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Speaker: Vidya Madhavan, University of Illinois Urbana-Champaign

Abstract: A well-known result from quantum mechanics is that when normal electrons scatter from a potential barrier, the probability of tunneling is suppressed with both barrier height and width. In a surprising result, Oskar Klein showed in 1929 that relativistic Dirac fermions can show completely unexpected tunneling behavior. For example, an electron encountering a barrier may be perfectly transmitted even for an infinitely tall barrier. Topological insulators hosting boundary modes with Dirac dispersion are ideal candidates to demonstrate the unique tunneling properties of relativistic fermions. However, such experiments have been difficult to carry out due to a combination of materials issues and geometric constraints. In this work we use nanowires of a special correlated topological insulator (SmB₆) mounted on the tip of a scanning tunneling microscope to demonstrate a tunneling process called `helical tunneling’ where the tunneling Dirac fermions are spin-polarized even in the absence of broken time-reversal symmetry. Our calculations show that the spin-polarization must reverse sign when traveling in the opposite direction, which we confirm in our experiments. Our findings show that the topological surface states in SmB₆ nanowires are conduits for robust spin-polarized currents and demonstrate another unexpected tunneling property of relativistic Dirac electrons.

 Host:  Eva Andrei