633 research outputs found
Quantum stabilization of classically unstable plateau structures
Motivated by the intriguing report, in some frustrated quantum
antiferromagnets, of magnetization plateaus whose simple collinear structure is
{\it not} stabilized by an external magnetic field in the classical limit, we
develop a semiclassical method to estimate the zero-point energy of collinear
configurations even when they do not correspond to a local minimum of the
classical energy. For the spin-1/2 frustrated square-lattice antiferromagnet,
this approach leads to the stabilization of a large 1/2 plateau with
"up-up-up-down" structure for J_2/J_1>1/2, in agreement with exact
diagonalization results, while for the spin-1/2 anisotropic triangular
antiferromagnet, it predicts that the 1/3 plateau with "up-up-down" structure
is stable far from the isotropic point, in agreement with the properties of
Cs_2CuBr_4.Comment: 6 pages, 4 figure
Topologically Protected Loop Flows in High Voltage AC Power Grids
Geographical features such as mountain ranges or big lakes and inland seas
often result in large closed loops in high voltage AC power grids. Sizable
circulating power flows have been recorded around such loops, which take up
transmission line capacity and dissipate but do not deliver electric power.
Power flows in high voltage AC transmission grids are dominantly governed by
voltage angle differences between connected buses, much in the same way as
Josephson currents depend on phase differences between tunnel-coupled
superconductors. From this previously overlooked similarity we argue here that
circulating power flows in AC power grids are analogous to supercurrents
flowing in superconducting rings and in rings of Josephson junctions. We
investigate how circulating power flows can be created and how they behave in
the presence of ohmic dissipation. We show how changing operating conditions
may generate them, how significantly more power is ohmically dissipated in
their presence and how they are topologically protected, even in the presence
of dissipation, so that they persist when operating conditions are returned to
their original values. We identify three mechanisms for creating circulating
power flows, (i) by loss of stability of the equilibrium state carrying no
circulating loop flow, (ii) by tripping of a line traversing a large loop in
the network and (iii) by reclosing a loop that tripped or was open earlier.
Because voltage angles are uniquely defined, circulating power flows can take
on only discrete values, much in the same way as circulation around vortices is
quantized in superfluids.Comment: 12 pages 6 figures + Supplementary Material, Accepted for publication
in New Journal of Physic
Phase diagram of the fully frustrated transverse-field Ising model on the honeycomb lattice
Motivated by the current interest in the quantum dimer model on the
triangular lattice, we investigate the phase diagram of the closely related
fully-frustrated transverse field Ising model on the honeycomb lattice using
classical and semi-classical approximations. We show that, in addition to the
fully polarized phase at large field, the classical model possesses a multitude
of phases that break the translational symmetry which in the dimer language,
correspond to a plaquette phase and a columnar phase separated by an infinite
cascade of mixed phases. The modification of the phase diagram by quantum
fluctuations has been investigated in the context of linear spin-wave theory.
The extrapolation of the semiclassical energies suggests that the plaquette
phase extends down to zero field for spin 1/2, in agreement with the
phase of the quantum dimer model on the triangular
lattice with only kinetic energy.Comment: 15 Pages, 11 Figures, Accepted for PR
Dynamical structure factors and excitation modes of the bilayer Heisenberg model
Using quantum Monte Carlo simulations along with higher-order spin-wave
theory, bond-operator and strong-coupling expansions, we analyse the dynamical
spin structure factor of the spin-half Heisenberg model on the square-lattice
bilayer. We identify distinct contributions from the low-energy Goldstone modes
in the magnetically ordered phase and the gapped triplon modes in the quantum
disordered phase. In the antisymmetric (with respect to layer inversion)
channel, the dynamical spin structure factor exhibits a continuous evolution of
spectral features across the quantum phase transition, connecting the two types
of modes. Instead, in the symmetric channel we find a depletion of the spectral
weight when moving from the ordered to the disordered phase. While the
dynamical spin structure factor does not exhibit a well-defined distinct
contribution from the amplitude (or Higgs) mode in the ordered phase, we
identify an only marginally-damped amplitude mode in the dynamical singlet
structure factor, obtained from interlayer bond correlations, in the vicinity
of the quantum critical point. These findings provide quantitative information
in direct relation to possible neutron or light scattering experiments in a
fundamental two-dimensional quantum-critical spin system.Comment: 19 pages, 15 figure
Magnon mode selective spin transport in compensated ferrimagnets
We investigate the generation of magnonic thermal spin currents and their
mode selective spin transport across interfaces in insulating, compensated
ferrimagnet/normal metal bilayer systems. The spin Seebeck effect signal
exhibits a non-monotonic temperature dependence with two sign changes of the
detected voltage signals. Using different ferrimagnetic garnets, we demonstrate
the universality of the observed complex temperature dependence of the spin
Seebeck effect. To understand its origin, we systematically vary the interface
between the ferrimagnetic garnet and the metallic layer, and by using different
metal layers we establish that interface effects play a dominating role. They
do not only modify the magnitude of the spin Seebeck effect signal but in
particular also alter its temperature dependence. By varying the temperature,
we can select the dominating magnon mode and we analyze our results to reveal
the mode selective interface transmission probabilities for different magnon
modes and interfaces. The comparison of selected systems reveals
semi-quantitative details of the interfacial coupling depending on the
materials involved, supported by the obtained field dependence of the signal
Effect of soil management and training system on negroamaro wine aroma
This study aimed to assess the impact of two soil managements and training systems on yield and wine aroma compounds of Negroamaro variety grown in a warm climate region (southern Italy). Cover crop (CC) and soil tillage (ST) as soil management, whilst bilateral Guyot (BG) and monolateral Guyot (MG) as training systems were compared. Free and bound volatile fractions were evaluated by GC-MS. ST and CC as well as BG and MG significantly affected yield parameters. In particular, yield was higher in ST and BG than in CC and MG, respectively; moreover, it was found to be positively influenced by interaction between BG and ST. Regarding aroma compounds, significant interactions between soil management and training system factors were observed. In case of free volatiles, the most positive interaction was found between BG and ST, whereas, for bound volatiles, the best interaction was represented by MG with both soil tillage and cover crop. Vine leaf area and development over vine growth stages along with water stress levels played an important role in determining the aroma profile as well as yield parameters. In conclusion, the training system significantly interacted with soil management and affected most of important aroma compounds in Negroamaro wine
Reação da cultivar Navelina ISA 315 (Citrus sinensis L. Osb.), à clorose variegada dos citros em condições de campo.
