3,238 research outputs found
N\'eel and Spin-Peierls ground states of two-dimensional SU(N) quantum antiferromagnets
The two-dimensional SU(N) quantum antiferromagnet, a generalization of the
quantum Heisenberg model, is investigated by quantum Monte Carlo simulations.
The ground state for is found to be of the N\'eel type with broken
SU(N) symmetry, whereas it is of the Spin-Peierls type for with broken
lattice translational invariance. No intermediate spin-liquid phase was
observed in contrast to previous numerical simulations on smaller lattices
[Santoro et al., Phys. Rev. Lett. {\bf 83} 3065 (1999)].Comment: 4 pages, 4 figure
Exchange interactions and magnetic properties of the layered vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9
We have performed ab-initio calculations of exchange couplings in the layered
vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9. The uniform susceptibility of the
Heisenberg model with these exchange couplings is then calculated by quantum
Monte Carlo method; it agrees well with the experimental measurements. Based on
our results we naturally explain the unusual magnetic properties of these
materials, especially the huge difference in spin gap between CaV2O5 and
MgV2O5, the unusual long range order in CaV3O7 and the "plaquette resonating
valence bond (RVB)" spin gap in CaV4O9
Interfacing Modbus Plus to EPICS for KEKB Accelerator Control System
The KEKB Accelerator control system[1] is based on EPICS(Experimental Physics and Industrial Control System)[2] and uses many PLCs in the magnet protec-tion systems and the radiation safety system. In order to monitor the interlock status, Modbus Plus[3] is adopted as the protocol between an IOC(Input/Output Controller) and PLCs. For this purpose, a device support and a driver support for Modbus Plus have been developed. The device/driver support modules allow an IOC to communicate with PLC-s by asynchronous I/O transactions, in such a manner that the GPIB devices do. With the software modules, an IOC works always as a master device on the Modbus Plus net-work to read the status of controlled devices from PLC memory. While the main use of the software is to read the interlock status, it is also used to reset the interlock sys-tems. Details of the software structure are described. An ap-plication of this software in the KEKB accelerator control system is also presented.
Contraction of cross-linked actomyosin bundles
Cross-linked actomyosin bundles retract when severed in vivo by laser
ablation, or when isolated from the cell and micromanipulated in vitro in the
presence of ATP. We identify the time scale for contraction as a viscoelastic
time tau, where the viscosity is due to (internal) protein friction. We obtain
an estimate of the order of magnitude of the contraction time tau ~ 10-100 s,
consistent with available experimental data for circumferential microfilament
bundles and stress fibers. Our results are supported by an exactly solvable,
hydrodynamic model of a retracting bundle as a cylinder of isotropic, active
matter, from which the order of magnitude of the active stress is estimated.Comment: To be published in Physical Biolog
The Heisenberg model on the 1/5-depleted square lattice and the CaV4O9 compound
We investigate the ground state structure of the Heisenberg model on the
1/5-depleted square lattice for arbitrary values of the first- and
second-neighbor exchange couplings. By using a mean-field Schwinger-boson
approach we present a unified description of the rich ground-state diagram,
which include the plaquette and dimer resonant-valence-bond phases, an
incommensurate phase and other magnetic orders with complex magnetic unit
cells. We also discuss some implications of ours results for the experimental
realization of this model in the CaV4O9 compound.Comment: 4 pages, Latex, 7 figures included as eps file
Origin of spin-gap in CaVO: effect of frustration and lattice distortion
We study the origin of spin-gap in recently discovered material CaVO.
We analyze the spin- Heisenberg model on the depleted square lattice
with nearest neighbor (nn) and next nearest neighbor (nnn) interactions, in
terms of the singlet and triplet states of the 4-spin plaquettes and 2-spin
dimers. Phase diagram of the model is obtained within a linear
``spin-wave"-like approximation, and is shown to agree well with the earlier
results of QMC simulations for nn interactions. We further propose that the
special lattice structure of CaVO naturally leads to lattice
distortions, which enhances the spin-gap via a spin-Peierls mechanism.Comment: 4 pages, RevTex, 2 postscript figures. Latex file and figures have
been uuencode
Weakly Coupled Antiferromagnetic Quantum Spin Chains
Quasi-one-dimensional quantum antiferromagnets formed by a d-dimensional
hypercubic lattice of weakly coupled spin-1/2 antiferromagnetic Heisenberg
chains are studied by combining exact results in one-dimension and
renormalization group analyses of the interchain correlations. It is shown that
d-dimensional magnetic long-range order develops at zero-temperature for
infinitesimal antiferromagnetic or ferromagnetic interchain couplings. In the
presence of weak bond alternations, the order-disorder transition occurs at a
finite interchain coupling. Relevances to the lightly doped quantum
antiferromagnets and multi-layer quantum Hall systems are discussed.Comment: 12 revtex pages, no figures, revised final version to appear in PR
A usage-centered approach for the promotion of institutional repository: a case of Chiba University
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