Angle-resolved photoemission spectroscopy (ARPES) is the most direct technique to probe the electronic structure of crystalline solids. While ARPES is typically used to map the bands’ dispersion, increasing the dimensionality of the measurements, and thus of the observables, have been shown to provide more subtle information about the electronic wavefunction of solids. In this joint experimental and theoretical work (in collaboration with J. Braun, H. Ebert, K. Hricovini, J. Minar and M. Schüler), we introduce a new observable in ARPES, Time-Reversal Dichroism in Photoelectron Angular Distributions (TRDAD). This novel observable quantifies the modulation of the photoemission intensity upon azimuthal crystal rotation which mimics a time-reversal operation. We demonstrate that this observable allows accessing the hidden orbital pseudospin texture in bulk 2H-WSe₂.
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On Wednesday, June-3, a virtual Workshop on Data Acquisition in Angle-Resolved Photoemission Spectroscopy will be organized. Registration is free of charge at the FAIR-DI Conference on a FAIR Data Infrastructure for Materials Genomics.
Scope of the workshop: Due to the advancement of both electron detectors and light sources, ARPES data is increasing in volume and complexity. This applies to ARPES performed at 3rd and 4th generation light sources as well as lab-based sources. We have reached a point where data handling, workflow management, visualization and analysis are a severe challenge and potentially become the bottleneck [more...]

Angle-resolved photoemission spectroscopy (ARPES) provides the most direct access to the electronic structure of solids. In collaboration with researchers in the fields of machine learning and electronic structure theory, we developed a computational method for reconstructing the band structure of the semiconductor tungsten diselenide from three-dimensional ARPES data.
preprint: Xian et al., arXiv:2005.10210

Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties. In collaboration with the research groups of M. Chergui, M. Grioni, N. Marzari (all EPFL) and M. Kovalenko (ETH Zürich), we find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr3 by means of angle-resolved photoelectron spectroscopy. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond.
Puppin et al., Phys. Rev. Lett. 124, 206402 (2020).

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...]

Black phosphorus is a layered semiconductor with an intriguing in-plane anisotropic structure. We studied its lattice response to photoexcitation using femtosecond electron diffraction and found that the anisotropic structure impacts the evolution of the atomic vibrations. After photoexcitation, the lattice remains in a nonthermal state up to about 60 picoseconds, which is characterized by less anisotropic atomic vibrations compared to equilibrium. Our results provide timescales for electron-phonon and phonon-phonon thermalization in black phosphorus and show that in the presence of an anisotropic crystal structure, nonthermal phonon populations can transiently change the anisotropy of the atomic vibrations. More information is available here and a video presentation is available here. [more...]

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.

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.
D. Kutnyakhov et al., First time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser.
Rev. Sci. Instrum. 91, 013109 (2020).

Collaborators from the group of Andreas Knorr at the TU Berlin developed a theoretical description of angle-resolved photoemission signals from transient excitonic states.
Christiansen et al., Phys. Rev. B 100, 205401 (2019).

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. Congratulations!

Thomas received his PhD in physics for his investigations of ultrafast energy flow and structural dynamics in nanoscale heterostructures with femtosecond electron diffraction.

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...]

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.

new paper: Puppin et al., Rev. Sci. Inst. 90, 023104 (2019).
open access: arXiv 1811.06939

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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].
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.

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.