Archive 2016

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. [more]

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I discuss the interplay between theory and experiment to design super functional carbon-based nanomaterials/nanodevices. These include intriguing organic nanostructures, graphene and functionalized carbon hybrid materials for energy harvesting, solar cells, fuel cells, gas storage, water remediation and medical treatment. Hyperresolution phenomena by nano-lensing, super-paramagnetism driven water remediation, and super-magnetoresitance & ultrafast DNA sequencing of graphene nanoribbon are addressed. [more]

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)]. [more]

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In this very informal talk I will first briefly present our view on superconductivity in alkali doped fullerenes. In particular, I will discuss how it is at all possible that M3C60 has an s-wave condensate despite the sizeable intra-molecular Coulomb repulsion relatively to the narrow molecular bandwidth. In the second part of the talk I will review what it is known about the MIR optical properties of doped fullerenes and try to build a link with the remarkable Hamburg observation of a large enhancement of Tc under intense THz pumping. [more]

Magnetism is a quantum effect that has fascinated mankind for millennia and also led to many modern-day applications, very prominently in information processing and storage. Now available polarized soft X-ray pulses from X-FELs with sub-100 fs duration allow observing magnetic interactions at work in their natural length and timescales. I will show how the challenges for performing time-resolved XMCD at the Linac Coherent Light Source have recently been overcome and give an overview how such studies help to understand the fundamental speed limits for nanoscale spin motion. [more]

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“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! [more]

Strong coupling of light and matter can give rise to a multitude of exciting physical effects through the formation of delocalized hybrid light-matter states. After introducing the fundamental concepts, examples of modified properties under strong coupling, such as enhanced charge transport in organic semiconductors and non-radiative energy transfer, will be given to illustrate the broad potential of light-matter states. [more]

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. [more]

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Nanocrystals are already used for many applications in technical products for everyday life. The talk will describe actual developments such as quantum dots in display and lighting technology, ultra-hard nanocomposite materials and materials for the visualization of biological events or malignant cells or tissues. Different chemical approaches and factors determining the biological response on fully synthetic nanocrystals will be reported. We will present approaches to improve targeting specificity and concepts to optimize physical proper ties to increase the sensitivity for imaging. [more]

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. [more]

Photophysical investigations of the canonical nucleobases that make up DNA and RNA show that in room-temperature solvents, their excited states decay with (sub)picosecond lifetimes (τ = 0.3–2 ps). The apparent generality of ultrafast excited-state deactivation in the canonical nucleobases has led to the hypothesis that nucleobases are molecular survivors of the harsh UV environment on the early Earth before the emergence of life. [more]

Strongly correlated quantum systems, where more traditional methods of quantum many-body physics fail, have attracted enormous attention over the last decades but still provide formidable problems for our understanding: High-T_c superconductors, frustrated quantum magnets, transition metal oxide and rear earth materials, ultracold atomic gases in optical lattices. Key numerical advances have been made using so-called tensor network methods, the best known of which is the density matrix renormalization group (DMRG). After an introduction into the methodology, I want to presentselected results from areas which in my view present particularly interesting challenges also in the future: non-equilibrium dynamics of correlated systems (here: ultracold atoms in lattices) and material properties of threedimensionaltransition metal oxides. [more]

Flexural and in-plane thermal fluctuations in crystalline membranes affect the band structure of the carriers, which has an effect on transport properties as well as carrier density of states of 2D systems. I consider a specific example of one-layer black phosphorus, which is a highly anisotropic material, and present our recent results on intrinsic carrier mobility. In contrast to graphene, where the mobility is determined by two-phonon (flexural) scattering, in black phosphorus one-phonon (in-plane) processes dominate. I also will show the results on DOS tail for holes in black phosphorus that have quasi-one-dimensional dispersion (my /mx » 1) and, as a result, an enhanced Van Hove singularity at the valence band top. Interaction with flexural phonons results in smearing of this singularity and to an appearing of a tail in DOS in the gap. The material parameters are determined by ab initio GW calculations and then are used for quantitative estimation of the above-mentioned effects. [more]

This course will provide an overview of selected techniques used to probe the microscopic properties of condensed matter. The course will cover the use of these methods at steady state as well as their extension to transient measurements of non-equilibrium phenomena in condensed matter. [more]

Like a tuning fork for light, optical resonators have a characteristic set of frequencies at which it is possible to confine light waves. At these frequencies, optical energy can be efficiently stored for lengths of time characterized by the resonator Q factor, roughly the storage time in cycles of oscillation. In the past there has been remarkable progress in boosting optical storage time in micro and millimeter-scale optical resonators with Q factors exceeding 100 billion. This opens up new opportunities to access a wide range of nonlinear phenomena and to create laser devices operating at low power. After reviewing the nonlinear physics of these devices, current efforts to miniaturize time standards and stable frequency sources for metrology and spectroscopy will be described. [more]

