410 research outputs found
DNA-directed in vitro synthesis of proteins involved in bacterial transcription and translation.
Solution of the Two-Channel Anderson Impurity Model - Implications for the Heavy Fermion UBe -
We solve the two-channel Anderson impurity model using the Bethe-Ansatz. We
determine the ground state and derive the thermodynamics, obtaining the
impurity entropy and specific heat over the full range of temperature. We show
that the low temperature physics is given by a line of fixed points decribing a
two-channel non Fermi liquid behavior in the integral valence regime associated
with moment formation as well as in the mixed valence regime where no moment
forms. We discuss relevance for the theory of UBe.Comment: 4 pages, 2 figures, (to be published in PRL
Intermediate coupling fixed point study in the overscreened regime of generalized multichannel SU(N) Kondo models
We study a generalized multichannel single-impurity Kondo model, in which the
impurity spin is described by a representation of the SU(N) group which
combines bosonic and fermionic degrees of freedom. The impurity spin states are
described by Abrikosov pseudofermions, and we make use of a method initiated by
Popov and Fedotov which allows a proper handling of the fermionic constraint.
The partition function is derived within a path integral approach. We use
renormalization group techniques to calculate the scaling function
perturbatively in powers of the Kondo coupling constant, which is justified in
the weak coupling limit. The truncated expansion is valid in the overscreened
(Nozieres-Blandin) regime, for an arbitrary SU(N) group and any value of the
parameters characterizing the impurity spin representation. The intermediate
coupling fixed point is identified. We derive the temperature dependence of
various physical quantities at low T, controlled by a unique critical exponent,
and show that the physics of the system in the overscreened regime governed by
the intermediate coupling fixed point is characterized by a non-Fermi liquid
behavior. Our results are in accordance with those obtained by other methods,
as Bethe ansatz and boundary conformal field theory, in the case of various
impurity spin symmetries. We establish in a unified way that the Kondo models
in which the impurity spin is described successively by a fundamental,
symmetric, antisymmetric and mixed symmetry representation yield all the same
low-energy physics in the overscreened regime. Possible generalizations of the
analysis we present to the case of arbitrary impurity spin representations of
SU(N) are also discussed.Comment: 21 pages, 7 figures, REVTeX; final version accepted for publicatio
Effects of seagrasses and algae of the Caulerpa family on hydrodynamics and particle-trapping rates
The widespread decline of seagrass beds within the Mediterranean often results in the replacement of seagrasses by opportunistic green algae of the Caulerpa family. Because Caulerpa beds have a different height, stiffness and density compared to seagrasses, these changes in habitat type modify the interaction of the seafloor with hydrodynamics, influencing key processes such as sediment resuspension and particle trapping. Here, we compare the effects on hydrodynamics and particle trapping of Caulerpa taxifolia, C. racemosa, and C. prolifera with the Mediterranean seagrasses Cymodocea nodosa and Posidonia oceanica. All macrophyte canopies reduced near-bed volumetric flow rates compared to bare sediment, vertical profiles of turbulent kinetic energy revealed peak values around the top of the canopies, and maximum values of Reynolds stress increased by a factor of between 1.4 (C. nodosa) and 324.1 (P. oceanica) when vegetation was present. All canopies enhanced particle retention rates compared to bare sediment. The experimental C. prolifera canopy was the most effective at particle retention (m2 habitat); however, C. racemosa had the largest particle retention capacity per structure surface area. Hence, in terms of enhancing particle trapping and reducing hydrodynamic forces at the sediment surface, Caulerpa beds provided a similar or enhanced function compared to P.oceanica and C. nodosa. However, strong seasonality in the leaf area index of C. racemosa and C. taxifolia within the Mediterranean, combined with a weak rhizome structure, suggests that sediments maybe unprotected during winter storms, when most erosion occurs. Hence, replacement of seagrass beds with Caulerpa is likely to have a major influence on annual sediment dynamics at ecosystem scales.This research was funded by the European Network of Excellence ‘‘Marine Biodiversity and Ecosystem Function’’ (MarBEF); FP6, EC contract no. 505446 and a grant from the Fundacio ´n BBVA. EPM was supported by a European Union Marie Curie host fellowship for transfer of knowledge, MTKD-CT-2004-509254, the Spanish national project EVAMARIA (CTM2005-00395/MAR) and the regional government of Andalusia project FUNDIV(P07-RNM-2516)
Thermodynamics of the dissipative two-state system: a Bethe Ansatz study
The thermodynamics of the dissipative two-state system is calculated exactly
for all temperatures and level asymmetries for the case of Ohmic dissipation.
We exploit the equivalence of the two-state system to the anisotropic Kondo
model and extract the thermodynamics of the former by solving the thermodynamic
Bethe Ansatz equations of the latter. The universal scaling functions for the
specific heat and static dielectric susceptibility
are extracted for all dissipation strengths for
both symmetric and asymmetric two-state systems. The logarithmic corrections to
these quantities at high temperatures are found in the Kondo limit , whereas for we find the expected power law temperature
dependences with the powers being functions of the dissipative coupling
. The low temperature behaviour is always that of a Fermi liquid.Comment: 24 pages, 32 PS figures. Typos corrected, final versio
Wanted dead or alive : high diversity of macroinvertebrates associated with living and ’dead’ Posidonia oceanica matte
The Mediterranean endemic seagrass Posidonia
oceanica forms beds characterised by a dense leaf canopy
and a thick root-rhizome ‘matte’. Death of P. oceanica
shoots leads to exposure of the underlying matte, which
can persist for many years, and is termed ‘dead’ matte.
