Speaker
Description
For the quantum spin Hall insulator bismuthene, robust helical edge states protected by a large topological band gap of 800 meV have been demonstrated, making the system interesting for potential room-temperature spintronic applications [1]. Here, we show that a single layer of elemental Bi, formed by intercalating an epitaxial graphene buffer layer on SiC(0001), can be transformed into bismuthene [2]. Specifically, the layer of atomic Bi can be reversibly switched between an electronically inactive precursor state and a state that exhibits the predicted band structure of a true two-dimensional bismuthene. This switching is accomplished by enabling/disabling a partial hydrogen-passivation of Si dangling bonds, which triggers a change of the Bi adsorption site.
This key finding describing the mechanisms behind bismuthene formation and switching, was achieved using normal incidence x-ray standing wave imaging (NIXSWI) [2]. Diffraction-based techniques, which are conventionally used to resolve atomic structure, are limited by the so-called "phase problem". Specifically, they are capable of determining the diffraction intensities, which represent the squared magnitudes of the complex structure factors. On the contrary, NIXSWI is a “direct method” for structure determination because it measures both the amplitudes and phases of the structure factors. It overcomes the phase problem by exploiting the direct relationship between the phase of the structure factor and the phase of an x-ray standing wave field generated by the interference of the incoming wave and the selected Bragg wave reflected from a crystalline substrate. Although this method was proposed already 40 years ago [3], it was raraely applied due to its demanding experimental requirements. In our study, NIXSWI provides chemically specific atomic densities, directly resolving the structural configuration of bismuth before and after transformation.
[1] L. Gehrig et al., Adv. Mater. 37, 2502412 (2025)
[2] N. Tilgner et al., 2D Mat. 12, 045020 (2025), and Nat. Commun. 16, 6171 (2025)
[3] M.J. Bedzyk, G. Materlik, Phys. Rev. B 32, 6456 (1985).