Surface Hall Effect and Nonlocal Transport in SmB 6 : Evidence for Surface Conduction D. J. Kim*, S. Thomas*, T. Grant, J. Botimer, Z. Fisk & Jing Xia Dept. of Physics and Astronomy, University of California, Irvine, California 92697, USA. 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 Bi 2 Se 3 , Bi 2 Te 3 and Sb 2 Te 3 ) due to impurity caused bulk conduction. We report in large crystals of topological Kondo insulator (TKI) candidate material SmB 6 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. R ecently discovered 1–7 three-dimensional (3D) topological insulators (TI) have generated great excitement. Strong spin-orbit coupling in a TI gives rise to a non-trivial and robust conducting surface state 4,5 , remin- iscent of the edge channel 8,9 found in quantum Hall (QH) 10,11 and quantum spin Hall (QSH) 12,13 states. However, such surface transport properties have remained challenging to separate from residual bulk impurity conduction 14–17 despite rapid recent improvements 7,18–21 , promoting us to search for TI materials with better bulk insulation. More importantly, most theoretical and experimental efforts to date have been dedicated to materials with the underlying physics of non-interacting electrons 1,4,5,12 . A big question thus concerns the role of electron interaction and competing orders in new TI materials with strong correlation, from which new types of topo- logical phases are expected to emerge 4,5 . Recent seminal theoretical works 22,23 have predicted in the strongly correlated material SmB 6 the existence of a topological insulator phase: topological Kondo insulator (TKI). SmB 6 is a heavy fermion material first studied 40 years ago 24 . In SmB 6 , highly renormalized f-electrons hybridize with conduction electrons and form a completely filled band of quasiparticles with a transport excitation gap D of about 40 Kelvin. However, its many exotic properties 25–27 still defy a satisfactory understanding within the framework of conventional insulators. One of these mysteries is the peculiar residual conduction at the lowest temperatures. Behaving electronically like an insulator at high temperatures, at low temperature its resistance mysteriously saturates: a curiosity that is usually attributed to the existence of density of states within the band gap. According to the recent TKI theory 22,23,28,29 , the hybridization and odd parity wavefunction lead to strong spin-orbit coupling in SmB 6 and give rise to a topo- logical surface state, which naturally explains the origin of the in-gap state and pinpoints its location to be on the surface of SmB 6 . Our recent capacitance measurements on high quality SmB 6 crystals revealed intriguing anom- alous capacitance effects 30 that could be explained by assuming the in-gap-states exist on the surface. In this paper we present evidence of thickness-independent surface Hall effect and nonlocal transport in high quality SmB 6 crystals of various geometries and from different growth batches. These results reveal in SmB 6 an insulating bulk and a conducting surface. Results Hall effect measurements were carried out in wedge-shaped SmB 6 crystals. As depicted in the inset in Fig. 1(a), the sample is placed in a perpendicular magnetic field B ! and current I flows between the two ends of the wedge. The Hall resistances R xy 5 V xy /I are measured at different thicknesses d to distinguish between surface and bulk conduction. For bulk conduction R xy /B / 1/d, while R xy /B is d-independent if surface conduction dominates. In both cases, one can define a Hall coefficient R H 5 E y /j x B independent of B, where j x is the current density (surface OPEN SUBJECT AREAS: TOPOLOGICAL INSULATORS ELECTRONIC PROPERTIES AND MATERIALS TWO-DIMENSIONAL MATERIALS Received 2 July 2013 Accepted 22 October 2013 Published 6 November 2013 Correspondence and requests for materials should be addressed to J.X. (xia.jing@uci.edu) * These authors contributed equally to this work. SCIENTIFIC REPORTS | 3 : 3150 | DOI: 10.1038/srep03150 1