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Exchange Parameters (exchange + parameter)
Kinds of Exchange Parameters Selected AbstractsPressure Dependence of Exchange Parameters and Neel Temperature in La2CuO4ISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2007Anatolii Ya. Microscopic mechanisms responsible for the observed pressure dependencies of the Neel temperature TN and parameters of isotropic and anisotropic exchange interaction in the orthorhombic antiferromagnet La2CuO4 are investigated. Within the framework of the Anderson microscopic theory of superexchange interaction, the expressions establishing interrelation between parameters of isotropic and anisotropic (both symmetric and antisymmetric) exchange interactions and by the structural parameters describing the Cu-O-Cu bonding angle and the Cu-O bond length in La2CuO4 are obtained. Experimentally determined pressure dependencies of structural parameters by H. Takahashi et al., allow one to present pressure dependencies of exchange parameters and TN in an apparent form. [source] Relationship between Carbon Isotope Discrimination, Mineral Content and Gas Exchange Parameters in Vegetative Organs of Wheat Grown under Three Different Water RegimesJOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 3 2010L. Zhu Abstract Carbon isotope discrimination (,) has been proposed as an indirect selection criterion for transpiration efficiency and grain yield in wheat. However, because of high cost for , analysis, attempts have been made to identify alternative screening criteria. Ash content (ma) has been proposed as an alternative criterion for , in wheat and barley. A pot experiment was conducted to analyse the relationship between ,, mineral content and gas exchange parameters in seedlings and leaves of bread wheat (Triticum aestivum L.). Plants of 10 genotypes were cultivated under three different water regimes corresponding to moderate (T3), intermediate (T2) and severe drought (T1) stress obtained by maintaining soil humidity at 75 %, 55 % and 45 % of the humidity at field capacity respectively. , and ma in seedlings and leaves showed significant differences among the three water treatments. Significant positive correlations were found between , and ma in seedlings and leaves at elongation and anthesis stages in severe drought stress (T1). , was negatively associated with potassium (K) content in intermediate drought stress (T2) and positively with magnesium (Mg) content in T2 and T3 (moderate drought stress) in flag leaf at anthesis. There were negative correlations between , and single-leaf intrinsic water-use efficiency (WT) in T2 and T3 at anthesis stage. Stronger positive associations were noted between , and stomatal conductance (gs) in T1 and T2 than in T3 at anthesis. These results suggested that , is a good trait as an indirect selection criterion for genotypic improvement in transpiration efficiency, while ma is a possible alternative criterion of , in wheat vegetative organs, especially in stressed environments. Significant association was found between , and K, Mg and Ca contents that would merit being better investigated. [source] The influence of roughness on some magnetic properties of layered structures of Fe, Co separated Cu or AuPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2006I. Staniucha Abstract The system of two magnetic (M) layers divided by nonmagnetic (N) spacer is considered. Roughness in interface region is introduced employing model proposed by Bruno and Chappert. Presence of roughness leads to modification of interface exchange parameter and interface anisotropy in comparison to samples with ideal interface. The magnetisation distribution and Curie temperature has been calculated using Green function formalism for systems consisting of Fe or Co standing for M and Cu or Au standing for N, respectively. Parameters corresponding to GaAs have been taken into account to characterize the substrate. The results obtained show decreasing of Curie temperature and shift of magnetisation curve with increasing of roughness parameter. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Hydrothermal Synthesis, Structure and Magnetic Properties of a One-Dimensional Iron Arsenate, 1,[NH3(CH2)2NH(CH2)2NH3][Fe2F4(HAsO4)2]EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 19 2004Sandip Chakrabarti Abstract A novel one-dimensional iron arsenate, 1,[NH3(CH2)2NH(CH2)2NH3][Fe2F4(HAsO4)2], built up exclusively from SBU-4 units (two octahedral units sharing an edge and connected through two tetrahedra) with Fe3+ ions in the high-spin state has been synthesized by hydrothermal methods. Structure elucidation reveals that the compound is a two-leg ladder with very weak antiferromagnetic exchanges along the chains. The magnetic susceptibility data show activated behaviour at T = 14 K, suggesting a finite spin gap between the nonmagnetic ground state and the excited triplet state. We have estimated the exchange parameters theoretically based on the S = 5/2 Majumdar,Ghosh model with frustrations and dimerisations. Although the magnetic gap is quite small, this is the first example, to the best of our knowledge, where the rungs with two S = 5/2 spins form strong singlet dimers. