8,886 research outputs found
Schwinger-Boson Mean-Field Theory of Mixed-Spin Antiferromagnet
The Schwinger-boson mean-field theory is used to study the three-dimensional
antiferromagnetic ordering and excitations in compounds , a large
family of quasi-one-dimensional mixed-spin antiferromagnet. To investigate
magnetic properties of these compounds, we introduce a three-dimensional
mixed-spin antiferromagnetic Heisenberg model based on experimental results for
the crystal structure of . This model can explain the experimental
discovery of coexistence of Haldane gap and antiferromagnetic long-range order
below N\'{e}el temperature. Properties such as the low-lying excitations,
magnetizations of and rare-earth ions, N\'{e}el temperatures of different
compounds, and the behavior of Haldane gap below the N\'{e}el temperature are
investigated within this model, and the results are in good agreement with
neutron scattering experiments.Comment: 12 pages, 6 figure
Angle-dependence of the Hall effect in HgBa2CaCu2O6 thin films
Superconducting compounds of the family Hg-Ba-Ca-Cu-O have been the subject
of intense study since the current record-holder for the highest critical
temperature of a superconductor belongs to this class of materials. Thin films
of the compound with two adjacent copper-oxide layers and a critical
temperature of about 120 K were prepared by a two-step process that consists of
the pulsed-laser deposition of precursor films and the subsequent annealing in
mercury-vapor atmosphere. Like some other high-temperature superconductors,
Hg-Ba-Ca-Cu-O exhibits a specific anomaly of the Hall effect, a double-sign
change of the Hall coefficient close to the superconducting transition. We have
investigated this phenomenon by measurements of the Hall effect at different
angles between the magnetic field direction and the crystallographic c-axis.
The results concerning the upper part of the transition, where the first sign
change occurs, are discussed in terms of the renormalized fluctuation model for
the Hall conductivity, adapted through the field rescaling procedure in order
to take into account the arbitrary orientation of the magnetic field.Comment: to be published in Phys. Rev.
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Clustered sparse Bayesian learning
Many machine learning and signal processing tasks involve computing sparse representations using an overcomplete set of features or basis vectors, with compressive sensing-based applications a notable example. While traditionally such problems have been solved individually for different tasks, this strategy ignores strong correlations that may be present in real world data. Consequently there has been a push to exploit these statistical dependencies by jointly solving a series of sparse linear inverse problems. In the majority of the resulting algorithms however, we must a priori decide which tasks can most judiciously be grouped together. In contrast, this paper proposes an integrated Bayesian framework for both clustering tasks together and subsequently learning optimally sparse representations within each cluster. While probabilistic models have been applied previously to solve these types of problems, they typically involve a complex hierarchical Bayesian generative model merged with some type of approximate inference, the combination of which renders rigorous analysis of the underlying behavior virtually impossible. On the other hand, our model subscribes to concrete motivating principles that we carefully evaluate both theoretically and empirically. Importantly, our analyses take into account all approximations that are involved in arriving at the actual cost function to be optimized. Results on synthetic data as well as image recovery from compressive measurements show improved performance over existing methods
Effective matrix model for deconfinement in pure gauge theories
We construct matrix models for the deconfining phase transition in SU(N)
gauge theories, without dynamical quarks, at a nonzero temperature T. We
generalize models with zero and one free parameter to study a model with two
free parameters: besides perturbative terms ~T^4, we introduce terms ~T^2 and
~T^0. The two N-dependent parameters are determined by fitting to data from
numerical simulations on the lattice for the pressure, including the latent
heat. Good agreement is found for the pressure in the semi-quark gluon plasma
(QGP), which is the region from Tc, the critical temperature, to about ~4 Tc.
Above ~1.2 Tc, the pressure is a sum of a perturbative term, ~ +T^4, and a
simple non-perturbative term, essentially just a constant times ~ -Tc^2 T^2.
