19,724 research outputs found
The role of singletons in compactifications
We derive the isometry irrep content of squashed seven-sphere
compactifications of eleven-dimensional supergravity, i.e., the left-squashed
() with and right-squashed () with supersymmetry, in a manner completely independent of the round sphere.
Then we compare this result with the spectrum obtained by Higgsing the round
sphere spectrum. This way we discover features of the spectra which makes it
possible to argue that the only way the round spectrum can be related by a
Higgs mechanism to the one of is if the singletons are included in the
round sphere spectrum. For this to work also in the case it seems that
the gravitino of the spectrum must be replaced by a fermionic singleton
present in the spectrum.Comment: 24 pages including appendix with 12 figure, v2 minor typos correcte
X-ray fluorescence spectra of metals excited below threshold
X-ray scattering spectra of Cu and Ni metals have been measured using
monochromatic synchrotron radiation tuned from far above to more than 10 eV
below threshold. Energy conservation in the scattering process is found to be
sufficient to explain the modulation of the spectral shape, neglecting momentum
conservation and channel interference. At excitation energies close to and
above threshold, the emission spectra map the occupied local partial density of
states. For the sub-threshold excitations, the high-energy flank of the
inelastic scattering exhibits a Raman-type linear dispersion, and an asymmetric
low energy tail develops. For excitation far below threshold the emission
spectra are proportional to a convolution of the occupied and unoccuppied local
partial densities of states.Comment: 10 pages, 3 figures,
http://link.aps.org/doi/10.1103/PhysRevB.68.04511
Initial experiments concerning quantum information processing in rare-earth-ion doped crystals
In this paper initial experiments towards constructing simple quantum gates
in a solid state material are presented. Instead of using specially tailored
materials, the aim is to select a subset of randomly distributed ions in the
material, which have the interaction necessary to control each other and
therefore can be used to do quantum logic operations. The experimental results
demonstrate that part of an inhomogeneously broadened absorption line can be
selected as a qubit and that a subset of ions in the material can control the
resonance frequency of other ions. This opens the way for the construction of
quantum gates in rare-earth-ion doped crystals.Comment: 24 pages, including 12 figure
Fast all-optical nuclear spin echo technique based on EIT
We demonstrate an all-optical Raman spin echo technique, using
Electromagnetically Induced Transparency (EIT) to create the different pulses
of the spin echo sequence: initialization, pi-rotation, and readout. The first
pulse of the sequence induces coherence directly from a mixed state, and the
technique is used to measure the nuclear spin coherence of an inhomogeneously
broadened ensemble of rare-earth ions (Pr). In contrast to previous
experiments it does not require any preparatory hole burning pulse sequences,
which greatly shortens the total duration of the sequence. The effect of the
different pulses is characterized by quantum state tomography and is compared
with simulations. We demonstrate two applications of the technique by using the
spin echo sequence to accurately compensate a magnetic field across our sample,
and to measure the coherence time at high temperatures up to 11 K, where
standard preparation techniques are difficult to implement. We explore the
potential of the technique and possible applications.Comment: 8 pages, 6 figure
Charge distribution and screening in layered graphene systems
The charge distribution induced by external fields in finite stacks of
graphene planes, or in semiinfinite graphite is considered. The interlayer
electronic hybridization is described by a nearest neighbor hopping term, and
the charge induced by the self consistent electrostatic potential is calculated
within linear response (RPA). The screening properties are determined by
contributions from inter- and intraband electronic transitions. In neutral
systems, only interband transitions contribute to the charge polarizability,
leading to insulating-like screening properties, and to oscillations in the
induced charge, with a period equal to the interlayer spacing. In doped
systems, we find a screening length equivalent to 2-3 graphene layers,
superimposed to significant charge oscillations.Comment: 8 page
A revision of the East African Nebrioporus abyssinicus group (Coleoptera, Dytiscidae)
The four species of the Nebrioporus abyssinicus (Sharp, 1882) group, confined to East Africa, are revised and a key is given for their identification. Lectotypes are designated for the following nominal species: Deronectes abyssinicus Sharp, 1882, Hydroporus tellinii Régimbart, 1904, Hydroporus atratus Régimbart, 1908, Hydroporus flavidus Régimbart, 1908, Deronectes cooperi Omer-Cooper, 1931, and Deronectes scotti Omer-Cooper, 1931. Hydroporus septemvittatus Régimbart, 1883, is listed as a species dubium. The third-instar larva of the group is described for the first time
On Making Good Games - Using Player Virtue Ethics and Gameplay Design Patterns to Identify Generally Desirable Gameplay Features
This paper uses a framework of player virtues to perform a
theoretical exploration of what is required to make a game
good. The choice of player virtues is based upon the view
that games can be seen as implements, and that these are
good if they support an intended use, and the intended use
of games is to support people to be good players. A collection of gameplay design patterns, identified through
their relation to the virtues, is presented to provide specific starting points for considering design options for this type of good games. 24 patterns are identified supporting the virtues, including RISK/REWARD, DYNAMIC ALLIANCES, GAME MASTERS, and PLAYER DECIDED RESULTS, as are 7 countering three or more virtues, including ANALYSIS
PARALYSIS, EARLY ELIMINATION, and GRINDING. The paper concludes by identifying limitations of the approach as well as by showing how it can be applied using other views of what are preferable features in games
Electronic structure of GaAs1-xNx alloy by soft-X-ray absorption and emission: Origin of the reduced optical efficiency
The local electronic structure of N atoms in a diluted GaAs1-xNx (x=3%)
alloy, in view of applications in optoelectronics, is determined for the first
time using soft-X-ray absorption (SXA) and emission (SXE). Deviations from
crystalline GaN, in particular in the conduction band, are dramatic. Employing
the orbital character and elemental specificity of the SXE/SXA spectroscopies,
we identify a charge transfer from the N atoms at the valence band maximum,
reducing the overlap with the wavefunction in conduction band minimum, as the
main factor limiting the optical efficiency of GaAs1-xNx alloys. Moreover, a
k-conserving process of resonant inelastic x-ray scattering involving the L1
derived valence and conduction states is discovered.Comment: 3 pages, physica status solidi (Rapid Research Notes), in pres
Electronic properties of graphene multilayers
We study the effects of disorder in the electronic properties of graphene
multilayers, with special focus on the bilayer and the infinite stack. At low
energies and long wavelengths, the electronic self-energies and density of
states exhibit behavior with divergences near half-filling. As a consequence,
the spectral functions and conductivities do not follow Landau's Fermi liquid
theory. In particular, we show that the quasiparticle decay rate has a minimum
as a function of energy, there is a universal minimum value for the in-plane
conductivity of order e^2/h per plane and, unexpectedly, the c-axis
conductivity is enhanced by disorder at low doping, leading to an enormous
conductivity anisotropy at low temperatures.Comment: 4 pages, 4 figure. Reference to exciting new ARPES results on
graphite added (we thank A. Lanzara for sharing the paper prior to its
publication
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