Alexandra Zampetaki - Helically confined Charged Particles: Dynamics and Vibrational Band Structure

Disputation

Chemie im Weltall – neue Herausforderungen und Methoden

Musik und Wissenschaft
Auch im Wissenschaftsjahr 2016*17 – Meere und Ozeane geht die MS Wissenschaft wieder auf Tour. Sie lädt ein zu einer Forschungsexpedition in diese faszinierende Welt. Kommen Sie an Bord. Erfahren Sie mehr über Vergangenheit und Zukunft der Ozeane, ihre Bedeutung für den Menschen, die Erforschung und den Schutz dieses größten und artenreichsten Lebensraums der Erde. [mehr]

Metal Ion Based Probes for Imaging

Disputation

Generation and control of super-octave-spanning spectra

Disputation

Nele Müller - Electron diffraction and controlled molecules

Disputation

An atomic perspective of the photodissociation and geminate recombination of triiodide in condensed phases

Disputation
Was passiert im Inneren von Zellen? Wie funktioniert der hellste Röntgenlaser der Welt und wie kommen Rechtsmediziner Verbrechern auf die Spur? Der erste Tag des Wissens gibt Einblicke in aktuelle Forschungsthemen. [mehr]

Nonequilibrium ultrafast excited state dynamics in DNA

Disputation
Mit der kostenlosen Veranstaltung „Forschen in Europa: Nationale und europäische Forschungsförderung“ wollen die Kooperationsstelle EU der Wissenschaftsorganisationen (KoWi) und die Universität Hamburg Graduierte, Doktoranden/innen, Postdoktoranden/innen und Nachwuchsgruppen über die vielfältigen nationalen und europäischen Unterstützungsmöglichkeiten informieren. [mehr]

Jhih-An You - Light-induced Electronic Hole Dynamics and its Application

Disputation

Nonlinear phononics and structural control of strongly correlated materials

Disputation

In-liquid Electron Microscopy and Diffraction for real-time observation and structural analysis

Disputation

20. Deutsche Physikerinnentagung

Tagung
Die jährliche Deutsche Physikerinnentagung wird von der Deutschen Physikalischen Gesellschaft (DPG) und ihrem Arbeitskreis Chancengleichheit veranstaltet. Dabei bekommen Physikerinnen aus allen Bereichen und Karrierestufen die Chance sich miteinander zu vernetzen und auszutauschen. Im Vordergrund steht dabei ein fächerübergreifender fachlicher Austausch, um einen Überblick aus allen Bereichen der Forschung und der Physik zu erhalten. Erfahrungsberichte aus dem Alltag verschiedener Berufe aus Industrie, Wirtschaft und Forschung geben einen Einblick über die vielfältigen Möglichkeiten von Physikerinnen und die Chance die Arbeitsituationen von Physikerinnen zu diskutieren.Zudem diskutieren wir in diesem Jahr in der Podiumsdiskussion die Ziele, die Umsetzung und was tatsächlich mit Frauenförderungsprogrammen erreicht wird. [mehr]

Matteo Lucchini - Attosecond dynamics in polycrystalline diamond

MPSD Seminar
The possibility to manipulate the electrical properties of matter with very short opticalpulses is a fascinating field of research with possible far reaching applications inmany relevant technological fields. The first step towards the realization of this goal is to understand the ultrafast dynamics at the basis of light-matter interaction. Short and intense pulses allowed us to investigate a very interesting regime where the photon energy becomes comparable to the cycle-averaged kinetic energy of the electrons in the field. As the optical response of the material transitions from a classical to quantum-mechanical description many intriguing effects co-exist in this regime and the importance of inter- versus intra-band transitions is still debated. We used attosecond transient absorption spectroscopy (ATAS) to study the optical response of polycrystalline diamond driven by few-femtosecond, intense (IIR ~1012 W/cm2) infrared (IR) pulses. We monitored the system response by looking at the induced change in the absorbance with a 250-as pulse centred around 40 eV. We observed the appearance of oscillating features which modulate at twice the IR frequency, ωIR, and fully recover after the interaction. Simultaneous photoelectron acquisition from a gas nozzle placed in front of the diamond target allowed us to study the phase relation of the oscillating features and the pumping IR field. We found that the timing of the diamond response changes significantly with the probing energy and does not always follow the IR field adiabatically. Ab initio calculations performed by coupling time-dependent density functional theory (TDDFT) in real time with Maxwell’s equations reproduced the experimental observations. Further comparison with a numerical two-band model allowed us to conclude that intra-band motion dominates over inter-band transitions, thus identifying the dynamical Franz- Keldysh effect as the dominant mechanism in this regime. Our analysis constitutes an important step towards a full understanding of the optical properties of dielectrics in the Petahertz regime. [mehr]

Cedric Weber - Many body effects in transition metal molecular systems

MPSD Seminar
Phenomena that are connected to quantum mechanics, such as magnetism, transport, and the effect of impurity atoms and disorder, and their relation to material design and energy needs are important for almost every branch of the industry. Density functional theory (DFT) was successful at making accurate Predictions for many materials, in particular compounds which have a metallic behaviour. DFT combines high accuracy and moderate computational cost, but the computational effort of performing calculations with conventional DFT approaches is still non negligible and scales with the cube of the number of atoms. A recent optimised implementation of DFT was however shown to scale linearly with the number of atoms (ONETEP), and opened the route to large scale DFT calculations for molecules and nano-structures. Nonetheless, one bottleneck of DFT and ONETEP, is that it fails at describing well some of the compounds where strong correlations are present, in particular because the computational scheme has to capture both the band-like character of the uncorrelated part of the compound and the Mott-like features emerging from the local strongly correlated centres. A recent progress has been made in this direction by the dynamical mean-field theory (DMFT), that allows to describe the two limits (metal and insulator) in a remarkable precise way when combined with DFT. The ONETEP+DMFT implementation and strategies to overcome the main bottlenecks of this type of calculations will be discussed, and its applications illustrated by a few case of studies, such as the role of quantum entanglement in Myoglobin and heme systems. [mehr]

Presentation skills

IMPRS-UFAST skills course
“Poets are born – speakers are made.” Public speaking is a necessity in scientific life. Take part in this two-days course, find out what your strengths are and develop your individual presentation profile. Take steps to learn how to lead the audience from your first appearance on stage until the last question in the discussion. Be authentic, enthusiastic and convincing! [mehr]

Annual General Meeting at MPSD

Informationsveranstaltung
Working within the Nonequilibrium Green's Function formalism, a formula for the two-time current correlation function is derived for the case of transport through a nanojunction in response to an arbitrary time-dependent bias. The one-particle Hamiltonian and the wide band limit approximation are assumed, enabling us to extract all necessary Green's functions and self-energies for the system, extending the analytic work presented previously [Ridley et al. Phys. Rev. B (2015)]. [mehr]

Julian Gebhardt: Ab initio modeling of perovskite materials and two-dimensional organic networks

MPSD Seminar
Perovskite materials with stoichiometry ABX3 are a rich family of ionic compounds with many technological interesting properties. They can be traditional inorganic, metal per- ovskites or hybrid perovskites with one organic cation. In this talk I want to discuss some results of a special modification of the latter kind, the so-called layered perovskites. This family of structures is currently rediscovered, and we report on the geometrical and electronic structure of one promising candidate ((C6H5C2H4NH3)2PbI4) with phenethylammonium cations on the A-site and compare the differences towards the benchmark case of three-dimensional (3D) hybrid perovskites, CH3PbI3. The influence of varying the cation as well as changing the dimensionality from 3D to 2D systems is discussed by comparing bulk and monolayer structures of both systems. In addition, insight into the optical behavior and the observed electron-phonon coupling will be given. [mehr]