Soft-X-ray photoelectron spectroscopy from liquid microjets has considerably contributed to an understanding of the electronic structure of aqueous solutions. Quantities that can be obtained include valence and core-level electron binding energies of both water solvent and atomic as well as molecular solutes. [more]

The course provides an overview of the working principles of nonlinear optics with a focus on the basic physical concepts. We start from introducing relevant topics in linear optics (e.g., Maxwell equation, Lorentz model, Crystal optics, Optical pulse and dispersion), and continue to discuss key concepts in nonlinear optics such as phase matching and quasi phase matching, second-order nonlinear effects, and third-order nonlinear effects. [more]

This symposium aims to enhance scientific cooperation between scientists from Argentina, Latin America and Max Planck Institutes. It is jointly organized by the International Center for Advanced Studies (ICAS) and the Max Planck Liason Office for Latin America, in cooperation with the Kolleg Programme of the Alexander von Humboldt Foundation. [more]

Watching a single molecule move on its intrinsic time scale has been a dream of modern nanoscience. We show how a single oscillation cycle of phase-stable infrared pulses can accelerate and recollide electrons in solids. By combining this idea with sub-angstrom spatial resolution of scanning tunnelling microscopy we manage to control the ultrafast quantum motion of individual electrons in a single orbital of one molecule. Such elementary quantum processes allow us to record first slow-motion movies of individual vibrating molecules. [more]

The talk will cover the resonance energy transfer in light-harvesting systems and entropy transfer in nanomechancial resonators.In the first part, I will present a classical formulation of the quantum multichromophoric theory of resonance energy transfer developed on the basis of classical electrodynamics. The theory allows for the identification of a variety of processes of different order in the interactions that contribute to the energy transfer in molecular aggregates with intracoupling in donors and acceptor chromophores. Enhanced rates in multichromophoric resonance energy transfer are shown to be well described by this theory. Specifically, in a coupling configuration between $N_A$ acceptors and $N_D$ donors, the theory correctly predicts an enhancement of the energy transfer rate dependent on the total number of donoracceptor pairs. As an example, the theory, applied to the transfer rate in light harvesting II, gives results in excellent agreement with experiment. Finally, it is explicitly shown that as long as linear response theory holds, the classical multichromophoric theory formally coincides with the quantum formulation.In the second part, I will present a sideband cooling strategy that incorporates (i) the dynamics induced by structured (non-Markovian) environments in the target and auxiliary systems and (ii) the optimally time-modulated interaction between them. For the context of cavity optomechanics, when non-Markovian dynamics are considered in the target system, ground state cooling is reached at much faster rates and at much lower phonon occupation number than previously reported. In contrast to similar current strategies, ground state cooling is reached here for coupling-strength rates that are experimentally accessible for the state-of-the-art implementations. After the ultrafast optimal-ground-state-cooling protocol is accomplished, an additional optimal control strategy is considered to maintain the phonon number as closer as possible to the one obtained in the cooling procedure. Contrary to the conventional expectation, when non-Markovian dynamics are considered in the auxiliary system, the efficiency of the cooling protocol is undermined. [more]

CUI invites to its fourth International Symposium on Thursday, 10 November 2016, in the Center for Free Electron Laser Science. In honor of the winner of this year’s Hamburg Prize for Theoretical Physics the symposium will cover modern trends in condensed matter physics. The prize is worth 40 000 Euro and will be presented by the Joachim Herz Stiftung together with CUI to Russian physicist Mikhail Katsnelson. [more]

In this talk I will describe the evolution of our understanding on how to describe transport through strongly correlated systems in the framework of density functional theory (DFT).A first indication that DFT might be useful to tackle this situation came with the realization that the Kondo plateau in the zero-bias conductance may already be captured at the level of standard Landauer theory combined with DFT. Later it has been shown how the description of Coulomb blockade in the zero-bias limit can be achieved within DFT. In a more recent development we have proposed a DFT formalism to describe electronic transport in the steady state which uses the density on the junction and the steady current as basic variables. In a finite window around zero bias, a one-to-one map is established between the basic variables and both local potential on as well as bias across the junction. The resulting Kohn-Sham system features two exchange-correlation (xc) potentials, a local xc potential and an xc contribution to the bias. For weakly coupled junctions the xc potentials exhibit steps in the density-current plane which are shown to be crucial to describe the Coulomb blockade diamonds.Finally, I will present a recent parametrization of the xc potentials for the single-impurity Anderson model which correctly incorporates both the Kondo and Coulomb blockade regimes, i.e., both zero and finite temperature. This parametrization allows for calculation of currents and differential conductances at arbitrary bias and temperature at negligible numerical cost but with the accuracy of sophisticated renormalization group methods. [more]

