Welcome to the Structural & Electronic Surface Dynamics Group!
We are an experimental research group investigating the electronic and atomic structure of solids and heterostructures in out-of-equilibrium conditions. We develop and use ultrafast techniques providing movies of the electronic and atomic structure in solids and nanostructures. From these time-resolved measurements, we infer information on coupling and correlation effects of electrons and atomic motion. Our techniques include time- and angle-resolved photoelectron spectroscopy (trARPES), femtosecond electron diffraction and microscopy, and time-resolved optical spectroscopy.
Thomas received his PhD in physics for his investigations of ultrafast energy flow and structural dynamics in nanoscale heterostructures with femtosecond electron diffraction.
Collaborators from the group of Andreas Knorr at the TU Berlin developed a theoretical description of angle-resolved photoemission signals from transient excitonic states.
A preprint is available at arXiv:1907.01842
Contrast enhancement is an important preprocessing technique for improving the performance of downstream tasks in image processing and computer vision. Our multidimensional photoemission spectroscopy results in densely sampled data of higher than three dimensions. The initial understanding of these complex multidimensional datasets often requires human intervention through visual examination, which may be hampered by the varying levels of contrast permeating through the dimensions. In collaboration with collaborators from the MPI for Intelligent Systems, a multidimensional extension of contrast-limited adaptive histogram equalization (MCLAHE) has been developed.
The algorithm is publicly available, a preprint of its description is avalable here: [more...]
The free-electron laser FLASH at DESY in Hamburg delivers femtosecond soft-x-ray pulses which allow unique applications in the field of time-resolved photoelectron spectroscopy. We participated in a larger consortium establishing time-resolved momentum microscopy with such 4th generation photon sources.
A preprint describing this technical development can be found here:
D. Kutnyakhov et al., First time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser.
new paper: Xian et al., Ultramicroscopy 202, 133 (2019).
OA: arXiv 1901.00312
An image symmetrization algorithm for symmetry criteria-based correction of volumetric data is described. Its use for the distortion correction of volumetric photoemission data is demonstrated. The code is provided as open source software package for sharing and reuse.
The CRC 951 – Hybrid Inorganic/Organic Systems for Opto-Electronics (HIOS) is a Berlin-based interdisciplinary collaborative research center. The DFG just approved a third 4-year funding period starting July-1 2019. We are new members of this collaborative research project and will investigate ultrafast [more...]
Structural stability of nanoscale building blocks is prone to ultrafast lattice motions that range from atomic vibrations, to translations and rotations of entire nanostructures. In this work, we establish femtosecond electron diffraction as goniometer of ultrafast nanocrystal rotations. To achieve our goal, we have combined size-selected synthesis of Au nanoclusters on graphene and femtosecond electron diffraction experiments with molecular dynamics and electron diffraction simulations. We have found that Au923 nanoclusters perform constrained rotational motions, termed librations, driven quasi-impulsively by graphene’s phonons in picosecond timescales. Our investigations aim for a more complete understanding of out-of-equilibrium conditions, heat- and mass-transport in nanoscale heterostructures. The article is now published in Nanoscale Horizons and it was the product of an international collaboration that involved, among others, [more...]
BiGmax is a Max Planck Research network on big-data-driven materials science. We collaborate with computer scientists from the MPI for Intelligent Systems to apply machine learning approaches to multidimensional photoemission data.
We are seeking a postdoctoral researcher for this interdisciplinary project merging condensed matter physics and computer science.
TrARPES with a laser-based 500 kHz XUV beamline: we report the result of a long-term experimental development project. We developed a high-repetition rate extreme ultraviolet laser source (photon energy: 22 eV, pulse duration: 20 fs) which is used as probe pulses in trARPES experiments. This experimental setup allows multidimensional photoemission spectroscopy: the electronic structure of solids in an excited state can be observed in energy, both parallel momentum directions, and time.
In collaboration with the group of Peter Baum, Univ. Konstanz, a short-pulse electron source providing few-electron bunches with a duration below 30 fs was developed.
