IMAGING INTERFEROMETRIC MICROSCOPY TO THE LIMITS OF AVAILABLE FREQUENCY SPACE

Imaging interferometric microscopy (IIM) is a synthetic aperture approach offering the potential of optical resolution to the linear systems limit of optics (~lambda/4n). IIM allows one to resolve structures not accessible in a conventional illumination setup, while using a low NA microscope objective and thus keeping the large working distance, depth of focus and field of view associated with the lower NA. The goal of this dissertation is to reach ultimate resolution limits of non-fluorescent microscopy by using IIM in new optical configurations realizing a solid immersion technique with immersion materials employed in advanced regimes unsuitable in other systems. The immersion advantages of IIM can be realized if the object is in close proximity to a solid-immersion medium. Illumination through the substrate involves photons propagating at angles beyond total internal reflection, collection of high frequencies, and decoupling this radiation by a grating on the medium surface opposite to the object. The spatial resolution as a function of the medium thickness and refractive index as well as the field-of-view of the optical system is derived and applied to simulations. Structural illumination technique allows aliasing high spatial frequency into the low frequency range and using conventional microscopes at high resolution. This technique may be useful for broad swath of technical applications, biological and medical research

Mobility traces and spreading of COVID-19

We use human mobility models, for which we are experts, and attach a virus infection dynamics to it, for which we are not experts but have taken it from the literature, including recent publications. This results in a virus spreading dynamics model. The results should be verified, but because of the current time pressure, we publish them in their current state. Recommendations for improvement are welcome. We come to the following conclusions: 1. Complete lockdown works. About 10 days after lockdown, the infection dynamics dies down. This assumes that lockdown is complete, which can be guaranteed in the simulation, but not in reality. Still, it gives strong support to the argument that it is never too late for complete lockdown. 2. As a rule of thumb, we would suggest complete lockdown no later than once 10% of hospital capacities available for COVID-19 are in use, and possibly much earlier. This is based on the following insights: a. Even after lockdown, the infection dynamics continues at home, leading to another tripling of the cases before the dynamics is slowed. b. There will be many critical cases coming from people who were infected before lockdown. Because of the exponential growth dynamics, their number will be large. c. Researchers with more detailed disease progression models should improve upon these statements. 3. Our simulations say that complete removal of infections at child care, primary schools, workplaces and during leisure activities will not be enough to sufficiently slow down the infection dynamics. It would have been better, but still not sufficient, if initiated earlier. 4. Infections in public transport play an important role. In the simulations shown later, removing infections in the public transport system reduces the infection speed and the height of the peak by approximately 20%. Evidently, this depends on the infection parameters, which are not well known. – This does not point to reducing public transport capacities as a reaction to the reduced demand, but rather use it for lower densities of passengers and thus reduced infection rates. 5. In our simulations, removal of infections at child care, primary schools, workplaces, leisure activities, and in public transport may barely have been sufficient to control the infection dynamics if implemented early on. Now according to our simulations it is too late for this, and (even) harsher measures will have to be initiated until possibly a return to such a restrictive, but still somewhat functional regime will again be possible. Evidently, all of these results have to be taken with care. They are based on preliminary infection parameters taken from the literature, used inside a model that has more transport/movement details than all others that we are aware of but still not enough to describe all aspects of reality, and suffer from having to write computer code under time pressure. Optimally, they should be confirmed independently. Short of that, given current knowledge we believe that they provide justification for “complete lockdown” at the latest when about 10% of available hospital capacities for COVID-19 are in use (and possibly earlier; we are no experts of hospital capabilities). What was not investigated in detail in our simulations was contact tracing, i.e. tracking down the infection chains and moving all people along infection chains into quarantine. The case of Singapore has so far shown that this may be successful. Preliminary simulation of that tactic shows that it is difficult to implement for COVID-19, since the incubation time is rather long, people are contagious before they feel sick, or maybe never feel sufficiently sick at all. We will investigate in future work if and how contact tracing can be used together with a restrictive, but not totally locked down regime. When opening up after lockdown, it would be important to know the true fraction of people who are already immune, since that would slow down the infection dynamics by itself. For Wuhan, the currently available numbers report that only about 0.1% of the population was infected, which would be very far away from “herd immunity”. However, there have been and still may be many unknown infections (Frankfurter Allgemeine Zeitung GmbH 2020)

