Speaker: Cagliyan Kurdak, University of Michigan Title: Robustness of the Insulating Bulk and the Conducting Surface in the Topological Kondo Insulator SmB6 Abstract: The prediction of the topological Kondo insulator behavior in samarium hexaboride (SmB6) and the verification of robust surface conduction below 4 K have led to a large effort to understand the surface states in this material. Moreover, recent magnetization measurements suggest that the material might have an unconventional bulk Fermi surface, even though the material is electrically insulating with a well-defined gap. The results are so far only observed in “special samples” that are grown by the floating zone method. The unconventional bulk Fermi surface, with a truly insulating gap, requires exotic excitations, such as composite excitons, neutral spinons, or Majorana fermions, which couple to a magnetic field but not to an electric field. Experimental demonstration of such novel low energy excitations would be a great achievement as it would indicate the bulk of SmB6 must harbor one of the most unusual quantum states of matter. In this work, we study the transport properties of SmB6 grown both floating zone and Al flux methods with the goal of resolving why these excitations occur in special samples. In addition to standard Hall and Corbino measurements, we have a new type of resistance measurement, inverted resistance measurement, to study electrical conduction in this material system. This new method allows us to characterize the bulk conductivity, even in situations where surface conduction dominates transport by many orders of magnitude. We find the bulk resistivity exhibits an activated behavior that extends over 10 orders of magnitude with an activation energy of 4.0 meV. This is a remarkable result as it would not happen in any other topological insulator or semiconductor. Furthermore, samples grown with off-stoichiometric growth conditions exhibit activated behavior with similar activation energies, suggesting that SmB6 is an ideal insulator that is immune to point defects. On the other hand. extended defects such as one-dimensional dislocations and surface cracks may still be present would provide additional current paths beside the topologically protected surface state in this material system. I will discuss some of the new theories emerging to discuss the robustness of this material.
Host: J. Paglione
Schedule: here Refreshments 1:30pm John S Toll Physics Bldg Room 1117