Twisting bilayers of two-dimensional topological insulators has the potential to create unique quantum states of matter. Here, we successfully synthesized a twisted bilayer of germanene on Ge2Pt(101) with a 21.8o degrees twist angle, corresponding to a commensurate (7×7) structure. Using scanning tunneling microscopy and spectroscopy, we unraveled the structural and electronic properties of this configuration, revealing a moir\'e-modulated band gap and a well-defined edge state. This band gap opens at AB/BA stacked sites and closes at AA stacked sites, a phenomenon attributed to the electric field induced by the scanning tunneling microscopy tip. Our study further revealed two van Hove singularities at -0.8 eV and +1.04 eV, resulting in a Fermi velocity of (8±1)×105 m/s. Our tight-binding results uncover a unique quantum state, where the topological properties could be regulated through an electric field, potentially triggering two topological phase transitions.
@article{arxiv.2403.14560,
title = {Moir\'e-modulated band gap and van Hove singularities in twisted bilayer germanene},
author = {Pantelis Bampoulis and Carolien Castenmiller and Dennis J. Klaassen and Jelle v. Mil and Paul L. de Boeij and Motohiko Ezawa and Harold J. W. Zandvliet},
journal= {arXiv preprint arXiv:2403.14560},
year = {2025}
}