928 research outputs found
Impurity Effects in Highly Frustrated Diamond Lattice Antiferromagnets
We consider the effects of local impurities in highly frustrated diamond
lattice antiferromagnets, which exhibit large but non-extensive ground state
degeneracies. Such models are appropriate to many A-site magnetic spinels. We
argue very generally that sufficiently dilute impurities induce an ordered
magnetic ground state, and provide a mechanism of degeneracy breaking. The
states which are selected can be determined by a "swiss cheese model" analysis,
which we demonstrate numerically for a particular impurity model in this case.
Moreover, we present criteria for estimating the stability of the resulting
ordered phase to a competing frozen (spin glass) one. The results may explain
the contrasting finding of frozen and ordered ground states in CoAl2O4 and
MnSc2S4, respectively.Comment: 13 pages, 7 figure
A classification of primitive permutation groups with finite stabilizers
We classify all infinite primitive permutation groups possessing a finite
point stabilizer, thus extending the seminal Aschbacher-O'Nan-Scott Theorem to
all primitive permutation groups with finite point stabilizers.Comment: Accepted in J. Algebra. Various changes, some due to the author, some
due to suggestions from readers and others due to the comments of anonymous
referee
CAP Bench: a benchmark suite for performance and energy evaluation of low-power many-core processors
International audienceSUMMARY The constant need for faster and more energy-efficient processors has been stimulating the development of new architectures, such as low-power many-core architectures. Researchers aiming to study these architectures are challenged by peculiar characteristics of some components such as Networks-on-Chip and lack of specific tools to evaluate their performance. In this context, the goal of this paper is to present a benchmark suite to evaluate state-of-the-art low-power many-core architectures such as the Kalray MPPA-256 low-power processor, which features 256 compute cores in a single chip. The benchmark was designed and used to highlight important aspects and details that need to be considered when developing parallel applications for emerging low-power many-core architectures. As a result, this paper demonstrates that the benchmark offers a diverse suite of programs with regard to parallel patterns, job types, communication intensity and task load strategies, suitable for a broad understanding of performance and energy consumption of MPPA-256 and upcoming many-core architectures
Fluctuations of a holographic quantum Hall fluid
We analyze the neutral spectrum of the holographic quantum Hall fluid
described by the D2-D8' model. As expected for a quantum Hall state, we find
the system to be stable and gapped and that, at least over much of the
parameter space, the lowest excitation mode is a magneto-roton. In addition, we
find magneto-rotons in higher modes as well. We show that these magneto-rotons
are direct consequences of level crossings between vector and scalar modes.Comment: 20 pages, 8 figures; v.2 figures improved, 2 figures added, and text
clarified particularly in Sec. 5, to appear in JHE
Influence of non-local damping on magnon properties of ferromagnets
We study the influence of non-local damping on magnon properties of Fe, Co,
Ni and FeCo () alloys. The Gilbert damping parameter
is typically considered as a local scalar both in experiment and in theoretical
modelling. However, recent works have revealed that Gilbert damping is a
non-local quantity that allows for energy dissipation between atomic sites.
