741 research outputs found
Optimum ground states for spin- chains
We present a set of {\em optimum ground states} for a large class of
spin- chains. Such global ground states are simultaneously ground
states of the local Hamiltonian, i.e. the nearest neighbour interaction in the
present case. They are constructed in the form of a matrix product. We find
three types of phases, namely a {\em weak antiferromagnet}, a {\em weak
ferromagnet}, and a {\em dimerized antiferromagnet}. The main physical
properties of these phases are calculated exactly by using a transfer matrix
technique, in particular magnetization and two spin correlations. Depending on
the model parameters, they show a surprisingly rich structure.Comment: LaTeX, 22 pages, 6 embedded Postscript figure
Spin-3/2 models on the Cayley tree -- optimum ground state approach
We present a class of optimum ground states for spin-3/2 models on the Cayley
tree with coordination number 3. The interaction is restricted to nearest
neighbours and contains 5 continuous parameters. For all values of these
parameters the Hamiltonian has parity invariance, spin-flip invariance, and
rotational symmetry in the xy-plane of spin space. The global ground states are
constructed in terms of a 1-parametric vertex state model, which is a direct
generalization of the well-known matrix product ground state approach. By using
recursion relations and the transfer matrix technique we derive exact
analytical expressions for local fluctuations and longitudinal and transversal
two-point correlation functions.Comment: LaTeX 2e, 8 embedded eps figures, 14 page
Mixed Heisenberg Chains. II. Thermodynamics
We consider thermodynamic properties, e.g. specific heat, magnetic
susceptibility, of alternating Heisenberg spin chains. Due to a hidden Ising
symmetry these chains can be decomposed into a set of finite chain fragments.
The problem of finding the thermodynamic quantities is effectively separated
into two parts. First we deal with finite objects, secondly we can incorporate
the fragments into a statistical ensemble. As functions of the coupling
constants, the models exhibit special features in the thermodynamic quantities,
e.g. the specific heat displays double peaks at low enough temperatures. These
features stem from first order quantum phase transitions at zero temperature,
which have been investigated in the first part of this work.Comment: 12 pages, RevTeX, 12 embedded eps figures, cf. cond-mat/9703206,
minor modification
Composition and degradation of organic matter in sediments from the Peru-Chile upwelling region
Exact ground states of quantum spin-2 models on the hexagonal lattice
We construct exact non-trivial ground states of spin-2 quantum
antiferromagnets on the hexagonal lattice. Using the optimum ground state
approach we determine the ground state in different subspaces of a general
spin-2 Hamiltonian consistent with some realistic symmetries. These states,
which are not of simple product form, depend on two free parameters and can be
shown to be only weakly degenerate. We find ground states with different types
of magnetic order, i.e. a weak antiferromagnet with finite sublattice
magnetization and a weak ferromagnet with ferrimagnetic order. For the latter
it is argued that a quantum phase transition occurs within the solvable
subspace.Comment: 7 pages, accepted for publication in Phys. Rev.
Photo-lability of deep ocean dissolved black carbon
Dissolved black carbon (DBC), defined here as condensed aromatics isolated from seawater via PPL solid phase extraction and quantified as benzenepolycarboxylic acid (BPCA) oxidation products, is a significant component of the oceanic dissolved organic carbon (DOC) pool. These condensed aromatics are widely distributed in the open ocean and appear to be tens of thousands of years old. As such DBC is regarded as highly refractory. In the current study, the photo-lability of DBC, DOC and coloured dissolved organic matter (CDOM; ultraviolet-visible absorbance) were determined over the course of a 28 day irradiation of North Atlantic Deep Water under a solar simulator. During the irradiation DBC fell from 1044 ± 164 nM-C to 55 ± 15 nM-C, a 20-fold decrease in concentration. Dissolved black carbon photo-degradation was more rapid and more extensive than for bulk CDOM and DOC. The concentration of DBC correlated with CDOM absorbance and the quality of DBC indicated by the ratios of different BPCAs correlated with CDOM absorbance spectral slope, suggesting the optical properties of CDOM may provide a proxy for both DBC concentrations and quality in natural waters. Further, the photo-lability of components of the DBC pool increased with their degree of aromatic condensation. These trends indicate that a continuum of compounds of varying photo-lability exists within the marine DOC pool. In this continuum, photo-lability scales with aromatic character, specifically the degree of condensation. Scaling the rapid photo-degradation of DBC to rates of DOC photo-mineralisation for the global ocean leads to an estimated photo-chemical half-life for oceanic DBC of less than 800 years. This is more than an order of magnitude shorter than the apparent age of DBC in the ocean. Consequently, photo-degradation is posited as the primary sink for oceanic DBC and the apparent survival of DBC molecules in the oceans for millennia appears to be facilitated not by their inherent inertness but by the rate at which they are cycled through the surface ocean's photic zone
