arXiv subjects

Jürgen P. Rabe

Publications and source records attributed to Jürgen P. Rabe.

12 recordsLinked to original sources

arXiv2026

Writing and erasing skyrmions by single ultrafast laser pulses in monolayer Janus 2D magnets

Skyrmions in 2D magnets are promising candidates for nonvolatile, low-power, and high-density spintronic memories. However, their experimental realization at the 2D limit remains challenging, owing to the difficulty in engineering the required chiral magnetic interactions. Here, we report the creation and direct imaging of Néel-type skyrmions in Janus 2D chromium chalcogenides using synchrotron X-ray photoemission electron microscopy, and scanning nitrogen-vacancy magnetometry, which exhibit field-free stability, nonvolatility, and size tunability. First-principles calculations and micromagnetic simulations reveal that Janus-surface-induced inversion-symmetry breaking enhances the Dzyaloshinskii-Moriya interaction, providing the microscopic mechanism for skyrmion stabilization and tunability. We further achieve reversible skyrmion writing and erasing using a single ultrafast laser pulse in a magnetic field as low as 300 Oe, demonstrating the excellent manipulability of this 2D magnetic system. These results establish Janus engineering as a route to creating and manipulating nonvolatile skyrmions in atomically thin magnets, with implications for skyrmion-based low-power spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Zero-energy band observation in an interfacial chalcogen-organic network

Structurally-defined molecule-based lattices such as covalent organic or metal-organic networks on substrates, have emerged as highly tunable, modular platforms for two-dimensional band structure engineering. The ability to grow molecule-based lattices on diverse platforms, such as metal dichalcogenides, would further enable band structure tuning and alignment to the Fermi level, which is crucial for the exploration and design of quantum matter. In this work, we study the emergence of a zero-energy band in a triarylamine-based network on semiconducting 1T-TiSe2 at low temperatures, by means of scanning probe microscopy and photoemission spectroscopy, together with density-functional theory. Hybridization between the position-selective nitrogens and selenium p-states results in CN-Se interfacial coordination motifs, leading to a hybrid molecule-semiconductor band at the Fermi level. Our findings introduce chalcogen-organic networks and showcase an approach for the engineering of organic-inorganic quantum matter.

physics.chem-ph↗

arXiv2024

Geometric phase and holonomy in the space of 2-by-2 symmetric operators

We present a non-trivial metric tensor field on the space of 2-by-2 real-valued, symmetric matrices whose Levi-Civita connection renders frames of eigenvectors parallel. This results in fundamental reimagining of the space of symmetric matrices as a curved manifold (rather than a flat vector space) and reduces the computation of eigenvectors of one-parameter-families of matrices to a single computation of eigenvectors at an initial point, while the rest are obtained by the parallel transport ODE. Our work has important applications to vibrations of physical systems whose topology is directly explained by the non-trivial holonomy of the spaces of symmetric matrices.

math.DG↗

arXiv2023

Collective States in Molecular Monolayers on 2D Materials

Collective excited states form in organic two-dimensional layers through the Coulomb coupling of the molecular transition dipole moments. They manifest as characteristic strong and narrow peaks in the excitation and emission spectra that are shifted to lower energies compared to the monomer transition. We study experimentally and theoretically how robust the collective states are against homogeneous and inhomogeneous broadening as well as spatial disorder that occur in real molecular monolayers. Using a microscopic model for a two-dimensional dipole lattice in real space we calculate the properties of collective states and their extinction spectra. We find that the collective states persist even for 1-10% random variation in the molecular position and in the transition frequency, with similar peak position and integrated intensity as for the perfectly ordered system. We measure the optical response of a monolayer of the perylene-derivative MePTCDI on two-dimensional materials. On the wide band-gap insulator hexagonal boron nitride it shows strong emission from the collective state with a line width that is dominated by the inhomogeneous broadening of the molecular state. When using the semimetal graphene as a substrate, however, the luminescence is completely quenched. By combining optical absorption, luminescence, and multi-wavelength Raman scattering we verify that the MePTCDI molecules form very similar collective monolayer states on hexagonal boron nitride and graphene substrates, but on graphene the line width is dominated by non-radiative excitation transfer from the molecules to the substrate. Our study highlights the transition from the localized molecular state of the monomer to a delocalized collective state in the two-dimensional molecular lattice that is entirely based on Coulomb coupling between optically active excitations of the electrons or molecular vibrations.

