7,549 research outputs found
Phase diagram of the one-dimensional Holstein model of spinless fermions
The one-dimensional Holstein model of spinless fermions interacting with
dispersionless phonons is studied using a new variant of the density matrix
renormalisation group. By examining various low-energy excitations of finite
chains, the metal-insulator phase boundary is determined precisely and agrees
with the predictions of strong coupling theory in the anti-adiabatic regime and
is consistent with renormalisation group arguments in the adiabatic regime. The
Luttinger liquid parameters, determined by finite-size scaling, are consistent
with a Kosterlitz-Thouless transition.Comment: Minor changes. 4 pages, 4 figures. To appear in Physical Review
Letters 80 (1998) 560
Ground-state properties of the One-dimensional Kondo Lattice at partial Band-filling
We compute the magnetic structure factor, the singlet correlation function
and the momentum distribution of the one-dimensional Kondo lattice model at the
density . The density matrix-renormalization group method is used.
We show that in the weak-coupling regime, the ground state is paramagnetic. We
argue that a Luttinger liquid description of the model in this region is
consistent with our calculations . In the strong-coupling regime, the ground
state becomes ferromagnetic. The conduction electrons show a spinless-fermion
like behavior.Comment: 8 pages, Latex, 5 figures included, to be published in PRB (Rapid
Communications
Density Matrix Renormalization Group Applied to the Ground State of the XY-Spin-Peierls System
We use the density matrix renormalization group (DMRG) to map out the ground
state of a XY-spin chain coupled to dispersionless phonons of frequency . We confirm the existence of a critical spin-phonon coupling for the onset of the spin gap bearing the signature of
a Kosterlitz-Thouless transition. We also observe a classical-quantum crossover
when the spin-Peierls gap is of order . In the classical
regime, , the mean-field parameters are strongly renormalized
by non-adiabatic corrections. This is the first application of the DMRG to
phonons.Comment: 10 pages, 5 figures. To be published in PR
Orbital Selective Magnetism in the Spin-Ladder Iron Selenides BaKFeSe
Here we show that the 2.80(8) {\mu}B/Fe block antiferromagnetic order of
BaFe2Se3 transforms into stripe antiferromagnetic order in KFe2Se3 with a
decrease in moment to 2.1(1) {\mu}B/Fe. This reduction is larger than expected
from the change in electron count from Ba to K, and occurs with
the loss of the displacements of Fe atoms from ideal positions in the ladders,
as found by neutron pair distribution function analysis. Intermediate
compositions remain insulating, and magnetic susceptibility measurements show a
suppression of magnetic order and probable formation of a spin-glass. Together,
these results imply an orbital-dependent selection of magnetic versus bonded
behavior, driven by relative bandwidths and fillings.Comment: Final versio
Mice with Reduced NMDA Receptor Expression Display Behaviors Related to Schizophrenia
AbstractN-methyl-D-aspartate receptors (NMDARs) represent a subclass of glutamate receptors that play a critical role in neuronal development and physiology. We report here the generation of mice expressing only 5% of normal levels of the essential NMDAR1 (NR1) subunit. Unlike NR1 null mice, these mice survive to adulthood and display behavioral abnormalities, including increased motor activity and stereotypy and deficits in social and sexual interactions. These behavioral alterations are similar to those observed in pharmacologically induced animal models of schizophrenia and can be ameliorated by treatment with haloperidol or clozapine, antipsychotic drugs that antagonize dopaminergic and serotonergic receptors. These findings support a model in which reduced NMDA receptor activity results in schizophrenic-like behavior and reveals how pharmacological manipulation of monoaminergic pathways can affect this phenotype
Fermi Surface of The One-dimensional Kondo Lattice Model
We show a strong indication of the existence of a large Fermi surface in the
one-dimensional Kondo lattice model. The characteristic wave vector of the
model is found to be , being the density of the
conduction electrons. This result is at first obtained for a variant of the
model that includes an antiferromagnetic Heisenberg interaction between
the local moments. It is then directly observed in the conventional Kondo
lattice , in the narrow range of Kondo couplings where the long
distance properties of the model are numerically accessible.Comment: 11 pages, 6 figure
Tools for understanding the agricultural production systems and their socio-economic context in target regions for the introduction of new banana cultivars: baseline intra-household survey.
Within the framework of the IITA-led project āImprovement of banana for smallholder farmers in the Great Lakes region of Africaā (also known as the āBreeding Better Bananasā project, http://breedingbetterbananas.org), Bioversity International and partners conducted baseline research in the target regions of Luwero and Mbarara in Uganda, and Bukoba, Meru, Moshi and Rungwe in Tanzania during 2015-2016, prior to conducting on-station and on-farm evaluations of the new NARITA banana cultivars.
Five tools were used to characterise the banana and agricultural production systems, and the socioeconomic context of these systems, in the target regions. The research used a mixed-methods, participatory and sex-disaggregated approach to ensure that the knowledge, experiences and opinions of as many people as possible were obtained.
The understanding gained from the baseline research will:
ā¢ Be fed into the banana breeding pipeline at multiple entry points to assist with breeding banana cultivars that better meet the requirements of the users. Some of these entry points are: setting of breeding targets; selection of parent material; evaluation in regional on-station and on-farm trials; participatory varietal selection taking into account the criteria (or ātrait preferencesā) that are important to multiple and different users; facilitating access to and adoption of the new cultivars by farmers and other end-users through scaling up the supply of clean planting materials and ensuring equitable distribution of these through the āseedā systems.
ā¢ Inform the ongoing adaptive management of the project activities to ensure fair participation and decision-making by people in the affected communities.
ā¢ Provide the baseline to evaluate, in conjunction with the endline, the impact of adoption of the new cultivars on households, and individuals within, in the target regions
Experimental study of digital image processing techniques for LANDSAT data
The author has identified the following significant results. Results are reported for: (1) subscene registration, (2) full scene rectification and registration, (3) resampling techniques, (4) and ground control point (GCP) extraction. Subscenes (354 pixels x 234 lines) were registered to approximately 1/4 pixel accuracy and evaluated by change detection imagery for three cases: (1) bulk data registration, (2) precision correction of a reference subscene using GCP data, and (3) independently precision processed subscenes. Full scene rectification and registration results were evaluated by using a correlation technique to measure registration errors of 0.3 pixel rms thoughout the full scene. Resampling evaluations of nearest neighbor and TRW cubic convolution processed data included change detection imagery and feature classification. Resampled data were also evaluated for an MSS scene containing specular solar reflections
Density Matrix Approach to Local Hilbert Space Reduction
We present a density matrix approach for treating systems with a large or
infinite number of degrees of freedom per site with exact diagonalization or
the density matrix renormalization group. The method is demonstrated on the 1D
Holstein model of electrons coupled to Einstein phonons. In this system, two or
three optimized phonon modes per site give results as accurate as with 10-100
bare phonon levels per site.Comment: 4 pages, 4 figure
Position Automaton Construction for Regular Expressions with Intersection
Positions and derivatives are two essential notions in the conversion methods from regular expressions to equivalent finite automata. Partial derivative based methods have recently been extended to regular expressions with intersection. In this paper, we present a position automaton construction for those expressions. This construction generalizes the notion of position making it compatible with intersection. The resulting automaton is homogeneous and has the partial derivative automaton as its quotient
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