DEPARTMENT OF
PHYSICAL CHEMISTRY
DEPARTMENT OF
PHYSICAL CHEMISTRY
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The functionality of a material and its macroscopic response to external stimuli are dictated by the microscopic interactions of its elementary degrees of freedom – electrons, lattice, and spin. Understanding the fundamental mechanisms and complex many-body interactions that determine relevant material properties is a main goal of modern solid state research. In spatially inhomogeneous crystals, atomic-scale variations of the crystal lattice and/or its electronic structure will alter the microscopic response – may it be due to imperfection in the presence of defects, or by design due to the growth of nanoscale heterostructures. Whereas femtosecond pump-probe techniques provide insight into microscopic couplings by following the non-equilibrium response and dynamics of a system in real-time, very few experimental approaches exist to date which are capable to resolve such dynamics locally on the relevant length scales down to a single atom.

Coherent phonon spectroscopy is a powerful tool to monitor ultrafast lattice dynamics under nonequilibrium conditions. Such collective, in-phase vibrations of a crystal lattice can be excited by ultrafast laser pulses and can be used, for example, to monitor a light-induced phase transition in strongly correlated materials in real time and even to control its outcome. Despite the important role of the local crystal structure for such dynamics, it has not been possible so far to measure coherent lattice vibrations with high, few nanometer, spatial resolution. In this work we demonstrate nanoscale coherent phonon spectroscopy by means of ultrafast laser-induced scanning tunneling microscopy (STM) in a plasmonic junction. Comparison of the CP spectra with tip-enhanced Raman spectroscopy allows us to identify the involved phonon modes. In the all-optical pump-probe STM approach, the laser-excited coherent phonons periodically modulate the photoinduced tunneling current generated by the probe pulse, for which it is crucially that the laser excitation is resonant with an optical transition in the electronic structure of the hybridized ZnO/Ag(111) system. Moreover, in contrast to the Raman spectra, the relative CP intensities exhibit strong nanoscale spatial variations, which correlate with changes in the local density of states recorded via scanning tunneling spectroscopy. A new aspect of this work is that the photoinduced tunneling current is not modulated by a periodic change of the tip-sample distance, but likely by coupling of the phonons to the local electronic structure, which in turn leads to a modulation of the resonantly excited photocurrent. Our work introduces a new approach to probe the ultrafast structural response of a photoexcited solid using STM, and opens up new possibilities to study and control of ultrafast dynamics at solid surfaces and in low-dimensional quantum materials.

Original publication: S. Liu, A. Hammud, I. Hamada, M. Wolf, M. Müller, T. Kumagai, Nanoscale coherent phonon spectroscopy, Science Advances.

Science Advances 21, eabq5682 (2022) • DOI: 10.1126/sciadv.abq5682

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Sebastian Maehrlein, leader of the THz Structural Dynamics group, has been awarded an independent Emmy Noether group by the German Research Foundation (DFG) for a funding period of 6 years. His research focus will be centered on actively steering lattice angular momentum in solids for ultrafast control of material properties.

Even though exchange of energy and linear momentum between lattice vibrations (phonons) and other degrees of freedom is a cornerstone of solid-state physics, phonon angular momentum is commonly just assumed to account for angular momentum conservation but its active control remains elusive. In this long-term project Sebastian F. Maehrlein and his team will prepare and coherently control phonon states with angular momentum to study and actively manipulate coupled electronic and spin degrees of freedoms. As a first step, the group will work towards polarization tailoring of highly intense few-cycle pulses in the THz and multi-THz (mid-infrared) spectral range. Once this control is established, the preparation and detection of circularly polarized (degenerate chiral) coherent phonons in benchmark materials will be tackled. The coupling of this coherent phonon angular momentum to ordered spin states in magnetic materials or atomically thin semiconductors bears a number of fundamental scientific questions and application potential. Eventually, the team aims to establish phonon angular momentum as a novel ultrafast tuning knob for comprehensive material control.

https://www.fhi.mpg.de/1103091/22-07-18-emmy-noether-group

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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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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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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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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.