1,474 research outputs found
Curvature Dependence of Hydrophobic Hydration Dynamics
We investigate the curvature-dependence of water dynamics in the vicinity of
hydrophobic spherical solutes using molecular dynamics simulations. For both,
the lateral and perpendicular diffusivity as well as for H-bond kinetics of
water in the first hydration shell, we find a non-monotonic solute-size
dependence, exhibiting extrema close to the well-known structural crossover
length scale for hydrophobic hydration. Additionally, we find an apparently
anomalous diffusion for water moving parallel to the surface of small solutes,
which, however, can be explained by topology effects. The intimate connection
between solute curvature, water structure and dynamics has implications for our
understanding of hydration dynamics at heterogeneous biomolecular surfaces.Comment: 10 pages, 9 figure
Solvent fluctuations induce non-Markovian kinetics in hydrophobic pocket-ligand binding
We investigate the impact of water fluctuations on the key-lock association
kinetics of a hydrophobic ligand (key) binding to a hydrophobic pocket (lock)
by means of a minimalistic stochastic model system. It describes the collective
hydration behavior of the pocket by bimodal fluctuations of a water-pocket
interface that dynamically couples to the diffusive motion of the approaching
ligand via the hydrophobic interaction. This leads to a set of overdamped
Langevin equations in 2D-coordinate-space, that is Markovian in each dimension.
Numerical simulations demonstrate locally increased friction of the ligand,
decelerated binding kinetics, and local non-Markovian (memory) effects in the
ligand's reaction coordinate as found previously in explicit-water molecular
dynamics studies of model hydrophobic pocket-ligand binding [1,2]. Our
minimalistic model elucidates the origin of effectively enhanced friction in
the process that can be traced back to long-time decays in the
force-autocorrelation function induced by the effective, spatially fluctuating
pocket-ligand interaction. Furthermore, we construct a generalized 1D-Langevin
description including a spatially local memory function that enables further
interpretation and a semi-analytical quantification of the results of the
coupled 2D-system
Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits
This work is concerned with Al/Al-oxide(AlO)/Al-layer systems which are
important for Josephson-junction-based superconducting devices such as quantum
bits. The device performance is limited by noise, which has been to a large
degree assigned to the presence and properties of two-level tunneling systems
in the amorphous AlO tunnel barrier. The study is focused on the
correlation of the fabrication conditions, nanostructural and nanochemical
properties and the occurrence of two-level tunneling systems with particular
emphasis on the AlO-layer. Electron-beam evaporation with two different
processes and sputter deposition were used for structure fabrication, and the
effect of illumination by ultraviolet light during Al-oxide formation is
elucidated. Characterization was performed by analytical transmission electron
microscopy and low-temperature dielectric measurements. We show that the
fabrication conditions have a strong impact on the nanostructural and
nanochemical properties of the layer systems and the properties of two-level
tunneling systems. Based on the understanding of the observed structural
characteristics, routes are derived towards the fabrication of
Al/AlO/Al-layers systems with improved properties.Comment: 28 pages, 4 figure
The ARGUS Vertex Trigger
A fast second level trigger has been developed for the ARGUS experiment which
recognizes tracks originating from the interaction region. The processor
compares the hits in the ARGUS Micro Vertex Drift Chamber to 245760 masks
stored in random access memories. The masks which are fully defined in three
dimensions are able to reject tracks originating in the wall of the narrow
beampipe of 10.5\,mm radius.Comment: gzipped Postscript, 27 page
Molecular gas heating in Arp 299
Understanding the heating and cooling mechanisms in nearby (Ultra) luminous
infrared galaxies can give us insight into the driving mechanisms in their more
distant counterparts. Molecular emission lines play a crucial role in cooling
excited gas, and recently, with Herschel Space Observatory we have been able to
observe the rich molecular spectrum. CO is the most abundant and one of the
brightest molecules in the Herschel wavelength range. CO transitions are
observed with Herschel, and together, these lines trace the excitation of CO.
