3,802 research outputs found

### Rotating quantum turbulence in superfluid 4He in the T=0 limit

Observations of quantum turbulence in pure superfluid 4He in a rotating
container are reported. New techniques of large-scale forcing (rotational
oscillations of the cubic container) and detecting (monitoring ion transport
along the axis of rotation) turbulence were implemented. Near the axial walls,
with increasing forcing the vortex tangle grows without an observable
threshold. This tangle gradually develops into bulk turbulence at a
characteristic amplitude of forcing that depends on forcing frequency and
rotation rate. At higher amplitudes, the total vortex line length increases
rapidly. Resonances of inertial waves are observed in both laminar and
turbulent bulk states. On such resonances, the turbulence appears at smaller
amplitudes of forcing.Comment: 5 pages, 5 figure

### Chirality of superfluid 3He-A

We have used torsional oscillators, containing disk-shaped slabs of
superfluid 3He-A, to probe the chiral orbital textures created by cooling into
the superfluid state while continuously rotating. Comparing the observed
flow-driven textural transitions with numerical simulations of possible
textures shows that an oriented monodomain texture with l antiparallel to the
angular velocity Omega_0 is left behind after stopping rotation. The bias
towards a particular chirality, while in the vortex state, is due to the
inequivalence of energies of vortices of opposite circulation. When spun-up
from rest, the critical velocity for vortex nucleation depends on the sense of
rotation, Omega, relative to that of l. A different type of vorticity,
apparently linked to the slab's rim by a domain wall, appears when Omega is
parallel to l.Comment: 8 pages, 6 figure

### The structure and stability of molecular cloud cores in external radiation fields

We have considered the thermal equilibrium in pre-protostellar cores in the
approximation where the dust temperature is independent of interactions with
the gas and where the gas is heated both by collisions with dust grains and
ionization by cosmic rays. We have then used these results to study the
stability of cores in the limit where thermal pressure dominates over magnetic
field and turbulence. We find that for cores with characteristics similar to
those observed, the gas and dust temperatures are coupled in the core interior.
As a consequence, the gas temperature like the dust temperature decreases
towards the center of these objects. The density structure computed taking into
account such deviations from isothermality are not greatly different from that
expected for an isothermal Bonnor-Ebert sphere. It is impossible in the
framework of these models to have a stable equilibrium core with mass above
about 5 solar masses and column density compatible with observed values. We
conclude from this that observed high mass cores are either supported by
magnetic field or turbulence or are already in a state of collapse. Lower mass
cores on the other hand have stable states and we conclude that the much
studied object B68 may be in a state of stable equilibrium if the internal gas
temperature is computed in self-consistent fashion. Finally we note that in
molecular clouds such as Ophiuchus and Orion with high radiation fields and
pressures, gas and dust temperatures are expected to be well coupled and hence
one expects temperatures to be relatively high as compared to low pressure
clouds like Taurus.Comment: 11 pages, 6 figures. Astronomy & Astrophysics, in pres

### Ultrafast nonlinear optics in semiconductors

The nonlinear optical phenomena which occur in semiconductor materials on a femtosecond to picosecond timescale have recently generated much interest, especially in the field of telecommunications where the development of all-optical switching devices based on semiconductors promises a considerable reduction in the complexity of design coupled with a large increase in the speed of operation. This thesis examines the underlying ultrafast physical processes with the aim of providing a clear understanding of the mechanisms involved. The two main regimes of operation are investigated, namely off-resonance excitation where virtual processes are important and on-resonance excitation where real carriers are photogenerated, and in each case a particular system of interest is studied. For the virtual regime of operation, a recent proposal is examined which suggests the use of bandstructure engineering for a semiconductor quantum well in order to enhance the nonlinear optical response by the introduction of additional resonant transitions between subbands. A number of descriptions of the device are presented, and it is concluded that the technique does not necessarily lead to an improved response. An example of on-resonance phenomena is provided by the modelling of the fast refractive index changes in semiconductor laser amplifiers which have been observed in recent experiments. A simple physical model is developed which predicts the behaviour seen in the experimental observations. The nonlinear optical response of the laser amplifier promises the development of fast all-optical switching based on these devices. The thesis also examines the difficulties associated with describing the interaction of semiconductor material and electromagnetic field, and in particular looks at the formulation of a gauge invariant procedure for calculating values of the susceptibility. The propagation of a light beam along the plane of a semiconductor quantum well is discussed, and the gauge invariance of susceptibility calculations performed in the so called A.p and E.r gauges is explicitly demonstrated. Finally, a brief exploration is undertaken of the effects of bandstructure on the optical response of a semiconductor, and two quantum well models for the calculation of a more realistic bandstructure are presented which employ infinite and finite wells respectively

