723 research outputs found
Diabatic and Adiabatic Collective Motion in a Model Pairing System
Large amplitude collective motion is investigated for a model pairing
Hamiltonian containing an avoided level crossing. A classical theory of
collective motion for the adiabatic limit is applied utilising either a
time-dependent mean-field theory or a direct parametrisation of the
time-dependent Schr\"odinger equation. A modified local harmonic equation is
formulated to take account of the Nambu-Goldstone mode. It turns out that in
some cases the system selects a diabatic path. Requantizing the collective
Hamiltonian, a reasonable agreement with an exact calculation for the low-lying
levels are obtained for both weak and strong pairing force. This improves on
results of the conventional Born-Oppenheimer approximation.Comment: 23 pages, 7 ps figures. Latex, uses revtex and graphic
Collective coordinates, shape transitions and shape coexistence: a microscopic approach
We investigate a description of shape-mixing and shape-transitions using
collective coordinates. To that end we apply a theory of adiabatic
large-amplitude motion to a simplified nuclear shell-model, where the
approximate results can be contrasted with exact diagonalisations. We find
excellent agreement for different regimes, and contrast the results with those
from a more standard calculation using a quadrupole constraint. We show that
the method employed in this work selects diabatic (crossing) potential energy
curves where these are appropriate, and discuss the implications for a
microscopic study of shape coexistence.Comment: 20 pages, including 6 ps file
On the nature of the phase transitions in two-dimensional type II superconductors
We have simulated the thermodynamics of vortices in a thin film of a type-II
superconductor. We make the lowest Landau level approximation, and use
quasi-periodic boundary conditions. Our work is consistent with the results of
previous simulations where evidence was found for an apparent first order
transition between the vortex liquid state and the vortex crystal state. We
show, however, that these results are just an artifact of studying systems
which are too small. There are substantial difficulties in simulating larger
systems using traditional approaches. By means of the optimal energy diffusion
algorithm we have been able to study systems containing up to about one
thousand vortices, and for these larger systems the evidence for a first order
transition disappears. By studying both crystalline and hexatic order, we show
that the KTHNY scenario seems to apply, where melting from the crystal is first
to the hexatic liquid state and next to the normal vortex liquid, in both cases
via a continuous transition.Comment: 26 pages, 26 composite figures. Pre-proof versio
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