other physical properties in SmB6 and also other correlated topological materials are remarked.

An STM Perspective on Hexaborides: Surface States of the Kondo Insulator SmB6

“…[128]. The hybridization gap obtained ranged between 14–20 meV, [ 127,128,140,165,167 ] in line with results of other measurements, [ 16–18,20,21,134,155,166,177–179 ] albeit smaller gaps were also reported. [ 23,122,123 ] The observation of co‐tunneling phenomena also on reconstructed [ 126 ] or less well‐defined surfaces is consistent with the bulk nature of the underlying Kondo effect.…”

Bulk and surface properties of SmB6

“…Planar tunneling spectroscopy measurements are also sensitive to the surface, and a Pb-SmB 6 junction has been used to probe the spectroscopic properties of SmB 6 [72,73]. The differential conductance on both ( 100) and ( 110) surfaces display a peak at about −21 mV attributed to the bulk hybridization gap, in agreement with scanning tunneling spectroscopy measurements discussed above.…”

Topological nonmagnetic impurity states in topological Kondo insulators

“…The bulk component of the resistivity is thermally activated and increases by ten orders of magnitude [15], indicating that the bulk is an exceptionally good insulator and that the surface is metallic. Surface states have also been observed in ARPES [16,17,18] and tunneling spectroscopies [19,20,21,22,23,24], which provides further evidence that is consistent with SmB 6 being topological. However, despite a number of experimental efforts by various groups, no direct evidence has been obtained which conclusively proves that SmB 6 has a non-trivial gauge topological.…”