There exists a size difference between molecules and visible wavelengths. Nanofabrication techniques can be adopted to obtain designed meta-molecules or meta-materials, with interesting optical properties. [more]

Nanomembranes are thin, flexible, transferable and can be assembled into 3D micro- and nanoarchitectures. This makes them attractive for a broad range of applications and scientific research fields ranging from strain-tunable heterostructure devices to ultra-compact 3D systems both on and off the chip. Rolled-up nanomembranes can be exploited to rigorously compact electronic circuitry, create novel optical components and open up pathways towards entirely new biomedical applications. [more]

Since 2000, we have been developing a real-time, real-space computationalmethod based on time-dependent density functional theory to describe electron dynamics in crystalline solids induced by light pulses. In a microscopic scale, we solve the time-dependent Kohn-Sham equation in a unit cell of solid treating the applied electric field is by the vector potential. We further combine the microscopic calculation with the dynamics of light electromagnetic field in a multiscale modeling, as describe in the figure. In my presentation, I first explain our method including some historical aspects. Then I will show some recent and on-going applications such as energy transfer from a femtosecond laser pulse to electrons in quartz and graphite, and ultrafast changes of dielectric properties of diamond by an intense laser pulse. [more]

Van der Waals heterostructures (vdWHs) represent a novel and largely unexplored class of materials. Since 2013, when Geim and Grigorieva first conceived the stacking of 2D (two-dimensional) materials to create artificial layered structures with tailored properties, a number of promising (opto)electronics devices, e.g. light emitting diodes, solar cells, ultra-fast photodetectors, transistors etc. have been successfully fabricated. It is well established that for isolated 2D semiconductors and vdWHs the optical response is governed by excitonic effects. A theoretical understanding of excitonic effects and of how the electronic screening is affected for the more complex case of multi-layer structures is still lacking due to the computational limitations of standard ab-initio methods. [more]

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. [more]

The progress in quantum optics utilizes a unique photon state configuration for engineering of the ultimate light-matter interactions with relatively simple material systems. It results in a broad range of photonic applications including radiation sources, quantum communication, information, computing and nanotechnology. The development of the ultrafast multidimensional nonlinear spectroscopy that has been enabled by progress in ultrafast optical technology provides a unique tool for probing complex molecules, semiconductors, nanomaterials by classical light fields. [more]

The second symposium of the ERC Synergy Grant Q-MAC (Frontiers in Quantum Materials’ Control) takes place at the Venice International University, Isola di San Servolo in Venice, Italy. [more]

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Our understanding of heterogeneous catalysis is to a large extent based on the investigation of metal single crystals as models. Increasing the complexity of the models, resembling a real disperse metal catalyst, allows one to catch some of the complexity that cannot be covered by metal single crystals alone. We have developed model systems based on metal deposits on single crystalline oxide films, which may be studied at the atomic level using the tools of surface science. [more]

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This talk will showcase algorithms that I have developed over the course of my PhD suited to X-ray free electron laser data processing. At present, generating merged datasets to sub-atomic resolution can be tricky at LCLS due to the limitations of the photon energy and detector distance. I will present some of the challenges faced by processing diffraction patterns, collected at LCLS, to generate a 1.15 Å resolution dataset, and how these new algorithms have proved useful to solve them. [more]

Welcome to the PIER Graduate Week 2016, an interdisciplinary lecture and workshop week for PhD students. The PIER Graduate Week offers a wide range of introductory and focus courses in the PIER research fields of Particle & Astroparticle Physics, Nanoscience, Photon Science, Infection & Structural Biology. It aims at PhD students, MSc students, postdoctoral researchers and other interested scientists. [more]

Spurred by recent progress in melting, enhancement and induction of electronic order out of equilibrium, a tantalizing prospect concerns instead accessing transient Floquet steady states via broad pump pulses, to affect electronic properties. [more]

The new paradigm of topology in condensed matter physics does not only pertain to insulators and semimetals but also to superconductors. Topological superconductors are predicted to show many novel physical properties. [more]

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. [more]

