Structural dynamics of surfaces by ultrafast electron crystallography: Experimental and multiple scattering theory

Recent studies in ultrafast electron crystallography (UEC) using a reflection diffraction geometry have enabled the investigation of a wide range of phenomena on the femtosecond and picosecond time scales. In all these studies, the analysis of the diffraction patterns and their temporal change after excitation was performed within the kinematical scattering theory. In this contribution, we address the question, to what extent dynamical scattering effects have to be included in order to obtain quantitative information about structural dynamics. We discuss different scattering regimes and provide diffraction maps that describe all essential features of scatterings and observables. The effects are quantified by dynamical scattering simulations and examined by direct comparison to the results of ultrafast electron diffraction experiments on an in situ prepared Ni(100) surface, for which structural dynamics can be well described by a two-temperature model. We also report calculations for graphite surfaces. The theoretical framework provided here allows for further UEC studies of surfaces especially at larger penetration depths and for those of heavy-atom materials

Primary structural dynamics in graphite

The structural dynamics of graphite and graphene are unique, because of the selective coupling between electron and lattice motions and hence the limit on electric and electro-optic properties. Here, we report on the femtosecond probing of graphite films (1–3 nm) using ultrafast electron crystallography in the transmission mode. Two time scales are observed for the dynamics: a 700 fs initial decrease in diffraction intensity due to lattice phonons in optically dark regions of the Brillouin zone, followed by a 12 ps decrease due to phonon thermalization near the Г and K regions. These results indicate the non-equilibrium distortion of the unit cells at early time and the subsequent role of long-wavelength atomic motions in the thermalization process. Theory and experiment are now in agreement regarding the nature of nuclear motions, but the results suggest that potential change plays a role in the lateral dynamics of the lattice

Electron Imaging of Nanoscale Charge Distributions Induced by Femtosecond Light Pulses

Surface charging is a phenomenon ubiquitously observable in in-situ transmission electron microscopy of non-conducting specimens as a result of electron-beam/sample interactions or optical stimuli and often limits the achievable image stability and spatial or spectral resolution. Here, we report on the electron-optical imaging of surface charging on a nanostructured surface following femtosecond-multiphoton photoemission. By quantitatively extracting the light-induced electrostatic potential and studying the charging dynamics on the relevant timescales, we gain insights into the details of the multi-photon photoemission process in the presence of a background field. We study the interaction of the charge distribution with the high-energy electron beam and secondary electrons and propose a simple model to describe the interplay of electron- and light-induced processes.Comment: 26 pages; Manuscript with 3 figures and Supporting Information with 1 additional figur

Banken werden immer größer

Die Lehman-Insolvenz förderte die hohe Interdependenz und Ansteckungsgefahr im Bankensektor zu Tage. Die Vielzahl der gegenseitigen Kredit- und Anleihegeschäfte macht insbesondere die Insolvenz einer Großbank zum unkalkulierbaren Risiko. Um dieser Geiselhaft zu entkommen, wollen Aufsicht und Politik seit längerem das fremdfinanzierte Wachstum der Banken begrenzen. Dieses Bemühen steht beispielsweise auch hinter der neu eingeführten Leverage Ratio im Baseler Regelwerk (Basel III). Das DIW Berlin hat für Deutschland, Großbritannien und die USA untersucht, ob dies bisher gelungen ist. Dabei zeigt sich: In den Jahren vor der Finanzkrise haben die Bankbilanzen in allen drei Ländern sehr viel schneller zugelegt als das Bruttoinlandsprodukt. Im Wesentlichen wurde das exzessive Bilanzwachstum durch Fremdkapital finanziert. Die Maßnahmen im Zuge der Finanz- und Wirtschaftskrise haben das Wachstum der Bankbilanzen nur kurzfristig gebändigt. Auch die neuen Eigenkapitalvorschriften (Basel III) sind voraussichtlich nicht in der Lage, das Bilanzwachstum einzudämmen.Too-big-to-fail, Banks, Basel III, Balance Sheet Growth

Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy

The control of optically driven high-frequency strain waves in nanostructured systems is an essential ingredient for the further development of nanophononics. However, broadly applicable experimental means to quantitatively map such structural distortion on their intrinsic ultrafast time and nanometer length scales are still lacking. Here, we introduce ultrafast convergent beam electron diffraction (U-CBED) with a nanoscale probe beam for the quantitative retrieval of the time-dependent local distortion tensor. We demonstrate its capabilities by investigating the ultrafast acoustic deformations close to the edge of a single-crystalline graphite membrane. Tracking the structural distortion with a 28-nm/700-fs spatio-temporal resolution, we observe an acoustic membrane breathing mode with spatially modulated amplitude, governed by the optical near field structure at the membrane edge. Furthermore, an in-plane polarized acoustic shock wave is launched at the membrane edge, which triggers secondary acoustic shear waves with a pronounced spatio-temporal dependency. The experimental findings are compared to numerical acoustic wave simulations in the continuous medium limit, highlighting the importance of microscopic dissipation mechanisms and ballistic transport channels

Der Stark-Effekt als Werkzeug zur Strukturaufklärung isolierter Cluster

In dieser Arbeit wird untersucht, inwieweit die experimentelle Bestimmung der Ablenkung isolierter Cluster in einem inhomogenen, elektrischen Feld dazu geeignet ist, deren geometrische Struktur aufzuklären. Die Molekularstrahlablenkung ist verknüpft mit dem Stark-Effekt der Teilchen und dadurch auch mit deren permanentem Dipolmoment und elektrischer Polarisierbarkeit. Durch Vergleich von so gewonnenen Dipolmomenten und Polarisierbarkeiten mit theoretischen Vorhersagen ist es prinzipiell möglich, experimentelle Strukturaussagen zu treffen. Dieses Vorgehen wird am Beispiel von kleinen Barium-Spezies, GeN-, SnN- und PbN-Clustern sowie bimetallischen MgMPbN-Cluster demonstriert. Es wird gezeigt, dass die experimentellen Ergebnisse konsistent mit quantenchemischen Rechnungen gedeutet werden können, sofern die untersuchten Spezies ausreichend rigide sind

Nanoscale mapping of ultrafast magnetization dynamics with femtosecond Lorentz microscopy

Novel time-resolved imaging techniques for the investigation of ultrafast nanoscale magnetization dynamics are indispensable for further developments in light-controlled magnetism. Here, we introduce femtosecond Lorentz microscopy, achieving a spatial resolution below 100 nm and a temporal resolution of 700 fs, which gives access to the transiently excited state of the spin system on femtosecond timescales and its subsequent relaxation dynamics. We demonstrate the capabilities of this technique by spatio-temporally mapping the light-induced demagnetization of a single magnetic vortex structure and quantitatively extracting the evolution of the magnetization field after optical excitation. Tunable electron imaging conditions allow for an optimization of spatial resolution or field sensitivity, enabling future investigations of ultrafast internal dynamics of magnetic topological defects on 10-nanometer length scales

Single-cell transcriptomics : a high-resolution avenue for plant functional genomics

Plant function is the result of the concerted action of single cells in different tissues. Advances in RNA-seq technologies and tissue processing allow us now to capture transcriptional changes at single-cell resolution. The incredible potential of single-cell RNA-seq lies in the novel ability to study and exploit regulatory processes in complex tissues based on the behaviour of single cells. Importantly, the independence from reporter lines allows the analysis of any given tissue in any plant. While there are challenges associated with the handling and analysis of complex datasets, the opportunities are unique to generate knowledge of tissue functions in unprecedented detail and to facilitate the application of such information by mapping cellular functions and interactions in a plant cell atlas. [Abstract copyright: Copyright © 2019 The Authors. Published by Elsevier Ltd.. All rights reserved.

Few-nm tracking of magnetic vortex orbits and their decay with ultrafast Lorentz microscopy

Transmission electron microscopy is one of the most powerful techniques to characterize nanoscale magnetic structures. In light of the importance of fast control schemes of magnetic states, time-resolved microscopy techniques are highly sought after in fundamental and applied research. Here, we implement time-resolved Lorentz imaging in combination with synchronous radio-frequency excitation using an ultrafast transmission electron microscope. As a model system, we examine the current-driven gyration of a vortex core in a 2 $\mathrm{\mu}$m-sized magnetic nanoisland. We record the trajectory of the vortex core for continuous-wave excitation, achieving a localization precision of $\pm$2nm with few-minute integration times. Furthermore, by tracking the core position after rapidly switching off the current, we find a temporal hardening of the free oscillation frequency and an increasing orbital decay rate attributed to local disorder in the vortex potential