Events: Fysik events at Chalmers University of TechnologyThu, 20 Feb 2020 14:17:48 +0100 Properties of Semiconductor Moire Crystals<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​​Welcome to a Graphene Centre Seminar with Xiaoqin (Elaine) Li, Austin, Texas USA</p><strong>Abstra​ct:</strong><div><span style="font-size:14px"><span></span>A new type of superlattice, known as the moiré superlattice, form when two monolayers of van der Waals materials are stacked to form a heterostructure. A periodic energy modulation, as well as distinct optical selection rules, give rise to rich optical properties that remain largely unexplored in semiconductor moiré crystals. I will discuss how the moiré potential in twisted transition metal dichalcogenide bilayers changes the exciton resonances and diffusion in a manner controllable by the twist angle. Additional insights are obtained by comparing heterostructures prepared with either chemical vapor deposition or mechanical stacking. There are many exciting opportunities for exploring fundamental condensed matter physics and novel optoelectronic devices such as an array of single-photon emitters in these moiré crystals.</span><br /></div> many-body localization transition<p>Nexus 4030, meeting room, Fysikgården 2B, Fysik Origo</p><p>​Welcome to attend a seminar by Abolfazl Bayat (UESTC).</p><p></p> <h2 class="chalmersElement-H2">Abstract:</h2> <div>Many-body localization has become an important phenomenon for illuminating a potential rift between non-equilibrium quantum systems and statistical mechanics. Unlike quantum phase transitions, the many-body <img src="/SiteCollectionImages/Centrum/Fysikcentrum/Blandade%20bilddimensioner/300_Seminarium%2026%20februari.jpg" class="chalmersPosition-FloatRight" alt="" style="margin:5px;width:300px;height:306px" /><br />localization is a property of the whole spectrum. However, the exact nature of the many-body localization transition remains an open question. An aspect which has been posited in various studies is the emergence of scale invariance around this point, however the direct observation of this phenomenon is still absent. Here we achieve this by studying the logarithmic negativity and mutual information between disjoint blocks of varying size across the many-body localization transition. The two length scales, block sizes and the distance between them, provide a clear quantitative probe of scale invariance across different length scales. We find that at the transition point, the logarithmic negativity obeys a scale invariant exponential decay with respect to the ratio of block separation to size, whereas the mutual information obeys a polynomial decay. The observed scale invariance of the quantum correlations in a microscopic model opens the direction to probe the fractal structure in critical eigenstates using tensor network techniques and provide constraints on the theory of the many-body localization transition.</div> <p></p> Yi, Nuclear Engineering<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>​ Title of thesis: A neutron noise solver based on a discrete ordinates method.</p><h2 class="chalmersElement-H2">Abstract:</h2> <div><span style="background-color:initial">A</span><span style="background-color:initial"> neutron noise transport modelling tool is presented in this thesis. The simulator allows to determine the static solution of a critical system and the neutron noise induced by a prescribed perturbation of the critical system. The simulator is based on the neutron balance equations in the frequency domain and for two-dimensional systems. The discrete ordinates method is used for the angular discretization and the diamond finite difference method for the treatment of the spatial variable. The energy dependence is modelled with two neutron energy groups. The conventional inner-outer iterative scheme is employed for solving the discretized neutron transport equations. For the acceleration of the iterative scheme, the diffusion synthetic acceleration is implemented.</span></div> <div><br /></div> <div> </div> <div>The convergence rate of the accelerated and unaccelerated versions of the simulator is studied for the case of a perturbed infinite homogeneous system. The theoretical behavior predicted by the Fourier convergence analysis agrees well with the numerical performance of the simulator. The diffusion synthetic acceleration decreases significantly the number of numerical iterations, but its convergence rate is still slow, especially for perturbations at low frequencies.<br /><br /></div> <div> </div> <div>The simulator is further tested on neutron noise problems in more realistic, heterogeneous systems and compared with the diffusion-based solver. The diffusion synthetic acceleration leads to a reduction of the computational burden by a factor of 20. In addition, the simulator shows results that are consistent with the diffusion-based approximation. However, discrepancies are found because of the local effects of the neutron noise source and the strong variations of material properties in the system, which are expected to be better reproduced by a higher-order transport method such as the one used in the new solver. </div> <div> </div> <div><br /></div> <div> </div> <div>Keywords: Neutron noise, nuclear reactor modelling, deterministic neutron transport methods, discrete ordinates, diffusion synthetic acceleration, convergence analysis</div> <div> </div> <div><br /></div> <div> </div> <div>​<br /></div> by Lai-Sheng Wang<p>Nexus 4030, meeting room, Fysikgården 2B, Fysik Origo</p><p>​Welcome to a seminar by Professor Lai-Sheng Wang, Brown