Complex quantum many-body systems are ubiquitous in nature, but their behaviour often remains very challenging to predict with analytical or numerical calculations - especially when it comes to dynamics. However, using ultracold atoms in optical lattices it is possible to create precisely tunable, yet very accessible artificial solids, which can be probed with a large arsenal of observables. Using this experimental set-up, we demonstrate how a periodically modulated lattice can be described by an effective Floquet-Hamiltonian on longer time scales - even when driving the system far from equilibrium. This allows for implementing Haldane's model for a topological insulator, and mapping out its topological transitions, by applying an oscillating force to a honeycomb lattice. Using an oscillating magnetic field gradient, we also engineer spin-dependent bands. By adding interactions to the optical lattice system, we create a pure realisation of the Hubbard model and study how the distribution of anti-ferromagnetic correlations therein depends on the geometry of the lattice. We investigate how fast correlations can re-arrange, by deforming the lattice geometry on time-scales ranging from the sudden to the adiabatic regime. Finally, we explore how an oscillating force applied to this interacting system may be used to tune and enhance the magnetic exchange energy beyond the regimes accessible within the Hubbard model. [more]

Dynamical phase transitions (DPTs) have gained a lot of interest in the past few years in a variety of quantum many-body systems, where a system in its groundstate is quenched by abruptly changing a control parameter of the Hamiltonian, such as interaction strength or external field. Afterwards, a DPT can be detected in one of at least two forms: a type-I DPT which is detected, after relaxation in time, through the nonanalyticity of an appropriate order parameter as a function of the control parameter through which the quench is effected; and a type-II DPT which is detected as a nonanalyticity of the Loschmidt echo return rate as a function in time, without giving care as to whether or not a stationary state has been reached in the time evolution. [more]

Time evolution in ensembles of quantum emitters (atoms, molecules, ions, Rydberg atoms, quantum dots etc) at low vs. high densities is fundamentally different. As particles get closer together, strong environment-mediated interactions start playing an important role both in the coherent (such as dipole-dipole interactions) as well as in the incoherent evolution (super- /subradiance). [more]

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The physics of the “ultra-fast” and “ultra-small” are often closely linked, motivating experiments that access these extremes. In this talk, I will discuss the application of ultrashort light pulses, from THz to X-rays, to the study of dynamics and emergent correlations in quantum materials. [more]

Molecular strong field ionization (SFI)is at the heart of several ultrafast imaging/spectroscopic techniques such aslaser induced electron diffraction and high harmonic spectroscopy. Whileproof-of-principle experiments are underway, it is still a long way before theycan evolve into a standard imaging techniques. The coupled motion of electronsand nuclei under the influence of a non-perturbative external laser field isnot fully understood yet and there is a strong demand for developing many bodymethods that can model these molecular SFI based processes. [more]

When an exciton transition and a resonant optical mode exchange energy faster than any competing dissipation process, it can lead to light-matter strong coupling. This brings about interesting properties possessed by neither the original exciton nor the optical mode and leads to new possibilities such as enhanced conductivity of organic semiconductors. In this case, the enhancement stems from the delocalized nature of the hybrid states over the spatial extent of the optical mode which is also expected to affect energy transport according to recent theoretical studies. [more]

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. [more]

Which features make the brain such a powerful and energy-efficient computing machine? Can we reproduce them in the solid state, and if so, what type of computing paradigm would we obtain? [more]

The particle-hole map (PHM) is a new visualization tool to analyze electronic excitations in molecules in the time- or frequency domain, to be used in conjunction with TDDFT or other ab initio methods [1–3]. The purpose of the PHM is to give detailed insight into electronic excitation processes which is not obtainable from local visualization methods such as transition densities, density differences, or natural transition orbitals. The PHM provides information on the origins, destinations, and coherences of charge fluctuations during an excitation process. In contrast with the transition density matrix, the PHM has a statistical interpretation involving joint probabilities of individual states and their transitions, and it is easier to read and interpret. [more]

Photochemical reactions have tremendous scientific importance, ranging from photosynthesis to atmospheric reactions, and technologies such as sensors or displays. Due to the intrinsic complexity of photochemical reactions, they remain the least understood type of chemical process. Nonadiabatic dynamics, ultrafast time-scales, quantum effects and conical intersections are known to be important, but a detailed comprehension remains elusive. However, new experimental techniques capable of monitoring photochemical processes in unprecedented detail are appearing. This includes the development of intense-laser techniques, the construction of free-electron lasers such as the XFEL in Europe and the LCLS in the USA, new sources of pulsed electrons, advanced detection techniques, and important advances in theoretical modelling of quantum dynamics. Many of these techniques are developed by research communities not traditionally concerned with photochemistry, but provide an opportunity to shed new light on photochemical dynamics. [more]

