193,594 research outputs found
Thermodynamics and kinetics of the Mg65Cu25Y10 bulk metallic glass forming liquid
The thermodynamics and kinetics of the bulk metallic glass forming Mg65Cu25Y10 liquid were investigated using differential scanning calorimetry and three-point beam bending. The experiments lead to the determination of the thermodynamic functions as well as the viscosity of the supercooled liquid. The viscosity shows a temperature dependence, which is consistent with that of a strong glass similar to Zr–Ti–Cu–Ni–Be bulk metallic glasses or sodium silicate glasses. This contrasts with more fragile conventional metallic glass formers or pure metals. The relatively weak temperature dependence of the thermodynamic functions of the supercooled liquid is related to these sluggish kinetics in the supercooled liquid. Entropy, viscosity, and kinetic glass transition are compared in the frameworks of the fragility concept and the Adam–Gibbs theory. Strong liquid behavior retards the formation of crystals kinetically and thermodynamically
Probing Nuclear Matter with Jet Conversions
We discuss the flavor of leading jet partons as a valuable probe of nuclear
matter. We point out that the coupling of jets to nuclear matter naturally
leads to an alteration of jet chemistry even at high transverse momentum .
In particular, QCD jets coupling to a chemically equilibrated quark gluon
plasma in nuclear collisions, will lead to hadron ratios at high transverse
momentum that can differ significantly from their counterparts in
collisions. Flavor measurements could complement energy loss as a way to study
interactions of hard QCD jets with nuclear matter. Roughly speaking they probe
the inverse mean free path , while energy loss probes the average
squared momentum transfer . We present some estimates for the
rate of jet conversions in a consistent Fokker-Planck framework and their
impact on future high- identified hadron measurements at RHIC and LHC. We
also suggest some novel observables to test flavor effects.Comment: 12 pages, 11 figures, version to appear in PR
Modelling Safety-Related Driving Behaviour - the Impact of Parameter Values
Traffic simulation models make assumptions about the safety-related behaviour of drivers. These
assumptions may or may not replicate the real behaviour of those drivers who adopt seemingly unsafe
behaviour, for example running red lights at signalised intersections or too closely following the vehicles
in front. Such behaviour results in the performance of the system that we observe but will often result in
conflicts and very occasionally in accidents. The question is whether these models should reflect safe behav-
iour or actual behaviour. Good design should seek to enhance safety, but is the safety of a design neces-
sarily enhanced by making unrealistically optimistic assumptions about the safety of drivers behaviour?
This paper explores the questions associated with the choice of values for safety-related parameters in
simulation models. The paper identifies the key parameters of traffic simulation models and notes that sev-
eral of them have been derived from theory or informed guesswork rather than observation of real behav-
iour and that, even where they are based on observations, these may have been conducted in circumstances
quite different to those which now apply. Tests with the micro-simulation model DRACULA demonstrate
the sensitivity of model predictions—and perhaps policy decisions—to the value of some of the key param-
eters. It is concluded that, in general, it is better to use values that are realistic-but-unsafe than values that
are safe-but-unrealistic. Although the use of realistic-but-unsafe parameter values could result in the adop-
tion of unsafe designs, this problem can be overcome by paying attention to the safety aspects of designs.
