Arno Ehresmann

We report on the epitaxial growth of antiferromagnetic Mn 2 Au on a Nb(001) substrate capped with a pseudomorphic layer of gold. We observe a layer-by-layer growth by means of medium-energy electron diffraction and confirm stoichiometry and surface structure by Auger electron spectroscopy and low-energy electron diffraction. Evaporation of 15 ML of ferromagnetic Fe on 12–17 ML of Mn 2 Au results in an exchange-coupled bilayer system with an exchange-bias shift that can be set by field-cooling from 400 K. Areas with and without exchange bias, with domain sizes in the range of tens of µ m , are identified by Kerr microscopy. Postannealing the sample at or above 450 K after Mn 2 Au layer growth decreases the fraction of areas where Fe magnetically couples to Mn 2 Au . We suggest that exchange coupling to an interfacial Fe layer depends on the interface termination of Mn 2 Au or on the chemical order in the Mn 2 Au layer. Our findings provide insight into the growth process of Mn 2 Au and the coupling to an Fe layer. Our results point out the importance of growth, interface quality, and termination for the magnetic properties of a Mn 2 Au / Fe bilayer, which may help to improve material properties for spintronic applications.

Using computer simulations and experiments, we demonstrate a mechanism for simultaneous size-specific control over the trajectories of isotropic colloidal particles. Magnetic microparticles of different diameters suspended above a periodic magnetic film are driven by loops traced by the orientation of a uniform external magnetic field. The uniform time-dependent magnetic field couples to the inhomogeneous static magnetic field of the pattern, generating a complex energy landscape for the microparticles. The energy landscape varies with height above the pattern, acting on particles of different sizes in distinct ways, as their centers are located at different elevations. At a specific elevation, distinct modulation loops exist that transport the particles to a neighboring unit cell of the pattern. The transport is topologically protected because it is determined only by a set of winding numbers of the modulation loop, which act as topological invariants. The set of winding numbers depends on the elevation, and hence, on the particle size, allowing simultaneous and independent driving of different-sized particles along arbitrarily complex and distinct trajectories.

In a multispectroscopic approach, we investigated the tailored chiral organosilicon compound sec-butoxytrimethylsilane in the gas phase and report its racemic and enantiopure synthesis. We characterized the structural and conformational flexibility using chirped-pulse Fourier transform microwave spectroscopy in a supersonic jet and identified three different conformers from the analysis of the rotational spectrum supported by quantum-chemical calculations. For the lowest energy conformer, characteristic splitting patterns are observed, matching the internal rotation of two nonequivalent methyl rotors, which can be attributed to two out of the three methyl groups attached to the silicon atom. Complementary synchrotron-based photoelectron spectroscopy provided the Si 2p core-level binding energies, characterizing the electronic environment of silicon in this chiral organosilicon framework. These results establish a detailed picture of both the structural dynamics and the electronic properties of this flexible silicon-containing molecule, providing a foundation for future studies of structure-property relationships in chiral organosilicon systems.

We report on the shift of energies released during Auger cascades in rare-gas ions, when the ions are not isolated but embedded in a homogeneous cluster. By combining high-resolution electron spectroscopy with multielectron coincidence spectroscopy of small Ar and Xe clusters, we investigate the dependence of Auger electron emission energies on the charge state of the emitting ion in the polarizable cluster medium up to charge states of + 4 . Our results show deviations from the predictions of the established electrostatic Born model by a factor of ≈ 2 for the final step of the cascades.

Understanding the on‐chip motion of magnetic particles in a microfluidic environment is key to realizing magnetic particle‐based Lab‐on‐a‐chip systems for medical diagnostics. In this work, a simulation model is established to quantify the trajectory of a single particle moving close to a polymer surface in a quiescent liquid. The simulations include hydrodynamic, magnetostatic, and Derjaguin–Landau–Verwey–Overbeek (DLVO) interactions. They are applied to particle motion driven by a dynamically changing magnetic field landscape created by engineered parallel‐stripe magnetic domains superposed by a homogeneous, time‐varying external magnetic field. The simulation model is adapted to experiments in terms of fluid‐particle interactions with the magnetic field landscape approximated by analytic equations under the assumption of surface charges. Varying simulation parameters, we especially clarify the impact of liquid‐mediated DLVO interactions, which are essential for diagnostic applications, on the 3D trajectory of the particle. A comparison to experimental results validates our simulation approach.

A common issue faced by magnetic particle-based lab-on-a-chip systems, e.g., for medical diagnostics, is the intrinsic fabrication-related polydispersity in particle sizes and magnetic properties. Therefore, to reduce this variation, it is prudent to integrate a pre-separation procedure for the particles into the overall workflow of the system. In this work, a concept for the controlled on-chip fractionation of micron-sized superparamagnetic beads (SPBs) is introduced, which is applicable for sorting magnetic particles according to their properties in a continuous operation mode. A specifically designed magnetic domain pattern is imprinted into an exchange-biased thin film system to generate a tailored magnetic stray field landscape (MFL), enabling lateral transport of SPBs when superposing the MFL with external magnetic field pulses. The domain pattern consists of parallel stripes with gradually increasing and decreasing width, resulting in a step-wise jumping motion of SPBs with increasing/decreasing jump distance. SPBs with different magnetophoretic mobilities, determined, among others, by the particle size and magnetic susceptibility, discontinue their lateral motion at different jump distances, i.e., different lateral positions on the substrate. Thorough analysis of the motion using optical microscopy and particle tracking revealed that an increasing stripe width not only leads to a larger jump distance but also to a lowered jump velocity. As a consequence, particles are spatially separated according to their magnetic and structural properties with a large throughput and time efficiency, as simultaneous sorting occurs for all particles present on the substrate using a constant sequence of short external field pulses.

We investigate the laboratory-frame angular emission distribution of photoelectrons, ejected by circularly polarized light from chiral molecules, theoretically beyond the electric-dipole approximation. We demonstrate that the helicity-dependent forward-backward asymmetry, which is governed by the electric-dipole interaction and known as photoelectron circular dichroism (PECD), can be altered significantly by the helicity-independent asymmetry, which is induced by nondipole contributions. Our findings are exemplified by numerical calculations on the inner-shell photoionization of the bromine atom in the epibromohydrin molecule. The uncovered effect is expected to be omnipresent in the deep inner-shell ionization and needs to be taken into consideration in the interpretation of experiments on PECD in chiral molecules.

Excited electronic states of fenchone, thiofenchone, and selenofenchone are characterized and assigned with different gas‐phase spectroscopic methods and ab initio quantum chemical calculations. With an increasing atomic number of the chalcogen, increasing bathochromic (red) shifts are observed, which vary in strength for Rydberg states, valence‐excited states, and ionization energies. The spectroscopic insight is used to state‐resolve the contributions in multiphoton photoelectron circular dichroism with femtosecond laser pulses. This is shown to be a sensitive observable of molecular chirality in all studied chalcogenofenchones. This work contributes new spectroscopic information, particularly on thiofenchone and selenofenchone. It may open a perspective for future coherent control experiments exploiting resonances in the visible and near‐ultraviolet spectral regions.