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.
New preprint: Nuclear dynamics of singlet exciton fission: a direct observation in pentacene single crystals
Singlet exciton fission (SEF) is a key process in the development of efficient opto-electronic devices. An aspect that is rarely probed directly, and yet has a tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. In this work we have directly observed the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. Together with theory modelling by Marcin Krynski (Mariana Rossi’s group), we have revealed the long-range motions accompanying the disintegration of the electronically correlated triplet pairs. Our work provides new insights into why SEF occurs on ultrafast timescales. Seiler et al., arXiv:2011.12016
New preprint: Ultrafast lattice of the antiferromagnet nickel oxide
We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce a Q-resolved effective lattice temperature and identify a nonthermal lattice state with a preferential displacement of O compared to Ni ions, which occurs within ~0.3 ps and persists for 25 ps. We associate this with transient changes to the antiferromagnetic exchange striction-induced lattice distortion, supported by the observation of a transient Q-asymmetry of Friedel pairs. Our observation highlights the role of spin-lattice coupling in routes towards ultrafast control of spin order.
DFG funds project within Priority Program 2D Materials
Two-dimensional (2D) materials are crystals with a thickness of only one or very few atoms. After the discovery of graphene, the most prominent representative of this class of materials, many other 2D crystals have been identified, often with intriguing properties that have no counterparts in three-dimensional solids. The German Science Foundation established the Priority Program SPP2244 2D Materials – Physics of van der Waals heterostructures. The Structural & Electronic Surface Dynamics Group participates in this consortium with the project Tailoring electronic correlations, excitonics and topological properties in van der Waals heterostructures on ultrafast timescales. This project aims at getting a quantum state-resolved, microscopic understanding of the role of electronic correlations, excitonics and topological properties [more...]
EU-project OPTOlogic to develop optical topological computing as a means to reduce energy consumption of electronic circuits
About 10 % of the world’s electricity production is used to power the information and communication technologies used for data networks, computing centres and personal digital devices. As this area is expected to take an even bigger share in the future, it is important to find ways to keep its energy costs as low as possible. The EU has recently funded the OPTOlogic project that aims to do exactly that: develop a computing architecture that makes these logic operations energy efficient, taking advantage of light-induced and controlled topological properties of materials. Topology is a mathematical concept for describing the shape of geometrical objects. It has been realized that the concept is extremely useful for describing exotic electronic properties of solids, a finding awarded with the 2016 Nobel Price in Physics. Electrons in topologically protected electronic states of materials can move with minimal loss of energy, which [more...]
Optoelectronic applications root in excited electronic states. In semiconductors, there are two types of excited states: many-body states like bound electron-hole states, so-called excitons, and simpler quasi-particle states, typically referred to as quasi-free carriers (QFCs). In general, both types coexist in a dynamic interplay of exciton and QFC populations. We present a novel, at first glance counter-intuitive approach for accessing exciton and QFC dynamics on ultrafast time scales: the photoemission lineshapes of core levels, i.e. states deep below the frontier orbitals, turn out to be sensitive probes of the electron dynamics occurring in the valence and conduction band. By combining time-resolved photoemission spectroscopy simultaneously probing excited states and core levels with a novel lineshape model, we retrieved how the character of excited states changes from excitonic to QFC-like. Our core finding is that the [more...]
new preprint: Lattice dynamics and ultrafast energy flow between electrons, spins, and phonons in a 3d ferromagnet
The response of magnetic materials to femtosecond laser excitation is governed by the interplay of electronic, magnetic and lattice degrees of freedom. While many experiments investigated the electronic or the spin response, here we studied the lattice response of ferromagnetic nickel using femtosecond electron diffraction. To interpret our results for the lattice heating, we compared them to DFT calculations in combination with models for the microscopic energy flow between the different subsystems. The comparison revealed that the energy cost of demagnetization has a strong impact on the lattice dynamics. We achieved a consistent description of the electron, spin and lattice dynamics by employing energy-conserving atomistic spin dynamics simulations. Our results provide a clear picture of the ultrafast energy flow between electronic, magnetic and lattice degrees of freedom. [more...]
new preprint: a momentum-resolved view of phonon dynamics in black phosphorus
It is only in the last few years that time- and momentum-resolved probes of phonons have become a reality. Here, we have employed the method of femtosecond electron inelastic scattering to reveal how the lattice of photoexcited black phosphorus thermalizes at the microscopic level. In collaboration with the research groups of C. Draxl (HU Berlin) and C. Carbogno (FHI Theory), we have developed a new approach to predict the influence of the non-equilibrium lattice dynamics on the structure factor based on many-body calculations of the electron-phonon and phonon-phonon interactions. By directly comparing the experimental and calculated structure factors, we have demonstrated that the anisotropic conduction band is at the origin of an anisotropic non-thermal phonon population and that our model reproduces the subsequent lattice thermalization. Seiler et al., arXiv:2006.12873
Time-Reversal Dichroism in ARPES
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 Dichroismin 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. [more...]
new preprint: A machine learning route between band mapping and band structure.
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
Observation of large polarons in the perovskite semiconductor CsPbBr3
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).
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
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 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.
First time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser.
Thomas received his PhD in physics for his investigations of ultrafast energy flow and structural dynamics in nanoscale heterostructures with femtosecond electron diffraction.
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...]
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
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
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.
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.