44,802 research outputs found
Quantum critical point in the spin glass-antiferromagnetism competition for fermionic Ising Models
The competition between spin glass () and antiferromagnetic order ()
is analyzed in two sublattice fermionic Ising models in the presence of a
transverse and a parallel magnetic fields. The exchange
interaction follows a Gaussian probability distribution with mean and
standard deviation , but only spins in different sublattices can
interact. The problem is formulated in a path integral formalism, where the
spin operators have been expressed as bilinear combinations of Grassmann
fields. The results of two fermionic models are compared. In the first one, the
diagonal operator has four states, where two eigenvalues vanish (4S
model), which are suppressed by a restriction in the two states 2S model. The
replica symmetry ansatz and the static approximation have been used to obtain
the free energy. The results are showing in phase diagrams ( is the
temperature) {\it versus} , , and . When is
increased, (transition temperature to a nonergodic phase) reduces and
the Neel temperature decreases towards a quantum critical point. The field
always destroys ; however, within a certain range, it favors the
frustration. Therefore, the presence of both fields, and , produces
effects that are in competition. The critical temperatures are lower for the 4S
model and it is less sensitive to the magnetic couplings than the 2S model.Comment: 15 pages, 6 figures, accepted in Physica
Spin Glass and ferromagnetism in disordered Kondo lattice
The competition among spin glass (SG), ferromagnetism and Kondo effect has
been analysed in a Kondo lattice model where the inter-site coupling
between the localized magnetic moments is given by a generalized Mattis model
\cite{Mattis} which represents an interpolation between ferromagnetism and a
highly disordered spin glass. Functional integral techniques with of Grassmann
fields has been used to obtain the partition function. The static approximation
and the replica symmetric ansatz has also been used. The solution of the
problem is presented as a phase diagram temperature {\it versus} (the
strength of the intra-site interaction). If is small, for decreasing
temperature there is a second order transition from a paramagnetic to a spin
glass phase For lower temperatures, a first order transition appears where
solutions for the spin glass order parameter and the local magnetizations are
simultaneously non zero. For very low temperatures, the local magnetizations
becomes thermodinamically stables. For high , the Kondo state is
dominating. These results could be helpful to clarify the experimental
situation of .Comment: 4 pages, 1 figure, accept to be published in Physica
The link between the ecology of the prokaryotic rare biosphere and its biotechnological potential
Current research on the prokaryotic low abundance taxa, the prokaryotic rare biosphere, is growing, leading to a greater understanding of the mechanisms underlying organismal rarity and its relevance in ecology. From this emerging knowledge it is possible to envision innovative approaches in biotechnology applicable to several sectors. Bioremediation and bioprospecting are two of the most promising areas where such approaches could find feasible implementation, involving possible new solutions to the decontamination of polluted sites and to the discovery of novel gene variants and pathways based on the attributes of rare microbial communities. Bioremediation can be improved through the realization that diverse rare species can grow abundant and degrade different pollutants or possibly transfer useful genes. Further, most of the prokaryotic diversity found in virtually all environments belongs in the rare biosphere and remains uncultivatable, suggesting great bioprospecting potential within this vast and understudied genetic pool. This Mini Review argues that knowledge of the ecophysiology of rare prokaryotes can aid the development of future, efficient biotechnology-based processes, products and services. However, this promise may only be fulfilled through improvements in (and optimal blending of) advanced microbial culturing and physiology, metagenomics, genome annotation and editing, and synthetic biology, to name a few areas of relevance. In the future, it will be important to understand how activity profiles relate with abundance, as some rare taxa can remain rare and increase activity, whereas other taxa can grow abundant. The metabolic mechanisms behind those patterns can be useful in designing biotechnological processes.info:eu-repo/semantics/publishedVersio
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