Ultrafast spintronics with terahertz radiation

MPSD Seminar
Terahertz (THz) electromagnetic radiation is located in the gap that separates the realms of electronics (<1 THz) and optics (>30 THz). Sub-picosecond THz pulses are capable of probing and even controlling numerous low-energy excitations such as phonons, excitons and Cooper pairs. Here, we consider experiments showing that THz radiation is also a very useful and versatile tool in the fields of spintronics and ultrafast magnetism. First, we optically launch ultrafast spin transport and study its conversion into charge currents by means of the inverse spin Hall effect [Nature Nanotech. 8, 256 (2013)]. The charge current can be detected by sampling the concomitantly emitted THz radiation. This approach allows us to monitor ultrafast spin currents and provides a quick and easy estimate of the strength of the spin Hall effect in a contact-free manner. In addition, optimization of the spintronic structure has led to new, efficient and scalable emitters of THz pulses that fully cover the range from 1 to 30 THz without gap [Nature Photon. 10, 483 (2016)]. Second, we address spin-lattice coupling by selective excitation of optical phonons in the model ferrimagnetic insulator Y3Fe5O12 (YIG) and find a quenching of magnetic order on a time scale as short as 1 ps. This observation attests to a highly efficient coupling of crystal lattice and electron spins in this material. [mehr]

Enrico Ronca - Theoretical Methods for Excited State Properties in Extended Systems

MPSD Seminar
Light matter interaction is involved in several fundamental processes in Chemistry, Physics and Biology and is at the basis of Spectroscopy, experimental technique representing probably the most important source of information about properties of matter. Due to the significant improvements introduced in the last decades, spectroscopic techniques became very accurate and the interpretation of spectra, just by observation of the experimental results, became sometimes extremely complicated. For this reason the need of accurate theoretical methods able to reproduce and interpret the experimental results is becoming more and more pressing. The accurate theoretical simulation of spectra is therefore a very a challenging task, in particular for extended systems for which only approximated methods can be used. [mehr]

Matteo Puviani - Strongly correlated Floquet systems: Cluster Perturbation Theory approach

CFEL Theory Seminar
Under the influence of periodic fields quantum systems may reach regimes inaccessible under equilibrium conditions and new phases may be engineered by a tunable control [1]. The coexistence of periodic driving forces and electron-electron correlation is particularly interesting for two main reasons: on one side the external driving effectively modulates the inter-site hopping enhancing the effects of the e-e repulsion and the tendency to an insulating behaviour. On the other hand, irradiation itself is responsible for a photo-doping consisting in an electronic energy dressing that may turn a Mott insulator into a metal. Due to these competing effects, novel phenomena are expected when strongly correlated quantum systems are exposed to time-dependent fields. [mehr]
Many-Body Perturbation Theory (MBPT) is a methodology routinely employed in computational spectroscopy to calculate photoemission and absorption spectra. However, usually these computational experiments are only possible for real nanostructures, solids, etc. by resorting to simple approximations in which, e.g., self-consistency is neglected. [mehr]
Density functional theory in finite basis sets tends to degenerate to one-body reduced (1RDM) functional theory. As all calculations are done in finite basis sets, a rigorous foundation of 1RDM functional theory is desirable. To avoid uniqueness problems in the potential to 1RDM mapping, I will discuss the foundations of 1RDM functional theory in finite basis sets at finite temperatures, both for fermions and bosons. The fermionic case turns out to be relatively straightforward, but the bosonic case requires more care. The main result is that we can rigorously proof v-representability and functional differentiability in this setting. [mehr]

Michael Sentef - Light-enhanced electron-phonon coupling from nonlinear electron-phonon coupling

MPSD Seminar
In light of recent experiments suggesting light-induced superconductivity [1] as well as light-enhanced electron-lattice coupling [2] for strongly driven IR phonons, it is natural to ask for a minimal and generic theoretical model that predicts such enhancement effects of important couplings in different material classes. One idea that comes to mind is nonlinear electron-phonon coupling [3,4]. A quadratic coupling term of the form " g2 nel x2IR " is generically the lowest order symmetry-allowed direct coupling of an IR-active phonon coordinate xIR to the electronic density nel in systems with inversion symmetry. In this talk I will present model evidence for light-enhanced electron-phonon coupling and light-induced effective attraction between electrons based on nonlinear electron-phonon coupling [3], the latter of which was already discussed in a similar context in [4]. [1] M. Mitrano et al., Nature 530, 461 (2016) [2] E. Pomarico et al., Phys. Rev. B 95, 024304 (2017) [3] M. A. Sentef, arXiv: 1702.00952 [4] D. M. Kennes et al., Nature Physics (2017) , doi:10.1038/nphys4024, arXiv:1609.03802 [mehr]

Tong Zhou - Quantum Spin-quantum Anomalous Hall Effect with Tunable Edge States in Sb Monolayer-based Materials

MPSD Seminar
The quantum anomalous Hall (QAH) effect, quantum spin Hall (QSH) effect and (quantum) valley Hall ((Q)VH) effect have attracted considerable attention in condensed matter physics and material science. Generally, only one of the QAH, QSH, and QVH effects can be realized in a specific system. It would be very interesting if these three effects can be achieved in one single system. In this talk, I shall represent this interesting imagination may be realized in Sb monolayer-based materials, where the QAH state occurs at one valley and the QSH state occurs at the other valley, called quantum spin-quantum anomalous Hall (QSQAH) effect. [mehr]

Claudius Hubig - Time Evolution with a Krylov Variant for MPS Time Evolution with a Krylov Variant for MPS

MPSD Seminar
I will briefly motivate the need for time-evolution methods on matrix-product states and provide an overview over the available methods. Focus will then be placed on the Krylov subspace method to evaluate exp(-tH)|ψ❭, starting from a naive translation of the standard method suitable for dense matrix-vector arithmetic to matrix-product arithmetic. Key improvements that can be made to the algorithm when applied to matrix-product arithmetics will then be illustrated, resulting in increased precision and reduced computational effort. [mehr]

Vahid Sandoghdar - Nano-Quantum-Optics: from single photons and emitters to cooperative effects

MPSD Seminar
I plan to start this presentation with an overview of our work over the past decade on the efficient coupling of light and single quantum emitters, leading to the single-photon communication of two individual molecules at long distances [1]. In this context, we will also discuss new results on a high-efficiency triggered source of single photons [2] and coherent nonlinear optical phenomena, which let a single organic molecule act as an efficient switch for weak beams of light [3]. The long-term goal of these projects is to establish a platform for nano-quantum-optical operations and cooperative interactions in a mesoscopic system of photons and quantum emitters [4, 5]. In order to achieve this, we have developed novel microcavity [6] and chip-based nanoguide circuitry [7] for use at cryogenic conditions.References:[1] Y. Rezus, et al., Phys. Rev. Lett. 108, 093601 (2012). [2] X-L. Chu, et al., Nature Photonics, 11, 58 (2017).[3] A. Maser, et al., Nature Photonics 10, 450 (2016).[4] S. Faez, et al., Phys. Rev. Lett. 113, 213601 (2014). [5] H. Haakh, et al., Phys. Rev. A, 94, 053840 (2016).[6] D. Wang, et al., Phys. Rev. X, under review.[7] P. Türschmann, et al., submitted. [mehr]

Noejung Park - Ab initio study of ultrafast dynamics of spin-valley polarized states in TMDC with a particular focus on the role of phonon

