2,535 research outputs found
One-dimensional Hubbard model at quarter filling on periodic potentials
Using the Hubbard chain at quarter filling as a model system, we study the
ground state properties of highly doped antiferromagnets. In particular, the
Hubbard chain at quarter filling is unstable against 2k_F- and 4k_F-periodic
potentials, leading to a large variety of charge and spin ordered ground
states. Employing the density matrix renormalization group method, we compare
the energy gain of the ground state induced by different periodic potentials.
For interacting systems the lowest energy is found for a 2k_F-periodic magnetic
field, resulting in a band insulator with spin gap. For strong interaction, the
4k_F-periodic potential leads to a half-filled Heisenberg chain and thus to a
Mott insulating state without spin gap. This ground state is more stable than
the band insulating state caused by any non-magnetic 2k_F-periodic potential.
Adding more electrons, a cluster-like ordering is preferred.Comment: 8 pages, 5 figures, accepted by Phys. Rev.
Stationary distributions of sums of marginally chaotic variables as renormalization group fixed points
We determine the limit distributions of sums of deterministic chaotic
variables in unimodal maps assisted by a novel renormalization group (RG)
framework associated to the operation of increment of summands and rescaling.
In this framework the difference in control parameter from its value at the
transition to chaos is the only relevant variable, the trivial fixed point is
the Gaussian distribution and a nontrivial fixed point is a multifractal
distribution with features similar to those of the Feigenbaum attractor. The
crossover between the two fixed points is discussed and the flow toward the
trivial fixed point is seen to consist of a sequence of chaotic band mergers.Comment: 7 pages, 2 figures, to appear in Journal of Physics: Conf.Series
(IOP, 2010
Magnetic, electronic and vibrational properties of metal and fluorinated metal phthalocyanines
The magnetic and electronic properties of metal phthalocyanines (MPc) and
fluorinated metal phthalocyanines (FMPc) are studied by means of spin
density functional theory (SDFT). Several metals (M) such as Ca, all first
d-row transition metals and Ag are investigated. By considering different open
shell transition metals it is possible to tune the electronic properties of
MPc, in particular the electronic molecular gap and total magnetic moment.
Besides assigning the structural and electronic properties of MPc and
FMPc, the vibrational modes analysis of the ScPc\textendash ZnPc series
have been studied and correlated to experimental measurements when available.Comment: 28 pages (preprint style), several figure
Biogeochemical variations at the Porcupine Abyssal Plain sustained Observatory in the northeast Atlantic Ocean, from weekly to inter-annual timescales
We present high-resolution autonomous measurements of carbon dioxide partial pressure p(CO2) taken in situ at the Porcupine Abyssal Plain sustained Observatory (PAP-SO) in the northeast Atlantic (49° N, 16.5° W; water depth of 4850 m) for the period 2010–2012. Measurements of p(CO2) made at 30 m depth on a sensor frame are compared with other autonomous biogeochemical measurements at that depth (including chlorophyll a fluorescence and nitrate concentration data) to analyse weekly to seasonal controls on p(CO2) flux in the inter-gyre region of the North Atlantic. Comparisons are also made with in situ regional time series data from a ship of opportunity and mixed layer depth (MLD) measurements from profiling Argo floats. There is a persistent under-saturation of CO2 in surface waters throughout the year which gives rise to a perennial CO2 sink. Comparison with an earlier data set collected at the site (2003–2005) confirms seasonal and inter-annual changes in surface seawater chemistry. There is year-to-year variability in the timing of deep winter mixing and the intensity of the spring bloom.The 2010–2012 period shows an overall increase in p(CO2) values when compared to the 2003–2005 period as would be expected from increases due to anthropogenic CO2 emissions. The surface temperature, wind speed and MLD measurements are similar for both periods of time. Future work should incorporate daily CO2 flux measurements made using CO2 sensors at 1 m depth and the in situ wind speed data now available from the UK Met Office Buoy
An algorithmic approach to the existence of ideal objects in commutative algebra
The existence of ideal objects, such as maximal ideals in nonzero rings,
plays a crucial role in commutative algebra. These are typically justified
using Zorn's lemma, and thus pose a challenge from a computational point of
view. Giving a constructive meaning to ideal objects is a problem which dates
back to Hilbert's program, and today is still a central theme in the area of
dynamical algebra, which focuses on the elimination of ideal objects via
syntactic methods. In this paper, we take an alternative approach based on
Kreisel's no counterexample interpretation and sequential algorithms. We first
give a computational interpretation to an abstract maximality principle in the