Traditionally, dead matte has been regarded as a degraded
habitat. To test whether this assumption was
true, the motile macroinvertebrates of adjacent living
(with shoots) and dead (without shoots) matte of
P. oceanica were sampled in four different plots located
at the same depth (5–6 m) in Mellieha Bay, Malta
(central Mediterranean). The total number of species
and abundance were significantly higher (ANOVA;
P<0.05 and P<0.01, respectively) in the dead matte
than in living P. oceanica matte, despite the presence of
the foliar canopy in the latter. Multivariate analysis
(MDS) clearly showed two main groups of assemblages,
corresponding to the two matte types. The amphipods
Leptocheirus guttatus and Maera grossimana, and the
polychaete Nereis rava contributed most to the dissimilarity
between the two different matte types. Several
unique properties of the dead matte contributing to the
unexpected higher number of species and abundance of
motile macroinvertebrates associated with this habitat
are discussed. The findings have important implications
for the conservation of bare P. oceanica matte, which
has been generally viewed as a habitat of low ecological
value.peer-reviewe
Prevalência de rotavírus do grupo A em fezes diarréicas de bezerros de corte em sistema semi-intensivo de produção.
Determinou-se a prevalência de rotavírus durante surto de diarréia em bezerros de um rebanho de corte, criado em regime semi-intensivo de produção. Analisaram-se, por meio de técnicas de eletroforese em gel de poliacrilamida (EGPA) e ensaio imunoenzimático (kit EIARA - Fiocruz), 69 amostras de fezes de bezerros, entre 30 e 60 dias de idade, colhidas em três estações de parição consecutivas (agosto a novembro/1999, janeiro a abril e agosto a novembro/2000). Pelo EIARA foram detectadas 63,8% (44/69) de amostras positivas. Na primeira estação de parição foi detectado rotavírus em 82,4% (14/17) dos bezerros que apresentaram quadro clínico de diarréia. No ano de 2000 a presença de rotavírus foi detectada em 41,7% (5/12) e 62,5% (25/40) do total de amostras examinadas. A análise do perfil eletroforético do genoma indicou grande diversidade, com quatro eletroferótipos distintos, todos com perfil longo, característico de rotavírus do grupo A
Intra and Inter-Population Morphological Variation of Shape and Size of the Chilean Magnificent Beetle, Ceroglossus chilensis in the Baker River Basin, Chilean Patagonia
The alteration of habitat generates different degrees of stress in insects. It has been suggested that the degrees of phenotypic disturbances reflect the ability of an individual to overcome the effects of stress. The Baker River Basin in the Aysén Region, Chilean Patagonia has a very fragmented landscape, due to the destruction of the native forest and the use of land for agriculture and animal husbandry. This alteration should generate different degrees of disturbances in the insect communities, whose effects may be quantified by geometric morphometric tools. We analyzed morphological differences in 244 males and 133 females of the the Chilean magnificent beetle, Ceroglossus chilensis (Eschscholtz) (Coleoptera: Carabidae) collected in January, 2007, in mixed forests of Nothofagus dombeyi Mirbel (Ørsted) (Fagales: Nothofagaceae) and N. nitida Hofmus and in Second-growth forest of N. pumilio (Poepp. & Endl.) Krasser. Males were generally wider in the pronotum, while females had wider abdominal sternites. Although there were significant differences in shape and size between mature forests and second-growth forest, these were less significant among the sites within each type of vegetal formation. Individuals had more shape variations in the mature forest. We suggest that differences in shape are due at least in part to the isolation of the habitat. The differences found between sexes raises the question of how morphological variations and sexual dimorphism may be affected spatially by natural selection
Transition from a strong-coupling fixed point to an intermediate-coupling fixed point in a single-channel SU(N) Kondo model: role of the filling and two-stage screening
We study an extended SU(N) single-impurity Kondo model in which the impurity
spin is described by a combination of Abrikosov fermions and Schwinger bosons.
Our aim is to describe both the quasiparticle-like excitations and the locally
critical modes observed in various physical situations, including non-Fermi
liquid behavior in heavy fermion systems in the vicinity of a quantum critical
point. We identify the strong coupling fixed point of the model and study its
stability within second order perturbation theory. Already in the single
channel case and in contrast with either the pure bosonic or the pure fermionic
case, the strong coupling fixed point is unstable against the conduction
electron kinetic term as soon as the amount of Abrikosov fermions reaches a
critical value. In the stability region, the partially screened, dressed
impurity at site 0 repels the conduction electrons on adjacent sites. In the
instability region, the impurity tends to attract conduction electrons
to the neighboring sites, giving rise to a two-stage Kondo effect with
additional screening.This result opens the route to the existence of an
intermediate coupling fixed point, characterized by non-Fermi liquid behavior
Forgotten pilots, airfields and aircraft:A transdisciplinary approach to the memory of the Republican Air Force during the Spanish Civil War (1936–39)
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