Time-reversal invariant two-dimensional topological insulators, often dubbed Quantum Spin Hall systems, possess helical edge modes whose ballistic transport is protected by physical symmetries. We argue that, though the time-reversal symmetry (TRS) of the bulk is needed for the existence of helicity, protection of the helical transport is actually provided by the spin conservation on the edges. This general statement is illustrated by some specific examples confirming the importance of the spin conservation. One of these examples demonstrates the ballistic conductance in the setup where the TRS on the edge is broken. It shows that attributing the symmetry protection exclusively to the TRS is not entirely correct.
- Pub Date:
- September 2019
- Condensed Matter - Mesoscale and Nanoscale Physics;
- Condensed Matter - Disordered Systems and Neural Networks
- several references and their discussion added, conclusions extended, 9 pages, 2 figures