A clorose variegada dos citros (CVC), causada pela bactéria Xylella fastidiosa, está presente no Estado de São Paulo desde 1987. Todas as variedades comerciais de laranjas doces são afetadas. Causa redução da produção, principalmente pela acentuada redução no tamanho dos frutos. (LARANJEIRA et al., 2005). A CVC é transmitida por meio de borbulhas contaminadas e por cigarrinhas das famÃlias Cicadellidae, em citros existem diversas espécies transmissoras de X. fastidiosa, porém a eficiência na transmissão é inferior a 15%, e este Ãndice pode variar entre espécies em função de mecanismos fisiológicos e comportamentais (YAMAMOTO, 2007; LOPES, 1996). Os sintomas caracterÃsticos da doença são cloroses internervais amareladas na face superior da folha com correspondente necroses de tons de marrom na face inferior, a frutificação tem tendência de ser em pencas onde os frutos tornam-se rÃgidos, pequenos e com uma concentração de açúcar maior que o normal (LARANJEIRA et al., 2005).pdf 12
Extended cardiolipin anchorage to cytochrome c: model for protein-mitochondrial membrane binding.
Two models have been proposed to explain the interaction of cytochrome c with cardiolipin (CL) vesicles. In one case, an acyl chain of the phospholipid accommodates into a hydrophobic channel of the protein located close the Asn52 residue, whereas the alternative model considers the insertion of the acyl chain in the region of the Met80-containing loop. In an attempt to clarify which proposal offers a more appropriate explanation of cytochrome c-CL binding, we have undertaken a spectroscopic and kinetic study of the wild type and the Asn52Ile mutant of iso-1-cytochrome c from yeast to investigate the interaction of cytochrome c with CL vesicles, considered here a model for the CL-containing mitochondrial membrane. Replacement of Asn52, an invariant residue located in a small helix segment of the protein, may provide data useful to gain novel information on which region of cytochrome c is involved in the binding reaction with CL vesicles. In agreement with our recent results revealing that two distinct transitions take place in the cytochrome c-CL binding reaction, data obtained here support a model in which two (instead of one, as considered so far) adjacent acyl chains of the liposome are inserted, one at each of the hydrophobic sites, into the same cytochrome c molecule to form the cytochrome c-CL complex
The Met80Ala and Tyr67His/Met80Ala mutants of human cytochrome cshed light on the reciprocal role of Met80 and Tyr67 in regulating ligand access into the heme pocket.
The spectroscopic and functional properties of the single Met80Ala and double Tyr67His/Met80Ala mutants of human cytochrome c have been investigated in their ferric and ferrous forms, and in the presence of different ligands, in order to clarify the reciprocal effect of these two residues in regulating the access of exogenous molecules into the heme pocket. In the ferric state, both mutants display an aquo high spin and a low spin species. The latter corresponds to an OH- ligand in Met80Ala but to a His in the double mutant. The existence of these two species is also reflected in the functional behavior of the mutants. The observation that (i) a significant peroxidase activity is present in the Met80Ala mutants, (ii) the substitution of the Tyr67 by His leads to only a slight increase of the peroxidase activity in the Tyr67His/Met80Ala double mutant with respect to wild type, while the Tyr67His mutant behaves as wild type, as previously reported, suggests that the peroxidase activity of cytochrome c is linked to an overall conformational change of the heme pocket and not only to the disappearance of the Fe-Met80 bond. Therefore, in human cytochrome c there is an interplay between the two residues at positions 67 and 80 that affects the conformation of the distal side of the heme pocket, and thus the sixth coordination of the hem
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