Spin currents, and correlations between spin and charge currents, are at the basis of Spintronics. When the spin and charge are transported through a small grain (a „micromagnet“) or when the grain is subject to Ferromagnetic Resonance, one needs to consider quantum fluctuations and strong deviations from equilibrium. I shall review recent developments in this field, stressing the importance of far-from-equilibrium regimes and of geometrical phase effects.
Whereas the quantum transport of electricity is being actively investigated since morethan three decades, the flow of heat is more challenging to access. In particular, there is no equivalent of the ammeter for the flow of heat. Only recently experimental observations are emerging, such as the universal thermal conductance quantum, or heat interferometry. After a general introduction of the field, I will present the experimental determination of the universal limit imposed to heat flow by quantum mechanics, and the observation of heat Coulomb blockade, a many-body quantum effect that can selectively apply to heat but not to electricity in violation of the standard Wiedemann-Franz law.
Thermal fluctuations play a crucial role for the ultimate sensitivity of instruments in highprecision optical metrology such gravitational wave detectors or ultra-stable laser cavities. The fluctuations in these optical systems result from the complex interplay of dissipation processes and optical fields. Particularly critical is thermal noise, for instance caused by the thermally driven motion of the atoms in the solid. In this lecture we introduce relevant noise mechanisms and discuss their coupling to the readout of the experiments. We illustrate that by a deliberate spatial distribution of optical and mechanical fields the readout of thermal noise can be minimized. To this end, optical micro-and nanostructures are promising to outperform conventional multilayer based optics by several orders of magnitude. This lays the foundation for gravitational wave astronomy and laser spectroscopy with unprecedented precision.
Mentioning of a random walk model typically comes together with an image of drunkard's walk as way of introducing a model or with a classical Brownian diffusion as an example of the corresponding physical process. In this talk, we will discuss models of random walks leading to stochastic transport processes which are markedly different from classical diffusion and thus often referred to as “anomalous”. In the so-called Lévy walk model, a random walking particle may undertake very long excursions with a diverging mean squared length, however, in doing so it always moves with a constant speed. This combination is the key feature of the model which made it so successful in describing a vast number of real-world dispersal phenomena in physics, biology and in our everyday life. I will describe the setup of the basic Lévy walk model and its generalisations, illustrate it by several applications and try to outline some open questions in the field.
Einstein once said: "What really interests me is whether God could have created the world any differently." Our existence depends on a variety of constants which appear to be extremely fine-tuned to allow for the existence of Life. These include the number of spatial dimensions, the strengths of the forces, the masses of the particles, the composition of the Universe and others.
Starting from Leibniz' question whether we live in the "Best of all Worlds" we discuss whether the hypothesis of a multiverse could explain the mysterious fine tuning of so many fundamental quantities. Anthropic arguments are critically reviewed.
The universe as we know is highly sensitive to the size of the neutron (n) - proton (p) mass difference. If it would be larger than the binding energy of the deuteron, no fusion would take place. If it would be only a little smaller, all hydrogen would have been burned. Though it is one of the most consequential parameters of physics, the n - p mass difference is not a primary quantity. The relevant theories for the calculation are QCD and QED. With strong and electromagnetic effects being of the same order of magnitude and strongly correlated, this makes a nonperturbative evaluation necessary. For the first time both QCD and QED are now included in the same nonperturbative calculation, which allows us to predict isospin breaking effects in the meson, baryon and quark sectors from first principles, and in particular the n - p mass difference.
A smartphone can be a fascinating measuring instrument. It features a wide range of sensors like an accelerometer, a magnetometer or a gyroscope. Ranging from demonstration experiments with data acquisition to individual experimentation for hundreds of students in a large course, these sensors can be used in physics education to discover the world of physics with a familiar tool. Readily available tools can be used to construct a pendulum, measure centrifugal acceleration in a salad spinner or determine the speed of sound with just two smartphone.
In this colloquium, the developer of the free app "phyphox" from the RWTH Aachen University will demonstrate its possibilities in several experiments. New features will be shown along with advanced interfaces to integrate phyphox with your own projects
Geometric constraints can profoundly affect pattern selection and topological defect formation in equilibrium and non-equilibrium systems. In this talk, I will summarize recent experimental and theoretical work that aims to understand (i) how substrate curvature controls symmetry breaking and defect statistics in elastic surface crystals, and (ii) how confinement geometry affects spontaneous flows of microbial suspensions. Our results show that minimal higher-order PDE models can accurately capture the experimentally observed pattern formation transitions in these systems. We first describe phenomenological parallels between 2D elastic and colloidal crystals on spherical surfaces that suggest some universality in the underlying nucleation processes. Subsequently, we demonstrate how microbial flow patterns can be controlled by microstructure to realize bacterial spin lattices. Building on these insights, we will consider generalized Navier-Stokes equations for active fluids, which also reveal a new invariant for the triad dynamics of classical turbulence.
We review an approach to macroscopic irreversibility from microscopic time reversal invariant dynamics, that can be traced back to Boltzmann; an approach that rests on the special initial conditions of the universe, and on its expansion. We discuss the notion of entropy as a measure of disorder, and how the (irreversible) Boltzmann equation can be harmonized with the (reversible) dynamics of classical particles. This way we illustrate Boltzmann's reply to the macroscopically pardoxical but dynamically justified observations on his theory, due to Loschmidt and Zermelo. We finally discuss possible advances with respect to this approach, highlighting points in which it may be reviewed, from both a mathematical and a physical standpoint, drawing from recent results on time reversibility in presence of magnetic fields, and on consequences of the so-called fluctuation theorems.
