Cluster expansion for dimerized spin systems

We have studied dimerized spin systems by realizing the cluster expansion to high order. We have extended our previous dimer expansion for one-dimensional systems to cover weakly interacting chains for a quantitative description of three dimensional materials like PHCC and KCuCl_3. By comparison with recent inelastic neutron scattering data we are able to determine the exchange energies between individual spins. We have further investigated the incommensurate region of zigzag chains with isotropic exchange coupling constants near the disorder-line where the dispersion curve exhibits a minimum at a finite wavevector. Our approach clearly shows the gradual transition between the minimum of the dispersion at wavevector 0 and wavevector Pi within this region. The extent of the incommensurate regime is given analytically in an expansion in the coupling constants.Comment: 3 pages, 3 figures; contribution to ICNS2001; uses svjour.clo, svglobal.clo (included

The colour-magnitude relation of Globular Clusters in Centaurus and Hydra - Constraints on star cluster self-enrichment with a link to massive Milky Way GCs

We investigate the colour-magnitude relation of metal-poor globular clusters, the 'blue tilt', in the Hydra and Centaurus galaxy clusters and constrain the primordial conditions for star cluster self-enrichment. We analyse U,I photometry for about 2500 globular clusters in the central regions of Hydra and Centaurus, based on FORS1@VLT data. We convert the measured colour-magnitude relations into mass-metallicity space and obtain a scaling of Z \propto M^{0.27 \pm 0.05} for Centaurus GCs and Z \propto M^{0.40 \pm 0.06} for Hydra GCs, consistent with results in other environments. We find that the GC mass-metallicity relation already sets in at present-day masses of a few 10^5 solar masses and is well established in the luminosity range of massive MW clusters like omega Centauri. We compare the mass-metallicity relation with predictions from the star cluster self-enrichment model by Bailin & Harris (2009). For this we include effects of dynamical and stellar evolution and a physically well motivated primordial mass-radius scaling. The self-enrichment model reproduces the observed relations well for average primordial half-light radii r_h ~ 1-1.5 pc, star formation efficiencies f_* ~ 0.3-0.4, and pre-enrichment levels of [Fe/H] ~ -1.7 dex. Within the self-enrichment scenario, the observed blue tilt implies a correlation between GC mass and width of the stellar metallicity distribution. We find that this implied correlation matches the trend of width with GC mass measured in Galactic GCs, including extreme cases like omega Cen and M54. We conclude that 1. A primordial star cluster mass-radius relation provides a significant improvement to the self-enrichment model fits. 2. Broadenend metallicity distributions as found in some massive MW globular clusters may have arisen naturally from self-enrichment processes, without the need of a dwarf galaxy progenitor.Comment: 15 pages, 13 figures. Language edited version of paper accepted for publication in Astronomy & Astrophysics. Colour-composite in Figure 1 reduced in resolutio

Monte Carlo Simulation of Long Chain Polymer Melts: Crossover from Rouse to Reptation Dynamics

We present data of Monte Carlo simulations for monodisperse linear polymer chains in dense melts with degrees of polymerization between N=16 and N=512. The aim of this study is to investigate the crossover from Rouse-like dynamics for short chains to reptation-like dynamics for long chains. To address this problem we calculate a variety of different quantities: standard mean-square displacements of inner monomers and of the chain's center of mass, the recently proposed cubic invariant, persistence of bond-vector orientation with time, and the auto-correlation functions of the bond vector, the end-to-end vector and the Rouse modes. This analysis reveals that the crossover from non- to entangled dynamics is very protracted. Only the largest chain length N=512, which is about 13 times larger than the entanglement length, shows evidence for reptation.Comment: 38 pages of REVTeX, 14 PostScript figure

Experimental measurements of the laminar separation bubble on an Eppler 387 airfoil at low Reynolds numbers

An experimental investigation was conducted to measure the flow velocity in the boundary layer of an Eppler 387 airfoil. In particular, the laminar separation bubble that this airfoil exhibits at low Reynolds numbers was the focus. Single component laser Doppler velocimetry data were obtained at a Reynolds number of 100,000 at an angle of attack of 2.0 degree. Static Pressure and flow visualization data for the Eppler 387 airfoil were also obtained. The difficulty in obtaining accurate experimental measurements at low Reynolds numbers is addressed. Laser Doppler velocimetry boundary layer data for the NACA 663-018 airfoil at a Reynolds number of 160,000 and angle of attack of 12 degree is also presented

Multi-parton correlations and "exclusive" cross sections

In addition to the inclusive cross sections discussed within the QCD-parton model, in the regime of multiple parton interactions, different and more exclusive cross sections become experimentally viable and may be suitably measured. Indeed, in its study of double parton collisions, the quantity measured by CDF was an "exclusive" rather than an inclusive cross section. The non perturbative input to the "exclusive" cross sections is different with respect to the non perturbative input of the inclusive cross sections and involves correlation terms of the hadron structure already at the level of single parton collisions. The matter is discussed in details keeping explicitly into account the effects of double and of triple parton collisions.Comment: 18 pages, no figures, corrected typo

Domain wall dynamics in a two-component Bose-Mott insulator

We model the dynamics of two species of bosonic atoms trapped in an optical lattice within the Mott regime by mapping the system onto a spin model. A field gradient breaks the cloud into two domains. We study how the domain wall evolves under adiabatic and diabatic changes of this gradient. We determine the timescales for adiabaticity, and study how temperature evolves for slow ramps. We show that after large, sudden changes of the field gradient, the system does not equilibrate on typical experimental timescales. We find interesting spin dynamics even when the initial temperature is large compared to the super-exchange energy. We discuss the implication of our results for experiments wishing to use such a two-component system for thermometry, or as part of a cooling scheme.Comment: 6 pages, 5 figures Minor typographical errors corrected. Figure labels changed. Added concluding statement

Non-Gaussian Correlations in the McLerran-Venugopalan Model

We argue that the statistical weight function W[rho] appearing in the McLerran-Venugopalan model of a large nucleus is intrinsically non-Gaussian, even if we neglect quantum corrections. Based on the picture where the nucleus of radius R consists of a collection of color-neutral nucleons, each of radius a<<R, we show that to leading order in alpha_s and a/R only the Gaussian part of W[rho] enters into the final expression for the gluon number density. Thus, the existing results in the literature which assume a Gaussian weight remain valid.Comment: 21 pages with 4 figures (revtex