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Pressure Dependence of Exchange Parameters and Neel Temperature in La2CuO4ISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2007Anatolii Ya. Microscopic mechanisms responsible for the observed pressure dependencies of the Neel temperature TN and parameters of isotropic and anisotropic exchange interaction in the orthorhombic antiferromagnet La2CuO4 are investigated. Within the framework of the Anderson microscopic theory of superexchange interaction, the expressions establishing interrelation between parameters of isotropic and anisotropic (both symmetric and antisymmetric) exchange interactions and by the structural parameters describing the Cu-O-Cu bonding angle and the Cu-O bond length in La2CuO4 are obtained. Experimentally determined pressure dependencies of structural parameters by H. Takahashi et al., allow one to present pressure dependencies of exchange parameters and TN in an apparent form. [source] Relationship between Carbon Isotope Discrimination, Mineral Content and Gas Exchange Parameters in Vegetative Organs of Wheat Grown under Three Different Water RegimesJOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 3 2010L. Zhu Abstract Carbon isotope discrimination (,) has been proposed as an indirect selection criterion for transpiration efficiency and grain yield in wheat. However, because of high cost for , analysis, attempts have been made to identify alternative screening criteria. Ash content (ma) has been proposed as an alternative criterion for , in wheat and barley. A pot experiment was conducted to analyse the relationship between ,, mineral content and gas exchange parameters in seedlings and leaves of bread wheat (Triticum aestivum L.). Plants of 10 genotypes were cultivated under three different water regimes corresponding to moderate (T3), intermediate (T2) and severe drought (T1) stress obtained by maintaining soil humidity at 75 %, 55 % and 45 % of the humidity at field capacity respectively. , and ma in seedlings and leaves showed significant differences among the three water treatments. Significant positive correlations were found between , and ma in seedlings and leaves at elongation and anthesis stages in severe drought stress (T1). , was negatively associated with potassium (K) content in intermediate drought stress (T2) and positively with magnesium (Mg) content in T2 and T3 (moderate drought stress) in flag leaf at anthesis. There were negative correlations between , and single-leaf intrinsic water-use efficiency (WT) in T2 and T3 at anthesis stage. Stronger positive associations were noted between , and stomatal conductance (gs) in T1 and T2 than in T3 at anthesis. These results suggested that , is a good trait as an indirect selection criterion for genotypic improvement in transpiration efficiency, while ma is a possible alternative criterion of , in wheat vegetative organs, especially in stressed environments. Significant association was found between , and K, Mg and Ca contents that would merit being better investigated. [source] Biomass and leaf-level gas exchange characteristics of three African savanna C4 grass species under optimum growth conditionsAFRICAN JOURNAL OF ECOLOGY, Issue 4 2009K. B. Mantlana Abstract C4 savanna grass species, Digitaria eriantha, Eragrostis lehmanniana and Panicum repens, were grown under optimum growth conditions with the aim of characterizing their above- and below-ground biomass allocation and the response of their gas exchange to changes in light intensity, CO2 concentration and leaf-to-air vapour pressure deficit gradient (Dl). Digitaria eriantha showed the largest above- and below-ground biomass, high efficiency in carbon gain under light-limiting conditions, high water use efficiency (WUE) and strong stomatal sensitivity to Dl (P = 0.002; r2 = 0.5). Panicum repens had a high aboveground biomass and attained high light saturated photosynthetic rates (Asat, 47 ,mol m,2 s,1), stomatal conductance, (gsat, 0.25 mol m,2 s,1) at relatively high WUE. Eragrostis lehmanniana had almost half the biomass of other species, and had similar Asat and gsat but were attained at lower WUE than the other species. This species also showed the weakest stomatal response to Dl (P = 0.19, r2 = 0. 