Quantum Link Models with Many Rishon Flavors and with Many Colors

Quantum link models are a novel formulation of gauge theories in terms of discrete degrees of freedom. These degrees of freedom are described by quantum operators acting in a finite-dimensional Hilbert space. We show that for certain representations of the operator algebra, the usual Yang-Mills action is recovered in the continuum limit. The quantum operators can be expressed as bilinears of fermionic creation and annihilation operators called rishons. Using the rishon representation the quantum link Hamiltonian can be expressed entirely in terms of color-neutral operators. This allows us to study the large N_c limit of this model. In the 't Hooft limit we find an area law for the Wilson loop and a mass gap. Furthermore, the strong coupling expansion is a topological expansion in which graphs with handles and boundaries are suppressed.Comment: Lattice2001(theorydevelop), poster by O. Baer and talk by B. Schlittgen, 6 page

2-loop Functional Renormalization for elastic manifolds pinned by disorder in N dimensions

We study elastic manifolds in a N-dimensional random potential using functional RG. We extend to N>1 our previous construction of a field theory renormalizable to two loops. For isotropic disorder with O(N) symmetry we obtain the fixed point and roughness exponent to next order in epsilon=4-d, where d is the internal dimension of the manifold. Extrapolation to the directed polymer limit d=1 allows some handle on the strong coupling phase of the equivalent N-dimensional KPZ growth equation, and eventually suggests an upper critical dimension of about 2.5.Comment: 4 pages, 3 figure

Cusps and shocks in the renormalized potential of glassy random manifolds: How Functional Renormalization Group and Replica Symmetry Breaking fit together

We compute the Functional Renormalization Group (FRG) disorder- correlator function R(v) for d-dimensional elastic manifolds pinned by a random potential in the limit of infinite embedding space dimension N. It measures the equilibrium response of the manifold in a quadratic potential well as the center of the well is varied from 0 to v. We find two distinct scaling regimes: (i) a "single shock" regime, v^2 ~ 1/L^d where L^d is the system volume and (ii) a "thermodynamic" regime, v^2 ~ N. In regime (i) all the equivalent replica symmetry breaking (RSB) saddle points within the Gaussian variational approximation contribute, while in regime (ii) the effect of RSB enters only through a single anomaly. When the RSB is continuous (e.g., for short-range disorder, in dimension 2 <= d <= 4), we prove that regime (ii) yields the large-N FRG function obtained previously. In that case, the disorder correlator exhibits a cusp in both regimes, though with different amplitudes and of different physical origin. When the RSB solution is 1-step and non- marginal (e.g., d < 2 for SR disorder), the correlator R(v) in regime (ii) is considerably reduced, and exhibits no cusp. Solutions of the FRG flow corresponding to non-equilibrium states are discussed as well. In all cases the regime (i) exhibits a cusp non-analyticity at T=0, whose form and thermal rounding at finite T is obtained exactly and interpreted in terms of shocks. The results are compared with previous work, and consequences for manifolds at finite N, as well as extensions to spin glasses and related models are discussed.Comment: v2: Note added in proo

Calcium adsorption and displacement: characterization of lipid monolayers and their interaction with membrane-active peptides/proteins

BACKGROUND: The first target of antimicrobial peptides (AMPs) is the bacterial membrane. In the case of Gram-negative bacteria this is the outer membrane (OM), the lipid composition of which is extremely asymmetric: Whereas the inner leaflet is composed of a phospholipid mixture, the outer leaflet is made up solely from lipopolysaccharides (LPSs). LPS, therefore, represents the first target of AMPs. The binding and intercalation of polycationic AMPs is driven by the number and position of negatively charged groups of the LPS. Also, proteins other than cationic AMPs can interact with LPS, e.g. leading eventually to a neutralization of the endotoxic effects of LPS. We compared different biophysical techniques to gain insight into the properties of the electrical surface potentials of lipid monolayers and aggregates composed of LPSs and various phospholipids and their interaction with peptides and proteins. RESULTS: The net negative charge calculated from the chemical structure of the phospholipid and LPS molecules is linearly correlated with the adsorption of calcium to two-dimensional lipid monolayers composed of the respective lipids. However, the ζ-potentials determined by the electrophoretic mobility of LPS aggregates can only be interpreted by assuming a dependence of the plane of shear on the number of saccharides and charged groups. Various peptides and proteins were able to displace calcium adsorbed to monolayers. CONCLUSION: To characterize the electrical properties of negatively charged phospholipids and LPSs and their electrostatic interaction with various polycationic peptides/proteins, the adsorption of calcium to and displacement from lipid monolayers is a suitable parameter. Using the calcium displacement method, the binding of peptides to monolayers can be determined even if they do not intercalate. The interpretation of ζ-potential data is difficulty for LPS aggregates, because of the complex three-dimensional structure of the LPS molecules. However, the influence of peptides/proteins on the ζ-potential can be used to characterize the underlying interaction mechanisms

Corals as Source of Bacteria with Antimicrobial Activity

In this study we examined marine bacteria associated with different corals (Porites lutea, Galaxea fascicularis, Acropora sp. and Pavona sp.) collected from vicinity of Panjang island, Jepara, North Java Sea, Indonesia for their antimicrobial activities against the bacteria Echerichia coli, Bacillus subtilis, Staphylococcus lentus and the yeast Candida glabrata. A total of 13 bacterial isolates belonged to the members of Bacillus, Vibrio, Micrococcus, Pseudoalteromonas, Arthrobacter and Pseudovibrio were found to inhibit the growth of at least one test strain. Further examinations among the biologically active strains by using PCR with specific primers of non-ribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) resulted in the presence of NRPS gene fragments in the 2 members of Bacillus and Micrococcus and PKS gene fragments in the 2 members of Bacillus and Vibrio. Following cloning and sequencing of the PCR products, the fragments from Bacillus BM1.5 and Micrococcus BJB showed sequence identity with peptide synthetase genes of Bacillus subtilis (61 %) and Actinoplanes teichomyceticus (62.4%). On the other hand, PKS-amplifying strains Bacillus BJ.7 and Vibrio MJ.5 showed closest sequence identity with polyketide synthase genes of Bacillus subtilis (73%) and Anabaena sp 90 (62%), respectively

