2,030 research outputs found
Spin-polarized stable phases of the 2-D electron fluid at finite temperatures
The Helmholtz free energy F of the interacting 2-D electron fluid is
calculated nonperturbatively using a mapping of the quantum fluid to a
classical Coulomb fluid [Phys. Rev. Letters, vol. 87, 206404 (2001)]. For
density parameters rs such that rs<~25, the fluid is unpolarized at all
temperatures t=T/EF where EF is the Fermi energy. For lower densities, the
system becomes fully spin polarized for t<~0.35, and partially polarized for
0.35<t< 2, depending on the density. At rs ~25-30, and t ~0.35, an ''ambispin''
phase where F is almost independent of the spin polarization is found. These
results support recent claims, based on quantum Monte Carlo results, for a
stable, fully spin-polarized fluid phase at T = 0 for rs larger than about
25-26.Comment: Latex manuscript (4-5 pages) and two postscript figures; see also
http://nrcphy1.phy.nrc.ca/ims/qp/chandre/chnc
Cosmic-ray propagation properties for an origin in SNRs
We have studied the impact of cosmic-ray acceleration in SNR on the spectra
of cosmic-ray nuclei in the Galaxy using a series expansion of the propagation
equation, which allows us to use analytical solutions for part of the problem
and an efficient numerical treatment of the remaining equations and thus
accurately describes the cosmic-ray propagation on small scales around their
sources in three spatial dimensions and time. We found strong variations of the
cosmic-ray nuclei flux by typically 20% with occasional spikes of much higher
amplitude, but only minor changes in the spectral distribution. The locally
measured spectra of primary cosmic rays fit well into the obtained range of
possible spectra. We further showed that the spectra of the secondary element
Boron show almost no variations, so that the above findings also imply
significant fluctuations of the Boron-to-Carbon ratio. Therefore the commonly
used method of determining CR propagation parameters by fitting
secondary-to-primary ratios appears flawed on account of the variations that
these ratios would show throughout the Galaxy.Comment: Accepted for publication in Ap
Measurement of forward neutral pion transverse momentum spectra for = 7TeV proton-proton collisions at LHC
The inclusive production rate of neutral pions in the rapidity range greater
than has been measured by the Large Hadron Collider forward (LHCf)
experiment during LHC \,TeV proton-proton collision operation in
early 2010. This paper presents the transverse momentum spectra of the neutral
pions. The spectra from two independent LHCf detectors are consistent with each
other and serve as a cross check of the data. The transverse momentum spectra
are also compared with the predictions of several hadronic interaction models
that are often used for high energy particle physics and for modeling
ultra-high-energy cosmic-ray showers.Comment: 18 Pages, 10 figures, submitted to Phys. Rev.
The 2-D electron gas at arbitrary spin polarizations and arbitrary coupling strengths: Exchange-correlation energies, distribution functions and spin-polarized phases
We use a recent approach [Phys. Rev. Letters, {\bf 84}, 959 (2000)] for
including Coulomb interactions in quantum systems via a classical mapping of
the pair-distribution functions (PDFs) for a study of the 2-D electron gas. As
in the 3-D case, the ``quantum temperature'' T_q of a classical 2-D Coulomb
fluid which has the same correlation energy as the quantum fluid is determined
as a function of the density parameter r_s. Spin-dependent exchange-correlation
energies are reported. Comparisons of the spin-dependent pair-distributions and
other calculated properties with any available 2-D quantum Monte Carlo (QMC)
results show excellent agreement, strongly favouring more recent QMC data. The
interesting novel physics brought to light by this study are: (a) the
independently determined quantum-temperatures for 3-D and 2-D are found to be
approximately the same, (i.e, universal) function of the classical coupling
constant Gamma. (b) the coupling constant Gamma increases rapidly with r_s in
2-D, making it comparatively more coupled than in 3-D; the stronger coupling in
2-D requires bridge corrections to the hyper- netted-chain method which is
adequate in 3-D; (c) the Helmholtz free energy of spin-polarized and
unpolarized phases have been calculated. The existence of a spin-polarized 2-D
liquid near r_s = 30, is found to be a marginal possibility. These results
pertain to clean uniform 2-D electron systems.Comment: This paper replaces the cond-mat/0109228 submision; the new version
include s more accurate numerical evaluation of the Helmholtz energies of the
para- and ferromagentic 2D fluides at finite temperatures. (Paper accepted
for publication in Phys. Rev. Lett.