Since the epoch of general relativity, scientists have dealt with higher-dimensional wonderlands. The contemporary field of topological condensed matter similarly deals with systems of arbitrary dimensions. Nowadays synthetic dimensions can be quantum engineered in the lab. For the first time, we have thus proposed and realized physics of the 4D Hall effect in experiments.
Scaling is a key concept in statistical physics. It implies a universal form of scaling functions and of scaling exponents – for instance, in the theory of equilibrium critical phenomena and in the classical description of Ostwald ripening. For two examples, droplet growth in binary fluid mixtures and jamming of granular flows, I describe how this relation between scaling and universality is challenged. My findings are underpinned by dedicated experimental and numerical studies.
Area laws were first discovered by Bekenstein and Hawking, who found that the entropy of a black hole grows proportional to its surface area, and not its volume. Entropy area laws have since become a fundamental part of modern physics, from the holographic principle in quantum gravity to ground state wavefunctions of quantum matter, where entanglement entropy is generically found to obey area law scaling. As no experiments are currently capable of directly probing the entanglement area law in naturally occurring many-body systems, evidence of its existence is based on studies of simplified theories. Using new exact microscopic path integral ground state Monte Carlo simulations of superfluid 4He, we demonstrate for the first time area law scaling of entanglement entropy in a real quantum liquid in three dimensions. We validate the fundamental principles underlying its physical origin, and present an "entanglement equation of state" showing how it depends on the density of the superfluid.
It is said that there are more possibilities in chess than there are atoms in the universe. However, size may not be all that matters. The different rules according to which the different chess pieces move, impose a highly nontrivial structure on the game's configuration space. Clearly, direct sampling of even small portions of this space is out of reach. Yet, the task of figuring out properties of a state space that is too vast to enumerate, is a familiar one for statistical physics. In this talk I will show how we applied transition-path sampling, an advanced Monte-Carlo simulation method that is usually used to study crystallization, protein folding or the flow behavior of polymers, to chess. The simulations show that chess' state space decomposes into a large number of weakly connected "pockets" that reflect the pawn structures emphasized by good chess players.
Quantum technologies allow for fully novel schemes of computing, simulation and sensing. For quantum computing, we employ trapped ions in modern segmented ion traps as scalable and freely reconfigurable qubit register [1]. I will give an overview of the recent progress, where gate fidelities of 99.995% (single bit) and 99.6% (two bit) are reached. Alternative platforms for quantum computers in solid state technology would largely benefit from determinsitic schemes to fabricate qubit registers with nm-accuray. I describe our deterministic ion source, which allows for delivering Ca+ ions on demand and focus it into a spot of a few nm [2]. The source can be operated with any other doping ion, which is co-trapped and sympathetically cooled together with a single Ca+ ion, eventually extracted and implanted. We have started structuring solid state samples such as diamond with N2+ molecular ions to generate NV centers, rate-earth Presodym or Cer ions [3] in YAG samples and will start implanting P+ ions into ultrapure Silicium [4], with the vision to fabricate devices for quantum information processing. [1] Kaufmann er al, Phys. Rev. Lett. 119, 150503 (2017) [2] Jacob et al, Phys. Rev. Lett. 117, 043001 (2016) [3] Kornher et al, Appl. Phys. Lett. 108, 053108 (2016), [4] van Donkelaar et al, J. Phys.: Condens. Matter 27, 154204 (2015), http://www.cqc2t.org/ Implantation pattern of Pr+ with distance 2µm and spot size of about 30nm. The confocal optical microscope has a 200nm resolution. Background spots are from imurity ions which have been before in the host crystal.
Living cells need to exert tight control over their lipid membranes to maintain their internal structure, to guard their outside boundary, to establish potential and concentration gradients as their energy source, or to transmit signals between their compartments and to the outside. As a consequence, elaborate molecular machineries have evolved that allow cells to sense and regulate both shapes and physical characteristics of their lipid membranes. The molecular modeling of these machineries faces significant challenges because of their complexity, size, and dynamic nature. To address these challenges, we combine atomistic and coarse-grained simulations of protein-membrane systems. In my talk, I will examine mechanisms used by eukaryotic cells to sense the physical state of their membranes, and to create their intricately shaped membrane structures. Remarkably, physical aspects of the membranes emerge as key factors: fluctuation characteristics and first-order-like transitions in shape space.
The quantum vacuum is one of the most counter-intuitive concepts of quantum electrodynamics. Whereas the classical vacuum refers to a region of space that is devoid of any particles or fields, its quantum counterpart contains fluctuating electromagnetic fields even in the most idealised case. As predicted by macroscopic quantum electrodynamics, the structure of these virtual photons can be significantly altered by the presence of magnetodielectric bodies or media. The signature of the quantum vacuum is manifest in the interaction of virtual photons with charged matter. To illustrate this, I will discuss Casimir effect and its possible use as a glue, the van der Waals force involving excited atoms and the elusive phenomenon of quantum friction.
The protein folding problem – i.e. predicting the three-dimensional structure of a protein from its amino acid sequence alone – is an unsolved problem in computational biology. I will present a novel computational approach to this problem that assembles protein tertiary structure from predicted secondary structure elements. I will demonstrate how this algorithm can be combined with limited EPR- and NMR-spectroscopic data to determine the structure of membrane proteins, an important class of proteins targeted by therapeutics. Structure- and Ligand-based computer aided drug discovery (CADD) algorithms are leveraged to develop small molecule therapeutics that bind to proteins. I will introduce ROSETTALIGAND, an algorithm that allows protein-small molecule docking with full protein and ligand flexibility. I will illustrate the application of machine learning algorithms for ligand-based CADD, specifically the discovery of allosteric modulators of human brain receptors for the treatment of schizophrenia and other neurological diseases.