### Kinematics of a hot massive accretion disk candidate

Characterizing rotation, infall and accretion disks around high-mass
protostars is an important topic in massive star formation research. With the
Australia Telescope Compact Array and the Very Large Array we studied a massive
disk candidate at high angular resolution in ammonia (NH3(4,4) & (5,5)) tracing
the warm disk but not the envelope. The observations resolved at ~0.4''
resolution (corresponding to ~1400AU) a velocity gradient indicative of
rotation perpendicular to the molecular outflow. Assuming a Keplerian accretion
disk, the estimated protostar-disk mass would be high, similar to the
protostellar mass. Furthermore, the position-velocity diagram exhibits
additional deviation from a Keplerian rotation profile which may be caused by
infalling gas and/or a self-gravitating disk. Moreover, a large fraction of the
rotating gas is at temperatures >100K, markedly different to typical low-mass
accretion disks. In addition, we resolve a central double-lobe cm continuum
structure perpendicular to the rotation. We identify this with an ionized,
optically thick jet.Comment: 5 pages, 3 figures, accepted for Astrophysical Journal Letters, a
high-resolution version of the draft can be found at
http://www.mpia.de/homes/beuther/papers.htm

### Interactions between unidirectional quantized vortex rings

We have used the vortex filament method to numerically investigate the
interactions between pairs of quantized vortex rings that are initially
traveling in the same direction but with their axes offset by a variable impact
parameter. The interaction of two circular rings of comparable radii produce
outcomes that can be categorized into four regimes, dependent only on the
impact parameter; the two rings can either miss each other on the inside or
outside, or they can reconnect leading to final states consisting of either one
or two deformed rings. The fraction of of energy went into ring deformations
and the transverse component of velocity of the rings are analyzed for each
regime. We find that rings of very similar radius only reconnect for a very
narrow range of the impact parameter, much smaller than would be expected from
geometrical cross-section alone. In contrast, when the radii of the rings are
very different, the range of impact parameters producing a reconnection is
close to the geometrical value. A second type of interaction considered is the
collision of circular rings with a highly deformed ring. This type of
interaction appears to be a productive mechanism for creating small vortex
rings. The simulations are discussed in the context of experiments on colliding
vortex rings and quantum turbulence in superfluid helium in the zero
temperature limit

### Reconnections of quantized vortex rings in superfluid $^4$He at very low temperatures

Collisions in a beam of unidirectional quantized vortex rings of nearly
identical radii $R$ in superfluid $^4$He in the limit of zero temperature (0.05
K) were studied using time-of-flight spectroscopy. Reconnections between two
primary rings result in secondary vortex loops of both smaller and larger
radii. Discrete steps in the distribution of flight times, due to the limits on
the earliest possible arrival times of secondary loops created after either one
or two consecutive reconnections, are observed. The density of primary rings
was found to be capped at the value $500{\rm \,cm}^{-2} R^{-1}$ independent of
the injected density. This is due to collisions between rings causing piling-up
of many other vortex rings. Both observations are in quantitative agreement
with our theory.Comment: 7 pages, 4 figures, includes supplementary materia

### Simplified Quantum Process Tomography

We propose and evaluate experimentally an approach to quantum process
tomography that completely removes the scaling problem plaguing the standard
approach. The key to this simplification is the incorporation of prior
knowledge of the class of physical interactions involved in generating the
dynamics, which reduces the problem to one of parameter estimation. This allows
part of the problem to be tackled using efficient convex methods, which, when
coupled with a constraint on some parameters allows globally optimal estimates
for the Kraus operators to be determined from experimental data. Parameterising
the maps provides further advantages: it allows the incorporation of mixed
states of the environment as well as some initial correlation between the
system and environment, both of which are common physical situations following
excitation of the system away from thermal equilibrium. Although the approach
is not universal, in cases where it is valid it returns a complete set of
positive maps for the dynamical evolution of a quantum system at all times.Comment: Added references to interesting related work by Bendersky et a

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