53,073 research outputs found
The subgroup growth spectrum of virtually free groups
For a finitely generated group denote by the growth
coefficient of , that is, the infimum over all real numbers such
that . We show that the growth coefficient of a virtually
free group is always rational, and that every rational number occurs as growth
coefficient of some virtually free group. Moreover, we describe an algorithm to
compute
Momentum isotropisation in random potentials
When particles are multiply scattered by a random potential, their momentum
distribution becomes isotropic on average. We study this quantum dynamics
numerically and with a master equation. We show how to measure the elastic
scattering time as well as characteristic isotropisation times, which permit to
reconstruct the scattering phase function, even in rather strong disorder.Comment: 5 pages, paper contributed to Lyon BEC 2012; v2 minor changes,
version published in prin
Symmetric Diblock Copolymers in Thin Films (I): Phase stability in Self-Consistent Field Calculations and Monte Carlo Simulations
We investigate the phase behavior of symmetric AB diblock copolymers confined
into a thin film. The film boundaries are parallel, impenetrable and attract
the A component of the diblock copolymer. Using a self-consistent field
technique [M.W. Matsen, J.Chem.Phys. {\bf 106}, 7781 (1997)], we study the
ordered phases as a function of incompatibility and film thickness in
the framework of the Gaussian chain model. For large film thickness and small
incompatibility, we find first order transitions between phases with different
number of lamellae which are parallel oriented to the film boundaries. At high
incompatibility or small film thickness, transitions between parallel oriented
and perpendicular oriented lamellae occur. We compare the self-consistent field
calculations to Monte Carlo simulations of the bond fluctuation model for chain
length N=32. In the simulations we quench several systems from to
and monitor the morphology into which the diblock copolymers
assemble. Three film thicknesses are investigated, corresponding to parallel
oriented lamellae with 2 and 4 interfaces and a perpendicular oriented
morphology. Good agreement between self-consistent field calculations and Monte
Carlo simulations is found.Comment: to appear in J.Chem.Phy
Spin effects in strong-field laser-electron interactions
The electron spin degree of freedom can play a significant role in
relativistic scattering processes involving intense laser fields. In this
contribution we discuss the influence of the electron spin on (i) Kapitza-Dirac
scattering in an x-ray laser field of high intensity, (ii) photo-induced
electron-positron pair production in a strong laser wave and (iii) multiphoton
electron-positron pair production on an atomic nucleus. We show that in all
cases under consideration the electron spin can have a characteristic impact on
the process properties and their total probabilities. To this end,
spin-resolved calculations based on the Dirac equation in the presence of an
intense laser field are performed. The predictions from Dirac theory are also
compared with the corresponding results from the Klein-Gordon equation.Comment: 9 pages, 6 figure
Strong Anderson localization in cold atom quantum quenches
Signatures of strong Anderson localization in the momentum distribution of a
cold atom cloud after a quantum quench are studied. We consider a quasi
one-dimensional cloud initially prepared in a well defined momentum state, and
expanding for some time in a disorder speckle potential. Anderson localization
leads to a formation of a coherence peak in the \emph{forward} scattering
direction (as opposed to the common weak localization backscattering peak). We
present a microscopic, and fully time resolved description of the phenomenon,
covering the entire diffusion--to--localization crossover. Our results should
be observable by present day technology.Comment: 4 pages, 2 figures, published versio
Echo spectroscopy of Anderson localization
We propose a conceptually new framework to study the onset of Anderson
localization in disordered systems. The idea is to expose waves propagating in
a random scattering environment to a sequence of short dephasing pulses. The
system responds through coherence peaks forming at specific echo times, each
echo representing a particular process of quantum interference. We suggest a
concrete realization for cold gases, where quantum interferences are observed
in the momentum distribution of matter waves in a laser speckle potential. This
defines a challenging, but arguably realistic framework promising to yield
unprecedented insight into the mechanisms of Anderson localization.Comment: 14 pages, 7 figures; published versio
Anomalous Viscosity of an Expanding Quark-Gluon Plasma
We argue that an expanding quark-gluon plasma has an anomalous viscosity,
which arises from interactions with dynamically generated color fields. We
derive an expression for the anomalous viscosity in the turbulent plasma domain
and apply it to the hydrodynamic expansion phase, when the quark-gluon plasma
is near equilibrium. The anomalous viscosity dominates over the collisional
viscosity for weak coupling and not too late times. This effect may provide an
explanation for the apparent ``nearly perfect'' liquidity of the matter
produced in nuclear collisions at the Relativistic Heavy Ion Collider without
the assumption that it is a strongly coupled state.Comment: Final version accepted for publicatio
Dynamics of quantum spin chains and multi-fermion excitation continua
We use the Jordan-Wigner representation to study dynamic quantities for the
spin-1/2 XX chain in a transverse magnetic field. We discuss in some detail the
properties of the four-fermion excitation continuum which is probed by the
dynamic trimer structure factor.Comment: Presented at the SCES '05 - The International Conference on Strongly
Correlated Electron Systems (Vienna, July 26-30, 2005
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