968 research outputs found
Numerical study of the ordering of the +-J XY spin-glass ladder
The properties of the domain-wall energy and of the correlation length are
studied numerically for the one-dimensional +-J XY spin glass on the two-leg
ladder lattice, focusing on both the spin and the chirality degrees of freedom.
Analytic results obtained by Ney-Niftle et al for the same model were confirmed
for asymptotically large lattices, while the approach to the asymptotic limit
is slow and sometimes even non-monotonic. Attention is called to the occurrence
of the SO(2)-Z_2 decoupling and its masking in spin correlations, the latter
reflecting the inequality between the SO(2) and Z_2 exponents. Discussion is
given concerning the behaviors of the higher-dimensional models.Comment: 14 pages, 10 figure
Numerical Studies of the Two Dimensional XY Model with Symmetry Breaking Fields
We present results of numerical studies of the two dimensional XY model with
four and eight fold symmetry breaking fields. This model has recently been
shown to describe hydrogen induced reconstruction on the W(100) surface. Based
on mean-field and renormalization group arguments,we first show how the
interplay between the anisotropy fields can give rise to different phase
transitions in the model. When the fields are compatible with each other there
is a continuous phase transition when the fourth order field is varied from
negative to positive values. This transition becomes discontinuous at low
temperatures. These two regimes are separated by a multicritical point. In the
case of competing four and eight fold fields, the first order transition at low
temperatures opens up into two Ising transitions. We then use numerical methods
to accurately locate the position of the multicritical point, and to verify the
nature of the transitions. The different techniques used include Monte Carlo
histogram methods combined with finite size scaling analysis, the real space
Monte Carlo Renormalization Group method, and the Monte Carlo Transfer Matrix
method. Our numerical results are in good agreement with the theoretical
arguments.Comment: 29 pages, HU-TFT-94-36, to appear in Phys. Rev. B, Vol 50, November
1, 1994. A LaTeX file with no figure
Transverse thermal depinning and nonlinear sliding friction of an adsorbed monolayer
We study the response of an adsorbed monolayer under a driving force as a
model of sliding friction phenomena between two crystalline surfaces with a
boundary lubrication layer. Using Langevin-dynamics simulation, we determine
the nonlinear response in the direction transverse to a high symmetry direction
along which the layer is already sliding. We find that below a finite
transition temperature, there exist a critical depinning force and hysteresis
effects in the transverse response in the dynamical state when the adlayer is
sliding smoothly along the longitudinal direction.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
The Atp-dependent Rna Helicase Hrpb Plays An Important Role In Motility And Biofilm Formation In Xanthomonas Citri Subsp Citri
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)RNA helicases are enzymes that catalyze the separation of double-stranded RNA (dsRNA) using the free energy of ATP binding and hydrolysis. DEAD/DEAH families participate in many different aspects of RNA metabolism, including RNA synthesis, RNA folding, RNA-RNA interactions, RNA localization and RNA degradation. Several important bacterial DEAD/DEAH-box RNA helicases have been extensively studied. In this study, we characterize the ATP-dependent RNA helicase encoded by the hrpB (XAC0293) gene using deletion and genetic complementation assays. We provide insights into the function of the hrpB gene in Xanthomonas citri subsp. citri by investigating the roles of hrpB in biofilm formation on abiotic surfaces and host leaves, cell motility, host virulence of the citrus canker bacterium and growth in planta. Results: The hrpB gene is highly conserved in the sequenced strains of Xanthomonas. Mutation of the hrpB gene (Delta hrpB) resulted in a significant reduction in biofilms on abiotic surfaces and host leaves. Delta hrpB also exhibited increased cell dispersion on solid medium plates. Delta hrpB showed reduced adhesion on biotic and abiotic surfaces and delayed development in disease symptoms when sprayed on susceptible citrus leaves. Quantitative reverse transcription-PCR assays indicated that deletion of hrpB reduced the expression of four type IV pili genes. The transcriptional start site of fimA (XAC3241) was determined using rapid amplification of 5'-cDNA Ends (5' RACE). Based on the results of fimA mRNA structure predictions, the fimA 5' UTR may contain three different loops. HrpB may be involved in alterations to the structure of fimA mRNA that promote the stability of fimA RNA. Conclusions: Our data show that hrpB is involved in adherence of Xanthomonas citri subsp. citri to different surfaces. In addition, to the best of our knowledge, this is the first time that a DEAH RNA helicase has been implicated in the regulation of type IV pili in Xanthomonas.16INCT CitrusCAPES/PSDE fellowship [99999.002657/2014-07]CNPqCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq
Association of caspase 3 activation and h2ax γ phosphorylation in the aging brain: Studies on untreated and irradiated mice
Phosphorylation of H2AX is a response to DNA damage, but γH2AX also associates with mitosis and/or apoptosis. We examined the effects of X-rays on DNA integrity to shed more light on the significance of H2AX phosphorylation and its relationship with activation of caspase 3 (CASP3), the main apoptotic effector. After administration of the S phase marker BrdU, brains were collected from untreated and irradiated (10 Gray) 24-month-old mice surviving 15 or 30 min after irradiation. After paraffin embedding, brain sections were single- or double-stained with antibodies against γH2AX, p53-binding protein 1 (53BP1) (which is recruited during the DNA damage response (DDR)), active CASP3 (cCASP3), 5-Bromo-2-deoxyuridine (BrdU), and phosphorylated histone H3 (pHH3) (which labels proliferating cells). After statistical analysis, we demonstrated that irradiation not only induced a robust DDR with the appearance of γH2AX and upregulation of 53BP1 but also that cells with damaged DNA attempted to synthesize new genetic material from the rise in BrdU immunostaining, with increased expression of cCASP3. Association of γH2AX, 53BP1, and cCASP3 was also evident in normal nonirradiated mice, where DNA synthesis appeared to be linked to disturbances in DNA repair mechanisms rather than true mitotic activity
Diluted Josephson-junction arrays in a magnetic field: phase coherence and vortex glass thresholds
The effects of random dilution of junctions on a two-dimensional
Josephson-junction array in a magnetic field are considered. For rational
values of the average flux quantum per plaquette , the superconducting
transition temperature vanishes, for increasing dilution, at a critical value
, while the vortex ordering remains stable up to , much
below the value corresponding to the geometric percolation threshold. For
, the array behaves as a zero-temperature vortex-glass.
Numerical results for from defect energy calculations are presented
which are consistent with this scenario.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Dynamical transitions and sliding friction in the two-dimensional Frenkel-Kontorova model
The nonlinear response of an adsorbed layer on a periodic substrate to an
external force is studied via a two dimensional uniaxial Frenkel-Kontorova
model. The nonequlibrium properties of the model are simulated by Brownian
molecular dynamics. Dynamical phase transitions between pinned solid, sliding
commensurate and incommensurate solids and hysteresis effects are found that
are qualitatively similar to the results for a Lennard-Jones model, thus
demonstrating the universal nature of these features.Comment: 11 pages, 12 figures, to appear in Phys. Rev.
Glassy phases and driven response of the phase-field-crystal model with random pinning
We study the structural correlations and the nonlinear response to a driving
force of a two-dimensional phase-field-crystal model with random pinning. The
model provides an effective continuous description of lattice systems in the
presence of disordered external pinning centers, allowing for both elastic and
plastic deformations. We find that the phase-field crystal with disorder
assumes an amorphous glassy ground state, with only short-ranged positional and
orientational correlations even in the limit of weak disorder. Under increasing
driving force, the pinned amorphous-glass phase evolves into a moving
plastic-flow phase and then finally a moving smectic phase. The transverse
response of the moving smectic phase shows a vanishing transverse critical
force for increasing system sizes
GMOS Spectroscopy of SCUBA Galaxies Behind A851
We have identified counterparts to two submillimeter (submm) sources, SMM
J09429+4659 and SMM J09431+4700, seen through the core of the z=0.41 cluster
Abell 851. We employ deep 1.4-GHz observations and the far-infrared/radio
correlation to refine the submm positions and then optical and near-infrared
imaging to locate their counterparts. We identify an extremely red counterpart
to SMM J09429+4659, while GMOS spectroscopy with Gemini-North shows that the
R=23.8 radio source identified with SMM J09431+4700 is a hyperluminous infrared
galaxy (L_FIR~1.5x10^13 L_sun) at z=3.35, the highest spectroscopic redshift so
far for a galaxy discovered in the submm. The emission line properties of this
galaxy are characteristic of a narrow-line Seyfert-1, although the lack of
detected X-ray emission in a deep XMM-Newton observation suggests that the bulk
of the luminosity of this galaxy is derived from massive star formation. We
suggest that active nuclei, and the outflows they engender, may be an important
part of the evolution of the brightest submm galaxies at high redshifts.Comment: to appear in the Oct 1 issue of ApJ Letter
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