8,014 research outputs found
Saddle-point van Hove singularity and the phase diagram of high-Tc cuprates
We examine the generic phase behavior of high-Tc cuprate superconductors in
terms a universal van Hove singularity in the strongly overdoped region. Using
a rigid ARPES-derived dispersion we solve the BCS gap equation and show that
the pairing interaction or pairing energy cutoff must be a rapidly declining
function of doping. This result is prejudicial to a phonon-based pairing
interaction and more consistent with a magnetic or magnetically enhanced
interaction.Comment: 5 pages, 2 figures, submitted to Physical Review
Spectral variation in the X-ray pulsar GX 1+4 during a low-flux episode
The X-ray pulsar GX 1+4 was observed with the RXTE satellite for a total of
51ks between 1996 July 19 - 21. During this period the flux decreased smoothly
from an initial mean level of ~ 6 X 10^36 erg/s to a minimum of ~ 4 X 10^35
erg/s (2-60 keV, assuming a source distance of 10 kpc) before partially
recovering towards the initial level at the end of the observation.
BATSE pulse timing measurements indicate that a torque reversal took place
approximately 10 d after this observation. Both the mean pulse profile and the
photon spectrum varied significantly. The observed variation in the source may
provide important clues as to the mechanism of torque reversals.
The single best-fitting spectral model was based on a component originating
from thermal photons with kT ~ 1 keV Comptonised by a plasma of temperature kT
\~ 7 keV. Both the flux modulation with phase during the brightest interval and
the evolution of the mean spectra over the course of the observation are
consistent with variations in this model component; with, in addition, a
doubling of the column density nH contributing to the mean spectral change.
A strong flare of duration 50 s was observed during the interval of minimum
flux, with the peak flux ~ 20 times the mean level. Although beaming effects
are likely to mask the true variation in Mdot thought to give rise to the
flare, the timing of a modest increase in flux prior to the flare is consistent
with dual episodes of accretion resulting from successive orbits of a locally
dense patch of matter in the accretion disc.Comment: 8 pages, 3 figures, submitted to MNRA
Pulse Profiles, Accretion Column Dips and a Flare in GX 1+4 During a Faint State
The Rossi X-ray Timing Explorer (RXTE) spacecraft observed the X-ray pulsar
GX 1+4 for a period of 34 hours on July 19/20 1996. The source faded from an
intensity of ~20 mCrab to a minimum of <~0.7 mCrab and then partially recovered
towards the end of the observation. This extended minimum lasted ~40,000
seconds. Phase folded light curves at a barycentric rotation period of
124.36568 +/- 0.00020 seconds show that near the center of the extended minimum
the source stopped pulsing in the traditional sense but retained a weak dip
feature at the rotation period. Away from the extended minimum the dips are
progressively narrower at higher energies and may be interpreted as
obscurations or eclipses of the hot spot by the accretion column. The pulse
profile changed from leading-edge bright before the extended minimum to
trailing-edge bright after it. Data from the Burst and Transient Source
Experiment (BATSE) show that a torque reversal occurred <10 days after our
observation. Our data indicate that the observed rotation departs from a
constant period with a Pdot/P value of ~-1.5% per year at a 4.5 sigma
significance. We infer that we may have serendipitously obtained data, with
high sensitivity and temporal resolution about the time of an accretion disk
spin reversal. We also observed a rapid flare which had some precursor
activity, close to the center of the extended minimum.Comment: 19 pages, 6 figures, accepted for publication in Astrophysical
Journal (tentatively scheduled for vol. 529 #1, 20 Jan 2000
Thermodynamic properties of Bi2Sr2CaCu2O8 calculated from the electronic dispersion
The electronic dispersion for Bi2Sr2CaCu2O(8+d) has been determined from
angle-resolved photoelectron spectroscopy (ARPES). From this dispersion we
calculate the entropy and superfluid density. Even with no adjustable
parameters we obtain an exceptional match with experimental data across the
entire phase diagram, thus indirectly confirming both the ARPES and
thermodynamic data. The van Hove singularity is crossed in the overdoped region
giving a distinctive linear-in-T temperature dependence in the superfluid
density there.Comment: 5 pages, 4 figures, submitted to Physical Review Letter
The 21cm Signature of the First Stars
We predict the 21-cm signature of the first metal-free stars. The soft X-rays
emitted by these stars penetrate the atomic medium around their host halos,
generating Lyman alpha photons that couple the spin and kinetic temperatures.
These creates a region we call the Lyman alpha sphere, visible in 21-cm against
the CMB, which is much larger than the HII region produced by the same star.
The spin and kinetic temperatures are strongly coupled before the X-rays can
substantially heat the medium, implying that a strong 21-cm absorption signal
from the adiabatically cooled gas in Hubble expansion around the star is
expected when the medium has not been heated previously. A central region of
emission from the gas heated by the soft X-rays is also present although with a
weaker signal than the absorption. The Lyman alpha sphere is a universal
signature that should be observed around any first star illuminating its
vicinity for the first time. The 21-cm radial profile of the Lyman alpha sphere
can be calculated as a function of the luminosity, spectrum and age of the
star. For a star of a few hundred solar masses and zero metallicity (as
expected for the first stars), the physical radius of the Lyman alpha sphere
can reach tens of kiloparsecs. The first metal-free stars should be strongly
clustered because of high cosmic biasing; this implies that the regions
producing a 21-cm absorption signal may contain more than one star and will
generally be irregular and not spherical, because of the complex distribution
of the gas. We discuss the feasiblity of detecting these Lyman alpha spheres,
which would be present at redshifts in the Cold Dark Matter model.