I will present the advances in multimodal linear, nonlinear, and spatio-temporal nano-imaging for the study of fundamental optical and plasmonic phenomena, coupled single molecule or quantum dynamics, with unprecedented nanometer spatial and femtosecond resolution, sensitivity and precision. To gain the desired simultaneous nanometer spatial resolution with spectroscopic specificity and femtosecond temporal resolution we combine plasmonic and optical antennaconcepts with ultrafast and shaped laser pulses to precisely control optical excitation on femtosecond time and nanometer length scales from the visible to THz spectral range. In the implementation with scattering scanning near-field microscopy (s-SNOM) or other tip-enhanced microscopy modalities with nonlinear, ultrafast, and IR and Raman vibrational spectroscopies, the resulting enhanced and qualitatively new forms of light-matter interaction enable deep-subwavelength spatially resolved imaging of heterogeneities and nano-confinement as they define the properties of most functional materials. I will present several new concepts extending tip-enhanced spectroscopy into the nonlinear and ultrafast regime for nano-scale imaging and spectroscopy of surface molecules and nano-solids. Examples include the adiabatic nano-focusing for nm-resolved imaging of the few-fs plasmon coherence, ultrafast and nonlinear probing of structure and dynamics in quantum materials, or vibrational nano-imaging of molecular structure, coupling, and dynamics down to the single molecule level.
Plasma technology is the unsung hero enabling the fabrication of many devices we take for granted such as smart phones, TVs, internet hardware, most of today’s solar panels, low-friction engines, airplane turbines, energy-efficient windows, cutting tools, hip implants, etc. It all started quite humble, with early observations in the 18th century, long before the term “plasma” was coined. Plasma generation and experimental observations were closely related to the development of electrical energy supply and storage. As storage improved and energy increased, it was actually difficult to avoid making plasmas. However, due to their complex nature, understanding, controlling, and using them represents challenges until today. I will provide a brief history of the roots of modern plasma technology and include some recent, rather surprising findings related to plasma instabilities in sputtering magnetrons, one of my fields of research.
Physical theories of matter are plagued with infinities. At the classical level these seem difficult to avoid, even at the conceptual level. In quantum physics the problem is attenuated since quantum objects are described as probability amplitudes only. The inifinities reappear in relativistic quantum field theory, due to the interplay of locality and the uncertainty principle. Renormalization theory permits to absorb the infinities in the physical parameters of the theory. Embedding renormalization theory into the renormalization group opens new perspectives on the problem, conceptually and mathematically.
The hydrodynamic approximation is an extremely powerful tool to describe the behavior of many-body systems such as gases. At the Euler scale, the approximation is based on the idea of local thermodynamic equilibrium: locally, within fluid cells, the system is in a Galilean or relativistic boost of a Gibbs equilibrium state. This is expected to arise in conventional gases thanks to ergodicity and Gibbs thermalization, which in the quantum case is embodied by the eigenstate thermalization hypothesis. However, integrable systems are well known not to thermalize in the standard fashion. The presence of infinitely-many conservation laws preclude Gibbs thermalization, and instead generalized Gibbs ensembles emerge. In this talk I will introduce the theory of generalized hydrodynamics (GHD), which applies the hydrodynamic ideas to systems with infinitely-many conservation laws. It describes the dynamics from inhomogeneous states and in inhomogeneous fields, and is valid both for quantum systems such as experimentally realized one-dimensional interacting Bose gases, and classical ones such as soliton gases. I will give an overview of what GHD is, its relation to quantum integrable systems and to gases of classical solitons, how it leads to exact results in transport problems, and if time permits some geometric ideas underlying it.
Topological materials have attracted much interest in recent years. Topological insulators and superconductors possess a bulk gap and topologically protected surface states with unusual properties. A prime example are the Majorana bound states at the ends of onedimensional topological superconductors, whose non-Abelian statistics could be exploited for topological quantum computation. Topological semimetals possess topologically protected band crossings in the bulk and so-called Fermi arcs on their boundaries, which also lead to interesting phenomena. Here we show that n-terminal Josephson junctions with conventional superconductors may provide a straightforward realization of tunable topological materials in n−1 dimensions, the independent superconducting phases playing the role of quasi-momenta. In particular, we find zero-energy Weyl points in the Andreev bound state spectrum of 4-terminal junctions. The topological properties of the junction may be probed experimentally by measuring the transconductance between two voltage-biased leads, which we predict to be quantized. Further, the analogy between the spectrum of Andreev bound states in an n-terminal Josephson junction and the bandstructure of an n-1-dimensional material opens the possibility of realizing topological phases in higher dimensions, not accessible in real materials.
Zeit ist einer der grundlegendsten Begriffe der Physik. Was aber ist Zeit? Wie wird sie gemessen oder definiert? Was ist eine "gute" Uhr? Mit welcher Qualität kann man Zeit realisieren? Und was hat man davon, diese möglichst genau zu messen? Und was hat Zeit mit Gravitation zu tun? Gibt es Zeitreisen? Auf all diese Frage wird in dem Vortrag eingegangen. Zunächst wird kurz in die Begrifflichkeit der Zeit eingeführt und dann in einem grundlegenden konstruktiven Zugang gezeigt, dass man eindeutig charakterisieren kann, was auf der begrifflichen Ebene eine "gute" Uhr ist. Danach wird vorgestellt, wie gut man heutzutage Zeit messen und Uhren vergleichen kann. Dies hat Implikationen für die Grundlagenphysik, z.B. für Tests der Grundlagen sowie der Konsequenzen der Speziellen und Allgemeinen Relativitätstheorie. Auch neue praktische Anwendungen genauer Zeitmessung in Positionierung, Geodäsie und Metrologie werden präsentiert. Insbesondere in Bezug auf die Tests und Anwendungen werden die aktuellen Entwicklungen vorgestellt.
