34 research outputs found
On-chip picosecond synchrotron pulse shaper
Synchrotrons are powerful and productive in revealing the spatiotemporal
complexities in matter. However, X-ray pulses produced by the synchrotrons are
predetermined in specific patterns and widths, limiting their operational
flexibility and temporal resolution. Here, we introduce the on-chip picosecond
synchrotron pulse shaper that shapes the sub-nm-wavelength hard X-ray pulses at
individual beamlines, flexibly and efficiently beyond the synchrotron pulse
limit. The pulse shaper is developed using the widely available
silicon-on-insulator technology, oscillates in torsional motion at the same
frequency or at harmonics of the storage ring, and manipulates X-ray pulses
through the narrow Bragg peak of the crystalline silicon. Stable pulse
manipulation is achieved by synchronizing the shaper timing to the X-ray timing
using electrostatic closed-loop control. Tunable shaping windows down to 40
are demonstrated, allowing X-ray pulse picking, streaking, and slicing in
the majority of worldwide synchrotrons. The compact, on-chip shaper offers a
simple but versatile approach to boost synchrotron operating flexibility and to
investigate structural dynamics from condensed matter to biological systems
beyond the current synchrotron-source limit
Duality of switching mechanisms and transient negative capacitance in improper ferroelectrics
The recent discovery of transient negative capacitance has sparked an intense
debate on the role of homogeneous and inhomogeneous mechanisms in polarizations
switching. In this work, we report observation of transient negative
capacitance in improper ferroelectric h-YbFeO3 films in a resistor-capacitor
circuit, and a concaved shape of anomaly in the voltage wave form, in the early
and late stage of the polarizations switching respectively. Using a
phenomenological model, we show that the early-stage negative capacitance is
likely due to the inhomogeneous switching involving nucleation and domain wall
motion, while the anomaly at the late stage, which appears to be a reminiscent
negative capacitance is the manifestation of the thermodynamically unstable
part of the free-energy landscape in the homogeneous switching. The complex
free-energy landscape in hexagonal ferrites may be the key to cause the abrupt
change in polarization switching speed and the corresponding anomaly. These
results reconcile the two seemingly conflicting mechanisms in the polarization
switching and highlight their different roles at different stages. The unique
energy-landscape in hexagonal ferrites that reveals the dual switching
mechanism suggests the promising application potential in terms of negative
capacitance.Comment: 14 pages,5 figure
A Ten-Fold Solvent Kinetic Isotope Effect for the Nonradiative Relaxation of the Aqueous Ferrate(VI) Ion
Hypervalent iron intermediates have been invoked in the catalytic cycles of many metalloproteins, and thus it is crucial to understand how the coupling between such species and their environment can impact their chemical and physical properties in such contexts. In this 2 work, we take advantage of the solvent kinetic isotope effect (SKIE) to gain insight into the nonradiative deactivation of electronic excited states of the aqueous ferrate(VI) ion. We observe an exceptionally large SKIE of 9.7 for the nanosecond-scale relaxation of the lowest energy triplet ligand field state to the ground state. Proton inventory studies demonstrate that a single solvent O-H bond is coupled to the ion during deactivation, likely due to the sparse vibrational structure of ferrate(VI). Such a mechanism is consistent with that reported for the deactivation of f-f excited states of aqueous trivalent lanthanides, which exhibit comparably large SKIE values. This phenomenon is ascribed entirely to dissipation of energy into a higher overtone of a solvent acceptor mode, as any impact on the apparent relaxation rate due to a change in solvent viscosity is negligible
X-ray induced electron and ion fragmentation dynamics in IBr
Characterization of the inner-shell decay processes in molecules containing
heavy elements is key to understanding x-ray damage of molecules and materials
and for medical applications with Auger-electron-emitting radionuclides. The 1s
hole states of heavy atoms can be produced by absorption of tunable x-rays and
the resulting vacancy decays characterized by recording emitted photons,
electrons, and ions. The 1s hole states in heavy elements have large x-ray
fluorescence yields that transfer the hole to intermediate electron shells that
then decay by sequential Auger-electron transitions that increase the ion's
charge state until the final state is reached. In molecules the charge is
spread across the atomic sites, resulting in dissociation to energetic atomic
ions. We have used x-ray/ion coincidence spectroscopy to measure charge states
and energies of I and Br atomic ions following 1s ionization at
the I and Br \textit{K}-edges of IBr. We present the charge states and kinetic
energies of the two correlated fragment ions associated with core-excited
states produced during the various steps of the cascades. To understand the
dynamics leading to the ion data, we develop a computational model that
combines Monte-Carlo/Molecular Dynamics simulations with a classical
over-the-barrier model to track inner-shell cascades and redistribution of
electrons in valence orbitals and nuclear motion of fragments