8,262 research outputs found
Determination of the wind response of Saturn 5 by statistical methods, volume 1
Statistical analysis of Saturn 5 launch vehicle wind response - Vol.
Prelaunch testing of the GEOS-3 laser reflector array
The prelaunch testing performed on the Geos-3 laser reflector array before launch was used to determine the lidar cross section of the array and the distance of the center of gravity of the satellite from the center of gravity of reflected laser pulses as a function of incidence angle. Experimental data are compared to computed results
An exactly solvable model of a superconducting to rotational phase transition
We consider a many-fermion model which exhibits a transition from a
superconducting to a rotational phase with variation of a parameter in its
Hamiltonian. The model has analytical solutions in its two limits due to the
presence of dynamical symmetries. However, the symmetries are basically
incompatible with one another; no simple solution exists in intermediate
situations. Exact (numerical) solutions are possible and enable one to study
the behavior of competing but incompatible symmetries and the phase transitions
that result in a semirealistic situation. The results are remarkably simple and
shed light on the nature of phase transitions.Comment: 11 pages including 1 figur
2015 researcher's mini-symposium
Postgraduate researchers from the Faculties of Science, Engineering, Medicine & Surgery and Health Sciences gathered for a forum to present their research interests. The symposium was held in the afternoon of 30
January 2015 in the Engineering Lecture Theatre.
The symposium promoted multi-disciplinary networking between various university faculties. Participants
were invited based on research topic diversity and
gender balance.peer-reviewe
Control of trapped-ion quantum states with optical pulses
We present new results on the quantum control of systems with infinitely
large Hilbert spaces. A control-theoretic analysis of the control of trapped
ion quantum states via optical pulses is performed. We demonstrate how resonant
bichromatic fields can be applied in two contrasting ways -- one that makes the
system completely uncontrollable, and the other that makes the system
controllable. In some interesting cases, the Hilbert space of the
qubit-harmonic oscillator can be made finite, and the Schr\"{o}dinger equation
controllable via bichromatic resonant pulses. Extending this analysis to the
quantum states of two ions, a new scheme for producing entangled qubits is
discovered.Comment: Submitted to Physical Review Letter
Investigation of the Liquid Fluorine-liquid Diborane Propellant Combination in a 100-pound-thrust Rocket Engine
The experimental performance of liquid fluorine and liquid diborane was investigated in a 100-pound-thrust engine at a combustion pressure of 300 pounds per square inch absolute. Methods of handling and transporting liquid fluorine were developed. It was extremely difficult to obtain satisfactory operation because of the high flame speed and high combustion chamber temperatures. The maximum performance obtained was 280 pound seconds per pound, 88 percent of the theoretical maximum. The theoretical performance was recalculated with revised thermodynamic data, indicating a maximum specific impulse of 311 pound seconds per pound as compared with the previously reported value of 323
Coexistence versus extinction in the stochastic cyclic Lotka-Volterra model
Cyclic dominance of species has been identified as a potential mechanism to
maintain biodiversity, see e.g. B. Kerr, M. A. Riley, M. W. Feldman and B. J.
M. Bohannan [Nature {\bf 418}, 171 (2002)] and B. Kirkup and M. A. Riley
[Nature {\bf 428}, 412 (2004)]. Through analytical methods supported by
numerical simulations, we address this issue by studying the properties of a
paradigmatic non-spatial three-species stochastic system, namely the
`rock-paper-scissors' or cyclic Lotka-Volterra model. While the deterministic
approach (rate equations) predicts the coexistence of the species resulting in
regular (yet neutrally stable) oscillations of the population densities, we
demonstrate that fluctuations arising in the system with a \emph{finite number
of agents} drastically alter this picture and are responsible for extinction:
After long enough time, two of the three species die out. As main findings we
provide analytic estimates and numerical computation of the extinction
probability at a given time. We also discuss the implications of our results
for a broad class of competing population systems.Comment: 12 pages, 9 figures, minor correction
The unrestricted Skyrme-tensor time-dependent Hartree-Fock and its application to the nuclear response from spherical to triaxial nuclei
The nuclear time-dependent Hartree-Fock model formulated in the
three-dimensional space,based on the full Skyrme energy density functional and
complemented with the tensor force,is presented for the first time. Full
self-consistency is achieved by the model. The application to the isovector
giant dipole resonance is discussed in the linear limit, ranging from spherical
nuclei (16O, 120Sn) to systems displaying axial or triaxial deformation (24Mg,
28Si, 178Os, 190W, 238U).
Particular attention is paid to the spin-dependent terms from the central
sector of the functional, recently included together with the tensor. They turn
out to be capable of producing a qualitative change on the strength
distribution in this channel. The effect on the deformation properties is also
discussed. The quantitative effects on the linear response are small and,
overall, the giant dipole energy remains unaffected.
Calculations are compared to predictions from the (quasi)-particle random
phase approximation and experimental data where available, finding good
agreement
Fowler-Nordheim-like local injection of photoelectrons from a silicon tip
Tunneling between a photo-excited p-type silicon tip and a gold surface is
studied as a function of tip bias, tip/sample distance and light intensity. In
order to extend the range of application of future spin injection experiments,
the measurements are carried out under nitrogen gas at room temperature. It is
found that while tunneling of valence band electrons is described by a standard
process between the semiconductor valence band and the metal, the tunneling of
photoelectrons obeys a Fowler-Nordheim-like process directly from the
conduction band. In the latter case, the bias dependence of the photocurrent as
a function of distance is in agreement with theoretical predictions which
include image charge effects. Quantitative analysis of the bias dependence of
the dark and photocurrent spectra gives reasonable values for the distance, and
for the tip and metal work functions. For small distances image charge effects
induce a vanishing of the barrier and the bias dependence of the photocurrent
is exponential. In common with many works on field emission, fluctuations in
the tunneling currents are observed. These are mainly attributed to changes in
the prefactor for the tunneling photocurrent, which we suggest is caused by an
electric-field-induced modification of the thickness of the natural oxide layer
covering the tip apex.Comment: 12 pages, 11 figures. Submitted to Phys. Rev.
Experimental investigation of planar ion traps
Chiaverini et al. [Quant. Inf. Comput. 5, 419 (2005)] recently suggested a
linear Paul trap geometry for ion trap quantum computation that places all of
the electrodes in a plane. Such planar ion traps are compatible with modern
semiconductor fabrication techniques and can be scaled to make compact, many
zone traps. In this paper we present an experimental realization of planar ion
traps using electrodes on a printed circuit board to trap linear chains of tens
of 0.44 micron diameter charged particles in a vacuum of 15 Pa (0.1 torr). With
these traps we address concerns about the low trap depth of planar ion traps
and develop control electrode layouts for moving ions between trap zones
without facing some of the technical difficulties involved in an atomic ion
trap experiment. Specifically, we use a trap with 36 zones (77 electrodes)
arranged in a cross to demonstrate loading from a traditional four rod linear
Paul trap, linear ion movement, splitting and joining of ion chains, and
movement of ions through intersections. We further propose an additional DC
biased electrode above the trap which increases the trap depth dramatically,
and a novel planar ion trap geometry that generates a two dimensional lattice
of point Paul traps.Comment: 11 pages, 20 figure
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