Optomechanics holds great promises to push the limits of experimental physics, opening new opportunities in ultra-weak force sensing, thermodynamics and giving us further insight on the transition between the quantum and classical worlds. In this talk, we introduce the use of a levitated nanoparticle in vacuum as a nano-optomechanical system with unprecedented performances. We first describe its unique linear and nonlinear mechanical properties including its high sensing capability and bi-stable dynamics. Subsequently, we present our efforts in cooling its motion towards mechanical ground state at room temperature. In particular, we report on an experiment that combines active parametric feedback cooling with passive resolved side band cooling in a macroscopic high finesse optical cavity. Finally, we discuss how the concept of levitation optomechanics can be extended to optical nanocavity by exploiting their subwavelength mode confinement.
The collective movement of epithelial cells drives essential multicellular organization during various fundamental physiological processes like embryonic morphogenesis, cancer, and wound healing. Two hallmarks of collective behavior in migrating cohesive epithelial cell sheets is the emergence of so called leader cells and the communication between adjacent cells to move correlated to each other. Here we discuss these two phenomena: (i) The geometry-based cue imposed by the matrix environment like local curvature of the collective’s perimeter is capable of triggering leader cell formation and promoting enhanced motility at defined positions. Cytoskeletal tension was found to be important for geometry induced leader cell formation. Together our findings suggest that high curvature leads to locally increased stress accumulation, mediated via cell-substrate interaction as well as via cytoskeleton tension. The stress accumulation in turn enhances the probability of leader cell formation as well as cell motility. (ii) Within this cohesive group each individual cell correlates its movement with that of its neighbours. We investigate the distinct molecular mechanism that links intercellular forces to collective cell movements in migrating epithelia. More specifically, we identified the molecular mechanism whereby Merlin, a tumor suppressor protein and Hippo pathway regulator that functions as a mechanochemical transducer, coordinates collective migration of tens of hundreds of cells. In the context of collective cell migration, the transmission and mediation of cellular tension is of major importance. References 1. T. Das, K. Safferling, S. Rausch, N. Grabe, H. Böhm, and J.P. Spatz: A molecular mechanotransduction pathway regulates collective migration of epithelial cells. Nature Cell Biology 2015, 17, 276-287. 2. S. Rausch, T. Das, J.R.D. Soiné, T.W. Hofmann, C.H.J. Böhm, U.S. Schwarz, H. Böhm, and J.P. Spatz: Polarizing cytoskeletal tension to induce leader cell formation during collective cell migration. Biointerphases 2013, 8, 32.
The Boltzmann distribution is the starting point for the description of equilibrium many-particle systems in statistical mechanics textbooks. For isolated many-particle systems the situation is actually both more complicated and more interesting than alluded to by conventional textbook treatments. This colloquium presents a closer look at the foundations of statistical mechanics; a topic that has seen a lot of interest in the past decade triggered by experiments on cold atomic gases. There is a plethora of interesting phenomena associated with this question ranging from the Fermi-Pasta-Ulam-Tsingou problem to spin echo experiments, the Unruh effect and black holes. I will give an overview of some key questions, current answers and future directions of research.
Within the last 25 years a remarkable increase of the Arctic near–surface air temperature exceeding the global warming by a factor of two has been observed. This phenomenon is commonly referred to as Arctic Amplification. The warming results in rather dramatic changes of a variety of Arctic climate parameters. For example, the Arctic sea ice has declined significantly. This ice retreat has been well identified by satellite measurements. However, coupled regional and global climate models still fail to reproduce it adequately; they tend to systematically underestimate the observed sea ice decline. Although several causes of the recent Arctic climate changes are known, their combined influence and relative importance for Arctic Amplification are complicated to quantify and difficult to disentangle. As a result, there is no consensus about the mechanisms dominating Arctic Amplification. To investigate these problems we combine the scientific expertise and competency of three German universities and two non–university research institutes in the framework of the Transregional Collaborative Research Centre TR 172. The presentation will give an overview of the planned activities within TR 172. In particular the role of clouds in Arctic Amplification will be discussed.
Breakup phenomena are ubiquitous in nature and technology. They span a vast range of time and length scales, including polymer degradation as well as collision induced fragmentation of asteroids. In geology, fragmentation results in the distribution of grain sizes observed in soils; fluids break up into droplets and fluid structures such as eddies undergo fragmentation. On the subatomic scale, excited atomic nuclei break up into fragments. Practical applications, such as mineral processing, ask for optimizations according to technological requirements and efficiency considerations. More generally, a wide range of structures from transport systems to social connections are described by complex networks, whose degree of resilience against fragmentation is a recent subject of intense scrutiny. In this talk I will give an introduction to fragmentation phenomena and show how they can be described in mean-field theory using a rate equation. Going beyond mean-field theory, I will analyze the fragmentation behavior of random clusters on the lattice. Using a combination of analytical and numerical techniques allows for a complete understanding of the critical properties of this system. Dynamical fragmentation with a size cut-off leads to broad distributions of fragment sizes, where the fragment size distribution encodes characteristic fingerprints of the fragmented objects. References [1] E. M. Elci, M.Weigel, and N. G. Fytas, Fragmentation of random fractal structures, Phys. Rev. Lett. 114, 115701 (2015). [2] E. M. Elci, M. Weigel, and N. G. Fytas, Bridges in the random-cluster model, Nucl. Phys. B 903, 19 (2016).
