38,251 research outputs found
Universal characteristics of resonant-tunneling field emission from nanostructured surfaces
We have performed theoretical and experimental studies of field emission from
nanostructured semiconductor cathodes. Resonant tunneling through
electric-field-induced interface bound states is found to strongly affect the
field-emission characteristics. Our analytical theory predicts power-law and
Lorentzian-shaped current-voltage curves for resonant-tunneling field emission
from three-dimensional substrates and two-dimensional accumulation layers,
respectively. These predicted line shapes are observed in field emission
characteristics from self-assembled silicon nanostructures. A simple model
describes formation of an accumulation layer and of the resonant level in these
systems.Comment: 5 pages, 4 figures, RevTex, to appear in J. Appl. Phy
Oxidation and crystallization of an amorphous Zr60Al15Ni25 alloy
The amorphous ternary metallic alloy Zr60Al15Ni25 was oxidized in dry oxygen in the temperature range 310 ±C to 410 ±C. Rutherford backscattering (RBS) and cross-sectional transmission electron microscopy (TEM) studies suggest that during this treatment an amorphous layer of zirconium-aluminum-oxide is formed at the surface. Nickel was depleted in the oxide and enriched in the amorphous alloy near the interface. The oxide layer thickness grows parabolically with annealing duration, with a transport constant of 2.8 x 10^-5 m^2/s x exp(-1.7 eV/kT). The oxidation rate may be controlled by the diffusion of Ni in the amorphous alloy. At later stages of the oxidation process, precipitates of nanocrystalline ZrO2 appear in the oxide near the interface. Finally, two intermetallic phases nucleate and grow simultaneously in the alloy, one at the interface and one within the alloy. An explanation involving preferential oxidation is proposed
Relativistic many-body calculations of the Stark-induced amplitude of the 6P1/2 -7P1/2 transition in thallium
Stark-induced amplitudes for the 6P1/2 - 7P1/2 transition in Tl I are
calculated using the relativistic SD approximation in which single and double
excitations of Dirac-Hartree-Fock levels are summed to all orders in
perturbation theory. Our SD values alpha S = 368 a0 3 and beta S= 298 a 0 3 are
in good agreement with the measurements alpha S=377(8) a 0 3$ and beta S =
313(8) a 0 3 by D. DeMille, D. Budker, and E. D. Commins [Phys. Rev. A 50, 4657
(1994)]. Calculations of the Stark shifts in the 6P1/2 - 7P1/2 and 6P1/2 -
7S1/2 transitions are also carried out. The Stark shifts predicted by our
calculations agree with the most accurate measured values within the
experimental uncertainties for both transitions
Third-order many-body perturbation theory calculations for the beryllium and magnesium isoelectronic sequences
Relativistic third-order MBPT is applied to obtain energies of ions with two
valence electrons in the no virtual-pair approximation (NVPA). A total of 302
third-order Goldstone diagrams are organized into 12 one-body and 23 two-body
terms. Only third-order two-body terms and diagrams are presented here, owing
to the fact that the one-body terms are identical to the previously studied
third-order terms in monovalent ions. Dominant classes of diagrams are
identified. The model potential is a Dirac-Hartree-Fock potential,
and B-spline basis functions in a cavity of finite radius are employed in the
numerical calculations. The Breit interaction is taken into account through
second order of perturbation theory and the lowest-order Lamb shift is also
evaluated. Sample calculations are performed for berylliumlike ions with Z =
4--7, and for the magnesiumlike ion P IV. The third-order energies are in
excellent agreement with measurement with an accuracy at 0.2% level for the
cases considered. Comparisons are made with previous second-order MBPT results
and with other calculations. The third-order energy correction is shown to be
significant, improving second-order correlation energies by an order of
magnitude
Small atom diffusion and breakdown of the StokesâEinstein relation in the supercooled liquid state of the Zr46.7Ti8.3Cu7.5Ni10Be27.5 alloy
Be diffusivity data in the bulk metallic glass forming alloy Zr46.7Ti8.3Cu7.5Ni10Be27.5 are reported for temperatures between 530 and 710 K, extending 85 K into the supercooled liquid state of the alloy. At the glass transition temperature Tg, a change in temperature dependence of the data is observed, and above Tg the diffusivity increases more quickly with temperature than below. The data in the supercooled liquid can be described by a modified Arrhenius expression based on a diffusion mechanism suggested earlier. The comparison with viscosity data in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5 reveals a breakdown of the StokesâEinstein relation, indicating a cooperative diffusion mechanism in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5
The Revealing Dust: Mid-Infrared Activity in Hickson Compact Group Galaxy Nuclei
We present a sample of 46 galaxy nuclei from 12 nearby (z<4500 km/s) Hickson
Compact Groups (HCGs) with a complete suite of 1-24 micron 2MASS+Spitzer
nuclear photometry. For all objects in the sample, blue emission from stellar
photospheres dominates in the near-IR through the 3.6 micron IRAC band.
