A topological insulator (TI) is an unusual quantum state in which the insulating bulk is topologically distinct from vacuum, resulting in a unique metallic surface that is robust against time-reversal invariant perturbations. The surface transport, however, remains difficult to isolate from the bulk conduction in most existing TI crystals (particularly Bi2Se3, Bi2Te3 and Sb2Te3) due to impurity caused bulk conduction. We report in large crystals of topological Kondo insulator (TKI) candidate material SmB6 the thickness-independent surface Hall effects and non-local transport, which persist after various surface perturbations. These results serve as proof that at low temperatures SmB6 has a metallic surface that surrounds an insulating bulk, paving the way for transport studies of the surface state in this proposed TKI material.

Strongly correlated Kondo insulator SmB 6 is known for its peculiar low temperature residual conduction, which has recently been demonstrated to arise from a robust metallic surface state, as predicted by the theory of topological Kondo insulator (TKI). Photoemission, quantum oscillation and magnetic doping experiments have provided evidence for the Dirac-like dispersion and topological protection. Questions arise as whether signatures of spin-momentum locking and electron interaction could be resolved in transport measurements. Here we report metallic conduction of surface state down to mK temperatures with saturation behaviors suggestive of Kondo effect. We observe in the surface state the weak-antilocalization (WAL) effect that is in agreement with a spin-momentum locked metallic surface. At larger perpendicular magnetic fields, the surface state exhibits an unusual linear magnetoresistance similar to those found in Bi-based topological insulators and in graphene.With its heavy f-electron degree of freedom, Kondo insulator (1) SmB 6 (2) behaves as a correlated metal at high temperatures. Below 40 K the bulk of SmB 6 becomes insulating with the opening of Kondo energy gap due to the hybridization between conduction electrons and the highly renormalized f-electrons. The theory (3) of topological Kondo insulator predicted the existence of a topologically protected surface state (TSS) within this Kondo gap, naturally explaining the mysterious resistance saturation below 4 K (1). Recent transport measurements (4-6) have confirmed the low temperature surface conduction and the robustness of the surface state (SS). This SS has been demonstrated (7) to vanish with a small amount of magnetic impurity but survives larger amount of non-magnetic doping, which is consistent with a TSS protected by time-reversal symmetry. Recent high resolution ARPES (8-10) and quantum oscillation (11) experiments have provided tentative evidence for the Dirac dispersion of the surface carriers, as expected in a TSS. Furthermore, unlike usual topological SS, first principle calculations (12, 13) have predicted three surface Dirac bands residing at Γ and X/Y points, which agrees with ARPES-measured surface electronic structure (8-10), although the anticipated spin-momentum locking (14, 15) awaits spin-resolved measurements. In this paper we perform transport studies of the SmB 6 SS down to 20 mK, searching for transport signatures of spinmomentum locking and electron interaction effects.Extending our previous work (6), we have verified the existence of SS in SmB 6 samples down to 20mK with non-local transport and thickness dependent Hall effect measurements. In particular, Fig. 1A demonstrates the non-local measurement from sample S11 with both (100) and (101)

We report a frequency coding limit cycle and anomalous capacitance in the Kondo insulator SmB6 at low temperatures where the insulating gap becomes fully opened. The limit cycle appears to be associated with local activity and autocatalytic temporal pattern formation, as occurs in biological systems. The measured anomalous capacitance may indicate surface and bulk separation, suggesting the formation of a surface conducting state. The biological analogy suggests lossless information transport and complex information coding, and the surface state with a superconductor would provide a possible venue for quantum computing resources without decoherence.

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