Edge-dependent topology in Kekul\'e lattices

Abstract

Topological states of matter are robust quantum phases, characterised by propagating or localised edge states in an insulating bulk. Topological boundary states can be triggered by various mechanisms, for example by strong spin-orbit coupling. In this case, the existence of topological states does not depend on the termination of the material. On the other hand, topological phases can also occur in systems without spin-orbit coupling, such as topological crystalline insulators. In these systems, the protection mechanism originates from the crystal symmetry. Here, we show that for topological crystalline insulators with the same bulk, different edge geometries can lead to topological or trivial states. We artificially engineer and investigate a 2D electronic dimerised honeycomb structure, known as the Kekul\'e lattice, on the nanoscale. The surface electrons of Cu(111) are confined into this geometry by positioning repulsive scatterers (carbon monoxide molecules) with atomic precision, using the tip of a scanning tunnelling microscope. We show experimentally and theoretically that for the same bulk, molecular zigzag and partially bearded edges lead to topological or trivial states in the opposite range of parameters, thus revealing a subtle link between topology and edge termination. Our results shed further light on the nature of topological states and might be useful for future manipulations of these states, with the aim of designing valves or other more complex devices.