Physikalisches Kolloquium

Programm für das Sommersemester 2019

Dienstags, 16 Uhr c.t.

Tee ab 15:45 Uhr

Ort: Institut für Kernphysik, Johann Joachim Becher Weg 45, HS KPH

16.04.19Prof. Dr. Diederik Wiersma, LENS, University of Florence, Italy
Nature provides a great source of inspiration for many fields in science. When looking at locomotion in the living world, for instance, amazing solutions can be found especially for tiny, sub-millimeter size creatures. On these length scales the laws of physics behave so differently from what we are used to in our meter scale world. In this contribution we will discuss how liquid crystalline elastomers can be used to create micro meter scale artificial creatures, taking inspiration from nature. We will show that it is possible to create microscopic robots with an overall size of hundreds of micrometers out of elastomers, that can walk on dry surfaces and swim in fluid environments and perform tasks like grabbing microscopic particles. The liquid crystalline elastomer constitutes the essential material in the realization of these microscopic robots, since it allows to use light as source of energy and control mechanism. We will give an overview of the recent progress that was made in this exciting adventure.
23.04.19Prof. Dr. Markus Valtiner, Vienna University of Technology, Institute for Applied Physics
Multiple beam interferometry (MBI) evolved as a powerful tool for the simultaneous evaluating of thin film thicknesses and refractive indices in Surface Forces Apparatus (SFA) measurements. However, analysis has relied on simplifications for providing fast or simplified analysis of recorded interference spectra. I will describe the implementation of new optics and a generalized fitting approach to 4x4 transfer matrix method simulations for the SFA and will describe a numerical approach for constructing transfer matrices for birefringent materials. This enables self-consistent fitting of thicknesses, birefringence and relative rotation of anisotropic layers, evaluation of reflection and transmission mode spectra, simultaneous fitting of thicknesses and refractive indices of ultrathin (molecular) layers confined between two transparent surfaces. I will showcase a variety of different topics including measuring refractive indices of confined fluids and organic thin films (e.g. lipid bilayers, liquid crystals and ionic liquids), the thickness of ultrathin films, the thickness of metal layers and the relative rotations of birefringent thin films during normal and shear load application.
30.04.19Dr. Eva Benckiser, Max Planck Institute for Solid State Research, Stuttgart
Transition-metal oxides with strong electron-electron correlations show a variety of interesting properties including metal-insulator transitions, different magnetic orders, and superconductivity. These phases are of technological interest, but often difficult to access because they only occur at very low temperatures, high external fields, or high pressures. Heterostructures offer promising new research approaches. Targeted interfacial reconstructions in epitaxial multilayers can stabilize novel phases that are not present in the bulk phase diagrams of the individual components. This is particularly relevant for multilayers with nanometre thin layers in which the properties are essentially governed by the interfacial reconstructions. Our research focuses on the investigation of such electronic reconstructions in the two-dimensional limit by means of x-ray spectroscopy. As a model system we have investigated perovskite-type nickelate heterostructures with composition RNiO3 (R = rare-earth ion). In my talk, I will present results of our studies on multilayers and will address layer-resolved orbital occupations, the unusual antiferromagnetic order and its interplay with the bond-order instability, and the feasibility of digital charge carrier doping.
07.05.19Dr. Dr. Justin Shaw, National Institute of Standards and Technology, Boulder/CO, USA
Modern spin-based technologies rely on multiple, simultaneous phenomena that originate from the spin-orbit interaction in magnetic systems. These include damping, magnetic anisotropy, orbital moments, and spin-orbit torques that are manifested in the spin-Hall and Rashba-Edelstein effects. While cavity based ferromagnetic resonance (FMR) spectroscopy has been used to characterize magnetic materials for many decades, recent advances in broadband and phase-sensitive FMR techniques have allowed further refinement, improved accuracy, and new measurement capability. In fact, broadband FMR techniques can now precisely measure spin-orbit torques at the thin-film level without the requirement of device fabrication. Broadband FMR measurements have also improved our fundamental understanding of magnetic damping. Numerous extrinsic relaxation mechanisms can obscure the measurement of the intrinsic damping of a material. This created a challenge to our understanding of damping because experimental data were not