Matthias Bode1 Paolo Sessi1 Domenico DiSante1 Andrzej Szczerbakow2 Florian Glott1 Stefan Wilfert1 Henrik Schmidt1 Thomas Bathon1 Piotr Dziawa2 Martin Greiter1 Titus Neupert3 Giorgio Sangiovanni1 Tomasz Story2 Ronny Thomale1

1, Univ. Wuerzburg, Hamburg, , Germany
2, Polish Academy of Sciences, Warsaw, , Poland
3, Univ. Zurich, Zurich, , Switzerland

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