5,206 research outputs found
Heavy-Fermions in a Transition-Metal Compound:
The recent discovery of heavy-Fermion properties in Lithium Vanadate and the
enormous difference in its properties from the properties of Lithium Titanate
as well as of the manganite compounds raise some puzzling questions about
strongly correlated Fermions. These are disscussed as well as a solution to the
puzzles provided.Comment: late
A Distributed Parameter Model for a Solid Oxide Fuel Cell: Simulating Realistic Operating Conditions
We present a detailed multiphysics model capable of simulating the dyn
amic behavior
of a solid oxide fuel cell (SOFC). This model includes a description of a
ll the important physical
and chemical processes in a fuel cell: fluid flow, mass and heat trans
fer, electronic and ionic
potential fields, as well as the chemical and electrochemical react
ions. The resulting highly
nonlinear, coupled system of differential equations is solved using a fi
nite volume discretization.
Our interest lies in simulating realistic operating conditions with the obj
ective of high efficiency
operation at high fuel utilization. While there are a number of studies
in the literature that
present multiphysics models for SOFCs, few have focused on simulat
ing operating conditions
that are necessary if SOFC systems are to realize their promise of h
igh efficiency conversion of
chemical energy to electrical energy. In this report we present s
imulation results at operating
conditions that approach the required ranges of power density an
d overall efficiency. Our results
include a) the temperature and composition profiles along a typical f
uel cell in a SOFC stack, b)
the dynamic response of the cell to step changes in the available inpu
t variables. Since models
such as the one presented here are fairly expensive computationa
lly and cannot be directly used
for online model predictive control, one generally looks to use simplifie
d reduced order models
for control. We briefly discuss the implications of our model results o
n the validity of using
reduced models for the control of SOFC stacks to show that avoid
ing operating regions where
well-known degradation modes are activated is non-trivial without u
sing detailed multiphysics
models
Heat and Mass Transfer in MHD Micropolar Fluid in The Presence of Diffusion Thermo and Chemical Reaction
This work is devoted to investigating the influence of diffusion thermo effect on hydromagnetic heat and mass transfer oscillatory flow of a micropolar fluid over an infinite moving vertical permeable plate in a saturated porous medium in the presence of transverse magnetic field and chemical reaction. The dimensionless equations are solved analytically using perturbation technique. The effects of the various fluid flow parameters entering into the problem on the velocity, microrotation, temperature and concentration fields within the boundary layer are discussed with the help of graphs. Also the local skin-friction coefficient, the wall couple stress coefficient, and the rates of heat and mass transfer coefficients are derived and shown in graphs. Comparison of the obtained numerical results is made with existing literature and is found to be in good agreement
Interactions and Scaling in a Disordered Two-Dimensional Metal
We show that a non-Fermi liquid state of interacting electrons in two
dimensions is stable in the presence of disorder and is a perfect conductor,
provided the interactions are sufficiently strong. Otherwise, the disorder
leads to localization as in the case of non-interacting electrons. This
conclusion is established by examining the replica field theory in the weak
disorder limit, but in the presence of arbitrary electron-electron interaction.
