2,879 research outputs found
Novel Properties of Frustrated Low Dimensional Magnets with Pentagonal Symmetry
In the context of magnetism, frustration arises when a group of spins cannot
find a configuration that minimizes all of their pairwise interactions
simultaneously. We consider the effects of the geometric frustration that
arises in a structure having pentagonal loops. Such five-fold loops can be
expected to occur naturally in quasicrystals, as seen for example in a number
of experimental studies of surfaces of icosahedral alloys. Our model considers
classical vector spins placed on vertices of a subtiling of the two dimensional
Penrose tiling, and interacting with nearest neighbors via antiferromagnetic
bonds. We give a set of recursion relations for this system, which consists of
an infinite set of embedded clusters with sizes that increase as a power of the
golden mean. The magnetic ground states of this fractal system are studied
analytically, and by Monte Carlo simulation.Comment: 7 pages, 7 figures, contribution to ICQ11 (Sapporo, Japan 2010)
conference proceeding
Contribution of Unresolved Point Sources to the Diffuse X-ray Background below 1 keV
We present here the analysis of X-rays point sources detected in several
observations available in the XMM-Newton public archive. We focused, in
particular, on energies below 1 keV, which are of particular relevance to the
understanding of the Diffuse X-ray Background. The average field of all the
exposures is 0.09 deg^-2. We reached an average flux sensitivity of 5.8x10^-16
erg s^-1 cm^-2 in the soft band (0.5-2.0 keV) and 2.5x10^-16 erg s^-1 cm^-2 in
the very soft band (0.4-0.6 keV). In this paper we discuss the logN-logS
results, the contribution to the integrated X-ray sky flux, and the properties
of the cumulative spectrum from all sources. In particular, we found an excess
flux at around 0.5 keV in the composite spectrum of faint sources. The excess
seems to be a general property of all the fields observed suggesting an
additional class of weak sources is contributing to the X-ray emission at these
energies. Combining our results with previous investigations we have also
quantified the contribution of the individual components of the diffuse X-ray
Background in the 3/4 keV band.Comment: Accepted for publication in ApJ; 27 pages, 8 figure
Penrose Matching Rules from Realistic Potentials in a Model System
We exhibit a toy model of a binary decagonal Al-Co quasicrystal -- closely
related to actual structures -- in which realistic pair potentials yield a
ground state which appears to perfectly implement Penrose's matching rules, for
Hexagon-Boat-Star (HBS) tiles of edge 2.45 A. The second minimum of the
potentials is crucial for this result.Comment: 7 pp, 2 figures; proc. "Quasicrystals: Silver Jubilee" (Tel Aviv,
2007), Phil. Mag. in pres
Method of fabricating an object with a thin wall having a precisely shaped slit
A method is described for making a structure with a cavity and a thin wall with a precisely shaped slit. An object with a cavity having two openings, one of which is to be closed by a thin wall with a slit, is placed on the surface of a fixture. The fixture surface has a slot conforming to the size and shape of the slit to be formed in the thin wall
Spin of ground state baryons
We calculate the quark spin contribution to the total angular momentum of
flavor octet and flavor decuplet ground state baryons using a spin-flavor
symmetry based parametrization method of quantum chromodynamics. We find that
third order SU(6) symmetry breaking three-quark operators are necessary to
explain the experimental result Sigma_1=0.32(10). For spin 3/2 decuplet baryons
we predict that the quark spin contribution is Sigma_3=3.93(22), i.e.
considerably larger than their total angular momentum.Comment: 8 page
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