16,453 research outputs found
Weak inter-band coupling in MgB: a specific heat analysis
The superconducting state of MgB is investigated by specific
heat measurements in detail. The specific heat in the normal state is analyzed
using a recently developed computer code. This allows for an extraction of the
electronic specific heat in the superconducting state with high accuracy and a
fair determination of the main lattice features. One of the two investigated
samples shows a hump in the specific heat at low temperatures within the
superconducting state, accompanied by an unusual low value of the small gap,
, pointing to a very weak inter-band coupling. This
sample allows for a detailed analysis of the contribution from the -band
to the electronic specific heat in the superconducting state. Therefore the
usual analysis method is modified, to include the individual conservation of
entropy of both bands. From analyzing the deviation function of
MgB, the theoretically predicted weak inter-band coupling scenario is
confirmed.Comment: major revision
Spin correlations among the charge carriers in an ordered stripe phase
We have observed a diffuse component to the low-energy magnetic excitation
spectrum of stripe-ordered La5/3Sr1/3NiO4 probed by neutron inelastic
scattering. The diffuse scattering forms a square pattern with sides parallel
and perpendicular to the stripe directions. The signal is dispersive, with a
maximum energy of ~10 meV. Probed at 2 meV the scattering decreases in strength
with increasing temperature, and is barely visible at 100 K. We argue that the
signal originates from dynamic, quasi- one-dimensional, antiferromagnetic
correlations among the stripe electrons.Comment: 4 pages, 4 figures. To be published in Physical Review Letter
Effects of an Early-Time Impact Generated Vapor Blast in the Martian Atmosphere: Formation of High-Latitude Pedestal Craters
Following impact, vapor expansion creates an intense airblast that interacts with the ambient atmosphere. The resulting hemi-spherical shock wave leaves a signature on the surface that is dependent on initial atmospheric and surface conditions. Here we propose that the formation of pedestal craters (craters surrounded by an erosion-resistant pedestal) may be a direct consequence of extreme winds and elevated temperatures generated by such an impact-induced atmospheric blast. Pedestal craters, first recognized in Mariner 9 data, are a unique feature on Mars and likely a signature of near-surface volatiles. They are found at high latitudes (small pedestals, Amazonian to Late Hesperian in age) and in thick equatorial mantling deposits (larger pedestals, early Hesperian to Noachian in age). Previously suggested mechanisms for pedestal crater formation (e.g., wind: ejecta curtain vortices or vapor blast; and ejecta dust: armoring) do not provide a complete picture. The clear evidence for near-surface volatiles at high latitudes requires a re-evaluation of these alternative models. The results presented here suggest that a combined atmospheric blast/thermal model provides a plausible formation hypothesis
Modeling Marsh‐Forest Boundary Transgression in Response to Storms and Sea‐Level Rise
The lateral extent and vertical stability of salt marshes experiencing rising sea levels depend on interacting drivers and feedbacks with potential for nonlinear behaviors. A two‐dimensional transect model was developed to examine changes in marsh and upland forest lateral extent and to explore controls on marsh inland transgression. Model behavior demonstrates limited and abrupt forest retreat with long‐term upland boundary migration rates controlled by slope, sea‐level rise (SLR), high water events, and biotic‐abiotic interactions. For low to moderate upland slopes the landward marsh edge is controlled by the interaction of these inundation events and forest recovery resulting in punctuated transgressive events. As SLR rates increase, the importance of the timing and frequency of water‐level deviations diminishes, and migration rates revert back to a slope‐SLR‐dominated process
Intrinsic Low Temperature Paramagnetism in B-DNA
We present experimental study of magnetization in -DNA in
conjunction with structural measurements. The results show the surprising
interplay between the molecular structures and their magnetic property. In the
B-DNA state, -DNA exhibits paramagnetic behaviour below 20 K that is
non-linear in applied magnetic field whereas in the A-DNA state, remains
diamagnetic down to 2 K. We propose orbital paramagnetism as the origin of the
observed phenomena and discuss its relation to the existence of long range
coherent transport in B-DNA at low temperature.Comment: 5 pages, 4 figures, submitted to Physical Review Letters October 200
Chaotic exploration and learning of locomotion behaviours
We present a general and fully dynamic neural system, which exploits intrinsic chaotic dynamics, for the real-time goal-directed exploration and learning of the possible locomotion patterns of an articulated robot of an arbitrary morphology in an unknown environment. The controller is modeled as a network of neural oscillators that are initially coupled only through physical embodiment, and goal-directed exploration of coordinated motor patterns is achieved by chaotic search using adaptive bifurcation. The phase space of the indirectly coupled neural-body-environment system contains multiple transient or permanent self-organized dynamics, each of which is a candidate for a locomotion behavior. The adaptive bifurcation enables the system orbit to wander through various phase-coordinated states, using its intrinsic chaotic dynamics as a driving force, and stabilizes on to one of the states matching the given goal criteria. In order to improve the sustainability of useful transient patterns, sensory homeostasis has been introduced, which results in an increased diversity of motor outputs, thus achieving multiscale exploration. A rhythmic pattern discovered by this process is memorized and sustained by changing the wiring between initially disconnected oscillators using an adaptive synchronization method. Our results show that the novel neurorobotic system is able to create and learn multiple locomotion behaviors for a wide range of body configurations and physical environments and can readapt in realtime after sustaining damage
The effect of parallel static and microwave electric fields on excited hydrogen atoms
Motivated by recent experiments we analyse the classical dynamics of a
hydrogen atom in parallel static and microwave electric fields. Using an
appropriate representation and averaging approximations we show that resonant
ionisation is controlled by a separatrix, and provide necessary conditions for
a dynamical resonance to affect the ionisation probability.