We develop a theory for light-induced superconductivity in underdoped cuprates in which the competing bond-density wave order is suppressed by driving phonons with light. Close to a bond-density wave instability in a system with a small Fermi surface, such as a fractionalized Fermi liquid, we show that the coupling of electrons to phonons is strongly enhanced at the bond-density wave ordering wavevectors, leading to a strong softening of phonons at these wavevectors. For a model of classical phonons with anharmonic couplings, we show that thecombination of strong softening and driving can lead to large phonon oscillations. When coupled to a phenomenological model describing the competition between bond-density wave order and superconductivity, these phonon oscillations melt bond-density wave order, thereby enhancing pairing correlations. [more]

The photoinduced charge-separation events occurring in photovoltaic and light harvesting systems have traditionally been interpreted in terms of the incoherent kinetics of optical excitations and of charge hopping. Although signatures of quantum coherence were recently observed in energy transfer in photosynthetic bacteria andalgae[1] still very little is known about the role of quantum coherence at room temperature in technologically relevant organic photovoltaic materials. Recent experiments found evidence for an ultrafast long-range charge separation in such systems but could not differentiate between coherent and incoherent charge-transfermodels.[2] [more]

The discovery of graphene established the possibility of obtaining stable two-dimensional solids, and it was soon realized that layered materials other than graphite can be used as bulk parents for novel two-dimensional materials. Among them, layered transition metal dichalcogenides have attracted considerable attention due to interesting physical properties and potential for electronics applications. While most of the research focused on mechanically exfoliated specimens, many proposed approaches to characterizing these materials, as well as potential applications, require large-area and high-quality samples. These can be achieved by epitaxial growth methods developed in our group. [more]

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Light-matter interactions have been extensively studied by physicist in quantum optics and condensed matter physics, [1] but there are only fewer attempts to understand this effect in molecular science. [2, 3] [more]

Due to experimental advances in the preparation and control of ultra-cold atomic gases, there is a widespread interest in the behaviour of quantum systems out of equilibrium. A common way to probe quantum systems for non-equilibrium phenomena is given by sudden quenches. [more]

Ultrafast vibrational spectroscopy can tell a lot about molecular dynamics. Quite diverse examples will be discussed, highlighting the importance of water as a very special substance: the ultrafast dynamics of bulk water and concentrated salt solution studied by THz photon echo spectroscopy, the catalytic cycle of an artificial photosynthetic system for light-driven water splitting, and the response of an allosteric protein whose dynamics is dictated by the water solvation layer. [more]

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Plasmons are the collective excitations of free electrons. They focus light into nanoscale volumes, increasing electromagnetic fields by orders of magnitude. Plasmonic enhancement in light scattering (SERS) leads to a 10⁸ increase in the cross section. I will present Raman measurements on graphene and nanotubes coupled to nanoplasmons. The enhancement will be described as a higher-order Raman process with striking consequences for our understanding of plasmon-matter interaction at the nanoscale. [more]

Join us at the CFEL Sommerfest! CFELians, partners and kids are more than welcome! [more]

The coupling and mutual dependence of electronic and vibrational degrees of freedom is at the heart of microscopic as well as macroscopic phenomena in condensed matter. Ultrafast pump-probe techniques provide experimental access to these coupling and correlation effects by revealing the response of electrons and lattice to specific excitation of a material. [more]

Recent developments in ultra-fast laser techniques allow experimentalists to reveal intriguing aspects of the dynamics of superconductors such as the Higgs oscillations (amplitude oscillations of the order parameter) and photo-induced superconductivity. These experiments demonstrate the interesting possibility of manipulating superconductivity in nonequilibrium settings. However, the properties of nonequilibrium superconductors are not fully understood, and further theoretical explorations are needed. [more]

Coupled cluster theory is the dominant method in wave function-based calculations in systems of small to moderate size. It provides exceptionally accurate predictions for a wide array of energetics and properties. Moreover, it is size extensive, meaning that it can fruitfully be applied to condensed systems, provided only that one has computational resources sufficient for the task. Unfortunately, coupled cluster theory often breaks down in the presence of strong correlations, such as those responsible for superconductivity or various magnetically-ordered states. [more]

Cold and ultracold molecules provide a sensitive way to search for new physics, e.g. variation of fundamental constants, dark energy, or new elementary particles. I will describe some of these ideas, with particular emphasis on the search for a permanent electric dipole moment of the electron, which already provides a strong constraint on possible supersymmetric theories of particle physics. Laser cooling can now be applied to molecules. I will discuss the recent advances in that area and the extraordinary sensitivity that this new approach can bring to tests of fundamental physics. [more]

