17,038 research outputs found
Video switcher for coupling video cameras to single TV monitor
Device couples up to 60 TV cameras to single monitor. Video switching is provided by diode matrix arranged in a 60-by-1 configuration. Switcher can be operated manually or automatically
Decorated Shastry-Sutherland lattice in the spin-1/2 magnet CdCu2(BO3)2
We report the microscopic magnetic model for the spin-1/2 Heisenberg system
CdCu2(BO3)2, one of the few quantum magnets showing the 1/2-magnetization
plateau. Recent neutron diffraction experiments on this compound [M. Hase et
al., Phys. Rev. B 80, 104405 (2009)] evidenced long-range magnetic order,
inconsistent with the previously suggested phenomenological magnetic model of
isolated dimers and spin chains. Based on extensive density-functional theory
band structure calculations, exact diagonalizations, quantum Monte Carlo
simulations, third-order perturbation theory, as well as high-field
magnetization measurements, we find that the magnetic properties of CdCu2(BO3)2
are accounted for by a frustrated quasi-2D magnetic model featuring four
inequivalent exchange couplings: the leading antiferromagnetic coupling J_d
within the structural Cu2O6 dimers, two interdimer couplings J_t1 and J_t2,
forming magnetic tetramers, and a ferromagnetic coupling J_it between the
tetramers. Based on comparison to the experimental data, we evaluate the ratios
of the leading couplings J_d : J_t1 : J_t2 : J_it = 1 : 0.20 : 0.45 : -0.30,
with J_d of about 178 K. The inequivalence of J_t1 and J_t2 largely lifts the
frustration and triggers long-range antiferromagnetic ordering. The proposed
model accounts correctly for the different magnetic moments localized on
structurally inequivalent Cu atoms in the ground-state magnetic configuration.
We extensively analyze the magnetic properties of this model, including a
detailed description of the magnetically ordered ground state and its evolution
in magnetic field with particular emphasis on the 1/2-magnetization plateau.
Our results establish remarkable analogies to the Shastry-Sutherland model of
SrCu2(BO3)2, and characterize the closely related CdCu2(BO3)2 as a material
realization for the spin-1/2 decorated anisotropic Shastry-Sutherland lattice.Comment: 16 pages, 13 figures, 2 tables. Published version with additional QMC
dat
Semiclassical approach to the ac-conductance of chaotic cavities
We address frequency-dependent quantum transport through mesoscopic
conductors in the semiclassical limit. By generalizing the trajectory-based
semiclassical theory of dc quantum transport to the ac case, we derive the
average screened conductance as well as ac weak-localization corrections for
chaotic conductors. Thereby we confirm respective random matrix results and
generalize them by accounting for Ehrenfest time effects. We consider the case
of a cavity connected through many leads to a macroscopic circuit which
contains ac-sources. In addition to the reservoir the cavity itself is
capacitively coupled to a gate. By incorporating tunnel barriers between cavity
and leads we obtain results for arbitrary tunnel rates. Finally, based on our
findings we investigate the effect of dephasing on the charge relaxation
resistance of a mesoscopic capacitor in the linear low-frequency regime
Absence of long-range order in a spin-half Heisenberg antiferromagnet on the stacked kagome lattice
We study the ground state of a spin-half Heisenberg antiferromagnet on the
stacked kagome lattice by using a spin-rotation-invariant Green's-function
method. Since the pure two-dimensional kagome antiferromagnet is most likely a
magnetically disordered quantum spin liquid, we investigate the question
whether the coupling of kagome layers in a stacked three-dimensional system may
lead to a magnetically ordered ground state. We present spin-spin correlation
functions and correlation lengths. For comparison we apply also linear spin
wave theory. Our results provide strong evidence that the system remains
short-range ordered independent of the sign and the strength of the interlayer
coupling
The spin 1/2 Heisenberg star with frustration II: The influence of the embedding medium
We investigate the spin 1/2 Heisenberg star introduced in J. Richter and A.
