243 research outputs found
Effects of semiclassical spiral fluctuations on hole dynamics
We investigate the dynamics of a single hole coupled to the spiral
fluctuations related to the magnetic ground states of the antiferromagnetic
J_1-J_2-J_3 Heisenberg model on a square lattice. Using exact diagonalization
on finite size clusters and the self consistent Born approximation in the
thermodynamic limit we find, as a general feature, a strong reduction of the
quasiparticle weight along the spiral phases of the magnetic phase diagram. For
an important region of the Brillouin Zone the hole spectral functions are
completely incoherent, whereas at low energies the spectral weight is
redistributed on several irregular peaks. We find a characteristic value of the
spiral pitch, Q=(0.7,0.7)\pi, for which the available phase space for hole
scattering is maximum. We argue that this behavior is due to the non trivial
interference of the magnon assisted and the free hopping mechanism for hole
motion, characteristic of a hole coupled to semiclassical spiral fluctuations.Comment: 6 pages, 5 figure
Quasiparticle excitations in frustrated antiferromagnets
We have computed the quasiparticle wave function corresponding to a hole
injected in a triangular antiferromagnet. We have taken into account
multi-magnon contributions within the self consistent Born approximation. We
have found qualitative differences, under sign reversal of the integral
transfer t, regarding the multi-magnon components and the own existence of the
quasiparticle excitations. Such differences are due to the subtle interplay
between magnon-assisted and free hopping mechanisms. We conclude that the
conventional quasiparticle picture can be broken by geometrical frustration
without invoking spin liquid phases.Comment: 5 pages, 4 figures, presented at " At the Frontiers of the condensed
Matter II, Buenos Aires. June, 2004 ". To be published in Physica
Spectral formation in a radiative shock: application to anomalous X-ray pulsars and soft gamma-ray repeaters
In the fallback disk model for the persistent emission of Anomalous X-ray
pulsars (AXPs) and soft gamma-ray repeaters (SGRs), the hard X-ray emission
arises from bulk- and thermal Comptonization of bremsstrahlung photons, which
are generated in the accretion column. The relatively low X-ray luminosity of
these sources implies a moderate transverse optical depth to electron
scattering, with photons executing a small number of shock crossings before
escaping sideways. We explore the range of spectral shapes that can be obtained
with this model and characterize the most important parameter dependencies. We
use a Monte Carlo code to study the crisscrossing of photons in a radiative
shock in an accretion column and compute the resulting spectrum. As expected,
high-energy power-law X-ray spectra are produced in radiative shocks with
photon-number spectral index larger than or about 0.5. We find that the
required transverse optical depth is between 1 and 7. Such spectra are observed
in low-luminosity X-ray pulsars. We demonstrate here with a simple model that
Compton upscattering in the radiative shock in the accretion column can produce
hard X-ray spectra similar to those seen in the persistent and transient
emission of AXPs and SGRs. In particular, one can obtain a high-energy
power-law spectrum, with photon-number spectral index ~ 1 and a cutoff at 100 -
200 keV, with a transverse Thomson optical depth of ~ 5, which is shown to be
typical in AXPs/SGRs.Comment: Accepted for publication in A&
Spin polaron in the J1-J2 Heisenberg model
We have studied the validity of the spin polaron picture in the frustrated
J1-J2 Heisenberg model. For this purpose, we have computed the hole spectral
functions for the Neel, collinear, and disordered phases of this model, by
means of the self-consistent Born approximation and Lanczos exact
diagonalization on finite-size clusters. We have found that the spin polaron
quasiparticle excitation is always well defined for the magnetically ordered
Neel and collinear phases, even in the vicinity of the magnetic quantum
critical points, where the local magnetization vanishes. As a general feature,
the effect of frustration is to increase the amplitude of the multimagnon
states that build up the spin polaron wave function, leading to the reduction
of the quasiparticle coherence. Based on Lanczos results, we discuss the
validity of the spin polaron picture in the disordered phase.Comment: 9 pages, 12 figure
Classical Antiferromagnetism in Kinetically Frustrated Electronic Models
We study the infinite U Hubbard model with one hole doped away half-filling,
in triangular and square lattices with frustrated hoppings that invalidate
Nagaoka's theorem, by means of the density matrix renormalization group. We
find that these kinetically frustrated models have antiferromagnetic ground
states with classical local magnetization in the thermodynamic limit. We
