Iterative Approach to Gravitational Lensing Theory

We develop an iterative approach to gravitational lensing theory based on approximate solutions of the null geodesic equations. The approach can be employed in any space-time which is ``close'' to a space-time in which the null geodesic equations can be completely integrated, such as Minkowski space-time, Robertson-Walker cosmologies, or Schwarzschild-Kerr geometries. To illustrate the method, we construct the iterative gravitational lens equations and time of arrival equation for a single Schwarzschild lens. This example motivates a discussion of the relationship between the iterative approach, the standard thin lens formulation, and an exact formulation of gravitational lensing.Comment: 27 pages, 2 figures, submitted to Phys.Rev.D, minor revisions, new reference

Disorder-Induced Shift of Condensation Temperature for Dilute Trapped Bose Gases

We determine the leading shift of the Bose-Einstein condensation temperature for an ultracold dilute atomic gas in a harmonic trap due to weak disorder by treating both a Gaussian and a Lorentzian spatial correlation for the quenched disorder potential. Increasing the correlation length from values much smaller than the geometric mean of the trap scale and the mean particle distance to much larger values leads first to an increase of the positive shift to a maximum at this critical length scale and then to a decrease.Comment: Author information under http://www.theo-phys.uni-essen.de/tp/ags/pelster_di

A Cellular Automaton Model for Diffusive and Dissipative Systems

We study a cellular automaton model, which allows diffusion of energy (or equivalently any other physical quantities such as mass of a particular compound) at every lattice site after each timestep. Unit amount of energy is randomly added onto a site. Whenever the local energy content of a site reaches a fixed threshold $E_{c1}$, energy will be dissipated. Dissipation of energy propagates to the neighboring sites provided that the energy contents of those sites are greater than or equal to another fixed threshold $E_{c2} (\leq E_{c1})$. Under such dynamics, the system evolves into three different types of states depending on the values of $E_{c1}$ and $E_{c2}$ as reflected in their dissipation size distributions, namely: localized peaks, power laws, or exponential laws. This model is able to describe the behaviors of various physical systems including the statistics of burst sizes and burst rates in type-I X-ray bursters. Comparisons between our model and the famous forest-fire model (FFM) are made.Comment: in REVTEX 3.0. Figures available on request. Extensively revised. Accepted by Phys.Rev.

Ultrashort pulse laser cutting of glass by controlled fracture propagation

International audienceLaser induced controlled fracture propagation has great potential in cutting brittle materials such as glass or sapphire. In this paper we demonstrate that the use of ultrashort pulse laser sources may be advantageous since it allows to overcome several restrictions of the convenient method

Particle emission rates during electrostatic spray deposition of TiO2 nanoparticle-based photoactive coating

A new method for the covalent and specific labeling of fusion proteins of carrier proteins (CPs) with small organic molecules has been developed in this work. This technology combines the convenience of expressing genetically tagged reporter proteins with the versatility of synthetic organic molecules. Moreover it promises to overcome some of the limitations of the currently used approaches. The method is based on the posttranslational modification of CPs by phosphopantetheine transferase (PPTase). In this reaction, the 4'-phosphopantetheine group of coenzyme A (CoA) is transferred to a serine residue of CP by PPTase. The PPTase can also use as substrates CoA derivatives that are modified in the thiol moiety by fluorophores or affinity reporter groups that are transferred to CP by PPTase in a covalent and irreversible manner. In this work, several CoA derivatives were synthesized by coupling of CoA with reporter groups functionalized by maleimide. The labeling method using the acyl carrier protein (ACP) and the PPTase (AcpS) from E. coli was applied to the in vitro labeling of purified proteins or in E. coli and yeast lysates, but also to the labeling of proteins expressed on cell surfaces of yeast and mammalian cells. The labeling reaction is fast, specific and quantitative. Pulse-chase labeling experiments with different fluorophores allowed the visualization of different protein generations on yeast cell surfaces. Thus, the method was demonstrated to be attractive for fluorescence microscopy. The second objective was to create a system for the selective labeling of different CPs with different CoA derivatives in the same sample, which requires PPTases with different specificities. The labeling must be performed sequentially, in order that each CP is labeled with only one CoA derivative. The pair peptidyl carrier protein (PCP) from B. brevis and the PPTase from B. subtilis (Sfp) was chosen as counterpart of the pair ACP / AcpS from E. coli. AcpS that is specific towards ACP is used for the first labeling reaction, and after a washing step to remove excess of substrate, the second labeling is performed with Sfp which is promiscuous. The system was successfully tested in vitro in solution and with proteins immobilized on microarrays, and on the surface of yeast and mammalian cells. Finally, the last objective was to reduce the size of the carrier protein (∼ 80 amino acids) to a minimal motif that is efficiently recognized by the PPTase. ACP and PCP were truncated before and after helix II whose residues are involved in the recognition by AcpS and Sfp. The fragments of ACP (aa 27-50) and PCP (aa 37-59) were labeled by AcpS and Sfp respectively, but the kinetics of labeling was slow. Two libraries were created with randomization of the six amino acids around the modified serine. Selections were performed using a phage display system based on the phagemid technology. Mt1 (32 aa) was modified by AcpS at the same rate as wild type ACP. Additional truncations of mt1 sequence yielded mt1.4 (16 aa) that was efficiently recognized by AcpS and weakly by Sfp. In conclusion, this labeling method should become an important tool for studies of cell surface proteins as well as for in vitro applications

String Field Theory Projectors for Fermions of Integral Weight

The interaction vertex for a fermionic first order system of weights (1,0) such as the twisted bc-system, the fermionic part of N=2 string field theory and the auxiliary \eta\xi system of N=1 strings is formulated in the Moyal basis. In this basis, the Neumann matrices are diagonal; as usual, the eigenvectors are labeled by \kappa\in\R. Oscillators constructed from these eigenvectors make up two Clifford algebras for each nonzero value of \kappa. Using a generalization of the Moyal-Weyl map to the fermionic case, we classify all projectors of the star-algebra which factorize into projectors for each \kappa-subspace. At least for the case of squeezed states we recover the full set of bosonic projectors with this property. Among the subclass of ghost number-homogeneous squeezed state projectors, we find a single class of BPZ-real states parametrized by one (nearly) arbitrary function of \kappa. This class is shown to contain the generalized butterfly states. Furthermore, we elaborate on sufficient and necessary conditions which have to be fulfilled by our projectors in order to constitute surface states. As a byproduct we find that the full star product of N=2 string field theory translates into a canonically normalized continuous tensor product of Moyal-Weyl products up to an overall normalization. The divergent factors arising from the translation to the continuous basis cancel between bosons and fermions in any even dimension.Comment: LaTeX, 1+23 pages, minor improvements, references adde