DEPARTMENT OF
PHYSICAL CHEMISTRY
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Singlet exciton fission (SEF) is a process occurring in several organic semiconductors by which an electronically excited singlet exciton spontaneously splits into two triplet excitons. The SEF properties, such as yields and rates, depend crucially on the molecular structure and intermolecular interactions. This calls for an experimental technique that can probe the structural changes accompanying the SEF process at the relevant time and length scales.

In a study published in Science Advances, we have performed femtosecond electron diffraction experiments to directly reveal the structural dynamics accompanying the SEF process in pentacene single crystals. We have observed 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. The theory analysis has shown that these motions can only be triggered through the coupling of electronic excitations to other more localized molecular motions, which in turn, couple to the sliding motions also observed in the experiments. 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 about the origin of the ultrafast timescales related to the fission.

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The head of “Structural & Electronic Surface Dynamics” from the Department of Physical Chemistry was appointed Professor by the Technische Universität Berlin at the Institut für Optik und Atomare Physik on June 1st, 2021.

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Phase transitions under quasi-equilibrium conditions, e.g., induced by a slow variation of temperature, are well described by Landau theory. In contrast, the situation far from equilibrium, e.g., after ultrafast laser excitation, differs fundamentally from a thermodynamic scenario, and it remains an open question how our understanding of static phase transitions in complex matter has to be adapted to capture a dynamical, photoinduced melting and recovery of order. In particular, even the thermal critical temperature might not provide a valid description in a system exhibiting strong non-equilibrium between different degrees of freedom, such as electrons and lattice.

In our study which was published in Nature Communications, we investigate an ultrafast charge-density-wave-to-metal transition after optical excitation by combining state-of-the-art time-resolved electronic and structural probes. By tracking the electronic and lattice dynamics of the charge density wave (CDW) and the transient electronic temperature throughout the full melting and recovery cycle, we reveal a surprising formation and stability of CDW order at electronic temperatures far greater than the thermal transition temperature, which we link to transiently suppressed lattice fluctuations in the nonthermal scenario of hot electrons coupled to an initially cold lattice. Additionally, we map the collective CDW amplitude excitations in unprecedented detail, which allows us to reconstruct the underlying transient potential energy surfaces using a time-dependent Ginzburg-Landau approach.

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Electrical resistance of magnets with and without electron collisions

Electrical resistance R of metals arises from collisions of the conduction electrons with obstacles in the surrounding material and from the mass of the electrons. Interestingly, in a magnetic metal, R is larger when the current flows parallel to the magnetic moment M (see Figure (a), left), and it is smaller when the electrons flow perpendicular to it (Figure (b), left). This “anisotropic magnetoresistance” (AMR) is a mature effect, which is usually explained by the assumption that electrons moving parallel to M collide with obstacles more frequently than electrons flowing perpendicularly (see Figures (a,b), middle column).

However, this assumption was recently challenged by researchers from the Czech Republic and Germany, who are also associated with the Collaborative Research Center TRR227 “Ultrafast spin dynamics”. They showed experimentally that AMR can be strong even without electron collisions.

“Our trick was to use ultrashort electromagnetic pulses in the terahertz frequency range. They allowed us to measure AMR at rates both slower and faster than the average time between two collision events of conduction electrons”, explains Dr. Lukas Nadvornik, then member of the Terahertz Physics Group. “In this way, we were able to separate contributions that are unrelated to electron collisions from those that arise from collisions.” The team found that AMR unrelated to collisions emerges because the electrons moving along the magnetization direction are effectively heavier than electrons moving perpendicular to it (see Figures (a,b), right column).

AMR is an important probe of the magnetic state of prototypical ferromagnetic and antiferromagnetic spintronic devices, which convey and store information using the magnetic moment of the electron rather than its charge. Lukas Nadvornik concludes: “Our findings show that collision-unrelated AMR has a large information bandwidth and, thus, is highly interesting for future ultrafast spintronic applications.”

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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.

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The research group of Julia Stähler (Fritz Haber Institute and Humboldt-Universität zu Berlin) has found out that semiconductors can be converted to metals and back more easily and more quickly than previously thought. This discovery may increase the processing speed and simplify the design of many common technological devices.

Band bending at semiconductor surfaces induced by chemical doping or electric fields can create metallic surfaces with properties not found in the bulk, such as high electron mobility, magnetism or superconductivity. Optical generation of such metallic surfaces on ultrafast timescales would be appealing for high-speed electronics. It was demonstrated that the ultrafast generation of a metal at the (10-10) surface of ZnO upon photoexcitation. Compared to hitherto known ultrafast photoinduced semiconductor-to-metal transitions that occur in the bulk of inorganic semiconductors, the metallization of the ZnO surface is launched by 3–4 orders of magnitude lower photon fluxes. Using time- and angle-resolved photoelectron spectroscopy, it was shown that the phase transition is caused by photoinduced downward surface band bending due to photodepletion of donor-type deep surface defects. The discovered mechanism is in analogy to chemical doping of semiconductor surfaces and presents a general route for controlling surface-confined metallicity on ultrafast timescales.

See press release here.

Here’s a video summary and the full paper!

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Characterization of the electronic band structure of solid state materials is routinely performed using photoemission spectroscopy. Recent advancements in short-wavelength light sources and electron detectors give rise to multidimensional photoemission spectroscopy, allowing parallel measurements of the electron spectral function simultaneously in energy, two momentum components and additional physical parameters with single-event detection capability. Efficient processing of the photoelectron event streams at a rate of up to tens of megabytes per second will enable rapid band mapping for materials characterization. We describe an open-source workflow that allows user interaction with billion-count single-electron events in photoemission band mapping experiments, compatible with beamlines at 3rd and 4rd generation light sources and table-top laser-based setups. The workflow offers an end-to-end recipe from distributed operations on single-event data to structured formats for downstream scientific tasks and storage to materials science database integration. Both the workflow and processed data can be archived for reuse, providing the infrastructure for documenting the provenance and lineage of photoemission data for future high-throughput experiments.

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Researchers from the Physical Chemistry Department at the Fritz Haber Institute have together with other European research institutions and companies formed the research consortium OPTOlogic. It aims to develop optical topological computing as a means to reduce energy consumption of electronic circuits. The EU funds the project with almost 4 Million Euros.

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 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 a material can move with minimal loss of energy, which enables the realization of dissipation-free quantum devices. To artificially induce and control topological protected states, the project will use spatially and temporally structured ultrafast pulses of light. These novel quantum devices will use minimal energy to move and store information, while increasing computing power. By increasing the energy efficiency and speed of logical operations, the project could have a significant economic, environmental and social impact.

Coordinated by Prof. Jens Biegert, the OPTOlogic consortium includes researchers at the Institute of Photonic Sciences (ICFO) near Barcelona, the Fritz Haber Institute of the Max Plank Society and the Max Born Institute in Berlin, the French Alternative Energies and Atomic Energy Commission (CEA) at Saclay, and the company LightOn,  thus uniting world-leading experimental, theoretical, and industrial expertise in condensed matter physics, ultrafast spectroscopy, attoscience, quantum optics and computing, machine learning and artificial intelligence.

The consortium aims at developing a new technological platform that leverages topology to avoid energy loss in electronic transport, light-wave-electronics to overcome limitations imposed by material properties, and quantum materials to realize novel information storage and processing. The project will use the latest technology in ultrafast techniques and attoscience, nanotechnology, and quantum computing to develop this new platform.

This project has received funding from the European Union’s “Horizon 2020” research and innovation programme under grant agreement No 899794.

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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.