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.
Description of our time- and angle-resolved photoemission spectroscopy setup published
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
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
Hot-electron induced disordering of gold nanoclusters revealed
Vasileiadis et al., ACS Nano 12, 7710 (2018), [doi: 10.1021/acsnano.8b01423]. OA: arXiv:1803.00074 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.
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
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-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...]
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).
Ralph Ernstorfer receives ERC Consolidator Grant
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.
Time- and angle-resolved study of the role of electrons and phonons in the charge density wave material TiSe2
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...]
Best Poster Award for Michele Puppin
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 graduates at Freie Universität Berlin
Electron-phonon Coupling and Energy Flow in a Simple Metal beyond the Two-Temperature Approximation
new paper: Waldecker et al., Phys. Rev. X 6, 021003 (2016) We revisited a basic problem: the exchange of energy between electrons and phonons in a simple metal like aluminum. Typically, this energy flow is described by the two-temperature model. We show that this model needs to be refined and propose the non-thermal lattice model.
Nanofocused Plasmon-Driven Sub-10 fs Electron Point Source
new paper: Müller et al., ACS Photonics 3, 611 (2016) We report a point source of single electron wave packets driven by plasmons. The duration of the electron wave packets is less than 10 femtoseconds. This source enables femtosecond electron holography, and potentially even femtosecond scanning tunneling microscopy. This work was performed in collaboration with the group of Markus Raschke at the University of Colorado.
First time- and angle-resolved photoelectron spectroscopy with our 0.5 MHz extreme ultraviolet laser
We developed a high-repetition rate extreme ultraviolet (XUV) laser delivering 500,000 pulses of photons with 22 eV energy and a duration of approximately 20 femtoseconds. These light pulses are used to photoemit electrons from a crystal in order to obtain a map of the material’s electronic structure. As we use a second, visible laser pulse to excite a small fraction of the material’s electrons into excited states shortly before the XUV pulses arrive, we are now able to do excited state mapping and to take movies of how electrons scatter in the band structure. This achievement required years of development of new laser sources (see Puppin et al., Opt. Exp. 23 1491, 2015), high-harmonic generation, and a sophisticated surface science apparatus. This novel technique is developed in close collaboration with the [more...]
Time-domain separation of optical properties from structural transitions in resonantly bonded materials