7,975 research outputs found
Electron Spin Resonance of defects in the Haldane System Y(2)BaNiO(5)
We calculate the electron paramagnetic resonance (EPR) spectra of the
antiferromagnetic spin-1 chain compound Y(2)BaNi(1-x)Mg(x)O(5) for different
values of x and temperature T much lower than the Haldane gap (~100K). The
low-energy spectrum of an anisotropic Heisenberg Hamiltonian, with all
parameters determined from experiment, has been solved using DMRG. The observed
EPR spectra are quantitatively reproduced by this model. The presence of
end-chain S=1/2 states is clearly observed as the main peak in the spectrum and
the remaining structure is completely understood.Comment: 5 pages, 4 figures include
A Quantum Approach to Classical Statistical Mechanics
We present a new approach to study the thermodynamic properties of
-dimensional classical systems by reducing the problem to the computation of
ground state properties of a -dimensional quantum model. This
classical-to-quantum mapping allows us to deal with standard optimization
methods, such as simulated and quantum annealing, on an equal basis.
Consequently, we extend the quantum annealing method to simulate classical
systems at finite temperatures. Using the adiabatic theorem of quantum
mechanics, we derive the rates to assure convergence to the optimal
thermodynamic state. For simulated and quantum annealing, we obtain the
asymptotic rates of and , for the temperature and magnetic field, respectively. Other
annealing strategies, as well as their potential speed-up, are also discussed.Comment: 4 pages, no figure
Electron-Doped Manganese Perovskites: The Polaronic State
Using the Lanczos method in linear chains we study the ground state of the
double exchange model including an antiferromagnetic super-exchange in the low
concentration limit. We find that this ground state is always inhomogeneous,
containig ferromagnetic polarons. The extention of the polaron spin distortion,
the dispersion relation and their trapping by impurities, are studied for
diferent values of the super exchange interaction and magnetic field. We also
find repulsive polaron polaron interaction.Comment: 4 pages, 6 embedded figure
Generalized Jordan-Wigner Transformations
We introduce a new spin-fermion mapping, for arbitrary spin generating
the SU(2) group algebra, that constitutes a natural generalization of the
Jordan-Wigner transformation for . The mapping, valid for regular
lattices in any spatial dimension , serves to unravel hidden symmetries in
one representation that are manifest in the other. We illustrate the power of
the transformation by finding exact solutions to lattice models previously
unsolved by standard techniques. We also present a proof of the existence of
the Haldane gap in 1 bilinear nearest-neighbors Heisenberg spin chains and
discuss the relevance of the mapping to models of strongly correlated
electrons. Moreover, we present a general spin-anyon mapping for the case .Comment: 5 pages, 1 psfigur
Singlet-triplet dispersion reveals additional frustration in the triangular dimer compound BaMnO
We present single crystal inelastic neutron scattering measurements of the
S=1 dimerized quasi-two-dimensional antiferromagnet BaMnO. The
singlet-triplet dispersion reveals nearest-neighbor and next-nearest-neighbor
ferromagnetic interactions between adjacent bilayers that compete against each
other. Although the inter-bilayer exchange is comparable to the intra-bilayer
exchange, this additional frustration reduces the effective coupling along the
c-axis and leads to a quasi-two dimensional behavior. In addition, the obtained
exchange values are able to reproduce the four critical fields in the phase
diagram.Comment: 4 pages, 3 color figures, submitted to an APS physical review journa
Isolamento de fungos filamentosos seção Nigri produtores de ocratoxina A em uvas tintas.
A colonização dos Aspergillusda Seção Nigri nas uvas durante o cultivo é a principal fonte de ocratoxina A (OTA) em uvas e seus derivados. Este estudo teve como objetivo avaliar a presença de espécies de Aspergillus Seção Nigri produtores de ocratoxina A em três variedades de uvas tintas utilizadas para a elaboração de vinho tinto e suco de uva, coletadas na região vitivinícola do Vale Submédio São Francisco em parceria com o setor de Enologia da Embrapa Semi-Árido localizada em Petrolina (PE). Dentre as variedades de uvas analisadas, Petit verdot apresentou maior nível de contaminação por Aspergillus Seção Nigri. A produção de ocratoxina A foi confirmada nas espécies Aspergillus carbonarius e Aspergillus niger. Palavras-chaves: Uvas Tintas, Aspergillus, Seção Nigri, Ocratoxina A
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