We study Arp 299, a colliding galaxy group, with one component harboring an AGN
and two more undergoing intense star formation. For Arp 299 A, we present PACS
spectrometer observations of high-J CO lines up to J=20-19 and JCMT
observations of CO and HCN to discern between UV heating and alternative
heating mechanisms. There is an immediately noticeable difference in the
spectra of Arp 299 A and Arp 299 B+C, with source A having brighter high-J CO
transitions. This is reflected in their respective spectral energy line
distributions. We find that photon-dominated regions (PDRs) are unlikely to
heat all the gas since a very extreme PDR is necessary to fit the high-J CO
lines. In addition, this extreme PDR does not fit the HCN observations, and the
dust spectral energy distribution shows that there is not enough hot dust to
match the amount expected from such an extreme PDR. Therefore, we determine
that the high-J CO and HCN transitions are heated by an additional mechanism,
namely cosmic ray heating, mechanical heating, or X-ray heating. We find that
mechanical heating, in combination with UV heating, is the only mechanism that
fits all molecular transitions. We also constrain the molecular gas mass of Arp
299 A to 3e9 Msun and find that we need 4% of the total heating to be
mechanical heating, with the rest UV heating
Excitation of the molecular gas in the nuclear region of M82
We present high resolution HIFI spectroscopy of the nucleus of the
archetypical starburst galaxy M82. Six 12CO lines, 2 13CO lines and 4
fine-structure lines are detected. Besides showing the effects of the overall
velocity structure of the nuclear region, the line profiles also indicate the
presence of multiple components with different optical depths, temperatures and
densities in the observing beam. The data have been interpreted using a grid of
PDR models. It is found that the majority of the molecular gas is in low
density (n=10^3.5 cm^-3) clouds, with column densities of N_H=10^21.5 cm^-2 and
a relatively low UV radiation field (GO = 10^2). The remaining gas is
predominantly found in clouds with higher densities (n=10^5 cm^-3) and
radiation fields (GO = 10^2.75), but somewhat lower column densities
(N_H=10^21.2 cm^-2). The highest J CO lines are dominated by a small (1%
relative surface filling) component, with an even higher density (n=10^6 cm^-3)
and UV field (GO = 10^3.25). These results show the strength of multi-component
modeling for the interpretation of the integrated properties of galaxies.Comment: Accepted for publication in A&A Letter
Radiative and mechanical feedback into the molecular gas of NGC 253
Starburst galaxies are undergoing intense periods of star formation.
Understanding the heating and cooling mechanisms in these galaxies can give us
insight to the driving mechanisms that fuel the starburst. Molecular emission
lines play a crucial role in the cooling of the excited gas. With SPIRE on the
Herschel Space Observatory we have observed the rich molecular spectrum towards
the central region of NGC 253. CO transitions from J=4-3 to 13-12 are observed
and together with low-J line fluxes from ground based observations, these lines
trace the excitation of CO. By studying the CO excitation ladder and comparing
the intensities to models, we investigate whether the gas is excited by UV
radiation, X-rays, cosmic rays, or turbulent heating. Comparing the CO
and CO observations to large velocity gradient models and PDR models we
find three main ISM phases. We estimate the density, temperature,and masses of
these ISM phases. By adding CO, HCN, and HNC line intensities, we are
able to constrain these degeneracies and determine the heating sources. The
first ISM phase responsible for the low-J CO lines is excited by PDRs, but the
second and third phases, responsible for the mid to high-J CO transitions,
require an additional heating source. We find three possible combinations of
models that can reproduce our observed molecular emission. Although we cannot
determine which of these are preferable, we can conclude that mechanical
heating is necessary to reproduce the observed molecular emission and cosmic
ray heating is a negligible heating source. We then estimate the mass of each
ISM phase; M for phase 1 (low-J CO lines), M for phase 2 (mid-J CO lines), and M for
phase 3 (high-J CO lines) for a total system mass of M
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