193 research outputs found
Instabilities leading to vortex lattice formation in rotating Bose-Einstein condensates
We present a comprehensive theoretical study of vortex lattice formation in
atomic Bose-Einstein condensates confined by a rotating elliptical trap. We
consider rotating solutions of the classical hydrodynamic equations, their
response to perturbations, as well as time-dependent simulations. We
discriminate three distinct, experimentally testable, regimes of instability:
{\em ripple}, {\em interbranch}, and {\em catastrophic}. Under
symmetry-breaking perturbations these instabilities lead to lattice formation
even at zero temperature. While our results are consistent with previous
theoretical and experimental results, they shed new light on lattice formation.Comment: 5 pages, 2 figure
Anisotropic and long-range vortex interactions in two-dimensional dipolar Bose gases
We perform a theoretical study into how dipole-dipole interactions modify the
properties of superfluid vortices within the context of a two-dimensional
atomic Bose gas of co-oriented dipoles. The reduced density at a vortex acts
like a giant anti-dipole, changing the density profile and generating an
effective dipolar potential centred at the vortex core whose most slowly
decaying terms go as and . These effects modify
the vortex-vortex interaction which, in particular, becomes anisotropic for
dipoles polarized in the plane. Striking modifications to vortex-vortex
dynamics are demonstrated, i.e. anisotropic co-rotation dynamics and the
suppression of vortex annihilation.Comment: PRL accepted, 6 pages, 5 figure
Dynamical Instability of a Rotating Dipolar Bose-Einstein Condensate
We calculate the hydrodynamic solutions for a dilute Bose-Einstein condensate
with long-range dipolar interactions in a rotating, elliptical harmonic trap,
and analyse their dynamical stability. The static solutions and their regimes
of instability vary non-trivially on the strength of the dipolar interactions.
We comprehensively map out this behaviour, and in particular examine the
experimental routes towards unstable dynamics, which, in analogy to
conventional condensates, may lead to vortex lattice formation. Furthermore, we
analyse the centre of mass and breathing modes of a rotating dipolar
condensate.Comment: 4 pages, including 2 figure
Collective excitation frequencies and stationary states of trapped dipolar Bose-Einstein condensates in the Thomas-Fermi regime
We present a general method for obtaining the exact static solutions and
collective excitation frequencies of a trapped Bose-Einstein condensate (BEC)
with dipolar atomic interactions in the Thomas-Fermi regime. The method
incorporates analytic expressions for the dipolar potential of an arbitrary
polynomial density profile, thereby reducing the problem of handling non-local
dipolar interactions to the solution of algebraic equations.
We comprehensively map out the static solutions and excitation modes,
including non-cylindrically symmetric traps, and also the case of negative
scattering length where dipolar interactions stabilize an otherwise unstable
condensate. The dynamical stability of the excitation modes gives insight into
the onset of collapse of a dipolar BEC. We find that global collapse is
consistently mediated by an anisotropic quadrupolar collective mode, although
there are two trapping regimes in which the BEC is stable against quadrupole
fluctuations even as the ratio of the dipolar to s-wave interactions becomes
infinite. Motivated by the possibility of fragmented BEC in a dipolar Bose gas
due to the partially attractive interactions, we pay special attention to the
scissors modes, which can provide a signature of superfluidity, and identify a
long-range restoring force which is peculiar to dipolar systems. As part of the
supporting material for this paper we provide the computer program used to make
the calculations, including a graphical user interface.Comment: 23 pages, 11 figure
Exact solutions and stability of rotating dipolar Bose-Einstein condensates in the Thomas-Fermi limit
We present a theoretical analysis of dilute gas Bose-Einstein condensates
with dipolar atomic interactions under rotation in elliptical traps. Working in
the Thomas-Fermi limit, we employ the classical hydrodynamic equations to first
derive the rotating condensate solutions and then consider their response to
perturbations. We thereby map out the regimes of stability and instability for
rotating dipolar Bose-Einstein condensates and in the latter case, discuss the
possibility of vortex lattice formation. We employ our results to propose
several novel routes to induce vortex lattice formation in a dipolar
condensate.Comment: 12 pages with 6 figure
The renewable energy and energy efficiency potential of Waitakere City : a thesis presented in partial fulfilment of the requirements for the degree of Masters of Technology in Energy Management at Massey University
Electricity restrictions and blackouts have occurred in Waitakere City in the past and are likely to occur again in the future unless the city can become more self reliant by meeting, at least in part, the increasing energy requirements for what is one of the fastest growing cities in New Zealand. In this study the potentials for energy conservation, energy efficiency and renewable energy resources have been broadly quantified and assessed using desktop analysis of publicly available data for stationary final use energy systems (i.e. excluding transportation) within the geographical area of Waitakere City and adjoining waters.
It was found that energy efficiency and energy conservation measures can consistently and predictably achieve overall energy savings and reduce daily and seasonal peak demand.
The best renewable energy resource potential exists with solar and geothermal for heating applications and wave, offshore and inshore wind and tidal currents for electricity generation. There is very limited potential for hydro and bioenergy systems beyond what already exists. PV solar and land based wind power generation are currently only feasible for limited off-grid applications.
This scoping study confirms the achievability of the vision expressed in Waitakere City Council's "Long Term Council Community Plan" (LTCCP) that by 2020 " Waitakere City will be an energy cell, not an energy sink. Air quality supports good health". A range of flagship projects have been identified to progress the achievement of this vision. Waitakere City Council can use this report as part of the development of a comprehensive energy management plan
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