Loop Quantum Gravity is an incarnation of the canonical approach towards a synthesis of the principles of General Relativity and Quantum Field Theory. It is based on a Yang- Mills theory formulation in terms of Wilson loop functionals. The talk offers an introduction to the many conceptual and technical challenges that arise and its partial solutions.
For fabrication of stimuli responsive coatings one challenge is to generate stable films which are still mobile and sensitive to outer parameters. The talk will focus on 2 different types of thin polymer films at solid interfaces: 1) films formed by deposition of hydrogel microgels and 2) polymer brushes. Both architectures have in common that they consist of N-isopropylacrylamide (NIPAM) monomers. During the last decades microgels made of N-isopropylacrylamide (NIPAM) have attracted much interest and were studied by several techniques like microscopy and light scattering. These polymer particles show thermoresponsive behaviour and can therefore be classified as “smart” materials. By copolymerisation with organic acids such as acrylic acid (AAc) the temperature of the volume phase transition as well as the swelling ratio can be influenced. Moreover charged copolymers are sensitive to changes in pH and ionic strength. Our work focuses on the fabrication of stimuli responsive films and on the effect of geometrical confinement on the phase volume transition of these microgel particles [1]. The effect of cross-linker and co-monomers on the swelling behaviour and on the elasticity is presented [2]. The second example is coating with PNIPAM brushes synthesized via ATRP and grafting from method [3]. Beside pH, temperature or solvent [4] light is a very efficient stimulus, since it can trigger quite fast and local the volume phase transition. Therefore additives like surfactants with azobenzene groups [5] and gold nanoparticles [6,7] are embedded within both microgels and brushes. In case of gold nanoparticles, the change in optical properties of microgels and brushes and the impact of gold nanoparticles as hot spots is studied.
Selected Publications:[1] A. Burmistrova, R. von Klitzing J. Mat. Chem, 2010, 20, 3502. [2] A. Burmistrova, M. Richter, C. Üzüm, R. von Klitzing, Coll. Polym. Sci. 2011, 289, 613.[3] S. Christau, S. Thurandt, Z. Yenice, R. von Klitzing Polymers 2014, 6 1877.[4] M. Richter, M. Hunnenmörder, R. von Klitzing Coll. Polymer Sci. 2014, 292 2439.[5] Y.Zakrevskyy, M. Richter, S. Zakrevska, N. Lomadze, R. von Klitzing, S. Santer Advanced Funtional Materials 2012, 22 5000.[6] K. Gawlitza, S. T. Turner, F. Polzer, S. Wellert, M. Karg, P. Mulvaney, R. von Klitzing PCCP 2013, 15 15623.[7] S. Christau, T. Möller, F. Brose, J. Genzer, O. Soltwedel, R.von Klitzing Polymer, 2016.
Over the last few years, transport phenomena at the nanoscale have become of paramount importance, not only because we have reached the end of the (classical) rope for Moore's law, but also because nanosize biological complexes might hold the key to building highly efficient technologies for solar energy conversion. In this talk, I will review several examples of charge and energy transport in systems ranging from biological light-harvesting complexes to topological superconductors and strongly correlated materials. I will discuss the crossover from classical to quantum transport that occurs at the nanoscale, and demonstrate how this crossover provides us with unprecedented opportunities to discover exciting new physical phenomena.
Die Verschmelzung leichter Atomkerne ist die Energiequelle der Sterne. Um diesen Prozess auf der Erde nutzbar zu machen müssen im Labor ähnliche Temperaturen wie im Sonneninneren erzeugt werden. Obwohl dies schon seit einiger Zeit verlässlich gelingt, ist der Schritt zur praktikablen Energiegewinnung noch weit. Der Vortrag zeigt den Fortschritt bei der Kernfusionsforschung mit magnetischem Plasmaeinschluss auf und diskutiert dabei die Rolle der Großexperimente ITER und W7-X von denen die Fusionsforscher sich entscheidende Beiträge zur Lösung dieses Problems erwarten.
Folding, association and aggregation of proteins are key processes in the biochemistry of cells, but often difficult to probe in experiments or computer simulations. The later suffer from the problem that these processes happen on time scales that in general are not accessible in atomistic simulations, and the required computational resources even increase exponentially with size of the molecules. In this talk, I will describe variants of replica exchange sampling designed to overcome this sampling problem in studies of amyloid oligomers and fibrils, protein assemblies that are associated with various diseases. I will present some of our recent results investigating the stability of such aggregates.
Dynamics of weakly nonlinear waves (such as small amplitude water or gravitational waves) depends crucially on whether the domains in which the waves propagate are unbounded (open) or bounded (closed). On unbounded domains the evolution of waves is stabilized by the dissipation of energy by dispersion, while on bounded domains the evolution is destabilized by weak turbulence. In my talk, I will illustrate these two kinds of behaviour first with simple examples and then with solutions of Einstein's equations.
The top quark, discovered 20 years ago, is now copiously produced at the CERN Large Hadron Collider, which allows detailed studies and interpretations of its properties. It remains the heaviest elementary particle ever found. Its strong interaction properties are those of any other quark, but its large coupling to the Higgs boson suggests a special role in electroweak symmetry breaking. I review recent efforts to compute top quark properties precisely, and discuss the more speculative questions: Does the top quark, elusive as it is, determine the fate of the Universe? Does naturalness require top partners, and what and where are they?