identify the mechanism of this kinetic antiferromagnetism with the release of
the kinetic energy frustration as the hole moves in the established
antiferromagnetic background. This release can occurs in two different ways: by
a non-trivial spin-Berry phase acquired by the hole or by the effective
vanishing of the hopping amplitude along the frustrating loops.Comment: 12 pages and 4 figures, with Supplementary Material. To be published
in Phys. Rev. Let
Magnons and Excitation Continuum in XXZ triangular antiferromagnetic model: Application to
We investigate the excitation spectrum of the triangular-lattice
antiferromagnetic model using series expansions and mean field Schwinger
bosons approaches. The single-magnon spectrum computed with series expansions
exhibits rotonic minima at the middle points of the edges of the Brillouin
zone, for all values of the anisotropy parameter in the range . Based on the good agreement with series expansions for the
single-magnon spectrum, we compute the full dynamical magnetic structure factor
within the mean field Schwinger boson approach to investigate the relevance of
the model for the description of the unusual spectrum found recently in
. In particular, we obtain an extended continuum above the spin
wave excitations, which is further enhanced and brought closer to those
observed in with the addition of a second neighbor exchange
interaction approximately 15% of the nearest-neighbor value. Our results
support the idea that excitation continuum with substantial spectral-weight are
generically present in two-dimensional frustrated spin systems and
fractionalization in terms of {\it bosonic} spinons presents an efficient way
to describe them.Comment: 8 pages, 4 figure
A test of the bosonic spinon theory for the triangular antiferromagnet spectrum
We compute the dynamical structure factor of the spin-1/2 triangular
Heisenberg model using the mean field Schwinger boson theory. We find that a
reconstructed dispersion, resulting from a non trivial redistribution of the
spectral weight, agrees quite well with the spin excitation spectrum recently
found with series expansions. In particular, we recover the strong
renormalization with respect to linear spin wave theory along with the
appearance of roton-like minima. Furthermore, near the roton-like minima the
contribution of the two spinon continuum to the static structure factor is
about 40 % of the total weight. By computing the density-density dynamical
structure factor, we identify an unphysical weak signal of the spin excitation
spectrum with the relaxation of the local constraint of the Schwinger bosons at
the mean field level. Based on the accurate description obtained for the static
and dynamic ground state properties, we argue that the bosonic spinon theory
should be considered seriously as a valid alternative to interpret the physics
of the triangular Heisenberg model.Comment: 6 pages, 5 figures, extended version including: a table with ground
state energy and magnetization; and the density-density dynamical structure
factor. Accepted for publication in Europhysics Letter
Influence of corn, Zea mays, phenological stages in Diatraea saccharalis F. (Lep. Crambidae) oviposition
Diatraea saccharalis F. is a major pest to maize, sorghum and sugarcane crops in Latin America and the most damaging insect pest of maize in Argentina. Female moths lay their eggs on lower and upper sides of leaves and sheaths of corn plants. Oviposition behaviour of different species of corn stem borers is influenced by host plant phenology in different ways. Host pubescence affects oviposition behaviour in different species of caterpillars. Oviposition preferences for phenological stages, leaf surface and vertical distribution in corn plants were investigated. Corn plots of 120 m2 were sown with a Pioneer pubescent cultivar in six planting dates, each one replicated three times according to a completely randomized design. Throughout the season 14 samplings were carried out, during which plants were randomly chosen within each of the six treatments and exhaustively examined in order to count the number of egg masses and their location in the leaf surface and in the vertical structure of the plant. Diatraea saccharalis preferred laying their eggs on the lower sides of leaves located in the middle stratum of corn plants whose phenological stages were older than V10. Our study suggests that attention should be paid to the influence of crop phenological stage in the location of egg masses in futures studies, as oviposition preference can change through plant maturation.Fil: More, Marcela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; ArgentinaFil: Trumper, E. V.. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Córdoba. Estación Experimental Agropecuaria Manfredi; ArgentinaFil: Prola, M. J.. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Córdoba. Estación Experimental Agropecuaria Manfredi; Argentin
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