Abstract for Wayne State Colloquium Talk – Feb. 28, 2013

Topologically protected surface conduction in SmB6 –
new solution to a thirty year old mystery

J. W. Allen

Department of Physics, Randall Laboratory, University of Michigan, Ann Arbor, MI 48109

Topological insulators are arguably the most exciting new development in condensed matter physics since initial theoretical work in 2005. These materials constitute a new state of matter in which the (three-dimensional) bulk is insulating but the surface is metallic due to topologically protected surface states that disperse across the Fermi energy EFinside the bulk insulator gap. The initial theory was done for insulators which are weakly correlated and hence well described by the usual band theory so that Coulomb interactions need not be specifically included. The theory has recently been extended to predict that the so-called Kondo insulators, which are strongly correlated heavy fermion or mixed valent rare earth compounds, can also have such topologically protected surface states. Pursuing this possibility [1, 2] for the mixed valent insulator SmB6 has led to a new solution of a more than thirty year old mystery [3] concerning the electrical transport of this material.

In mixed valent materials hybridization between the f-shell and conduction band states enables quantum fluctuations of the f-shell occupation in spite of the very large f-shell Coulomb interaction that normally forces integer valence and binds the f-electrons into atomic multiplet states. SmB6 is one of the earliest known such materials. More than thirty years ago it came as a surprise to find that in spite of these quantum fluctuations a small insulating gap is indicated by an exponential increase of its resistivity with decreasing temperature (T), and it was then even more perplexing that at the lowest T the resistivity rise saturates as for a metal but at a value so high that within the framework of bulk conduction it has never been rationalized as arising either from intrinsic or impurity band states in the gap. [3] Numerous efforts over the years to eliminate this anomalously large residual resistivity have failed. Through novel electrical measurements we have now shown that this low T conductivity resides on the surface. [2] Our finding is consistent with a theoretical prediction [1] that SmB6 is a topological insulator and also resolves the old mystery in a surprising way – theresidual conductivity is intrinsic but not in the bulk. We argue that the robustness of the conductivity is a signature of the topological protection of the surface states.

[1] M. Dzero, K. Sun, V. Galitski and P. Coleman, "Topological Kondo Insulators," Phys. Rev. Lett. 104, 106408 (2010).

[2] Steven Wolgast, Çağliyan Kurdak, Kai Sun, J. W. Allen, Dae-Jeong Kim and Zachary Fisk, "Discovery of the First Topological Kondo Insulator: Samarium Hexaboride,"arXiv:1211.5104v2.

[3] J. W. Allen, B. Batlogg and P. Wachter, "Large Low-Temperature Hall Effect and Resistivity in Mixed-Valent SmB6," Phys. Rev. B 20, 4807 (1979).