Physikalisches Kolloquium

Programm für das Wintersemester 2018/2019

Dienstags, 16 Uhr c.t.

Tee ab 15:45 Uhr

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

16.10.18Prof. Dr. Tanja Weil, Max-Planck-Institute for Polymer Research, Mainz
Fluorescent nanodiamonds (FNDs) are emerging as promising quantum materials for bio-medical applications and precision sensing due to their unique optical and magnetic proper-ties.[1] They are obtained by implementing elemental defects into the carbon lattice, such as the nitrogen vacancy (N-V), giving unconditionally stable fluorescence without bleaching or blinking even after several months of continuous excitation. The emission wavelength of FNDs is not size-dependent and is tuneable from the visible to the near infrared region according to the elemental defects. In addition, the N-V center in FNDs serves as single-spin sensor[2] that locally detects various physical properties offering great potential for atomic resolution imaging under physiological conditions. The advent of diamond quantum sensing promises solving the longstanding goal of single molecule detection with atomic resolution under ambient condi-tions[1] There is currently no other nanomaterial that would offer such features. There is an urgent need to prepare high quality N-V diamonds nanodiamonds in a controlled fashion to customize diamond sizes and lattice defects. We present the synthesis of nanodia-monds that paves the way to tailored quantum materials with precisely defined and positioned lattice defects. In addition, functionalization of nanodiamonds is crucial for various applications in biology and medicine. Nanodiamond surface coatings based on biopolymers[3-4] and pro-teins[2,5] will be discussed that provide the basis for quantum sensing and drug delivery in living biological environments. In addition, functionalization of N-V diamonds with proteins or DNA provides access to precisely assembled diamonds on DNA origami to access sophisti-cated quantum devices.
30.10.18Prof. Dr. Jan-Michael Rost, Max-Planck-Institut für Physik komplexer Systeme, Dresden
Highly excited "Rydberg" systems play an important role for progress in theory, experiment, and more recently for potential applications in quantum computing. Rydberg systems connect ultracold physics, condensed and atomic/molecular physics and also non-linear (semi-)classical dynamics on the theoretical side. In the talk I will illustrate this progress with exotic Rydberg systems from antiprotonic helium to ultralong-range Rydberg molecules with several thousand atomic units bond length. Immersed in their natural environment of an ultracold gas those molecules thrive through the presence of many randomly located gas atoms - a surprising and counterintuitive result. It is rooted in a novel scarring phenomenon of excited quantum wave functions and the fact that a random gas contains clusters of atoms, a phenomenon more broadly known as "birthday paradoxon".
06.11.18Prof. Dr. Naohito Saito, Japan Proton Accelerator Research Complex (J-PARC), Japan
A precision measurement of muon dipole moments have been playing important role to test the standard model of particle physics in the past and to shape the new physics in recent years. We plan to launch an experiment to perform precision measurements of the muon anomalous magnetic moment (aka "g-2") and electric dipole moment with a novel technique at Japan Proton Accelerator Research Complex, J-PARC. This experiment requires a muon source with very small emittance, muon acceleration, spiral injection of the muon beam into super-precision magnetic field with 66-cm diameter and high-rate tracking system. The current status and prospect of the experiment will be described in this seminar.
27.11.18Prof. Dr. Regine von Klitzing, Technische Universität Darmstadt
The presentation addresses ordering phenomena of Silica suspensions under confinement in a Colloidal Probe AFM (CP-AFM). The wavelength λ scales with the particle number density as λ=ρ-1/3. An extrapolation towards high volume fractions shows that the ρ-1/3 scaling law for λ ends up into a cubic lattice found for one-component systems like organic solvents, where λ=d (d: diameter of molecules, particles etc.). A deviation from the exponentially decaying cosine function was found and can be described by an additional repulsion term [2,3] which will be discussed. Furthermore, it will be shown how oscillatory forces can be switched on and off by external stimuli [4]. Partially hydrophobized Silica particles order laterally at the interfaces of a free-standing film. They can form percolation networks which seem to stabilize foam films and even foams, i.e. so called Pickering foams
04.12.18Prof. Dr. Steven Johnson, ETH Zürich Institute for Quantum Electronics
The original experiments showing what is now called the Einstein-de Haas effect were a critical demonstration in early modern physics of the equivalence of angular momentum in electronic spins and mechanical angular momentum. Here I discuss a recent experiment where we investigate using femtosecond x-ray diffraction the dynamics of the Einstein-de Haas effect on picosecond time scales, along the way showing definitively that the long-studied but poorly understood phenomenon of “ultrafast demagetisation” in ferromagnetic iron involves a sub-picosecond time scale transfer of angular momentum from the spin system to the lattice.
11.12.18Prof. Lawrence Wald, Ph.D., Harvard Medical School & Massachusetts General Hospital
