Veranstaltungsarchiv 2017

Hier finden Sie vergangene Veranstaltungen des MPSDs.

MPSD Seminar

11778 1512642696

Hélène Seiler - Development of an “optical NMR” spectrometer to investigate electronic couplings in molecules and nanostructures

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]

MPSD Seminar

11637 1511190809

Michael Zürch - Photoexcitation dynamics in semiconductors and two-dimensional semiconductors observed by attosecond core-level spectroscopy

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]

MPSD Seminar

11752 1512401673

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

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]

MPSD Seminar

11435 1512642302

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


MPSD Seminar

11434 1509550553

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


MPSD Seminar

11716 1511959262

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

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]

MPSD ARD Seminar

11596 1510835281

Thomas Renger - Structure-based theory of light-harvesting in photosynthesis

Two challenges in the simulation of excitation energy transfer and optical spectra of pigment-protein complexes are the equal magnitude of the excitonic and the exciton-vibrational coupling and the structure-based parametrization of the Hamiltonian (for review see ref. 1). We have developed quantum chemical/electrostatics/normal mode analysis (NMA) schemes to calculate optical transition energies of pigments in their binding site in the protein (site energies), interpigment excitonic couplings and the spectral density of the pigment-protein coupling. Our NMA of the spectral density shows that the modulation of site energies is an order of magnitude stronger than that of the excitonic couplings and that also in the basis of delocalized exciton states the diagonal exciton-vibrational coupling dominates [2]. This result explains the good performance of our earlier time-local lineshape theory [3], in which the diagonal elements are treated exactly and the off-diagonal elements in Markov and secular approximations, and triggered a new development that takes into account the finite relaxation time of nuclei during exciton relaxation [4]. I will give a summary of our theory development and present applications on a small model system (water soluble chlorophyll binding protein -WSCP) and large photosystem II core particles. In the case of WSCP we have developed and applied a theory of holeburning spectroscopy [5] that goes beyond the standard two-level system approach and allows for a quantitative description of experimental data, revealing the lifetime of the upper exciton state, in excellent agreement with results from 2D electronic spectroscopy and our earlier prediction [6]. In the case of photosystem II I will present results of the parametrization of the exciton Hamiltonian of its subunits [7-9]. These parameters were used to describe VIS/IR pump-probe data on single crystals of PSII core particles [10] that allowed for a verification of our earlier prediction [11] on the relative timescale of excitation energy and charge transfer in this system. The results are discussed in terms of photoprotection scenarios that allow photosystem II to switch between a light harvesting and an excitation energy quenching mode protecting the reaction center. [mehr]

MPSD Seminar

11397 1509550688

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


MPSD Seminar

11522 1510136653

Michael Schüler: Nonequilibrium topological states traced by transient spectroscopies

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]

MPSD Seminar

11396 1509550707

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


MPSD Seminar

11395 1509550508

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



11249 1507637464

Gengji Zhou - Power scaling of ultrafast mid-IR source enabled by high-power fiber laser technology



11077 1505816704



IMPRS-UFAST skills course

11224 1516104770

IMPRS Job Seminar

You are not sure yet what to do after your PhD? You want to know what kind of jobs are out there in the real world? You wonder what makes you interesting for industry? Then this seminar is the right fit for you!In this seminar you have the chance to quiz interesting people from outside of academia. Find your dream job! Or at least know where to look for it. [mehr]

IMPRS-UFAST core course

9888 1503484514

Basics of chemistry and biochemistry - IMPRS-UFAST core course

In this course, chemistry will mainly be understood as reactions. The course gives an overview about the basics of reaction chemistry and discuss what is already known and what can be measured in the laboratory nowadays (i.e. describing the current frontiers and where the research performed at CFEL can make a difference). In the biochemistry part, the basic principles of nucleic acids (DNA, RNA, their replication etc.) and proteins, their structure and function etc. will be discussed. It will be interesting to work out where the new coherent sources can advance the field. [mehr]

IMPRS-UFAST skills course

10759 1503484206

Moderation training

Learn powerful and effective methods for conducting all types of meeting from an experienced expert. [mehr]

