Magneto-structural properties of the layered quasi-2D triangular-lattice antiferromagnets Cs2_2CuCl4−x_{4-x}Brx_x for x{x} = 0,1,2 and 4

We present a study of the magnetic susceptibility $\chi_{mol}$ under variable hydrostatic pressure on single crystals of Cs$_2$CuCl$_{4-x}$Br$_x$. This includes the border compounds \textit{x} = 0 and 4, known as good realizations of the distorted triangular-lattice spin-1/2 Heisenberg antiferromagnet, as well as the isostructural stoichiometric systems Cs$_2$CuCl$_{3}$Br$_1$ and Cs$_2$CuCl$_{2}$Br$_2$. For the determination of the exchange coupling constants $J$ and $J^{\prime}$, $\chi_{mol}$ data were fitted by a $J-J^{\prime}$ model \cite{Schmidt2015}. Its application, validated for the border compounds, yields a degree of frustration $J^{\prime}$/$J$ = 0.47 for Cs$_2$CuCl$_3$Br$_1$ and $J^{\prime}$/$J$ $\simeq$ 0.63 - 0.78 for Cs$_2$CuCl$_2$Br$_2$, making these systems particular interesting representatives of this family. From the evolution of the magnetic susceptibility under pressure up to about 0.4\,GPa, the maximum pressure applied, two observations were made for all the compounds investigated here. First, we find that the overall energy scale, given by $J_c = (J^2$ + $J^{\prime 2}$)$^{1/2}$, increases under pressure, whereas the ratio $J^{\prime}$/$J$ remains unchanged in this pressure range. These experimental observations are in accordance with the results of DFT calculations performed for these materials. Secondly, for the magnetoelastic coupling constants, extraordinarily small values are obtained. We assign these observations to a structural peculiarity of this class of materials

Specific Heat Study of 1D and 2D Excitations in the Layered Frustrated Quantum Antiferromagnets Cs2_2CuCl4−x_{4-x}Brx_x

We report an experimental and theoretical study of the low-temperature specific heat $C$ and magnetic susceptibility $\chi$ of the layered anisotropic triangular-lattice spin-1/2 Heisenberg antiferromagnets Cs$_2$CuCl$_{4-x}$Br$_x$ with $x$ = 0, 1, 2, and 4. We find that the ratio $J'/J$ of the exchange couplings ranges from 0.32 to $\approx 0.78$, implying a change (crossover or quantum phase transition) in the materials' magnetic properties from one-dimensional (1D) behavior for $J'/J < 0.6$ to two-dimensional (2D) behavior for $J'/J \approx 0.78$ behavior. For $J'/J < 0.6$, realized for $x$ = 0, 1, and 4, we find a magnetic contribution to the low-temperature specific heat, $C_{\rm m} \propto T$, consistent with spinon excitations in 1D spin-1/2 Heisenberg antiferromagnets. Remarkably, for $x$ = 2, where $J'/J \approx 0.78$ implies a 2D magnatic character, we also observe $C_{\rm m} \propto T$. This finding, which contrasts the prediction of $C_{\rm m} \propto T^2$ made by standard spin-wave theories, shows that Fermi-like statistics also plays a significant role for the magnetic excitations in frustrated spin-1/2 2D antiferromagnets

Condiciones de germinación de semillas de "Briza subaristata" Lam. recolectadas en diferentes sitios de la Pampa Deprimida (Argentina)

Briza subaristata Lam. es una poácea perenne nativa, de valor forrajero en pastizales naturales de la pradera pampeana (Argentina). El objetivo de este ensayo fue establecer las condiciones de germinación de semillas de diferente origen geográfico. Para las condiciones de este ensayo se concluyó que el sitio de cosecha tuvo un efecto significativo en cuanto a la respuesta a los pre-tratamientos y temperaturas de germinación aplicados. En general, la pre-refrigeración no mejoró el porcentaje de germinación. En algunos sitios mejoró la germinación al utilizar KNO3 para humedecer el sustrato de germinación. Para estimar la calidad fisiológica de cariopsis de Briza subaristata con ocho o más meses poscosecha, temperaturas alternas 25-20 C en períodos de oscuridad/luz de 16/8 h, ausencia de pre-refrigeración y de KNO3, se presentaron como las más adecuadas para todas las muestras, independientemente del origen analizado

