1,834 research outputs found
COMPARATIVE ANALYSIS OF THE KINEMATICS AND KINETICS OF FORWARD AND BACKWARD HUMAN LOCOMOTION
Although common in occurrence one of the least studied modes
of human motion is backward locomotion. Backward walking and jogging are currently being used as components of physical therapy for persons who have suffered trauma or submitted to surgery on the lower extremity or back as well as stroke patients. (Gray, 1985, Bates and McCaw, 1986; Kramer and Reid, 1981; Bates, Morrison, Hamill, 1984).
Anecdotal reports of reduced noxious stress and beneficial
proprioceptive stimulation have perpetuated the use of this form of physical therapy. Position specific sport training necessitates backward locomotion to engender efficient game performances. In numerous sport activities the defensive techniques employed entail retreating or "jockeying" maneuvers. Evasive offensive techniques likewise include backward travel. Some coaches and athletes are supplementing their conventional fitness training regimens with backwards running claiming enhanced hip extensor overloading with the resulting strength adaptation (Morton, 1985)
Deformed Skyrme Crystals
The Skyrme crystal, a solution of the Skyrme model, is the lowest
energy-per-charge configuration of skyrmions seen so far. Our numerical
investigations show that, as the period in various space directions is changed,
one obtains various other configurations, such as a double square wall, and
parallel vortex-like solutions. We also show that there is a sudden "phase
transition" between a Skyrme crystal and the charge 4 skyrmion with cubic
symmetry as the period is gradually increased in all three space directions.Comment: 13 pages, 6 figures. To be published in JHE
Engineering ultralong spin coherence in two-dimensional hole systems at low temperatures
For the realisation of scalable solid-state quantum-bit systems, spins in
semiconductor quantum dots are promising candidates. A key requirement for
quantum logic operations is a sufficiently long coherence time of the spin
system. Recently, hole spins in III-V-based quantum dots were discussed as
alternatives to electron spins, since the hole spin, in contrast to the
electron spin, is not affected by contact hyperfine interaction with the
nuclear spins. Here, we report a breakthrough in the spin coherence times of
hole ensembles, confined in so called natural quantum dots, in narrow
GaAs/AlGaAs quantum wells at temperatures below 500 mK. Consistently,
time-resolved Faraday rotation and resonant spin amplification techniques
deliver hole-spin coherence times, which approach in the low magnetic field
limit values above 70 ns. The optical initialisation of the hole spin
polarisation, as well as the interconnected electron and hole spin dynamics in
our samples are well reproduced using a rate equation model.Comment: 16 pages, 6 figure
Geometric phase effects for wavepacket revivals
The study of wavepacket revivals is extended to the case of Hamiltonians
which are made time-dependent through the adiabatic cycling of some parameters.
It is shown that the quantal geometric phase (Berry's phase) causes the revived
packet to be displaced along the classical trajectory, by an amount equal to
the classical geometric phase (Hannay's angle), in one degree of freedom. A
physical example illustrating this effect in three degrees of freedom is
mentioned.Comment: Revtex, 11 pages, no figures
Parameter identification in a semilinear hyperbolic system
We consider the identification of a nonlinear friction law in a
one-dimensional damped wave equation from additional boundary measurements.
Well-posedness of the governing semilinear hyperbolic system is established via
semigroup theory and contraction arguments. We then investigte the inverse
problem of recovering the unknown nonlinear damping law from additional
boundary measurements of the pressure drop along the pipe. This coefficient
inverse problem is shown to be ill-posed and a variational regularization
method is considered for its stable solution. We prove existence of minimizers
for the Tikhonov functional and discuss the convergence of the regularized
solutions under an approximate source condition. The meaning of this condition
and some arguments for its validity are discussed in detail and numerical
results are presented for illustration of the theoretical findings
Spin and orbital mechanisms of the magneto-gyrotropic photogalvanic effects in GaAs/AlGaAs quantum well structures
We report on the study of the linear and circular magneto-gyrotropic
photogalvanic effect (MPGE) in GaAs/AlGaAs quantum well structures. Using the
fact that in such structures the Land\'e-factor g* depends on the quantum well
(QW) width and has different signs for narrow and wide QWs, we succeeded to
separate spin and orbital contributions to both MPGEs. Our experiments show
that, for most quantum well widths, the PGEs are mainly driven by spin-related
mechanisms, which results in a photocurrent proportional to the g* factor. In
structures with a vanishingly small g* factor, however, linear and circular
MPGE are also detected, proving the existence of orbital mechanisms.Comment: 10 pages, 10 figure
Electromagnetic Calorimeter for HADES
We propose to build the Electromagnetic calorimeter for the HADES di-lepton
spectrometer. It will enable to measure the data on neutral meson production
from nucleus-nucleus collisions, which are essential for interpretation of
dilepton data, but are unknown in the energy range of planned experiments (2-10
GeV per nucleon). The calorimeter will improve the electron-hadron separation,
and will be used for detection of photons from strange resonances in elementary
and HI reactions.
Detailed description of the detector layout, the support structure, the
electronic readout and its performance studied via Monte Carlo simulations and
series of dedicated test experiments is presented.
The device will cover the total area of about 8 m^2 at polar angles between
12 and 45 degrees with almost full azimuthal coverage. The photon and electron
energy resolution achieved in test experiments amounts to 5-6%/sqrt(E[GeV])
which is sufficient for the eta meson reconstruction with S/B ratio of 0.4% in
Ni+Ni collisions at 8 AGeV. A purity of the identified leptons after the hadron
rejection, resulting from simulations based on the test measurements, is better
than 80% at momenta above 500 MeV/c, where time-of-flight cannot be used.Comment: 40 pages, 38 figures version2 - the time schedule added, information
about PMTs in Sec.III update
Skyrme Crystal In A Two-Dimensional Electron Gas
The ground state of a two-dimensional electron gas at Landau level filling
factors near is a Skyrme crystal with long range order in the
positions and orientations of the topologically and electrically charged
elementary excitations of the ferromagnetic ground state. The lowest
energy Skyrme crystal is a square lattice with opposing postures for
topological excitations on opposite sublattices. The filling factor dependence
of the electron spin-polarization, calculated for the square lattice Skyrme
crystal, is in excellent agreement with recent experiments.Comment: 3 pages, latex, 3 figures available upon request from
[email protected]
Thermal bremsstrahlung probing the thermodynamical state of multifragmenting systems
Inclusive and exclusive hard-photon (E 30 MeV) production in five
different heavy-ion reactions (Ar+Au, Ag, Ni,
C at 60{\it A} MeV and Xe+Sn at 50{\it A} MeV) has been
studied coupling the TAPS photon spectrometer with several charged-particle
multidetectors covering more than 80% of 4. The measured spectra, slope
parameters and source velocities as well as their target-dependence, confirm
the existence of thermal bremsstrahlung emission from secondary nucleon-nucleon
collisions that accounts for roughly 20% of the total hard-photon yield. The
thermal slopes are a direct measure of the temperature of the excited nuclear
systems produced during the reaction.Comment: 4 pages, 3 figures, Proceedings CRIS 2000, 3rd Catania Relativistic
Ion Studies, "Phase Transitions in Strong Interactions: Status and
Perspectives", Acicastello, Italy, May 22-26, 2000 (to be published in Nuc.
Phys. A
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