Discovery of a hybrid topological quantum state in an elemental solid

Topology and interactions are foundational concepts in the modernunderstanding of quantum matter. Their nexus yields three significant researchdirections: competition between distinct interactions, as in the multipleintertwined phases, interplay between interactions and topology that drives thephenomena in twisted layered materials and topological magnets, and thecoalescence of multiple topological orders to generate distinct novel phases.The first two examples have grown into major areas of research, while the lastexample remains mostly untouched, mainly because of the lack of a materialplatform for experimental studies. Here, using tunneling microscopy,photoemission spectroscopy, and theoretical analysis, we unveil a "hybrid" andyet novel topological phase of matter in the simple elemental solid arsenic.Through a unique bulk-surface-edge correspondence, we uncover that arsenicfeatures a conjoined strong and higher-order topology, stabilizing a hybridtopological phase. While momentum-space spectroscopy measurements show signs oftopological surface states, real-space microscopy measurements unravel a uniquegeometry of topology-induced step edge conduction channels revealed on variousforms of natural nanostructures on the surface. Using theoretical models, weshow that the existence of gapless step edge states in arsenic relies on thesimultaneous presence of both a nontrivial strong Z2 invariant and a nontrivialhigher-order topological invariant, providing experimental evidence for hybridtopology and its realization in a single crystal. Our discovery highlightspathways to explore the interplay of different kinds of band topology andharness the associated topological conduction channels in future engineeredquantum or nano-devices.