1,742 research outputs found
Unconventional density wave in CeCoIn_5?
Very recently large Nernst effect and Seebeck effect were observed above the
superconducting transition temperature 2.3K in a heavy fermion superconductor
CeCoIn_5. We shall interpret this large Nernst effect in terms of
unconventional density wave (UDW), which appears around T=18K. Also the
temperature dependence of the Seebeck coefficient below T=18K is described in
terms of UDW. Another hallmark for UDW is the angular dependent
magnetoresistance, which should be readily accessible experimentally.Comment: 4 pages, 7 figure
Measurements with the Chandra X-Ray Observatory's flight contamination monitor
NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor
(FCM), a system of 16 radioactive calibration sources mounted to the inside of
the Observatory's forward contamination cover. The purpose of the FCM is to
verify the ground-to-orbit transfer of the Chandra flux scale, through
comparison of data acquired during the ground calibration with those obtained
in orbit, immediately prior to opening the Observatory's sun-shade door. Here
we report results of these measurements, which place limits on the change in
mirror--detector system response and, hence, on any accumulation of molecular
contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7
The interplay of millets and rice in Neolithic central China: Integrating phytoliths into the archaeobotany of Baligang
Baligang is a Neolithic site with a long occupation, from before 6300. BC up to the first millennium BC, although the bulk of excavated finds and archaeobotanical evidence from the site comes from the Yangshao, Qujialing, Shijiahe and Longshan (4300-1800. BC). The cultural group affiliation of the site varies between northern (Yangshao and Longshan) and southern (Qujialing and Shijiahe) cultural connections. The earliest occupation of the site represents a pre-Yangshao society with early cultivation of rice (Oryza). In later periods Baligang has evidence for mixed farming of both rice and millets (Setaria italica and Panicum miliaceum), although rice is the most prominent crop in the phytolith record throughout the occupation. Wetland rice cultivation is indicated throughout the Yangshao, Qujialing, Shijiahe and Late Longshan periods. However, there is a shift towards better watered rice in the Qujialing and Shijiahe phytolith assemblages, indicated by a decline in sedges (Cyperaceae) alongside occurrence of sponge spicules and diatoms. These data suggest deeper flooding of rice fields in order to suppress weeds and increase productivity, indicating that the ecology of rice cultivation changed over time. In the Late Longshan period, when millet became more prominent and the cultural influence shifted northwards, it appears that more sedge-infested and weedy rice fields became the norm, suggesting a decline in rice cultivation intensity, perhaps connected to influences of cultivation practices from the north. In addition, we can infer aspects of the organisation of crop-processing from the phytolith evidence. In the Yangshao period the remains consist of mostly dehusking waste from the final processing, suggesting storage of a more processed crop and therefore larger scale, more communal post-harvest processing. By contrast this declined in the subsequent period with more evidence for primary winnowing waste indicating a shift towards smaller social scales of harvesting and processing, such as smaller household groups replacing a more communal approach. The household-level of processing is most evident in the Late Longshan period
The Development and Scientific Impact of the Chandra X-Ray Observatory
I review the operational capabilities of the Chandra X-ray Observatory,
including some of the spectacular results obtained by the general observer
community. A natural theme of this talk is that Chandra is revealing outflows
of great quantities of energy that were not previously observable. I highlight
the Chandra studies of powerful X-ray jets. This subject is only possible due
to the sub-arcsecond resolution of the X-ray telescope.Comment: 8 pages, 4 figures. Talk given at the International Workshop on
Astronomy and Relativistic Astrophysics (IWARA2003), Olinda, Brasil, October
2003. To appear in the International Journal of Modern Physics
Temporal and Spectral Correlations of Cyg X-1
Temporal and spectral properties of X-ray rapid variability of Cyg X-1 are
studied by an approach of correlation analysis in the time domain on different
time scales. The correlation coefficients between the total intensity in 2-60
keV and the hardness ratio of 13-60 keV to 2-6 keV band on the time scale of
about 1 ms are always negative in all states. For soft states, the correlation
coefficients are positive on all the time scales from about 0.01 s to 100 s,
which is significantly different with that for transition and low states.
