429 research outputs found
Magnetic characterization of the frustrated three-leg ladder compound [(CuCl2tachH)3Cl]Cl2
We report the magnetic features of a new one-dimensional stack of
antiferromagnetically coupled equilateral copper(II) triangles. High-field
magnetization measurements show that the interaction between the copper
triangles is of the same order of magnitude as the intra-triangle exchange
although only coupled via hydrogen bonds. The infinite chain turns out to be an
interesting example of a frustrated cylindrical three-leg ladder with competing
intra- and inter-triangle interactions. We demonstrate that the ground state is
a spin singlet which is gaped from the triplet excitation.Comment: 6 pages, 9 figures, revised version submitted to Phys. Rev. B. More
information at http://obelix.physik.uni-osnabrueck.de/~schnack
Statistical Transfer Matrix Study of the Multileg Ising Ladders and Tubes
Finite temperature properties of symmetric multileg Ising ladders and
tubes are investigated using the statistical transfer matrix method. The
temperature dependences of the specific heat and entropy are calculated. In the
case of tubes, it is found that the ground state entropy shows an even-odd
oscillation with respect to the number of legs. The same type of oscillation is
also found in the ground state energy. On the contrary, these oscillations do
not take place in ladders. From the temperature-dependence of the specific
heat, it is found that the lowest excitation energy is 4J for even-leg ladders
while it is 2J otherwise, The physical origin of these behaviors is discussed
based on the structure of excitations.Comment: 6 pages, 9 figure
A unitary correlation operator method
The short range repulsion between nucleons is treated by a unitary
correlation operator which shifts the nucleons away from each other whenever
their uncorrelated positions are within the replusive core. By formulating the
correlation as a transformation of the relative distance between particle
pairs, general analytic expressions for the correlated wave functions and
correlated operators are given. The decomposition of correlated operators into
irreducible n-body operators is discussed. The one- and two-body-irreducible
parts are worked out explicitly and the contribution of three-body correlations
is estimated to check convergence. Ground state energies of nuclei up to mass
number A=48 are calculated with a spin-isospin-dependent potential and single
Slater determinants as uncorrelated states. They show that the deduced energy-
and mass-number-independent correlated two-body Hamiltonian reproduces all
"exact" many-body calculations surprisingly well.Comment: 43 pages, several postscript figures, uses 'epsfig.cls'. Submitted to
Nucl. Phys. A. More information available at http://www.gsi.de/~fm
Active Selection of Classification Features
Some data analysis applications comprise datasets, where explanatory
variables are expensive or tedious to acquire, but auxiliary data are readily
available and might help to construct an insightful training set. An example is
neuroimaging research on mental disorders, specifically learning a
diagnosis/prognosis model based on variables derived from expensive Magnetic
Resonance Imaging (MRI) scans, which often requires large sample sizes.
Auxiliary data, such as demographics, might help in selecting a smaller sample
that comprises the individuals with the most informative MRI scans. In active
learning literature, this problem has not yet been studied, despite promising
results in related problem settings that concern the selection of instances or
instance-feature pairs.
Therefore, we formulate this complementary problem of Active Selection of
Classification Features (ASCF): Given a primary task, which requires to learn a
model f: x-> y to explain/predict the relationship between an
expensive-to-acquire set of variables x and a class label y. Then, the
ASCF-task is to use a set of readily available selection variables z to select
these instances, that will improve the primary task's performance most when
acquiring their expensive features z and including them to the primary training
set.
We propose two utility-based approaches for this problem, and evaluate their
performance on three public real-world benchmark datasets. In addition, we
illustrate the use of these approaches to efficiently acquire MRI scans in the
context of neuroimaging research on mental disorders, based on a simulated
study design with real MRI data.Comment: Accepted for publication at the 19th Intelligent Data Analysis
Symposium, 2021. The final authenticated publication will be made available
online at springer.co
Free Energy of the Eight Vertex Model with an Odd Number of Lattice Sites
We calculate the bulk contribution for the doubly degenerated largest
eigenvalue of the transfer matrix of the eight vertex model with an odd number
of lattice sites N in the disordered regime using the generic equation for
roots proposed by Fabricius and McCoy. We show as expected that in the
thermodynamic limit the result coincides with the one in the N even case.Comment: 11 pages LaTeX New introduction, Method change
- …