Proof of phase separation in the binary-alloy problem: the one-dimensional spinless Falicov-Kimball model

The ground states of the one-dimensional Falicov-Kimball model are investigated in the small-coupling limit, using nearly degenerate perturbation theory. For rational electron and ion densities, respectively equal to $\frac{p}{q}$, $\frac{p_i}{q}$, with $p$ relatively prime to $q$ and $\frac{p_i}{q}$ close enough to $\frac{1}{2}$, we find that in the ground state the ion configuration has period $q$. The situation is analogous to the Peierls instability where the usual arguments predict a period-$q$ state that produces a gap at the Fermi level and is insulating. However for $\frac{p_i}{q}$ far enough from $\frac{1}{2}$, this phase becomes unstable against phase separation. The ground state is a mixture of a period-$q$ ionic configuration and an empty (or full) configuration, where both configurations have the same electron density to leading order. Combining these new results with those previously obtained for strong coupling, it follows that a phase transition occurs in the ground state, as a function of the coupling, for ion densities far enough from $\frac{1}{2}$.Comment: 22 pages, typeset in ReVTeX and one encapsulated postscript file embedded in the text with eps

Regulation of Wages and Hours Prior to 1938

Direct numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure

Dynamical mean-field theory for light fermion--heavy boson mixtures on optical lattices

We theoretically analyze Fermi-Bose mixtures consisting of light fermions and heavy bosons that are loaded into optical lattices (ignoring the trapping potential). To describe such mixtures, we consider the Fermi-Bose version of the Falicov-Kimball model on a periodic lattice. This model can be exactly mapped onto the spinless Fermi-Fermi Falicov-Kimball model at zero temperature for all parameter space as long as the mixture is thermodynamically stable. We employ dynamical mean-field theory to investigate the evolution of the Fermi-Bose Falicov-Kimball model at higher temperatures. We calculate spectral moment sum rules for the retarded Green's function and self-energy, and use them to benchmark the accuracy of our numerical calculations, as well as to reduce the computational cost by exactly including the tails of infinite summations or products. We show how the occupancy of the bosons, single-particle many-body density of states for the fermions, momentum distribution, and the average kinetic energy evolve with temperature. We end by briefly discussing how to experimentally realize the Fermi-Bose Falicov-Kimball model in ultracold atomic systems.Comment: 10 pages with 4 figure

Lower bound for the segregation energy in the Falicov-Kimball model

In this work, a lower bound for the ground state energy of the Falicov-Kimball model for intermediate densities is derived. The explicit derivation is important in the proof of the conjecture of segregation of the two kinds of fermions in the Falicov-Kimball model, for sufficiently large interactions. This bound is given by a bulk term, plus a term proportional to the boundary of the region devoid of classical particles. A detailed proof is presented for density n=1/2, where the coefficient 10^(-13) is obtained for the boundary term, in two dimensions. With suitable modifications the method can also be used to obtain a coefficient for all densities.Comment: 8 pages, 2 figure

Structure of a translocation signal domain mediating conjugative transfer by Type IV secretion systems

Relaxases are proteins responsible for the transfer of plasmid and chromosomal DNA from one bacterium to another during conjugation. They covalently react with a specific phosphodiester bond within DNA origin of transfer sequences, forming a nucleo-protein complex which is subsequently recruited for transport by a plasmid-encoded type IV secretion system. In previous work we identified the targeting translocation signals presented by the conjugative relaxase TraI of plasmid R1. Here we report the structure of TraI translocation signal TSA. In contrast to known translocation signals we show that TSA is an independent folding unit and thus forms a bona fide structural domain. This domain can be further divided into three sub-domains with striking structural homology with helicase sub-domains of the SF1B family. We also show that TSA is part of a larger vestigial helicase domain which has lost its helicase activity but not its single-stranded DNA binding capability. Finally, we further delineate the binding site responsible for translocation activity of TSA by targeting single residues for mutations. Overall, this study provides the first evidence that translocation signals can be part of larger structural scaffolds, overlapping with translocation-independent activities

Influence of peptidylarginine deiminase type 4 genotype and shared epitope on clinical characteristics and autoantibody profile of rheumatoid arthritis.