MPSD Seminar
The valley degree of freedom and the possibility of spin-valley coupling of solid materials have attracted growing interest, and the relaxation dynamics of spin- and valley-polarized states has become an important focus of recent studies. In spin-orbit-coupled inversion-asymmetric two-dimensional materials, such as MoS2, it has been found that the spin randomization is characteristically faster than the time scales for inter- and intra-valley scatterings. In this talk, I present our recent study of the ultrafast non-collinear spin dynamics of the electron in a valley of monolayer MoS2 by using real-time propagation time-dependent density functional theory. We found that the spin precession is sharply selectively coupled only with the particular optical phonon that lifts the in-plane mirror symmetry. We suggest that the observed spin randomization can be attributed to this spin-phonon interaction. Further, our results imply that flipping of spins in a spin-orbit-coupled system can be achieved by the control over phonons. In a later part of the talk, I would also describe the feature of the computational package we have developed and have used for the spin-phonon dynamics, which is based on the plane-wave basis set and various types of pseudopotentials. [mehr]

Peter Deák - How to choose the correct hybrid functional for defect calculations

MPSD Seminar
The electronic and optical properties of a material critically depend on its defects, and understanding that requires substantial and accurate input from theory. Defect calculations in traditional semiconductors have relied on the local and semi/local approximations of density functional theory, which in wide band gap materials may lead to fatal errors. Since first-principle total energy methods beyond these approximations cannot yet be carried out with sufficient accuracy for the supercells needed in defect calculations, nowadays semi-empirical hybrid functionals are often applied instead. In my talk I will analyze the performance of the HSE06 screened hybrid functional on defects in Group-IV semiconductors and in TiO2, and show that its success is the result of error compensation between semi-local and non-local exchange, resulting in a proper derivative discontinuity (reproduction of the band gap) and a total energy which is a linear function of the fractional occupation numbers (removing most of the electron self-interaction). As it is well known, however, HSE06 does not work equally well for all materials. On the example of Ga2O3, I will show that tuning both the mixing and the screening parameter of HSE for the given material allows to ensure the same error compensation. Unless the electronic screening is strongly direction- or orbital-dependent (as in ZnO), the optimized HSE hybrid is nearly self-interaction free and provides a band structure on par with GW. Since the total energy can also be calculated, the real equilibrium structure of a defect can be found and the levels are in good agreement with experimental observations. [mehr]

Sven Ahrens - Relativistic quantum dynamics and the electron spin in standing light waves

MPSD Seminar
Strong laser beams allow for the coherent control of electrons, as for example electron diffraction in standing light waves [1]. In my talk I will discuss the possible manipulation of the electron spin in X-ray diffraction, in which the interacting standing wave of light can be formed from intensive and coherent X-ray beams from free-electron lasers [2]. The quantum dynamics of this process is theoretically modeled by inserting a plane wave ansatz for the electro-magnetic field and the electron wave function into the Dirac equation [3]. Based on this relativistic quantum description and substantiated by numeric and analytic solutions I will talk about the necessary kinematic conditions for generating spin effects in light-matter interaction. To the end I will present spin dynamics in circularly polarized laser fields [4,5] and discuss how circular polarization can be used for generating spin-dependent diffraction and spin polarization [6]. [1] D. L. Freimund, K. Aflatooni, H. Batelaan, Nature 413, 142 (2001). [2] S. Ahrens, H. Bauke, C. H. Keitel, and C. Müller, Phys. Rev. Lett. 109, 043601 (2012). [3] S. Ahrens, H. Bauke, C. H. Keitel, and C. Müller, Phys. Rev. A 88, 012115 (2013). [4] H. Bauke, S. Ahrens, C. H Keitel, R. Grobe, New J. Phys. 16, 103028 (2014). [5] H. Bauke, S. Ahrens, R. Grobe Phys. Rev. A 90, 052101 (2014). [6] S. Ahrens, arXiv:1604.06201 [quant-ph] (2016). [mehr]

Eryin Wang - Novel and tailored electronic structures in 2D material heterostructures

MPSD Seminar
The big family of 2D materials provides variable and interesting stacking blocksfor constructing 2D heterostructures to achieve novel electronic propertiesdistinct from its constitute materials. So far, the 2D material heterostructureshave been an emerging research area with increasing scientific interest. In thistalk, I will present the angle-resolved photoemission spectroscopy studies ontwo novel 2D heterostructures, Bi2Se3/BSCCO [1] and graphene/h-BN [2,3]. Iwill show how the proximity effect (in Bi2Se3/BSCCO) and moire superlatticepotential (in graphene/h-BN) tune the electronic properties and further lead tothe realization of many novel quantum phenomena. The variety of 2D materialsgenerates great possibilities in 2D heterostructures which are waiting for moreresearch investigations. [mehr]

Jorge Jover - Open quantum systems: a geometric description

MPSD Seminar
In this talk, I will present some of the results in my PhD dissertation, whose main topic is the geometric description of open quantum systems. Differential geometry allows for an intrinsic formulation of mathematical models, thus achieving a better characterisation of their properties. I will analyse from a geometric perspective the manifold of pure and mixed states of quantum systems and its properties, such as its stratification in terms of the rank of states [1]. The algebraic properties of observables allow to define a Poisson and a symmetric tensor fields on the manifold, which are necessary in order to describe features such as dissipation and Markovian dynamics in an intrinsic way. Applications to Molecular Dynamics, in particular the Hamiltonian description of the Ehrenfest model, will also be discussed [2,3]. [1] Grabowski, Kus, Marmo. Symmetries, group actions and entanglement. Open Syst. Inf. Dyn. 13, 343–362 (2006) [2] Alonso et al. Statistics and Nosé formalism for Ehrenfest dynamics. J. Phys. A Math. Theor. 44 395004 (2011) [3] Alonso et al. Ehrenfest dynamics is purity non-preserving: a necessary ingredient for decoherence. J. Chem. Phys. 137 54106 (2012) [mehr]

Dante Kennes - Transient superconductivity from electronic squeezing of optically pumped phonons

MPSD Seminar
Advances in light sources and time resolved spectroscopy have made it possible to excite specific atomic vibrations in solids and to observe the resulting changes in electronic properties but the mechanism by which phonon excitation causes qualitative changes in electronic properties, has remained unclear. Here we show that the dominant symmetry-allowed coupling between electron density and dipole active modes implies an electron density-dependent squeezing of the phonon state which provides an attractive contribution to the electron-electron interaction, independent of the sign of the bare electron-phonon coupling and with a magnitude proportional to the degree of laser-induced phonon excitation. Reasonable excitation amplitudes lead to non-negligible attractive interactions that may cause significant transient changes in electronic properties including superconductivity. The mechanism is generically applicable to a wide range of systems, offering a promising route to manipulating and controlling electronic phase behavior in novel materials. [mehr]

Simon Wall - Spins, Phonons and Phase Separation in Correlated Materials

MPSD Seminar
Electrons, phonons and spins are the key ingredients that make up correlated materials and understanding how these parameters interact is vital for determining their relative interactions. In this talk I will discuss our recent experiments on how to measure these interactions on a range of time and length-scales. I will discuss demagnetization of the antiferromagnetic Mott insulator Cr2O3 as measured through second harmonic generation, in which the demagnetization pathway is dictated by phonons. Then I will discuss the insulator to metal transition in VO2, both in terms of static nano-scale measurements of phase separation measured with resonant soft X-ray holography and dynamic measurements of how the phonon degree of freedom evolves away from the zone centre using time-resolved thermal diffuse scattering. [mehr]
From 1 – 15 June 2017 the cluster of excellence CUI presents images from the micro world in Hamburg’s City Hall. The exhibition is centered around the beauty of visualizations of scientific phenomena and structures researched by the Hamburg Centre for Ultrafast Imaging, flanked by photographs of experiments and labs. [mehr]

Yang Peng - Boundary Green functions of topological insulators and Superconductors

