2,263 research outputs found
Fate of dissipative hierarchy of timescales in the presence of unitary dynamics
The generic behavior of purely dissipative open quantum many-body systems
with local dissipation processes can be investigated using random matrix
theory, revealing a hierarchy of decay timescales of observables organized by
their complexity as shown in [Wang et al.,
. This hierarchy is reflected in distinct
eigenvalue clusters of the Lindbladian. Here, we analyze how this spectrum
evolves when unitary dynamics is present, both for the case of strongly and
weakly dissipative dynamics. In the strongly dissipative case, the unitary
dynamics can be treated perturbatively and it turns out that the locality of
the Hamiltonian determines how susceptible the spectrum is to such a
perturbation. For the physically most relevant case of (dissipative) two-body
interactions, we find that the correction in the first order of the
perturbation vanishes, leading to the relative robustness of the spectral
features. For weak dissipation, the spectrum flows into clusters with
well-separated eigenmodes, which we identify to be the local symmetries of the
Hamiltonian
Extended point defects in crystalline materials: Ge and Si
B diffusion measurements are used to probe the basic nature of
self-interstitial 'point' defects in Ge. We find two distinct self-interstitial
forms - a simple one with low entropy and a complex one with entropy ~30 k at
the migration saddle point. The latter dominates diffusion at high temperature.
We propose that its structure is similar to that of an amorphous pocket - we
name it a 'morph'. Computational modelling suggests that morphs exist in both
self-interstitial and vacancy-like forms, and are crucial for diffusion and
defect dynamics in Ge, Si and probably many other crystalline solids
Identifying Primordial Substructure in NGC 2264
We present new Spitzer Space Telescope observations of the young cluster
NGC2264. Observations at 24 micron with the Multiband Imaging Photometer has
enabled us to identify the most highly embedded and youngest objects in
NGC2264. This letter reports on one particular region of NGC2264 where bright
24 micron sources are spatially configured in curious linear structures with
quasi-uniform separations. The majority of these sources (~60% are found to be
protostellar in nature with Class I spectral energy distributions. Comparison
of their spatial distribution with sub-millimeter data from Wolf-Chase (2003)
and millimeter data from Peretto et al. (2005) shows a close correlation
between the dust filaments and the linear spatial configurations of the
protostars, indicating that star formation is occurring primarily within dense
dusty filaments. Finally, the quasi-uniform separations of the protostars are
found to be comparable in magnitude to the expected Jeans length suggesting
thermal fragmentation of the dense filamentary material.Comment: Accepted for publication in ApJL, 5 pages, 4 figures. Color version
available from the following webpages: http://cfa-www.harvard.edu/~pteixeir/
and http://cfa-www.harvard.edu/~clada
Attoclock Ptychography
Dedicated simulations show that the application of time-domain ptychography to angular photo-electron streaking data allows shot-to-shot reconstruction of individual X-ray free electron laser pulses. Specifically, in this study, we use an extended ptychographic iterative engine to retrieve both the unknown X-ray pulse and the unknown streak field. We evaluate the quality of reconstruction versus spectral resolution, signal-to-noise and sampling size of the spectrogram
A self-referenced in-situ arrival time monitor for X-ray free-electron lasers
We present a novel, highly versatile, and self-referenced arrival time
monitor for measuring the femtosecond time delay between a hard X-ray pulse
from a free-electron laser and an optical laser pulse, measured directly on the
same sample used for pump-probe experiments. Two chirped and picosecond long
optical supercontinuum pulses traverse the sample with a mutually fixed time
delay of 970 fs, while a femtosecond X-ray pulse arrives at an instant in
between both pulses. Behind the sample the supercontinuum pulses are temporally
overlapped to yield near-perfect destructive interference in the absence of the
X-ray pulse. Stimulation of the sample with an X-ray pulse delivers non-zero
contributions at certain optical wavelengths, which serve as a measure of the
relative arrival time of the X-ray pulse with an accuracy of better than 25 fs.
We find an excellent agreement of our monitor with the existing timing
diagnostics at the SACLA XFEL with a Pearson correlation value of 0.98. We
demonstrate a high sensitivity to measure X-ray pulses with pulse energies as
low as 30 J. Using a free-flowing liquid jet as interaction sample ensures
the full replacement of the sample volume for each X-ray/optical event, thus
enabling its utility even at MHz repetition rate XFEL sources
Bayesian inferencing and deterministic anisotropy for the retrieval of the molecular geometry in gas-phase diffraction experiments
Currently, our general approach to retrieve the molecular geometry from
ultrafast gas-phase diffraction heavily relies on complex geometric simulations
to make conclusive interpretations. In this manuscript, we develop a broadly
applicable ultrafast gas-phase diffraction method that approximates the
molecular frame geometry distribution using Bayesian
Inferencing. This method does not require complex molecular dynamics simulation
and can identify the unique molecular structure. We demonstrate this method's
viability by retrieving the ground state geometry distribution
for both simulated stretched NO and measured ground
state NO. Due to our statistical interpretation, we retrieve a
coordinate-space resolution on the order of 100~fm, depending on signal
quality, an improvement of order 100 compared to commonly used Fourier
transform based methods. By directly measuring the width of
, this is generally only accessible through simulation,
we open ultrafast gas-phase diffraction capabilities to measurements beyond
current analysis approaches. Our method also leverages deterministic ensemble
anisotropy; this provides an explicit dependence on the molecular frame angles.
This method's ability to retrieve the unique molecular structure with high
resolution, and without complex simulations, provides the potential to
effectively turn gas-phase ultrafast diffraction into a discovery oriented
technique, one that probes systems that are prohibitively difficult to
simulate.Comment: 16 pages, 8 figures, 2 tables. Please find the analysis code and
templates for new molecules at https://github.com/khegazy/BIG
Dynamically Driven Evolution of the Interstellar Medium in M51
We report the highest-fidelity observations of the spiral galaxy M51 in CO
emission, revealing the evolution of giant molecular clouds (GMCs) vis-a-vis
the large-scale galactic structure and dynamics. The most massive GMCs
(so-called GMAs) are first assembled and then broken up as the gas flow through
the spiral arms. The GMAs and their H2 molecules are not fully dissociated into
atomic gas as predicted in stellar feedback scenarios, but are fragmented into
smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as
the chains of GMCs that emerge from the spiral arms into interarm regions. The
kinematic shear within the spiral arms is sufficient to unbind the GMAs against
self-gravity. We conclude that the evolution of GMCs is driven by large-scale
galactic dynamics --their coagulation into GMAs is due to spiral arm streaming
motions upon entering the arms, followed by fragmentation due to shear as they
leave the arms on the downstream side. In M51, the majority of the gas remains
molecular from arm entry through the inter-arm region and into the next spiral
arm passage.Comment: 6 pages, including 3 figures. Accepted, ApJ
Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules
Observing the motion of the nuclear wave packets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wave packet in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 Å and temporal resolution of 230 fs full width at half maximum. The method is not only sensitive to the position but also the shape of the nuclear wave packet
Femtosecond gas phase electron diffraction with MeV electrons
We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution
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