Quantum transport is often discussed in the steady-state regime where the characteristics of the system are described in terms of the energy-dependent transmission or conductance. There is, however, no guarantee that this description would capture the essential physics in, say, atomic-scale junction operating at high frequencies. Therefore, we look for an accurate theory for describing the full time-dependence. The time-dependence also provides us with ''transient spectroscopy'' which can give detailed information about the nanosystems out of equilibrium. [more]

In this talk, I will first give an introduction to Majorana bound states (MBS), zero-energy modes predicted to appear in exotic spin-polarized p-wave superconductors. MBS satisfy non-Abelian statistics and, in addition, encode quantum information in a topologically protected manner, which makes them highly interesting for quantum computation applications. The required p-wave superconductivity seems to be hard to find in nature, but recent theoretical works have shown that it can instead be artificially engineered, for example in a semiconductor nanowire with strong spin-orbit coupling which is covered by a superconductor and exposed to a magnetic field. I will also discuss the recent experimental progress towards creating and detecting MBS. [more]

Hybrid light-matter networks offer the promise for delivering robust quantum information processing technologies, from sensor arrays to secure communications to quantum simulators and eventually to a quantum computer. Photonics plays a major role in delivering these new enhanced performance applications. I will describe recent science and engineering progress towards build a resilient, scalable photonic quantum network. [more]

The iridium oxide family of correlated electron systems is predicted to host a variety of exotic electronic phases owing to a unique interplay of strong electron-electron interactions and spin-orbit coupling. There is particular interest in the perovskite iridate Sr₂IrO₄ due to its striking structural and electronic similarities to the parent compound of high-Tc cuprates La₂CuO₄. Recent observations of Fermi arcs with a pseudogap behavior in doped Sr₂IrO₄ and the emergence of a d-wave gap at low temperatures further strengthen their phenomenological parallels. [more]

Graphene is an ideal material to study new processes in the ultrafast carrier kinetics of a two-dimensional system: Its linear energy dispersion and the vanishing bandgap allow new and surpising electron scattering processes, suppressed in conventional semiconductors: A typical, fascinating example is a process which generates two optically excited electrons out of one photon. [more]

The high-temperature superconductor YBa₂Cu₃O₇ is a very interesting material for many device applications ranging from wiring for energy transmission and high field magnets to superconducting filters and high frequency electronics in the THz regime. [more]

Quantum-chemical calculations are essential for the assignment and interpretation of spectra of small molecules as well as complex molecular systems. Large molecular systems present a significant challenge not only because of the increasing computational effort that is required for an accurate quantum-chemical description, but also require new approaches for the interpretation of the large amount of data provided by such calculations. On the other hand, small molecular systems with a complicated electronic structure, such as transition metal complexes, pose additional challenges and their theoretical spectroscopy also requires novel quantum-chemical approaches. [more]

Competition between ordered phases, and their associated phase transitions, are significant in the study of strongly correlated systems. [more]

TiSe₂ is one of the simplest charge density wave (CDW) materials, forming a 2×2 superlattice below a transition temperature T_C = 200 K, but the origin of this phase is controversial. Its nearly inverted band structure led early authors to identify TiSe₂ as an “excitonic insulator,” which is an electronic instability involving spontaneous proliferation of excitons. The problem is that the CDW also exhibits a sizeable lattice distortion, leading later authors to identify it as a conventional Peierls phase. That said, an excitonic phase would also create an incidental lattice distortion, since the interaction with phonons can’t be switched off. The arguments on the matter have gone in circles for decades. [more]

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All life on Earth needs water to survive. But increasingly it seems that the characteristics that make water a solvent for life are also those that make it the weirdest of liquids. Some of these quirks of ‘life’s matrix’ are well understood; others are still being debated, sometimes furiously and controversially. I will explore what we do and don’t know about water, ending with a consideration both of how its behaviour in living cells can offer clues for new purification technologies and of whether its unique role for life on Earth makes it a prerequisite for life on other worlds. [more]

The unusual properties of water, including the thermodynamic anomalies of the liquid, the existence of more than one amorphous ice form, and the abnormal mobility of the water ions, derive from the tetrahedral network of hydrogen bonds that hold the molecules together. Computer simulations are an essential tool to understand the microscopic origin of these fascinating properties, complementing experiment and theory. [more]

The workshop covers advances and challenges in the simulation of materials and the description of light-matter interactions in molecules and solids. [more]