For the pressure, the details of the matrix model only enter within a very
narrow window, from Tc to ~1.2 Tc, whose width does not change significantly
with N. Without further adjustment, the model also agrees well with lattice
data for the 't Hooft loop. This is notable, because in contrast to the
pressure, the 't Hooft loop is sensitive to the details of the matrix model
over the entire semi-QGP. For the (renormalized) Polyakov loop, though, our
results disagree sharply with those from the lattice. Matrix models provide a
natural and generic explanation for why the deconfining phase transition in
SU(N) gauge theories is of first order not just for three, but also for four or
more colors. Lastly, we consider gauge theories where there is no strict order
parameter for deconfinement, such as for a G(2) gauge group. To agree with
lattice measurements, in the G(2) matrix model it is essential to add terms
which generate complete eigenvalue repulsion in the confining phase.Comment: 80 pages, 26 figure
Gene and Pathway-Based Analysis: Second Wave of Genome-wide Association Studies
Despite great success of GWAS in identification of common genetic variants associated with complex diseases, the current GWAS have focused on single SNP analysis. However, single SNP analysis often identifies a number of the most significant SNPs that account for only a small proportion of the genetic variants and offers limited understanding of complex diseases. To overcome these limitations, we propose gene and pathway-based association analysis as a new paradigm for GWAS. As a proof of concept, we performed a comprehensive gene and pathway-based association analysis for thirteen published GWAS. Our results showed that the proposed new paradigm for GWAS not only identified the genes that include significant SNPs found by single SNP analysis, but also detected new genes in which each single SNP conferred small disease risk, but their joint actions were implicated in the development of diseases. The results also demonstrated that the new paradigm for GWAS was able to identify biologically meaningful pathways associated with the diseases which were confirmed by gene-set rich analysis using gene expression data
Early Science with the Large Millimetre Telescope: Molecules in the Extreme Outflow of a proto-Planetary Nebula
Extremely high velocity emission likely related to jets is known to occur in
some proto-Planetary Nebulae. However, the molecular complexity of this
kinematic component is largely unknown. We observed the known extreme outflow
from the proto-Planetary Nebula IRAS 16342-3814, a prototype water fountain, in
the full frequency range from 73 to 111 GHz with the RSR receiver on the Large
Millimetre Telescope. We detected the molecules SiO, HCN, SO, and CO.
All molecular transitions, with the exception of the latter are detected for
the first time in this source, and all present emission with velocities up to a
few hundred km s. IRAS 16342-3814 is therefore the only source of this
kind presenting extreme outflow activity simultaneously in all these molecules,
with SO and SiO emission showing the highest velocities found of these species
in proto-Planetary Nebulae. To be confirmed is a tentative weak SO component
with a FWHM 700 km s. The extreme outflow gas consists of dense
gas (n 10--10 cm), with a mass larger than
0.02--0.15 M. The relatively high abundances of SiO and SO may
be an indication of an oxygen-rich extreme high velocity gas.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Letter
Electrodynamics of the vanadium oxides VO2 and V2O3
The optical/infrared properties of films of vanadium dioxide (VO2) and
vanadium sesquioxide (V2O3) have been investigated via ellipsometry and
near-normal incidence reflectance measurements from far infrared to ultraviolet
frequencies. Significant changes occur in the optical conductivity of both VO2
and V2O3 across the metal-insulator transitions at least up to (and possibly
beyond) 6 eV. We argue that such changes in optical conductivity and electronic
spectral weight over a broad frequency range is evidence of the important role
of electronic correlations to the metal-insulator transitions in both of these
vanadium oxides. We observe a sharp optical transition with possible final
state (exciton) effects in the insulating phase of VO2. This sharp optical
transition occurs between narrow a1g bands that arise from the
quasi-one-dimensional chains of vanadium dimers. Electronic correlations in the
metallic phases of both VO2 and V2O3 lead to reduction of the kinetic energy of
the charge carriers compared to band theory values, with paramagnetic metallic
V2O3 showing evidence of stronger correlations compared to rutile metallic VO2.Comment: 11 pages, 7 figure
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