Simultaneous dynamic characterization of charge and structural motion during ferroelectric switching

Monitoring structural changes in ferroelectric thin films during electric field-induced polarization switching is important for a full microscopic understanding of the coupled motion of charges, atoms and domain walls. We combine standard ferroelectric test-cycles with time-resolved x-ray diffraction to investigate the response of a nanoscale ferroelectric oxide capacitor upon charging, discharging and switching. Piezoelectric strain develops during the electronic RC time constant and additionally during structural domain-wall creep. The complex atomic motion during ferroelectric polarization reversal starts with a negative piezoelectric response to the charge flow triggered by voltage pulses. Incomplete screening limits the compressive strain. The piezoelectric modulation of the unit cell tweaks the energy barrier between the two polarization states. Domain wall motion is evidenced by a broadening of the in-plane components of Bragg reflections. Such simultaneous measurements on a working device elucidate and visualize the complex interplay of charge flow and structural motion and challenges theoretical modelling

Following Strain-Induced Mosaicity Changes of Ferroelectric Thin Films by Ultrafast Reciprocal Space Mapping

We investigate coherent phonon propagation in a thin film of ferroelectric PbZr0.2Ti0.8O3 (PZT) by ultrafast x-ray diffraction (UXRD) experiments, which are analyzed as time-resolved reciprocal space mapping (RSM) in order to observe the in- and out-of-plane structural dynamics simultaneously. The mosaic structure of the PZT leads to a coupling of the excited out-of-plane expansion to in-plane lattice dynamics on a picosecond timescale, which is not observed for out-of-plane compression.Comment: 5 pages, 4 figure

Thermoelastic study of nanolayered structures using time-resolved x-ray diffraction at high repetition rate

We investigate the thermoelastic response of a nanolayered sample composed of a metallic SrRuO3 (SRO) electrode sandwiched between a ferroelectric Pb(Zr0.2Ti0.8)O3 (PZT) film with negative thermal expansion and a SrTiO3 substrate. SRO is rapidly heated by fs-laser pulses with 208 kHz repetition rate. Diffraction of x-ray pulses derived from a synchrotron measures the transient out-of-plane lattice constant c of all three materials simultaneously from 120 ps to 5 mus with a relative accuracy up to Delta c/c = 10^-6. The in-plane propagation of sound is essential for understanding the delayed out of plane expansion.Comment: 5 pages, 3 figure

Photodynamics and Ground State Librational States of ClF Molecule in solid Ar. Comparison of experiment and theory

Photodynamics calculations of a ClF molecule in solid Ar are compared to experimental results and a new interpretation is given for the observed femtosecond-pump-probe signal modulation. We analyze the round-trip and depolarization times for the excited state wave-packet motion and discuss the incorporation of lattice cage motions that partially explain the time dependence of the measured signal. Librational eigenstates and -energies are calculated by solving the rotational Schrödinger equation in the previously computed [T. Kiljunen, M. Bargheer, M. Gühr, and N. Schwentner, Phys. Chem. Chem. Phys. 6 (9), 2185-2197 (2004)] octahedral potentials that hinder free molecular rotation in the solids. The obtained level structure is compared to infrared-spectroscopic results. We comment on the correspondence between temperature effects in the classical dynamics of the nuclei and the quantum mechanical probability distributions. We find the combinative treatment of different simulation temperatures congruous for interpreting the experimental results at cryogenic conditions

Localized excited charge carriers generate ultrafast inhomogeneous strain in the multiferroic BiFeO3_3

We apply ultrafast X-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO$_3$ after above-bandgap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO$_3$: the relevant excited charge carriers must remain localized to be consistent with the data

Particle transport phenomena in low temperature solids

We review different approaches to measure the transport of F atoms and ions in rare gas matrices and compare the experimental results to simulations. Static measurements on sandwich structures and co-doped matrices yield rather long travel ranges beyond 2 nm, in accord with early classical simulations which predict a channeling of the F atoms in rare gas matrices. Nonadiabatic simulations show a rapid energy loss, fast nonadiabatic dynamics and only short travel ranges of typically 1 unit cell. The rapid energy loss, fast nonadiabatic transitions and the timescale for direct dissociation (~ 250 fs) are verified by fs-pump-probe experiments. It remains a challenge to account for the long-range migration when nonadiabatic processes are allowed in simulations, and to measure the long distance flights directly by ultrafast spectroscopy

UDKM1DSIM A simulation toolkit for 1D ultrafast dynamics in condensed matter

The UDKM1DSIM toolbox is a collection of MATLAB MathWorks Inc. classes and routines to simulate the structural dynamics and the according X ray diffraction response in one dimensional crystalline sample structures upon an arbitrary time dependent external stimulus, e.g. an ultrashort laser pulse. The toolbox provides the capabilities to define arbitrary layered structures on the atomic level including a rich database of corresponding element specific physical properties. The excitation of ultrafast dynamics is represented by an N temperature model which is commonly applied for ultrafast optical excitations. Structural dynamics due to thermal stress are calculated by a linear chain model of masses and springs. The resulting X ray diffraction response is computed by dynamical X ray theory. The UDKM1DSIM toolbox is highly modular and allows for introducing user defined results at any step in the simulation procedur

A new route to gold nanoflowers

Catanionic vesicles spontaneously formed by mixing the anionic surfactant bis(2-ethylhexyl) sulfosuccinate sodium salt with the cationic surfactant cetyltrimethylammonium bromide were used as a reducing medium to produce gold clusters, which are embedded and well-ordered into the template phase. The gold clusters can be used as seeds in the growth process that follows by adding ascorbic acid as a mild reducing component. When the ascorbic acid was added very slowly in an ice bath round-edged gold nanoflowers were produced. When the same experiments were performed at room temperature in the presence of Ag+ ions, sharp-edged nanoflowers could be synthesized. The mechanism of nanoparticle formation can be understood to be a non-diffusion-limited Ostwald ripening process of preordered gold nanoparticles embedded in catanionic vesicle fragments. Surface-enhanced Raman scattering experiments show an excellent enhancement factor of 1.7 · 105 for the nanoflowers deposited on a silicon wafer.Deutsche Forschungsgemeinschaft https://doi.org/10.13039/501100001659Peer Reviewe