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.
Ultrafast Light-Induced Lifshitz Transition
In crystalline solids, electrons fill quantum-mechanically allowed states from the lowest possible energy upwards, a consequence of the Pauli exclusion principle. The energy of the highest occupied state is known as the Fermi energy. Because electrons within solids have well-defined three-dimensional momenta, one can plot components of these momenta against each other, for electron lying at the Fermi energy, leading to characteristic and often beautiful shape, bounded by a so-called Fermi surface.
The Fermi surface is “the stage where the drama of the life of the electron is played out,” wrote famous physicists Lifshitz and Kaganov, in 1980. Indeed, the shape of the Fermi surface governs most of the properties of metals and strongly correlated many-body systems. Equilibrium tuning of macroscopic parameters such as temperature, pressure, strain or doping has recently been established as robust tools to modify the Fermi [more...]
Anisotropic Nonequilibrium Lattice Dynamics of Black Phosphorus
Tommaso Pincelli receives the Postdoctoral Humboldt Research Fellowship
Tommaso Pincelli was awarded the Humboldt Fellowship for Postdoctoral Researchers from the Alexander von Humboldt Foundation
. Tommaso will focus on understanding the effects of interfacing on the electronic structure of 2D materials, aiming at discovering across-interface transport mechanisms and conserved quantities such as spin and valley polarization.
Ivana Lapsanska receives best poster price
Attending the Workshop on Nano and Ultrafast Surface Science (NUSS)
at the Institute for Advanced Study in Garching, Ivana returned with the prize for the best poster
Thomas Vasileiadis graduates at FU Berlin
Thomas received his PhD in physics for his investigations of ultrafast energy flow and structural dynamics in nanoscale heterostructures with femtosecond electron diffraction.
Theory of exciton signatures in trARPES.
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
Algorithm for multidimensional contrast enhancement developed.
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 available here: [more...]
First time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser.
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.arXiv:1906.12155
An algorithm for symmetry-guided non-rigid registration
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.
New DFG-funded project in Collaborative Reserach Center
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...]
Femtosecond electron diffraction established as goniometer for ultrafast nanocrystal rotations
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 project approved / postdoctoral research opportunity
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.
Description of our time- and angle-resolved photoemission spectroscopy setup published
new paper: Puppin et al., Rev. Sci. Inst. 90, 023104 (2019).
open access: arXiv 1811.06939
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.
THz compression of electron pulses to sub-30 fs pulse duration
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 receives SNF Postdoctoral Research Grant
Helene Seiler was awarded a postdoc mobility grant by the Swiss National Science Foundation
. Helene will study ultrafast structural dynamics in photovoltaic materials.
Beyond the molecular movie: dynamics of bands and bonds during a photo-induced phase transition
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 receives the Carl-Ramsauer-Preis der Physikalischen Gesellschaft zu Berlin
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!
Hot-electron induced disordering of gold nanoclusters revealed
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.
Start of a project in the Max Planck-EPFL Center
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
DPG Condensed Matter Dissertation Price for Lutz Waldecker
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!
Reinstallation of the trARPES experiment in the new clean-room laboratory
Lars Gundlach joins group as visiting scientist
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.
Start of DFG-funded project on ultrafast spin dynamics in semiconductor-metal heterostructures
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...]
A momentum-resolved view on phonon dynamics in WSe2
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.
Move of the experiments to the new clean-room laboratory
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.
Melanie Müller receives doctorate from Freie Universität Berlin
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.
Coherent and Incoherent Structural Dynamics in Laser-Excited Antimony
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.
Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2
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
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 PhD thesis award
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!
Control of current by the electric field of a short laser pulses
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).