arXiv subjects

M. Fuchs

Publications and source records attributed to M. Fuchs.

At least 55 records · Page 3Linked to original sources

arXiv2008

Theory of Thermodynamic Stresses in Colloidal Dispersions at the Glass Transition

We discuss the nonlinear rheology of dense colloidal dispersions at the glass transition. A first principles approach starting with interacting Brownian particles in given arbitrary homogeneous (incompressible) flow neglecting hydrodynamic interactions is sketched. It e.g. explains steady state flow curves for finite shear rates measured in dense suspensions of thermosensitive core-shell particles consisting of a polystyrene core and a crosslinked poly(N-isopropylacrylamide)(PNIPAM) shell. The exponents of simple and generalized Herschel Bulkley laws are computed for hard spheres.

cond-mat.soft↗

arXiv2008

Quasi-free photoproduction of eta-mesons of the neutron

Quasi-free photoproduction of eta-mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta-mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasi-free n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta-production off the neutron shows a pronounced bump-like structure at W=1.68 GeV (E_g ~ 1 GeV), which is absent for the proton.

nucl-ex↗

arXiv2008

Shear stresses of colloidal dispersions at the glass transition in equilibrium and in flow

We consider a model dense colloidal dispersion at the glass transition, and investigate the connection between equilibrium stress fluctuations, seen in linear shear moduli, and the shear stresses under strong flow conditions far from equilibrium, viz. flow curves for finite shear rates. To this purpose thermosensitive core-shell particles consisting of a polystyrene core and a crosslinked poly(N-isopropylacrylamide)(PNIPAM) shell were synthesized. Data over an extended range in shear rates and frequencies are compared to theoretical results from integrations through transients and mode coupling approaches. The connection between non-linear rheology and glass transition is clarified. While the theoretical models semi-quantitatively fit the data taken in fluid states and the predominant elastic response of glass, a yet unaccounted dissipative mechanism is identified in glassy states.

cond-mat.soft↗

arXiv2008

Modification of the $ω$-Meson Lifetime in Nuclear Matter

The photo production of $ω$ mesons on the nuclei C, Ca, Nb and Pb has been measured using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the $ω$ meson cross section on the nuclear mass number has been compared with three different types of models, a Glauber analysis, a BUU analysis of the Giessen theory group and a calculation by the Valencia theory group. In all three cases, the inelastic $ω$ width is found to be $130-150 \rm{MeV/c^2}$ at normal nuclear matter density for an average 3-momentum of 1.1 GeV/c. In the restframe of the $ω$ meson, this inelastic $ω$ width corresponds to a reduction of the $ω$ lifetime by a factor $\approx 30$. For the first time, the momentum dependent $ω$N cross section has been extracted from the experiment and is in the range of 70 mb.

nucl-ex↗

arXiv2007

The MAGIC Project: Contributions to ICRC 2007

The MAGIC Project: Contributions to ICRC 2007, Merida, Mexico. Contents pages for the Contribution on behalf of the MAGIC Collaboration to the 30th ICRC that took place in July 2007 in Merida, Mexico. The contents are in html form with clickable links to the papers that exist on the Astrophysics archive. We hope that this will make it easier to access the output of the conference in a systematic way. Comments on how useful this is/ how it could be improved should be sent to michela.demaria@iuav.it.

astro-ph↗

arXiv2007

N* and Delta* decays into N pi0 pi0

Decays of baryon resonances in the second and the third resonance region into N pi0 pi0 are studied by photoproduction of two neutral pions off protons. Partial decay widths of N* and Delta* resonances decaying into Delta(1232) pi, N(ππ)_{S}, N(1440)P_{11} pi, and N(1520)D_{13} pi are determined in a partial wave analysis of this data, and data from other reactions. Several partial decay widths were not known before. Interesting decay patterns are observed which are not even qualitatively reproduced by quark model calculations. In the second resonance region, decays into Delta(1232) pi dominate clearly. The N(ππ)_{S}-wave provides a significant contribution to the cross section, especially in the third resonance region. The P_{13}(1720) properties found here are at clear variance to PDG values.

hep-ph↗

arXiv2007

MAGIC upper limits on the very high energy emission from GRBs

The fast repositioning system of the MAGIC Telescope has allowed during its first data cycle, between 2005 and the beginning of year 2006, observing nine different GRBs as possible sources of very high energy gammas. These observations were triggered by alerts from Swift, HETE-II, and Integral; they started as fast as possible after the alerts and lasted for several minutes, with an energy threshold varying between 80 and 200 GeV, depending upon the zenith angle of the burst. No evidence for gamma signals was found, and upper limits for the flux were derived for all events, using the standard analysis chain of MAGIC. For the bursts with measured redshift, the upper limits are compatible with a power law extrapolation, when the intrinsic fluxes are evaluated taking into account the attenuation due to the scattering in the Metagalactic Radiation Field (MRF).

