18,618 research outputs found
Perturbation theory for the effective diffusion constant in a medium of random scatterer
We develop perturbation theory and physically motivated resummations of the
perturbation theory for the problem of a tracer particle diffusing in a random
media. The random media contains point scatterers of density uniformly
distributed through out the material. The tracer is a Langevin particle
subjected to the quenched random force generated by the scatterers. Via our
perturbative analysis we determine when the random potential can be
approximated by a Gaussian random potential. We also develop a self-similar
renormalisation group approach based on thinning out the scatterers, this
scheme is similar to that used with success for diffusion in Gaussian random
potentials and agrees with known exact results. To assess the accuracy of this
approximation scheme its predictions are confronted with results obtained by
numerical simulation.Comment: 22 pages, 6 figures, IOP (J. Phys. A. style
Effective diffusion constant in a two dimensional medium of charged point scatterers
We obtain exact results for the effective diffusion constant of a two
dimensional Langevin tracer particle in the force field generated by charged
point scatterers with quenched positions. We show that if the point scatterers
have a screened Coulomb (Yukawa) potential and are uniformly and independently
distributed then the effective diffusion constant obeys the
Volgel-Fulcher-Tammann law where it vanishes. Exact results are also obtained
for pure Coulomb scatterers frozen in an equilibrium configuration of the same
temperature as that of the tracer.Comment: 9 pages IOP LaTex, no figure
Complex coupled-cluster approach to an ab-initio description of open quantum systems
We develop ab-initio coupled-cluster theory to describe resonant and weakly
bound states along the neutron drip line. We compute the ground states of the
helium chain 3-10He within coupled-cluster theory in singles and doubles (CCSD)
approximation. We employ a spherical Gamow-Hartree-Fock basis generated from
the low-momentum N3LO nucleon-nucleon interaction. This basis treats bound,
resonant, and continuum states on equal footing, and is therefore optimal for
the description of properties of drip line nuclei where continuum features play
an essential role. Within this formalism, we present an ab-initio calculation
of energies and decay widths of unstable nuclei starting from realistic
interactions.Comment: 4 pages, revtex
Medium-mass nuclei from chiral nucleon-nucleon interactions
We compute the binding energies, radii, and densities for selected
medium-mass nuclei within coupled-cluster theory and employ the "bare" chiral
nucleon-nucleon interaction at order N3LO. We find rather well-converged
results in model spaces consisting of 15 oscillator shells, and the doubly
magic nuclei 40Ca, 48Ca, and the exotic 48Ni are underbound by about 1 MeV per
nucleon within the CCSD approximation. The binding-energy difference between
the mirror nuclei 48Ca and 48Ni is close to theoretical mass table evaluations.
Our computation of the one-body density matrices and the corresponding natural
orbitals and occupation numbers provides a first step to a microscopic
foundation of the nuclear shell model.Comment: 5 pages, 5 figure
Symmetry Relations for Trajectories of a Brownian Motor
A Brownian Motor is a nanoscale or molecular device that combines the effects
of thermal noise, spatial or temporal asymmetry, and directionless input energy
to drive directed motion. Because of the input energy, Brownian motors function
away from thermodynamic equilibrium and concepts such as linear response
theory, fluctuation dissipation relations, and detailed balance do not apply.
The {\em generalized} fluctuation-dissipation relation, however, states that
even under strongly thermodynamically non-equilibrium conditions the ratio of
the probability of a transition to the probability of the time-reverse of that
transition is the exponent of the change in the internal energy of the system
due to the transition. Here, we derive an extension of the generalized
fluctuation dissipation theorem for a Brownian motor for the ratio between the
probability for the motor to take a forward step and the probability to take a
backward step
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