Identification of cultivars of foxtail Mollet (Setaria italica (L.) Beauv.) from morphological and biochemical traits of their seed

The objective of this research was to characterise three cultivars of Setaria italica (L.) Beauv. "Foxtail millet": Ñandú INTA, Carapé INTA and Yaguané INTA by means of their seeds. The methodologies applied were: description of seed size, shape and weight, chemical changes to the phenol test, ferrous sulphate test and fluorescence tests, and polyacrylamide gel electrophoresis of proteins. ANOVA and comparison means test (Tukey, p<0.05) were performed. This study revealed that, with the exception of the fluorescence test, Yaguané, Carapé and Ñandú cultivars could be identified by using the set of methodologies considered.Fil: Pallares, I. N.. Universidad Nacional de Lomas de Zamora. Facultad de Ciencias Agrarias; ArgentinaFil: Ferrari, L.. Universidad Nacional de Lomas de Zamora. Facultad de Ciencias Agrarias; ArgentinaFil: Postulka, E. B.. Universidad Nacional de Lomas de Zamora. Facultad de Ciencias Agrarias; ArgentinaFil: Ritta, Monica Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Universidad Nacional de Lomas de Zamora. Facultad de Ciencias Agrarias; Argentin

Spin-dimer networks: engineering tools to adjust the magnetic interactions in biradicals

Magneto-structural correlations in stable organic biradicals have been studied on the example of weakly exchange coupled models with nitronyl nitroxide and imino nitroxide spin-carrying entities. Here, heteroatom substituted 2,2′-diaza- and 3,3′-diaza-tolane bridged biradicals were compared with the hydrocarbon analogue, while a biphenyl model with its 2,2′-bipyridine counterpart. For a 3,3′-diazatolane bridge the torsional angle between the nitronyl nitroxides and the pyridyl rings increased heavily (∼52–54°) leading to a smaller theoretical intra-dimer exchange coupling value. However, a very large antiferromagnetic coupling was obtained experimentally. This could be appropriately explained by the presence of dominating inter-dimer exchange between the molecules. For the bis(imino nitroxide) with tolane bridge a field induced ordered state between 1.8 to 4.3 T in AC-susceptibility measurements was observed. In terms of a Bose Einstein condensate (BEC) of triplons this phenomenon could be described as a magnetic field induced ordered phase with 3D character

Susceptibility measurements on a weakly coupled spin-dimersystem consisting of nitronyl-nitroxid stable biradicals

We present low-temperature susceptibility data on a spinS=12dimersystem consisting of nitronyl-nitroxid radicals which are bridged viatolan molecules. The material, exhibiting a double-layer structure,can be regarded as a quasi- 2D system. From SQUID magnetisationmeasurements on high-quality single crystals, an intra-dimer couplingconstant ofJintra/kB≈9.6 K can be determined. In addition, thedata reveal indications for an inter-dimer coupling of the order of 1 K.In order to look for magnetic field-induced ordering phenomena, AC-susceptibility measurements were performed down to temperatures aslow asT= 34 mK and magnetic fields up toB= 10.5 T. For temperatures belowT= 250 mK, a double-peak structure was observedat fields between 8 T and 9.5 T consistent with the expectation basedon the above-mentioned coupling constants. Furthermore, in the sametemperature range another anomaly aroundB= 4.5 T appears. Fora detailed understanding especially with regard to the dimensionalityof the interactions and the implications for the type of order that isrealized here, Quantum Monte Carlo simulations are underway