With the Gilbert damping parameters calculated from a state-of-the-art
real-space electronic structure method, magnon lifetimes are evaluated from
spin dynamics and linear response, where a good agreement is found between
these two methods. It is found that non-local damping affects the magnon
lifetimes in different ways depending on the system. Specifically, we find that
in Fe, Co, and Ni the non-local damping decreases the magnon lifetimes, while
in and FeCo an opposite, non-local damping
effect is observed, and our data show that it is much stronger in the former
Tunable and robust room-temperature magnon-magnon entanglement
Although challenging, realizing controllable high-temperature entanglement is
of immense importance for practical applications as well as for fundamental
research in quantum technologies. Here, we report the existence of entangled
steady states in bipartite quantum magnonic systems at high temperatures. We
consider dissipative dynamics of two magnons in a bipartite antiferromagnet or
ferrimagnet subjected to a vibrational phonon mode and an external rotating
magnetic field. To quantify the bipartite magnon-magnon entanglement, we use
the entanglement negativity and compute its dependence on the temperature and
magnetic field. We show that, for any given phonon frequency and magnon-phonon
coupling rates, there are always ranges of the magnetic field amplitudes and
frequencies, for which bipartite magnon-magnon entanglement persists up to and
above the room temperature. The generality of the result allows for
experimental observation in a variety of crystals and synthetic bipartite
antiferromagnetic and ferrimagnetic materials.Comment: 6 pages, 5 figure
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Preliminary field evaluation of high efficiency steel filters
The authors have conducted an evaluation of two high efficiency steel filters in the exhaust of an uranium oxide grit blaster at the Y-12 Plant in Oak Ridge Tennessee. The filters were installed in a specially designed filter housing with a reverse air-pulse cleaning system for automatically cleaning the filters in-place. Previous tests conducted on the same filters and housing at LLNL under controlled conditions using Arizona road dust showed good cleanability with reverse air pulses. Two high efficiency steel filters, containing 64 pleated cartridge elements housed in the standard 2{prime} x 2{prime} {times} l{prime} HEPA frame, were evaluated in the filter test housing using a 1,000 cfm slip stream containing a high concentration of depleted uranium oxide dust. One filter had the pleated cartridges manufactured to the authors specifications by the Pall Corporation and the other by Memtec Corporation. Test results showed both filters had a rapid increase in pressure drop with time, and reverse air pulses could not decrease the pressure drop. The authors suspected moisture accumulation in the filters was the problem since there were heavy rains during the evaluations, and the pressure drop of the Memtec filter decreased dramatically after passing clean, dry air through the filter and after the filter sat idle for one week. Subsequent laboratory tests on a single filter cartridge confirmed that water accumulation in the filter was responsible for the increase in filter pressure drop and the inability to lower the pressure drop by reverse air pulses. No effort was made to identify the source of the water accumulation and correct the problem because the available funds were exhausted
A pulsed source of continuous variable polarization entanglement
We have experimentally demonstrated polarization entanglement using
continuous variables in an ultra-short pulsed laser system at telecommunication
wavelengths. Exploiting the Kerr-nonlinearity of a glass fibre we generated a
polarization squeezed pulse with S2 the only non-zero Stokes parameter thus S1
and S3 being the conjugate pair. Polarization entanglement was generated by
interference of the polarization squeezed field with a vacuum on a 50:50 beam
splitter. The two resultant beams exhibit strong quantum noise correlations in
S1 and S3. The sum noise signal of S3 was at the respective shot noise level
and the difference noise signal of S1 fell 2.9dB below this value
Twisted Backgrounds, PP-Waves and Nonlocal Field Theories
We study partially supersymmetric plane-wave like deformations of string
theories and M-theory on brane backgrounds. These deformations are dual to
nonlocal field theories. We calculate various expectation values of
configurations of closed as well as open Wilson loops and Wilson surfaces in
those theories. We also discuss the manifestation of the nonlocality structure
in the supergravity backgrounds. A plane-wave like deformation of little string
theory has also been studied.Comment: 46 pages, changed to JHEP forma
Two Mode Quantum Systems: Invariant Classification of Squeezing Transformations and Squeezed States
A general analysis of squeezing transformations for two mode systems is given
based on the four dimensional real symplectic group Sp(4,\Re)\/. Within the
framework of the unitary metaplectic representation of this group, a
distinction between compact photon number conserving and noncompact photon
number nonconserving squeezing transformations is made. We exploit the
Sp(4,\Re)-SO(3,2)\/ local isomorphism and the U(2)\/ invariant squeezing
criterion to divide the set of all squeezing transformations into a two
parameter family of distinct equivalence classes with representative elements
chosen for each class. Familiar two mode squeezing transformations in the
literature are recognized in our framework and seen to form a set of measure
zero. Examples of squeezed coherent and thermal states are worked out. The need
to extend the heterodyne detection scheme to encompass all of U(2)\/ is
emphasized, and known experimental situations where all U(2)\/ elements can
be reproduced are briefly described.Comment: Revtex 37 pages, Latex figures include
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