440,946 research outputs found
Visualization of flows in a motored rotary combustion engine using holographic interferometry
The use of holographic interferometry to view the small- and large-scale flow field structures in the combustion chamber of a motored Wankel engine assembly is described. In order that the flow patterns of interest could be observed, small quantities of helium were injected with the intake air. Variation of the air flow patterns with engine speed, helium flow rate, and rotor position are described. The air flow at two locations within the combustion chamber was examined using this technique
Coulomb blockade in silicon based structures at temperatures up to 50 K
Coulomb blockade has been observed in the current-voltage characteristics of structures fabricated in silicon germanium delta-doped material at temperatures up to 50 K. This is consistent with the estimated effective tunnel capacitance of 10 aF which is significantly smaller than the reported capacitances of tunnel junctions made from Al or GaAs/AlGaAs heterostructures
Nonlinear current response of one- and two-band superconductors
We have calculated the nonlinear current of a number of single band s-wave
electron-phonon superconductors. Among issues considered were those of
dimensionality, strong electron-phonon coupling, impurities, and comparison
with BCS. For the case of two bands, particular attention is paid to the role
of anisotropy, the integration effects of the off-diagonal electron-phonon
interaction, as well as inter- and intraband impurities. For the specific case
of MgB2, we present results based on the known microscopic parameters of band
theory.Comment: 10 pages, 6 figure
Partial spin freezing in the quasi-two-dimensional La2(Cu,Li)O4
In conventional spin glasses, the magnetic interaction is not strongly
anisotropic and the entire spin system freezes at low temperature. In
La2(Cu,Li)O4, for which the in-plane exchange interaction dominates the
interplane one, only a fraction of spins with antiferromagnetic correlations
extending to neighboring planes become spin-glass. The remaining spins with
only in-plane antiferromagnetic correlations remain spin-liquid at low
temperature. Such a novel partial spin freezing out of a spin-liquid observed
in this cold neutron scattering study is likely due to a delicate balance
between disorder and quantum fluctuations in the quasi-two dimensional S=1/2
Heisenberg system.Comment: 4 pages, 4 figure
Detection of an X-ray Pulsar Wind Nebula and Tail in SNR N157B
We report Chandra X-ray observations of the supernova remnant N157B in the
Large Magellanic Cloud, which are presented together with an archival HST
optical image and a radio continuum map for comparison. This remnant contains
the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly
rotating young pulsar known.
Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar
to within an uncertainty of less than 1 arcsec. PSR J0537-6910 is not detected
in any other wavelength band. The X-ray observations resolve three distinct
features: the pulsar itself, a surrounding compact wind nebula which is
strongly elongated and a feature of large-scale diffuse emission trailing from
the pulsar. This latter comet tail-shaped feature coexists with enhanced radio
emission and is oriented nearly perpendicular to the major axis of the pulsar
wind nebula. We propose the following scenario to explain these features. The
bright, compact nebula is likely powered by a toroidal pulsar wind of
relativistic particles which is partially confined by the ram-pressure from the
supersonic motion of the pulsar. The particles, after being forced out from the
compact nebula (the head of the ``comet''), are eventually dumped into a bubble
(the tail), which is primarily responsible for the extended diffuse X-ray and
radio emission. The ram-pressure confinement also allows a natural explanation
for the observed X-ray luminosity of the compact nebula and for the unusually
small X-ray to spin-down luminosity ratio, compared to similarly energetic
pulsars. We estimate the pulsar wind Lorentz factor of N157B as about 4 times
10^6 (with an uncertainty of a factor about 2, consistent with that inferred
from the modeling of the Crab Nebula.Comment: 15 pages plus 4 figures. The postscript file of the whole paper is
available at http://xray.astro.umass.edu/wqd/papers/n157b/n157b.ps. accepted
for publication in Ap
The frustrated Heisenberg antiferromagnet on the honeycomb lattice: -- model
We study the ground-state (gs) phase diagram of the frustrated spin-1/2
-- antiferromagnet with () on the
honeycomb lattice, using the coupled-cluster method. We present results for the
ground-state energy, magnetic order parameter and plaquette valence-bond
crystal (PVBC) susceptibility. We find a paramagnetic PVBC phase for
, where and . The transition at
to the N\'{e}el phase seems to be a continuous deconfined
transition (although we cannot exclude a very narrow intermediate phase in the
range ), while that at is of
first-order type to another quasiclassical antiferromagnetic phase that occurs
in the classical version of the model only at the isolated and highly
degenerate critical point . The spiral phases that are present
classically for all values are absent for all .Comment: 6 pages, 5 figure
Evaluation of reforestation using remote sensing techniques
The utilization of remotely sensed orbital data for forestry inventory. The study area (approximately 491,100 ha) encompasses the municipalities of Ribeirao Preto, Altinopolis, Cravinhos, Serra Azul, Luis Antonio, Sao Simao, Sant Rita do Passa Quatro and Santa Rosa do Viterbo (Sao Paulo State). Materials used were LANDSAT data from channels 5 and 7 (scale 1:250,000) and CCT's. Visual interpretation of the imagery showed that for 1977 a total of 37,766.00 ha and for 1979 38,003.75 ha were reforested with Pinus and Eucalyptus within the area under study. The results obtained show that LANDSAT data can be used efficiently in forestry inventory studies
Shot Noise Enhancement in Resonant Tunneling Structures in a Magnetic Field
We have observed that the shot noise of tunnel current, I, in
GaSb-AlSb-InAs-AlSb-GaSb double-barrier structure under a magnetic field can
exceed 2qI. The measurements were done at T=4K in fields up to 5T parallel to
the current. The noise enhancement occurred at each of the several
negative-differential conductance regions induced by the tunneling of holes
through Landau levels in the InAs quantum well. The amount of the enhancement
increased with the strength of the negative conductance and reached values up
to 8qI. These results are explained qualitatively by fluctuations of the
density of states in the well, but point out the need for a detailed theory of
shot noise enhancement in resonant tunneling devices.Comment: 4 pages, RevTex, 3 figure
The frustrated Heisenberg antiferromagnet on the honeycomb lattice: A candidate for deconfined quantum criticality
We study the ground-state (gs) phase diagram of the frustrated spin-1/2
-- antiferromagnet with on the
honeycomb lattice, using coupled-cluster theory and exact diagonalization
methods. We present results for the gs energy, magnetic order parameter,
spin-spin correlation function, and plaquette valence-bond crystal (PVBC)
susceptibility. We find a N\'eel antiferromagnetic (AFM) phase for , a collinear striped AFM phase for , and a paramagnetic PVBC phase for . The transition at
appears to be of first-order type, while that at is
continuous. Since the N\'eel and PVBC phases break different symmetries our
results favor the deconfinement scenario for the transition at
- …