4,950 research outputs found
New results for the t-J model in ladders: Changes in the spin liquid state with applied magnetic field. Implications for the cuprates
Exact Diagonalization calculations are presented for the t-J model in the
presence of a uniform magnetic field. Results for 2xL ladders (L=8,10,12) and
4x4 square clusters with 1 and 2 holes indicate that the diamagnetic response
to a perpendicular magnetic field tends to induce a spin liquid state in the
spin background. The zero-field spin liquid state of a two-leg ladder is
reinforced by the magnetic field: a considerable increase of rung
antiferromagnetic correlations is observed for J/t up to 0.6, for 1 and 2
holes. Pair-breaking is also clearly observed in the ladders and seems to be
associated in part with changes promoted by the field in the spin correlations
around the zero-field pair. In the 4x4 cluster, the numerical results seem to
indicate that the field-induced spin liquid state competes with the zero-field
antiferromagnetic short-range-order, the spin liquid state being favored by
higher doping and smaller values of J/t. It is interesting to note that the
field-effect can also be observed in a 2x2 plaquette with 1 and 2 holes. This
opens up the possibility of gaining a qualitative understanding of the effect.Comment: 16 pages, 7 figures, latex New results adde
Braided Cyclic Cocycles and Non-Associative Geometry
We use monoidal category methods to study the noncommutative geometry of
nonassociative algebras obtained by a Drinfeld-type cochain twist. These are
the so-called quasialgebras and include the octonions as braided-commutative
but nonassociative coordinate rings, as well as quasialgebra versions
\CC_{q}(G) of the standard q-deformation quantum groups. We introduce the
notion of ribbon algebras in the category, which are algebras equipped with a
suitable generalised automorphism , and obtain the required
generalisation of cyclic cohomology. We show that this \emph{braided cyclic
cocohomology} is invariant under a cochain twist. We also extend to our
generalisation the relation between cyclic cohomology and differential calculus
on the ribbon quasialgebra. The paper includes differential calculus and cyclic
cocycles on the octonions as a finite nonassociative geometry, as well as the
algebraic noncommutative torus as an associative example.Comment: 36 pages latex, 9 figure
Melhoramento da população de milho CMS 14.
A ausencia de sistemas de produção tecnificados, a não utilizacao de insumos modernos e, principalmente a não disponibilidade de sementes melhoradas, são fatores que concorrem de maneira decisiva para as baixas produtividades de milho no Amapá. As cultivares tradicionalmente plantadas como Pretinha e Palha Roxa não apresentaram rendimentos satisfatorios. Com o objetivo de se obter uma nova variedade de milho, a partir da população CMS 14 (Pool 25), a mais produtiva que a original, realizou-se nesta população o primeiro ciclo de seleção massal estratificada. Os trabalhos foram conduzidos no município de Mazagão, em solo Gley Pouco Humico hidromorfico.bitstream/item/65042/1/AP-1985-melhoramento-populacao-milho.pd
Astrophysical Neutrino Event Rates and Sensitivity for Neutrino Telescopes
Spectacular processes in astrophysical sites produce high-energy cosmic rays
which are further accelerated by Fermi-shocks into a power-law spectrum. These,
in passing through radiation fields and matter, produce neutrinos. Neutrino
telescopes are designed with large detection volumes to observe such
astrophysical sources. A large volume is necessary because the fluxes and
cross-sections are small. We estimate various telescopes' sensitivities and
expected event rates from astrophysical sources of high-energy neutrinos. We
find that an ideal detector of km^2 incident area can be sensitive to a flux of
neutrinos integrated over energy from 10^5 and 10^{7} GeV as low as 1.3 *
10^(-8) * E^(-2) (GeV/cm^2 s sr) which is three times smaller than the
Waxman-Bachall conservative upper limit on potential neutrino flux. A real
detector will have degraded performance. Detection from known point sources is
possible but unlikely unless there is prior knowledge of the source location
and neutrino arrival time.Comment: Section added +modification
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