The possibility of using traffic simulation models to culties involved in doing so are discussed
-adic exponential sums of polynomials in one variable
The -adic exponential sum of a polynomial in one variable is studied. An
explicit arithmetic polygon in terms of the highest two exponents of the
polynomial is proved to be a lower bound of the Newton polygon of the
-function of the T-adic exponential sum. This bound gives lower bounds for
the Newton polygon of the -function of exponential sums of -power order
A Generalised Sidelobe Canceller Architecture Based on Oversampled Subband Decompositions
Adaptive broadband beamforming can be performed in oversampled subband signals, whereby an independent beamformer is operated in each frequency band. This has been shown to result in a considerably reduced computational complexity. In this paper, we primarily investigate the convergence behaviour of the generalised sidelobe canceller (GSC) based on normalised least mean squares algorithm (NLMS) when operated in subbands. The minimum mean squared error can be limited, amongst other factors, by the aliasing present in the subbands. With regard to convergence speed, there is strong indication that the subband-GSC converges faster than a fullband counterpart of similar modelling capabilities. Simulations are presented
Quantum Manifestation of Elastic Constants in Nanostructures
Generally, there are two distinct effects in modifying the properties of
low-dimensional nanostructures: surface effect (SS) due to increased
surface-volume ratio and quantum size effect (QSE) due to quantum confinement
in reduced dimension. The SS has been widely shown to affect the elastic
constants and mechanical properties of nanostructures. Here, using Pb nanofilm
and graphene nanoribbon as model systems, we demonstrate the QSE on the elastic
constants of nanostructures by first-principles calculations. We show that
generally QSE is dominant in affecting the elastic constants of metallic
nanostructures while SS is more pronounced in semiconductor and insulator
nanostructures. Our findings have broad implications in quantum aspects of
nanomechanics
Quantum Statistical Entropy and Minimal Length of 5D Ricci-flat Black String with Generalized Uncertainty Principle
In this paper, we study the quantum statistical entropy in a 5D Ricci-flat
black string solution, which contains a 4D Schwarzschild-de Sitter black hole
on the brane, by using the improved thin-layer method with the generalized
uncertainty principle. The entropy is the linear sum of the areas of the event
horizon and the cosmological horizon without any cut-off and any constraint on
the bulk's configuration rather than the usual uncertainty principle. The
system's density of state and free energy are convergent in the neighborhood of
horizon. The small-mass approximation is determined by the asymptotic behavior
of metric function near horizons. Meanwhile, we obtain the minimal length of
the position which is restrained by the surface gravities and the
thickness of layer near horizons.Comment: 11pages and this work is dedicated to the memory of Professor Hongya
Li
Helium Recombination Lines as a Probe of Abundance and Temperature Problems
The paper presents a simplified formula to determine an electron temperature,
Te(He I), for planetary nebulae (PNe) using the He I 7281/6678 line flux ratio.
In our previous studies of Te(He I) (Zhang et al. 2005), we used the He I line
emission coefficients given by Benjamin et al. (1999). Here we examine the
results of using more recent atomic data presented by Porter et al. (2005). A
good agreement is shown, suggesting that the effect of uncertainties of atomic
data on the resultant Te(He I) is negligible. We also present an analytical
formula to derive electron temperature using the He I discontinuity at 3421 A.
Our analysis shows that Te(He I) values are significantly lower than electron
temperatures deduced from the Balmer jump of H I recombination spectra, Te(H
I), and that inferred from the collisionally excited [O III] nebular-to-auroral
forbidden line flux ratio, Te([O III]). In addition, Te(H I) covers a wider
range of values than either Te(He I) or Te([O III]). This supports the
two-abundance nebular model with hydrogen-deficient material embedded in
diffuse gas of a ``normal'' chemical composition (i.e. ~solar).Comment: 5 pages, 3 figures. To appear in the RevMexAA proceedings of "The
Ninth Texas-Mexico Conference on Astrophysics
DarkHistory: A code package for calculating modified cosmic ionization and thermal histories with dark matter and other exotic energy injections
We present a new public Python package, DarkHistory, for computing the
effects of dark matter annihilation and decay on the temperature and ionization
history of the early universe. DarkHistory simultaneously solves for the
evolution of the free electron fraction and gas temperature, and for the
cooling of annihilation/decay products and the secondary particles produced in
the process. Consequently, we can self-consistently include the effects of both
astrophysical and exotic sources of heating and ionization, and automatically
take into account backreaction, where modifications to the
ionization/temperature history in turn modify the energy-loss processes for
injected particles. We present a number of worked examples, demonstrating how
to use the code in a range of different configurations, in particular for
arbitrary dark matter masses and annihilation/decay final states. Possible
applications of DarkHistory include mapping out the effects of dark matter
annihilation/decay on the global 21cm signal and the epoch of reionization, as
well as the effects of exotic energy injections other than dark matter
annihilation/decay. The code is available at
https://github.com/hongwanliu/DarkHistory with documentation at
https://darkhistory.readthedocs.io . Data files required to run the code can be
downloaded at https://doi.org/10.7910/DVN/DUOUWA .Comment: 40 pages, 17 figure
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