5,480 research outputs found
Velocity dependence of friction of confined polymers
We present molecular dynamics friction calculations for confined hydrocarbon
solids with molecular lengths from 20 to 1400 carbon atoms. Two cases are
considered: (a) polymer sliding against a hard substrate, and (b) polymer
sliding on polymer. We discuss the velocity dependence of the frictional shear
stress for both cases. In our simulations, the polymer films are very thin
(approx. 3 nm), and the solid walls are connected to a thermostat at a short
distance from the polymer slab. Under these circumstances we find that
frictional heating effects are not important, and the effective temperature in
the polymer film is always close to the thermostat temperature. In the first
setup (a), for hydrocarbons with molecular lengths from 60 to 1400 carbon
atoms, the shear stresses are nearly independent of molecular length, but for
the shortest hydrocarbon C20H42 the frictional shear stress is lower. In all
cases the frictional shear stress increases monotonically with the sliding
velocity. For polymer sliding on polymer [case (b)] the friction is much
larger, and the velocity dependence is more complex. For hydrocarbons with
molecular lengths from 60 to 140 C-atoms, the number of monolayers of lubricant
increases (abruptly) with increasing sliding velocity (from 6 to 7 layers),
leading to a decrease of the friction. Before and after the layering
transition, the frictional shear stresses are nearly proportional to the
logarithm of sliding velocity. For the longest hydrocarbon (1400 C-atoms) the
friction shows no dependence on the sliding velocity, and for the shortest
hydrocarbon (20 C-atoms) the frictional shear stress increases nearly linearly
with the sliding velocity.Comment: 10 pages, 14 figure
Photon generation in an electromagnetic cavity with a time-dependent boundary
We report the observation of photon generation in a microwave cavity with a
time-dependent boundary condition. Our system is a microfabricated quarter-wave
coplanar waveguide cavity. The electrical length of the cavity is varied using
the tunable inductance of a superconducting quantum interference device. It is
measured in the quantum regime, where the temperature is significantly less
than the resonance frequency (~ 5 GHz). When the length is modulated at
approximately twice the static resonance frequency, spontaneous oscillations of
the cavity field are observed. Time-resolved measurements of the dynamical
state of the cavity show multiple stable states. The behavior is well described
by theory. Connections to the dynamical Casimir effect are discussed.Comment: 5 pages, 3 Figure
The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars
(Abridged) The water deuterium fractionation (HDO/HO abundance ratio) has
traditionally been used to infer the amount of water brought to Earth by
comets. Measuring this ratio in deeply-embedded low-mass protostars makes it
possible to probe the critical stage when water is transported from clouds to
disks in which icy bodies are formed. We present sub-arcsecond resolution
observations of HDO in combination with HO from the PdBI toward the
three low-mass protostars NGC 1333-IRAS 2A, IRAS 4A-NW, and IRAS 4B. The
resulting HDO/HO ratio is for IRAS 2A,
for IRAS 4A-NW, and for IRAS
4B. Derived ratios agree with radiative transfer models within a factor of 2-4
depending on the source. Our HDO/HO ratios for the inner regions (where
K) of four young protostars are only a factor of 2 higher than those
found for pristine, solar system comets. These small differences suggest that
little processing of water occurs between the deeply embedded stage and the
formation of planetesimals and comets.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Observation of resonance trapping in an open microwave cavity
The coupling of a quantum mechanical system to open decay channels has been
theoretically studied in numerous works, mainly in the context of nuclear
physics but also in atomic, molecular and mesoscopic physics. Theory predicts
that with increasing coupling strength to the channels the resonance widths of
all states should first increase but finally decrease again for most of the
states. In this letter, the first direct experimental verification of this
effect, known as resonance trapping, is presented. In the experiment a
microwave Sinai cavity with an attached waveguide with variable slit width was
used.Comment: to be published in Phys. Rev. Let
Phase transitions in open quantum systems
We consider the behaviour of open quantum systems in dependence on the
coupling to one decay channel by introducing the coupling parameter
being proportional to the average degree of overlapping. Under critical
conditions, a reorganization of the spectrum takes place which creates a
bifurcation of the time scales with respect to the lifetimes of the resonance
states. We derive analytically the conditions under which the reorganization
process can be understood as a second-order phase transition and illustrate our
results by numerical investigations. The conditions are fulfilled e.g. for a
picket fence with equal coupling of the states to the continuum. Energy
dependencies within the system are included. We consider also the generic case
of an unfolded Gaussian Orthogonal Ensemble. In all these cases, the
reorganization of the spectrum occurs at the critical value of
the control parameter globally over the whole energy range of the spectrum. All
states act cooperatively.Comment: 28 pages, 22 Postscript figure
Theory of friction: contribution from fluctuating electromagnetic field
We calculate the friction force between two semi-infinite solids in relative
parallel motion (velocity ), and separated by a vacuum gap of width . The
friction force result from coupling via a fluctuating electromagnetic field,
and can be considered as the dissipative part of the van der Waals interaction.
We consider the dependence of the friction force on the temperature , and
present a detailed discussion of the limiting cases of small and large and
.Comment: 15 pages, No figure
A photoelectron spectroscopy study of the electronic structure evolution in CuInSe2-related compounds at changing copper content
Evolution of the valence-band structure at gradually increasing copper content has been analysed by x-ray photoelectron spectroscopy (XPS) in In2Se3, CuIn5Se8, CuIn3Se5, and CuInSe2 single crystals. A comparison of these spectra with calculated total and angular-momentum resolved density-of-states (DOS) revealed the main trends of this evolution. The formation of the theoretically predicted gap between the bonding and non-bonding states has been observed in both experimental XPS spectra and theoretical DOS
Evidence for the absence of regularization corrections to the partial-wave renormalization procedure in one-loop self energy calculations in external fields
The equivalence of the covariant renormalization and the partial-wave
renormaliz ation (PWR) approach is proven explicitly for the one-loop
self-energy correction (SE) of a bound electron state in the presence of
external perturbation potentials. No spurious correctio n terms to the
noncovariant PWR scheme are generated for Coulomb-type screening potentia ls
and for external magnetic fields. It is shown that in numerical calculations of
the SE with Coulombic perturbation potential spurious terms result from an
improper treatment of the unphysical high-energy contribution. A method for
performing the PWR utilizing the relativistic B-spline approach for the
construction of the Dirac spectrum in external magnetic fields is proposed.
This method is applied for calculating QED corrections to the bound-electron
-factor in H-like ions. Within the level of accuracy of about 0.1% no
spurious terms are generated in numerical calculations of the SE in magnetic
fields.Comment: 22 pages, LaTeX, 1 figur
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