The textbook formulation of MRI is typically framed with uniform fields, well-controlled linear gradients and stationary objects. If violated, corrections are imposed to nudge the data back into the proper form. If instead we replace image reconstruction with a more general optimization-based strategy, the increased computational burden can buy us important benefits. We show the potential of moving in this direction in three experiments; a fast imaging method otherwise compromised by small gradient control errors, a reconstruction that jointly estimates both the image and patient motion, thus producing artifact-free images of moving patients, and a 120 kg but inhomogeneous brain MRI that forms images with no switching gradients (and is thus also silent). We also show the potential of the method to eliminate motion and gradient mis-calibration artifacts from MR images. Finally, Machine Learning approaches appear poised to either completely take over the model-based reconstruction, offering a very general form of model, or perhaps less scary, do some of the more difficult and computation-time consuming steps.
18.12.18Prof. Dr. Ulrich Achatz, Goethe Universität Frankfurt/Institut für Atmosphäre u. Umwelt
Even with present and foreseeable computational capabilities, the spatial resolution of atmospheric weather-forecast and climate models is and will remain insufficient to capture many essential processes. Next to clouds and turbulence, subgrid-scale waves and their parameterization are one of the grand challenges of the field. Here, especially buoyancy-driven gravity waves are in the focus. The talk will give an overview of the fundamental properties and atmospheric impacts of these waves. It will describe the lead issues in their handling in models, and it will discuss recent developments towards their solution, ranging from laboratory experiments over theory to atmospheric modeling.
08.01.19Dr. Hendrik Ohldag, SLAC National Accelerator Laboratory, CA, USA
The goal of this talk is to present an introduction to the field of synchrotron based soft x-ray microscopy, which is becoming a tool available at every synchrotron. Particular focus will be given to time resolved measurements by employing the pulsed nature of the synchrotron source providing x-ray flashes with 50 ps duration. The general introduction will be followed by a set of examples, that includes the visualization of spin injection from non-magnet into ferromagnets, spin wave modes in ferromagnetic resonance of structured samples and spin waves excited by spin-torque nano-oscillators.
15.01.19Prof. Dr. Harald Giessen, University of Stuttgart, 4th Physics Institute
We use single crystalline gold flakes on atomically flat silicon substrates to generate ideally suitable interfaces for plasmon propagation. By electrochemical means, the thickness is tunable from a few tens to over 100 nm. Using sub-20 fs laser pulses around 800 nm, we excite surface plasmons, whose dynamics can be observed using time-resolved two-photon excitation electron emission (PEEM). Plotting the dispersion of surface plasmons in a thin gold slab on silicon, one finds that excitation at 800 nm can lead to extreme wavelength reduction due to the dispersion slop of over five. Using focused ion beam for cutting rings with appropriate periodicity into the samples (see left image), we can excite concentric surface plasmons that create a nanofocus of only 60 nm width for 800 nm excitation. Archimedean spirals with broken n-fold radial symmetry excite surface plasmons with angular orbital momentum on the gold flakes. This leads in case of 4-fold symmetry to cloverleaf-type nanofoci on the order of 100 nm, which rotate during four optical cycles by 360 degrees. Two-pulse experiments with a subwavelength-stabilized Michelson interferometer, allow investigation of the surface pattern dynamics with (sub-) femtosecond resolution, thus giving insight into the dynamics of the nanofocus formation as well as on the plasmonic spin-orbit coupling.
22.01.19Prof. Dr. Urs Wiedemann, CERN Theoretical Physics Department, Genf, Schweiz
Large azimuthal momentum asymmetries vn (a.k.a. flow harmonics) have been measured since long in nucleus-nucleus (AA) collisions. They indicate collectivity in the sense that they characterize a phenomenon shared by all soft particles produced in the collision. Such vn -signals are not obtained in models of the underlying event of proton-proton (pp) collisions, such as those implemented in multi-purpose event generators. However, large asymmetries vn have been measured in recent years in proton-nucleus and in high multiplicity proton-proton collisions. In this talk, I shall first shortly review the experimental state of the art of characterizing flow harmonics vn as a function of system size. I shall then give a critical assessment of the dynamical mechanisms that are studied to account for the observed phenomenology.
05.02.19Prof. Dr. Thomas Becher, University of Bern, Institute for Theoretical Physics, Bern, Schweiz
In the past hadron colliders were considered discovery machines at which one is able to search for striking signals such as new resonance peaks, but which are not suited for detailed studies and precision measurements. The LHC is strongly challenging this view as we are now using this collider for precision physics. I will review some of the recent progress — as well as the challenges — in the precise theoretical prediction of collider processes. Important tools are automated computations, resummations of perturbation theory, and effective field theory.
Prof. Dr. F. Schmidt-Kaler
Institut für Physik
Prof. Dr. Niklaus Berger
Institut für Kernphysik
Andrea Graham