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Komnik A.,University of Heidelberg | Thorwart M.,University of Hamburg | Thorwart M.,The Hamburg Center for Ultrafast Imaging
European Physical Journal B | Year: 2016

We study the impact of a time-dependent external driving of the lattice phonons in a minimal model of a BCS superconductor. Upon evaluating the driving-induced vertex corrections of the phonon-mediated electron-electron interaction, we show that parametric phonon driving can be used to elevate the critical temperature Tc, while a dipolar phonon drive has no effect. We provide simple analytic expressions for the enhancement factor of Tc. Furthermore, a mean-field analysis of a nonlinear phonon-phonon interaction also shows that phonon anharmonicities further amplify Tc. Our results hold universally for the large class of normal BCS superconductors. © 2016, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Bocklage L.,German Electron Synchrotron | Bocklage L.,The Hamburg Center for Ultrafast Imaging
Journal of Magnetism and Magnetic Materials | Year: 2017

Transient magnetization dynamics are calculated from the Landau-Lifshitz-Gilbert equation. The internal fields are described by partial derivatives of the sample's free energy density. This description allows the calculation of the magnetization trajectory of homogeneously magnetized samples for all kinds of energy contributions. It provides a general solution for transient magnetization dynamics. The model is used to determine the magnetization response to excitation profiles like rectangular or Gaussian pulses, frequency bursts, wavelet excitations, and chirped THz pulses. © 2017 Elsevier B.V.


Achilleos V.,National and Kapodistrian University of Athens | Frantzeskakis D.J.,National and Kapodistrian University of Athens | Kevrekidis P.G.,University of Massachusetts Amherst | Schmelcher P.,University of Hamburg | And 2 more authors.
Romanian Reports in Physics | Year: 2015

We present a unified description of different types of matter-wave solitons that can emerge in quasi one-dimensional spin-orbit coupled (SOC) Bose-Einstein condensates (BECs). This description relies on the reduction of the original two-component Gross-Pitaevskii SOC-BEC model to a single nonlinear Schr¨odinger equation, via a multiscale expansion method. This way, we find approximate bright and dark soliton solutions, for attractive and repulsive interatomic interactions respectively, for different regimes of the SOC interactions. Beyond this, our approach also reveals “negative mass” regimes, where corresponding “negative mass” bright or dark solitons can exist for repulsive or attractive interactions, respectively. Such a unique opportunity stems from the structure of the excitation spectrum of the SOC-BEC. Numerical results are found to be in excellent agreement with our analytical predictions. © 2015, Editura Academiei Romane. All rights reserved.


Mujica-Martinez C.A.,University of Hamburg | Mujica-Martinez C.A.,The Hamburg Center for Ultrafast Imaging | Nalbach P.,University of Hamburg | Nalbach P.,The Hamburg Center for Ultrafast Imaging
Annalen der Physik | Year: 2015

We study the quantum exciton dynamics in light-harvesting complexes under the influence of underdamped vibrational molecular modes. We observe prolonged coherent population oscillations between different pigments due to an underdamped mode connected to the originally excited pigment. At the same time, an underdamped vibration at the exit site of the complex provides additional channels for the excitation energy transfer towards the reaction center which shortens the transfer times. © 2015 by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


Han P.,Max Planck Institute for Solid State Research | Bester G.,Max Planck Institute for Solid State Research | Bester G.,University of Hamburg | Bester G.,The Hamburg Center for Ultrafast Imaging
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

The three-dimensional confinement characterizing a nanocrystal (NC) leads to the formation of discrete electronic states. The energy gap between these states in colloidal NCs can be up to an order of magnitude larger than the vibrational energy of the host material. This large energetic mismatch (not given in self-assembled quantum dots) leads to the expectation that an electron occupying an excited state would be unable to release its energy to vibrations and a "phonon bottleneck" should finally be observed. Using large-scale ab initio calculations and a time-dependent formalism, we show that on the contrary, a phonon bottleneck can be observed only in a narrow window of diameters for CdSe and InAs NCs and should not occur at all in Si NCs. Two relaxation pathways enable fast carrier relaxation. For smaller structures (below 20-Å radius), the coupling strength and energy detuning are such that quantum mechanics allows us to effectively bridge electronic gaps much larger than the vibronic energy. For larger structures, the coupling to passivant modes, although very weak, leads to an efficient picosecond carrier relaxation. This work provides insight into the nature of carrier relaxation in colloidal nanostructures and highlights that defects, of any kind, are not necessary to explain the observed fast carrier relaxation. © 2015 American Physical Society.