physics.atm-clus↗

arXiv2019

Microstructure and Elastic Constants of Transition Metal Dichalcogenide Monolayers from Friction and Shear Force Microscopy

Optical and electrical properties of two-dimensional transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. Here we demonstrate how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy (LFM) and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS2) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle-dependent TSM measurements enable us to acquire the fourth-order elastic constants of monolayer WS2 experimentally. Our results facilitate high-throughput and nondestructive microstructure visualization of monolayer TMDCs, insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such two-dimensional semiconductors.

cond-mat.mtrl-sci↗

arXiv2019

Pulsed thermal deposition of binary and ternary transition metal dichalcogenide monolayers and heterostructures

Application of transition metal dichalcogenides (TMDC) in photonic, optoelectronic or valleytronic devices requires the growth of continuous monolayers, heterostructures and alloys of different materials in a single process. We present a facile pulsed thermal deposition method which provides precise control over layer thickness and stoichiometry of two-dimensional systems. The versatility of the method is demonstrated on ternary monolayers of Mo$_{1-x}$W$_{x}$S$_{2}$ and on heterostructures combining metallic TaS$_{2}$ and semiconducting MoS$_{2}$ layers. The fabricated ternary monolayers cover the entire composition range of $x$ = 0...1 without phase separation. Band gap engineering and control over the spin-orbit coupling strength is demonstrated by absorption and photoluminescence spectroscopy. Vertical heterostructures are grown without intermixing. The formation of clean and atomically abrupt interfaces is evidenced by high-resolution transmission electron microscopy. Since both the metal components as well as the chalcogenides are thermally evaporated complex alloys and heterostructures can thus be prepared.

cond-mat.mtrl-sci↗

arXiv2018

Adsorption of polyelectrolytes onto the oppositely charged surface of tubular J-aggregates of a cyanine dye

The adsorption of three different polycations at the negatively charged surface of tubular J-aggregates of the amphiphilic cyanine dye 3,3'-bis(2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine (C8S3) is investigated by means of cryogenic electron microscopy and optical absorption spectroscopy. All three polycations could be adsorbed at the aggregates without flocculation or precipitation when added in molar amounts of monomers sufficiently smaller than of the dye molecules. Preferably the aggregates are either coated by the polycations in total or left completely uncoated. For the coated aggregates, the adsorption leads to charge reversal of the aggregate surface as supported by zeta-potential measurements. The morphology of the coating differs significantly for the three polycations: The branched polycation polyethlyenimine (PEI) attaches to the tubular aggregate by hit-and-stick adsorption of the coiled state in solution forming irregular clot-like coatings; the flexible and weakly cationic poly(allylamine hydrochloride) (PAH) forms a more homogeneous coating but destroys the integrity of the dye aggregate; the more hydrophobic and strong polycation poly(diallyldimethylammonium chloride) (PDADMAC) forms a thin and homogeneous layer, supposedly by wrapping around the tubular aggregate. For the latter, growth of a second dye double layer of the aggregates seems to grow. The different morphologies of the coatings are rather due to the details of the chemical structure of the polycations than due to physical parameters. The possible adsorption of polyelectrolytes at these amphiphilic tubular structures, stabilized by non-covalent bonds only, is far from obvious but demonstrates an applicable route to the build-up of more complex nanostructures by means of a self-assembly process.

