| We study thin strips of three dimensional topological insulators in the presence of a spin-splitting Zeeman field. We show that under certain conditions the topological edge states at the sides of a strip remain robust against a time-reversal symmetry breaking Zeeman field. For a particle-hole symmetric system with Zeeman field perpendicular to the strip we strictly proof that the dispersion and the spin-orbital structure of the edge states remains unchanged. When the Zeeman field lies parallel to the strip, the Dirac spectrum becomes flat, but remains intact. Above a critical value of the Zeeman field a completely flat surface band develops. We present numerical calculations for a realistic tight-binding model of Bi2Se3. These calculations show that even though particle-hole symmetry is not strictly fulfilled in this system, these special features are still present. |
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