Son S.-K.,German Electron Synchrotron | Son S.-K.,The Hamburg Center for Ultrafast Imaging | Thiele R.,German Electron Synchrotron | Thiele R.,The Hamburg Center for Ultrafast Imaging | And 8 more authors.
Physical Review X | Year: 2014

The charged environment within a dense plasma leads to the phenomenon of ionization-potential depression (IPD) for ions embedded in the plasma. Accurate predictions of the IPD effect are of crucial importance for modeling atomic processes occurring within dense plasmas. Several theoretical models have been developed to describe the IPD effect, with frequently discrepant predictions. Only recently, first experiments on IPD in Al plasma have been performed with an x-ray free-electron laser, where their results were found to be in disagreement with the widely used IPD model by Stewart and Pyatt. Another experiment on Al, at the Orion laser, showed disagreement with the model by Ecker and Kröll. This controversy shows a strong need for a rigorous and consistent theoretical approach to calculate the IPD effect. Here, we propose such an approach: a two-step Hartree-Fock-Slater model. With this parameter-free model, we can accurately and efficiently describe the experimental Al data and validate the accuracy of standard IPD models. Our model can be a useful tool for calculating atomic properties within dense plasmas with wide-ranging applications to studies on warm dense matter, shock experiments, planetary science, inertial confinement fusion, and nonequilibrium plasmas created with x-ray free-electron lasers.


Klumpp A.,University of Hamburg | Liebchen B.,University of Edinburgh | Schmelcher P.,University of Hamburg | Schmelcher P.,The Hamburg Center for Ultrafast Imaging
Physics Letters, Section A: General, Atomic and Solid State Physics | Year: 2016

We explore the non-equilibrium dynamics of two coupled zig-zag chains of trapped ions in a double well potential. Following a quench of the potential barrier between both wells, the induced coupling between both chains due to the long-range interaction of the ions leads to the complete loss of order in the radial direction. The resulting dynamics is however not exclusively irregular but leads to phases of motion during which various ordered structures appear with ions arranged in arcs, lines and crosses. We quantify the emerging order by introducing a suitable measure and complement our analysis of the ion dynamics using a normal mode analysis showing a decisive population transfer between only a few distinguished modes. © 2016 Elsevier B.V.


Jurek Z.,German Electron Synchrotron | Jurek Z.,The Hamburg Center for Ultrafast Imaging | Son S.-K.,German Electron Synchrotron | Son S.-K.,The Hamburg Center for Ultrafast Imaging | And 6 more authors.
Journal of Applied Crystallography | Year: 2016

Rapid development of X-ray free-electron laser (XFEL) science has taken place in recent years owing to the consecutive launch of large-scale XFEL instruments around the world. Research areas such as warm dense matter physics and coherent X-ray imaging take advantage of the unprecedentedly high intensities of XFELs. A single XFEL pulse can induce very complex dynamics within matter initiated by core-hole photoionization. Owing to this complexity, theoretical modeling revealing details of the excitation and relaxation of irradiated matter is important for the correct interpretation of the measurements and for proposing new experiments. XMDYN is a computer simulation tool developed for modeling dynamics of matter induced by high-intensity X-rays. It utilizes atomic data calculated by the ab initio XATOM toolkit. Here these tools are discussed in detail. © 2016 International Union of Crystallography.


PubMed | The Hamburg Center for Ultrafast Imaging and University of Hamburg
Type: | Journal: Scientific reports | Year: 2016

The analysis of isolated spin-wave packets is crucial for the understanding of magnetic transport phenomena and is particularly interesting for applications in spintronic and magnonic devices, where isolated spin-wave packets implement an information processing scheme with negligible residual heat loss. We have captured microscale magnetization dynamics of single spin-wave packets in metallic ferromagnets in space and time. Using an optically driven high-current picosecond pulse source in combination with time-resolved scanning Kerr microscopy probed by femtosecond laser pulses, we demonstrate phase-sensitive real-space observation of spin-wave packets in confined permalloy (Ni80Fe20) microstripes. Impulsive excitation permits extraction of the dynamical parameters, i.e. phase- and group velocities, frequencies and wave vectors. In addition to well-established Damon-Eshbach modes our study reveals waves with counterpropagating group- and phase-velocities. Such unusual spin-wave motion is expected for backward volume modes where the phase fronts approach the excitation volume rather than emerging out of it due to the negative slope of the dispersion relation. These modes are difficult to excite and observe directly but feature analogies to negative refractive index materials, thus enabling model studies of wave propagation inside metamaterials.


PubMed | The Hamburg Center for Ultrafast Imaging
Type: | Journal: Scientific reports | Year: 2015

Based on micromagnetic simulations, we report on a novel magnetic helix in a soft magnetic film that is sandwiched between and exchange-coupled to two hard magnetic layers with different anisotropies. We show that such a confined helix stays stable without the presence of an external magnetic field. The magnetic stability is determined by the energy minimization and is a result of an internal magnetic field created by the exchange interaction. We show that this internal field stores a magnetic energy density of a few kJ/m(3). We also find that it dramatically modifies ferromagnetic resonances, such that the helix can be used as a ferromagnetic resonance filter and a fast acting attenuator.

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