49,515 research outputs found
Mode Switching and Subpulse Drifting in PSR B0826-34
We present high-quality observations of PSR B0826-34 at 1374 MHz. The
emission from this pulsar exhibits strong bursts of pulses followed by long
periods of `null' pulses. When it is strong, the radiation extends through the
whole pulse period. We show for the first time that there is weak emission
during the `null' phases, which should therefore be considered to be a
different mode rather than a null. During this weak mode the profile is similar
to that observed in the strong mode at low radio frequency. Using a
phase-tracking method, the pattern of drifting subpulses during the strong mode
is seen to be coherent across the whole profile. The drift rate is variable and
includes positive and negative values. Thirteen subpulse bands have been
directly observed, covering the whole longitude range. The subpulses and their
spacings () are wider in one half of the profile than those in the other
half. This difference, and the variation of observed within the two
regions, can be accounted for if the magnetic pole is inclined to the rotation
axis by about 0.5 degrees. These two regions appear to represent radiation from
outer and inner cones. The intensity modulation of subpulses in all longitude
ranges is related to the magnitude of the drift rate.Comment: 8 pages, 10 figures. Accepted for publication in MNRA
Observation of ferromagnetism above 900 K in Cr-GaN and Cr-AlN
We report the observation of ferromagnetism at over 900K in Cr-GaN and Cr-AlN
thin films. The saturation magnetization moments in our best films of Cr-GaN
and Cr-AlN at low temperatures are 0.42 and 0.6 u_B/Cr atom, respectively,
indicating that 14% and 20%, of the Cr atoms, respectively, are magnetically
active. While Cr-AlN is highly resistive, Cr-GaN exhibits thermally activated
conduction that follows the exponential law expected for variable range hopping
between localized states. Hall measurements on a Cr-GaN sample indicate a
mobility of 0.06 cm^2/V.s, which falls in the range characteristic of hopping
conduction, and a free carrier density (1.4E20/cm^3), which is similar in
magnitude to the measured magnetically-active Cr concentration (4.9E19/cm^3). A
large negative magnetoresistance is attributed to scattering from loose spins
associated with non-ferromagnetic impurities. The results indicate that
ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange
mechanism as a result of hopping between near-midgap substitutional Cr impurity
bands.Comment: 14 pages, 4 figures, submitted to AP
Search for a Radio Pulsar in the Remnant of Supernova 1987A
We have observed the remnant of supernova SN~1987A (SNR~1987A), located in
the Large Magellanic Cloud (LMC), to search for periodic and/or transient radio
emission with the Parkes 64\,m-diameter radio telescope. We found no evidence
of a radio pulsar in our periodicity search and derived 8 upper bounds
on the flux density of any such source of Jy at 1.4~GHz and
Jy at 3~GHz. Four candidate transient events were detected with
greater than significance, with dispersion measures (DMs) in the
range 150 to 840\,cmpc. For two of them, we found a second pulse at
slightly lower significance. However, we cannot at present conclude that any of
these are associated with a pulsar in SNR~1987A. As a check on the system, we
also observed PSR~B054069, a young pulsar which also lies in the LMC. We
found eight giant pulses at the DM of this pulsar. We discuss the implications
of these results for models of the supernova remnant, neutron star formation
and pulsar evolution.Comment: 7 pages, 3 figures, 2 tables. Accepted for publication in MNRA
Electron localization : band-by-band decomposition, and application to oxides
Using a plane wave pseudopotential approach to density functional theory we
investigate the electron localization length in various oxides. For this
purpose, we first set up a theory of the band-by-band decomposition of this
quantity, more complex than the decomposition of the spontaneous polarization
(a related concept), because of the interband coupling. We show its
interpretation in terms of Wannier functions and clarify the effect of the
pseudopotential approximation. We treat the case of different oxides: BaO,
-PbO, BaTiO and PbTiO. We also investigate the variation of the
localization tensor during the ferroelectric phase transitions of BaTiO as
well as its relationship with the Born effective charges
