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Robust spin-polarized midgap states at step edges of topological crystalline insulators

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Science  09 Dec 2016:
Vol. 354, Issue 6317, pp. 1269-1273
DOI: 10.1126/science.aah6233

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An edge that is hard to get rid of

A distinguishing characteristic of topological insulators (TIs) is that they have conducting states on their boundary—a surface for a three-dimensional (3D) TI or a line edge for a 2D TI. Sessi et al. used scanning tunneling spectroscopy to discover unusual 1D states in a 3D crystalline TI. The states appeared on the edge of a particular kind of step in the crystal and survived large magnetic fields and increased temperatures. This robustness bodes well for the potential use of these states in practical applications.

Science, this issue p. 1269

Abstract

Topological crystalline insulators are materials in which the crystalline symmetry leads to topologically protected surface states with a chiral spin texture, rendering them potential candidates for spintronics applications. Using scanning tunneling spectroscopy, we uncover the existence of one-dimensional (1D) midgap states at odd-atomic surface step edges of the three-dimensional topological crystalline insulator (Pb,Sn)Se. A minimal toy model and realistic tight-binding calculations identify them as spin-polarized flat bands connecting two Dirac points. This nontrivial origin provides the 1D midgap states with inherent stability and protects them from backscattering. We experimentally show that this stability results in a striking robustness to defects, strong magnetic fields, and elevated temperature.

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