Ehberger et al., Phys. Rev. Applied 11, 024034 (2019).
Helene Seiler was awarded a postdoc mobility grant by the Swiss National Science Foundation. Helene will study ultrafast structural dynamics in photovoltaic materials.
new paper: Nicholson et al., Science 362, 821 (2018)
open access: arXiv 1803.11022
Watching the motions of atoms in the course of a chemical reaction is generally thought of as the Holy Grail for understanding chemical transformations or phase transitions in solids. While recordings of such “molecular movies” have been achieved in recent years, the atomic motion does not reveal the whole story of why specific bonds break and others form. This is dictated by the arrangement of the electrons as the atoms move along gradients on an energy landscape defined by the electrons. It is therefore necessary to observe the dynamics of the electronic structure, which means to record an “electron movie”, to obtain a complete [more...]
Chris Nicholson, former collaborator from the Dynamics of Correlated Materials Group, receives the Carl-Ramsauer-Preis for his PhD studies on ultrafast electron dynamics in low-dimensional systems. Congrats, Chris!
Vasileiadis et al., ACS Nano 12, 7710 (2018), [doi: 10.1021/acsnano.8b01423].
We investigated the flow of energy in laser-excited gold nanoclusters arranged on different thin film substrates with femtosecond electron diffraction. This experiment revealed an ultrafast disordering of the nanocluster’s surface atoms which only occurs in the presence of hot electrons with a temperature exceeding 3000 K. These findings result from a collaboration with the group of Richard Palmer, Swansea University.
We received funding for a collaboration with the group of Majed Chergui for the investigation of perovskite materials with ultrafast techniques. This project is embedded in the Max Planck-EPFL Center for Molecular Nanoscience and Technology.
Lutz Waldecker receives the Dissertationspreis of the Condensed Matter Physics Section of the German Physical Society for his PhD studies of ultrafast phonon dynamics in solids. Congratulations, Lutz!
Lars Gundlach, professor for Chemistry and Physics at the University in Delaware, receives an Alexander von Humboldt Research Fellowship. We will collaborate with Lars on ultrafast microscopy and phonon dynamics in nanomaterials.
We are participating in the trans-regional collaborative research center TRR227. 18 projects located at the Freie Universität Berlin, the Martin-Luther-Universität Halle-Wittenberg, the Helmholtz-Zentrum Berlin, the Max-Born-Institut and the Fritz-Haber-Institut will study ultrafast spin dynamics with different approaches. We will use trARPES for the investigation of charge and spin currents in [more...]
new paper: Waldecker et al., Phys. Rev. Lett. 119, 036803 (2017).
open access: arXiv:1703.03496
The interaction of electrons and phonons dictate fundamental processes in solids: the conductivity of charge carriers and heat, energy dissipation, etc. We investigate the basic mechanism of electron-phonon interaction with ultrafast techniques, in particular femtosecond electron diffraction. By tacking snapshots of the atomic structure of a sample, movies of ultrafast structural dynamics can be obtained. In this work, we investigate the layered semiconductor material tungsten diselenide and show that the electrons interact preferentially with phonons with large momentum vector.
After the completion of the new high-precision labs of the Department of Physical Chemistry, we finally merge all experiments in a single lab. The biggest logistic challenge is the move of the trARPES lab from the old hospital building Fabeckstraße, which requires getting the heavy equipment out through a window.
The investigation of the motion of electrons and atoms in nanostructures requires ultrafast measurement techniques with a high sensitivity to tiny sample volumes. Low-energy electrons have the highest scattering cross section and interact strongly with electric and magnetic fields. During her PhD studies, Melanie Müller developed a novel ultrafast electron microscopy technique based on femtosecond single-electron wave packets emitted from a sharp metallic needle. Utilizing this technique, Melanie demonstrated that the photocurrent arising inside an InP nanowire after optical excitation e can be filmed with femtosecond resolution. Melanie received her PhD with distinction.
new paper: Waldecker et al., Physical Review B 95, 54302 (2017).
