232,088 research outputs found
Low-energy parameters and spin gap of a frustrated spin- Heisenberg antiferromagnet with on the honeycomb lattice
The coupled cluster method is implemented at high orders of approximation to
investigate the zero-temperature phase diagram of the frustrated
spin- ---- antiferromagnet on the honeycomb lattice.
The system has isotropic Heisenberg interactions of strength ,
and between nearest-neighbour, next-nearest-neighbour and
next-next-nearest-neighbour pairs of spins, respectively. We study it in the
case , in the window
that contains the classical tricritical point (at ) of maximal frustration, appropriate to the limiting value of the spin quantum number. We present results for the magnetic
order parameter , the triplet spin gap , the spin stiffness
and the zero-field transverse magnetic susceptibility for the
two collinear quasiclassical antiferromagnetic (AFM) phases with N\'{e}el and
striped order, respectively. Results for and are given for the
three cases , and , while those for
and are given for the two cases and . On
the basis of all these results we find that the spin- and spin-1
models both have an intermediate paramagnetic phase, with no discernible
magnetic long-range order, between the two AFM phases in their phase
diagrams, while for there is a direct transition between them. Accurate
values are found for all of the associated quantum critical points. While the
results also provide strong evidence for the intermediate phase being gapped
for the case , they are less conclusive for the case . On
balance however, at least the transition in the latter case at the striped
phase boundary seems to be to a gapped intermediate state
The DSUB Approximation Scheme for the Coupled Cluster Method and Applications to Quantum Magnets
A new approximate scheme, DSUB, is described for the coupled cluster
method. We then apply it to two well-studied (spin-1/2 Heisenberg
antiferromagnet) spin-lattice models, namely: the and the models on
the square lattice in two dimensions. Results are obtained in each case for the
ground-state energy, the sublattice magnetization and the quantum critical
point. They are in good agreement with those from such alternative methods as
spin-wave theory, series expansions, quantum Monte Carlo methods and those from
the CCM using the LSUB scheme.Comment: 18 pages, 10 figure
Darwinian Data Structure Selection
Data structure selection and tuning is laborious but can vastly improve an
application's performance and memory footprint. Some data structures share a
common interface and enjoy multiple implementations. We call them Darwinian
Data Structures (DDS), since we can subject their implementations to survival
of the fittest. We introduce ARTEMIS a multi-objective, cloud-based
search-based optimisation framework that automatically finds optimal, tuned DDS
modulo a test suite, then changes an application to use that DDS. ARTEMIS
achieves substantial performance improvements for \emph{every} project in
Java projects from DaCapo benchmark, popular projects and uniformly
sampled projects from GitHub. For execution time, CPU usage, and memory
consumption, ARTEMIS finds at least one solution that improves \emph{all}
measures for () of the projects. The median improvement across
the best solutions is , , for runtime, memory and CPU
usage.
These aggregate results understate ARTEMIS's potential impact. Some of the
benchmarks it improves are libraries or utility functions. Two examples are
gson, a ubiquitous Java serialization framework, and xalan, Apache's XML
transformation tool. ARTEMIS improves gson by \%, and for
memory, runtime, and CPU; ARTEMIS improves xalan's memory consumption by
\%. \emph{Every} client of these projects will benefit from these
performance improvements.Comment: 11 page
Covariant gaussian approximation in Ginzburg - Landau model
Condensed matter systems undergoing second order transition away from the
critical fluctuation region are usually described sufficiently well by the mean
field approximation. The critical fluctuation region, determined by the
Ginzburg criterion, , is narrow even
in high superconductors and has universal features well captured by the
renormalization group method. However recent experiments on magnetization,
conductivity and Nernst effect suggest that fluctuations effects are large in a
wider region both above and below . In particular some "pseudogap"
phenomena and strong renormalization of the mean field critical temperature
can be interpreted as strong fluctuations effects that are
nonperturbative (cannot be accounted for by "gaussian fluctuations"). The
physics in a broader region therefore requires more accurate approach. Self
consistent methods are generally "non - conserving" in the sense that the Ward
identities are not obeyed. This is especially detrimental in the symmetry
broken phase where, for example, Goldstone bosons become massive. Covariant
gaussian approximation remedies these problems. The Green's functions obey all
the Ward identities and describe the fluctuations much better. The results for
the order parameter correlator and magnetic penetration depth of the Ginzburg -
Landau model of superconductivity are compared with both Monte Carlo
simulations and experiments in high cuprates.Comment: 24 pages, 7 figure
Frustrated spin- Heisenberg magnet on a square-lattice bilayer: High-order study of the quantum critical behavior of the ---- model
The zero-temperature phase diagram of the spin-
---- model on an -stacked square-lattice
bilayer is studied using the coupled cluster method implemented to very high
orders. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor
Heisenberg exchange interactions, of strengths and , respectively, are included in each layer. The two layers are
coupled via a NN interlayer Heisenberg exchange interaction with a strength
. The magnetic order parameter (viz.,
the sublattice magnetization) is calculated directly in the thermodynamic
(infinite-lattice) limit for the two cases when both layers have
antiferromagnetic ordering of either the N\'{e}el or the striped kind, and with
the layers coupled so that NN spins between them are either parallel (when
) to one another. Calculations
are performed at th order in a well-defined sequence of approximations,
which exactly preserve both the Goldstone linked cluster theorem and the
Hellmann-Feynman theorem, with . The sole approximation made is to
extrapolate such sequences of th-order results for to the exact limit,
. By thus locating the points where vanishes, we calculate
the full phase boundaries of the two collinear AFM phases in the
-- half-plane with . In particular, we provide the
accurate estimate, (), for the
position of the quantum triple point (QTP) in the region . We also
show that there is no counterpart of such a QTP in the region ,
where the two quasiclassical phase boundaries show instead an ``avoided
crossing'' behavior, such that the entire region that contains the nonclassical
paramagnetic phases is singly connected
decays
Effective chiral theory of mesons is applied to study the four decay modes of
. Theoretical values of the branching ratios are in
agreement with the data. The theory predicts that the resonance plays a
dominant role in these decays. There is no new parameter in this study.Comment: 12 pages and one figur
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