MPSD Seminar
Topological insulators and superconductors are characterized by their gapless boundary modes. In this talk, we develop a recursive approach to the boundary Green function which encodes this nontrivial boundary physics. Our approach describes the various topologically trivial and nontrivial phases as fixed points of a recursion and provides direct access to the phase diagram, the localization properties of the edge modes, as well as topological indices. We illustrate our approach in the context of various familiar models such as the Su-Schrieffer-Heeger model, the Kitaev chain, and a model for a Chern insulator. We also show that the method provides an intuitive approach to understand recently introduced topological phases which exhibit gapless corner states. Ref: Yang Peng, Yimu Bao, Felix von Oppen, arXiv:1704.05862 (2017) [mehr]

Michael Lubasch - Density Functional and Tensor Network Theory

MPSD Seminar
In my talk I will discuss this question: How can density functional and tensor network theory be combined in such a way that they benefit from each other. In particular I will present our publication [1] in which we developed a systematic procedure for the approximation of density functionals in density functional theory that consists of two parts. In the first part, for the efficient approximation of a general density functional, we introduced an efficient ansatz whose non-locality can be increased systematically. In the second part, we presented a fitting strategy that is based on systematically increasing a reasonably chosen set of training densities. I will present our results from reference [1] for strongly correlated fermions on a one-dimensional lattice. In this context we focused on the exchange-correlation energy and demonstrated how an efficient approximation can be found that includes and systematically improves beyond the local density approximation. Remarkably, we could show this systematic improvement for target densities that are quite different from the training densities. [1] M. Lubasch, J. I. Fuks, H. Appel, A. Rubio, J. I. Cirac, and M.-C. Banuls, New Journal of Physics 18, 083039 (2016)." [mehr]

Philipp Strasberg - Strong coupling thermodynamics and Markovian embedding strategies

Whenever a small-scale system is weakly coupled to macroscopic and Markovian reservoirs, it is possible to establish a consistent thermodynamic framework -- even for systems far away from equilibrium and even at the level of single, fluctuating trajectories. But outside the Markovian and weak coupling regime already the definition of basic quantities such as internal energy or heat becomes problematic. After reviewing the phenomenology of non-equilibrium thermodynamics, I will discuss a technique which allows to map a strongly coupled, non-Markovian system to a weakly coupled, Markovian one by appropriately including environmental degrees of freedom in the description of the system. Thus, by redefining the system-environment partition, it is possible to carry over a consistent thermodynamic framework to the strong coupling situation. [mehr]

Ming Lei - Capping the Ends: Structure and Function of Telomere Proteins

MPSD Seminar
Telomeres, the natural ends of linear eukaryotic chromosomes, are specialized protein-DNA complexes that play essential roles in cell viability and genome integrity. The long-term goal of my research is to understand how telomeres protect chromosome ends and mediate their replication by telomerase. A six-protein complex, called shelterin, associates with telomeres and protects the ends of human chromosomes. A major gap in our knowledge of the shelterin complex is how its protein components organize at telomeres. I will present our recent studies that reveal the molecular architecture and functional significance of the shelterin complex. [mehr]
Spectroscopic mapping by STEM/EELS has proven to be a powerful technique for determining the structure, chemistry and bonding of interfaces, reconstructions, and defects. So far, most efforts in the physical sciences have focused on room temperature measurements where atomic resolution mapping of composition and bonding has been demonstrated [1-3]. For many materials, including those that exhibit electronic and structural phase transitions below room temperature and systems that involve liquid/solid interfaces, STEM/EELS measurements at low temperature are required. Operating close to liquid nitrogen temperature gives access to a range of emergent electronic states in solid materials and allows us to study processes at liquid/solid interfaces immobilized by rapid freezing [4,5]. [mehr]

Ryan Requist - Reduced formula for the macroscopic polarization including quantum Fluctuations

MPSD Seminar
The macroscopic polarization of a solid is an fundamental quantity from which permittivity and piezoelectric tensors can be derived. The Berry phase formula of King-Smith and Vanderbilt expresses the macroscopic polarization in terms of the Bloch states of a mean-field band structure, almost invariably taken from density functional theory. Although this procedure has been successful for many materials, quantum fluctuations cause it to break down in strongly correlated systems. [mehr]

Lukas Müchler - Exploring topological phenomena in Molecules

MPSD Seminar
The use of topological methods has revolutionized the field of condensed matter physics both theoretically and experimentally. Many new exotic states of matter with unremovable surface states that can carry dissipationless currents have been predicted and quickly been verified experimentally. Moreover, topological techniques that characterize periodic Hamiltonians according to their spatial and non-spatial symmetries have lead to an almost complete classification of all possible non-magnetic states of matter that can be realized in crystals. [mehr]

Sangwan Sim - Ultrafast optical spectroscopy of topological insulators and two-dimensional transition metal dichalcogenides

MPSD Seminar
Ultrafast optical spectroscopy of quantum materials uncovers their intrinsic physical properties such as light-matter interactions and dynamics of particles and quasi-particles. In this presentation, I will present our ultrafast optical studies of two different electronic systems: topological insulators (TIs) and two-dimensional transition metal Dichalcogenides (2D TMDs). In TIs, where Dirac-like topological surface states (TSSs) coexist with an underlying bulk insulator, we have investigated ultrafast dynamics of TSS Dirac fermions and plasmons, and their interactions with phonon by using optical-pump terahertz-probe spectroscopy. We have found that, unlike Dirac electrons in graphene, TSS Dirac electrons exhibit unique dynamic features originating from interactions with coexisting bulk insulator. In the studies of 2D TMDs, we have performed ultrafast optical pump-probe spectroscopy of anisotropic excitons in group-VII TMDs. We discuss coherent light-matter interactions such as excitonic optical Stark effect and quantum beats, both of which exhibit significant laser-polarization dependence, resulting from anisotropic nature of the excitons. [mehr]

Hideo Aoki - Quantum phases induced by polarised light --- Floquet-topological states and Higgs modes

MPSD Seminar
Putting materials out of equilibrium can produce new quantum phenomena. When driven by a circularly-polarised laser, a massless Dirac system such as graphene can be converted into the Floquet topological insulator of photon-dressed electrons, which realises the quantum anomalous Hall effect. The phase diagram becomes increasingly intricate for lower laser frequencies where transitions between different Chern numbers take place. If we consider electron correlation, we can predict transitions between Floquet topological insulators and Mott insulator[1]. In another avenue a linearlypolarised light can induce the collective Higgs amplitude mode in superconductors, a condensed-matter analogue of the renowned particle. Then a resonantly strong third-harmonic generation arises, where the Higgs mode contribution is shown to dominate[2] unlike in the BCS approximation.[1] T. Mikami et al, Phys. Rev. B 93, 144307 (2016).[2] R. Matsunaga et al, Phys. Rev. B 96, 020505(R) (2017). [mehr]

Paolo G. Radaelli - Lecture 1: Introduction to symmetry in CMP

MPSD Seminar

Paolo G. Radaelli - Lecture 2: Crystallographic point groups and group theory

MPSD Seminar

Michael Schüler: Nonequilibrium topological states traced by transient spectroscopies

MPSD Seminar
Chern insulators exhibit fascinating properties which originate from the topologically nontrivial state characterized by the Chern number. How these properties are affected and manifest in the presence time-dependent perturbations is still a sparsely explored field of research. This applies,in particular, to quantum quenches between topologically distinct phases. Identifying robust measures of topology which are applicable in nonequilibrium scenarios — ideally also for correlated materials or dissipative systems — is thus a nontrivial task. [mehr]

Paolo G. Radaelli - Lecture 3: Introduction to the theory of representations

MPSD Seminar

Paolo G. Radaelli - Lecture 4: Key theorems about irreducible representations

MPSD Seminar

Paolo G. Radaelli - Lecture 5: Applications of representations to physics problems

MPSD Seminar
See 'more' for link for lecture notes and supportin material. [mehr]