The semimetal antimony is a model system for studying electron-lattice correlation and coherent phonons. The electronic structure of antimony induces a static lattice distortion, a so-called Peierls distortion. Optical excitation of the electrons with a short laser pulse impulsively reduces the mechanism of the Peierls distortion, leading to a collective oscillation of all atoms in the crystal. In addition, the energy given to the electrons by the laser pulse dissipates by incoherent scattering of the electrons with all other lattice vibrations. Applying our femtosecond electron diffraction apparatus, we were able to simultaneously observe and distinguish both phenomena.
new paper: Bertoni et al., Physical Review Letters 117, 277201 (2016).
A range of transition metal dichalcogenides (TMDCs) are semiconductors with layered crystalline structure. In the form of monolayers, these TMDCs are 2D materials with peculiar optoelectronic properties and an unusual spin texture of the electronic structure, i.e. a spin-valley correlation. In contrast, bilayers and bulk crystals of 2H-WSe2 are centrosymmetric, which causes all electronic states to be spin-degenerate. We show, however, that spin-polarized excited carriers can be generated in bulk crystals of the non-magnetic layered material WSe2. This is a consequence of the hidden spin polarization in this class of materials: optical excitations generates excited states with 2D character, i.e. localized to an [more...]
Lutz Waldecker receives the Carl Ramsauer Award of the Physikalische Gesellschaft zu Berlin (PGzB) in recognition of his PhD thesis Electron-Lattice Interactions and Ultrafast Structural Dynamics of Solids. The price is awarded for outstanding doctoral research studies in physics or related fields at the three Berlin universities and the University of Potsdam. Congrats, Lutz!
In collaboration with researchers at the MPI for Quantum Optics, both Munich universities and Monash University, Australia, the generation and control of electric current in a semiconductor on time scales short than the oscillation period of visible light (~2 femtoseconds) has been demonstrated. This study extends the previously achieved current control in dielectrics to the material class of semiconductors. The study reveals a crossover in the mechanism of current generation from the multiphoton to the tunneling regime depending on the intensity of the employed laser pulses.
Publication: Paasch-Colberg et al., Optica 3, 1358 (2016).
The European Research Council (ERC) funds the 5-year project FLATLAND with 2.6 million €.
FLATLAND is an experimental research project addressing the exotic spin-valley-layer correlations in few-layer thick transition metal dichalcogenides (TMDC) crystals and related heterostructures. Microscopic coupling and correlation effects, both within pure materials as well as across the interface of heterostructures, will be accessed by time- and angle-resolved extreme ultraviolet-photoelectron spectroscopy, femtosecond electron diffraction, and time-resolved optical spectroscopies. The project promises unprecedented insight into the microscopic coupling mechanisms governing the performance of van der Waals-bonded devices.
At room temperature, titanium diselenide is a metallic crystal with pronounced electron-electron as well as electron-phonon correlations. At low temperature, a metal-insulator phase transition occurs concurrently with the formation of a charge density wave and a periodic lattice distortion. In collaboration with the research group Dynamics of Correlated Materials, we investigated the response of the electronic structure of this material to infrared and infrared optical excitation. The transient electronic structure was measured with time-, energy- and momentum-resolution by time- and angle-resolved photoelectron spectroscopy (trARPES) employing femtosecond extreme-ultraviolet laser pulses.
full publication: Monney et al., Phys. [more...]
Michele Puppin receives the Best Poster Prize at IMPACT 2016 (Electronic States and Phases Induced by Electric or Optical Impacts). The poster describes how to map the conduction band of a semiconductor in the whole Brillouin zone with the help of high-repetition rate XUV time- and angle-resolved photoemission spectroscopy (trARPES). The study directly visualizes the valence and conduction bands of the bi-dimensional semiconductor WSe2, a member of the transition metal dichalcogenide family.
Lutz Waldecker developed a femtosecond electron diffractometer and employed this apparatus to study Electron-Lattice Interactions and Ultrafast Structural Dynamics of Solids. He received his PhD with distinction from FU Berlin.