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### 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.

### News

Review on ARPES

Jul 2022

Angle-resolved photoemission spectroscopy (ARPES) is a method of investigating the energy- and momentum-resolved electronic structure of a material. As the physical properties of condensed matter depend strongly on the electronic structure, ARPES can be used to study the physics of crystalline solids with a variety of applications. We contributed to a tutorial-style review introducing the key aspects of ARPES principles, instrumentation, data analysis, and representative scientific cases to demonstrate the power of the method.

Zhang et al., Nature Reviews Methods Primers 2, 54 (2022)

Zhang et al., Nature Reviews Methods Primers 2, 54 (2022)

New paper: Electron dynamics in a two-dimensional topological insulator.

Jun 2022

Two-dimensional quantum spin Hall (QSH) insulators are a promising material class for spintronic applications based on topologically protected spin currents in their edges. Yet, they have not lived up to their technological potential, as experimental realizations are scarce and limited to cryogenic temperatures. These constraints have also severely restricted the characterization of their dynamical properties. Here, we report on the electron dynamics of the novel room-temperature QSH candidate bismuthene after photoexcitation using time- and angle-resolved photoemission spectroscopy. We map the transiently occupied conduction band and track the full relaxation pathway of hot photocarriers. Intriguingly, we observe photocarrier lifetimes much shorter than those in conventional semiconductors. This is ascribed to the presence of topological in-gap states already established by local probes. Indeed, we find spectral signatures consistent with [more...]

New preprint: Coherent Light Control of a Metastable Hidden Phase.

Jun 2022

Metastable phases present a promising route to expand the functionality of complex materials. Of particular interest are light-induced metastable phases that are inaccessible under equilibrium conditions, as they often host new, emergent properties switchable on ultrafast timescales. However, the processes governing the trajectories to such hidden phases remain largely unexplored. Here, using time- and angle-resolved photoemission spectroscopy, we investigate the ultrafast dynamics of the formation of a hidden quantum state in the layered dichalcogenide 1T-TaS2 upon photoexcitation. Our results reveal the nonthermal character of the transition governed by a collective charge-density-wave excitation. Utilizing a double-pulse excitation of the structural mode, we show vibrational coherent control of the phase-transition efficiency. Our demonstration of exceptional control, switching speed, and stability of the hidden phase are key for device [more...]

New preprint: orbital-resolved movie of the singlet fission process.

Apr 2022

Singlet fission may boost photovoltaic efficiency by transforming a singlet exciton into two triplet excitons and thereby doubling the number of excited charge carriers. The primary step of singlet fission is the ultrafast creation of the correlated triplet pair. While several mechanisms have been proposed to explain this step, none has emerged as a consensus. The challenge lies in tracking the transient excitonic states.

We use time- and angle-resolved photoemission spectroscopy to observe the primary step of singlet fission in crystalline pentacene and show that it occurs in a charge-transfer mediated mechanism. We gained intimate knowledge about the localization and the

orbital character of the exciton wavefunctions recorded in momentum maps. This allowed us to directly compare the localization of singlet and bitriplet excitons and decompose energetically overlapping states based on their orbital character. Orbital- and localization-resolved [more...]

We use time- and angle-resolved photoemission spectroscopy to observe the primary step of singlet fission in crystalline pentacene and show that it occurs in a charge-transfer mediated mechanism. We gained intimate knowledge about the localization and the

orbital character of the exciton wavefunctions recorded in momentum maps. This allowed us to directly compare the localization of singlet and bitriplet excitons and decompose energetically overlapping states based on their orbital character. Orbital- and localization-resolved [more...]

New paper: Excited-state band mapping.

Feb 2022

Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. We extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet femtosecond light source to probe optically excited samples. The wide-ranging applicability and power of this approach are demonstrated by measurements on the two-dimensional semiconductorWSe2, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view, not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electronic screening. Our work establishes [more...]

New paper: monitoring the ultrafast flow of energy in a magnet.

Feb 2022

Ultrafast magnetization dynamics are governed by energy flow between electronic, magnetic, and lattice degrees of freedom. A quantitative understanding of these dynamics must be based on a model that agrees with experimental results for all three subsystems. However, ultrafast dynamics of the lattice remain largely unexplored experimentally. Here we combine femtosecond electron diffraction experiments of the lattice dynamics with energy-conserving atomistic spin dynamics (ASD) simulations and ab initio calculations to study the intrinsic energy flow in the 3d ferromagnets cobalt (Co) and iron (Fe). The simulations yield a good description of experimental data, in particular an excellent description of our experimental results for the lattice dynamics. We find that the lattice dynamics are influenced significantly by the magnetization dynamics due to the energy cost of demagnetization. Our results highlight the role of the spin system as [more...]

New paper: towards Bloch wavefunction reconstruction.

