1,969 research outputs found
Aging to Equilibrium Dynamics of SiO2
Molecular dynamics computer simulations are used to study the aging dynamics
of SiO2 (modeled by the BKS model). Starting from fully equilibrated
configurations at high temperatures T_i =5000K/3760K the system is quenched to
lower temperatures T_f=2500K, 2750K, 3000K, 3250K and observed after a waiting
time t_w. Since the simulation runs are long enough to reach equilibrium at
T_f, we are able to study the transition from out-of-equilibrium to equilibrium
dynamics. We present results for the partial structure factors, for the
generalized incoherent intermediate scattering function C_q(t_w, t_w+t), and
for the mean square displacement msd(t_w,t_w+t). We conclude that there are
three different t_w regions: (I) At very short waiting times, C_q(t_w, t_w+t)
decays very fast without forming a plateau. Similarly msd(t_w,t_w+t) increases
without forming a plateau. (II) With increasing t_w a plateau develops in
C_q(t_w, t_w+t) and msd(t_w,t_w+t). For intermediate waiting times the plateau
height is independent of t_w and T_i. Time superposition applies, i.e.
C_q=C_q(t/t_r) where t_r=t_r(t_w) is a waiting time dependent decay time.
Furthermore C_q=C(q,t_w,t_w+t) scales as C_q=C(q,z(t_w,t) where z is a function
of t_w and t only, i.e. independent of q. (III) At large t_w the system reaches
equilibrium, i.e. C_q(t_w,t_w+t) and msd(t_w,t_w+t) are independent of t_w and
T_i. For C_q(t_w,t_w+t) we find that the time superposition of intermediate
waiting times (II) includes the equilibrium curve (III).Comment: 9 pages, 11 figures, submission to PR
Optimized stray-field-induced enhancement of the electron spin precession by buried Fe gates
The magnetic stray field from Fe gates is used to modify the spin precession
frequency of InGaAs/GaAs quantum-well electrons in an external magnetic field.
By using an etching process to position the gates directly in the plane of the
quantum well, the stray-field influence on the spin precession increases
significantly compared with results from previous studies with top-gated
structures. In line with numerical simulations, the stray-field-induced
precession frequency increases as the gap between the ferromagnetic gates is
reduced. The inhomogeneous stray field leads to additional spin dephasing.Comment: 4 pages, 2 figure
The CO\u3csub\u3e2\u3c/sub\u3e Concentration of Air Trapped in Greenland Ice Sheet Project 2 Ice Formed During Periods of Rapid Climate Change
The CO2 content of air occluded in Greenland Ice Sheet Project 2 (GISP2) ice formed over two separate intervals of rapidly changing climate, centered at approximately 46 and 63 kyr B. P., is as much as 90 ppm more during warm periods (interstadials) than during cold periods (stadials). These CO2 variations are superimposed on changes in annual layer thickness and δ18O of the ice and do not show the 200- to 700-year offsets which would be expected for concurrent variations in the atmosphere and the ice. The CO2 concentrations during the stadials are similar to the atmospheric values recorded by Antarctic ice of the same age, so processes occurring in the ice after bubble enclosure must be enriching the air trapped in GISP2 ice formed during the interstadials. This conclusion is supported by Ca content and electrical conductivity measurements of the ice, which show that adequate carbonate is present to produce these enrichments and that CO2 content is high only when the electrical conductivity (a proxy for H+ concentration) is high. High-resolution mapping of one 4-cm section of ice shows a 200-ppm increase in the CO2 content of the trapped air, from approximately 275 to 475 ppm. Analyses of the total inorganic carbon of ice from both the LGM and Holocene show that most of the Ca in the ice is from CaCO3 and that the δ13CO2 approaches that of soil and marine carbonates. These results show that the CO2 record preserved in ice can be altered by in situ decarbonation reactions and that only ice containing either abundant carbonate or essentially no carbonate contains a reliable record of paleoatmospheric CO2
Nonlinear interfacial waves in a constant-vorticity planar flow over variable depth
Exact Lagrangian in compact form is derived for planar internal waves in a
two-fluid system with a relatively small density jump (the Boussinesq limit
taking place in real oceanic conditions), in the presence of a background shear
current of constant vorticity, and over arbitrary bottom profile. Long-wave
asymptotic approximations of higher orders are derived from the exact
Hamiltonian functional in a remarkably simple way, for two different
parametrizations of the interface shape.Comment: revtex, 4.5 pages, minor corrections, summary added, accepted to JETP
Letter
Historical Biomass Burning: Late 19th Century Pioneer Agriculture Revolution in Northern Hemisphere Ice Core Data and its Atmospheric Interpretation
