A hybrid topological quantum state in elemental solid arsenic

Nature 628, 527–533 (2024)

We have demonstrated the hybridization between the gapped edge state and the gapless surface state. In the first (left) column, the free-standing monolayer exhibits a gapped edge state. The second (middle) column demonstrates that the clean surface of gray arsenic (As) features a gapless Rashba surface state near the Fermi level. In the third (right) column, with the introduction of monolayer step edges, the edge state undergoes hybridization with the gapless surface states, resulting in the emergence of gapless edge states at the step edges.

An international team of scientists including dr Rajibul Islam from the Institute of Physics P.A.S. has demonstrated that the observed step-edge states in α-As, unlike surface or hinge states, are not expected for either first or higher-order topological insulators separately but only for hybrid materials in which both kinds of band topology are present.

The discovery of new quantum phases of materials that exhibit exotic properties, such as dissipationless transport through helical edge channels, attracts much attention in condensed matter physics. In this paper, we demonstrate the coalescence of several topological orders to generate distinct novel phases called “Hybrid topological phase”. Employing in-depth first-principles calculations, IFPAN researcher Rajibul Islam (ON6.5) in collaboration with experimental groups from Princeton, Imperial College, Beihang University, Nanyang Technological University,Florida State University, and the University of Zurich, discovered such a new quantum phase - the “Hybrid topological phase” - in grey elemental Arsenic materials.

Topology and interactions are fundamental concepts in the modern understanding of quantum matter. Their nexus yields several research directions including: the competition between distinct interactions, as in several intertwined phases; the interplay between interactions and topology that drives phenomena in twisted layered materials and topological magnets; and the coalescence of several topological orders to generate distinct novel phases. The first two examples have grown into major areas of research, although the last example remains mostly unexplored, mainly because of the lack of a material platform for experimental studies till recently. Here, using tunnelling microscopy, photoemission spectroscopy and theoretical analysis, we unveil a ‘hybrid’ topological phase of matter in the simple elemental solid arsenic. Through a unique bulk-surface-edge correspondence, we uncover that arsenic features a conjoined strong and higher-order topology that stabilizes a hybrid topological phase. Although momentum-space spectroscopy measurements show signs of topological surface states, real-space microscopy measurements unravel a unique geometry of topologically induced step-edge conduction channels revealed on various natural nanostructures on the surface. Using theoretical models, we show that the existence of gapless step-edge states in arsenic relies on the simultaneous presence of both a non-trivial strong Z₂ invariant and a non-trivial higher-order topological invariant, which provide experimental evidence for hybrid topology.

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Publications

A hybrid topological quantum state in an elemental solid

Md Shafayat Hossain, Frank Schindler, Rajibul Islam, Zahir Muhammad, Yu-Xiao Jiang, Zi-Jia Cheng, Qi Zhang, Tao Hou, Hongyu Chen, Maksim Litskevich, Brian Casas, Jia-Xin Yin, Tyler A. Cochran, Mohammad Yahyavi, Xian P. Yang, Luis Balicas, Guoqing Chang, Weisheng Zhao, Titus Neupert & M. Zahid Hasan

Nature 628, 527–533 (2024)

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