Paolo G. Radaelli - Lecture 6: Projectors, subduction and group product

MPSD Seminar

Binghai Yan - Nonlinear optical responses of Weyl semimetal materials

MPSD Seminar
In the band structure of a Weyl semimetal (WSM), the conduction and valence bands cross each linearly through Weyl points that are usually treated as “monopoles” of the Berry curvature. As a second-order response, WSMs were very recently demonstrated to show strong nonlinear optical effects including an exotic nonlinear Hall effect. This is caused by the non-equilibrium distribution of the Berry curvature, described as the “dipole” of the Berry curvature. In this talk, I will talk about our recent computational results on nonlinear response for representative WSM materials TaAs and MoTe2. [mehr]

Daniele Fausti - Optical control and quasiparticle witnessing in strongly correlated electron systems

MPSD Seminar
The prospect of “forcing” the formation of quantum coherent states in matter, by means of pulsed electromagnetic fields, discloses a new regime of physics where thermodynamic limits can be bridged and quantum effects can, in principle, appear at ambient temperatures. In this presentation I will introduce the field of optical control of correlated electron systems. I will focus on the possibility of coherently driving low-lying excitations of quantum many body systems making light-based control of quantum phases in real materials feasible. I will review the recent results in archetypal strongly correlated cuprate superconductors and introduce our new approach to go beyond mean photon number observables. I will show that quantum features of light can provide a richer statistical information than standard linear and non-linear optical spectroscopies. This will potentially uncover with unprecedented detail the evolution and properties of light-induced transient states of matter.ReferencesScience 331 (6014), 189 (2011)Nature Comm. 6, 10249 (2015)Nature Comm. 5, 5112 (2014)New J. Phys. 16 043004 (2014)Nature materials 12 (10), 882-886(2013) [mehr]

Paolo G. Radaelli - Lecture 7: Tensors and tensor products of representations

MPSD Seminar

Paolo G. Radaelli - Lecture 8: “Physical” tensors

MPSD Seminar

Eric Heller - Graphene Spectroscopy and Ultrafast Pump-Probe Experiments and Theory

MPSD Seminar
Graphene is a key reduced dimensionality solid with many promising applications. Its spectroscopy is vital to understanding the quantum physics of graphene and to evaluate some of the potential uses of graphene. We have found that new and very different interpretations of graphene spectroscopy and ultrafast pump-probe experiments are required. This will be explained. They are very informative about the role of phonon assisted processes in solids and the role of the electronic transition matrix elements. [mehr]
Investigation of the ultrafast photoexcited electronic response in semiconductors has provided invaluable insights into carrier dynamics. Germanium and its alloys with Si have promise for creating multi-junction solar cells with higher efficiency and mid-infrared optoelectronics. However, the dynamics are complicated by multiple energetically similar valleys, rendering an understanding of carrier thermalization and population inversion following photoexcitation difficult. Attosecond transient absorption spectroscopy (ATAS) has recently been employed to probe ultrafast electron and hole dynamics in germanium at the M4,5-edge (~30 eV). In the experiment, a 5 fs VIS-NIR pump pulse excites carriers across the direct band gap and the dynamics are probed with a time-delayed broadband extreme ultraviolet pulse generated by high harmonic generation in xenon spanning ~20-45 eV. The observed transient absorption signal contains the energetic distribution of both carriers, electrons and holes, due to state blocking as well as spectroscopic features induced by bandshifts (e.g. due to band gap renormalization) and broadening (e.g. due to many body effects). By iterative procedures the measured signal can be successfully decoupled into these contributions resolving the carrier and band dynamics with excellent time and energy resolution. Hot carrier relaxation on a 100-fs time scale and carrier recombination on a 1-ps time scale are observed in nanocrystalline Germanium. Going from bulk semiconductor to two-dimensional layers, long-lived core-exciton states are observed at the MoN2,3 edge between 32 and 35 eV in MoS2. Comparing the XUV absorption spectra of bulk and monolayer MoS2, a ~4 eV red-shift suggests a tightly bound core-exciton. The lifetime of the core-exciton states can be directly measured in the time domain. Furthermore, transient Stark shifts, coherences, and coherent population transfer between different core-exciton states are observed. [mehr]

Franco Bonafé - Time-dependent electron-nuclear dynamics in DFTB+: theory and applications

MPSD Seminar
This seminar will be focused on an implementation of electron-nuclear real-time dynamics within the Density Functional Tight-Binding (DFTB) formalism in the DFTB+ package[1], as a result of a collaboration between the Quantum Dynamics Group (University of Córdoba) and the BCCMS (University of Bremen). Some theory details will be presented as well as its application to explain the launching of mechanical oscillations in metal nanoparticles under plasmon-resonant laser illumination[2]. [mehr]
Coherent Multi-Dimensional Spectroscopy (CMDS) is a powerful technique that is directly sensitive to couplings between quantum states. In the optical regime, the technique is well-suited to investigate interactions between the electronic degrees of freedom in systems such as biological light-harvesting complexes and nanostructures. Following a general introduction on Multi-Dimensional Spectroscopy, I will present an ultrafast optical two-dimensional spectrometer based on a hollow-core fiber for broadband visible continuum generation and two acousto-optic pulse shapers arranged in a Mach-Zehnder interferometer for the production of fully-coherent pulse trains. The setup can easily switch between a pump-probe geometry and a collinear geometry with polarization shaping capabilities. The methodological improvements presented here represent important enabling steps towards the longstanding goal of achieving an ”Optical NMR”, and extends the realm of all-optical Multi-Dimensional Spectroscopy to spatially heterogeneous samples. The methods developed are then applied on two classes of systems. The model system Nile Blue is used to validate the performance of the instrument. The spectrometer is also used to reveal new processes in colloidal semiconductor CdSe nanocrystals. One of the most fascinating aspects of semiconductor nanocrystals is their ability to host multiple excitations per particle. When multiple excitons are created in the same nanocrystal, bound quasi-particles called multiexcitons form. In contrast to the single exciton, the structural and dynamic properties of multiexcitons remains, to this day, relatively poorly understood due to their complexity. In the last part of the seminar, I will discuss new insights gained on the structure of the ground state biexciton thanks to the optical CMDS method. [mehr]

Mitsuharo Uemoto - TDDFT+Maxwell multiscale method for lightpropagation calculation in semiconducting media

MPSD Seminar

Takashi Oka - Applied Floquet engineering

MPSD Seminar

Emanuele Dalla Torre - From Floquet engineering to prethermalization of peridically driven systems

MPSD Seminar

Polariton photophysics and photochemistry: theoretical perspectives

CFEL Theory Seminar
Organic molecules interact strongly with confined electromagnetic fields in plasmonicarrays or optical microcavities owing to their bright transition dipole moments. Thisinteraction gives rise to molecular polaritons, hybrid light-matter quasiparticles.Molecular polaritonics opens new room-temperature opportunities for the nontrivialcontrol of energy transport in the nano and mesoscales and modification of physicochemicalproperties of molecular assemblies. In this talk, I’ll showcase some of theseopportunities that we have been theoretically exploring in the past few years within thecontext of physical chemistry. I’ll start by briefly mentioning our work on topologicallynontrivial phases in excitonic and polaritonic systems of organic dye molecules [1,2].Next, I will discuss recent work on how polaritons can enhance singlet-fissionprocesses [3] or how excitation energy can be transferred across mesoscopicdistances of hundreds of nanometers to micron lengthscales [4]. If time permits, I’llconclude by explaining what we can learn about molecular polaritons using twodimensionalspectroscopy [5,6].[1] J. Yuen-Zhou et al., Nature Mater. 13, 1026 (2014).[2] J. Yuen-Zhou et al., Plexcitons: Dirac points andtopological modes, Nat. Commun. 7, 11783 (2016).[3] L. A. Martínez-Martínez, et al., Polariton-assistedsinglet fission in acene aggregates, under review in J.Phys. Chem. Lett., arXiV:1711.11264.[4] M. Du et al., Polariton-assisted remote energy transfer(PARET), under review in Chem. Sci., arXiv:1711.11576.[5] B. Xiang et al., Revealing hidden vibration polaritoninteractions by 2D IR spectroscopy, under review in Proc.Nat. Acad. Sci., arXiv:1711.11222.[6] R. F. Ribeiro et al., Theory for nonlinear spectroscopyof vibrational polaritons, submitted to J. Phys. Chem.Lett., arXiv:1711.11576. [mehr]