Ice core data from Yukon and Greenland spanning from ∼1750 to 1950 indicate that between ∼1850 and ≤1910 a clear atmospheric signal exists of an episodic biomass burning event that is referred to as the Pioneer Agriculture Revolution. This is best seen in NH4+ ion and particulate concentrations but also in some limited black carbon concentration data, where for all three quantities maximum levels reach about 3 times the prerevolution background concentrations. Tree cellulose δ13C data and some early, controversial, French, air CO2 data, occurring within the same time interval, are interpreted as providing other independent evidence for the same, mainly North American, late 19th century biomass burning event. Some hitherto problematic northern hemisphere ice core derived CO2 concentration data may now be interpreted as containing a biomass burn signal, and these data are compared, especially as to the time of occurrence, with all the other results. A global carbon cycle model simulation of atmospheric CO2 mixing ratios using a maximum input of 3 Gt(C)/yr at northern midlatitudes produces “anomalous” CO2 levels close to some of the ice core carbon dioxide values. However, other values in this data set do not reasonably represent fully mixed atmospheric values. This suggests that these values might be transients but still “tracers” for biomass burning. Nevertheless, it appears possible that interhemispheric CO2 gradients of similar magnitude to the present one could have existed briefly late last century
Synthesizing gas-filled fiber Raman lines enables access to the molecular fingerprint region
The synthesis of multiple narrow optical spectral lines, precisely and
independently tuned across the near- to mid-infrared (IR) region, is a pivotal
research area that enables selective and real-time detection of trace gas
species within complex gas mixtures. However, existing methods for developing
such light sources suffer from limited flexibility and very low pulse energy,
particularly in the mid-IR domain. Here, we introduce a new concept based on
the gas-filled anti-resonant hollow-core fiber (ARHCF) technology that enables
the synthesis of multiple independently tunable spectral lines with high pulse
energy of >1 {\mu}J and a few nanoseconds pulse width in the near- and mid-IR
region. The number and wavelengths of the generated spectral lines can be
dynamically reconfigured. A proof-of-concept laser beam synthesized of two
narrow spectral lines at 3.99 {\mu}m and 4.25 {\mu}m wavelengths is
demonstrated and combined with photoacoustic (PA) modality for real-time SO2
and CO2 detection. The proposed concept also constitutes a promising way for IR
multispectral microscopic imaging.Comment: 39 page
Record Drilling Depth Struck in Greenland
On July 1, 1993, after 5 years of drilling, the Greenland Ice Sheet Project (GISP2) penetrated several meters of silty ice and reached bedrock at a depth of 3053.4 m. It then penetrated 1.5 m into the bedrock, producing the deepest ice core ever recovered (Figure 1).
In July 1992, a nearby European ice coring effort, the Greenland Ice Core Project (GRIP), reached an ice depth of 3028.8 m, providing more than 250,000 years of record. Comparisons between these ice core records have already demonstrated the remarkable reproducibility of the upper ∼90% of the records unparalleled view of climatic and environmental change
The design of organic catalysis for epoxidation by hydrogen peroxide
The potential of various organic species to catalyze epoxidation of ethene by hydrogen peroxide is explored with B3LYP/6-31G* DFT calculations
Determination of the b quark mass at the M_Z scale with the DELPHI detector at LEP
An experimental study of the normalized three-jet rate of b quark events with
respect to light quarks events (light= \ell \equiv u,d,s) has been performed
using the CAMBRIDGE and DURHAM jet algorithms. The data used were collected by
the DELPHI experiment at LEP on the Z peak from 1994 to 2000. The results are
found to agree with theoretical predictions treating mass corrections at
next-to-leading order. Measurements of the b quark mass have also been
performed for both the b pole mass: M_b and the b running mass: m_b(M_Z). Data
are found to be better described when using the running mass. The measurement
yields: m_b(M_Z) = 2.85 +/- 0.18 (stat) +/- 0.13 (exp) +/- 0.19 (had) +/- 0.12
(theo) GeV/c^2 for the CAMBRIDGE algorithm. This result is the most precise
measurement of the b mass derived from a high energy process. When compared to
other b mass determinations by experiments at lower energy scales, this value
agrees with the prediction of Quantum Chromodynamics for the energy evolution
of the running mass. The mass measurement is equivalent to a test of the
flavour independence of the strong coupling constant with an accuracy of 7
permil.Comment: 24 pages, 10 figures, Accepted by Eur. Phys. J.
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