Proteins are biological nanomachines which operate at many length and time scales. We combined single molecule, x-ray crystallographic, and cryo-EM data with atomistic simulations to elucidate how these functions are performed at the molecular level. Examples include the mechanics of energy conversion in F-ATP synthase and tRNA translocation within the ribosome. We will further demonstrate how atomistic simulations enable one to mimic, one-to-one, single molecule FRET distance measurements, and thereby to markedly enhance their resolution and accuracy. We will, finally, take a more global view on the 'universe' of protein dynamics motion patterns and demonstrate that a systematic coverage of this 'dynasome' allows one to predict protein function.
Deposition of ingested volcanic ash (VA) within gas turbine aeroengines presents an increasing level of hazard as turbine entry temperatures (TET) continue to be raised, and can cause severe engine damage. The key issue is whether ingested particulate adheres to surfaces inside the engine, with the low softening temperature of many VAs making this more likely. Such adhesion has been studied both in a small turbojet aeroengine (using a borescope) and in a customized plasma torch-based set-up designed to simulate a turbine combustion chamber. Deposition in the engine mainly occurs on static components. Numerical modelling has been used to predict particle flight histories in the customized set-up and correlations established with observed rates of particle deposition. Particle size is important, since the Stokes number of small (~40 μm) particles remain relatively cool. Unfortunately, VA particles in the intermediate size range commonly reach the turbine. The composition of VA, which varies significantly between different volcanoes, is also important, particularly insofar as it affects the glass transition temperature, Tg, and the glass content. Many VAs have very low softening temperatures (
Optical phenomena visible to everyone have been central to the development of, and abundantly illustrate, important concepts in science and mathematics. The phenomena considered include rainbows, sparkling reflections on water, mirages, green flashes, earthlight on the moon, glories, daylight, crystals, and the squint moon. The concepts include refraction, caustics (focal singularities of ray optics), wave interference, numerical experiments, mathematical asymptotics, dispersion, complex angular momentum (Regge poles), polarization singularities, Hamilton’s conical intersections of eigenvalues (‘Dirac points’), geometric phases, and visual illusions.
Black holes are the most interesting prediction of General Relativity, Einstein's theory of gravity. They play a central role in astrophysics. Developments in string theory have led to new roles for black holes. Often these involve considering black holes with more than three dimensions of space. This has motivated a lot of recent research into the properties of black holes in higher dimensions. It turns out that higher-dimensional black holes can behave very differently from three dimensional black holes. In this talk I will explain some of the motivation for studying higher dimensional black holes. I will then compare and contrast the properties of black holes in three and higher dimensions.
Cells contain a variety of complex nanoscale machines. They share a number of common properties with macroscopic machines, such as their ability to produce directed motion and to generate forces. However, a fundamental difference is imposed by thermal fluctuations that significantly influence the operation of nanoscale machines. It is becoming more and more clear that biomolecular machines not only cope but even exploit fluctuations to full-fill their functions. Here I will exemplify this concept at the example of CRISPRCas enzymes, a recently discovered machinery that is currently revolutionizing genome engineering applications in biotechnology. I will show how such enzymes make use of fluctuations to find and verify a certain DNA target sequence and how they can speed up the target search process by a simple kinetic proofreading mechanism. In parallel I will also introduce and explain the techniques that we develop in the laboratory to study with high spatiotemporal resolution the behavior of enzymes and enzyme systems at the single molecule level. Furthermore, my laboratory tries to use the understanding about biomolecular machines to develop artificial nanoscale systems. This employs self-assembled DNA nanostructures, which are used as rigid 3-dimensional building blocks. As an example, I will demonstrate how such structures can be used to dictate the growth of inorganic nanoparticles, which is an important step towards the biomimetic fabrication of nanoelectronic devices.
Proteins are amazing molecular machines that can fold into a complex three dimensional structure in a self-organization process called protein folding. Even though powerful structural methods have allowed us taking still-photographs of protein structures in atomic detail, the knowledge about the folding pathways and dynamics as well as material properties of those structures is rather limited. Over the past 15 years, our group has developed single mechanical methods to study the dynamics and mechanics of protein structures. In my talk I will discuss how these methods can be used to investigate and control the conformational mechanics of individual proteins. Examples include protein folding as well as protein-protein interactions and enzyme mechanics.
Complex quantum systems out of equilibrium are at the basis of a number of the most intriguing puzzles in physics. This talk will be concerned with recent progress on understanding how quantum many-body systems out of equilibrium eventually come to rest and thermalise. The first part of the talk will highlight theoretical progress on this question, taking in several ways a quantum information view - employing ideas of Lieb-Robinson bounds, quantum central limit theorems and of concentration of measure. These findings will be complemented by experimental work with ultra-cold atoms in optical lattices, in setups constituting dynamical "quantum simulators", allowing to probe physical questions that are not only out of reach for state-of-the-art numerical techniques based on matrixproduct states, but also relate to classically computationally hard problems.
