807 research outputs found
Capabilities of a lobster eye telescope in the outer solar system
Current lobster eye telescopes demonstrate that it’s possible to produce light-weight, large field of view instruments for observing X-rays for both planetary science and astronomy. Jupiter is the most powerful particle accelerator in the solar system and the other outer planets have intricate and complicated magnetospheres which their moons often orbit within. Particle bombardment of the surfaces of their moons induces the emission of characteristic X-rays which enables their composition to be studied. An orbiting X-ray instrument would transform our understanding of the moons’ composition, as well as the aurorae, atmosphere, plasma tori of outer planet systems and could also enable direct imaging of the entire radiation belt. Lobster eye telescopes are perfect for this application due to their light weight and wide field of view. This paper begins to identify a lobster eye telescope design to fulfil these science goals
A-STAR: The All-Sky Transient Astrophysics Reporter
The small mission A-STAR (All-Sky Transient Astrophysics Reporter) aims to
locate the X-ray counterparts to ALIGO and other gravitational wave detector
sources, to study the poorly-understood low luminosity gamma-ray bursts, and to
find a wide variety of transient high-energy source types, A-STAR will survey
the entire available sky twice per 24 hours. The payload consists of a coded
mask instrument, Owl, operating in the novel low energy band 4-150 keV, and a
sensitive wide-field focussing soft X-ray instrument, Lobster, working over
0.15-5 keV. A-STAR will trigger on ~100 GRBs/yr, rapidly distributing their
locations.Comment: Accepted for the European Astronomical Society Publications Series:
Proceedings of the Fall 2012 Gamma-Ray Burst Symposium held in Marbella,
Spain, 8-12 Oct 201
Magnetic field independence of the spin gap in YBa_2Cu_3O_{7-delta}
We report, for magnetic fields of 0, 8.8, and 14.8 Tesla, measurements of the
temperature dependent ^{63}Cu NMR spin lattice relaxation rate for near
optimally doped YBa_2Cu_3O_{7-delta}, near and above T_c. In sharp contrast
with previous work we find no magnetic field dependence. We discuss
experimental issues arising in measurements of this required precision, and
implications of the experiment regarding issues including the spin or pseudo
gap.Comment: 4 pages, 3 figures, as accepted for publication in Physical Review
Letter
Incommensurate Charge and Spin Fluctuations in d-wave Superconductors
We show analytic results for the irreducible charge and spin
susceptibilities, , where is the momentum
transfer between the nodes in d-wave superconductors. Using the BCS theory and
a circular Fermi surface, we find that the singular behavior of the irreducible
charge susceptibility leads to the dynamic incommensurate charge collective
modes. The peaks in the charge structure factor occur at a set of wave vectors
which form an ellipse around and in
momentum space with momentum dependent spectral weight. It is also found that,
due to the non-singular irreducible spin susceptibility, an extremely strong
interaction via random phase approximation is required to support the magnetic
peaks near . Under certain conditions, the peaks in the magnetic
structure factor occur near and .Comment: 5 pages, 3 figure
The Superconducting Condensation Energy and an Antiferromagnetic Exchange Based Pairing Mechanism
For the traditional low T_c superconductors, the superconducting condensation
energy is proportional to the change in energy of the ionic lattice between the
normal and superconducting state, providing a clear link between pairing and
the electron-ion interaction. Here, for the t-J model, we discuss an analogous
relationship between the superconducting condensation energy and the change in
the exchange energy between the normal and superconducting states. We point out
the possibility of measuring this using neutron scattering and note that such a
measurement, while certainly difficult, could provide important evidence for an
exchange interaction-based pairing mechanism.Comment: Replaced with revised versio
Nonlinear Magneto-Optical Response of - and -Wave Superconductors
The nonlinear magneto-optical response of - and -wave superconductors
is discussed. We carry out the symmetry analysis of the nonlinear
magneto-optical susceptibility in the superconducting state. Due to the surface
sensitivity of the nonlinear optical response for systems with bulk inversion
symmetry, we perform a group theoretical classification of the superconducting
order parameter close to a surface. For the first time, the mixing of singlet
and triplet pairing states induced by spin-orbit coupling is systematically
taken into account. We show that the interference of singlet and triplet
pairing states leads to an observable contribution of the nonlinear
magneto-optical Kerr effect. This effect is not only sensitive to the
anisotropy of the gap function but also to the symmetry itself. In view of the
current discussion of the order parameter symmetry of High-T
superconductors, results for a tetragonal system with bulk singlet pairing for
various pairing symmetries are discussed.Comment: 21 pages (REVTeX) with 8 figures (Postscript
Critical Josephson Current in a Model Pb/YBa_2Cu_3O_7 Junction
In this article we consider a simple model for a c--axis
Pb/YBa_2Cu_3O_{7-\delta} Josephson junction. The observation of a nonzero
current in such a junction by Sun et al. [A. G. Sun, D. A. Gajewski, M. B.