1). The potential ecological significance of the contrasting patterns of biomass allocation and variations in gas exchange parameters among the species are discussed. Résumé On a fait pousser des espèces herbeuses de savane de type C4, Digitaria eriantha, Eragrostis lehmanniana et Panicum repens, dans des conditions optimales dans le but de caractériser l'allocation de leur biomasse aérienne et racinaire et la réponse de leur échange gazeux à des changements d'intensité de la lumière, de concentrations de CO2, et à un gradient déficitaire (Dl) de pression de vapeur feuille-air. D. eriantha montrait la plus grande biomasse aérienne et racinaire, une grande efficience de l'assimilation de carbone dans des conditions de luminosité limitée, une grande efficience d'utilisation de l'eau (WUE) et une forte sensibilité des stomates à Dl (P = 0,002; r2 = 0,5). P. repens avait une grande biomasse aérienne et atteignait des taux photosynthétiques élevés en lumière saturée (Asat, 47 ,mol m,2 s,1), et une conductance stomatique (gsat 0.25 mol m,2 s,1) à une WUE relativement élevée. E. lehmanniana avait une biomasse qui était presque la moitié de celle des autres espèces et avait un Asat et un gsat similaires mais qui étaient atteints à une WUE plus basse que les autres espèces. Cette espèce montrait aussi la plus faible réponse stomatique àDl (P = 0,19, r2 = 0,1). L'on discute de la signification écologique potentielle de ces schémas contrastés d'allocations de biomasse et des variations des paramètres des échanges gazeux entre les espèces. [source] Evidence that Branch Cuvettes are Reasonable Surrogates for Estimating O3 Effects in Entire Tree CrownsPLANT BIOLOGY, Issue 2 2007C. Then& Abstract: Within the scope of quantifying ozone (O3) effects on forest tree crowns it is still an open question whether cuvette branches of adult trees are reasonable surrogates for O3 responses of entire tree crowns and whether twigs exhibit autonomy in defense metabolism in addition to carbon autonomy. Therefore, cuvette-enclosed branches of mature beech (Fagus sylvatica) trees were compared with branches exposed to the same and different ozone regimes by a free-air fumigation system under natural stand conditions by means of a vice versa experiment. For this purpose, cuvettes receiving 1 × O3 air were mounted in trees exposed to 2 × O3 and cuvettes receiving 2 × O3 air were mounted in trees exposed to 1 × O3 in the upper sun crown. At the end of the fumigation period in September 2004, leaves were examined for differences in gas exchange parameters, pigments, antioxidants, carbohydrates, and stable isotope ratios. No significant differences in foliar gas exchange, total carbohydrates, stable isotope ratios, pigment, and antioxidant contents were found as a consequence of cuvette enclosure (cuvette versus free-air branches) of the same O3 concentrations besides increase of glucose inside the cuvettes and reduction of the de-epoxidation state of the xanthophyll cycle pigments. No significant ozone effect was found for the investigated gas exchange and most biochemical parameters. The total and oxidized glutathione level of the leaves was increased by the 2 × O3 treatment in the cuvette and the free-air branches, but this effect was significant only for the free-air branches. From these results we conclude that cuvette branches are useful surrogates for examining the response of entire tree crowns to elevated O3 and that the defence metabolism of twigs seems to be at least partially autonomous. [source] A coupled dispersion and exchange model for short-range dry deposition of atmospheric ammoniaTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 618 2006Benjamin Loubet Abstract The MODDAS-2D model (MOdel of Dispersion and Deposition of Ammonia over the Short-range in two dimensions) is presented. This stationary model couples a two-dimensional Lagrangian stochastic model for short-range dispersion, with a leaf-scale bi-directional exchange model for ammonia (NH3), which includes cuticular uptake and a stomatal compensation point. The coupling is obtained by splitting the upward and downward components of the flux, which can be generalized for any trace gas, and hence provides a way of simply incorporating bi-directional exchanges in existing deposition velocity models. The leaf boundary-layer resistance is parametrized to account for mixed convection in the canopy, and the model incorporates a stability correction for the Lagrangian time-scale for vertical velocity, which tends to increase the Lagrangian time-scale in very stable conditions compared with usual parametrizations. The model is validated against three datasets, where concentrations of atmospheric NH3 were measured at several distances from a line source. Two datasets are over grassland and one is over maize, giving a range of canopy structure. The model correctly simulates the concentration in one situation, but consistently overestimates it at further distances or underestimates it at small distances in the two other situations. It is argued that these discrepancies are mainly due to the lack of length of one of the line sources and non-aligned winds. Analysis shows that the surface exchange parameters and the turbulent mixing at the source level are the predominant factors controlling short-range deposition of NH3. Copyright © 2006 Royal Meteorological Society [source] Sexual dimorphism and seasonal changes