Temporal structures in high energy band are narrower than that in low energy
band in quite a few cases. The delay of high energy photons relative to low
energy ones in the X-ray variations has also been revealed by the correlation
analysis. The implication of observed temporal and spectral characteristics to
the production region and mechanism of Cyg X-1 X-ray variations is discussed.Comment: 17 pages, 6 figures included, to appear in Ap
Low energy polarization sensitivity of the Gas Pixel Detector
An X-ray photoelectric polarimeter based on the Gas Pixel Detector has been
proposed to be included in many upcoming space missions to fill the gap of
about 30 years from the first (and to date only) positive measurement of
polarized X-ray emission from an astrophysical source. The estimated
sensitivity of the current prototype peaks at an energy of about 3 keV, but the
lack of readily available polarized sources in this energy range has prevented
the measurement of detector polarimetric performances.
In this paper we present the measurement of the Gas Pixel Detector
polarimetric sensitivity at energies of a few keV and the new, light, compact
and transportable polarized source that was devised and built to this aim.
Polarized photons are produced, from unpolarized radiation generated with an
X-ray tube, by means of Bragg diffraction at nearly 45 degrees.
The employment of mosaic graphite and flat aluminum crystals allow the
production of nearly completely polarized photons at 2.6, 3.7 and 5.2 keV from
the diffraction of unpolarized continuum or line emission. The measured
modulation factor of the Gas Pixel Detector at these energies is in good
agreement with the estimates derived from a Monte Carlo software, which was up
to now employed for driving the development of the instrument and for
estimating its low energy sensitivity. In this paper we present the excellent
polarimetric performance of the Gas Pixel Detector at energies where the peak
sensitivity is expected. These measurements not only support our previous
claims of high sensitivity but confirm the feasibility of astrophysical X-ray
photoelectric polarimetry.Comment: 15 pages, 12 figures. Accepted for publication in NIM
The AXAF technology program: The optical flats tests
The results of a technology program aimed at determining the limits of surface polishing for reflecting X-ray telescopes is presented. This program is part of the major task of developing the Advanced X-ray Astrophysical Facility (AXAF). By studying the optical properties of state-of-the-art polished flat surfaces, conclusions were drawn as to the potential capability of AXAF. Surface microtopography of the flats as well as their figure are studied by X-ray, visual, and mechanical techniques. These techniques and their results are described. The employed polishing techniques are more than adequate for the specifications of the AXAF mirrors
QED vacuum fluctuations and induced electric dipole moment of the neutron
Quantum fluctuations in the QED vacuum generate non-linear effects, such as
peculiar induced electromagnetic fields. In particular, we show here that an
electrically neutral particle, possessing a magnetic dipole moment, develops an
induced electric dipole-type moment with unusual angular dependence, when
immersed in a quasistatic, constant external electric field. The calculation of
this effect is done in the framework of the Euler-Heisenberg effective QED
Lagrangian, corresponding to the weak field asymptotic expansion of the
effective action to one-loop order. It is argued that the neutron might be a
good candidate to probe this signal of non-linearity in QED.Comment: A misprint has been corrected, and three new references have been
adde
An open quantum system approach to EPR correlations in K0-K0 system
We find the time evolution of the system of two non-interacting unstable
particles, distinguishable as well as identical ones, in arbitrary reference
frame having only the Kraus operators governing the evolution of its components
in the rest frame. We than calculate in the rigorous way
Einstein-Podolsky-Rosen quantum correlation functions for K0-K0 system in the
singlet state taking into account CP-violation and decoherence and show that
the results are exactly the same despite the fact we treat kaons as
distinguishable or identical particles which means that the statistics of the
particles plays no role, at least in considered cases.Comment: 14 pp. no fig
Geometry of spin-field coupling on the worldline
We derive a geometric representation of couplings between spin degrees of
freedom and gauge fields within the worldline approach to quantum field theory.
We combine the string-inspired methods of the worldline formalism with elements
of the loop-space approach to gauge theory. In particular, we employ the loop
(or area) derivative operator on the space of all holonomies which can
immediately be applied to the worldline representation of the effective action.
This results in a spin factor that associates the information about spin with
"zigzag" motion of the fluctuating field. Concentrating on the case of quantum
electrodynamics in external fields, we obtain a purely geometric representation
of the Pauli term. To one-loop order, we confirm our formalism by rederiving
the Heisenberg-Euler effective action. Furthermore, we give closed-form
worldline representations for the all-loop order effective action to lowest
nontrivial order in a small-N_f expansion.Comment: 18 pages, v2: references added, minor changes, matches PRD versio
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