Functional renormalization group for anisotropic depinning and relation to branching processes

Using the functional renormalization group, we study the depinning of elastic objects in presence of anisotropy. We explicitly demonstrate how the KPZ-term is always generated, even in the limit of vanishing velocity, except where excluded by symmetry. We compute the beta-function to one loop taking properly into account the non-analyticity. This gives rise to additional terms, missed in earlier studies. A crucial question is whether the non-renormalization of the KPZ-coupling found at 1-loop order extends beyond the leading one. Using a Cole-Hopf-transformed theory we argue that it is indeed uncorrected to all orders. The resulting flow-equations describe a variety of physical situations. A careful analysis of the flow yields several non-trivial fixed points. All these fixed points are transient since they possess one unstable direction towards a runaway flow, which leaves open the question of the upper critical dimension. The runaway flow is dominated by a Landau-ghost-mode. For SR elasticity, using the Cole-Hopf transformed theory we identify a non-trivial 3-dimensional subspace which is invariant to all orders and contains all above fixed points as well as the Landau-mode. It belongs to a class of theories which describe branching and reaction-diffusion processes, of which some have been mapped onto directed percolation.Comment: 20 pages, 30 figures, revtex

Glassy trapping of manifolds in nonpotential random flows

We study the dynamics of polymers and elastic manifolds in non potential static random flows. We find that barriers are generated from combined effects of elasticity, disorder and thermal fluctuations. This leads to glassy trapping even in pure barrier-free divergenceless flows $v {f \to 0}{\sim} f^\phi$ ($\phi > 1$). The physics is described by a new RG fixed point at finite temperature. We compute the anomalous roughness $R \sim L^\zeta$ and dynamical $t\sim L^z$ exponents for directed and isotropic manifolds.Comment: 5 pages, 3 figures, RevTe

The mycocidal, membrane-active complex of Cryptococcus humicola is a new type of cellobiose lipid with detergent features

AbstractThe chemical composition of the mycocidal complex (formerly known as microcin) secreted by Cryptococcus humicola was investigated by chemical, mass spectrometric and nuclear magnetic resonance methods. The results indicate that the mycocidal complex is composed of glycolipids with a highly acetylated (up to five acetyl groups) cellobiose backbone [β-D-Glcp-(1′→4)-β-D-Glcp] linked to the ω-hydroxyl group of α,ω-dihydroxy palmitate [16:0-α,ω-di-OH] with an unsubstituted carboxyl group. The acyl chain forming aglycon can be replaced by [18:0-(α,ω-di-OH)], [18:0-(α,ω-1,ω-tri-OH)], and [18:0-(α,ω-2,ω-tri-OH)]. The complex has a comparatively high surface activity; 0.5 mg/ml of it reduced the surface tension of 0.1 M NaHCO3 from 71 mN/m to 37 mN/m and interfacial tension against n-hexadecane from 39 mN/m to 10 mN/m. The critical micelle concentration of the complex at pH 4.0, determined by the fluorometric method with N-phenyl-1-naphthylamine as fluorescent probe and by the De Nouy ring method, was 2×10−5 M (taking the average molecular mass of the complex to be 750); it did not depend on the presence of 100 mM KCl and was an order of magnitude higher at pH 7.0. By fluorescence resonance energy transfer spectroscopy with N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-phosphatidylethanolamine as energy donor and N-(rhodamine B sulfonyl)-phosphatidylethanolamine as energy acceptor the complex was shown to intercalate into the liposomal lipid matrix. Primary lesions caused by the complex in planar lipid bilayers were revealed as short-living current fluctuations of a broad spectrum of amplitudes. The mycocidal effect of the complex is suggested to be associated with its detergent-like properties

Quantum vs. Geometric Disorder in a Two-Dimensional Heisenberg Antiferromagnet

We present a numerical study of the spin-1/2 bilayer Heisenberg antiferromagnet with random interlayer dimer dilution. From the temperature dependence of the uniform susceptibility and a scaling analysis of the spin correlation length we deduce the ground state phase diagram as a function of nonmagnetic impurity concentration p and bilayer coupling g. At the site percolation threshold, there exists a multicritical point at small but nonzero bilayer coupling g_m = 0.15(3). The magnetic properties of the single-layer material La_2Cu_{1-p}(Zn,Mg)_pO_4 near the percolation threshold appear to be controlled by the proximity to this new quantum critical point.Comment: minor changes, updated figure

Google Summer of Code: Student Motivations and Contributions

Several open source software (OSS) projects expect to foster newcomers' onboarding and to receive contributions by participating in engagement programs, like Summers of Code. However, there is little empirical evidence showing why students join such programs. In this paper, we study the well-established Google Summer of Code (GSoC), which is a 3-month OSS engagement program that offers stipends and mentors to students willing to contribute to OSS projects. We combined a survey (students and mentors) and interviews (students) to understand what motivates students to enter GSoC. Our results show that students enter GSoC for an enriching experience, not necessarily to become frequent contributors. Our data suggest that, while the stipends are an important motivator, the students participate for work experience and the ability to attach the name of the supporting organization to their resum\'es. We also discuss practical implications for students, mentors, OSS projects, and Summer of Code programs.Comment: 30 page