The Path Integral Monte Carlo Calculation of Electronic Forces
We describe a method to evaluate electronic forces by Path Integral Monte
Carlo (PIMC). Electronic correlations, as well as thermal effects, are included
naturally in this method. For fermions, a restricted approach is used to avoid
the ``sign'' problem. The PIMC force estimator is local and has a finite
variance. We applied this method to determine the bond length of H and the
chemical reaction barrier of H+HH+H. At low
temperature, good agreement is obtained with ground state calculations. We
studied the proton-proton interaction in an electron gas as a simple model for
hydrogen impurities in metals. We calculated the force between the two protons
at two electronic densities corresponding to Na () and Al
() using a supercell with 38 electrons. The result is compared to
previous calculations. We also studied the effect of temperature on the
proton-proton interaction. At very high temperature, our result agrees with the
Debye screening of electrons. As temperature decreases, the Debye theory fails
both because of the strong degeneracy of electrons and most importantly, the
formation of electronic bound states around the protons.Comment: 18 pages, 10 figure
The performance of the LHCf detector for hadronic showers
The Large Hadron Collider forward (LHCf) experiment has been designed to use
the LHC to benchmark the hadronic interaction models used in cosmic-ray
physics. The LHCf experiment measures neutral particles emitted in the very
forward region of LHC collisions. In this paper, the performances of the LHCf
detectors for hadronic showers was studied with MC simulations and beam tests.
The detection efficiency for neutrons is from 60% to 70% above 500 GeV. The
energy resolutions are about 40% and the position resolution is 0.1 to 1.3mm
depend on the incident energy for neutrons. The energy scale determined by the
MC simulations and the validity of the MC simulations were examined using 350
GeV proton beams at the CERN-SPS.Comment: 15pages, 19 figure
Measurement of zero degree single photon energy spectra for sqrt(s) = 7TeV proton-proton collisions at LHC
In early 2010, the Large Hadron Collider forward (LHCf) experiment measured
very forward neutral particle spectra in LHC proton-proton collisions. From a
limited data set taken under the best beam conditions (low beam-gas background
and low occurance of pile-up events), the single photon spectra at sqrt(s)=7TeV
and pseudo-rapidity (eta) ranges from 8.81 to 8.99 and from 10.94 to infinity
were obtained for the first time and are reported in this paper. The spectra
from two independent LHCf detectors are consistent with one another and serve
as a cross check of the data. The photon spectra are also compared with the
predictions of several hadron interaction models that are used extensively for
modeling ultra high energy cosmic ray showers. Despite conservative estimates
for the systematic errors, none of the models agree perfectly with the
measurements. A notable difference is found between the data and the DPMJET
3.04 and PYTHIA 8.145 hadron interaction models above 2TeV where the models
predict higher photon yield than the data. The QGSJET II-03 model predicts
overall lower photon yield than the data, especially above 2TeV in the rapidity
range 8.81<eta<8.99
Circadian rhythm of hepatic cytosolic and nuclear estrogen receptors
The distribution of estrogen receptor between the cytosolic and nuclear compartments were evaluated in liver of male rats to determine whether a circadian rhythm exists. Cytosolic receptor reached a maximum level at 400 hours and a minimum at 2000 and 2400 hr. Nuclear receptor reached a maximum level at 800 hr and was lowest at 1600 and 2000 hr. Serum estradiol levels were also highest at 800 hr and lowest at 1600 hr. The variations in cytosolic and nuclear receptors are not reciprocal; in fact, the overall content of receptor in the liver is not constant and also displays a circadian rhythm. © 1986 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted
Thermal Density Functional Theory in Context
This chapter introduces thermal density functional theory, starting from the
ground-state theory and assuming a background in quantum mechanics and
statistical mechanics. We review the foundations of density functional theory
(DFT) by illustrating some of its key reformulations. The basics of DFT for
thermal ensembles are explained in this context, as are tools useful for
analysis and development of approximations. We close by discussing some key
ideas relating thermal DFT and the ground state. This review emphasizes thermal
DFT's strengths as a consistent and general framework.Comment: Submitted to Spring Verlag as chapter in "Computational Challenges in
Warm Dense Matter", F. Graziani et al. ed
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