Energy-level ordering and ground-state quantum numbers for frustrated two-leg spin-1/2 ladder model

The Lieb-Mattis theorem about antiferromagnetic ordering of energy levels on bipartite lattices is generalized to finite-size two-leg spin-1/2 ladder model frustrated by diagonal interactions. For reflection-symmetric model with site-dependent interactions we prove exactly that the lowest energies in sectors with fixed total spin and reflection quantum numbers are monotone increasing functions of total spin. The nondegeneracy of most levels is proved also. We also establish the uniqueness and obtain the spin value of the lowest-level multiplet in the whole sector formed by reflection-symmetric (antisymmetric) states. For a wide range of coupling constants, we prove that the ground state is a unique spin singlet. For other values of couplings, it may be also a unique spin triplet or may consist of both multiplets. Similar results have been obtained for the ladder with arbitrary boundary impurity spin. Some partial results have also been obtained in the case of periodical boundary conditions.Comment: 17 page

Determination of exchange energies in the sawtooth spin ring {Mo75V20} by ESR

The magnetism of the polyoxometalate cluster {Mo75V20}, containing a sawtooth ring of 10 corner-sharing triangles located on the equator of the barrel-shaped molecule, has remained debatable since it is masked by contributions from impurities as well as temperature-independent paramagnetism. In this article we demonstrate the usefulness of ESR measurements since the temperature dependence of the ESR intensity can discriminate between impurity and molecular contributions. We determine the exchange parameters and therefore also the low-lying spectrum of {Mo75V20}, especially the low-lying singlet states which so far have been probed solely by specific heat measurements.Comment: 5 pages, 5 figures, submitted to PR

Metamagnetic phase transition of the antiferromagnetic Heisenberg icosahedron

The observation of hysteresis effects in single molecule magnets like Mn$_{12}$-acetate has initiated ideas of future applications in storage technology. The appearance of a hysteresis loop in such compounds is an outcome of their magnetic anisotropy. In this Letter we report that magnetic hysteresis occurs in a spin system without any anisotropy, specifically, where spins mounted on the vertices of an icosahedron are coupled by antiferromagnetic isotropic nearest-neighbor Heisenberg interaction giving rise to geometric frustration. At T=0 this system undergoes a first order metamagnetic phase transition at a critical field \Bcrit between two distinct families of ground state configurations. The metastable phase of the system is characterized by a temperature and field dependent survival probability distribution.Comment: 4 pages, 4 figures, submitted to Physical Review Letter

Statistical Transfer Matrix Study of the ±J\pm J Multileg Ising Ladders and Tubes

Finite temperature properties of symmetric $\pm J$ multileg Ising ladders and tubes are investigated using the statistical transfer matrix method. The temperature dependences of the specific heat and entropy are calculated. In the case of tubes, it is found that the ground state entropy shows an even-odd oscillation with respect to the number of legs. The same type of oscillation is also found in the ground state energy. On the contrary, these oscillations do not take place in ladders. From the temperature-dependence of the specific heat, it is found that the lowest excitation energy is 4J for even-leg ladders while it is 2J otherwise, The physical origin of these behaviors is discussed based on the structure of excitations.Comment: 6 pages, 9 figure

Saturation of Magnetorotational Instability through Magnetic Field Generation

The saturation mechanism of Magneto-Rotational Instability (MRI) is examined through analytical quasilinear theory and through nonlinear computation of a single mode in a rotating disk. We find that large-scale magnetic field is generated through the alpha effect (the correlated product of velocity and magnetic field fluctuations) and causes the MRI mode to saturate. If the large-scale plasma flow is allowed to evolve, the mode can also saturate through its flow relaxation. In astrophysical plasmas, for which the flow cannot relax because of gravitational constraints, the mode saturates through field generation only.Comment: 9 pages, 10 figures to appear in ApJ, Jun 2009, 10 v69

Calculating the energy spectra of magnetic molecules: application of real- and spin-space symmetries