Spin Seebeck and Spin Nernst Effects of Magnons in Noncollinear Antiferromagnetic Insulators

Our joint theoretical and computer experimental study of heat-to-spin conversion reveals that noncollinear antiferromagnetic insulators are promising materials for generating magnon spin currents upon application of a temperature gradient: they exhibit spin Seebeck and spin Nernst effects. Using Kubo theory and spin dynamics simulations, we explicitly evaluate these effects in a single kagome sheet of potassium iron jarosite, KFe$_3$(OH)$_6$(SO$_4$)$_2$, and predict a spin Seebeck conversion factor of $0.2 \mu\mathrm{V}/\mathrm{K}$ at a temperature of $20 \mathrm{K}$.Comment: 6 pages, 3 figure

Molecular Clouds as the Origin of the Fermi Gamma-Ray GeV-Excess

The so-called "GeV-excess" of the diffuse Galactic gamma-ray emission is studied with a spectral template fit based on energy spectra. The spectral templates can be obtained in a data-driven way from the gamma-ray data, which avoids the use of emissivity models to subtract the standardbackground processes from the data. Instead, one can determine these backgrounds simultaneously with any "signals" in any sky direction, including the Galactic disk and the Galactic center. Using the spectral template fit two hypothesis of the "GeV-excess" were tested: the dark matter (DM) hypothesis assuming the excess is caused by DM annihilation and the molecular cloud (MC) hypothesis assuming the "GeV-excess" is related to a depletion of gamma-rays below 2 GeV, as is directly observed in the Central Molecular Zone (CMZ). Both hypotheses provide acceptable fits, if one considers a limited field-of-view centered within 20$^\circ$ around the Galactic center and applies cuts on the energy range and/or excludes low latitudes, cuts typically applied by the proponents of the DM hypothesis. However, if one considers the whole gamma-ray sky and includes gamma-ray energies up to 100 GeV we find that the MC hypothesis is preferred over the DM hypothesis for several reasons: i) The MC hypothesis provides significantly better fits; ii) The morphology of the "GeV-excess" follows the morphology of the CO-maps, a tracer of MCs, i.e. there exists a strong "GeV-excess" in the Galactic disk also at large longitudes; iii) The massive CMZ with a rectangular field-of-view of $l \times b = 3.5^{\circ} \times 0.5^{\circ}$ shows the maximum of the energy flux per log bin in the diffuse gamma-ray spectrum at 2 GeV, i.e. the "GeV-excess", already in the raw data without any analysis. The rectangular profile contradicts the spherical morphology expected for DM annihilation.Comment: 53 pages, 8 figures (+ 42 figures in Appendices), extended version of arXiv:1610.08926 accepted for publication in PR

Implications of a temperature-dependent magnetic anisotropy for superparamagnetic switching

The macroscopic magnetic moment of a superparamagnetic system has to overcome an energy barrier in order to switch its direction. This barrier is formed by magnetic anisotropies in the material and may be surmounted typically after 10^9 to 10^12 attempts per second by thermal fluctuations. In a first step, the associated switching rate may be described by a Neel-Brown-Arrhenius law, in which the energy barrier is assumed as constant or a given temperature. Yet, magnetic anisotropies in general depend on temperature themselves which is known to modify the Neel-Brown-Arrhenius law. We illustrate quantitatively the implications of a temperature-dependent anisotropy on the switching rate and in particular for the interpretation of the prefactor as an attempt frequency. In particular, we show that realistic numbers for the attempt frequency are obtained when the temperature dependence of the anisotropy is taken into account.Comment: 15 pages, 5 figure