DEPARTMENT OF
PHYSICAL CHEMISTRY
DEPARTMENT OF
PHYSICAL CHEMISTRY
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After the traditional New Year’s coffee had to be cancelled for two years due to corona, the FHI employees were happy to finally come together again with a sparkling drink, finger food, a delicious bratwurst and wonderful weather at the Haber lime tree.

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When photoexcited by an ultrafast laser pulse, antiferromagnets allow direct angular momentum transfer between opposing spins, promising faster functionality than ferromagnets, which are inherently limited because their net angular momentum must be dissipated into the lattice. The process of angular momentum transfer is closely related to the type of magnetic coupling in the system.

In lanthanides, magnetic 4f exchange is mediated indirectly via the conduction electrons (Ruderman-Kittel-Kasuya-Yosida interaction, RKKY), and the effects of such conditions on antiferromagnetic direct spin transfer are largely unexplored. The Ruderman-Kittel-Kasuya-Yosida interaction (RKKY interaction, after Malvin Avram Ruderman, Charles Kittel, Tadao Kasuya and Kei Yosida) describes the indirect exchange between the localised magnetic moments of the atoms of a metal. If an electron comes close to a magnetic atom, it aligns its spin with it. If the electron now moves further through the solid, the spin polarisation of the electron can in turn cause an alignment of the magnetic moment of one of the nearest atoms.

Here resonant ultrafast X-ray diffraction are utilized to study ultrafast magnetization dynamics in a series of 4f antiferromagnets, and systematically varied the 4f occupation, thereby altering the magnitude of RKKY. Combined with with ab-initio calculations, the researchers find that the rate of angular momentum transfer between opposing moments is directly determined by the magnitude of RKKY. Given the direct relationship between RKKY and the conduction electrons, the results offer a novel approach for controlling the speed of magnetic devices.

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The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase. Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light–matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales. Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response. Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic and hexagonal crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals, many common oxides and organic crystals, greatly expanding the material base and extending design opportunities for compact photonic devices.

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Window of Science now Highlights
Beautiful Data

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A combined experimental and theoretical study of relaxation dynamics in the charge-density-wave system TbTe3 shows, how a dynamical insulator-to-metal transition affects fundamental interactions, such as electron-electron and electron-phonon scattering. Time- and angle-resolved photoemission spectroscopy utilizes optical excitation to transiently alter the energy gap and reveals a concurrent, highly unusual transient modulation of the relaxation rates of excited photocarriers.
State-of-the-art calculations based on non-equilibrium Green’s functions provide a microscopic view onto the interplay of quasiparticle scattering and the transiently modified electronic band structure, highlighting the critical role of the phase space of electron-electron interaction. The results vividly demonstrate the possibility of controlling quasiparticle relaxation rates by transiently tuning the electronic band structure using light pulses.

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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.”