12,768 research outputs found
Pressure dependence of the superconducting transition and electron correlations in Na_xCoO_2 \cdot 1.3H_2O
We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on
the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under
hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure
at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the
decrease of T_c is accompanied by a weakening of the spin correlations at a
finite wave vector and a reduction of the density of states (DOS) at the Fermi
level. At high pressures above 1.93 GPa, however, the decrease of T_c is mainly
due to a reduction of the DOS. These results indicate that the
electronic/magnetic state of Co is primarily responsible for the
superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a
T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling
evidence for the presence of line nodes in the superconducting gap function.Comment: published on 19, Sept. 2007 on Phys. Rev.
Knowledge discovery for friction stir welding via data driven approaches: Part 2 – multiobjective modelling using fuzzy rule based systems
In this final part of this extensive study, a new systematic data-driven fuzzy modelling approach has been developed, taking into account both the modelling accuracy and its interpretability (transparency) as attributes. For the first time, a data-driven modelling framework has been proposed designed and implemented in order to model the intricate FSW behaviours relating to AA5083 aluminium alloy, consisting of the grain size, mechanical properties, as well as internal process properties. As a result, ‘Pareto-optimal’ predictive models have been successfully elicited which, through validations on real data for the aluminium alloy AA5083, have been shown to be accurate, transparent and generic despite the conservative number of data points used for model training and testing. Compared with analytically based methods, the proposed data-driven modelling approach provides a more effective way to construct prediction models for FSW when there is an apparent lack of fundamental process knowledge
A general condition of inflationary cosmology on trans-Planckian physics
We consider a more general initial condition satisfying the minimal
uncertainty relationship. We calculate the power spectrum of a simple model in
inflationary cosmology. The results depend on perturbations generated below a
fundamental scale, e.g. the Planck scale.Comment: 7 pages, References adde
Improving Orbit Estimates for Incomplete Orbits with a New Approach to Priors -- with Applications from Black Holes to Planets
We propose a new approach to Bayesian prior probability distributions
(priors) that can improve orbital solutions for low-phase-coverage orbits,
where data cover less than approximately 40% of an orbit. In instances of low
phase coverage such as with stellar orbits in the Galactic center or with
directly-imaged exoplanets, data have low constraining power and thus priors
can bias parameter estimates and produce under-estimated confidence intervals.
Uniform priors, which are commonly assumed in orbit fitting, are notorious for
this. We propose a new observable-based prior paradigm that is based on
uniformity in observables. We compare performance of this observable-based
prior and of commonly assumed uniform priors using Galactic center and
directly-imaged exoplanet (HR 8799) data. The observable-based prior can reduce
biases in model parameters by a factor of two and helps avoid under-estimation
of confidence intervals for simulations with less than about 40% phase
coverage. Above this threshold, orbital solutions for objects with sufficient
phase coverage such as S0-2, a short-period star at the Galactic center with
full phase coverage, are consistent with previously published results. Below
this threshold, the observable-based prior limits prior influence in regions of
prior dominance and increases data influence. Using the observable-based prior,
HR 8799 orbital analyses favor lower eccentricity orbits and provide stronger
evidence that the four planets have a consistent inclination around 30 degrees
to within 1-sigma. This analysis also allows for the possibility of
coplanarity. We present metrics to quantify improvements in orbital estimates
with different priors so that observable-based prior frameworks can be tested
and implemented for other low-phase-coverage orbits.Comment: Published in AJ. 23 pages, 14 figures. Monte Carlo chains are
available in the published article, or are available upon reques
Analytical solution of the dynamical spherical MIT bag
We prove that when the bag surface is allowed to move radially, the equations
of motion derived from the MIT bag Lagrangian with massless quarks and a
spherical boundary admit only one solution, which corresponds to a bag
expanding at the speed of light. This result implies that some new physics
ingredients, such as coupling to meson fields, are needed to make the dynamical
bag a consistent model of hadrons.Comment: Revtex, no figures. Submitted to Journal of Physics
Na content dependence of superconductivity and the spin correlations in Na_{x}CoO_{2}\cdot 1.3H_{2}O
We report systematic measurements using the ^{59}Co nuclear quadrupole
resonance(NQR) technique on the cobalt oxide superconductors Na_{x}CoO_{2}\cdot
1.3H_{2}O over a wide Na content range x=0.25\sim 0.34. We find that T_c
increases with decreasing x but reaches to a plateau for x \leq0.28. In the
sample with x \sim 0.26, the spin-lattice relaxation rate 1/T_1 shows a T^3
variation below T_c and down to T\sim T_c/6, which unambiguously indicates the
presence of line nodes in the superconducting (SC) gap function. However, for
larger or smaller x, 1/T_1 deviates from the T^3 variation below T\sim 2 K even
though the T_c (\sim 4.7 K) is similar, which suggests an unusual evolution of
the SC state. In the normal state, the spin correlations at a finite wave
vector become stronger upon decreasing x, and the density of states at the
Fermi level increases with decreasing x, which can be understood in terms of a
single-orbital picture suggested on the basis of LDA calculation.Comment: version published in J. Phys. Condens. Matter (references updated and
more added
Shallow BF2 implants in Xe-bombardment-preamorphized Si: the interaction between Xe and F
Si(100) samples, preamorphized to a depth of ~30 nm using 20 keV Xe ions to a nominal fluence of 2×1014 cm-2 were implanted with 1 and 3 keV BF2 ions to fluences of 7×1014 cm-2. Following annealing over a range of temperatures (from 600 to 1130 °C) and times the implant redistribution was investigated using medium-energy ion scattering (MEIS), secondary ion mass spectrometry (SIMS), and energy filtered transmission electron microscopy (EFTEM). MEIS studies showed that for all annealing conditions leading to solid phase epitaxial regrowth, approximately half of the Xe had accumulated at depths of 7 nm for the 1 keV and at 13 nm for the 3 keV BF2 implant. These depths correspond to the end of range of the B and F within the amorphous Si. SIMS showed that in the preamorphized samples, approximately 10% of the F migrates into the bulk and is trapped at the same depths in a ~1:1 ratio to Xe. These observations indicate an interaction between the Xe and F implants and a damage structure that becomes a trapping site. A small fraction of the implanted B is also trapped at this depth. EXTEM micrographs suggest the development of Xe agglomerates at the depths determined by MEIS. The effect is interpreted in terms of the formation of a volume defect structure within the amorphized Si, leading to F stabilized Xe agglomerates or XeF precipitates
Vortex avalanches and self organized criticality in superconducting niobium
In 1993 Tang proposed [1] that vortex avalanches should produce a self
organized critical state in superconductors, but conclusive evidence for this
has heretofore been lacking. In the present paper, we report extensive
micro-Hall probe data from the vortex dynamics in superconducting niobium,
where a broad distribution of avalanche sizes scaling as a power-law for more
than two decades is found. The measurements are combined with magneto-optical
imaging, and show that over a widely varying magnetic landscape the scaling
behaviour does not change, hence establishing that the dynamics of
superconducting vortices is a SOC phenomenon.Comment: 3 pages + 4 figures, a reference added, citation typos fixe
- …