arXiv subjects

Paul L. de Boeij

Publications and source records attributed to Paul L. de Boeij.

3 recordsLinked to original sources

arXiv2026

Decorated electronic kagome lattice in twisted bilayer germanene

Artificial kagome lattices provide a route to electronic flat bands, geometric frustration, and correlation driven phases, but their realization in atomically controlled two-dimensional materials remains scarce. Here, we show that commensurate twisted bilayer germanene on Ge2Pt produces two electronically distinct large-angle moiré phases. Scanning tunneling microscopy measurements and density functional theory calculations reveal that commensurate twisted bilayers that are odd under an exchange of sublattices are semiconducting, whereas the twisted bilayers with an even parity are metallic. The twisted bilayers with an even parity host an empty state resonance that exhibits an emergent decorated kagome structure with a C3 symmetry. These results establish large-angle twisted germanene as a platform for engineering kagome-like electronic states in a buckled two-dimensional material.

cond-mat.mes-hall↗

arXiv2024

Moiré-modulated band gap and van Hove singularities in twisted bilayer germanene

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 21.8$^o$ degrees twist angle, corresponding to a commensurate ($\sqrt{7} \times \sqrt{7}$) structure. Using scanning tunneling microscopy and spectroscopy, we unraveled the structural and electronic properties of this configuration, revealing a moiré-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 $(8 \pm 1) \times 10^5$ 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.

cond-mat.mes-hall↗

arXiv2017

Spatially resolved electronic structure of twisted graphene

We have used scanning tunneling microscopy and spectroscopy to resolve the spatial variation of the density of states of twisted graphene layers on top of a highly oriented pyrolytic graphite substrate. Owing to the twist a moire pattern develops with a periodicity that is substantially larger than the periodicity of a single layer graphene. The twisted graphene layer has electronic properties that are distinctly different from that of a single layer graphene due to the nonzero interlayer coupling. For small twist angles (about 1-3.5 degree) the integrated differential conductivity spectrum exhibits two well-defined Van Hove singularities. Spatial maps of the differential conductivity that are recorded at energies near the Fermi level exhibit a honeycomb structure that is comprised of two inequivalent hexagonal sub-lattices. For energies |E-E_F|>0.3 eV the hexagonal structure in the differential conductivity maps vanishes. We have performed tight-binding calculations of the twisted graphene system using the propagation method, in which a third graphene layer is added to mimic the substrate. This third layer lowers the symmetry and explains the development of the two hexagonal sub-lattices in the moire pattern. Our experimental results are in excellent agreement with the tight-binding calculations.

cond-mat.mes-hall↗