168 research outputs found
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
Edge modes and non local conductance in graphene superlattices
We study the existence of edge modes in gapped Moir\'e superlattices in
graphene monolayer ribbons. We find that the superlattice bands acquire finite
Chern numbers, which lead to a Valley Hall Effect. The presence of dispersive
edge modes is confirmed by calculations of the band structure of realistic
nanoribbons using tight binding methods. These edge states are only weakly
sensitive to disorder, as short-range scattering processes lead to mean free
paths of the order of microns. The results explain the existence of edge
currents when the chemical potential lies within the bulk superlattice gap, and
offer an explanation for existing non-local resistivity measurements in
graphene ribbons on boron nitride
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