countable setting via an intuitive, state based algorithm. We then carry out a
concrete case study, in which we give an algorithmic account of the result that
in any commutative ring, the intersection of all prime ideals is contained in
its nilradical
The Enhanced Sensitivity of the Transmission Phase of a Quantum Dot to Kondo Correlations
The strong sensitivity of the transmission phase through a quantum dot
embedded into one arm of a two-wave Aharonov-Bohm interferometer to the Kondo
effect is explained. The enhancement takes place because of the buildup of the
exchange scattering on the dot due to Kondo correlations even much above
. The enhanced exchange competes with the potential scattering, which is
always weak. Both cases of the Anderson impurity model and a multilevel quantum
dot are considered. In the latter case in addition to the description of
peculiar phase behavior a mechanism leading to ferromagnetic Kondo coupling in
quantum dots is proposed.Comment: 4 pages, 2 figure
Renormalization group structure for sums of variables generated by incipiently chaotic maps
We look at the limit distributions of sums of deterministic chaotic variables
in unimodal maps and find a remarkable renormalization group (RG) structure
associated to the operation of increment of summands and rescaling. In this
structure - where the only relevant variable is the difference in control
parameter from its value at the transition to chaos - the trivial fixed point
is the Gaussian distribution and a novel nontrivial fixed point is a
multifractal distribution that emulates the Feigenbaum attractor, and is
universal in the sense of the latter. The crossover between the two fixed
points is explained and the flow toward the trivial fixed point is seen to be
comparable to the chaotic band merging sequence. We discuss the nature of the
Central Limit Theorem for deterministic variables.Comment: 14 pages, 5 figures, to appear in Journal of Statistical Mechanic
Fluctuations of Spatial Patterns as a Measure of Classical Chaos
In problems where the temporal evolution of a nonlinear system cannot be
followed, a method for studying the fluctuations of spatial patterns has been
developed. That method is applied to well-known problems in deterministic chaos
(the logistic map and the Lorenz model) to check its effectiveness in
characterizing the dynamical behaviors. It is found that the indices
are as useful as the Lyapunov exponents in providing a quantitative measure of
chaos.Comment: 10 pages + 7 figures (in ps file), LaTex, Submitted to Phys. Rev.
Damage spreading and dynamic stability of kinetic Ising models
We investigate how the time evolution of different kinetic Ising models
depends on the initial conditions of the dynamics. To this end we consider the
simultaneous evolution of two identical systems subjected to the same thermal
noise. We derive a master equation for the time evolution of a joint
probability distribution of the two systems. This equation is then solved
within an effective-field approach. By analyzing the fixed points of the master
equation and their stability we identify regular and chaotic phases.Comment: 4 pages RevTeX, 2 Postscript figure
A fluorescent polarization-based assay for the identification of disruptors of the RCAN1/calcineurin A protein complex
5 pages, 4 figures, a table. 19891949 [PubMed]Calcineurin is a Ca(2+)/calmodulin-dependent serine/threonine protein phosphatase involved in many biological processes and developmental programs, including immune response. One of the most studied substrates of calcineurin is the transcription factor NFAT (nuclear factor of activated T cells) responsible for T-cell activation. Different anticalcineurin drugs, such as cyclosporine A and FK506, are the most commonly used immunosuppressants in transplantation therapies. Unfortunately, their mechanism of action, completely blocking the calcineurin phosphatase activity while also requiring continuous administration, bears severe side effects. During recent years, the family of regulators of calcineurin (RCAN) has been described and studied extensively as modulators of calcineurin signaling pathways. The RCAN1 region, spanning amino acids 198 to 218 and responsible for inhibiting the calcineurin-NFAT signaling pathway in vivo, has been identified. An RCAN1-derived peptide spanning this sequence interferes with the calcineurin-NFAT interaction without affecting the general calcineurin phosphatase activity. Here we report the development of an optimized in vitro high-throughput fluorescence polarization assay based on the disruption of the RCAN1(198-218)-CnA interaction for identifying molecules with immunosuppressant potential. This approach led us to identify dipyridamole as a disruptor of such interaction. Moreover, three small molecules with a potential immunosuppressive effect were also identifiedThis work was supported by grants from Fundació La Marató de TV3 (Ref. 030830), the Spanish Ministry of Education and Science (SAF2006-04815, BIO2004-00998, BIO2007-60066, CTQ2005-00995/BQU), the Fundación Mutua Madrileña 2007 and from the Generalitat de Catalunya (Ref. 2006 BE 00051)Peer reviewe
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