Numerically exact full counting statistics of the Anderson impurity model

MPSD Seminar
The full characterization of charge transfer processes in molecular junctions requires techniques for evaluating not only the first and second moments of charge currents, but also higher-order statistical cumulants of the charge transfer process. The complete set of cumulants gives access to the full counting statistics (FCS) through the so-called generating function [1]. [mehr]

Slow-Electrons Interacting with Light and Matter

MPSD Seminar

Sub-optical-cycle control of light and matter

MPSD Seminar

Liquid-Phase Electron Microscopy of Cells and Nanomaterials in Liquid

MPSD Seminar
Transmission electron microscopy (TEM) has traditionally been associated with the study of thin solid samples in vacuum. With the availability of reliable thin membranes of silicon nitride, TEM of liquid specimens has become accessible with nanoscale resolution in the past decade [1]. The usage of scanning transmission electron microscopy (STEM) presents a novel concept to study membrane proteins within whole mammalian cells in their native liquid environment [2]. The cells in liquid are placed in a microfluidic chamber enclosing the sample in the vacuum of the electron microscope, and are then imaged with STEM. It is not always necessary to enclose the cells in the microfluidic chamber. For many studies, it is sufficient to obtain information from the thin outer regions of the cells, and those can be imaged with high resolution using environmental scanning electron microscopy (ESEM) with STEM detector [3]. A third option is to cover a liquid specimen under a thin membrane of graphene providing the thinnest possible layer [4]. Liquid STEM was used to explore the formation of HER2 homodimers at the single-molecule level in intact SKBR3 breast cancer cells in liquid state [3]. HER2 is a membrane protein and plays an important role in breast cancer aggressiveness and progression. Data analysis based on calculating the pair correlation function from individual HER2 positions revealed remarkable differences its functional state between rare- and bulk cancer cells with relevance for studying the role of cancer cell heterogeneity in drug response. We discovered a small sub-populations of cancer cells with a different response to a prescription drug [5]. Liquid STEM was also used to explore the behavior of nanoparticles in liquid in time-lapse experiments. It was discovered that nanoparticle movement in close proximity of the supporting silicon nitride membrane was three orders of magnitude slower than what was expected on the basis of Brownian motion for a bulk liquid [6], pointing to the existence of a layer of highly ordered liquid at the membrane. References [1] de Jonge, N. and Ross, F.M. Nat. Nanotechnol., 6, 695-704 (2011) [2] de Jonge, N., et al. Proc. Natl. Acad. Sci., 106, 2159-2164 (2009) [3] Peckys, D.B., et al. Sci. Adv., 1, e1500165 (2015) [4] Dahmke, I.N., et al. ACS Nano, 11, 11108-11117 (2017) [5] Peckys, D.B., et al. Mol. Biol. Cell, 28, 3193-3202 (2017) [6] Verch, A., et al. Langmuir, 31, 6956–6964 (2015) [mehr]

Orbital-dependent improvements to density-functional approximations: Application of the FLO-SIC method

MPSD Seminar

Resonant Thermalization of periodically driven strongly correlated electrons

MPSD Seminar

Short Course on: Ultrafast Spectroscopy of Phonons and Spin Excitations in Solids - Lecture I

MPSD Seminar
Lecture notes see 'more'. [mehr]

Short Course on: Ultrafast Spectroscopy of Phonons and Spin Excitations in Solids - Lecture II

MPSD Seminar
Lecture notes see 'more'. [mehr]

Short Course on: Ultrafast Spectroscopy of Phonons and Spin Excitations in Solids - Lecture III

MPSD Seminar
Lecture notes see 'more' [mehr]

Ultrafast Electron Diffraction and Microscopy with High-Coherence Beams

MPSD Seminar
Time-resolved electron imaging, diffraction and spectroscopy are exceptional laboratory-based tools to trace non-equilibrium dynamics in materials with a sensitivity to structural, electronic and electromagnetic degrees of freedom. The capabilities of these approaches are largely governed by the quality of the beam of electrons used.This talk will discuss recent advances made by employing high-coherence ultrashort electron pulses from nanoscale field emitters, which substantially enhance the achievable image resolution in both real and reciprocal space. Two complementary developments with ultimate surface sensitivity and spatial resolution, respectively, will be presented, namely Ultrafast Low-Energy Electron Diffraction (ULEED) and Ultrafast Transmission Electron Microscopy (UTEM). Several recent examples of applying these methods to the observation of phase-ordering kinetics, the excitation of strongly-coupled fluctuation modes and the control of metastable states will be given. [mehr]

Short Course on: Ultrafast Spectroscopy of Phonons and Spin Excitations in Solids - Lecture IV

MPSD Seminar
Lecture IV Abstract will follow. [mehr]

Short Course on: Ultrafast Spectroscopy of Phonons and Spin Excitations in Solids - Lecture V

MPSD Seminar
Lecture V Abstract will follow. [mehr]

Short Course on: Ultrafast Spectroscopy of Phonons and Spin Excitations in Solids - Lecture VI

MPSD Seminar
Lecture VI Abstract will follow. [mehr]

Quantum control and dynamics with x-rays

MPSD Seminar
More than fifty years ago, it was the invention of the laser that revolutionized atomic physics and laid the foundations for quantum optics and coherent control. With only optical frequencies available, the interaction of coherent light with matter was for a long time mainly restricted to atomic transitions. Only recently have novelhigh-frequency light sources rendered x-ray quantum optics possible. In this higher frequency regime, atomic nuclei rise as natural candidates for the interaction with coherent light creating a new bridge between atomic physics, quantum optics and nuclear condensed matter physics. Nuclei are very clean quantum systems, well isolated from the environment and benefiting from long coherence times. Combining the advantages of x-rays and nuclei, a prominent incentive is to exploit x-rays as the future quantum information carriers or for novel probing technologies based on quantum effects. Furthermore, the control of nuclear transitions would open the possibility to use long-lived nuclear excited states as a compact and clean energy storage solution. The lecture will follow the developments on the emerging field of x-ray quantum optics and focus on the mutual control of coherent x-ray radiation and nuclear transitions in this new regime of laser-matter interactions. [mehr]

Gapless excitations in the ground state of 1T-TaS2

MPSD Seminar
1T-TaS2 is a layered transition metal dichalcogenide with a very rich phase diagram, which was investigated since the early 1970s. At T=180K it undergoes a metal to Mott insulator transition. Mott insulators usually display anti-ferromagnetic ordering in the insulating phase but 1T-TaS2 was never shown to order magnetically. We have shown that 1T-TaS2 has a large paramagnetic contribution to the magnetic susceptibility but it does not show any sign of magnetic ordering or freezing down to 20mK, as probed by muSR, possibly indicating a quantum spin liquid ground state. Although 1T-TaS2 exhibits a strong resistive behavior both in and out of plane at low temperatures we find a linear term in the heat capacity suggesting the existence of a Fermi-surface, which has an anomalously strong magnetic field dependence. [mehr]

Probing (and Changing) the Mechanical Properties of Cell Membranes

MPSD Seminar
Cell membranes are formed of lipid bilayers, and separate the interiors of all living cells from the surroundings. They have an integral role in maintaining the internal environment of cells. Anaesthetics have been shown to have a potency directly proportional to their affinity for lipid substances, strongly implying that the effect of anaesthesia is due to the action on cell membranes of the anaesthetic molecules. Previous studies have demonstrated the effects of anaesthetics on lipid melting points, but no experiments have looked at changes in mechanical properties. [mehr]

