Bulk Hybridization Gap and Surface Conduction in the Kondo Insulator SmB 6 Richard L. Greene, University of Maryland College Park, DMR Recently, topological insulators have attracted enormous scientific interest because of their novel surface electronic states that have the potential to lead to new types of low power spintronic devices. The spin and the momentum of these topological surface states are inter-locked with each other which prevents electron scattering and thus protected charge and spin transport. Topological insulators in contact with a superconductor are theoretically predicted to host Majorana fermions that could act as protected qubits for quantum computation. To date, most topological insulators show bulk conducting behavior which complicates the direct investigation of the properties of the topological conducting surface states. However, a recent theoretical study [Dzero et al., Phys. Rev. Lett. 104, (2010)] suggested that topologically protected surface states could also be found in some Kondo insulators, such as SmB 6. The presence of topological surface states would explain the decades old puzzle of the unusual low temperature residual conductivity of SmB 6, which was previously interpreted as conduction by bulk in-gap impurity states. By using the bulk sensitive point-contact spectroscopy (PCS) technique, we demonstrated for the first time the evolution of the hybridization gap in a Kondo insulator as the correlation between electrons and local spins increases with decreasing temperature. A strongly asymmetric Fano-like PCS conductance spectrum appears at temperatures below the Kondo temperature at about 100 K (Fig. a). Below about 30K the interactions between samarium ions and the conducting d-electrons leads to the formation of a bulk insulating Kondo hybridization gap. Significantly, the excellent low-temperature fit shown in Fig. (a) excludes the presence of in-gap bound states in the bulk of the material. This strongly suggests that the low-temperature electrical conductivity is the result of a metallic surface channel, which supports the idea of the predicted topological surface state (Fig. b). Our study provides an important new understanding of the exotic properties of the Kondo insulator SmB 6. What is needed next are experiments to determine if the surface conducting states are topological. Reference: X. H. Zhang, N. P. Butch, P. Syers, S. Ziemak, R. L. Greene, and J. Paglione, Phys. Rev. X 3, (2013). a) Point contact spectroscopy of the Kondo insulator SmB 6 at selected temperatures: Conductance (dI/dV) spectra (circles) can be well described by a Kondo tunneling model [red lines, Maltseva et al., Phys. Rev. Lett. 103, (2009)], suggesting a truly insulating bulk state for SmB 6 at low temperatures. b) A simple double-channel model for the transport in SmB 6 : at low temperatures, the metallic surface conduction surpasses the bulk electronic transport which follows a simple activation law.

Bulk Hybridization Gap and Surface Conduction in the Kondo Insulator SmB 6 Richard L. Greene, University of Maryland College Park, DMR Education A postdoctoral researcher (Xiaohang Zhang) contributed to this work. Dr. Zhang continues his research at Maryland to better understand superconductors and topological insulators. This work involved a collaboration with Dr. Johnpierre Paglione’s group at the University of Maryland (supported by a different NSF grant) and with Dr. N. P. Butch at the Lawrence Livermore National Laboratory. Broader Impacts The release of our results immediately attracted tremendous interest in the scientific community. For instance, our research was highlighted in Nature News (Nature 492, 165 (2012) “Hopes surface for exotic insulator” and by the American Physical Society in a Physics Synopsis ( for/ /PhysRevX ). A better understanding of topological insulators promises to lead to the development of various next generation electronic and spintronic devices.