Topological phase transition in chiral graphene nanoribbons: from edge bands to end states

Precise control over the size and shape of graphene nanostructures allows engineering spin-polarized edge and topological states, representing a novel source of non-conventional π-magnetism with promising applications in quantum spintronics. A prerequisite for their emergence is the existence of robust gapped phases, which are difficult to find in extended graphene systems. Here we show that semi-metallic chiral GNRs (chGNRs) narrowed down to nanometer widths undergo a topological phase transition. We fabricated atomically precise chGNRs of different chirality and size by on surface synthesis using predesigned molecular precursors. Combining scanning tunneling microscopy (STM) measurements and theory simulations, we follow the evolution of topological properties and bulk band gap depending on the width, length, and chirality of chGNRs. Our findings represent a new platform for producing topologically protected spin states and demonstrate the potential of connecting chiral edge and defect structure with band engineering.We gratefully acknowledge financial support from the Agencia Estatal de Investigación (AEI) through projects No MAT2016-78293, PID2019-107338RB, and FIS2017-83780-P, and the Maria de Maeztu Units of Excellence Programme MDM-2016-0618, from the Xunta de Galicia (Centro singular de investigación de Galicia, accreditation 2016–2019, ED431G/09), from the University of the Basque Country (Grant IT1246-19) and the Basque Departamento de Educación (PhD scholarship no. PRE_2019_2_0218 of S.S.), and from the European Regional Development Fund. We also acknowledge funding from the European Union (EU) H2020 program through the ERC (grant agreement No. 635919) and FET Open project SPRING (grant agreement No. 863098).Peer reviewe