The development of efficient ways to exploit the energy from the sun is an issue of major importance. Among possible solutions, the employment of solar energy to promote chemical reactions has the advantage of addressing the problems of harvesting, converting and storing energy at the same time. In this context, water splitting plays a central role both for direct hydrogen production and for the production of hydrocarbons. Therefore, great attention has been recently devoted to hydrogen production by means of photoelectrochemical (PEC) cells, via water splitting to molecular hydrogen and oxygen. The main challenge is to develop anode materials for these cells that can split water efficiently. [more]

Monolayer transition-metal dichalcogenides such as MoS₂ and WS₂ are prime examples of atomically thin semiconducting crystals that exhibit remarkable electronic properties. They have a pair of valleys that can serve as a new degree of freedom, but these valleys are energetically degenerate, protected by time-reversal symmetry. [more]

After introducing general aspects of photovoltaics this talk will illustrate how spectroscopy with soft X-rays can help developing new materials and new designs for solar cells. Starting with the most general layout of a solar cell, the focus will be on combining its three components with atomic precision into one molecular complex. A dream experiment will be discussed where the movement of photo-generated carriers through such a complex is tracked in real time at the latest X-ray sources. [more]

Laser ablation has proved to be an extremely versatile sampling technique for non‐volatile samples in a range of gas phase experiments. [more]

Ultracold atoms in optical lattices form a versatile platform for studying many-body physics, with the potential of addressing some of the most important issues in strongly correlated matter. I will present experimental results on the characterization of the BEC-BCS crossover with ultracold atoms, the phases of a spin-imbalanced Fermi gas in one and three dimensions, and finally the detection of anti-ferromagnetic order in the 3D Hubbard model, a paradigm model of strong correlations. [more]

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. [more]

A characteristic of ferroic materials is the emergence of a temporally static finite expectation value of an order parameter. Here, we introduce a new mechanism [1] for ferroic order, in which a non-zero quasi-static magnetoelectric quadrupolar order appears, mediated by a strong coupling of spin and phonon fluctuations. We show that our proposed mechanism is consistent, to our knowledge, with many experimental observations for the onset of the pseudo-gap phase in cuprate superconductors and therefore propose the quasi-static magnetoelectric quadrupole as a possible pseudo-gap order parameter. By using first-principles calculations in combination with our recent developed formalism [2,3], to calculate multipole moments within a Berry phase approach, we calculate the magnitude of the effect for the the prototypical cuprate superconductor, HgBa2CuO4+δ. Using these results we finally show that our mechanism embraces several key findings of experimental reports and in addition also aspects of previous theoretical models. [more]

In this talk I will discuss the transient dynamics of a d-wave BCS model after a quantum quench of the interaction parameter. The motivation comes from recent ultrafast pump-probe experiments on high-temperature superconductors. [more]

Condensed matter provides us deep insights into quantum physics. Giving just two examples, wave-corpuscle duality manifests itself in spectroscopy of strongly correlated systems as coexistence of itinerant and atomic-like features, and graphene and other Dirac materials provide a natural playground to study vacuum reconstruction, Klein tunneling and other fundamental quantum relativistic phenomena. Electron-photon interaction is the key tool to understand this rich and nontrivial physics. [more]

Detailed understanding and control of the intermolecular forces that govern molecular assembly are necessary to engineer structure and function at the nanoscale. [more]

The alkali-doped fullerides (A₃C₆₀, A = K, Rb, Cs) show a highest superconducting transition temperature (Tc) among molecular solids. In the phase diagram, s-wave superconductivity (SC) lies next to Mott insulating phase. This adjacency is similar to the cuprates (d-wave SC) but is more surprising because s-wave SC is believed to be severely suppressed by strong correlations. [more]

Laser light with a 'squeezed' quantum uncertainty shows less quantum noise and allows for improved optical measurements. The most prominent example is the use of squeezed light in gravitational wave detectors. Soon, also LIGO will be equipped with squeezed light. In principle, it allows for ultra-sensitive measurements in the so-called quantum non-demolition regime. But what is squeezed light, how is it produced and what are its other potential applications? This talk will answer these questions. [more]

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The charge-induced instabilities of conducting and dielectric drops in an electric field have been studied over a century, starting from the seminar works of Lord Rayleigh, J. Zeleny, G. I. Taylor. The significance of the study is found in the presence of charged droplets in jets, electrospray mass spectrometry methods and atmospheric aerosols. In low temperature physics, applications of the confinement of electrons on the surface of liquid helium add new perspective on the significance of the study of the instabilities. [more]

Hydration has a drastic impact on the structure and function of flexible biomolecules, such as aromatic ethylamino neurotransmitters. [more]

