43,019 research outputs found
Skyrmions in a Doped Antiferromagnet
Magnetization and magnetoresistance have been measured in insulating
antiferromagnetic La_{2}Cu_{0.97}Li_{0.03}O_{4} over a wide range of
temperatures, magnetic fields, and field orientations. The magnetoresistance
step associated with a weak ferromagnetic transition exhibits a striking
nonmonotonic temperature dependence, consistent with the presence of skyrmions.Comment: 4+ pages, 3 figures (some low resolution), supplementary material (3
pages); discussion expanded, references added; as publishe
Interplay between disorder, quantum and thermal fluctuations in ferromagnetic alloys: The case of UCu2Si(2-x)Ge(x)
We consider, theoretically and experimentally, the effects of structural
disorder, quantum and thermal fluctuations in the magnetic and transport
properties of certain ferromagnetic alloys.We study the particular case of
UCu2Si(2-x)Ge(x). The low temperature resistivity, rho(T,x), exhibits Fermi
liquid (FL) behavior as a function of temperature T for all values of x, which
can be interpreted as a result of the magnetic scattering of the conduction
electrons from the localized U spins. The residual resistivity, rho(0,x),
follows the behavior of a disordered binary alloy. The observed non-monotonic
dependence of the Curie temperature, Tc(x), with x can be explained within a
model of localized spins interacting with an electronic bath whose transport
properties cross-over from ballistic to diffusive regimes. Our results clearly
show that the Curie temperature of certain alloys can be enhanced due to the
interplay between quantum and thermal fluctuations with disorder.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Geotecnologias conciliando preservação ambiental e fortalecimento das atividades produtivas na região do Araripe-PE.
A região do Araripe, no Estado de Pernambuco, tem tido cada vez mais a matéria prima de base florestal como principal insumo na matriz energética. As empresas ali instaladas fazem uso, quase que exclusivamente, da vegetação nativa para o eneficiamento e transformação da gipsita, constituindo assim o maior pólo gesseiro do Brasil e segundo do mundo. Assim, existe a necessidade de se investir na oferta de madeira através das praticas de reflorestamentos, manejo da vegetação nativa e recuperação de areas degradadas para suprir o déficit energético. Deste modo, este artigo, utilizando-se das geotecnologias baseadas em imagens de satélites e geoprocessamento, teve como objetivo caracterizar ambientalmente os municípios da região, ontemplando os solos, a cobertura vegetal e uso antrópico, com vista à realização de uma espacialização dos diferentes ambientes para execução das praticas mencionadas. A analise realizada remete as seguintes indicações: a área indicada para pratica de manejo florestal é da ordem 470.000 ha; a área indicada para florestam ento/reflorestamento é da ordem de 110.000 ha; a área indicada para recuperação é da ordem de 267.000 ha e indicadas para Reserva legal é da ordem de 6.000 ha
Extraordinary magnetoresistance in graphite: experimental evidence for the time-reversal symmetry breaking
The ordinary magnetoresistance (MR) of doped semiconductors is positive and
quadratic in a low magnetic field, B, as it should be in the framework of the
Boltzmann kinetic theory or in the conventional hopping regime. We observe an
unusual highly-anisotropic in-plane MR in graphite, which is neither quadratic
nor always positive. In a certain current direction MR is negative and linear
in B in fields below a few tens of mT with a crossover to a positive MR at
higher fields, while in a perpendicular current direction we observe a giant
super-linear and positive MR. These extraordinary MRs are respectively
explained by a hopping magneto-conductance via non-zero angular momentum
orbitals, and by the magneto-conductance of inhomogeneous media. The linear
orbital NMR is a unique signature of the broken time-reversal symmetry (TRS) in
graphite. While some local paramagnetic centers could be responsible for the
broken TRS, the observed large diamagnetism suggests a more intriguing
mechanism of this breaking, involving superconducting clusters with
unconventional (chiral) order parameters and spontaneously generated
normal-state current loops in graphite.Comment: 4 pages, 5 figure
Growth of seven perennial plant species adapted to the Brazilian Semi-Arid.
Growth of seven perennial plant species adapted to the Brazilian Semi-Arid. Abstract - Measuring plant height (y) and canopy diameter (x) in trees may not be an easy task, but stem diameter (z) can be more easily evaluated. This work's objective was two-fold: evaluate the growth of species adapted to the Brazilian Semi-Arid Region in the first two years of age, and obtain linear equations to estimate y and x from z, in those species. A random block design with four replications was employed. The values for x, z, and y were measured biannually from October/2003 to March/2005. Prosopis juliflora and Mimosa caesalpiniaefolia showed the highest stem diameter and plant height values, respectively, and both showed the highest canopy diameter. In the equations to estimate plant height from the stem diameter the value of the coefficient of determination (R2) ranged from 0.76 (Tamarindus indica and Leucaena leucocephala) to 0.92 (Prosopis juliflora and Azadirachta indica). In the equations that allowed to estimate the crown diameter from the stem diameter the R2 value ranged from 0.70 (Leucaena leucocephala) to 0.92 (Azadirachta indica). Crescimento de sete espécies vegetais perenes adaptadas ao Semi-Árido brasileiro. Resumo - A mensuração da altura da planta (y) e do diâmetro da copa (x) em árvores pode não ser tarefa fácil, mas o diâmetro do caule (z) pode ser avaliado mais facilmente. Este trabalho teve por objetivos avaliar o crescimento de espécies adaptadas ao Semi-Árido brasileiro, nos primeiros dois anos de idade, e obter equações lineares para estimar y e x a partir de z, nessas espécies. Utilizou-se o delineamento de blocos ao acaso com quatro repetições. Os valores de x, z e y foram medidos semestralmente de outubro/2003 a março/2005. Prosopis juliflora e Mimosa caesalpiniaefolia apresentaram os maiores diâmetros do caule e altura da planta, respectivamente, e ambas apresentaram o maior diâmetro da copa. Nas equações para estimar a altura da planta a partir do diâmetro do caule, o valor do coeficiente de determinação (R2) variou de 0,76 (Tamarindus indica and Leucaena leucocephala) a 0,92 (Prosopis juliflora and Azadirachta indica). Nas equações que permitiram estimar o diâmetro da copa a partir do diâmetro do caule, o valor de R2 variou de 0,70 (Leucaena leucocephala) a 0,92 (Azadirachta indica)
Notes on the Two-brane Model with Variable Tension
Motivated by possible extensions of the braneworld models with two branes, we
investigate some consequences of a variable brane tension using the well
established results on consistency conditions. By a slight modification of the
usual stress-tensor used in order to derive the braneworld sum rules, we find
out some important constraints obeyed by time dependent brane tensions. In
particular it is shown that the tensions of two Randall-Sundrum like branes
obeying, at the same time, an Eotvos law, aggravate the fine tuning problem.
Also, it is shown that if the hidden brane tension obeys an Eotvos law, then
the visible brane has a mixed behavior allowing a bouncing-like period at early
times while it is dominated by an Eotvos law nowadays. To finalize, we discuss
some qualitative characteristics which may arise in the scope of dynamical
brane tensions, as anisotropic background and branons production.Comment: 7 pages, 1 figure, accepted for publication in Physical Review
Produção de inóculo de fungos micorrízicos arbusculares (FMA) utilizando argila expandida (Terrcottem).
Este trabalho teve como objetivo avaliar a influência de três tipos de substratos na esporulação de três espécies de fungos micorrízicos. O experimento foi conduzido em casa de vegetação, da Embrapa Semi-Árido, Petrolina-PE
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