2,776 research outputs found
Real time evolution using the density matrix renormalization group
We describe an extension to the density matrix renormalization group method
incorporating real time evolution into the algorithm. Its application to
transport problems in systems out of equilibrium and frequency dependent
correlation functions is discussed and illustrated in several examples. We
simulate a scattering process in a spin chain which generates a spatially
non-local entangled wavefunction.Comment: 4 pages, 4 eps figures, some minor corrections in text and Eq.(3
Quantum data compression, quantum information generation, and the density-matrix renormalization group method
We have studied quantum data compression for finite quantum systems where the
site density matrices are not independent, i.e., the density matrix cannot be
given as direct product of site density matrices and the von Neumann entropy is
not equal to the sum of site entropies. Using the density-matrix
renormalization group (DMRG) method for the 1-d Hubbard model, we have shown
that a simple relationship exists between the entropy of the left or right
block and dimension of the Hilbert space of that block as well as of the
superblock for any fixed accuracy. The information loss during the RG procedure
has been investigated and a more rigorous control of the relative error has
been proposed based on Kholevo's theory. Our results are also supported by the
quantum chemistry version of DMRG applied to various molecules with system
lengths up to 60 lattice sites. A sum rule which relates site entropies and the
total information generated by the renormalization procedure has also been
given which serves as an alternative test of convergence of the DMRG method.Comment: 8 pages, 7 figure
Finite Temperature Density Matrix Renormalization using an enlarged Hilbert space
We apply a generalization of the time-dependent DMRG to study finite
temperature properties of several quantum spin chains, including the frustrated
model. We discuss several practical issues with the method, including
use of quantum numbers and finite size effects. We compare with transfer-matrix
DMRG, finding that both methods produce excellent results.Comment: 4 pages and 4 figure
Efficient dynamical downscaling of general circulation models using continuous data assimilation
Continuous data assimilation (CDA) is successfully implemented for the first
time for efficient dynamical downscaling of a global atmospheric reanalysis. A
comparison of the performance of CDA with the standard grid and spectral
nudging techniques for representing long- and short-scale features in the
downscaled fields using the Weather Research and Forecast (WRF) model is
further presented and analyzed. The WRF model is configured at 25km horizontal
resolution and is driven by 250km initial and boundary conditions from
NCEP/NCAR reanalysis fields. Downscaling experiments are performed over a
one-month period in January, 2016. The similarity metric is used to evaluate
the performance of the downscaling methods for large and small scales.
Similarity results are compared for the outputs of the WRF model with different
downscaling techniques, NCEP reanalysis, and Final Analysis. Both spectral
nudging and CDA describe better the small-scale features compared to grid
nudging. The choice of the wave number is critical in spectral nudging;
increasing the number of retained frequencies generally produced better
small-scale features, but only up to a certain threshold after which its
solution gradually became closer to grid nudging. CDA maintains the balance of
the large- and small-scale features similar to that of the best simulation
achieved by the best spectral nudging configuration, without the need of a
spectral decomposition. The different downscaled atmospheric variables,
including rainfall distribution, with CDA is most consistent with the
observations. The Brier skill score values further indicate that the added
value of CDA is distributed over the entire model domain. The overall results
clearly suggest that CDA provides an efficient new approach for dynamical
downscaling by maintaining better balance between the global model and the
downscaled fields
MAPPING SEA CLIFFS ON DOMINICA USING PHOTO MOSAICS
Mapping on islands covered by rain forest presents challenges due to the extremely limited exposure of bedrock. In general, exposures are limited to road cuts, quarries, and sea cliffs. While the first two are easily accessible, the last one provides the most reliable series of exposures for most islands, and generally forms the largest exposures. However, these outcrops are frequently difficult to impossible to reach from land, due to a lack of roads and/or strong surf right to the bases of the cliffs. Therefore, in July 2007, we chartered a boat to circumnavigate the island of Dominica in the Lesser Antilles to map and photograph the sea cliffs all around the island. The results provide modifications to the published geological map of the island and hitherto unknown details on the geology of the Miocene, Pliocene, and Pleistocene-to-Recent volcanic centers. For example, an area previously mapped as part of the oldest sequence on the island (Miocene), has been identified as a megabreccia that is part of the Pleistocene sequence of the Grande Soufriere Hills volcanic center, and is now identified as much more extensive than was known from exposures accessible from land. Detailed stratigraphic sections of selected sequences will be presented to illustrate the effectiveness of this technique
Psychometric properties of multicomponent tools designed to assess frailty in older adults: a systematic review
Background
Frailty is widely recognised as a distinct multifactorial clinical syndrome that implies vulnerability. The links between frailty and adverse outcomes such as death and institutionalisation have been widely evidenced. There is currently no gold standard frailty assessment tool; optimizing the assessment of frailty in older people therefore remains a research priority. The objective of this systematic review is to identify existing multi-component frailty assessment tools that were specifically developed to assess frailty in adults aged ≥60 years old and to systematically and critically evaluate the reliability and validity of these tools.