Maple, R. C. Dynes, Phys. Rev. Lett. 72, 2267 (1994)] has been taken as
evidence against d--wave superconductivity in YBa_2Cu_3O_{7-\delta}. We
suggest, however, that the pairing interaction in the CuO_2 planes may well be
d--wave but that the CuO chains destroy the tetragonal symmetry of the system.
We examine two ways in which this happens. In a simple model of an incoherent
junction, the chains distort the superconducting condensate away from
d_{x^2-y^2} symmetry. In a specular junction the chains destroy the tetragonal
symmetry of the tunneling matrix element. In either case, the loss of
tetragonal symmetry results in a finite Josephson current. Our calculated
values of the critical current for specular junctions are in good agreement
with the results of Sun and co-workers.Comment: Latex File, 21 pages, 6 figures in uuencoded postscript, In Press
(Phys. Rev. B
The Superconducting Instabilities of the non half-filled Hubbard Model in Two Dimensions
The problem of weakly correlated electrons on a square lattice is formulated
in terms of one-loop renormalization group. Starting from the action for the
entire Brillouin zone (and not with a low-energy effective action) we reduce
successively the cutoff about the Fermi surface and follow the
renormalization of the coupling as a function of three energy-momenta. We
calculate the intrinsic scale where the renormalization group flow
crosses over from the regime () where the electron-electron
(e-e) and electron-hole (e-h) terms are equally important to the regime
() where only the e-e term plays a role. In the low energy
regime only the pairing interaction is marginally relevant, containing
contributions from all renormalization group steps of the regime . After diagonalization of , we identify its most
attractive eigenvalue . At low filling,
corresponds to the representation ( symmetry), while near half
filling the strongest attraction occurs in the representation
( symmetry). In the direction of the van Hove singularities, the
order parameter shows peaks with increasing strength as one approaches half
filling. Using the form of pairing and the structure of the renormalization
group equations in the low energy regime, we give our interpretation of ARPES
experiments trying to determine the symmetry of the order parameter in the
Bi2212 high- compound.Comment: 24 pages (RevTeX) + 11 figures (the tex file appeared incomplete
Polarization of X-ray lines from galaxy clusters and elliptical galaxies - a way to measure tangential component of gas velocity
We study the impact of gas motions on the polarization of bright X-ray
emission lines from the hot intercluster medium (ICM). The polarization
naturally arises from resonant scattering of emission lines owing to a
quadrupole component in the radiation field produced by a centrally peaked gas
density distribution. If differential gas motions are present then a photon
emitted in one region of the cluster will be scattered in another region only
if their relative velocities are small enough and the Doppler shift of the
photon energy does not exceed the line width. This affects both the degree and
the direction of polarization. The changes in the polarization signal are in
particular sensitive to the gas motions perpendicular to the line of sight. We
calculate the expected degree of polarization for several patterns of gas
motions, including a slow inflow expected in a simple cooling flow model and a
fast outflow in an expanding spherical shock wave. In both cases, the effect of
non-zero gas velocities is found to be minor. We also calculate the
polarization signal for a set of clusters, taken from large-scale structure
simulations and evaluate the impact of the gas bulk motions on the polarization
signal. We argue that the expected degree of polarization is within reach of
the next generation of space X-ray polarimeters.Comment: 25 pages, 18 figures, accepted to MNRA
Integration of professional judgement and decision-making in high-level adventure sports coaching practice
This study examined the integration of professional judgement and decision-making processes in adventure sports coaching. The study utilised a thematic analysis approach to investigate the decision-making practices of a sample of high-level adventure sports coaches over a series of sessions. Results revealed that, in order to make judgements and decisions in practice, expert coaches employ a range of practical and pedagogic management strategies to create and opportunistically use time for decision-making. These approaches include span of control and time management strategies to facilitate the decision-making process regarding risk management, venue selection, aims, objectives, session content, and differentiation of the coaching process. The implication for coaches, coach education, and accreditation is the recognition and training of the approaches that“create time” for the judgements in practice, namely“creating space to think”. The paper concludes by offering a template for a more expertise-focused progression in adventure sports coachin
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