Twenty-five of 46 (54%) galaxy nuclei show red, mid-IR continua characteristic
of hot dust powered by ongoing star formation and/or accretion onto a central
black hole. We introduce alpha_{IRAC}, the spectral index of a power-law fit to
the 4.5-8.0 micron IRAC data, and demonstrate that it cleanly separates the
mid-IR active and non-active HCG nuclei. This parameter is more powerful for
identifying low to moderate-luminosity mid-IR activity than other measures
which include data at rest-frame lambda<3.6 micron that may be dominated by
stellar photospheric emission. While the HCG galaxies clearly have a bimodal
distribution in this parameter space, a comparison sample from the Spitzer
Nearby Galaxy Survey (SINGS) matched in J-band total galaxy luminosity is
continuously distributed. A second diagnostic, the fraction of 24 micron
emission in excess of that expected from quiescent galaxies, f_{24D}, reveals
an additional 3 nuclei to be active at 24 micron. Comparing these two mid-IR
diagnostics of nuclear activity to optical spectroscopic identifications from
the literature reveals some discrepancies, and we discuss the challenges of
distinguishing the source of ionizing radiation in these and other lower
luminosity systems. We find a significant correlation between the fraction of
mid-IR active galaxies and the total HI mass in a group, and investigate
possible interpretations of these results in light of galaxy evolution in the
highly interactive system of a compact group environment.Comment: 20 pages, 17 figures (1 color), uses emulateapj. Accepted for
publication by Ap
-Scale Decoupling of the Mechanical Relaxation and Diverging Shear Wave Propagation Lengthscale in Triphenylphosphite
We have performed depolarized Impulsive Stimulated Scattering experiments to
observe shear acoustic phonons in supercooled triphenylphosphite (TPP) from
10 - 500 MHz. These measurements, in tandem with previously performed
longitudinal and shear measurements, permit further analyses of the relaxation
dynamics of TPP within the framework of the mode coupling theory (MCT). Our
results provide evidence of coupling between the shear and
longitudinal degrees of freedom up to a decoupling temperature = 231 K. A
lower bound length scale of shear wave propagation in liquids verified the
exponent predicted by theory in the vicinity of the decoupling temperature
A probabilistic analysis of argument cogency
This paper offers a probabilistic treatment of the conditions for argument cogency as endorsed in informal logic: acceptability, relevance, and sufficiency. Treating a natural language argument as a reason-claim-complex, our analysis identifies content features of defeasible argument on which the RSA conditions depend, namely: change in the commitment to the reason, the reasonâs sensitivity and selectivity to the claim, oneâs prior commitment to the claim, and the contextually determined thresholds of acceptability for reasons and for claims. Results contrast with, and may indeed serve to correct, the informal understanding and applications of the RSA criteria concerning their conceptual dependence, their function as update-thresholds, and their status as obligatory rather than permissive norms, but also show how these formal and informal normative approachs can in fact align
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