astro-ph↗

arXiv2007

Bond formation and slow heterogeneous dynamics in adhesive spheres with long--ranged repulsion: Quantitative test of Mode Coupling Theory

A colloidal system of spheres interacting with both a deep and narrow attractive potential and a shallow long-ranged barrier exhibits a prepeak in the static structure factor. This peak can be related to an additional mesoscopic length scale of clusters and/or voids in the system. Simulation studies of this system have revealed that it vitrifies upon increasing the attraction into a gel-like solid at intermediate densities. The dynamics at the mesoscopic length scale corresponding to the prepeak represents the slowest mode in the system. Using mode coupling theory with all input directly taken from simulations, we reveal the mechanism for glassy arrest in the system at 40% packing fraction. The effects of the low-q peak and of polydispersity are considered in detail. We demonstrate that the local formation of physical bonds is the process whose slowing down causes arrest. It remains largely unaffected by the large-scale heterogeneities, and sets the clock for the slow cluster mode. Results from mode-coupling theory without adjustable parameters agree semi-quantitatively with the local density correlators but overestimate the lifetime of the mesoscopic structure (voids).

cond-mat.soft↗

arXiv2007

Dynamic Glass Transition in Two Dimensions

The question about the existence of a structural glass transition in two dimensions is studied using mode coupling theory (MCT). We determine the explicit d-dependence of the memory functional of mode coupling for one-component systems. Applied to two dimensions we solve the MCT equations numerically for monodisperse hard discs. A dynamic glass transition is found at a critical packing fraction phi_c^{d=2} = 0.697 which is above phi_c^{d=3} = 0.516 by about 35%. phi^d_c scales approximately with phi^d_{\rm rcp} the value for random close packing, at least for d=2, 3. Quantities characterizing the local, cooperative 'cage motion' do not differ much for d=2 and d=3, and we e.g. find the Lindemann criterion for the localization length at the glass transition. The final relaxation obeys the superposition principle, collapsing remarkably well onto a Kohlrausch law. The d=2 MCT results are in qualitative agreement with existing results from MC and MD simulations. The mean squared displacements measured experimentally for a quasi-two-dimensional binary system of dipolar hard spheres can be described satisfactorily by MCT for monodisperse hard discs over four decades in time provided the experimental control parameter Gamma (which measures the strength of dipolar interactions) and the packing fraction phi are properly related to each other.

cond-mat.soft↗

arXiv2006

Design considerations for table-top, laser-based VUV and X-ray free electron lasers

A recent breakthrough in laser-plasma accelerators, based upon ultrashort high-intensity lasers, demonstrated the generation of quasi-monoenergetic GeV-electrons. With future Petawatt lasers ultra-high beam currents of ~100 kA in ~10 fs can be expected, allowing for drastic reduction in the undulator length of free-electron-lasers (FELs). We present a discussion of the key aspects of a table-top FEL design, including energy loss and chirps induced by space-charge and wakefields. These effects become important for an optimized table-top FEL operation. A first proof-of-principle VUV case is considered as well as a table-top X-ray-FEL which may open a brilliant light source also for new ways in clinical diagnostics.

physics.acc-ph↗

arXiv2006

Competition between glass transition and liquid-gas separation in attracting colloids

We present simulation results addressing the phenomena of colloidal gelation induced by attractive interactions. The liquid-gas transition is prevented by the glass arrest at high enough attraction strength, resulting in a colloidal gel. The dynamics of the system is controlled by the glass, with little effect of the liquid-gas transition. When the system separates in a liquid and vapor phases, even if the denser phase enters the non-ergodic region, the vapor phase enables the structural relaxation of the system as a whole.

cond-mat.soft↗

arXiv2006

Dense colloidal suspensions under time-dependent shear

We consider the nonlinear rheology of dense colloidal suspensions under a time-dependent simple shear flow. Starting from the Smoluchowski equation for interacting Brownian particles advected by shearing (ignoring fluctuations in fluid velocity) we develop a formalism which enables the calculation of time-dependent, far-from-equilibrium averages. Taking shear-stress as an example we derive exactly a generalized Green-Kubo relation, and an equation of motion for the transient density correlator, involving a three-time memory function. Mode coupling approximations give a closed constitutive equation yielding the time-dependent stress for arbitrary shear rate history. We solve this equation numerically for the special case of a hard sphere glass subject to step-strain.

cond-mat.soft↗

arXiv2005

Role of semicore states in the electronic structure of group-III nitrides: An exact exchange study