Chlorin Index: A new parameter for organic matter freshness in sediments
Total chlorins, comprising degradation products of chlorophyll, have been used recently to reconstruct paleoproductivity from marine sediment cores. Here, we report on a new index, the Chlorin Index (CI), that proves to be a helpful tool for rapidly estimating organic matter freshness in marine sediments. The CI is a ratio between the fluorescence intensity of a sediment extracted with acetone and treated with hydrochloric acid and the original sediment extract. It represents the ratio of chlorophyll and its degradation products deposited in the sediments that could still be chemically transformed and those that are inert to chemical attack. The ratio is lower in sediments that include freshly deposited phytoplankton material and higher in older, more degraded sediments. We measured this new parameter on surface sediments, and sediments from several short and a long sediment core from different oceanic settings. CI values range from 0.2 for chlorophyll a to 0.36–0.56 for fresh material deposited on the shelf off Namibia to values around 0.67 in sediments off Chile and Peru to values up to 0.97 for sediments in a deep core from the northeastern slope of the Arabian Sea. We have compared the CI to rates of bacterial sulfate reduction, as a direct measure of organic matter reactivity and to other degradation indices based on amino acid composition. We conclude that the CI is a reliable and simple tool for the characterization of organic material freshness in sediments in respect to its degradation state
Phase diagram of the asymmetric tetrahedral Ising-Heisenberg chain
The asymmetric tetrahedron is composed by all edges of tetrahedron
represented by Ising interaction except one, which has a Heisenberg type
interaction. This asymmetric tetrahedron is arranged connecting a vertex which
edges are only Ising type interaction to another vertex with same structure of
another tetrahedron. The process is replicated and this kind of lattice we call
the asymmetric Ising-Heisenberg chain. We have studied the ground state phase
diagram for this kind of models. Particularly we consider two situations in the
Heisenberg-type interaction, (i) The asymmetric tetrahedral spin(1/2,1/2)
Ising-XYZ chain, and (ii) the asymmetric tetrahedral spin-(1/2,1) Ising-XXZ
chain, where we have found a rich phase diagram and a number of multicritical
points. Additionally we have also studied their thermodynamics properties and
the correlation function, using the decorated transformation. We have mapped
the asymmetric tetrahedral Ising-Heisenberg chain in an effective Ising chain,
and we have also concluded that it is possible to evaluate the partition
function including a longitudinal external magnetic field.Comment: 14 pages, 8 figures. Accepted in Journal of Physics: Condensed Matte
Организация и ведение поисково и аварийно-спасательных работ на водоемах
Проведение поисковых и аварийно-спасательных работ на воде с применением спасательных судов при возникновении чрезвычайных ситуаций является одной из основных задач Единой государственной системы предупреждения и ликвидации чрезвычайных ситуаций (РСЧС), позволяющих уменьшить жертвы и сохранить здоровье людей. Спасение терпящих бедствие на воде должно быть организовано своевременно, оперативно и комплексно, то есть в полном объеме по всём необходимым в конкретной обстановке видам работ по спасению пострадавших.Conducting search and rescue operations on the water with the use of rescue vessels in the event of emergencies is one of the main tasks of the Unified State System for the Prevention and Elimination of Emergencies (RSES), which makes it possible to reduce casualties and preserve people's health. Salvation of those in distress on the water must be organized in a timely manner, promptly and in a comprehensive manner, that is, in full volume for all kinds of rescue work for the victims
Stripe Ansatzs from Exactly Solved Models
Using the Boltzmann weights of classical Statistical Mechanics vertex models
we define a new class of Tensor Product Ansatzs for 2D quantum lattice systems,
characterized by a strong anisotropy, which gives rise to stripe like
structures. In the case of the six vertex model we compute exactly, in the
thermodynamic limit, the norm of the ansatz and other observables. Employing
this ansatz we study the phase diagram of a Hamiltonian given by the sum of XXZ
Hamiltonians along the legs coupled by an Ising term. Finally, we suggest a
connection between the six and eight-vertex Anisotropic Tensor Product Ansatzs,
and their associated Hamiltonians, with the smectic stripe phases recently
discussed in the literature.Comment: REVTEX4.b4 file, 10 pages, 2 ps Figures. Revised version to appear in
PR
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