5,552 research outputs found
Visualizing Quantum Well State Perturbations of Metallic Thin Films near Stacking Fault Defects
We demonstrate that quantum well states (QWS) of thin Pb films are highly
perturbed within the proximity of intrinsic film defects. Scanning Tunneling
Spectroscopy (STM/STS) measurements indicate that the energy of these states
have a strong distance dependence within 4 nm of the defect with the strongest
energetic fluctuations equaling up to 100 meV. These localized perturbations
show large spatially-dependent asymmetries in the LDOS around the defect site
for each corresponding quantum well state. These energetic fluctuations can be
described by a simple model which accounts for fluctuations in the confinement
potential induced by topographic changes.Comment: Updated Versio
A Two-Coordinate Nickel Imido Complex That Effects CāH Amination
An exceptionally low coordinate nickel imido complex, (IPr*)NiāN(dmp) (2) (dmp = 2,6-dimesitylphenyl), has been prepared by the elimination of N_2 from a bulky aryl azide in its reaction with (IPr*)Ni(Ī·^6-C_7H_8) (1). The solid-state structure of 2 features two-coordinate nickel with a linear CāNiāN core and a short NiāN distance, both indicative of multiple-bond character. Computational studies using density functional theory showed a NiāN bond dominated by Ni(dĻ)āN(pĻ) interactions, resulting in two nearly degenerate singly occupied molecular orbitals (SOMOs) that are NiāN Ļ* in character. Reaction of 2 with CO resulted in nitrene-group transfer to form (dmp)NCO and (IPr*)Ni(CO)_3 (3). Net CāH insertion was observed in the reaction of 2 with ethene, forming the vinylamine (dmp)NH(CHāCH_2) (5) via an azanickelacyclobutane intermediate, (IPr*)Ni{N,C:Īŗ^2-N(dmp)CH_2CH_2} (4)
Equation-free implementation of statistical moment closures
We present a general numerical scheme for the practical implementation of
statistical moment closures suitable for modeling complex, large-scale,
nonlinear systems. Building on recently developed equation-free methods, this
approach numerically integrates the closure dynamics, the equations of which
may not even be available in closed form. Although closure dynamics introduce
statistical assumptions of unknown validity, they can have significant
computational advantages as they typically have fewer degrees of freedom and
may be much less stiff than the original detailed model. The closure method can
in principle be applied to a wide class of nonlinear problems, including
strongly-coupled systems (either deterministic or stochastic) for which there
may be no scale separation. We demonstrate the equation-free approach for
implementing entropy-based Eyink-Levermore closures on a nonlinear stochastic
partial differential equation.Comment: 7 pages, 2 figure
Probing the Inner Regions of Protoplanetary Disks with CO Absorption Line Spectroscopy
Carbon monoxide (CO) is the most commonly used tracer of molecular gas in the
inner regions of protoplanetary disks. CO can be used to constrain the
excitation and structure of the circumstellar environment. Absorption line
spectroscopy provides an accurate assessment of a single line-of-sight through
the protoplanetary disk system, giving more straightforward estimates of column
densities and temperatures than CO and molecular hydrogen emission line
studies. We analyze new observations of ultraviolet CO absorption from the
Hubble Space Telescope along the sightlines to six classical T Tauri stars. Gas
velocities consistent with the stellar velocities, combined with the
moderate-to-high disk inclinations, argue against the absorbing CO gas
originating in a fast-moving disk wind. We conclude that the far-ultraviolet
observations provide a direct measure of the disk atmosphere or possibly a slow
disk wind. The CO absorption lines are reproduced by model spectra with column
densities in the range N(^{12}CO) ~ 10^{16} - 10^{18} cm^{-2} and N(^{13}CO) ~
10^{15} - 10^{17} cm^{-2}, rotational temperatures T_{rot}(CO) ~ 300 - 700 K,
and Doppler b-values, b ~ 0.5 - 1.5 km s^{-1}. We use these results to
constrain the line-of-sight density of the warm molecular gas (n_{CO} ~ 70 -
4000 cm^{-3}) and put these observations in context with protoplanetary disk
models.Comment: 12 pages, 14 figures, ApJ - accepte
Synthesis and Characterization of Three-Coordinate Ni(III)-Imide Complexes
A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the NiāN core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane
Multispectral Recovery of a Fragment of Richard FitzRalphās Summa de Questionibus Armenorum from University of Rochester, D.460 1000-03
Multispectral imagingāthe process of obtaining image data from a range of both visible and invisible wavelengthsāis a new frontier in medieval studies, raising the possibility of recovering damaged or palimpsested texts that have been illegible for centuries. In this paper we show the remarkable results of applying this technology to University of X, MS D.460 1000-003, a previously unidentified single-folio fragment that was gifted to the university in 1968. Formerly used as a limp vellum binding for a seventeenth-century volume, the text has become so worn that it is all but completely unreadable to the naked eye. The fragment has consequently received little scholarly attention prior to our investigation. Our team recovered nearly all of the lost text and identified the fragment as an excerpt from Richard FitzRalphās Summa de Questionibus Armenorum. Although this text survives in 45 other manuscripts and fragments, our discovery is highly significant because the Rochester fragment is the only copy of any of FitzRalphās works in a non-European collection. Moreover, the fragment, whose handwriting dates to no later than 1370, may be the oldest extant copy of the Summa by at least half a decade. We present the process of this discovery, our conclusions about the text, and the potential for multispectral imaging to unlock new information hidden in known but understudied fragments held in archival collections around the world
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