In recent years the tailoring of magnetic properties by means of ion irradiation and implantation techniques has become fashionable. Early investigations relied on the fact that the perpendicular magnetic anisotropy of Co/Pt multilayers depend sensitively on the interface sharpness [1]. Subsequently also the ion induced modification of exchange bias phenomena as well as interlayer exchange coupling have been investigated [2]. For single magnetic films ion implantation has been used to reduce the Curie temperature and hence the saturation magnetization [3]. Nowadays also the reverse process, i.e. the creation of nanomagnets within special binary alloys is employed [4,5]. In combination with lithography or with focused ion beams a pure magnetic patterning becomes possible [6] leading to hybrid magnetic materials [7] with properties different from both, the ion irradiated as well as the untreated material. Even ion induced chemical reduction can be employed to create a nanomagnetic pattern [8,9]. An overview of the present status in this research field will be given.References: 1. C. Chappert et al., Science 280, 1919 (1998).2. J. Fassbender, D. Ravelosona, Y. Samson, J. Phys. D 37, R179 (2004).3. J. Fassbender, J. McCord, Appl. Phys. Lett. 88, 252501 (2006).4. E. Menendez et al., Small 5, 229 (2009).5. R. Bali et al., Nano Lett. 14, 435 (2014).6. J. Fassbender and J. McCord, J. Magn. Magn. Mater. 320, 579 (2008).7. J. McCord, L. Schultz, J. Fassbender, Adv. Mater. 20, 2090 (2008).8. S. Kim et al., Nature Nanotechnology 7, 567 (2012).9. J. Fassbender, Nature Nanotechnology 7, 554 (2012).
Insulating materials are ‘topological’ if their band structure encapsulates a non-trivial topological index. While the bulk properties of topological insulators are not remarkable (a fact that has prevented their discovery for decades), their most striking signature is the presence of conducting surface states — the celebrated bulk/boundary correspondence of topological matter. How do topological insulators respond to the inevitable presence of static disorder? Given that, by definition, topological structures are protected against ‘weak deformations’, a tentative answer might be: not by much. In this talk, we will argue that the contrary is true and that disorder takes an unexpectedly strong influence on the properties of topological matter: in the bulk, even weak amounts of impurities compromise the insulating band gaps crucial to our understanding of topological matter. We will discuss how this band closure is accompanied by the emergence of the topological Anderson insulator, a material class characterised by distinct types of bulk quantum critical phenomena, and highly universal surface properties. A number of concrete ramifications of the presence of disorder, notably with regard to the current search for Majorana fermions states in semiconductor quantum wires will be addressed.
Due to their amorphous structure, finite temperature glass-forming melts and glasses show a competition between an immense number of atomic arrangements, a feature not found in a similar pronounced way in crystalline samples. Changes between the arrangements lead to sluggish, non-exponential decay of configurations, typical for this class of materials and leading to glass formation. For metallic glasses and their melts, we consider findings from actual computer simulations of glass dynamics, which allow relating the time-decay characteristics of configurations with structural changes on the microscopic, atomistic level. Thereby we bridge more than eight orders in time, from atomic vibrations on the sub-pico second range to meso-scale micro second evolutions. In the presentation, key topics are shortand medium-range order of the amorphous structure, basic vibration induced excitations, the accumulation principle, and structure conserving correlations between excitations.
Understanding the normal and diseased human brain crucially depends on reliable knowledge of its anatomical microstructure and functional micro-organization (e.g., cortical layers and columns of 200-1000μm dimension). Even small changes in this microstructure can cause debilitating diseases. Until now, the microstructure can only be reliably determined using invasive methods, e.g., ex-vivo histology. This limits neuroscience, clinical research and diagnosis. I will discuss how an interdisciplinary approach developing novel MRI acquisition methods, image processing methods and integrated biophysical models aims to achieve quantitative histological measures of brain tissue, leading to the emerging field of in vivo histology using MRI. In particular, I will present recent methodological advances in quantitative MRI and related biophysical modelling. Examples will include: characterization of cortical myelination and its relation to function; mapping of the axonal g-ratio in a population; changes due to spinal cordinjury; age-related brain changes. The presentation will conclude with an outlook on future developments, applications and the potential impact of in-vivo histology using MRI.
The concept of the vacuum is analyzed, and it is shown that the idea of an empty space is in contradiction to basic principles of quantum field theory.
We review the notion of statistical forces in macroscopic physics. We know them from irreversible thermodynamics as gradients of thermodynamic potentials, but what is their nature and meaning away from equilibrium. We give a statistical mechanical framework leading to non-gradient and non-additive effects for statistical forces, revealing aspects of the thermodynamics and kinetics of driven media. There is a close connection with recent developments in nonequilibrium response theory, giving importance to the notion of dynamical activity.
The Cosmic Microwave Background (CMB), the fossil light of the Big Bang, is the oldest light that one can ever hope to observe in our Universe. The CMB provides us with a direct image of the Universe when it was still an "infant" - 380,000 years old - and has enabled us to obtain a wealth of cosmological information, such as the composition, age, geometry, and history of the Universe. Yet, can we go further and learn about the primordial universe, when it was much younger than 380,000 years old, perhaps as young as a tiny fraction of a second? If so, this gives us a hope to test competing theories about the origin of the Universe at ultra high energies. In this talk I present the results from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite that I contributed, and then discuss the recent results from the Planck satellite (in which I am not involved). Finally, I discuss future prospects on our quest to probe the physical condition of the very early Universe.
Almost one hundred years after Einstein formulated General Relativity, the central role of its most fundamental and fascinating objects --- the black holes --- is nowadays recognized in many areas of physics, even beyond astrophysics and cosmology. Still, solving the theory that governs their dynamics remains a formidable challenge that continues to demand new ideas. I will argue that, from many points of view, it is natural to consider the number of space time dimensions, D, as an adjustable parameter in the theory. Then we can use it for a perturbative expansion of the theory around the limit of very many dimensions, that is, considering 1/D as a small number. We will see that in this limit the gravitational field of a black hole simplifies greatly and its equations often turn out to be analytically tractable. A simple picture emerges in which the shape of the black hole is determined by the same equations that describe soap bubbles.