physics.chem-ph↗

arXiv2016

Reconstructing Interaction Potentials in Thin Films from Real-Space Images

We demonstrate that an inverse Monte Carlo approach allows to reconstruct effective interaction potentials from real-space images. The method is exemplified on monomolecular ethanol-water films imaged with scanning force microscopy (SFM), which provides the spatial distribution of the molecules. Direct Monte Carlo simulations with the reconstructed potential allow for obtaining characteristics of the system which are unavailable in the experiment, such as the heat capacity of the monomolecularly thin film, and for a prediction of the critical temperature of the demixing transition.

cond-mat.stat-mech↗

arXiv2015

Nucleation, growth, and dissolution of Ag nanostructures formed in nanotubular J-aggregates of amphiphilic cyanine dyes

The synthesis of silver nanowires in solution phase is of great interest because of their applicability for fabrication of plasmonic devices. Silver nanowires with diameters of 6.5 nm and length exceeding microns are synthesized in aqueous solution by reduction of silver ions within the nanotubular J-aggregates of an amphiphilic cyanine dye. The time scale of the growth of the nanowires is of the order of hours and days which provides the unique possibility to investigate the nucleation, growth, and dissolution of the nanowires by direct imaging using transmission electron microscopy. It is found that the initial nucleation and formation of seeds of silver nanostructures occurs randomly at the outer surface of the aggregates or within the hollow tube. The growth of the seeds within the inner void of the tubular structures to nanowires indicates transport of silver atoms, ions, or clusters through the bilayer wall of the molecular aggregates. This permeability of the aggregates for silver can be utilized to dissolve the preformed silver wires by oxidative etching using Cl- ions from dissolved NaCl. Although the nanosystem presented here is a conceptual rather simple organic-inorganic hybrid, it exhibits growth and dissolution phenomena not expected for a macroscopic system. These mechanisms are of general importance for both, the growth and the usage of such metal nanowires, e.g. for plasmonic applications.

physics.chem-ph↗

arXiv2015

Nanohybrids from Nanotubular J-Aggregates and Transparent Silica Nanoshells

Organic-inorganic nanohybrids have been synthesized by in-situ coating of self-assembled, nanotubular J-aggregates with helically wound silica ribbons. The J-aggregates retain their outstanding optical properties in the nanohybrids, but with higher mechanical stiffness, better processability, and an improved stability against photo-bleaching and chemical ambients.

physics.chem-ph↗

arXiv2014

Site-dependence of van der Waals interaction explains exciton spectra of double-walled tubular J-aggregates

The simulation of the optical properties of supramolecular aggregates requires the development of methods, which are able to treat a large number of coupled chromophores interacting with the environment. Since it is currently not possible to treat large systems by quantum chemistry, the Frenkel exciton model is a valuable alternative. In this work we show how the Frenkel exciton model can be extended in order to explain the excitonic spectra of a specific double-walled tubular dye aggregate explicitly taking into account dispersive energy shifts of ground and excited states due to van der Waals interaction with all surrounding molecules. The experimentally observed splitting is well explained by the site-dependent energy shift of molecules placed at the inner or outer side of the double-walled tube, respectively. Therefore we can conclude, that inclusion of the site-dependent dispersive effect in the theoretical description of optical properties of nanoscaled dye aggregates is mandatory.

physics.chem-ph↗

arXiv2014

Evidence for graphene plasmons in the visible spectral range probed by molecules

Graphene is considered to be plasmon active only up to the infrared based on combined tight binding model and random phase approximation calculations. Here we show that the optical properties of graphene as measured by ellipsometry and simulated by density functional theory imply the existence of strongly localized graphene plasmons in the visible with a line width of 0.1 eV. Using small emitters that provide the high wavevectors necessary to excite graphene plasmons at optical frequencies we demonstrate graphene plasmon induced excitation enhancement by nearly 3 orders of magnitude.

cond-mat.mes-hall↗