14 research outputs found
Sub-nanometer free electrons with topological charge
The holographic mask technique is used to create freely moving electrons with
quantized angular momentum. With electron optical elements they can be focused
to vortices with diameters below the nanometer range. The understanding of
these vortex beams is important for many applications. Here we present a theory
of focused free electron vortices. The agreement with experimental data is
excellent. As an immediate application, fundamental experimental parameters
like spherical aberration and partial coherence are determined.Comment: 4 pages, 5 figure
Coexisting multi-states in catalytic hydrogen oxidation on rhodium
Catalytic hydrogen oxidation on a polycrystalline rhodium foil used as a surface structure library is studied by scanning photoelectron microscopy (SPEM) in the 10−6 mbar pressure range, yielding spatially resolved X-ray photoemission spectroscopy (XPS) measurements. Here we report an observation of a previously unknown coexistence of four different states on adjacent differently oriented domains of the same Rh sample at the exactly same conditions. A catalytically active steady state, a catalytically inactive steady state and multifrequential oscillating states are simultaneously observed. Our results thus demonstrate the general possibility of multi-states in a catalytic reaction. This highly unusual behaviour is explained on the basis of peculiarities of the formation and depletion of subsurface oxygen on differently structured Rh surfaces. The experimental findings are supported by mean-field micro-kinetic modelling. The present observations raise the interdisciplinary question of how self-organising dynamic processes in a heterogeneous system are influenced by the permeability of the borders confining the adjacent regions
Pattern Formation in Catalytic H<sub>2</sub> Oxidation on Rh: Zooming in by Correlative Microscopy
Spatio-temporal nonuniformities in H2 oxidation on individual Rh(h k l) domains of a polycrystalline Rh foil were studied in the 10–6 mbar pressure range by photoemission electron microscopy (PEEM), X-ray photoemission electron microscopy (XPEEM), and low-energy electron microscopy (LEEM). The latter two were used for in situ correlative microscopy to zoom in with significantly higher lateral resolution, allowing detection of an unusual island-mediated oxygen front propagation during kinetic transitions. The origin of the island-mediated front propagation was rationalized by model calculations based on a hybrid approach of microkinetic modeling and Monte Carlo simulations
Exploiting the Acceleration Voltage Dependence of EMCD
Energy-loss magnetic chiral dichroism (EMCD) is a versatile method for measuring magnetism down to the atomic scale in transmission electron microscopy (TEM). As the magnetic signal is encoded in the phase of the electron wave, any process distorting this characteristic phase is detrimental for EMCD. For example, elastic scattering gives rise to a complex thickness dependence of the signal. Since the details of elastic scattering depend on the electron’s energy, EMCD strongly depends on the acceleration voltage. Here, we quantitatively investigate this dependence in detail, using a combination of theory, numerical simulations, and experimental data. Our formulas enable scientists to optimize the acceleration voltage when performing EMCD experiments
Imaging the dynamics of free-electron Landau states
Landau levels and states of electrons in a magnetic field are fundamental quantum entities underlying the quantum Hall and related effects in condensed matter physics. However, the real-space properties and observation of Landau wave functions remain elusive. Here we report the real-space observation of Landau states and the internal rotational dynamics of free electrons. States with different quantum numbers are produced using nanometre-sized electron vortex beams, with a radius chosen to match the waist of the Landau states, in a quasi-uniform magnetic field. Scanning the beams along the propagation direction, we reconstruct the rotational dynamics of the Landau wave functions with angular frequency ~100 GHz. We observe that Landau modes with different azimuthal quantum numbers belong to three classes, which are characterized by rotations with zero, Larmor and cyclotron frequencies, respectively. This is in sharp contrast to the uniform cyclotron rotation of classical electrons, and in perfect agreement with recent theoretical predictions
Switching on surface conduction in a topological insulator
The protected surface conductivity of topological insulators, carried by
ultra-relativistic Dirac fermions, is in high demand for the next generation of
electronic devices. Progress in the unambiguous identification of this surface
contribution and, in a second step, its control are needed to move forward.
Here we demonstrate both, with a combined transport and spectroscopy study of
high-quality single crystals and mesoscopic devices of the topological
insulator TlBiSe2. We show how various external stimuli-from thermal radiation,
via low-intensity light, to high-intensity laser pumping and current
driving-can boost the surface contribution, thereby making it both
unambiguously detectable and potentially exploitable for applications. Once
switched on, the extra surface contribution is persistent, with lifetimes of
hundreds of years at low temperatures. We understand this effect in terms of
the well-known concept of surface charge accumulation via a Schottky barrier
formation, and propose that the same mechanism underlies also the slow
relaxations seen with spectroscopic probes in our and other materials, which
might thus also be persistent. We expect our technique to be readily
transferable to other materials and probes, thereby shedding light on
unexplained slow relaxations in transport and beyond