45,981 research outputs found
Non-Fermi liquid states in the pressurized system: two critical points
In the archetypal strongly correlated electron superconductor CeCuSi
and its Ge-substituted alloys CeCu(SiGe) two quantum
phase transitions -- one magnetic and one of so far unknown origin -- can be
crossed as a function of pressure \cite{Yuan 2003a}. We examine the associated
anomalous normal state by detailed measurements of the low temperature
resistivity () power law exponent . At the lower critical point
(at , ) depends strongly on Ge
concentration and thereby on disorder level, consistent with a
Hlubina-Rice-Rosch scenario of critical scattering off antiferromagnetic
fluctuations. By contrast, is independent of at the upper quantum
phase transition (at , ), suggesting critical
scattering from local or Q=0 modes, in agreement with a density/valence
fluctuation approach.Comment: 4 pages, including 4 figures. New results added. Significant changes
on the text and Fig.
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Three-dimensional simulation of a new cooling strategy for proton exchange membrane fuel cell stack using a non-isothermal multiphase model
In this study, a new cooling strategy for a proton exchange membrane (PEM) fuel cell stack is investigated using a three-dimensional (3D) multiphase non-isothermal model. The new cooling strategy follows that of the Honda's Clarity design and further extends to a cooling unit every five cells in stacks. The stack consists of 5 fuel cells sharing the inlet and outlet manifolds for reactant gas flows. Each cell has 7-path serpentine flow fields with a counter-flow configuration arranged for hydrogen and air streams. The coolant flow fields are set at the two sides of the stack and are simplified as the convective heat transfer thermal boundary conditions. This study also compares two thermal boundary conditions, namely limited and infinite coolant flow rates, and their impacts on the distributions of oxygen, liquid water, current density and membrane hydration. The difference of local temperature between these two cooling conditions is as much as 6.9 K in the 5-cell stack, while it is only 1.7 K in a single cell. In addition, the increased vapor concentration at high temperature (and hence water saturation pressure) dilutes the oxygen content in the air flow, reducing local oxygen concentration. The higher temperature in the stack also causes low membrane hydration, and consequently poor cell performance and non-uniform current density distribution, as disclosed by the simulation. The work indicates the new cooling strategy can be optimized by increasing the heat transfer coefficient between the stack and coolant to mitigate local overheating and cell performance reduction
Optical spectroscopy study on single crystalline LaFeAsO
Millimeter-sized single crystals of LaFeAsO were grown from NaAs flux and the
in-plane optical properties were studied over a wide frequency range. A sizable
electronic correlation effect was indicated from the analysis of the
free-carrier spectral weight. With decreasing temperature from 300 K, we
observed a continuous suppression of the spectral weight near 0.6 eV. But a
spin-density-wave gap formation at lower energy scale was seen only in the
broken-symmetry state. We elaborate that both the itinerancy and local spin
interactions of Fe\emph{3d} electrons are present for the FeAs-based systems;
however, the establishment of the long-range magnetic order at low temperature
has a dominantly itinerant origin.Comment: 4 figures, 5 page
Horizon Mass Theorem
A new theorem for black holes is found. It is called the horizon mass
theorem. The horizon mass is the mass which cannot escape from the horizon of a
black hole. For all black holes: neutral, charged or rotating, the horizon mass
is always twice the irreducible mass observed at infinity. Previous theorems on
black holes are: 1. the singularity theorem, 2. the area theorem, 3. the
uniqueness theorem, 4. the positive energy theorem. The horizon mass theorem is
possibly the last general theorem for classical black holes. It is crucial for
understanding Hawking radiation and for investigating processes occurring near
the horizon.Comment: A new theorem for black holes is establishe
Investigation of the field-induced ferromagnetic phase transition in spin polarized neutron matter: a lowest order constrained variational approach
In this paper, the lowest order constrained variational (LOCV) method has
been used to investigate the magnetic properties of spin polarized neutron
matter in the presence of strong magnetic field at zero temperature employing
potential. Our results indicate that a ferromagnetic phase transition
is induced by a strong magnetic field with strength greater than ,
leading to a partial spin polarization of the neutron matter. It is also shown
that the equation of state of neutron matter in the presence of magnetic field
is stiffer than the case in absence of magnetic field.Comment: 23 pages, 9 figures Phys. Rev. C (2011) in pres
A comparative study of optical/ultraviolet variability of narrow-line Seyfert 1 and broad-line Seyfert 1 active galactic nuclei
The ensemble optical/ultraviolet variability of narrow-line Seyfert 1 (NLS1)
type active galactic nuclei (AGNs) is investigated, based on a sample selected
from the Sloan Digital Sky Survey (SDSS) Stripe-82 region with multi-epoch
photometric scanning data. As a comparison a control sample of broad-line
Seyfert 1 (BLS1) type AGNs is also incorporated. To quantify properly the
intrinsic variation amplitudes and their uncertainties, a novel method of
parametric maximum-likelihood is introduced, that has, as we argued, certain
virtues over previously used methods. The majority of NLS1-type AGNs exhibit
significant variability on timescales from about ten days to a few years with,
however, on average smaller amplitudes compared to BLS1-type AGNs. About 20
NLS1- type AGNs showing relatively large variations are presented, that may
deserve future monitoring observations, for instance, reverberation mapping.
