12,287 research outputs found
Spectroscopic fingerprints of a surface Mott-Hubbard insulator: the case of SiC(0001)
We discuss the spectroscopic fingerprints that a surface Mott-Hubbard
insulator should show at the intra-atomic level. The test case considered is
that of the Si-terminated SiC(0001) sqrt{3}xsqrt{3} surface, which is known
experimentally to be insulating. We argue that, due to the Mott-Hubbard
phenomenon, spin unpaired electrons in the Si adatom dangling bonds are
expected to give rise to a Si-2p core level spectrum with a characteristic
three-peaked structure, as seen experimentally. This structure results from the
joint effect of intra-atomic exchange, spatial anisotropy, and spin-orbit
coupling. Auger intensities are also discussed.Comment: 4 pages, 2 figures, ECOSS-18 conferenc
Microscopic theory of vibronic dynamics in linear polyenes
We propose a novel approach to calculate dynamical processes at ultrafast
time scale in molecules in which vibrational and electronic motions are
strongly mixed. The relevant electronic orbitals and their interactions are
described by a Hubbard model, while electron-phonon interaction terms account
for the bond length dependence of the hopping and the change in ionic radii
with valence charge. The latter term plays a crucial role in the non-adiabatic
internal conversion process of the molecule. The time resolved photoelectron
spectra are in good qualitative agreement with experiments.Comment: 3 figures, other comment
Dynamical chiral symmetry breaking in sliding nanotubes
We discovered in simulations of sliding coaxial nanotubes an unanticipated
example of dynamical symmetry breaking taking place at the nanoscale. While
both nanotubes are perfectly left-right symmetric and nonchiral, a nonzero
angular momentum of phonon origin appears spontaneously at a series of critical
sliding velocities, in correspondence with large peaks of the sliding friction.
The non-linear equations governing this phenomenon resemble the rotational
instability of a forced string. However, several new elements, exquisitely
"nano" appear here, with the crucial involvement of Umklapp and of sliding
nanofriction.Comment: To appear in PR
Faster annealing schedules for quantum annealing
New annealing schedules for quantum annealing are proposed based on the
adiabatic theorem. These schedules exhibit faster decrease of the excitation
probability than a linear schedule. To derive this conclusion, the asymptotic
form of the excitation probability for quantum annealing is explicitly obtained
in the limit of long annealing time. Its first-order term, which is inversely
proportional to the square of the annealing time, is shown to be determined
only by the information at the initial and final times. Our annealing schedules
make it possible to drop this term, thus leading to a higher order (smaller)
excitation probability. We verify these results by solving numerically the
time-dependent Schrodinger equation for small size systemsComment: 10 pages, 5 figures, minor correction
Isothermal and cyclic oxidation at 1000 and 1100 deg C of four nickel-base alloys: NASA-TRW VIA, B-1900, 713C, and 738X
The isothermal and cyclic oxidation resistance of four cast Ni-base gamma + gamma prime alloys, NASA-TRW Via, B-1900, 713C, and 738X, was determined in still air at 1000 and 1100 C. The oxidation process was evaluated by specific sample weight change with time, sample thickness change, X-ray diffraction of the scales, and sample metallography. The behavior is discussed in terms of the Cr, Al, and refractory metal contents of the alloys
Static friction on the fly: velocity depinning transitions of lubricants in motion
The dragging velocity of a model solid lubricant confined between sliding
periodic substrates exhibits a phase transition between two regimes,
respectively with quantized and with continuous lubricant center-of-mass
velocity. The transition, occurring for increasing external driving force F_ext
acting on the lubricant, displays a large hysteresis, and has the features of
depinning transitions in static friction, only taking place on the fly.
Although different in nature, this phenomenon appears isomorphic to a static
Aubry depinning transition in a Frenkel-Kontorova model, the role of particles
now taken by the moving kinks of the lubricant-substrate interface. We suggest
a possible realization in 2D optical lattice experiments.Comment: 5 pages, 4 figures, revtex, in print in Phys. Rev. Let
SiC(0001): a surface Mott-Hubbard insulator
We present ab-initio electronic structure calculations for the Si-terminated
SiC(0001) surface. While local density approximation
(LDA) calculations predict a metallic ground state with a half-filled narrow
band, Coulomb effects, included by the spin-polarized LDA+U method, result in a
magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the
experimental value of 2.0 eV. The calculated value of the inter-site exchange
parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except
at very low temperatures. The observed Si 2p surface core level doublet can
naturally be explained as an on-site exchange splitting.Comment: RevTex, 4 pages, 4 eps-figure
Optimization by Quantum Annealing: Lessons from Simple Cases
This paper investigates the basic behavior and performance of simulated
quantum annealing (QA) in comparison with classical annealing (CA). Three
simple one dimensional case study systems are considered, namely a parabolic
well, a double well, and a curved washboard. The time dependent Schr\"odinger
evolution in either real or imaginary time describing QA is contrasted with the
Fokker Planck evolution of CA. The asymptotic decrease of excess energy with
annealing time is studied in each case, and the reasons for differences are
examined and discussed. The Huse-Fisher classical power law of double well CA
is replaced with a different power law in QA. The multi-well washboard problem
studied in CA by Shinomoto and Kabashima and leading classically to a
logarithmic annealing even in the absence of disorder, turns to a power law
behavior when annealed with QA. The crucial role of disorder and localization
is briefly discussed.Comment: 16 pages, 9 figure
The effects of trace impurities in coal-derived liquid fuels on deposition and accelerated high temperature corrosion of cast superalloys
The effects of trace metal impurities in coal-derived liquids on deposition, high temperature corrosion and fouling were examined. Alloys were burner rig tested from 800 to 1100 C and corrosion was evaluated as a function of potential impurities. Actual and doped fuel test were used to define an empirical life prediction equation. An evaluation of inhibitors to reduce or eliminate accelerated corrosion was made. Barium and strontium were found to limit attack. Intermittent application of the inhibitors or silicon additions were found to be effective techniques for controlling deposition without losing the inhibitor benefits. A computer program was used to predict the dew points and compositions of deposits. These predictions were confirmed in deposition test. The potential for such deposits to plug cooling holes of turbine airfoils was evaluated. Tests indicated that, while a potential problem exists, it strongly depended on minor impurity variations
Adiabatic dynamics in a spin-1 chain with uniaxial single-spin anisotropy
We study the adiabatic quantum dynamics of an anisotropic spin-1 XY chain
across a second order quantum phase transition. The system is driven out of
equilibrium by performing a quench on the uniaxial single-spin anisotropy, that
is supposed to vary linearly in time. We show that, for sufficiently large
system sizes, the excess energy after the quench admits a non trivial scaling
behavior that is not predictable by standard Kibble-Zurek arguments for
isolated critical points or extended critical regions. This emerges from a
competing effect of many accessible low-lying excited states, inside the whole
continuous line of critical points.Comment: 17 pages, 8 figures, published versio
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