University, Rhode Island, USA Associate Editor of the Journal of Chemical Physics Title:  Resonant Photoelectron Spectroscopy of Cyrogenically-Cooled Anions via Dipole-Bound Excited States”</p><h2 class="chalmersElement-H2">​Abstract:</h2> <div>Research in the Wang lab focuses on the investigation of the electronic structure and chemical <span style="background-color:initial">bonding of <img src="/SiteCollectionImages/Centrum/Fysikcentrum/Blandade%20bilddimensioner/300_Lai-Sheng%20Wang.jpg" class="chalmersPosition-FloatRight" alt="" style="margin:5px;width:135px;height:176px" /><br />nanoclusters and anions from solution samples. We first introduced electrospray </span><span style="background-color:initial">i</span><span style="background-color:initial">onization into spectroscopy and developed an experimental apparatus integrating an electrospray </span><span style="background-color:initial">io</span><span style="background-color:initial">nization source with photoelectron spectroscopy, which allowed negatively-charged ions present in </span><span style="background-color:initial">solution to be studied in the gas phase, including multiply-charged anions, solvated anions, organic </span><span style="background-color:initial">and inorganic anions, and biological molecules. In this talk, I will first give a brief overview of our </span><span style="background-color:initial">r</span><span style="background-color:initial">esearch program. </span><span style="background-color:initial">The main topic will be on our recent development of resonant photoelectron </span><span style="background-color:initial">spectroscopy. I will review our early effort in the studies of multiply-charged anions, followed by our </span><span style="background-color:initial">development of a cryogenic ion trap to create vibrationally-cold anions, which are important to obtain </span><span style="background-color:initial">high-resolution photoelectron spectra free of vibrational hot bands. I will describe our recent effort </span><span style="background-color:initial">in high-resolution photoelectron imaging of cryogenically-cooled anions using a tunable laser, </span><span style="background-color:initial">l</span><span style="background-color:initial">eading to the observations of dipole-bound excited states and vibrational autodetachment. I will </span><span style="background-color:initial">present resonantly-enhanced photoelectron imaging via vibrational levels of the dipole-bound states </span><span style="background-color:initial">that yields highly non-Franck-Condon photoelectron spectra and rich vibrational information.</span></div> <span style="background-color:initial"><div> </div></span><div><span style="background-color:initial">I will </span><span style="background-color:initial">show that resonant photoelectron spectroscopy via vibrational autodetachment of dipole-bound </span><span style="background-color:initial">excited states is an unique technique to obtain vibrational information of neutral radicals, including </span><span style="background-color:initial">conformation-selective vibrational information.​</span></div> Åvall, Physics<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>​ Title of doctoral thesis: &quot;Structure and dynamics in liquid battery electrolytes.</p>​<br /><strong>Abstract</strong>: To be announced.<br /> of changing glaciers - processes and mechanisms of ice sheets<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>​Welcome to a colloquim by Angelika Humbert, Alfred Wegener Institute.</p><font color="#212121"><span style="font-size:20px">Abstract:</span></font><img src="/SiteCollectionImages/Centrum/Fysikcentrum/Blandade%20bilddimensioner/Angelika.JPG" class="chalmersPosition-FloatRight" alt="" style="margin:5px;height:215px;width:165px" /><div><div><span style="background-color:initial">Ice sheets are complex systems with processes from the micro-scale to </span><span style="background-color:initial">the kilometer scale are interacting with many of those interactions </span><span style="background-color:initial">being non-linear. Two processes with major influence on the dynamics of </span><span style="background-color:initial">the entire system are still lacking physically based process </span><span style="background-color:initial">description: calving of icebergs and sliding of glaciers across the </span><span style="background-color:initial">bedrock. Both mechanisms are of crucial for simulating future change of </span><span style="background-color:initial">ice sheets in Greenland and Antarctica. After a brief introduction into </span><span style="background-color:initial">the continuum mechanics of ice sheets, we will be discussing climatic </span><span style="background-color:initial">forcing of ice sheets, the role of calving, sliding and ice sheet </span><span style="background-color:initial">simulations for projecting future sea level change.</span></div></div> Ångqvist, Physics<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>Title of thesis: &quot;Atomic scale modeling of ordering phenomena</p>​<br />Abstract: To be announced pathways of hot electrons in graphene<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Welcome to a Graphene Centre Seminar with Bernard Placais, CNRS, Paris, France.</p> QED with atomic hydrogen<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>Welcome to a colloquium by Professor​ Thomas Udem, Max-Planck Institut of Quantum Optics.