Voigt, J. Phys. A: Math. Gen. {\bf 27}, 1139 (1994). The model is defined by
; , . In extension to the Ref. we consider a more general
describing the properties of the spins surrounding the
central spin . The Heisenberg star may be considered as an essential
structure element of a lattice with frustration (namely a spin embedded in a
magnetic matrix ) or, alternatively, as a magnetic system with a
perturbation by an extra spin. We present some general features of the
eigenvalues, the eigenfunctions as well as the spin correlation of the model. For being a linear chain, a square
lattice or a Lieb-Mattis type system we present the ground state properties of
the model in dependence on the frustration parameter .
Furthermore the thermodynamic properties are calculated for being a
Lieb--Mattis antiferromagnet.Comment: 16 pages, uuencoded compressed postscript file, accepted to J. Phys.
A: Math. Ge
Effect of magnetic pair breaking on Andreev bound states and resonant supercurrent in quantum dot Josephson junctions
We propose a model for resonant Josephson tunneling through quantum dots that
accounts for Cooper pair-breaking processes in the superconducting leads caused
by a magnetic field or spin-flip scattering. The pair-breaking effect on the
critical supercurrent and the Josephson current-phase relation
is largely due to the modification of the spectrum of Andreev bound states
below the reduced (Abrikosov-Gorkov) quasiparticle gap. For a quantum dot
formed in a quasi-one-dimensional channel, both and can show a
significant magnetic field dependence induced by pair breaking despite the
suppression of the orbital magnetic field effect in the channel. This case is
relevant to recent experiments on quantum dot Josephson junctions in carbon
nanotubes. Pair-breaking processes are taken into account via the relation
between the Andreev scattering matrix and the quasiclassical Green functions of
the superconductors in the Usadel limit.Comment: 5 pages, 6 eps figures, new results adde
Resistance Breeding in Apple at Dresden-Pillnitz
Resistance breeding in apple has a long tradition at the Institute of Fruit Breeding now Julius Kuehn-institute in Dresden-Pillnitz. The breeding was aimed at the production of multiple resistance cultivars to allow a more sustainable and environmentally friendly production of apple. In the last decades a series of resistant cultivars (Re®-cultivars) bred in Dresden-Pillnitz has been released, ‘Recolor’ and ‘Rekarda’ in 2006. The main topic in the resistance breeding programme was scab resistance and the donor of scab resistance in most cultivars was Malus x floribunda 821. Due to the development of strains that are able to overcome resistance genes inherited by M. x floribunda 821 and due to the fact that single resistance genes can be broken easily, pyramiding of resistance genes is necessary. Besides scab, fire blight and powdery mildew are the main disease for which a pyramiding of genes is aspired in Pillnitz. Biotechnical approaches are necessary for the early detection of pyramided resistance genes in breeding clones. This paper will give an overview of the resistance breeding of apple in Pillnitz and the methods used
Contribuição da bracatinga (Mimosa scabrella) para a produção de serapilheira em sistema silvipastoril agroecológico.
O presente trabalho teve por objetivo analisar a produção e o conteúdo de nutrientes da serapilheira produzida por árvores de bracatinga em uma área de pastagem agroecológica, localizada no CPRA ? Centro Paranaense de Referência em Agroecologia. As coletas de material foram efetuadas mensalmente, entre fevereiro de 2010 e julho de 2012. Com base na quantidade de matéria seca e dos teores de N, P, K, Ca e Mg foi obtida a estimativa da quantidade destes elementos aportados à pastagem pela serapilheira. A produção média de serapilheira foi de 2.006 kg de matéria seca por hectare por ano, correspondendo a uma incorporação ao solo de 30 kg de N, 0,4 kg de P, 3,8 kg de K, 7,4 kg de Ca e 3,2 kg de Mg
Sistemas silvipastoriles agroecológicos con espécies nativas como estratégia de conservassem de la biodiversidad.
IUFROLAT 2013. Disponibilizado online. Resumen
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