The determination of the energy spectra of small spin systems as for instance given by magnetic molecules is a demanding numerical problem. In this work we review numerical approaches to diagonalize the Heisenberg Hamiltonian that employ symmetries; in particular we focus on the spin-rotational symmetry SU(2) in combination with point-group symmetries. With these methods one is able to block-diagonalize the Hamiltonian and thus to treat spin systems of unprecedented size. In addition it provides a spectroscopic labeling by irreducible representations that is helpful when interpreting transitions induced by Electron Paramagnetic Resonance (EPR), Nuclear Magnetic Resonance (NMR) or Inelastic Neutron Scattering (INS). It is our aim to provide the reader with detailed knowledge on how to set up such a diagonalization scheme.Comment: 29 pages, many figure

Estudio del pleuston y bafon de las lagunas de Chascomus y Yalca

Se estudia la mesofauna asociada a la vegetación flotante y sumergida en las lagunas del epígrafe, realizándose un análisis cuali y cuantitativo de la misma y del sustrato, así como también su variación estacional. Se describe el método utilizado en la obtención de las muestras y extracción de la mesofauna, así como también se pone de manifiesto el criterio seguido para la selección de las “asociaciones vegetales” que sirven de sustrato a la fauna en cuestión. Tres asociaciones flotantes fueron seleccionadas y se individualizaron con el nombre de la especie dominante: Ricciocarpus natans; Azolla filiculoides y Salvinia rotundi- folia; siendo dos las sumergidas: Ceratophyllum demersum y Potamogetón striatus. Se describe cada asociación respecto de su extensión, permanencia y fluctuaciones a lo largo de un año calendario, así como también se da a conocer la lista de presencias de las distintas taxias discriminadas por asociación. Se analizan los pleustontes y bafontes de cada asociación, y sus posibles relaciones interespecíficas y con el sustrato, demostrándose la existencia de especies caracterizantes en cada uno de los mismos, siendo algunas exclusivas; se analizan, además, mediante correlaciones, las relaciones predator-presa y animal sustrato en cada asociación. La aplicación del índice de diversidad específica pone de manifiesto la madurez de las distintas asociaciones flotantes consideradas y la mayor o menor influencia del sustrato como elemento morigerador de los agentes externos en relación con el ciclo anual de los componentes de su mesofauna, evidenciándose en dos de ellos un comportamiento estacional similar al del plancton. De las consideraciones de estas poblaciones y sus relaciones con el ambiente acuático se propone dividir al pleuoton en: eupleuston y hemipleuston y un extrapleuston: pseudopleuston.In the Chascomús and Yalca lagoons, the mesofauna associated with floating and submerged vegetation was studied through a cualitative- quantitative analysis which also included the substratum and the seasonal variations. After a description of the sampling technique employed and the method of extraction of mesofauna, there follow-s an explanation of the criterium adopted in selecting the “vegetable associations’’ which act as a subtratum for the analysed fauna. Three floating associations each named after the dominant species, were selected: Ricciocarpus natans, Azolla filiculoides and Salvinia rotundifolia; two submerged associations were also studied: Ceratophillum demersum and Potamogetón stria- tus. Each association is described in relation to extensión, permanency and fluctuation through a whole calender year, and a list of presences of different taxias discriminated by association is presented. In each association an analysis of the “pleustontes” and ‘ bafontes” and of their interespecific relations as well as those related to the substratum, which have shown the existence of species characteristic of each substratum, some of them exclusive. Furthermore, through correlations, each association was determined the relationships predator-prey and animal-substratum. The application of the “Índex of specific diversity” has shown the maturity of the different floating associations and the greater or lesser influence of the substratum as a moderating element of external agents related to annual cycle of the constituents in the meso- fauna. In two of them a seasonal behavior, similar to that of plankton, has been evinced. From a consideration of these populations and their relation-ships to the acqueos environmets, it is proposed to divide pleuston into: eupleuston, hemipleuston and a extrapleuston: pseudopleuston

Наноалмазы как идеальные наноносители для циансодежащих цитостатиков

Цианосодержащие цитостатики - новый класс открытых нами лекарств, которые благодаря цианогруппам хорошо закрепляются на наноалмазах, с увеличением активности