Correlation between Bulk Thermodynamic Measurements and the Low-Temperature-Resistance Plateau in SmB6

Abstract

Topological insulators are materials characterized by dissipationless, spin-polarized surface states resulting from nontrivial band topologies. Recent theoretical models and experiments suggest that SmB6 is the first topological Kondo insulator, in which the topologically nontrivial band structure results from electron-electron interactions via Kondo hybridization. Here, we report that the surface conductivity of SmB6 increases systematically with bulk carbon content. Further, addition of carbon is linked to an increase in n -type carriers, larger low-temperature electronic contributions to the specific heat with a characteristic temperature scale of T*=17K , and a broadening of the crossover to the insulating state. Additionally, x-ray absorption spectroscopy shows a change in Sm valence at the surface. Our results highlight the importance of phonon dynamics in producing a Kondo insulating state and demonstrate a correlation between the bulk thermodynamic state and the low-temperature resistance of SmB6 .

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Plain Language Summary

Samarium hexaboride, SmB6 , is a material that undergoes a transition from a metallic to an insulating state below about 40 K. At even lower temperatures, about 5–10 K, a resistance plateau indicative of residual metallicity appears. The origins of this remnant conduction have remained enigmatic for more than 40 years. Previously attributed to impurities and secondary phases, recent theory and experiments suggest that the low-temperature resistivity plateau is due to topologically protected metallic surface states, arising from a nontrivial band topology in the insulating state of SmB6 . We show a correlation between thermodynamic observables (e.g., specific heat) and the low-temperature electrical conduction in SmB6 .

Since most samples of SmB6 are grown in aluminum flux or with the aid of carbon-based binders, we dope some of our SmB6 samples with these impurities—aluminum and carbon. We find that the surface conductivity of our samples increases with bulk carbon content, suggesting that the resistance can be controlled via the addition of carbon. In contrast, aluminum does not dramatically affect the resistance plateau.

These results serve as a foundation for resolving the origins of, and controlling, the surface states in SmB6 , a material appealing for spintronics and quantum computation applications. Our results demonstrate a correlation between the bulk thermodynamic properties of SmB6 and the low-temperature resistivity plateau arising from surface-dominated conduction, a necessary prerequisite for topological Kondo insulating behavior.