The bandstructure of the zinc-blende phase of AlN, GaN, InN is calculated employing the exact-exchange (EXX) Kohn-Sham density-functional theory and a pseudopotential plane-wave approach. The cation semicore d electrons are treated both as valence and as core states. The EXX bandgaps of AlN and GaN (obtained with the Ga 3d electrons included as core states) are in excellent agreement with previous EXX results, GW calculations and experiment. Inclusion of the semicore d electrons as valence states leads to a large reduction in the EXX bandgaps of GaN and InN. Contrary to common belief, the removal of the self-interaction, by the EXX approach, does not account for the large disagreement for the position of the semicore d electrons between the LDA results and experiment.

cond-mat.mtrl-sci↗

arXiv2004

Describing static correlation in bond dissociation by Kohn-Sham density functional theory

We show that density functional theory within the RPA (random phase approximation for the exchange-correlation energy) provides a correct description of bond dissociation in H$_2$ in a spin-restricted Kohn-Sham formalism, i.e. without artificial symmetry breaking. We present accurate adiabatic connection curves both at equilibrium and beyond the Coulson-Fisher point. The strong curvature at large bond length implies important static (left-right) correlation, justifying modern hybrid functional constructions but also demonstrating their limitations. Although exact at infinite and accurate around the equilibrium bond length, the RPA dissociation curve displays unphysical repulsion at larger but finite bond lengths. Going beyond the RPA by including the exact exchange kernel (RPA+X), we find a similar repulsion. We argue that this deficiency is due to the absence of double excitations in adiabatic linear response theory. Further analyzing the H$_2$ dissociation limit we show that the RPA+X is not size-consistent, in contrast to the RPA.

cond-mat.other↗

arXiv2004

Tagged-particle dynamics in a hard-sphere system: mode-coupling theory analysis

The predictions of the mode-coupling theory of the glass transition (MCT) for the tagged-particle density-correlation functions and the mean-squared displacement curves are compared quantitatively and in detail to results from Newtonian- and Brownian-dynamics simulations of a polydisperse quasi-hard-sphere system close to the glass transition. After correcting for a 17% error in the dynamical length scale and for a smaller error in the transition density, good agreement is found over a wide range of wave numbers and up to five orders of magnitude in time. Deviations are found at the highest densities studied, and for small wave vectors and the mean-squared displacement. Possible error sources not related to MCT are discussed in detail, thereby identifying more clearly the issues arising from the MCT approximation itself. The range of applicability of MCT for the different types of short-time dynamics is established through asymptotic analyses of the relaxation curves, examining the wave-number and density-dependent characteristic parameters. Approximations made in the description of the equilibrium static structure are shown to have a remarkable effect on the predicted numerical value for the glass-transition density. Effects of small polydispersity are also investigated, and shown to be negligible.

cond-mat.stat-mech↗

arXiv2004

Theory and simulation of gelation, arrest and yielding in attracting colloids

We present some recent theory and simulation results addressing the phenomena of colloidal gelation at both high and low volume fractions, in the presence of short-range attractive interactions. We discuss the ability of mode-coupling theory and its adaptations to address situations with strong heterogeneity in density and/or dynamics. We include a discussion of the effect of attractions on the shear-thinning and yield behaviour under flow.

cond-mat.soft↗

arXiv2003

Schematic mode coupling theories for shear thinning, shear thickening, and jamming

Mode coupling theory (MCT) appears to explain several, though not all, aspects of the glass transition in colloids (particularly when short-range attractions are present). Developments of MCT, from rational foundations in statistical mechanics, account qualitatively for nonlinear flow behaviour such as the yield stress of a hard-sphere colloidal glass. Such theories so far only predict shear thinning behaviour, whereas in real colloids both shear thinning and shear thickening can be found. The latter observation can, however, be rationalised by postulating an MCT vertex that is not only a decreasing function of strain rate (as found from first principles) but also an increasing function of stress. Within a highly simplified, schematic MCT model this can lead not only to discontinuous shear thickening but also to complete arrest of a fluid phase under the influence of an external stress (`full jamming').

cond-mat.soft↗

arXiv2003

Jamming transitions in a schematic model of suspension rheology

We study the steady-state response to applied stress in a simple scalar model of sheared colloids. Our model is based on a schematic (F2) model of the glass transition, with a memory term that depends on both stress and shear rate. For suitable parameters, we find transitions from a fluid to a nonergodic, jammed state, showing zero flow rate in an interval of applied stress. Although the jammed state is a glass, we predict that jamming transitions have an analytical structure distinct from that of the conventional mode coupling glass transition. The static jamming transition we discuss is also distinct from hydrodynamic shear thickening.

cond-mat.soft↗