MPSD Seminar

10945 1504190938

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

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]

MPSD Seminar

10996 1504601420

Lukas Müchler - Exploring topological phenomena in Molecules

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]

CFEL Molecular Physics Seminar

10466 1500018011

Pedro de la Torre Luque - Study of cosmic neutrinos diffuse flux with Antares neutrino telescope: track channel

This project is based on the study of signals which ANTARES neutrino telescope have detected between 2007 and 2015. Firstly, an introduction about the sources of neutrinos and what we know about the origins of cosmic neutrinos is showed. Then, It will be explained the process of detection of neutrinos used by ANTARES neutrino telescope. Secondly, the analysis of the detected data in the 9 years mentioned before is carried out. The first part of the analysis consists in a comparison between detected data and simulated data will be shown for some variables, like neutrino direction or energy of them. The Monte Carlo simulated data is done in order to reproduce the observed Background and reduce it by applying cut-offs. Also the simulation of the expected signals from cosmic neutrinos is done to compare to the fitted background. The second part consists in the search of the cut-offs which will help to improve the signal/noise ratio. These cut-offs will be selected by mean of the analysis of some parameters or applying statistical methods, like Feldmann-Cousin method. Finally, the obtained signal is showed to extract conclusions. [mehr]

MPSD Seminar

10409 1498811861

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

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]

10317 1497615596

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]


9882 1494248783

Alessandra Picchiotti - Coherent two-dimensional electronic broad-band UV-spectroscopy of DNA and its nucleobases


MPSD Seminar

10218 1496918622

Michael Lubasch - Density Functional and Tensor Network Theory

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]

MPSD Seminar

10121 1496325567

Yang Peng - Boundary Green functions of topological insulators and Superconductors

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]

MPSD Seminar

9975 1494502987

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

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]


9417 1494332458

Workshops for PhD students on Bahrenfeld Campus - The job interview - Business theatre with trained actors

A job interview is one of the most drawn-out and intimidating ways of making first impression. However, it’s also your opportunity to get on an employer’s good sight, which can give you a distinct edge over even those applicants whose credentials are better than yours. [mehr]

MPSD Seminar

9986 1494589193

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

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]

MPSD Seminar

9985 1494589059

Jorge Jover - Open quantum systems: a geometric description

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]

MPSD Seminar

9822 1493737277

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

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]

IMPRS-UFAST core course

7686 1518537449

Solid State Physics

From a microscopic point of view, a solid is just a regular arrangement of atoms, embedded in a soup of electrons. Yet, a remarkably rich manifold of phenomena emerges from this simple starting point, ranging from simple metals and semiconductors to multiple kinds of magnetic order or superconductivity. In this course we will discuss basic properties of solids and their microscopic understanding. [mehr]

MPSD Seminar

9683 1492611281

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

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]

MPSD Seminar

9631 1492007837

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

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]

Nonlinear harmonic generation and ultrafast studies of perovskite iridates

9492 1490711760

Hao Chu - Nonlinear harmonic generation and ultrafast studies of perovskite iridates

The perovskite iridate Srn+1IrnO3n+1 realizes an interesting analogue to the cuprate high Tc superconductor. When electron-doped, the single-layer Sr2IrO4 is shown to manifest a pseudogap phase and possibly d-wave superconductivity. Using second harmonic generation rotational anisotropy, we show that near the antiferromagnetic transition there exists a magnetic multipolar order that breaks inversion symmetry in hole-doped Sr2IrO4. One candidate for this order is the loop current order that is predicted to exist in the pseudogap region of cuprate. In bilayer Sr3Ir2O7, electron-doping has been found to induce an insulator-to-metal transition in addition to a structural phase transition at low temperature. There has been no prior experimental evidence of electronic instabilities in the bilayer system. Using time-resolved optical reflectivity, we detect a charge-density-wave-like instability in metallic samples of electron-doped Sr3Ir2O7. The absence of signatures of a new spatial periodicity from diffraction, scanning tunnelling and photoemission based probes suggests an unconventional and possibly short-ranged nature of this density wave order. [mehr]

MPSD Seminar

9630 1492007594

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

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]

