Electric-Field Control of Zero-Dimensional Topological States in Ultranarrow Germanene Nanoribbons

Reversible, all-electric control of symmetry-protected zero-dimensional modes has been a long-standing goal. In buckled honeycomb lattices, a perpendicular field couples to the staggered sublattice potential providing the required handle. We combine scanning tunneling microscopy and tight-binding theory to switch zero-dimensional topological end states reversibly on and off in ultranarrow germanene nanoribbons by tuning the electric field in the tunnel junction. Increasing the field switches off the end modes of topological two-hexagon-wide ribbons, while the same field switches on zero-dimensional states in initially trivial three- and four-hexagon-wide ribbons. This atomic scale platform realizes a proof of principle for a zero-dimensional topological field effect device, opening a path for ultrasmall memory, controllable qubits, and neuromorphic architectures.