Würzburg ToCoTronics Colloquium

Understanding topological phase transitions, particularly from two-dimensional (2D) to one-dimensional (1D) systems, is crucial for discovering new quantum states and advancing topological devices. Here, we theoretically and experimentally demonstrate that germanene exhibits complex, non-monotonic topological behavior influenced by size, spin-orbit coupling, and staggered mass. Low-buckled epitaxial germanene, a quantum spin Hall insulator with a large bulk gap and metallic edges, undergoes a topological phase transition under a critical perpendicular electric field. At this field, the topological gap closes, transforming germanene into a Dirac semimetal, and at higher fields, into a trivial insulator. Moreover, by controlling the growth process, we fabricate germanene nanoribbons with variable widths, enabling exploration of width-dependent electronic properties. In the nanoribbons, we observe a transition from a 2D to a 1D topological insulator phase below ~2 nm, where 1D edge states vanish, giving rise to zero-dimensional (0D) end states.