of leaf gas exchange in the dioecious tree Ilex paraguariensis grown in two contrasted cultivation typesANNALS OF APPLIED BIOLOGY, Issue 2 2009M. Rakocevic Abstract Yerba maté (Ilex paraguariensis, Aquifoliaceae) is a subtropical, evergreen, dioecious, South American tree. In one preliminary study, it was observed that the functional strategy of yerba mate females, aiming to finish reproductive process, was increased transpiration relative to photosynthetic rates compared with males, on self-shaded leaves. We hypothesised that the long-term gas exchange response of males and females can evolve independently of phenological stage and cultivation type. In this spirit, the primary aim of the study was to analyse the physiological sexual dimorphism of this species, evaluating fluctuations of gas exchanges related to microclimate and phenological stages. A field study was conducted on adult plants of yerba maté cultivated in monoculture (MO) and in forest understorey (FUS), and measurements carried out in situ on microclimate and leaf gas exchange parameters. The photosynthetic photon flux density that was attained at leaf level in FUS was reduced roughly 10-fold compared with that in MO. Various leaf age populations were observed during a 2-year period at 2-month intervals and grouped into four categories: young, young-fully-expanded, fully-expanded and old. Young and young-fully-expanded leaves were the most active in photosynthesis. Leaves of female plants showed greater photosynthetic rate than those of male plants, which was expressed on all leaf age categories in MO, but only during vegetative stages previous to flowering and fruit ripening. The photosynthesis of young-fully-expanded leaves of females grown in FUS was superior to males but only during winter growth pause. The stomatal conductance differed in relation to cultivation type and leaf age but did not show the sexual differentiation. Physiological sexual dimorphism in yerba mate is shown to be plastic, responding to environmental conditions. The cost associated to the reproduction of yerba maté could be most easily met showing physiological differentiation of both sexes. A higher reproductive investment of females might be compensated for by exhibiting greater leaf photosynthesis than males that occurs in vegetative stages that precede flowering and fruit ripening. [source] Fe2 and Fe4 Clusters Encapsulated in Vacant Polyoxotungstates: Hydrothermal Synthesis, Magnetic and Electrochemical Properties, and DFT CalculationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2008Céline Pichon Abstract While the reaction of [PW11O39]7, with first row transition-metal ions Mn+ under usual bench conditions only leads to monosubstituted {PW11O39M(H2O)} anions, we have shown that the use of this precursor under hydrothermal conditions allows the isolation of a family of novel polynuclear discrete magnetic polyoxometalates (POMs). The hybrid asymmetric [FeII(bpy)3][PW11O39Fe2III(OH)(bpy)2],12,H2O (bpy=bipyridine) complex (1) contains the dinuclear {Fe(,-O(W))(,-OH)Fe} core in which one iron atom is coordinated to a monovacant POM, while the other is coordinated to two bipyridine ligands. Magnetic measurements indicate that the FeIII centers in complex 1 are weakly antiferromagnetically coupled (J=,11.2,cm,1, H=,JS1S2) compared to other {Fe(,-O)(,-OH)Fe} systems. This is due to the long distances between the iron center embedded in the POM and the oxygen atom of the POM bridging the two magnetic centers, but also, as shown by DFT calculations, to the important mixing of bridging oxygen orbitals with orbitals of the POM tungsten atoms. The complexes [Hdmbpy]2[FeII(dmbpy)3]2[(PW11O39)2Fe4IIIO2(dmbpy)4],14,H2O (2) (dmbpy=5,5,-dimethyl-2,2,-bipyridine) and H2[FeII(dmbpy)3]2[(PW11O39)2Fe4IIIO2(dmbpy)4],10,H2O (3) represent the first butterfly-like POM complexes. In these species, a tetranuclear FeIII complex is sandwiched between two lacunary polyoxotungstates that are pentacoordinated to two FeIII cations, the remaining paramagnetic centers each being coordinated to two dmbpy ligands. The best fit of the ,MT=f(T) curve leads to Jwb=,59.6,cm,1 and Jbb=,10.2,cm,1 (H=,Jwb(S1S2+S1S2*+S1*S2+S1*S2*),Jbb(S2S2*)). While the Jbb value is within the range of related exchange parameters previously reported for non-POM butterfly systems, the Jwb constant is significantly lower. As for complex 1, this can be justified considering FewO distances. Finally, in the absence of a coordinating ligand, the dimeric complex [N(CH3)4]10[(PW11O39FeIII)2O],12,H2O (4) has been isolated. In this complex, the two single oxo-bridged FeIII centers are very strongly antiferromagnetically coupled (J=,211.7,cm,1, H=,JS1S2). The electrochemical behavior of compound 1 both in dimethyl sulfoxide (DMSO) and in the solid state is also presented, while the electrochemical properties of complex 2, which is insoluble in common solvents, have been studied in the solid state. [source] | ||||||||||||||||