Jan 2022

Angle-resolved photoemission spectroscopy (ARPES) is the most powerful technique to investigate the electronic band structure of crystalline solids. To completely characterize the electronic structure of topological materials, one needs to go beyond band structure mapping and access information about the momentum-resolved Bloch wave function, namely, orbitals, Berry curvature, and topological invariants. However, because phase information is lost in the process of measuring photoemission intensities, retrieving the complex-valued Bloch wave function from photoemission data has yet remained elusive. We introduce a novel measurement methodology and associated observable in extreme ultraviolet angle-resolved photoemission spectroscopy, based on continuous modulation of the ionizing radiation polarization axis.

This novel measurement methodology in ARPES, which is articulated around the manipulation of the photoionization transition dipole [more...]

This novel measurement methodology in ARPES, which is articulated around the manipulation of the photoionization transition dipole [more...]

New paper: Unveiling the orbital texture using intrinsic linear dichroism in multidimensional photoemission spectroscopy

Nov 2021

The momentum-dependent orbital character in crystalline solids, referred to as orbital texture, is of capital importance in the emergence of symmetry-broken collective phases, such as charge density waves as well as superconducting and topological states of matter. By performing extreme ultraviolet multidimensional angle-resolved photoemission spectroscopy for two different crystal orientations linked to each other by mirror symmetry, we isolate and identify the role of orbital texture in photoemission from the transition metal dichalcogenide 1T-TiTe

_{2}. By comparing our experimental results with theoretical calculations based on both a quantitative one-step model of photoemission and an intuitive tight-binding model, we unambiguously demonstrate the link between the momentum-dependent orbital orientation and the emergence of strong intrinsic linear dichroism in the photoelectron angular distributions. Our results represent an important [more...]New paper: a global fitting approach for time-resolved polycrystalline diffraction data.

Nov 2021

Quantitative knowledge of electron-phonon coupling is important for many applications as well as for the fundamental understanding of nonequilibrium relaxation processes. Time-resolved diffraction provides direct access to this knowledge through its sensitivity to laser-induced lattice dynamics. Here, we present an approach for analyzing time-resolved polycrystalline diffraction data. A two-step routine is used to minimize the number of time-dependent fit parameters. The lattice dynamics are extracted by finding the best fit to the full transient diffraction pattern rather than by analyzing transient changes of individual Debye–Scherrer rings.

Publication: Zahn et al., Structural Dynamics

Publication: Zahn et al., Structural Dynamics

**8**, 064301 (2021)[more...]New papers: Efficient First-Principles Methodology for the Calculation of the All-Phonon Inelastic Scattering in Solids

Nov 2021

Femtosecond electron diffuse scattering (FEDS) has emerged as a powerful technique to probe phonon dynamics in momentum space. FEDS data are rich in information, but are typically complex to interpret. A first important step in data analysis is to model accurately the thermal diffuse scattering background of materials. In this joint submission, we developed a first-principles methodology to fully address a fundamental process in solids, namely the phonon-induced inelastic scattering of x-rays, electrons, or neutrons. Besides obtaining excellent agreement between theory and experiment, we demonstrate that multi-phonon effects can drastically modify scattering signals for a large range of scattering wavevectors. The present method opens the way for large-scale high-throughput calculations, enabling the accurate interpretation of FEDS experiments.

Full publications: M. Zacharias et [more...]

Full publications: M. Zacharias et [more...]

New paper: Accessing the Anisotropic Nonthermal Phonon Populations in Black Phosphorus

Jul 2021

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

Jul 2021

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.

Jul 2021

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

Jun 2021

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.

Jun 2021

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 WSe

_{2}. 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.

Jun 2021

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

May 2021

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).

Full publication: Maklar et al., Nature Communications 12:2499 (2021).

New paper: Ultrafast modulation of a material's Fermi surface topology

Apr 2021

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+

Beaulieu et al., Science Advances 7, eabd9275 (2021). Link [more...]

*U*simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated type-II Weyl semimetal*T*_{d}-MoTe_{2}. 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

Apr 2021

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 WSe

_{2}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

Apr 2021

[more...]

New paper: Ultrafast lattice dynamics of the antiferromagnet nickel oxide

Apr 2021

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)

Windsor et al, Phys. Rev. Lett. 126, 147202 (2021)

DFG funds project within Priority Program 2D Materials

Oct 2020

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

The Structural & Electronic Surface Dynamics Group participates in this consortium with the project

*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

Sep 2020

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...]

Time-resolved core-level spectroscopy reveals exciton dynamics.

Aug 2020

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...]

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

Jun 2020

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,

[more...]

*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-WSe_{2}.[more...]

Workshop on data aspects in ARPES

May 2020

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...]

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...]

new preprint: A machine learning route between band mapping and band structure.

May 2020

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

preprint: Xian

*et al*., arXiv:2005.10210Observation of large polarons in the perovskite semiconductor CsPbBr3

May 2020

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).

Puppin et al., Phys. Rev. Lett. 124, 206402 (2020).

Ultrafast Light-Induced Lifshitz Transition

Mar 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...]

Anisotropic Nonequilibrium Lattice Dynamics of Black Phosphorus

Feb 2020

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...]