24,243 research outputs found
Generation of bipartite spin entanglement via spin-independent scattering
We consider the bipartite spin entanglement between two identical fermions
generated in spin-independent scattering. We show how the spatial degrees of
freedom act as ancillas for the creation of entanglement to a degree that
depends on the scattering angle, . The number of Slater determinants
generated in the process is greater than 1, corresponding to genuine quantum
correlations between the identical fermions. The maximal entanglement
attainable of 1 ebit is reached at . We also analyze a simple
dependent Bell's inequality, which is violated for
. This phenomenon is unrelated to the symmetrization
postulate but does not appear for unequal particles.Comment: 5 pages and 3 figures. Accepted in PR
Clustering of spectra and fractals of regular graphs
We exhibit a characteristic structure of the class of all regular graphs of
degree d that stems from the spectra of their adjacency matrices. The structure
has a fractal threadlike appearance. Points with coordinates given by the mean
and variance of the exponentials of graph eigenvalues cluster around a line
segment that we call a filar. Zooming-in reveals that this cluster splits into
smaller segments (filars) labeled by the number of triangles in graphs. Further
zooming-in shows that the smaller filars split into subfilars labelled by the
number of quadrangles in graphs, etc. We call this fractal structure,
discovered in a numerical experiment, a multifilar structure. We also provide a
mathematical explanation of this phenomenon based on the Ihara-Selberg trace
formula, and compute the coordinates and slopes of all filars in terms of
Bessel functions of the first kind.Comment: 10 pages, 5 figure
Earth-like sand fluxes on Mars
Strong and sustained winds on Mars have been considered rare, on the basis of surface meteorology measurements and global circulation models, raising the question of whether the abundant dunes and evidence for wind erosion seen on the planet are a current process. Recent studies showed sand activity, but could not determine whether entire dunes were moving—implying large sand fluxes—or whether more localized and surficial changes had occurred. Here we present measurements of the migration rate of sand ripples and dune lee fronts at the Nili Patera dune field. We show that the dunes are near steady state, with their entire volumes composed of mobile sand. The dunes have unexpectedly high sand fluxes, similar, for example, to those in Victoria Valley, Antarctica, implying that rates of landscape modification on Mars and Earth are similar
Emergence of long-range order in BaTiO3 from local symmetry-breaking distortions
By using a symmetry motivated basis to evaluate local distortions against
pair distribution function data (PDF), we show without prior bias, that the
off-centre Ti displacements in the archetypal ferroelectric BaTiO3 are zone
centred and rhombohedral-like in nature across its known ferroelectric and
paraelectric phases. With our newly-gained insight we construct a simple Monte
Carlo (MC) model which captures our main experimental findings and demonstrate
how the rich crystallographic phase diagram of BaTiO3 emerges from correlations
of local symmetry-breaking distortions alone. Our results strongly support the
order-disorder picture for these phase transitions, but can also be reconciled
with the soft-mode theory of BaTiO3 that is supported by some spectroscopic
techniques.Comment: 5 pages, 3 figure
Magnetic and vibrational properties of high-entropy alloys
The magnetic properties of high-entropy alloys based on equimolar FeCoCrNi were investigated using vibrating sample magnetometry to determine their usefulness in high-temperature magnetic applications. Nuclear resonant inelastic x-ray scattering measurements were performed to evaluate the vibrational entropy of the ^(57)Fe atoms and to infer chemical order. The configurational and vibrational entropy of alloying are discussed as they apply to these high-entropy alloys
Adiabatic Electron-Phonon Interaction and High-Temperature Thermodynamics of A15 Compounds
Inelastic neutron scattering was used to measure the phonon densities of states of the A15 compounds V_3Si, V_3Ge, and V_3Co at temperatures from 10 to 1273 K. It was found that phonons in V_3Si and V_3Ge, which are superconducting at low temperatures, exhibit an anomalous stiffening with increasing temperature, whereas phonons in V_3Co have a normal softening behavior. First-principles calculations show that this anomalous increase in phonon frequencies at high temperatures originates with an adiabatic electron-phonon coupling mechanism. The anomaly is caused by the thermally induced broadening of sharp peaks in the electronic density of states of V_3Si and V_3Ge, which tends to decrease the electronic density at the Fermi level. These results show that the adiabatic electron-phonon coupling can influence the phonon thermodynamics at temperatures exceeding 1000 K
Consistent Gravitationally-Coupled Spin-2 Field Theory
Inspired by the translational gauge structure of teleparallel gravity, the
theory for a fundamental massless spin-2 field is constructed. Accordingly,
instead of being represented by a symmetric second-rank tensor, the fundamental
spin-2 field is assumed to be represented by a spacetime (world) vector field
assuming values in the Lie algebra of the translation group. The flat-space
theory naturally emerges in the Fierz formalism and is found to be equivalent
to the usual metric-based theory. However, the gravitationally coupled theory,
with gravitation itself described by teleparallel gravity, is shown not to
present the consistency problems of the spin-2 theory constructed on the basis
of general relativity.Comment: 16 pages, no figures. V2: Presentation changes, including addition of
a new sub-section, aiming at clarifying the text; version accepted for
publication in Class. Quantum Grav
PIRATE: A Remotely-Operable Telescope Facility for Research and Education
We introduce PIRATE, a new remotely-operable telescope facility for use in
research and education, constructed from 'off-the-shelf' hardware, operated by
The Open University. We focus on the PIRATE Mark 1 operational phase where
PIRATE was equipped with a widely- used 0.35m Schmidt-Cassegrain system (now
replaced with a 0.425m corrected Dall Kirkham astrograph). Situated at the
Observatori Astronomic de Mallorca, PIRATE is currently used to follow up
potential transiting extrasolar planet candidates produced by the SuperWASP
North experiment, as well as to hunt for novae in M31 and other nearby
galaxies. It is operated by a mixture of commercially available software and
proprietary software developed at the Open University. We discuss problems
associated with performing precision time series photometry when using a German
Equatorial Mount, investigating the overall performance of such 'off-the-shelf'
solutions in both research and teaching applications. We conclude that PIRATE
is a cost-effective research facility, and also provides exciting prospects for
undergraduate astronomy. PIRATE has broken new ground in offering practical
astronomy education to distance-learning students in their own homes.Comment: Accepted for publication in PASP. 14 pages, 11 figure
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