Their observation would represent a direct proof of the detection of a first
star.Comment: replaced with ApJ accepted version. Many minor revisions and
additional references, major results unchange
Effluent sampling of Scout D and Delta launch vehicle exhausts
Characterization of engine-exhaust effluents (hydrogen chloride, aluminum oxide, carbon dioxide, and carbon monoxide) has been attempted by conducting field experiments monitoring the exhaust cloud from a Scout-Algol III vehicle launch and a Delta-Thor vehicle launch. The exhaust cloud particulate size number distribution (total number of particles as a function of particle diameter), mass loading, morphology, and elemental composition have been determined within limitations. The gaseous species in the exhaust cloud have been identified. In addition to the ground-based measurements, instrumented aircraft flights through the low-altitude, stabilized-exhaust cloud provided measurements which identified CO and HCI gases and Al2O3 particles. Measurements of the initial exhaust cloud during formation and downwind at several distances have established sampling techniques which will be used for experimental verification of model predictions of effluent dispersion and fallout from exhaust clouds
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Enterprise and entrepreneurship in the Caribbean region: introduction to the special issue
Entrepreneurship as a pseudo-discipline has matured to the point where it has begun to question the myths which have developed around it. As a panacea for the development ills of capitalism, studies have spanned various ideological and methodological viewpoints. Spatially, entrepreneurship studies have grown to include countries of the Global South and emerging economies, particularly those of Eastern Europe. This special issue extends this reach to the small developing states of the Caribbean and particularly those with a British colonial legacy rooted in the remnants of the plantation economy. The commencement of political independence in the 1960s has not resulted in any significant economic independence for the region as it remains dependent on foreign investment, whilst its key sectors remain subject to the volatility of the economies of the global north. The papers in this special issue identify domestic and enterprise level constraints to the development of entrepreneurship in the region. This Introduction places these, mostly micro-level studies, in a wider context, concluding that policy-makers need to better understand the concept of entrepreneurship and its role in achieving developmental goals. Our challenging recommendation is that those formulating and delivering these policies and practices should do so with an entrepreneurial mind-set
UNSWIRF: A Tunable Imaging Spectrometer for the Near-Infrared
We describe the specifications, characteristics, calibration, and analysis of
data from the University of New South Wales Infrared Fabry-Perot (UNSWIRF)
etalon. UNSWIRF is a near-infrared tunable imaging spectrometer, used primarily
in conjunction with IRIS on the AAT, but suitable for use as a visitor
instrument at other telescopes. The etalon delivers a resolving power in excess
of 4000 (corresponding to a velocity resolution ~75 km/s), and allows imaging
of fields up to 100" in diameter on the AAT at any wavelength between 1.5 and
2.4 microns for which suitable blocking filters are available.Comment: 16 pages, 10 figures, uses psfig.sty and html.sty (included). To
appear in Publications of the Astronomical Society of Australi
Isotope Effect in the Superfluid Density of HTS Cuprates: Stripes, Pseudogap and Impurities
Underdoped cuprates exhibit a normal-state pseudogap, and their spins and
doped carriers tend to spatially separate into 1- or 2-D stripes. Some view
these as central to superconductivity, others as peripheral and merely
competing. Using LaSrCuZnO we show that an oxygen
isotope effect in and in the superfluid density can be used to
distinguish between the roles of stripes and pseudogap and also to detect the
presence of impurity scattering. We conclude that stripes and pseudogap are
distinct, and both compete and coexist with superconductivity.Comment: Revised submission to PRL with added appendix on a possible isotope
effect in the effective mass, 4 pages, 3 figure
The effect of collisional enhancement of Balmer lines on the determination of the primordial helium abundance
This paper describes a new determination of the primordial helium abundance
(Y_P), based on the abundance analysis of five metal-poor extragalactic HII
regions. For three regions of the sample (SBS 0335-052, I Zw 18, and H29) we
present tailored photoionization models based on improved calculations with
respect to previous models. In particular, we use the photoionization models to
study quantitatively the effect of collisional excitation of Balmer lines on
the determination of the helium abundance (Y) in the individual regions. This
effect is twofold: first, the intensities of the Balmer lines are enhanced with
respect to the pure recombination value, mimicking a higher hydrogen abundance;
second, the observed reddening is larger than the true extinction, due to the
differential effect of collisions on different Balmer lines. In addition to
these effects, our analysis takes into account the following features of HII
regions: (i) the temperature structure, (ii) the density structure, (iii) the
presence of neutral helium, (iv) the collisional excitation of the HeI lines,
(v) the underlying absorption of the HeI lines, and (vi) the optical thickness
of the HeI lines. The object that shows the highest increase in Y after the
inclusion of collisional effects in the analysis is SBS 0335-052, whose helium
abundance has been revised by Delta Y = +0.0107. The revised Y values for the
five objects in our sample yield an increase of +0.0035 in Y_P, giving Y_P =
0.2391 +/- 0.0020.Comment: 59 pages, 8 figures. AAS Latex. Accepted for publication in the
Astrophysical Journa
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