Internally coupled ears, for short ICE, provide a powerful means of enhancing (in a direction-dependent fashion) the input difference between left and right ear due to an external sound source. ICE occurs in many animal groups, such as frogs, lizards, birds, and crocodilia. It will be shown that two factors play a key role. First, the physical geometry of the air-filled cavity connecting the two ear drums. Second, the fundamental frequency and hence the elastic properties of the tympani connecting the outside auditory world with the air-filled interior connection. Treated together, these factors allow physical and hence mechanistic insight into how ICE works.
Der Vortrag befasst sich mit der Möglichkeit eines Qualitätsmanagements für die universitäre Wissenschaft. In der Vergangenheit gab es zahlreiche Vorfälle wie von Guttenberg, Jan Hendrik Schön oder den Pathologieskandal der Uni Köln, die dringend einen Richtungswechsel nahelegen. Hinzu kommt, dass die Ergebnisse der modernen Forschung alleine auf Grund der hohen Anzahl von Publikationen kaum noch überprüfbar sind. Am Beispiel der Elmos Semiconductor AG, die als Zulieferer für kritische Bauelemente wie Airbagsysteme eine 0 ppm Toleranz fordert, sollen wesentlich Grundlagen des Qualitätsmanagement dargelegt werden. In einem zweiten Schritt wird diskutiert, ob und wie diese Methode auf die Bedingungen der Universität übertragen werden können und welche Voraussetzung eine Universität erfüllen sollte, damit Studierende diese in allen Bereichen der Industrie gebräuchlichen Standards erlernen können.
Cells attached to walls or in tissues can propel themselves by a variety of mechanisms. These are generally discussed in terms of the complicated biochemical feedbacks present in every cell. Here I will instead explore a physics-based approach: what is the simplest combination of physical ingredients that can allow cells to swim or to crawl through their surroundings? I will present a minimal model of cell propulsion based on an emulsion droplet of active polar liquid crystal. This object can swim through a bulk fluid by a mechanism that may (but need not) involve spontaneous symmetry breaking. When attached to a wall and subjected to suitable boundary influences, the droplet can also crawl. These results are possibly suggestive of a 'motility engine' whose function, although controlled by the cell's complex biochemical feedback networks, does not depend upon these for its operational principles.
In embryogenesis, vertebrate cells assemble into organized tissues. In cancer, metastasis tumor cells spreading in the circulatory system use mechanisms of adhesion to establish new tumors. At the root of these life-forming or life-threatening biological phenomena is cell adhesion, the binding of a biological cell to other cells or to a material substrate or scaffold. The most obvious fundamental question to ask is then as follows: What factors control or govern cell adhesion? For a long time, the paradigmatic answer to this question was that specific protein molecules embedded in the cell wall (or membrane) were responsible for cell adhesion, in either a key-lock fashion (in cell-cell adhesion) or a suction-cup fashion (in cell-substrate adhesion). But, a new realization has emerged during the past two decades that physical mechanisms, promoted by the the cell membrane, play an unavoidable, yet not fully understood role. Although these physical elements do not at all depend on any specific proteins, they can have a major impact on the protein-mediated adhesion and can be viewed as mechanism that control the binding affinity to the cell-adhesion molecules. In my talk I will show how these mechanisms can be studied in mimetic models both experimentally and theoretically, the result of which can be discussed in the cellular context.
In 1916, Einstein predicted the existence of gravitational radiation, a fundamental consequence of his general theory of relativity. By the end of this decade, we expect to make the first direct observations of gravitational waves, using ground-based instruments (LIGO in the USA, VIRGO in Italy, GEO in Germany, KAGRA in Japan, LIGO in India). I describe the status and capabilities of the detectors, and outline the different types of astrophysical sources which we hope to detect. We expect that the first direct detections of gravitational waves (perhaps as early as 2017) will be from the coalescence and merger of binary neutron star pairs. Such events may also be accompanied by electromagnetic gamma-ray bursts. I will also outline our hopes for the longer-term future of the field, both for ground- and space-based detectors.
Im Standardmodell ist das Higgsboson das direkte Signal fuer den Mechanismus, der die elektroschwache Symmetrie bricht und dadurch den fundamentalen Teilchen ihre Masse verleiht. Das vor kurzem am LHC gefundene neue Teilchen koennte das langgesuchte Higgsboson sein, wenn die weiteren experimentellen Bestimmungen seiner Eigenschaften dies bestaetigen, womit das Standardmodell komplett waere.
Das beobachtete Signal kann jedoch auch ein erstes Anzeichen von Physik "jenseits des Standardmodells" darstellen, wie sie in Erweiterungen des Standardmodells mit zusaetzlichen neuen Teilchen erwartet werden und von denen man sich Antwort auf grundlegende Fragen erhofft, die vom Standardmodell offen gelassen werde, wie z.B. die nach einer weiteren Vereinheitlichung der fundamentalen Kraefte.
Der Vortrag gibt einen Überblick ueber die Rolle des Higgsteilchens in der Theorie der fundamentalen Wechselwirkungen, ueber die seit den 1990er Jahren immer genauer werdende indirekte Information zu seiner Masse via Quanteneffekte und Praezisionsdaten, sowie den Weg zu seiner direkten Entdeckung. Darueber hinaus werden theoretisch motivierte Erweiterungen bzw. Alternativen zum Standardmodell vorgestellt und deren experimentelle Ueberpruefungen diskutiert, die in den kommenden Jahren die Physik am LHC massgeblich praegen werden.