Non-equilibrium control of the effective free energy landscape in a frustrated magnet

MPSD Seminar
Geometrically frustrated magnets often possess accidentally degenerate ground states at zero temperature. At low temperature, thermal fluctuations lift the accidental degeneracy and tend to stabilize ground states with maximal entropy. This phenomenon, known as “order by disorder”, underlines the fluctuation contribution to the free energy landscape in frustrated magnets.In this talk, I show that one can control such free energy landscape in a non-equilibrium setting. In a frustrated magnet with precessional dynamics, the system’s slow drift motion within the degenerate ground state manifold is governed by the fast modes out of the manifold. Exciting these fast modes generates a tuneable effective free energy landscape with minima located at thermodynamically unstable portions of the ground state manifold. I demonstrate this phenomenon on pyrochlore XY antiferromagnet, where a magnetic field pulse is sufficient for controlling the effective free energy landscape at nonequilibrium. [mehr]

Many-body dynamics in pump and probe experiments: From light amplification to terahertz STM

MPSD Seminar
I will discuss new theoretical approaches for analyzing pump and probe experiments in solid state systems. The focus will be on combining theoretical techniques from condensed matter physics and quantum optics. Several examples will be discussed, including light amplification in photo-excited superconductors and insulators, ultrafast molecular dynamics in terahertz-STM experiments. [mehr]

Quantum enhanced super-resolution microscopy

MPSD Seminar
Although the principles of quantum optics have yielded multiple ideas to surpass the classical limitations in optical microscopy, their application in life science imaging has remained extremely challenging. In this talk, I will present two works that apply measurements of photon correlations for the benefit of localization microscopy and image scanning microscopy (ISM). The first uses photon antibunching measurement to estimate the number of emitters in a fluctuating scene and can potentially speed-up super-resolution techniques based on localization microscopy [1]. In the second work, we employ photon antibunching as the imaging contrast itself. Measuring the spatial distribution of ‘missing’ photon pairs in an ISM architecture may enhance lateral resolution four time beyond the diffraction limit [2]. The robustness of the antibunching signal enabled super-resolved imaging of fixed cells, relying solely on a quantum contrast. [mehr]

Shedding New Light on Dirac Materials with Nonlinear Optics

MPSD Seminar
Nonlinear optics has recently emerged as an attractive approach for both probing topological properties and driving Dirac materials into new states. Here, I will describe our use of ultrafast nonlinear optics to study three representative Dirac materials: graphene micro-ribbons, topological insulators, and Weyl semimetals. [mehr]

Unconventional Charge Density Wave Transitions

MPSD Seminar
Historically charge density waves have been associated with the notions of Fermi surface nesting and, at the transition temperature, a soft phonon mode. In this talk, I will present two cases that defy this common theme. First, I will show that TiSe2 undergoes a transition due to exciton condensation, which exhibits a soft mode of a different, electronic variety. Second, when driving the system away from equilibrium, the phase transition is mediated by topological defects. These defects allow for the formation of a charge density wave that does not occur in equilibrium. This light-induced charge density wave shows some unique properties that suggest that it is not just a trivial extension of an equilibrium one. [mehr]

Electrical control of quantum spins

MPSD Seminar
Magnetic fields are challenging to localise to short length scales because their sources are electrical currents. Conversely, electric fields can be applied using electrostatic gates on scales limited only by lithography. This has important consequences for the design of spin-based information technologies: while the Zeeman interaction with a magnetic field provides a convenient tool for manipulating spins, it is difficult to achieve local control of individual spins on the length scale anticipated for useful quantum technologies. This motivates the study of electric field control of spin Hamiltonians [1]. Mn2+ defects in ZnO exhibit extremely long spin coherence times and a small axial zero-field splitting. Their environment is inversion-symmetry-broken, and the zero-field splitting shows a linear dependence on an externally-applied electric field. This control over the spin Hamiltonian offers a route to controlling the phase of superpositions of spin states using d.c. electric field pulses, and to driving spin transitions using microwave electric fields [2]. Experiments on Mn defects in ZnO provide insights into how to achieve manipulation of individual spins on surfaces using a scanning tunnelling microscope. A high-frequency voltage applied to the tip can drive electron spin resonance in Fe atoms on MgO surfaces via modulation of the crystal field experienced by the Fe atom [3]. It has been proposed theoretically that frustrated exchange-coupled molecular clusters might offer sensitivity to externally-applied electric fields [4]. Experiments on an antiferromagnetically-coupled Cu3 compound reveal a small linear electric field effect. A comparable sensitivity is exhibited by the heterometallic S = 1 antiferromagnetic ring Cr7Mn, but no effect is found for the S = 1/2 Cr7Ni [5]. [mehr]

Tuning quantum materials out of equilibrium: A FIB-microstructuring approach

MPSD Seminar
“Quantum materials” loosely defines a broad collection of materials whose ground states are defined by unusual quantum properties. This research largely focuses on macroscopic single crystals, yet naturally interesting quantum phenomena lie beyond their equilibrium state. My group works towards reducing the sample size onto the sub-mm length scale, following the general idea that small samples can be driven more strongly and react faster than on the macro scale. Our main tool is Focused Ion Beam machining capable of cutting single crystals into high quality quantum devices. I will present two concrete research projects showcasing how new quantum states out of equilibrium can be accessed and investigated in FIB-prepared microcrystal structures. The first concerns the heavy fermion superconductor, CeIrIn5 (Tc~400mK). When a mm-sized structure is firmly coupled to a mm-sized substrate of different thermal expansion, the microstructure is under significant strain at low temperatures. By precisely controlling its shape, the emergent strain field can be controlled. The key difference to other approaches, such as uniaxial strain, is that complex, yet well-controlled, spatially varying strain fields can be achieved. In collaboration with Katja Nowack (Cornell), we have experimentally mapped out the resulting superconducting landscape in the devices using scanning-SQUID microscopy, and show that this spatial modulation can be well captured by finite element simulations. [1] Second, I will present our ongoing efforts to experimentally identify pseudo-magnetic fields in 3D Dirac semi-metals [2,3]. Owing to their Dirac dispersion, deformation of the crystal structure does not open a gap at the nodes, but shifts the location of the nodes in k-space and hence playing the role of a “pseudo-magnetic field”, B5. I will show how microstructuring gives us unprecedented control of such a process, and discuss how future. [mehr]

Electronic and Vibrational Properties of Colloidal Nanocrystals

MPSD Seminar
Colloidal nanocrystals (CNCs) are nanometer sized crystals grown in solution. Due to their size-tunable optical properties, CNCs have emerged as a novel material platform for numerous applications such as displays, photovoltaics, and biological tagging. However, the colloidal growth process results in an unavoidable distribution of CNC size that inhomogeneously broadens optical absorption/luminescence lineshapes. 2-D spectroscopy is a technique capable of circumventing inhomogeneous broadening by correlating absorption and emission dynamics. In this talk I will present our results from applying 2-D spectroscopy to CNCs at cryogenic temperatures. I will first discuss our experiments on conventional CdSe CNCs, in which we have simultaneously observed both bulk-like acoustic phonons and acoustic vibrations discretized by the nanocrystal geometry for the first time. Next, I discuss our experiments on perovskite CNCs, which are a new class of materials first synthesized in 2015. We demonstrate that coherences due to vibrational coupling exhibit anomalous dephasing dynamics, which we attribute to a cascaded coherence transfer process. Finally, I discuss our observations of coherences between so-called bright-triplet exciton states, which are robust at high temperatures and polarization-selective. [mehr]
High harmonic generation (HHG) from crystalline solids has become a playground in ultrafast phenomena. In contrast to noble gases, crystalline solids have rich physical properties, e.g. anharmonic energy dispersion, anisotropy depending on crystalline axis, strong electron-hole correlation, and so on. While the three-step model for HHG and its generalizations are successfully applicable to several situations, a deviation from the theoretical prediction is one of the most interesting physics in this field. To understand such deviations in solid-state HHG experiments, we need to go beyond the three-step model or along different directions. I will mainly talk about our recent trials to understand solid-state HHG, electron-hole attraction inclusion based on Hartree-Fock theory for 1D model crystal, and an ab-initio approach based on density-functional theory for 3D bulk solid comparing with experiments. [mehr]