always directly comparable to theory. As a result of the improved ability to quantify all of these relaxation mechanisms, many theoretical models have been refined. In fact, this has recently led to both the prediction and discovery of new materials with ultra-low magnetic damping that will be essential for future technologies based on spintronics, magnonics, spin-logic and high-frequency devices. I will begin this lecture with a basic introduction to spin-orbit phenomena, followed by an overview of modern broadband FMR techniques and analysis methods. I will then discuss some recent successes in applying broadband FMR to improve our ability to control damping in metals and half-metals, quantify spin-orbit torques and spin-diffusion lengths in multilayers, and determine the interrelationships among damping, orbital moments, and magnetic anisotropy. The impact of these result on specific technologies will also be discussed.
14.05.19Univ.-Prof. Dr. Monika Ritsch-Marte, Medical University of Innsbruck, Dept. for Medical Physics
Optical wavefront shaping by means of spatial light modulators (SLMs) based on liquid crystal (LC) panels, has become a powerful tool in Biophotonics. “Holographic optical tweezers” are well-known and widespread, but an SLM can also be integrated into optical imaging systems. This makes the microscope programmable and adaptable with respect to the needs of a specific sample. A particular strength of the Synthetic Holography approach with programmable phase masks is the possibility to multiplex, which means that one can ‘pack’ several tasks into one computer-generated hologram. One can, for instance, create images which are composed of sub-images belonging to different microscopy modalities, to different depths inside the volumetric sample, or to different parameter settings. Moreover, if the phase modulation range is not restricted to 2π, the wealth of possibilities significantly increases: Several computer-generated holograms can be read out at different wavelengths from one and the same input pattern sent to the LC panel. In this way holographically modified imaging in the visible can be accommodated in the same phase mask that is used for holographically controlled trapping in the near-infrared.
21.05.19Dr. Chiara Caprini, Laboratoire Astroparticule et Cosmologie, Paris
LIA (Laser Interferometer Space Antenna) is the space mission of the European Space Agency to observe gravitational waves. After an introduction to gravitational waves and to the recent direct detections made by Earth-based interferometers, we will describe the scientific potential of the LISA mission, in particular for what concerns cosmology and the physics of the very early universe.
28.05.19Prof. Dr. Thomas Birner, LMU München, Meteorologisches Institut
Earth’s tropical belt can be defined by the band of rainy equatorial regions bordered by the arid subtropics to the north and the south. Because of the strong latitudinal gradients in temperature and precipitation at the edges of the tropical belt, any shift in its edges could drive major local changes in surface climate. Theoretical arguments and experiments with climate models suggest that increasing greenhouse gas concentrations should lead to a widening of the tropical belt alongside a poleward shift of the mid-latitude jet stream. However, observationally-based estimates of changes in tropical width have resulted in disparate rates of expansion, some of which much higher than those expected based on experiments with climate models. In this talk, I will first discuss the morphology of the tropical belt in terms of its thermodynamic and circulation characteristics, and the resulting metrics that can be used to define its edges. By studying the interrelationships across different metrics and accounting for methodological differences, the tropics are found to have widened by about 2 degrees of latitude over the last four decades. However, it is too early to detect robust anthropogenically induced widening imprints due to large unforced variability. I will then discuss the coupling between large-scale atmospheric disturbances originating along the mid-latitude jet and the tropical overturning circulation (the Hadley cell), which gives rise to year-to-year variability in tropical edge latitudes and is fundamental for the tropical width response to increased greenhouse gas concentrations.
04.06.19Prof. Dr. Ulrich Heinz, Ohio State University, Columbus/Ohio, USA