</p><h2 class="chalmersElement-H2">​Abstract:</h2> <div><span style="background-color:initial">Precise determination of transition frequencies of simple atomic systems are required for a</span></div> <div> </div> <div>number of fundamental applications such as tests of quantum electrodynamics (QED), the</div> <div> </div> <div>determination of fundamental constants and nuclear charge radii. The sharpest transition in</div> <div> </div> <div>atomic hydrogen occurs between the metastable 2S state and the 1S ground state with a</div> <div> </div> <div>natural line width of only 1.3 Hz. Its transition frequency has been measured with almost 15</div> <div> </div> <div>digits accuracy using an optical frequency comb and a cesium atomic clock as a reference [1].</div> <div> </div> <div>A measurement of the Lamb shift in muonic hydrogen is in significant contradiction to the</div> <div> </div> <div>hydrogen data if QED calculations are assumed to be correct [2]. In order to shed light on</div> <div> </div> <div>this discrepancy the transition frequency of one of the broader lines in atomic hydrogen has</div> <div> </div> <div>to be measured with very good accuracy [3].</div> <div> </div> <div><br /></div> <div> </div> <div>[1] C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011).</div> <div> </div> <div>[2] A. Antognini et al., Science 339, 417, (2013).</div> <div> </div> <div>[3] A. Beyer et al., Science 358, 79 (2017).</div> <div> </div> <div>​</div> by Patrice Simon<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>​ Welcome to a GPC Colloquium by Dr, Distinguished Professor​ Patrice Simon, University Paul Sabatier Toulouse, France Title: Electrochemistry at nanoporous electrodes: 2- and 3-D electrodes for Electrochemical Capacitor applications</p><h2 class="chalmersElement-H2">​Abstract:</h2> <div>This presentation will give an overview of the research work we achieved on capacitive (porous carbon) and pseudocapacitive materials and the challenges/limitations associated with the development of these materials. Starting with porous carbons, we will present the state-of-the art of the fundamental of ion adsorption mechanism in porous carbons and its practical applications. Moving from double layer to pseudocapacitive materials, we will show how the control of the electrodes structure can help in preparing high energy electrodes using 2-Dimensional MXene materials.​</div> and reversing a quantum jump mid-flight<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>Joint Linnaeus and GPC Colloquium with Michel Devoret, Yale University, USA​</p><div><span style="font-weight:700"><img src="/SiteCollectionImages/Institutioner/MC2/Föreläsningar/M%20Devoret.jpg" class="chalmersPosition-FloatRight" alt="" style="margin:5px;height:235px;width:200px" />Abstract:</span></div> <div>Measurements in quantum physics, unlike their classical physics counterparts, can fundamentally yield discrete and random results. Historically, Niels Bohr was the first to hypothesize that quantum jumps occurred between two discrete energy levels of an atom. Experimentally, quantum jumps were only directly observed many decades later, in an atomic ion driven by a weak deterministic force under strong continuous energy measurement. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Despite the non-deterministic character of quantum physics, is it possible to know if a quantum jump is about to occur? </div> <div>Our work<span style="font-size:10.5px;line-height:0;vertical-align:baseline;top:-0.5em">1</span> provides a positive answer to this question: we experimentally show that the jump from the ground state to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable “flight” by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the evolution of the jump — once completed — is continuous, coherent, and deterministic. Based on these insights and aided by real-time monitoring and feedback, we then pinpoint and reverse one such quantum jump “mid-flight”, thus deterministically preventing its completion. Our findings, which agree with theoretical predictions essentially without adjustable parameters, lend support to the modern formulation of quantum trajectory theory; most importantly, they may provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as the early detection of error syndromes.</div> <div><br /></div> <div>1.<span style="white-space:pre"> </span>Z. Minev et al., Nature 570, 200–204 (2019)</div> <div><br /></div> imaging of exoplanets<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>Welcome to a colloquium by Markus Janson, Stockholm University.</p><h2 class="chalmersElement-H2">​Abstract: </h2> <div>Since the first discoveries of plants in orbit around other stars in the early/mid-1990s, the field of exoplanet research has rapidly expanded, and now encompasses thousands of exoplanet detections, as well characterization of the detected systems to various degrees of depth. In this talk, I will discuss the different techniques used to detect and study exoplanets, with a particular emphasis on high-contrast imaging, which allows to probe both orbital, physical and atmospheric properties of the planet. High-contrast imaging also allows to study planets during their actual phase of formation, and their interactions with the surrounding protoplanetary disk in which they form. While the sensitivities with currently existing telescopes generally only allow for the detection of classes of planets quite unlike the Earth, improvements in technology are continuously enhancing these sensitivities. I will discuss the pathways toward the ability to detect and characterize true Earth analogs, which will (most likely) be required to assess the frequency and distribution of habitability and life in the universe.​</div>