Abstract
As an exemplary Kondo insulator, has been studied for several decades. However, direct evidence for the development of the Kondo coherent state and for the evolution of the electronic structure in the material has not been obtained due to the compound’s rather complicated electronic and thermal transport behavior. Recently, these open questions have attracted increasing attention as the emergence of a time-reversal-invariant topological surface state in the Kondo insulator has been suggested. Here, we use point-contact spectroscopy to reveal the temperature dependence of the electronic states in . We demonstrate that is a model Kondo insulator: Below 100 K, the conductance spectra reflect the Kondo hybridization of Sm ions, but, below about , signatures of inter-ion correlation effects clearly emerge. Moreover, we find evidence that the low-temperature insulating state of this exemplary Kondo-lattice compound harbors conduction states on the surface, in support of predictions of nontrivial topology in Kondo insulators.
- Received 24 November 2012
DOI:https://doi.org/10.1103/PhysRevX.3.011011
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Published by the American Physical Society
Synopsis
Surface Protection
Published 14 February 2013
Surface spectroscopy shows that a material long known as a Kondo insulator also exhibits the metallic surface states of a topological insulator.
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Popular Summary
The electrical behavior of a compound discovered in 1969, samarium hexaboride (), has been a 40-year mystery in condensed matter physics. The material was thought to be a Kondo lattice, an insulator in which screening of its localized spins associated with the ion lattice by the itinerant electrons was expected to lead to a temperature-dependent evolution of its electronic structure and “freezing” of the electrons at low temperatures. Yet, a finite electrical conductance was observed at very low temperatures. A theoretical solution to this mystery was proposed two years ago in the form of the novel concept of topological Kondo insulators, materials or physical states where nontrivial surface electrical conduction coexists with the Kondo effect. However, clear evidence for the evolution of the material’s electronic structure with temperature has not been obtained, nor has there been experimental confirmation of the predicted surface conduction. In this paper, we report experimental results that not only show for the first time the systematic Kondo-type evolution with temperature of the electronic structure of but also support the prediction of as a topological Kondo insulator.
Our experimental results are obtained by the use of point-contact spectroscopy. Soft point-contact junctions between and silver are fabricated on a single crystal. Conductance through such a junction as the function of the bias voltage across it—a conductance spectrum—probes directly the electronic structure of the bulk crystal. An asymmetric conductance spectrum observed in our junctions is a clear indication of the existence of the expected Kondo screening effect in . An analysis of the spectra confirms that, by its electronic structure, the single crystal is indeed a bulk insulator, a conclusion that strongly suggests that the finite low-temperature conductance observed earlier should be attributed to the conduction of surface states, in other words, that is a topological Kondo insulator.
The experimental findings reported here provide a simple but clear example for the existing understanding of the correlation between the electronic structure and the screening effect in Kondo lattices. In addition, the implication of the presence of conducting surface states in should also add new incentive to the study of topological insulators and their applications.