471 research outputs found
Phase diagram of an exactly solvable t-J ladder model
We study a system of one-dimensional t-J models coupled to a ladder system. A
special choice of the interaction between neighbouring rungs leads to an
integrable model with supersymmetry, which is broken by the presence of rung
interactions. We analyze the spectrum of low-lying excitations and ground state
phase diagram at zero temperature.Comment: LaTeX, 8 pp. incl. 1 figur
On the dynamics of coupled S=1/2 antiferromagnetic zig-zag chains
We investigate the elementary excitations of quasi one-dimensional S=1/2
systems built up from zig-zag chains with general isotropic exchange constants,
using exact (Lanczos) diagonalization for 24 spins and series expansions
starting from the decoupled dimer limit. For the ideal one-dimensional zig-zag
chain we discuss the systematic variation of the basic (magnon) triplet
excitation with general exchange parameters and in particular the presence of
practically flat dispersions in certain regions of phase space. We extend the
dimer expansion in order to include the effects of 3D interactions on the
spectra of weakly interacting zig-zag chains. In an application to KCuCl_3 we
show that this approach allows to determine the exchange interactions between
individual pairs of spins from the spectra as determined in recent neutron
scattering experiments.Comment: 8 pages, 9 figures; some changes, figure added; final versio
As experiências em proteção florestal da divisão florestal da Rigesa Meadwestvaco.
As experiências em proteção florestal da divisão florestal da Rigesa MeadWestvaco. A Divisão Florestal da Rigesa foi instalada em Três Barras SC em 1955 e atualmente possui unidades de manejo em 17 municípios no norte de Santa Catarina e sul do Paraná. A atividade florestal está voltada para o plantio, manejo e colheita de madeira plantada de Pinus e Eucalyptus. Na área de Proteção Florestal ao longo dos anos surgiram diversos desafios, para os quais a empresa tem criado ações específicas desenvolvidas por seu corpo técnico através de projetos cooperativos com Universidades e Instituições de Pesquisa, com o objetivo de encontrar a melhor solução para cada situação. Neste trabalho estão relatadas as últimas experiências e soluções encontradas pela empresa para os seguintes assuntos; Manejo Integrado do Pulgão-do- Pinus Cinara spp, detecção de Pissodes castaneus em florestas de Pinus; Cálculo da FMA ? Fórmula de Monte Alegre através da Estação Meteorológica Automática1 CD-ROM. Palestra (Pragas florestais). Co-promoção: Embrapa Florestas
Localization length of a soliton from a non-magnetic impurity in a general double-spin-chain model
A localization length of a free-spin soliton from a non-magnetic impurity is
deduced in a general double-spin-chain model ( model). We have
solved a variational problem which employs the nearest-neighbor singlet-dimer
basis. The wave function of a soliton is expressed by the Airy function, and
the localization length is found to obey a power law of the
dimerization with an exponent -1/3; .
This explains why NaV_2O_5 does not show the antiferromagnetic order, while
CuGeO_3 does by impurity doping. When the gap exists by the bond-dimerization,
a soliton is localized and no order is expected. Contrary, there is a
possibility of the order when the gap is mainly due to frustration.Comment: 4 pages, REVTeX, Figures are in eps-file
Specific Heat Study on a Novel Spin-Gapped System : (CH_3)_2NH_2CuCl_3
Specific heat measurements down to 120mK have been performed on a
quasi-one-dimensional spin-gapped system (CH)NHCuCl in
a magnetic field up to 8 T. This compound has a characteristic magnetization
curve which shows a gapless ground state and a plateau at 1/2 of the saturation
value. We have observed a spontaneous antiferromagnetic ordering and a
field-induced one below and above the 1/2 plateau field range, respectively.
The field versus temperature phase diagram is quite unusual and completely
different from those of the other quantum spin systems investigated so far. In
the plateau field range, a double-structure in the specific heat is observed,
reflecting the coexistence of ferromagnetic and antiferromagnetic excitations.
These behaviors are discussed on the basis of a recently proposed novel quantum
spin chain model consisting of weakly coupled ferromagnetic and
antiferromagnetic dimers.Comment: 4 pages, 3 figures, submitted to J. Phys. Soc. Jp
Thermodynamics of the (1,1/2) Ferrimagnet in Finite Magnetic Fields
We investigate the specific heat and magnetisation of a ferrimagnet with gS=1
and S=1/2 spins in a finite magnetic field using the transfer matrix DMRG down
to T=0.025J. Ferromagnetic gapless and antiferromagnetic gapped excitations for
H=0 lead to rich thermodynamics for H > 0. While the specific heat is
characterized by a generic double peak structure, magnetisation reveals two
critical fields, Hc1=1.76(1) and Hc2=3.00(1) with square-root behaviour in the
T=0 magnetisation. Simple analytical arguments allow to understand these
experimentally accessible findings.Comment: 5 pages, 7 eps figures, uses RevTeX, submitted to PR
Dynamical Structure Factors for Dimerized Spin Systems
We discuss the transition strength between the disordered ground state and
the basic low-lying triplet excitation for interacting dimer materials by
presenting theoretical calculations and series expansions as well as inelastic
neutron scattering results for the material KCuCl_3. We describe in detail the
features resulting from the presence of two differently oriented dimers per
unit cell and show how energies and spectral weights of the resulting two modes
are related to each other. We present results from the perturbation expansion
in the interdimer interaction strength and thus demonstrate that the wave
vector dependence of the simple dimer approximation is modified in higher
orders. Explicit results are given in 10th order for dimers coupled in 1D, and
in 2nd order for dimers coupled in 3D with application to KCuCl_3 and TlCuCl_3.Comment: 17 pages, 6 figures, part 2 is based on cond-mat/021133
Low-Energy Structure of Heisenberg Ferrimagnetic Spin Chains
Static and dynamic structure factors of Heisenberg ferrimagnetic spin chains
are numerically investigated. There exist two distinct branches of elementary
excitations, which exhibit ferromagnetic and antiferromagnetic aspects. The
ferromagnetic feature is smeared out with the increase of temperature, whereas
the antiferromagnetic one persists up to higher temperatures. The scattering
intensity is remarkably large at lower boundaries of the ferromagnetic and
antiferromagnetic spectra. All these observations are consistent with the
ferromagnetic-to-antiferromagnetic crossover in the thermal behavior which has
recently been reported.Comment: 3 pages, 5 PS figures, to appear in J. Phys. Soc. Jpn. Vol. 67, No.
11 (1998
Phase diagram and hidden order for generalized spin ladders
We investigate the phase diagram of antiferromagnetic spin ladders with
additional exchange interactions on diagonal bonds by variational and numerical
methods. These generalized spin ladders interpolate smoothly between the
chain with competing nn and nnn interactions, the chain with
alternating exchange and the antiferromagnetic chain. The Majumdar-Ghosh
ground states are formulated as matrix product states and are shown to exhibit
the same type of hidden order as the af chain. Generalized matrix product
states are used for a variational calculation of the ground state energy and
the spin and string correlation functions. Numerical (Lanczos) calculations of
the energies of the ground state and of the low-lying excited states are
performed, and compare reasonably with the variational approach. Our results
support the hypothesis that the dimer and Majumdar-Ghosh points are in the same
phase as the af chain.Comment: 23 pages, REVTEX, 7 figure
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