636 research outputs found
The Glass Transition in Driven Granular Fluids: A Mode-Coupling Approach
We consider the stationary state of a fluid comprised of inelastic hard
spheres or disks under the influence of a random, momentum-conserving external
force. Starting from the microscopic description of the dynamics, we derive a
nonlinear equation of motion for the coherent scattering function in two and
three space dimensions. A glass transition is observed for all coefficients of
restitution, epsilon, at a critical packing fraction, phi_c(epsilon), below
random close packing. The divergence of timescales at the glass-transition
implies a dependence on compression rate upon further increase of the density -
similar to the cooling rate dependence of a thermal glass. The critical
dynamics for coherent motion as well as tagged particle dynamics is analyzed
and shown to be non-universal with exponents depending on space dimension and
degree of dissipation.Comment: 16 pages, 9 figure
Erzeugung und Charakterisierung ultrakurzer Lichtimpulse für die Generation Hoher Harmonischer Strahlung
In atomic and molecular physics experiments extremely short laser pulses, mostly in the sub ten-femtosecond range, are of strong interest. The shorter the pulses are and correspondingly, with growing bandwidth, the more important dispersion control and management becomes. In this diploma thesis a new setup for spectral broadening involving self phase modulation via lamentation and subsequent recompression is presented. Moreover pulse characterization through a dedicated interferometric autocorrelation setup with nearly zero dispersion has been achieved. The initial pulses reveal a duration of 32 fs at a repetition rate of 8000 Hz and a single pulse energy of about 1 mJ. An extensive investigation of important quantities, in uencing the lamentation process, like pressure, focussing parameters and the interaction length was performed. Their optimization yielded a broadening of the fundamental spectra of about a factor of 5 supporting lightpulses down to a fourierlimited duration of 4 fs with 1.5 optical cycles. These values, as well as the appearance of smaller pre- and postpulse structures were con rmed by autocorrelation measurements of the pulses. Even though the full reconstruction of the time dependent electric eld of the pulses is impossible via autocorrelation, one can retrieve quantitative information about the pulse chirp by simulating the complete autocorrelation signal, including the second-order dispersion parameter
The Cole-Cole Law for Critical Dynamics in Glass-Forming Liquids
Within the mode-coupling theory (MCT) for glassy dynamics, the asymptotic
low-frequency expansions for the dynamical susceptibilities at critical points
are compared to the expansions for the dynamic moduli; this shows that the
convergence properties of the two expansions can be quite different. In some
parameter regions, the leading-order expansion formula for the modulus
describes the solutions of the MCT equations of motion outside the transient
regime successfully; at the same time, the leading- and next-to-leading order
expansion formulas for the susceptibility fail. In these cases, one can derive
a Cole-Cole law for the susceptibilities; and this law accounts for the
dynamics for frequencies below the band of microscopic excitations and above
the high-frequency part of the alpha-peak. It is shown that this scenario
explains the optical-Kerr-effect data measured for salol and benzophenone
(BZP). For BZP it is inferred that the depolarized light-scattering spectra
exhibit a wing for the alpha-peak within the Gigahertz band. This wing results
from the crossover of the von Schweidler-law part of the alpha-peak to the
high-frequency part of the Cole-Cole peak; and this crossover can be described
quantitatively by the leading-order formulas of MCT for the modulus.Comment: 15 pages, 9 figure
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).Comment: 10 pages, 8 figure
Evolution of unoccupied resonance during the synthesis of a silver dimer on Ag(111)
Silver dimers were fabricated on Ag(111) by single-atom manipulation using
the tip of a cryogenic scanning tunnelling microscope. An unoccupied electronic
resonance was observed to shift toward the Fermi level with decreasing
atom-atom distance as monitored by spatially resolved scanning tunnelling
spectroscopy. Density functional calculations were used to analyse the
experimental observations and revealed that the coupling between the adsorbed
atoms is predominantly direct rather than indirect via the Ag(111) substrate.Comment: 9 pages, 3 figure
The Jamming Transition in Granular Systems
Recent simulations have predicted that near jamming for collections of
spherical particles, there will be a discontinuous increase in the mean contact
number, Z, at a critical volume fraction, phi_c. Above phi_c, Z and the
pressure, P are predicted to increase as power laws in phi-phi_c. In
experiments using photoelastic disks we corroborate a rapid increase in Z at
phi_c and power-law behavior above phi_c for Z and P. Specifically we find
power-law increase as a function of phi-phi_c for Z-Z_c with an exponent beta
around 0.5, and for P with an exponent psi around 1.1. These exponents are in
good agreement with simulations. We also find reasonable agreement with a
recent mean-field theory for frictionless particles.Comment: 4 pages, 4 figures, 2 pages supplement; minor changes and
clarifications, 2 addtl. refs., accepted for publication in Phys. Rev. Let
Critical Decay at Higher-Order Glass-Transition Singularities
Within the mode-coupling theory for the evolution of structural relaxation in
glass-forming systems, it is shown that the correlation functions for density
fluctuations for states at A_3- and A_4-glass-transition singularities can be
presented as an asymptotic series in increasing inverse powers of the logarithm
of the time t: , where
with p_n denoting some polynomial and x=ln (t/t_0). The results are
demonstrated for schematic models describing the system by solely one or two
correlators and also for a colloid model with a square-well-interaction
potential.Comment: 26 pages, 7 figures, Proceedings of "Structural Arrest Transitions in
Colloidal Systems with Short-Range Attractions", Messina, Italy, December
2003 (submitted
Glass glass transition and new dynamical singularity points in an analytically solvable p-spin glass like model
We introduce and analytically study a generalized p-spin glass like model
that captures some of the main features of attractive glasses, recently found
by Mode Coupling investigations, such as a glass/glass transition line and
dynamical singularity points characterized by a logarithmic time dependence of
the relaxation. The model also displays features not predicted by the Mode
Coupling scenario that could further describe the attractive glasses behavior,
such as aging effects with new dynamical singularity points ruled by
logarithmic laws or the presence of a glass spinodal line
Enhanced granular medium-based tube press hardening
Active and passive control strategies of internal pressure for hot forming of
tubes and profiles with granular media are described. Force transmission and
plastic deformation of granular medium is experimentally investigated. Friction
between tube, granular medium and die as also the external stress field are
shown to be essential for the process understanding. Wrinkling, thinning and
insufficient forming of the tube establishes the process window for the active
pressure process. By improving the punch geometry and controlling tribological
conditions, the process limits are extended. Examples for the passive pressure
process reveal new opportunities for hot forming of tubes and profiles.Comment: 4 pages, 11 figure
- …