Electric-Field Control of Topology in Germanene

We demonstrate that germanene, a one-atom-thick sheet of germanium, can be driven between distinct topological phases in two-dimensions (2D) and one-dimension (1D). In 2D epitaxial germanene films, we observe quantum spin Hall behavior with a sizable energy gap and metallic edges. A perpendicular electric field switches the system from this topological insulator phase to a Dirac semimetal and then to an ordinary (non-topological) insulator [1]. Reducing by one dimension, we fabricated zigzag-terminated germanene nanoribbons and identified the critical width (~2 nm) at which the two-dimensional topological phase breaks down, giving rise to the one-dimensional topological insulator with symmetry-protected end states [2]. In this 1D limit, we achieved reversible all-electric switching of the zero-dimensional modes, providing atomic-scale control of topological states [3]. Together, these results establish germanene as a versatile platform for exploring dimensionality effects in topology and for developing novel low-energy, topological field-effect devices, with prospects for room-temperature electronics, quantum information processing, and neuromorphic architectures.

References

[1] Bampoulis, P., Castenmiller, C., Klaassen, D. J., van Mil, J., Liu, Y., Liu, C.-C., Yao, Y., Ezawa, M., Rudenko, A. N. & Zandvliet, H. J. W. Quantum Spin Hall States and Topological Phase Transition in Germanene. Phys. Rev. Lett. 130, 196401 (2023).

[2] Klaassen, D. J., Eek, L., Rudenko, A. N., van ’t Westende, E. D., Castenmiller, C., Zhang, Z., de Boeij, P. L., van Houselt, A., Ezawa, M., Zandvliet, H. J. W., Morais Smith, C., Bampoulis, P. Realization of a one-dimensional topological insulator in ultrathin germanene nanoribbons. Nat. Commun. 16, 2059 (2025).

[3] Eek, L., van ’t Westende, E. D., Klaassen, D. J., Zandvliet, H. J. W., Bampoulis, P., & Morais Smith, C. Electric-field control of zero-dimensional topological states in ultranarrow germanene nanoribbons. Phys. Rev. Lett (2025).