Relativistic ultra-intense laser-plasma physics: from classical to QED regimes

MPSD Seminar
Ultra-intense lasers deliver unprecedented energy densities within microscopic volumes and shortest time spans, as exemplified by last year’s Physics Nobel Prize. Today, these lasers facilitate many compact technical applications such as particle accelerators and sources of intense electromagnetic radiation. And the next development stages promise significant technical advancements as well as deep insights into fundamental science ranging from nonlinear quantum field theory to studying the complex quantum vacuum itself. [mehr]
A talk about how to succeed in challenging environments. Dr. Gregor Wittke, occupational psychologist shares his experience and know-how on coping with stress and high demands in the work place.The talk features a short overview of scientific findings on the topic as well as practical hands on strategies to apply in everyday life. Enjoy a mix of facts and numbers with practical thought experiments and even minimal movement exercises to gain confidence and composure or recover energy and determination.Besides those aspects concerning a helpful attitude to succeed in challenging environments and how to gain and maintain it, this talk will also include a kind of manual for delimiting yourself from an overwhelming amount of job tasks and focus on your own priorities. [mehr]

Transient Chirality in Chemistry and Biology: Capturing the Structural Evolution of Molecules in Solution

Most biological functions and many chemical processes are driven by chiral nanoscale molecular machines in solution, whose structures evolve on multiple time and length scales: from the ultrafast rotations of photo-driven synthetic molecular motors to the global conformational changes of proteins on the microsecond time scale. Yet capturing the associated conformational transitions in real-time continues to be a formidable experimental challenge, as prominent established methods come with their own limitations: solution nuclear magnetic resonance is limited to millisecond real-time resolution, whilst solution X-Ray scattering requires large-scale X-Ray facilities. A promising laboratory-based alternative is circular dichroism (CD), the absorption difference of left- and right-handed circularly polarized light, which is sensitive to the chiral geometrical arrangement of light-absorbing chemical groups within a molecular system. Steady-state CD is already a well-established tool in the far and middle ultraviolet (UV) < 300 nm, where equilibrium structures of proteins, DNA and functional chiral organic complexes are routinely characterized. However, pushing this technique into the time-domain has remained a challenge for over three decades, with only few isolated reports with sub-nanosecond resolution [1]. In this talk, I will present a technological breakthrough with the first time-resolved CD (TRCD) spectrometer that combines highly sensitive broadband UV-detection (250-370 nm) with pulsed laser sources and sub-picosecond time-resolution [2]. With this instrument, it is now possible to extract broadband CD spectra of photo-excited molecular states and follow their transient chirality changes with femtosecond resolution. This is opening a new avenue for capturing solution-phase structural dynamics in chemical and biological systems that I will illustrate with two examples: the coupling of electronic and structural dynamics in a chiral supramolecular metal-complex [3], and the application of a site-specific CD-label to track conformational changes of the peptide backbone [4]. On this basis I will present future developments that will establish TRCD as a complementary method for research in protein dynamics and chiral photochemistry, where the chirality of excited electronic states is the key design feature of chiral organic light-emitting diode materials and unidirectional molecular motors, for example. [mehr]

Coupled cluster theory with applications to conical intersections and quantum electrodynamics

MPSD Seminar
I will review different aspects of coupled cluster theory with focus on recent developments. In particular, similarity constrained coupled cluster theory1 for conical intersections and nonadiabatic dynamics, pump-probe simulations using time-dependent coupled cluster theory2 and coupled cluster theory for strong light-matter interactions (Cavity QED chemistry).3 [mehr]
We present a comprehensive theoretical framework for interaction of an ultrashort light pulse with a thin material based on the time-dependent density functional theory (TDDFT) [1] . We introduce a microscopic description solving the Maxwell equations for the light electromagnetic fields and the time-dependent Kohn-Sham equation for the electron dynamics simultaneously in the time domain on a common real-space grid. This scheme can simulate the light-matter interaction in thin films irrespective of the film thickness and the light intensity. [mehr]

Controlling superconductivity with the electromagnetic vacuum

MPSD Seminar
Exploiting light-matter interactions to control collective quantum phenomena in solids is an ongoing broad research effort. In particular, it is known that superconductivity can be strongly modified using an external radiation [1-3]. In the absence of the latter, it is an interesting question whether superconductivity induced by conventional Cooper pairing may be also affected by coupling phonons to the electromagnetic field confined in a cavity with enhanced vacuum fluctuations [4,5]. [mehr]

Cavity Optomagnonics

MPSD Seminar
Optomagnonics studies the quantum-coherent coupling of light to collective magnetic excitations in solid state systems. The magnetic material hosting the magnetic excitations can be also used as an optical cavity if patterned appropriately. This not only enhances the magnon-photon coupling (making these systems promising for applications in quantum technologies) but also allows studying cavity-modified light-matter interaction in a novel platform. In my talk I will go over the basics of cavity optomagnonics and present results on recent theory developments in my group, including optomagnonics with magnetic textures, optical heralding of magnon Fock states, and antiferromagnetic cavity optomagnonics.

Near-Field Physics and Chemistry in Plasmonic STM junctions

MPSD Seminar
Plasmonic cavities exhibit many intriguing properties and phenomena resulting from strong field enhancement and confinement. Recently, both experimental and theoretical studies have revealed crucial roles of atomistic structures and quantum mechanical effects in plasmonic nanostructures [1-3]. [mehr]

Density-potential inversion from Moreau-Yosida regularization

MPSD Seminar
For a quantum-mechanical system and a given density, the Zhao-Morrison-Parr method allows to infer the effective potential that yields precisely this density. I will discuss how this can be understood as a limit procedure from Moreau-Yosida-regularized density functionals to the unregularized case. This sheds new insight on the relevance of regularization in density-functional theory and allows to systematically improve density-potential inversion. [mehr]

An Epistemology of Oppression: How Science Created Social Hierarchies -- Women's Career Day

MPSD Equal Opportunities
The Keynote will unpack issues related to intersectionality and oppression, including diversity, equity, inclusion, discrimination and implicit bias in science. Using concrete examples, the keynote will bring clarity on these topics and invite the participants to widen and deepen their perspective, perceptions and beliefs on social injustice, allowing them to reflect on the positive role they can play towards a society free of systemic oppression. Registration: https://indico.desy.de/e/wcd2022 [mehr]

Bringing light into electron microscopy: From ultrafast plasmonics to heralded single-photon sources

MPSD Seminar
The interaction of intense light fields with matter is pivotal in various physical contexts, ranging from coherent control schemes in atomic physics to steering the flow of energy in solid-state systems. In particular, joining free-electron beams with ultrafast lasers facilitates the probing of nanoscale dynamics in ultrafast transmission electron microscopy (UTEM) and opens the field of free-electron quantum optics. In my talk, I will briefly introduce the UTEM methodology that combines state-of-the-art TEM with optical pump-probe spectroscopy [1,2], show selected applications in the study of ultrafast dynamics, and discuss the coherent coupling of electrons and photons down to the single particle level. [mehr]

Women‘s Career Day

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