The position of the dynamical resonance is computed using a high-order
perturbation series, and estimate its radius of convergence. We show that the
position of the dynamical resonance does not coincide precisely with the
ionisation maxima, and that the field switch-on time can dramatically affect
the ionisation signal which, for long switch times, reflects the shape of an
incipient homoclinic. Similarly, the resonance ionisation time can reflect the
time-scale of the separatrix motion, which is therefore longer than
conventional static field Stark ionisation. We explain why these effects should
be observed in the quantum dynamics.
PACs: 32.80.Rm, 33.40.+f, 34.10.+x, 05.45.Ac, 05.45.MtComment: 47 pages, 20 figure
Workload accomplished in phase III cardiac rehabilitation
Exercise training is an important component of clinical exercise programs. Although there are recognized guidelines for the amount of exercise to be accomplished (≥70,000 steps per week or ≥150 min per week at moderate intensity), there is virtually no documentation of how much exercise is actually accomplished in contemporary exercise programs. Having guidelines without evidence of whether they are being met is of limited value. We analyzed both the weekly step count and the session rating of perceived exertion (sRPE) of patients (n = 26) enrolled in a community clinical exercise (e.g., Phase III) program over a 3-week reference period. Step counts averaged 39,818 ± 18,612 per week, with 18% of the steps accomplished in the program and 82% of steps accomplished outside the program. Using the sRPE method, inside the program, the patients averaged 162.4 ± 93.1 min per week, at a sRPE of 12.5 ± 1.9 and a frequency of 1.8 ± 0.7 times per week, for a calculated exercise load of 2042.5 ± 1244.9 AU. Outside the program, the patients averaged 144.9 ± 126.4 min, at a sRPE of 11.8 ± 5.8 and a frequency of 2.4 ± 1.5 times per week, for a calculated exercise load of 1723.9 ± 1526.2 AU. The total exercise load using sRPE was 266.4 ± 170.8 min per week, at a sRPE of 12.6 ± 3.8, and frequency of 4.2 ± 1.1 times per week, for a calculated exercise load of 3359.8 ± 2145.9 AU. There was a non-linear relationship between steps per week and the sRPE derived training load, apparently attributable to the amount of non-walking exercise accomplished in the program. The results suggest that patients in a community clinical exercise program are achieving American College of Sports Medicine guidelines, based on the sRPE method, but are accomplishing less steps than recommended by guidelines
Electron Spin Dynamics and Hyperfine Interactions in Fe/Al_0.1Ga_0.9As/GaAs Spin Injection Heterostructures
We have studied hyperfine interactions between spin-polarized electrons and
lattice nuclei in Al_0.1Ga_0.9As/GaAs quantum well (QW) heterostructures. The
spin-polarized electrons are electrically injected into the semiconductor
heterostructure from a metallic ferromagnet across a Schottky tunnel barrier.
The spin-polarized electron current dynamically polarizes the nuclei in the QW,
and the polarized nuclei in turn alter the electron spin dynamics. The
steady-state electron spin is detected via the circular polarization of the
emitted electroluminescence. The nuclear polarization and electron spin
dynamics are accurately modeled using the formalism of optical orientation in
GaAs. The nuclear spin polarization in the QW is found to depend strongly on
the electron spin polarization in the QW, but only weakly on the electron
density in the QW. We are able to observe nuclear magnetic resonance (NMR) at
low applied magnetic fields on the order of a few hundred Oe by electrically
modulating the spin injected into the QW. The electrically driven NMR
demonstrates explicitly the existence of a Knight field felt by the nuclei due
to the electron spin.Comment: 19 Figures - submitted to PR
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