773 research outputs found
Renormalization group maps for Ising models in lattice gas variables
Real space renormalization group maps, e.g., the majority rule
transformation, map Ising type models to Ising type models on a coarser
lattice. We show that each coefficient of the renormalized Hamiltonian in the
lattice gas variables depends on only a finite number of values of the
renormalized Hamiltonian. We introduce a method which computes the values of
the renormalized Hamiltonian with high accuracy and so computes the
coefficients in the lattice gas variables with high accuracy. For the critical
nearest neighbor Ising model on the square lattice with the majority rule
transformation, we compute over 1,000 different coefficients in the lattice gas
variable representation of the renormalized Hamiltonian and study the decay of
these coefficients. We find that they decay exponentially in some sense but
with a slow decay rate. We also show that the coefficients in the spin
variables are sensitive to the truncation method used to compute them.Comment: 22 pages, 9 color postscript figures; minor revisions in version
Does language dominance affect cognitive performance in bilinguals? Lifespan evidence from preschoolers through older adults on card sorting, Simon, and metalinguistic tasks
Salt-metal feedstocks for the creation of stochastic cellular structures with controlled relative density by powder bed fabrication
A novel type of metallic feedstock material for powder-bed additive manufacturing (AM) processes is proposed that enables the manufacture of cellular structures without the time consuming and computationally intensive step of digitally representing the internal geometry of a part. The feedstock is a blend of metal and salt particles and, following Selective Laser Melting (SLM) processing, the salt is dissolved to leave a metallic, cellular structure. The conditions for successfully processing the feedstock are first demonstrated, followed by an investigation into how the feedstock composition can be used to control the relative density of the cellular material. Mechanical testing reveals that the strength and stiffness of the cellular structures can be tuned through control of feedstock composition, and hence, relative density. This presents a significant enhancement to the state-of-the-art for materials preparation for AM since, for the first time, cellular structures can be created with specific properties without explicitly defining or analysing the unit cell geometry
Diseases, lesions and malformations in the long-beaked common dolphin <i>Delphinus capensis</i> from the Southeast Pacific
Miscellaneous lesions of the head, skull, teeth, trunk, appendages, skin and genital tract were observed in 120 of 930 long-beaked common dolphins Delphinus capensis taken in fisheries off Peru between 1985 and 2000. Seven subsamples were defined according to the varying field sampling protocols. Forty-two dolphins showed at least 2 types of injuries or diseases affecting 1 or more organs. The majority (5 of 7) of traumas encountered were diagnosed as caused by violent, fisheries-related interactions, and the skin in 20.4% of specimens (n = 54) showed healed scars from such interactions. Prevalences of malformations and traumas of crania (n = 103) were 2.9 and 1.9%, respectively. Lytic cranial lesions were present in 31.1% of dolphins (n = 103) and accounted for 84.2% of all bone injuries. Skull damage diagnostic for Crassicauda sp. infestation was encountered in 26.5% of dolphins (n = 98) and did not differ among sex and age classes. Crassicauda sp. and tooth infections were responsible for, respectively, 78.8 and 6.1% of the lytic lesions. Adult dolphins showed a high prevalence of worn and broken teeth (35%, n = 20) as well as damaged alveoli (20%, n = 70). Prevalence of ‘paired teeth’, a congenital condition, was 9.4% (n = 32). Lesions of the head, body and appendages were present in 10 dolphins and included traumas, deformations (e.g. scoliokyphosis and brachygnathia) and chronic mastitis. Ovarian cysts suggestive of follicular cysts were observed in 1 of 24 females. Chronic orchitis affected 1 of 78 males. Of 12 dolphins 2 had vesicular lesions of the penis. Prevalence of cutaneous lesions, abnormalities and scars ranged between 1.8% (n = 56) and 48.2% (n = 27)
Generation of graded porous structures by control of process parameters in the selective laser melting of a fixed ratio salt-metal feedstock
The demonstration of salt dissolution incorporated within laser powder-bed fusion fabrication processes has allowed the creation of complex porous structures without the need for sophisticated design algorithms. This serves to simplify the process, for porous structure creation in powder-bed fabrication techniques, creating a new opportunity for the realisation of optimised structures. A new methodology is presented here in which modulation of the energy density while using a single feedstock material enables three-dimensional control of porosity, ranging from 20 % to 49 %. Through structured experimentation, the response of the material to varying the process parameters in selective laser melting is evaluated and nested structures of distinct densities and morphologies are created. Correlation of the process parameters with modulus and ultimate compressive stress are established. A simple-assembly algorithm was used to generate complex parts consisting of locally assigned porosities having characteristic properties