The Little Bangs created in ultra-relativistic heavy-ion collisions share many characteristic features with the cosmological evolution after the Big Bang. They create a quark-gluon plasma - an extremely dense state of strongly interacting matter that flows like an almost perfect fluid. This allows to describe such heavy-ion collisions with dissipative relativistic fluid dynamics, supplemented by an early pre-hydrodynamic and a late kinetic freeze-out stage. Similar to the Big Bang, fluctuations in the initial state create structures in the final state which can be measured and used to reconstruct the initial state. I will demonstrate how quantum fluctuations in the initial state of the Little Bang propagate into the experimentally observed final momentum distributions, manifesting themselves as fluctuations in the final flow pattern. A harmonic analysis of the final flows, their transverse momentum dependence and their flow angles (the "Little Bang flow fluctuation spectrum") provides detailed experimental information from which theory allows to extract with precision the spectrum of gluon fluctuations in the initial state, together with the transport coefficients of the quark-gluon plasma fluid created in the collisions.
11.06.19Dr. Sabine Hossenfelder, FIAS Frankfurt Institute for Advanced Studies
To develop fundamentally new laws of nature, theoretical physicists often rely on arguments from beauty. Simplicity and naturalness in particular have been strongly influential guides in the foundations of physics ever since the development of the standard model of particle physics. In this lecture I argue that arguments from beauty have led the field into a dead end and discuss what can be done about it.
18.06.19Prof. Dr. Guglielmo Tino, LENS, University of Florence, Italy
The ability to control the quantum degrees of freedom of atoms using laser light opened the way to precision measurements of fundamental physical quantities. I will describe experiments for precision tests of gravitational physics using new quantum devices based on ultracold atoms, namely, atom interferometers and optical clocks. I will report on the measurement of the gravitational constant G with a Rb Raman interferometer, on experiments based on Bloch oscillations of Sr atoms confined in an optical lattice for gravity measurements at small spatial scales, and on new tests of the Einstein equivalence principle. I will also discuss prospects to use atoms as new detectors for gravitational waves and for experiments in space.
25.06.19Prof. Dr. Udo Seifert, Universität Stuttgart
All processes in cell and molecular biology, like transport by molecular motors and replication machinery, are subject to thermal noise. Still, life relies on the fact that the result of such processes comes with a small enough uncertainty, i.e., large enough precision. The same holds for (wet) micro- and nano-robotics. While absolute precision is impossible in an environment of finite temperature, an obvious question is whether or not there is a fundamental trade-off between precision and the (free energy) cost of generating or running such processes. After recalling the principles of stochastic thermodynamics, I will introduce the recently discovered thermodynamic uncertainty relation that provides a universal lower bound on the precision any process in steady-state conditions can achieve for a given energy budget. A variant of this relation allows us to extract from experimental data a model-free upper bound on the efficiency of molecular motors. Likewise, for steady-state heat engines, this relation shows that Carnot efficiency can be reached at finite power, in principle, but only at the cost of diverging power fluctuations. I will close with recent insights into the minimal requirements for generating coherent oscillations in (biochemical) networks.
02.07.19Prof. Dr. Immanuel Bloch, Max-Planck-Institut fr Quantenoptik
Recent experiments with quantum gas microscopes allow for an unprecedented view and control of quantum matter in new parameter regimes and with new probes. In our fermionic quantum gas microscope, we can detect both charge and spin degrees of freedom simultaneously, thereby gaining maximum information on the intricate interplay between the two in the paradigmatic Hubbard model. In my talk, I will show how we can reveal hidden magnetic order, directly image individual polarons or probe the fractionalisation of spin and charge in dynamical experiments. For the first time we therefore have access to non-local hidden correlation properties of quantum matter. Furthermore, I will show how quantum gas microscopy can open new avenues for the for field of quantum chemistry when probing and controlling the formation of huge Rydberg macrodimers in optical lattices.
09.07.19Prof. Dr. Stephan Paul, TU München, Fakultät für Physik
The study of multibody hadronic final states plays a major role in the spectroscopy of hadrons and the search for exotic states, e.g. at the COMPASS experiment at CERN. New analysis technique thereby offer an unprecedented view into the underlying dynamics and correlations and in addition reveal information on the initial states. Such techniques are now to be employed in the analysis of of heavy mesons and tau leptons. They promise a new measurement technique for the polarization of tau leptons, one of the prerogatives for measuring dipole moments of the heaviest lepton at the Super B-factory.
Prof. Dr. F. Schmidt-Kaler
Institut für Physik
Prof. Dr. Niklaus Berger
Institut für Kernphysik
Andrea Graham