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 paper: Accessing the Anisotropic Nonthermal Phonon Populations in Black Phosphorus
Photo-induced non-radiative energy dissipation pathways in nanoscale materials are ubiquitous. They are the dominant loss channels in most opto-electronic devices, and offer new opportunities for optical control of quantum materials. We combine femtosecond electron diffuse scattering experiments and first-principles calculations of the coupled electron–phonon dynamics to provide a detailed momentum-resolved picture of lattice thermalization in black phosphorus. The measurements reveal the emergence of highly anisotropic nonthermal phonon populations persisting for several picoseconds after exciting the electrons with a light pulse. Ultrafast dynamics simulations based on the time-dependent Boltzmann formalism are supplemented by calculations of the structure factor, defining an approach to reproduce the experimental signatures of nonequilibrium structural dynamics. The combination of experiments and theory enables us to identify highly [more...]
New paper: Nuclear dynamics of singlet exciton fission in pentacene single crystals
Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. We have performed femtosecond electron diffraction experiments to directly probe the structural dynamics accompanying the SEF process in pentacene single crystals. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. By combining real-time time–dependent density-functional theory, molecular dynamics simulations and experimental structure factor analysis, we have identified the coherent motions as collective motions of the pentacene molecules along their long axis. These long-range intermolecular motions heavily modify the excitonic coupling between adjacent molecules. In doing so, they efficiently neutralize the forces that keep the two triplet excitons together right after they have been generated, providing a possible explanation [more...]
New review paper on applying machine learning in spectroscopy and scattering experiments.
Neutron and x-ray scattering represent two classes of state-of-the-art materials characterization techniques that measure materials’ structural and dynamical properties with high precision. These techniques play critical roles in understanding a wide variety of materials systems from catalysts to polymers, nanomaterials to macromolecules, and energy materials to quantum materials. In recent years, neutron and x-ray scattering have received a significant boost due to the development and increased application of machine learning to materials problems. This article reviews the recent progress in applying machine learning techniques to augment various neutron and x-ray techniques, including neutron scattering, x-ray absorption, x-ray scattering, and photoemission. We highlight the integration of machine learning methods into the typical workflow of scattering experiments, focusing on problems that challenge traditional analysis approaches [more...]
New paper: Wave-Mechanical Electron-Optical Modeling of Electron Sources
In 1924 Louis de Broglie postulated that electrons possess a wave nature, and only three years later this was confirmed in experiments by Davisson and Germer. In modern high-resolution electron microscopy the wave nature of electrons plays a central role in image formation. Yet in the conventional theory of electron sources, which are crucial components in determining the performance of these instruments, electrons are still treated as classical particles. In this publication we address this problem by introducing a wave-mechanical electron-optical model of electron sources. Using our model we investigate a low-energy electron microscopy technique that is a direct implementation of Gabor’s concept of in-line holography and show how the spatial resolution is determined by the coherence and aberration properties of the source. The simulated in-line holograms of an infinitely sharp edge (see figure) show that the coherence of the electron [more...]
New paper in Natural Sciences: Full experimental characterization of an exciton.
When a material absorbs visible light, an electron is lifted to a higher energy level. In semiconductors and molecular crystals, the excited electron and the hole it leaves behind attract each other. Such Coulomb-bound electron-hole pairs are considered to be new particles called excitons, which govern the optoelectronic properties of semiconductors. Although optical signatures of excitons have been studied extensively, experimental access to the excitonic wave function itself has been elusive. Using multidimensional photoemission spectroscopy, we present a momentum-, energy-, and time-resolved perspective on excitons in the layered semiconductor WSe2. By tuning the excitation wavelength, we determine the energy-momentum signature of bright exciton formation and its difference from conventional single-particle excited states. The multidimensional data allow to retrieve fundamental exciton properties like the binding energy and [more...]
Ralph Ernstorfer appointed professor at Technische Universität Berlin.
Ralph Ernstorfer, head of the Structural & Electronic Surface Dynamics Group at the Department of Physical Chemistry, was appointed W3 professor at the Technical University Berlin on June-01 2021. His new research group Ultrafast Nanoscience is part of the Institute of Optics and Atomic Physics. Both research groups at the Fritz Haber Institute and the TU Berlin will closely collaborate and advance experimental approaches for the investigation of the electronic and atomic structure and their dynamics in energized nanoscopic solids and heterostructures.
New paper: Nonequilibrium Charge-Density-Wave Order Beyond the Thermal Limit
The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. We demonstrate nonthermal charge-density-wave (CDW) order at electronic temperatures far greater than the thermodynamic transition temperature. Using time- and angle-resolved photoemission spectroscopy and time-resolved X-ray diffraction, we investigate the electronic and structural order parameters of an ultrafast photoinduced CDW-to-metal transition. Tracking the dynamical CDW recovery as a function of electronic temperature reveals a behaviour markedly different from equilibrium, which we attribute to the suppression of lattice fluctuations in the transient nonthermal phonon distribution. Full publication: Maklar et al., Nature Communications 12:2499 (2021).
New paper: Ultrafast modulation of a material's Fermi surface topology
The transport of electrons is governed by the shape of the Fermi surface. We found that the topology of the Fermi surface of a semimetal can be manipulated on ultrafast timescales through optical excitation. A change in the Fermi surface topology, also called Lifshitz transition, can lead to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. Combining time-resolved multidimensional photoemission spectroscopy and TDDFT+U simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated type-II Weyl semimetal Td-MoTe2. We show that this non-equilibrium topological transition finds its microscopic origin in the dynamical modification of the electronic correlations. Beaulieu et al., Science Advances 7, eabd9275 (2021).Link [more...]
New preprint: Bloch Wavefunction Reconstruction using Multidimensional Photoemission Spectroscopy
The most advanced experimental technique to measure the electronic band structure of solids is angle-resolved photoemission spectroscopy (ARPES). While ARPES directly maps the momentum-resolved electronic eigenvalues (energy bands), topological properties are often hidden in the complex-valued Bloch wavefunction, which is not directly accessible in standard photoemission experiments. In a recent joint experimental and theoretical work in collaboration with Dr. Michael Schüler and Prof. Thomas Devereaux from Stanford University, we have found a novel approach to reconstruct the Bloch wavefunction of WSe2 from polarization-modulated ARPES, with minimal theory input (arXiv:2103.17168).
New paper: 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.
New paper: Ultrafast lattice dynamics 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. Windsor et al, Phys. Rev. Lett. 126, 147202 (2021)
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...]
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.