The averaged structure functions of variability, constructed using the same
maximumlikelihood method, show remarkable similarity in shape for the two types
of AGNs on timescales longer than about 10 days, which can be approximated by a
power-law or an exponential function. This, along with other similar
properties, such as the wavelength-dependent variability, are indicative of a
common dominant mechanism responsible for the long-term optical/UV variability
of both NLS1- and BLS1-type AGNs. Towards the short timescales, however, there
is tentative evidence that the structure function of NLS1-type AGNs continues
declining, whereas that of BLS1-type AGNs flattens with some residual
variability on timescales of days. If this can be confirmed, it may suggest
that an alternative mechanism, such as X-ray reprocessing, starts to become
dominating in BLS1-type AGNs, but not in NLS1-, on such timescales.Comment: 53 pages, 13 figures, 3 tables, accepted for pulication in A
meson effects on neutron stars in the modified quark-meson coupling model
The properties of neutron stars are investigated by including meson
field in the Lagrangian density of modified quark-meson coupling model. The
population with meson is larger than that without
meson at the beginning, but it becomes smaller than that without meson
as the appearance of . The meson has opposite effects on
hadronic matter with or without hyperons: it softens the EOSes of hadronic
matter with hyperons, while it stiffens the EOSes of pure nucleonic matter.
Furthermore, the leptons and the hyperons have the similar influence on
meson effects. The meson increases the maximum masses of
neutron stars. The influence of on the meson effects
are also investigated.Comment: 10 pages, 6 figures, 4 table
From Jeff=1/2 insulator to p-wave superconductor in single-crystal Sr2Ir1-xRuxO4 (0 < x< 1)
Sr2IrO4 is a magnetic insulator assisted by strong spin-orbit coupling (SOC)
whereas the Sr2RuO4 is a p-wave superconductor. The contrasting ground states
have been shown to result from the critical role of the strong SOC in the
iridate. Our investigation of structural, transport, and magnetic properties
reveals that substituting 4d Ru4+ (4d4) ions for 5d Ir4+(5d5) ions in Sr2IrO4
directly adds holes to the t2g bands, reduces the SOC and thus rebalances the
competing energies in single-crystal Sr2Ir1-xRuxO4. A profound effect of Ru
doping driving a rich phase diagram is a structural phase transition from a
distorted I41/acd to a more ideal I4/mmm tetragonal structure near x=0.50 that
accompanies a phase transition from an antiferromagnetic-insulating state to a
paramagnetic-metal state. We also make a comparison drawn with Rh doped
Sr2IrO4, highlighting important similarities and differences.Comment: 18 pages,7 figure
Spectrum of low-lying configurations with negative parity
Spectrum of low-lying five-quark configurations with strangeness quantum
number and negative parity is studied in three kinds of constituent
quark models, namely the one gluon exchange, Goldstone Boson exchange, and
instanton-induced hyperfine interaction models, respectively. Our numerical
results show that the lowest energy states in all the three employed models are
lying at 1800 MeV, about 200 MeV lower than predictions of various
quenched three-quark models. In addition, it is very interesting that the state
with the lowest energy in one gluon exchange model is with spin 3/2, but 1/2 in
the other two models.Comment: Version published in Phys. Rev.
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