34,003 research outputs found
Dynamical scaling in Ising and vector spin glasses
We have studied numerically the dynamics of spin glasses with Ising and XY
symmetry (gauge glass) in space dimensions 2, 3, and 4. The nonequilibrium
spin-glass susceptibility and the nonequilibrium energy per spin of samples of
large size L_b are measured as a function of anneal time t_w after a quench to
temperatures T. The two observables are compared to the equilibrium spin-glass
susceptibility and the equilibrium energy, respectively, measured as functions
of temperature T and system size L for a range of system sizes. For any time
and temperature a nonequilibrium time-dependent length scale L*(t_w,T) can be
defined by comparing equilibrium and nonequilibrium quantities. Our analysis
shows that for all systems studied, an "effective dynamical critical exponent"
parametrization L*(t_w,T) = A(T) t^(1/z(T)) fits the data well at each
temperature within the whole temperature range studied, which extends from well
above the critical temperature to near T = 0 for dimension 2, or to well below
the critical temperature for the other space dimensions studied. In addition,
the data suggest that the dynamical critical exponent z varies smoothly when
crossing the transition temperature.Comment: 14 pages, 13 figures, 9 table
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SLS Materials Development Method for Rapid Manufacturing
As soon as SFF technology development began to make Rapid Prototyping possible the
interest in Rapid Manufacturing (RM) began to grow. The advantages in terms of
functional integration, elimination of tooling and fixtures and mass customization make a
compelling case for RM, leading some in the field to call it the next industrial revolution.
Yet without the materials properties necessary to provide the function and variety
currently available from mass production methods, the application of RM will remain
limited. Developing new materials for the SLS process, one immediate step toward a
larger portfolio of RM materials, is very challenging. The formation of high quality SLS
parts relies on appropriate powder characteristics, thermal cycles and sintering behavior.
Based on a brief examination of the key factors in SLS processing and a research project
to develop a new binder material for Silicon Carbide composites, a systematic materials
development method is proposed in this paper. The method provides guidance for
introducing new SLS materials, support for educating new SLS users and researchers and
direction for several future research projects.Mechanical Engineerin
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Rapid Manufacturing of Silicon Carbide Composites
From the earliest days of SFF technology development, a viable technique for the direct
manufacture of fully-functional parts has been a major technology goal. While direct metal
methods have been demonstrated for a variety of metals including aluminum, steel and titanium,
they have not reached wide commercial application due to processing speed, final material
properties and surface finish. In this paper the development of an SLS-based rapid
manufacturing (RM) platform is reviewed. The core of this platform is a thermosetting binder
system for preform parts in contrast to the thermoplastic materials currently available for SLS.
The preforms may include metal and/or ceramic powders. A variety of fully functional parts
can be prepared from different combinations of materials and post processing steps including
binder pyrolysis, free-standing alloy infiltration, room temperature polymer infiltration and
machining. The main issues of these steps are reviewed followed by a discussion about the
support of RM. This paper is an intermediate report additional materials, applications, process
models and product design strategies will be incorporated into the project in the next year.Mechanical Engineerin
Kinetics of viral self-assembly: the role of ss RNA antenna
A big class of viruses self-assemble from a large number of identical capsid
proteins with long flexible N-terminal tails and ss RNA. We study the role of
the strong Coulomb interaction of positive N-terminal tails with ss RNA in the
kinetics of the in vitro virus self-assembly. Capsid proteins stick to
unassembled chain of ss RNA (which we call "antenna") and slide on it towards
the assembly site. We show that at excess of capsid proteins such
one-dimensional diffusion accelerates self-assembly more than ten times. On the
other hand at excess of ss RNA, antenna slows self-assembly down. Several
experiments are proposed to verify the role of ss RNA antenna.Comment: 4 pages, 3 figures, several experiments are proposed, a new idea of
experiment is adde
Io's radar properties
Arecibo 13 cm wavelength radar observations during 1987-90 have yielded echoes from Io on each of 11 dates. Whereas Voyager imaged parts of the satellite at resolutions of several km and various visible/infrared measurements have probed the surfaces's microscale properties, the radar data yield new information about the nature of the surface at cm to km scales. Our observations provide fairly thorough coverage and reveal significant heterogeneity in Io's radar properties. A figure is given showing sums of echo spectra from 11 dates
Spin-dependent Seebeck coefficients of Ni_{80}Fe_{20} and Co in nanopillar spin valves
We have experimentally determined the spin-dependent Seebeck coefficient of
permalloy (Ni_{80}Fe_{20}) and cobalt (Co) using nanopillar spin valve devices.
The devices were specifically designed to completely separate heat related
effects from charge related effects. A pure heat current through the nanopillar
spin valve, a stack of two ferromagnetic layers (F) separated by a non-magnetic
layer (N), leads to a thermovoltage proportional to the spin-dependent Seebeck
coefficient S_{S}=S_{\uparrow}-S_{\downarrow} of the ferromagnet, where
S_{\uparrow} and S_{\downarrow} are the Seebeck coefficient for spin-up and
spin-down electrons. By using a three-dimensional finite-element model (3D-FEM)
based on spin-dependent thermoelectric theory, whose input material parameters
were measured in separate devices, we were able to accurately determine a
spin-dependent Seebeck coefficient of -1.8 microvolt/Kelvin and -4.5
microvolt/Kelvin for cobalt and permalloy, respectively corresponding to a
Seebeck coefficient polarization P_{S}=S_{S}/S_{F} of 0.08 and 0.25, where
S_{F} is the Seebeck coefficient of the ferromagnet. The results are in
agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent
Seebeck and spin-dependent Peltier measurements in Ni_{80}Fe_{20}/Cu spin valve
structures
Existence of multi-site intrinsic localized modes in one-dimensional Debye crystals
The existence of highly localized multi-site oscillatory structures (discrete
multibreathers) in a nonlinear Klein-Gordon chain which is characterized by an
inverse dispersion law is proven and their linear stability is investigated.
The results are applied in the description of vertical (transverse, off-plane)
dust grain motion in dusty plasma crystals, by taking into account the lattice
discreteness and the sheath electric and/or magnetic field nonlinearity.
Explicit values from experimental plasma discharge experiments are considered.
The possibility for the occurrence of multibreathers associated with vertical
charged dust grain motion in strongly-coupled dusty plasmas (dust crystals) is
thus established. From a fundamental point of view, this study aims at
providing a first rigorous investigation of the existence of intrinsic
localized modes in Debye crystals and/or dusty plasma crystals and, in fact,
suggesting those lattices as model systems for the study of fundamental crystal
properties.Comment: 12 pages, 8 figures, revtex forma
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