Background: Recent evidence suggests that distinction of subsets of rheumatoid arthritis (RA) depending on anticyclic citrullinated peptide antibody (anti-CCP) status may be helpful in distinguishing distinct aetiopathologies and in predicting the course of disease. HLA-DRB1 shared epitope (SE) and peptidylarginine deiminase type 4 (PADI4) genotype, both of which have been implicated in anti-CCP generation, are assumed to be associated with RA. Objectives: To elucidate whether PADI4 affects the clinical characteristics of RA, and whether it would modulate the effect of anti-CCPs on clinical course. The combined effect of SE and PADI4 on autoantibody profile was also analysed. Methods: 373 patients with RA were studied. SE, padi4_94C.T, rheumatoid factor, anti-CCPs and antinuclear antibodies (ANAs) were determined. Disease severity was characterised by cumulative therapy intensity classified into ordinal categories (CTI-1 to CTI-3) and by Steinbrocker score. Results: CTI was significantly associated with disease duration, erosive disease, disease activity score (DAS) 28 and anti-CCPs. The association of anti-CCPs with CTI was considerably influenced by padi4_94C.T genotype (C/C: ORadj=0.93, padj=0.92; C/T: ORadj=2.92, padj=0.093; T/T: ORadj=15.3, padj=0.002). Carriage of padi4_94T exhibited a significant trend towards higher Steinbrocker scores in univariate and multivariate analyses. An association of padi4_94C.T with ANAs was observed, with noteworthy differences depending on SE status (SE2: ORadj=6.20, padj,0.04; SE+: ORadj=0.36, padj=0.02) and significant heterogeneity between the two SE strata (p=0.006). Conclusions: PADI4 genotype in combination with anti- CCPs and SE modulates clinical and serological characteristics of RA

Studies on the Cherenkov Effect for Improved Time Resolution of TOF-PET

With the newly gained interest in the time of flight method for positron emission tomography (TOF-PET), many options for pushing the time resolution to its borders have been investigated. As one of these options the exploitation of the Cherenkov effect has been proposed, since it allows to bypass the scintillation process and therefore provides almost instantaneous response to incident 511keV annihilation photons. Our simulation studies on the yield of Cherenkov photons, their arrival rate at the photon detector and their angular distribution reveal a significant influence by Cherenkov photons on the rise time of inorganic scintillators - a key-parameter for TOF in PET. A measurement shows the feasibility to detect Cherenkov photons in this low energy range

Discovery of pulsations in the X-ray transient 4U 1901+03

We describe observations of the 2003 outburst of the hard-spectrum X-ray transient 4U 1901+03 with the Rossi X-ray Timing Explorer. The outburst was first detected in 2003 February by the All-Sky Monitor, and reached a peak 2.5-25 keV flux of 8x10^-9 ergs/cm^2/s (around 240 mCrab). The only other known outburst occurred 32.2 yr earlier, likely the longest presently known recurrence time for any X-ray transient. Proportional Counter Array (PCA) observations over the 5-month duration of the 2003 outburst revealed a 2.763 s pulsar in a 22.58 d orbit. The detection of pulsations down to a flux of 3x10^-11 ergs/cm^2/s (2.5-25 keV), along with the inferred long-term accretion rate of 8.1x10^-11 M_sun/yr (assuming a distance of 10 kpc) suggests that the surface magnetic field strength is below ~5x10^11 G. The corresponding cyclotron energy is thus below 4 keV, consistent with the non-detection of resonance features at high energies. Although we could not unambiguously identify the optical counterpart, the lack of a bright IR candidate within the 1' RXTE error circle rules out a supergiant mass donor. The neutron star in 4U 1901+03 probably accretes from the wind of a main-sequence O-B star, like most other high-mass binary X-ray pulsars. The almost circular orbit e=0.036 confirms the system's membership in a growing class of wide, low-eccentricity systems in which the neutron stars may have received much smaller kicks as a result of their natal supernova explosions.Comment: 7 pages, 6 figures, accepted by ApJ. Very minor addition in response to referee's comment; updated author affiliatio

Phase separation due to quantum mechanical correlations

Can phase separation be induced by strong electron correlations? We present a theorem that affirmatively answers this question in the Falicov-Kimball model away from half-filling, for any dimension. In the ground state the itinerant electrons are spatially separated from the classical particles.Comment: 4 pages, 1 figure. Note: text and figure unchanged, title was misspelle

Semantic Transformation of Web Services

Web services have become the predominant paradigm for the development of distributed software systems. Web services provide the means to modularize software in a way that functionality can be described, discovered and deployed in a platform independent manner over a network (e.g., intranets, extranets and the Internet). The representation of web services by current industrial practice is predominantly syntactic in nature lacking the fundamental semantic underpinnings required to fulfill the goals of the emerging Semantic Web. This paper proposes a framework aimed at (1) modeling the semantics of syntactically defined web services through a process of interpretation, (2) scop-ing the derived concepts within domain ontologies, and (3) harmonizing the semantic web services with the domain ontologies. The framework was vali-dated through its application to web services developed for a large financial system. The worked example presented in this paper is extracted from the se-mantic modeling of these financial web services