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 $\sqrt{12}\times\sqrt{12}$ 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 famílias Cicadellidae, em citros existem diversas espécies transmissoras de X. fastidiosa, porém a eficiência na transmissão é inferior a 15%, e este índice pode variar entre espécies em função de mecanismos fisiológicos e comportamentais (YAMAMOTO, 2007; LOPES, 1996). Os sintomas característicos 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 rígidos, 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