Recent advances in technology and instrumentation have made it possible to generate vortex beams of electrons and neutrons with phase singularities at their cores, where the beam intensity is zero and the phase is undefined. These new types of beams, apart from the spin angular momentum, carry a quantized orbital angular momentum (OAM) along their axes of propagation resulting in a twisted wavefronts, pretty much like quantum tornadoes. The OAM of electron is capable of fundamentally altering the physics of beams and is already being used for application purposes in electron microscopy. [more]

Fluctuations of the atomic positions are at the core of a large class of unusual material properties ranging from quantum para-electricity and charge density wave to, possibly, high temperature superconductivity. Their measurement in solids is the subject of an intense scientific debate focused on the research of a methodology capable of establishing a direct link between the variance of the ionic displacements and experimentally measurable observables. In this presentation I will introduce our new approach to address quantum and thermal fluctuation in complex materials. By means of non-equilibrium optical experiments performed in shot-noise limited regime we could reveal that the variance of the time dependent atomic positions and momenta is directly mapped into the quantum fluctuations of the photon number of the scattered probing light. A fully quantum description of the non-linear interactions between photonic and phononic fields pave the way for a direct measurement of fluctuation in complex systems. [more]

Three-dimensional structures of membrane proteins are of paramount value for understanding protein function on a molecular level. However, in vitro studies such as structure determination are impaired by the necessity to purify membrane proteins with the aid of detergents that often compromise protein stability and function. [more]

Our Universe displays a vast panoply of exotic objects like the Earth, black holes, or neutron stars. With a mass of twice the Sun mass and a radius of ~10 km, a neutron star is a very dense object. At a temperature of 10^8 K, the spin 1/2 neutrons are believed to be in a superfluid state. We will show how dilute gases prepared by laser techniques at the other extreme of the temperature scale, in the Nano kelvin range, can help us to understand the superfluid state of neutron stars. [more]

Extended molecules can be solvated at various inequivalent sites and it is useful to learn from experiment on a kJ/mol accuracy scale which solvent docking site is energetically more attractive. Theoretical relative energy predictions are rather easy to make and could thus be rigorously benchmarked by such experimental ratings of docking sites. However, experimental comparison of solvent-solute pair binding energies is not trivial, because it requires low temperature for complex stabilization but still sufficiently high mobility for equilibration. For the class of simple aromatic ethers solvated by methanol, the docking competition is very subtle [1] and we were able to tune it by chemical substitution, switching at will between classical OH-O and more diffuse OH-π docking preferences [2]. This new docking balance experiment profits from linear infrared absorption spectroscopy in supersonic jets [3] and allows for a fairly rigorous test of state-of-the-art quantum chemical predictions for relative hydrogen bond energies. The popular M06-2X density functional is shown to fail quite badly. [more]

Fermionic quantum matter is ubiquitous in nature and has many technological applications. After a general introduction into the physics of ultracold fermionic quantum gases, the talk will highlight two striking examples. Superfluidity in Fermi gases has been subject of intense research for more than a decade with many spectacular observations. Recent work has provided unique access to quasiparticles in the Landau-Fermi liquid regime, whose fast dynamics can now be observed on the Fermi time scale. [more]

The conformational dynamics of a protein are often essential for its function. In this talk I will present how time-resolved X-ray scattering and vibrational spectroscopy can be used to investigate protein dynamics based on two examples: the light sensing protein phytochrome and green fluorescent protein. In particular, I will show how computational chemistry can support the design and analysis of these experiments and how Molecular dynamics simulations can be used to predict the structural stability of small cyclic peptides. [more]

Magnetic vortices occur in soft magnetic nanodisks with suitable dimensions. The spin structure curls in the plane around the center where it points out-of-plane either up or down, defining the polarization P = ±1. Thus, the vortex core represents a binary digit that can be used for magnetic storage devices. Ferromagnetic resonance spectroscopy is used to investigate the high-frequency spectra of two stacked vortices depending on the relative polarizations. In this work, it is shown that magnetic field bursts with frequencies in the Gigahertz range can be used to control the polarization on a sub-nanosecond time scale. The demonstrated writing times are more than two magnitudes faster compared to settling times achieved with the gyrotropic mode. Furthermore, polarization dependent spin-wave modes that lead to a selective spin-wave mediated vortex core reversal are imaged by scanning transmission X-ray microscopy. [more]

Non-equilibrium pump-probe time-domain spectroscopy opens new perspectives in studying the dynamical properties of strongly correlated electron systems. New effects, such as transient superconductivity or Higgs oscillations of the superconducting condensate can be obtained. Using various methods I present a theoretical study of the non-equilibrium dynamics in superconductors. [more]

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