854 research outputs found
Evaporation and Step Edge Diffusion in MBE
Using kinetic Monte-Carlo simulations of a Solid-on-Solid model we
investigate the influence of step edge diffusion (SED) and evaporation on
Molecular Beam Epitaxy (MBE). Based on these investigations we propose two
strategies to optimize MBE-growth. The strategies are applicable in different
growth regimes: during layer-by-layer growth one can reduce the desorption rate
using a pulsed flux. In three-dimensional (3D) growth the SED can help to grow
large, smooth structures. For this purpose the flux has to be reduced with time
according to a power law.Comment: 5 pages, 2 figures, latex2e (packages: elsevier,psfig,latexsym
The dynamical equivalence of modified gravity revisited
We revisit the dynamical equivalence between different representations of
vacuum modified gravity models in view of Legendre transformations. The
equivalence is discussed for both bulk and boundary space, by including in our
analysis the relevant Gibbons-Hawking terms. In the f(R) case, the Legendre
transformed action coincides with the usual Einstein frame one. We then
re-express the R+f(G) action, where G is the Gauss-Bonnet term, as a second
order theory with a new set of field variables, four tensor fields and one
scalar and study its dynamics. For completeness, we also calculate the
conformal transformation of the full Jordan frame R+f(G) action. All the
appropriate Gibbons-Hawking terms are calculated explicitly.Comment: 17 pages; v3: Revised version. New comments added in Sections 3 & 5.
New results added in Section 6. Version to appear in Class. Quantum Gravit
Theory for the ultrafast ablation of graphite films
The physical mechanisms for damage formation in graphite films induced by
femtosecond laser pulses are analyzed using a microscopic electronic theory. We
describe the nonequilibrium dynamics of electrons and lattice by performing
molecular dynamics simulations on time-dependent potential energy surfaces. We
show that graphite has the unique property of exhibiting two distinct laser
induced structural instabilities. For high absorbed energies (> 3.3 eV/atom) we
find nonequilibrium melting followed by fast evaporation. For low intensities
above the damage threshold (> 2.0 eV/atom) ablation occurs via removal of
intact graphite sheets.Comment: 5 pages RevTeX, 3 PostScript figures, submitted to Phys. Re
Creation of the universe with a stealth scalar field
The stealth scalar field is a non-trivial configuration without any
back-reaction to geometry, which is characteristic for non-minimally coupled
scalar fields. Studying the creation probability of the de Sitter universe with
a stealth scalar field by the Hartle and Hawking's semi-classical method, we
show that the effect of the stealth field can be significant. For the class of
scalar fields we consider, creation with a stealth field is possible for a
discrete value of the coupling constant and its creation probability is always
less than that with a trivial scalar field. However, those creation rates can
be almost the same depending on the parameters of the theory.Comment: 7 pages; v2, references added; v3, creation of the open universe
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REMOTE OPERATION OF THE WEST COAST AND ALASKA TSUNAMI WARNING CENTER
The remote control of real time derivation of earthquake location and magnitude and the issuance of tsunami and earthquake bulletins was done using off-the-shelf remote control software and hardware. Such remote operation of the West Coast/Alaska Tsunami Warning Center can decrease the time needed to respond to an earthquake by eliminating travel from the duty standers’ home to the tsunami warning center
Silicon clusters produced by femtosecond laser ablation: Non-thermal emission and gas-phase condensation
Neutral silicon clusters Si_n (up to n = 7) and their cations Si_n+ (up to n
= 10) have been produced by femtosecond laser ablation of bulk silicon in
vacuum and investigated using time-of-flight mass spectrometry. Two populations
of the Si_n+ clusters with different velocity and abundance distributions in
the ablation plume have been clearly distinguished. Possible mechanisms of
cluster formation (Coulomb explosion, gas-phase condensation, phase explosion)
are discussed
Assessment of Alertness and Cognitive Performance of Closed Circuit Rebreather Divers With the Critical Flicker Fusion Frequency Test in Arctic Diving Conditions
Introduction: Cold water imposes many risks to the diver. These risks include decompression illness, physical and cognitive impairment, and hypothermia. Cognitive impairment can be estimated using a critical flicker fusion frequency (CFFF) test, but this method has only been used in a few studies conducted in an open water environment. We studied the effect of the cold and a helium-containing mixed breathing gas on the cognition of closed circuit rebreather (CCR) divers. Materials and Methods: Twenty-three divers performed an identical dive with controlled trimix gas with a CCR device in an ice-covered quarry. They assessed their thermal comfort at four time points during the dive. In addition, their skin temperature was measured at 5-min intervals throughout the dive. The divers performed the CFFF test before the dive, at target depth, and after the dive. Results: A statistically significant increase of 111.7% in CFFF values was recorded during the dive compared to the pre-dive values (p < 0.0001). The values returned to the baseline after surfacing. There was a significant drop in the divers' skin temperature of 0.48 degrees C every 10 min during the dive (p < 0.001). The divers' subjectively assessed thermal comfort also decreased during the dive (p = 0.01). Conclusion: Our findings showed that neither extreme cold water nor helium-containing mixed breathing gas had any influence on the general CFFF profile described in the previous studies from warmer water and where divers used other breathing gases. We hypothesize that cold-water diving and helium-containing breathing gases do not in these diving conditions cause clinically relevant cerebral impairment. Therefore, we conclude that CCR diving in these conditions is safe from the perspective of alertness and cognitive performance.Peer reviewe
Spectroscopy with the Engineering Development Array: cold H at 63 MHz towards the Galactic Centre
The Engineering Development Array (EDA) is a single test station for Square
Kilometre Array (SKA) precursor technology. We have used the EDA to detect
low-frequency radio recombination lines (RRLs) from the Galactic Centre region.
Low-frequency RRLs are an area of interest for future low-frequency SKA work as
these lines provide important information on the physical properties of the
cold neutral medium. In this project we investigate the EDA, its bandpass and
the radio frequency interference environment for low-frequency spectroscopy. We
present line spectra from 30 to 325 MHz for the Galactic Centre region. The
decrease in sensitivity for the EDA at the low end of the receiver prevents
carbon and hydrogen RRLs to be detected below 40 and 60 MHz respectively. RFI
strongly affects frequencies in the range 276-292, 234-270, 131-138, 95-102 and
below 33 MHz. Cn RRLs were detected in absorption for quantum levels n
= 378 to 550 (39-121 MHz) and in emission for n = 272 to 306 (228-325 MHz).
Cn lines were detected in absorption for n = 387 to 696 (39-225 MHz).
Hn RRLs were detected in emission for n = 272 to 480 (59-325 MHz).
Hn lines were detected for n = 387 to 453 (141-225 MHz). The stacked
Hn detection at 63 MHz is the lowest frequency detection made for
hydrogen RRLs and shows that a cold (partially) ionized medium exists along the
line of sight to the Galactic Centre region. The size and velocity of this cold
H gas indicates that it is likely associated with the nearby
Riegel-Crutcher cloud.Comment: 18 pages, 6 figures and 5 table
The conformal frame freedom in theories of gravitation
It has frequently been claimed in the literature that the classical physical
predictions of scalar tensor theories of gravity depend on the conformal frame
in which the theory is formulated. We argue that this claim is false, and that
all classical physical predictions are conformal-frame invariants. We also
respond to criticisms by Vollick [gr-qc/0312041], in which this issue arises,
of our recent analysis of the Palatini form of 1/R gravity.Comment: 9 pages, no figures, revtex; final published versio
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