Thus, a disordered two-dimensional metal is a perfect metal, but not a Fermi
liquid.Comment: 4 pages, RevTe
Diarrhea, CD4 counts and enteric infections in a hospital - based cohort of HIV-infected patients around Varanasi, India
Background: As most of the studies in HIV patients with diarrhea were cross sectional, focusing on the etiological agents, we are
reporting data on the rate of diarrhea, associations between diarrhea and CD4 counts and variation in frequency of identifying a pathogen
with consistency of diarrhea and duration in a prospective hospital based study. Methods: Stool specimens were obtained between Jan
2001 and April 2003 from HIV infected adults with diarrhea presenting to Infectious Disease clinic, Banaras Hindu University, Varanasi. In
all patients with diarrhea, specimens were examined by microscopy and cultures to identify pathogens. Results: During the study, 630
person years of observations with diarrhea were analyzed. 140 stool samples were collected representing 43% of episodes of reported
diarrhea. Positivity of finding a pathogen from watery stools and formed stools were 40%&24% respectively (p < 0.01) probably
due to associated inflammation is more in watery diarrhea. Patients having chronic diarrhea are 2.25 (95%CI 1.52-2.81) times at more risk
of developing other opportunistic infections compared to those who don't have. However this is not true with the acute diarrhea where risk
of harboring the opportunistic infections remain same. Conclusion: Diarrhea was most strongly associated with low CD4 counts. Over
two-thirds of diarrheal episodes were undiagnosed, suggesting that unidentified agents or primary HIV enteropathy are important causes of
diarrhea in this population. There is a strong negative association between duration of diarrhea and CD4 levels
Up-down instability of binary black holes in numerical relativity
Binary black holes with spins that are aligned with the orbital angular momentum do not precess. However, post-Newtonian calculations predict that "up-down" binaries, in which the spin of the heavier (lighter) black hole is aligned (antialigned) with the orbital angular momentum, are unstable when the spins are slightly perturbed from perfect alignment. This instability provides a possible mechanism for the formation of precessing binaries in environments where sources are preferentially formed with (anti) aligned spins. In this paper, we present the first full numerical relativity simulations capturing this instability. These simulations span orbits and - precession cycles before merger, making them some of the longest numerical relativity simulations to date. Initialized with a small perturbation of -, the instability causes a dramatic growth of the spin misalignments, which can reach near merger. We show that this leaves a strong imprint on the subdominant modes of the gravitational wave signal, which can potentially be used to distinguish up-down binaries from other sources. Finally, we show that post-Newtonian and effective-one-body approximants are able to reproduce the unstable dynamics of up-down binaries extracted from numerical relativity
Multi-connected Momentum Distribution and Fermion Condensation
The structure of the ground state beyond the instability point of the
quasiparticle system with Fermi-step momentum distribution is studied within
the model of a Fermi liquid with a strong repulsive interaction. A ground state
rearrangement occurs as the interaction strength is increased beyond a definite
critical value. Numerical investigation of the initial stage of this structural
transition shows that there are two temperature regions, corresponding to
different scenarios of the rearrangement. While for temperature T larger than
some characteristic temperature T_0 the behaviour of the system is the same as
that in the case of the fermion condensation, for T<T_0 the intermediate
structure with multi-connected quasiparticle momentum distribution arises. The
transition of this structure to the fermion condensate at increasing
interaction strength is discussed.Comment: 10 pages, 8 postscript figure
Theory of the Optical Conductivity in the Cuprate Superconductors
We present a study of the normal state optical conductivity in the cuprate
superconductors using the nearly antiferromagnetic Fermi liquid (NAFL)
description of the magnetic interaction between their planar quasiparticles. We
find that the highly anisotropic scattering rate in different regions of the
Brillouin zone, both as a function of frequency and temperature, a benchmark of
NAFL theory, leads to an average relaxation rate of the Marginal Fermi Liquid
form for overdoped and optimally doped systems, as well as for underdoped
systems at high temperatures. We carry out numerical calculations of the
optical conductivity for several compounds for which the input spin fluctuation
parameters are known. Our results, which are in agreement with experiment on
both overdoped and optimally doped systems, show that NAFL theory explains the
anomalous optical behavior found in these cuprate superconductors.Comment: REVTEX file, 8 PostScript figure
Low-Frequency Quantum Oscillations due to Strong Electron Correlations
The normal-state energy spectrum of the two-dimensional - model in a
homogeneous perpendicular magnetic field is investigated. The density of states
at the Fermi level as a function of the inverse magnetic field
reveals oscillations in the range of hole concentrations . The
oscillations have both high- and low-frequency components. The former
components are connected with large Fermi surfaces, while the latter with van
Hove singularities in the Landau subbands, which traverse the Fermi level with
changing . The singularities are related to bending the Landau subbands due
to strong electron correlations. Frequencies of these components are of the
same order of magnitude as quantum oscillation frequencies observed in
underdoped cuprates.Comment: 10 pages, 3 figures, Proc. NSS-2013, Yalta. arXiv admin note: text
overlap with arXiv:1308.056
- …