Power-law carrier dynamics in semiconductor nanocrystals at nanosecond time scales
We report the observation of power law dynamics on nanosecond to microsecond
time scales in the fluorescence decay from semiconductor nanocrystals, and draw
a comparison between this behavior and power-law fluorescence blinking from
single nanocrystals. The link is supported by comparison of blinking and
lifetime data measured simultaneously from the same nanocrystal. Our results
reveal that the power law coefficient changes little over the nine decades in
time from 10 ns to 10 s, in contrast with the predictions of some diffusion
based models of power law behavior.Comment: 3 pages, 2 figures, compressed for submission to Applied Physics
Letter
Broad Line Emission in Low-Metallicity Blue Compact Dwarf Galaxies: Evidence for Stellar Wind, Supernova and Possible AGN Activity
We present spectra of a large sample of low-metallicity blue compact dwarf
galaxies which exhibit broad components in their strong emission lines, mainly
in Hbeta, [O III]4959, 5007 and Halpha. Twenty-three spectra have been obtained
with the MMT, 14 of which show broad emission. The remaining 21 spectra with
broad emission have been selected from the Data Release 5 of the Sloan Digital
Sky Survey. The most plausible origin of broad line emission is the evolution
of massive stars and their interaction with the circumstellar and interstellar
medium. The broad emission with the lowest H luminosities (10^36 -
10^39 erg/s) is likely produced in circumstellar envelopes around hot Ofp/WN9
and/or LBV stars. The broad emission with the highest Halpha luminosities
(10^40 - 10^42 erg/s) probably arises from type IIp or type IIn supernovae
(SNe). It can also come from active galactic nuclei (AGN) containing
intermediate-mass black holes, although we find no strong evidence for hard
non-thermal radiation in our sample galaxies. The oxygen abundance in the host
galaxies with SN candidates is low and varies in the range 12 + log O/H = 7.36
- 8.31. However, type IIn SN / AGN candidates are found only in galaxies with
12 + log O/H < 7.99. Spectroscopic monitoring of these type IIn SN / AGN
candidates over a time scale of several years is necessary to distinguish
between the two possibilities.Comment: 50 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
Potential of a Neutrino Detector in the ANDES Underground Laboratory for Geophysics and Astrophysics of Neutrinos
The construction of the Agua Negra tunnels that will link Argentina and Chile
under the Andes, the world longest mountain range, opens the possibility to
build the first deep underground labo- ratory in the Southern Hemisphere. This
laboratory has the acronym ANDES (Agua Negra Deep Experiment Site) and its
overburden could be as large as \sim 1.7 km of rock, or 4500 mwe, providing an
excellent low background environment to study physics of rare events like the
ones induced by neutrinos and/or dark matter. In this paper we investigate the
physics potential of a few kiloton size liquid scintillator detector, which
could be constructed in the ANDES laboratory as one of its possible scientific
programs. In particular, we evaluate the impact of such a detector for the
studies of geoneutrinos and galactic supernova neutrinos assuming a fiducial
volume of 3 kilotons as a reference size. We emphasize the complementary roles
of such a detector to the ones in the Northern Hemisphere neutrino facilities
through some advantages due to its geographical location.Comment: 20 pages, 16 figures and 9 table
Nanotube Piezoelectricity
We combine ab initio, tight-binding methods and analytical theory to study
piezoelectric effect of boron nitride nanotubes. We find that piezoelectricity
of a heteropolar nanotube depends on its chirality and diameter and can be
understood starting from the piezoelectric response of an isolated planar
sheet, along with a structure specific mapping from the sheet onto the tube
surface. We demonstrate that coupling between the uniaxial and shear
deformation are only allowed in the nanotubes with lower chiral symmetry. Our
study shows that piezoelectricity of nanotubes is fundamentally different from
its counterpart in three dimensional (3D) bulk materials.Comment: 4 pages, with 3 postscript figures embedded. Uses REVTEX4 macros.
Also available at
http://www.physics.upenn.edu/~nsai/preprints/bn_piezo/index.htm
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