MPSD Seminar

9551 1491314406

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

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]

MPSD Seminar

9535 1490969094

Michael Lorke - Carrier dynamics in MoS2

In the context of the current interest in transition-metal dichalcogenides, we study the optical generation and relaxation of excited carriers and their influence on optical properties. [mehr]

MPSD Seminar

9499 1490797894

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

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]


7740 1503491563




9191 1488379539

Kemal Shafak - Large-scale laser-microwave synchronization for attosecond photon science facilities


IMPRS-UFAST core course

7687 1485872543

Ultrafast techniques

The course focuses on the use of modern light/ X-ray/ electron sources for investigating the physics/ chemistry/ biology phenomena. We will discuss scattering and image reconstruction techniques, spectroscopy and their use for time-resolved measurements . Key questions addressed are which techniques exist, how to use them, and which method is best used to reach a certain goal. [mehr]

MPSD Seminar

9128 1487771964

Ra’anan I. Tobey - Transient Grating Spectroscopy in Magnetic Thin Films: Elastic Excitation of a Transient Magnonic Crystal

Control of material properties is one of the driving forces in ultrafast optical sciences. The notion that light can influence material parameters is founded on a wide range of experiments demonstrating optomagnetic control, light induced superconductivity, and the photo induced insulator to metal transition in a wide range of materials. A recent addition to the tool chest of control methodologies is the excitation of acoustic waves, and their effect on intrinsic materials properties; particularly the material magnetization via magnetostrictive effects. [mehr]


9034 1487062128

Eliza Casandruc - Nonlinear Optical Control of Josephson Coupling in Cuprates


MPSD Seminar

9220 1488812739

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

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]

MPSD Seminar

9133 1487855304

Markus Kowalewski - Coherent Signatures of Conical Intersections in Ultrafast X-Ray Spectroscopy

The rates and outcomes of virtually all photochemical and photobiological processes are dominated by conical intersections (CIs), which provide a fast sub-100-femtosecond nonradiative pathway back to the ground state. At a CI, the electronic and nuclear degrees of freedom frequencies are comparable and strongly mix due to the breakdown of the Born-Oppenheimer approximation. [mehr]

MPSD Seminar

9129 1487774342

Kota Ido: " Time-dependent variational Monte Carlo method for nonequilibrium strongly correlated electron systems and its application "

Strongly correlated electron systems driven out of equilibrium have attracted much attention because of potential routes to realizing intriguing phenomena that are not attainable in the equilibrium. To treat such systems, we proposed a time-dependent trial wave function with many variational parameters for the time-dependent variational Monte Carlo method. To show the accuracy of our trial wave function in nonequilibrium states, we present our benchmark results for relaxation dynamics during and after interaction quench protocols of fermionic Hubbard models. We find that our trial wave function well reproduces the exact results for the time evolution of physical quantities such as momentum distribution and superconducting correlations. As its application, I will show the dynamics of d-wave superconductivity in two-dimensional correlated electron system under a laser irradiation. Our results show that the superconductivity can be enhanced by irradiating strong laser. I will discuss the origins of this enhancement associated with the laser effect and the relation between superconductivity and charge inhomogeneous phases. [mehr]

MPSD Seminar

9067 1487173496

Klaas Giesbertz - One-body reduced density matrix to potential mapping in finite basis sets at elevated temperatures One-body reduced density matrix to potential mapping in finite basis sets at elevated temperatures

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]

IMPRS-UFAST core course

7685 1480331440

Source technology

The course provides an overview of the working principles of modern light/ X-ray/electron sources, including the respective physics background and their currentstrengths and limitations. The focus will be on techniques and technical basics. [mehr]

IMPRS-UFAST skills course

7739 1484145442

Preparing for your PhD defence

In this workshop participants have the opportunity to strategically engage with the upcoming oral examination. They develop individual strategies for preparing themselves and learn techniques so they can direct the discussion even at critical points. [mehr]

CFEL Theory Seminar

8895 1486037106

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

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]

MPSD Seminar

8395 1484667961

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

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]

MPSD Seminar

8291 1483606225

Ultrafast spintronics with terahertz radiation

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]

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