Creating diamond color centers for quantum optical applications
Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state
qubits. This is because of their large dipole moment, convenient level
structure and very long room-temperature coherence times. In general, a
combination of ion irradiation and subsequent annealing is used to create the
centers, however for the rigorous demands of quantum computing all processes
need to be optimized, and decoherence due to the residual damage caused by the
implantation process itself must be mitigated. To that end we have studied
photoluminescence (PL) from NV, NV and GR1 centers formed by ion
implantation of 2MeV He ions over a wide range of fluences. The sample was
annealed at C to minimize residual vacancy diffusion, allowing for
the concurrent analysis of PL from NV centers and irradiation induced vacancies
(GR1). We find non-monotic PL intensities with increasing ion fluence,
monotonic increasing PL in NV/NV and GR1/(NV + NV) ratios, and
increasing inhomogeneous broadening of the zero-phonon lines with increasing
ion fluence. All these results shed important light on the optimal formation
conditions for NV qubits. We apply our findings to an off-resonant photonic
quantum memory scheme using vibronic sidebands
Distribution of exchange energy in a bond-alternating S=1 quantum spin chain
The quasi-one-dimensional bond-alternating S=1 quantum antiferromagnet NTENP
is studied by single crystal inelastic neutron scattering. Parameters of the
measured dispersion relation for magnetic excitations are compared to existing
numerical results and used to determine the magnitude of bond-strength
alternation. The measured neutron scattering intensities are also analyzed
using the 1st-moment sum rules for the magnetic dynamic structure factor, to
directly determine the modulation of ground state exchange energies. These
independently determined modulation parameters characterize the level of spin
dimerization in NTENP. First-principle DMRG calculations are used to study the
relation between these two quantities.Comment: 10 pages, 10 figure
Detection of Extrasolar Planets by Gravitational Microlensing
Gravitational microlensing provides a unique window on the properties and
prevalence of extrasolar planetary systems because of its ability to find
low-mass planets at separations of a few AU. The early evidence from
microlensing indicates that the most common type of exoplanet yet detected are
the so-called "super-Earth" planets of ~10 Earth-masses at a separation of a
few AU from their host stars. The detection of two such planets indicates that
roughly one third of stars have such planets in the separation range 1.5-4 AU,
which is about an order of magnitude larger than the prevalence of gas-giant
planets at these separations. We review the basic physics of the microlensing
method, and show why this method allows the detection of Earth-mass planets at
separations of 2-3 AU with ground-based observations. We explore the conditions
that allow the detection of the planetary host stars and allow measurement of
planetary orbital parameters. Finally, we show that a low-cost, space-based
microlensing survey can provide a comprehensive statistical census of
extrasolar planetary systems with sensitivity down to 0.1 Earth-masses at
separations ranging from 0.5 AU to infinity.Comment: 43 pages. Very similar to chapter 3 of Exoplanets: Detection,
Formation, Properties, Habitability, John Mason, ed. Springer (April 3, 2008
Fractal iso-contours of passive scalar in smooth random flows
We consider a passive scalar field under the action of pumping, diffusion and
advection by a smooth flow with a Lagrangian chaos. We present theoretical
arguments showing that scalar statistics is not conformal invariant and
formulate new effective semi-analytic algorithm to model the scalar turbulence.
We then carry massive numerics of passive scalar turbulence with the focus on
the statistics of nodal lines. The distribution of contours over sizes and
perimeters is shown to depend neither on the flow realization nor on the
resolution (diffusion) scale for scales exceeding . The scalar
isolines are found fractal/smooth at the scales larger/smaller than the pumping
scale . We characterize the statistics of bending of a long isoline by the
driving function of the L\"owner map, show that it behaves like diffusion with
the diffusivity independent of resolution yet, most surprisingly, dependent on
the velocity realization and the time of scalar evolution
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