147,421 research outputs found
Production of Millisecond Dips in Sco X-1 Count Rates by Dead Time Effects
Chang et al. (2006) reported millisecond duration dips in the X-ray intensity
of Sco X-1 and attributed them to occultations of the source by small
trans-Neptunian objects (TNOs). We have found multiple lines of evidence that
these dips are not astronomical in origin, but rather the result of high-energy
charged particle events in the RXTE PCA detectors. Our analysis of the RXTE
data indicates that at most 10% of the observed dips in Sco X-1 could be due to
occultations by TNOs, and, furthermore, we find no positive or supporting
evidence for any of them being due to TNOs. We therefore believe that it is a
mistake to conclude that any TNOs have been detected via occultation of Sco
X-1.Comment: Submitted to ApJ; uses emulateapj.cls, 8 pages with 8 figure
Nonlinear Sigma Model Analysis of the AFM Phase Transition of the Kondo Lattice
We have studied the antiferromagnetic quantum phase transition of a 2D
Kondo-Heisenberg square lattice using the non-linear sigma model. A
renormalization group analysis of the competing Kondo -- RKKY interaction was
carried out to 1-loop order in the expansion, and a new quantum
critical point is found, dominated by Kondo fluctuations. In addition, the
spin-wave velocity scales logarithmically near the new QCP, i.e breakdown of
hydrodynamic behavior. The results allow us to propose a new phase diagram near
the AFM fixed point of this 2D Kondo lattice model.Comment: 4 pages, 4 figure
Mode switching in the nearby Mira-like variable R Doradus
We discuss visual observations spanning nearly 70 years of the nearby
semiregular variable R Doradus. Using wavelet analysis, we show that the star
switches back and forth between two pulsation modes having periods of 332 days
and about 175 days, the latter with much smaller amplitude. Comparison with
model calculations suggests that the two modes are the first and third radial
overtone, with the physical diameter of the star making fundamental mode
pulsation unlikely. The mode changes occur on a timescale of about 1000 d,
which is too rapid be related to a change in the overall thermal structure of
the star and may instead be related to weak chaos.
The Hipparcos distance to R Dor is 62.4 +/- 2.8 pc which, taken with its
dominant 332-day period, places it exactly on the period-luminosity relation of
Miras in the Large Magellanic Cloud. Our results imply first overtone pulsation
for all Miras which fall on the P-L relation. We argue that semiregular
variables with long periods may largely be a subset of Miras and should be
included in studies of Mira behaviour. The semiregulars may contain the
immediate evolutionary Mira progenitors, or stars may alternate between periods
of semiregular and Mira behaviour.Comment: 12 pages, latex with figures, accepted by MNRA
Photodetection in silicon beyond the band edge with surface states
Silicon is an extremely attractive material platform for integrated optics at
telecommunications wavelengths, particularly for integration with CMOS
circuits. Developing detectors and electrically pumped lasers at telecom
wavelengths are the two main technological hurdles before silicon can become a
comprehensive platform for integrated optics. We report on the generation of
free carriers in unimplanted SOI ridge waveguides, which we attribute to
surface state absorption. By electrically contacting the waveguides, a
photodetector with a responsivity of 36 mA/W and quantum efficiency of 2.8% is
demonstrated. The photoconductive effect is shown to have minimal falloff at
speeds of up to 60 Mhz
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High Percentages of Reclaimed Asphalt Affect the Performance of Asphalt Binder
More than 90 percent of the road and highway network in the United States is paved with asphalt concrete. Maintenance and periodic rehabilitation require a continuous supply of aggregates and asphalt binder, both of which are becoming increasingly scarce and expensive. Recycling and reusing these resources can reduce costs and improve sustainability. The most common recyclable material used in road construction is reclaimed asphalt pavement (RAP), which is milled asphalt surface layers that have been removed from existing pavements before new asphalt overlay is placed. Reclaimed asphalt roofing shingles (RAS) are another potential source of asphalt binder.There is growing interest in allowing significantly higher percentages of RAP and RAS in asphalt mixes used on state and local roadways. However, making this change has raised concerns regarding how these composite binders may influence the performance and durability of asphalt mixes, depending on the blends of different virgin and reused binders. Researchers at the UC Pavement Research Center investigated the use of higher percentages of RAP and RAS as a partial replacement for the virgin binder in new asphalt mixes and their effect on pavement performance in California. This research brief summarizes findings from that study.View the NCST Project Webpag
Photon Frequency Mode Matching using Acousto-Optic Frequency Beam Splitters
It is a difficult engineering task to create distinct solid state single
photon sources which nonetheless emit photons at the same frequency. It is also
hard to create entangled photon pairs from quantum dots. In the spirit of
quantum engineering we propose a simple optical circuit which can, in the right
circumstances, make frequency distinguishable photons frequency
indistinguishable. Our circuit can supply a downstream solution to both
problems, opening up a large window of allowed frequency mismatches between
physical mechanisms. The only components used are spectrum analysers/prisms and
an Acousto-Optic Modulator. We also note that an Acousto-Optic Modulator can be
used to obtain Hong-Ou-Mandel two photon interference effects from the
frequency distinguishable photons generated by distinct sources.Comment: 4 pages, 4 figure
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