1,765 research outputs found
Radial and angular rotons in trapped dipolar gases
We study Bose-Einstein condensates with purely dipolar interactions in oblate
(pancake) traps. We find that the condensate always becomes unstable to
collapse when the number of particles is sufficiently large. We analyze the
instability, and find that it is the trapped-gas analogue of the
``roton-maxon'' instability previously reported for a gas that is unconfined in
two dimensions. In addition, we find that under certain circumstances, the
condensate wave function attains a biconcave shape, with its maximum density
away from the center of the gas. These biconcave condensates become unstable
due to azimuthl excitation - an angular roton.Comment: 4 pages, 3 figure
Dipolar Bose gases: Many-body versus mean-field description
We characterize zero-temperature dipolar Bose gases under external spherical
confinement as a function of the dipole strength using the essentially exact
many-body diffusion Monte Carlo (DMC) technique. We show that the DMC energies
are reproduced accurately within a mean-field framework if the variation of the
s-wave scattering length with the dipole strength is accounted for properly.
Our calculations suggest stability diagrams and collapse mechanisms of dipolar
Bose gases that differ significantly from those previously proposed in the
literature
Stability of fermionic Feshbach molecules in a Bose-Fermi mixture
In the wake of successful experiments in Fermi condensates, experimental
attention is broadening to study resonant interactions in degenerate Bose-Fermi
mixtures. Here we consider the properties and stability of the fermionic
molecules that can be created in such a mixture near a Feshbach resonance (FR).
To do this, we consider the two-body scattering matrix in the many-body
environment, and assess its complex poles. The stability properties of these
molecules strongly depend on their centre-of-mass motion, because they must
satisfy Fermi statistics. At low centre-of-mass momenta the molecules are more
stable than in the absence of the environment (due to Pauli-blocking effects),
while at high centre-of-mass momenta nontrivial many body effects render them
somewhat less stable
In Defence of Modest Doxasticism About Delusions
Here I reply to the main points raised by the commentators on the arguments put forward in my Delusions and Other Irrational Beliefs (OUP, 2009). My response is aimed at defending a modest doxastic account of clinical delusions, and is articulated in three sections. First, I consider the view that delusions are in-between perceptual and doxastic states, defended by Jacob Hohwy and Vivek Rajan, and the view that delusions are failed attempts at believing or not-quite-beliefs, proposed by Eric Schwitzgebel and Maura Tumulty. Then, I address the relationship between the doxastic account of delusions and the role, nature, and prospects of folk psychology, which is discussed by Dominic Murphy, Keith Frankish, and Maura Tumulty in their contributions. In the final remarks, I turn to the continuity thesis and suggest that, although there are important differences between clinical delusions and non-pathological beliefs, these differences cannot be characterised satisfactorily in epistemic terms. \u
Wave Mechanics of a Two Wire Atomic Beamsplitter
We consider the problem of an atomic beam propagating quantum mechanically
through an atom beam splitter. Casting the problem in an adiabatic
representation (in the spirit of the Born-Oppenheimer approximation in
molecular physics) sheds light on explicit effects due to non-adiabatic passage
of the atoms through the splitter region. We are thus able to probe the fully
three dimensional structure of the beam splitter, gathering quantitative
information about mode-mixing, splitting ratios,and reflection and transmission
probabilities
Dipolar Bose-Einstein condensates with dipole-dependent scattering length
We consider a Bose-Einstein condensate of polar molecules in a harmonic trap,
where the effective dipole may be tuned by an external field. We demonstrate
that taking into account the dependence of the scattering length on the dipole
moment is essential to reproducing the correct energies and for predicting the
stability of the condensate. We do this by comparing Gross-Pitaevskii
calculations with diffusion Monte Carlo calculations. We find very good
agreement between the results obtained by these two approaches once the dipole
dependence of the scattering length is taken into account. We also examine the
behavior of the condensate in non-isotropic traps
Parallel pumping of magnetic vortex gyrations in spin-torque nano-oscillators
We experimentally demonstrate that large magnetic vortex oscillations can be
parametrically excited in a magnetic tunnel junction by the injection of
radio-frequency (rf) currents at twice the natural frequency of the gyrotropic
vortex core motion. The mechanism of excitation is based on the parallel
pumping of vortex motion by the rf orthoradial field generated by the injected
current. Theoretical analysis shows that experimental results can be
interpreted as the manifestation of parametric amplification when rf current is
small, and of parametric instability when rf current is above a certain
threshold. By taking into account the energy nonlinearities, we succeed to
describe the amplitude saturation of vortex oscillations as well as the
coexistence of stable regimes.Comment: Submitted to Phys. Rev. Let
Geometry of General Hypersurfaces in Spacetime: Junction Conditions
We study imbedded hypersurfaces in spacetime whose causal character is
allowed to change from point to point. Inherited geometrical structures on
these hypersurfaces are defined by two methods: first, the standard rigged
connection induced by a rigging vector (a vector not tangent to the
hypersurface anywhere); and a second, more physically adapted, where each
observer in spacetime induces a new type of connection that we call the rigged
metric connection. The generalisation of the Gauss and Codazzi equations are
also given. With the above machinery, we attack the problem of matching two
spacetimes across a general hypersurface. It is seen that the preliminary
junction conditions allowing for the correct definition of Einstein's equations
in the distributional sense reduce to the requirement that the first
fundamental form of the hypersurface be continuous. The Bianchi identities are
then proven to hold in the distributional sense. Next, we find the proper
junction conditions which forbid the appearance of singular parts in the
curvature. Finally, we derive the physical implications of the junction
conditions: only six independent discontinuities of the Riemann tensor are
allowed. These are six matter discontinuities at non-null points of the
hypersurface. For null points, the existence of two arbitrary discontinuities
of the Weyl tensor (together with four in the matter tensor) are also allowed.Comment: Latex, no figure
Inverse Spin Hall Effect in nanometer-thick YIG/Pt system
High quality nanometer-thick (20 nm, 7 nm and 4 nm) epitaxial YIG films have
been grown on GGG substrates using pulsed laser deposition. The Gilbert damping
coefficient for the 20 nm thick films is 2.3 x 10-4 which is the lowest value
reported for sub-micrometric thick films. We demonstrate Inverse spin Hall
effect (ISHE) detection of propagating spin waves using Pt. The amplitude and
the lineshape of the ISHE voltage correlate well to the increase of the Gilbert
damping when decreasing thickness of YIG. Spin Hall effect based
loss-compensation experiments have been conducted but no change in the
magnetization dynamics could be detected
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