Methods
A systematic literature review was conducted using the standardised COnsensus‐based Standards for the selection of health Measurement INstruments (COSMIN) checklist to assess the methodological quality of included studies.
Results
Five thousand sixty-three studies were identified in total: 73 of which were included for review. 38 multi-component frailty assessment tools were identified: Reliability and validity data were available for 21 % (8/38) of tools. Only 5 % (2/38) of the frailty assessment tools had evidence of reliability and validity that was within statistically significant parameters and of fair-excellent methodological quality (the Frailty Index-Comprehensive Geriatric Assessment [FI-CGA] and the Tilburg Frailty Indicator [TFI]).
Conclusions
The TFI has the most robust evidence of reliability and validity and has been the most extensively examined in terms of psychometric properties. However, there is insufficient evidence at present to determine the best tool for use in research and clinical practice. Further in-depth evaluation of the psychometric properties of these tools is required before they can fulfil the criteria for a gold standard assessment tool
Creating artificial magnetic fields for cold atoms by photon-assisted tunneling
This paper proposes a simple setup for introducing an artificial magnetic
field for neutral atoms in 2D optical lattices. This setup is based on the
phenomenon of photon-assisted tunneling and involves a low-frequency periodic
driving of the optical lattice. This low-frequency driving does not affect the
electronic structure of the atom and can be easily realized by the same means
which employed to create the lattice. We also address the problem of detecting
this effective magnetic field. In particular, we study the center of mass
wave-packet dynamics, which is shown to exhibit certain features of cyclotron
dynamics of a classical charged particle.Comment: EPL-style, 8 pages, 4 figure
Nonequilibrium Current in the One Dimensional Hubbard Model at Half-Filling
Nonlinear transport in the one dimensional Hubbard model at half-filling
under a finite bias voltage is investigated by the adaptive time-dependent
density matrix renormalization group method. For repulsive on-site interaction,
dielectric breakdown of the Mott insulating ground state to a current-carrying
nonequilibrium steady state is clearly observed when the voltage exceeds the
charge gap. It is found that by increasing the voltage further the
current-voltage characteristics are scaled only by the charge gap and the
scaling curve exhibits almost linear dependence on the voltage whose slope is
suppressed by the electron correlation. In the case of attractive interaction
the linear conductance is the perfect one which agrees with the
prediction by the Luttinger liquid theory.Comment: 4 pages, 7 figure
Time evolution of Matrix Product States
In this work we develop several new simulation algorithms for 1D many-body
quantum mechanical systems combining the Matrix Product State variational
ansatz with Taylor, Pade and Arnoldi approximations to the evolution operator.
By comparing all methods with previous techniques based on Trotter
decompositions we demonstrate that the Arnoldi method is the best one, reaching
extremely good accuracy with moderate resources. Finally we apply this
algorithm to studying the formation of molecules in an optical lattices when
crossing a Feschbach resonance with a cloud of two-species hard-core bosons.Comment: More extensive comparison with all nearest-neighbor spin s=1/2
models. The results in this manuscript have been superseded by a more
complete work in cond-mat/061021
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