7,758 research outputs found
Orientation dependence of the elastic instability on strained SiGe films
At low strain, SiGe films on Si substrates undergo a continuous
nucleationless morphological evolution known as the Asaro-Tiller-Grinfeld
instability. We demonstrate experimentally that this instability develops on
Si(001) but not on Si(111) even after long annealing. Using a continuum
description of this instability, we determine the origin of this difference.
When modeling surface diffusion in presence of wetting, elasticity and surface
energy anisotropy, we find a retardation of the instability on Si(111) due to a
strong dependence of the instability onset as function of the surface
stiffness. This retardation is at the origin of the inhibition of the
instability on experimental time scales even after long annealing.Comment: 3 pages, 4 figure
Short to long-range charge-transfer excitations in the zincbacteriochlorin-bacteriochlorin complex: a Bethe-Salpeter study
We study using the Bethe-Salpeter formalism the excitation energies of the
zincbacteriochlorinbacteriochlorin dyad, a paradigmatic photosynthetic complex.
In great contrast with standard timedependent density functional theory
calculations with (semi)local kernels, charge transfer excitations are
correctly located above the intramolecular Q-bands transitions found to be in
excellent agreement with experiment. Further, the asymptotic Coulomb behavior
towards the true quasiparticle gap for charge transfer excitations at long
distance is correctly reproduced, showing that the present scheme allows to
study with the same accuracy intramolecular and charge transfer excitations at
various spatial range and screening environment without any adjustable
parameter.Comment: 5 pages, 2 figures, 1 tabl
Universal decay of scalar turbulence
The asymptotic decay of passive scalar fields is solved analytically for the
Kraichnan model, where the velocity has a short correlation time. At long
times, two universality classes are found, both characterized by a distribution
of the scalar -- generally non-Gaussian -- with global self-similar evolution
in time. Analogous behavior is found numerically with a more realistic flow
resulting from an inverse energy cascade.Comment: 4 pages, 3 Postscript figures, submitted to PR
A practical guide to density matrix embedding theory in quantum chemistry
Density matrix embedding theory (DMET) provides a theoretical framework to
treat finite fragments in the presence of a surrounding molecular or bulk
environment, even when there is significant correlation or entanglement between
the two. In this work, we give a practically oriented and explicit description
of the numerical and theoretical formulation of DMET. We also describe in
detail how to perform self-consistent DMET optimizations. We explore different
embedding strategies with and without a self-consistency condition in hydrogen
rings, beryllium rings, and a sample S2 reaction. The source code
for the calculations in this work can be obtained from
\url{https://github.com/sebwouters/qc-dmet}.Comment: 41 pages, 10 figure
On The Complexity and Completeness of Static Constraints for Breaking Row and Column Symmetry
We consider a common type of symmetry where we have a matrix of decision
variables with interchangeable rows and columns. A simple and efficient method
to deal with such row and column symmetry is to post symmetry breaking
constraints like DOUBLELEX and SNAKELEX. We provide a number of positive and
negative results on posting such symmetry breaking constraints. On the positive
side, we prove that we can compute in polynomial time a unique representative
of an equivalence class in a matrix model with row and column symmetry if the
number of rows (or of columns) is bounded and in a number of other special
cases. On the negative side, we show that whilst DOUBLELEX and SNAKELEX are
often effective in practice, they can leave a large number of symmetric
solutions in the worst case. In addition, we prove that propagating DOUBLELEX
completely is NP-hard. Finally we consider how to break row, column and value
symmetry, correcting a result in the literature about the safeness of combining
different symmetry breaking constraints. We end with the first experimental
study on how much symmetry is left by DOUBLELEX and SNAKELEX on some benchmark
problems.Comment: To appear in the Proceedings of the 16th International Conference on
Principles and Practice of Constraint Programming (CP 2010
Statistical conservation laws in turbulent transport
We address the statistical theory of fields that are transported by a
turbulent velocity field, both in forced and in unforced (decaying)
experiments. We propose that with very few provisos on the transporting
velocity field, correlation functions of the transported field in the forced
case are dominated by statistically preserved structures. In decaying
experiments (without forcing the transported fields) we identify infinitely
many statistical constants of the motion, which are obtained by projecting the
decaying correlation functions on the statistically preserved functions. We
exemplify these ideas and provide numerical evidence using a simple model of
turbulent transport. This example is chosen for its lack of Lagrangian
structure, to stress the generality of the ideas
The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within about
30 pc of the Sun is determined from optical and ultraviolet absorption line
data, after omitting from the sample stars with circumstellar disks or variable
emission lines and the active variable HR 1099. Ninety-six velocity components
towards the remaining 60 stars yield a streaming velocity through the local
standard of rest of -17.0+/-4.6 km/s, with an upstream direction of l=2.3 deg,
b=-5.2 deg (using Hipparcos values for the solar apex motion). The velocity
dispersion of the interstellar matter (ISM) within 30 pc is consistent with
that of nearby diffuse clouds, but present statistics are inadequate to
distinguish between a Gaussian or exponential distribution about the bulk flow
velocity. The upstream direction of the bulk flow vector suggests an origin
associated with the Loop I supernova remnant. Groupings of component velocities
by region are seen, indicating regional departures from the bulk flow velocity
or possibly separate clouds. The absorption components from the cloudlet
feeding ISM into the solar system form one of the regional features. The
nominal gradient between the velocities of upstream and downstream gas may be
an artifact of the Sun's location near the edge of the local cloud complex. The
Sun may emerge from the surrounding gas-patch within several thousand years.Comment: Typographical errors corrected; Five tables, seven figures;
Astrophysical Journal, in pres
Emergence of chaotic scattering in ultracold Er and Dy
We show that for ultracold magnetic lanthanide atoms chaotic scattering
emerges due to a combination of anisotropic interaction potentials and Zeeman
coupling under an external magnetic field. This scattering is studied in a
collaborative experimental and theoretical effort for both dysprosium and
erbium. We present extensive atom-loss measurements of their dense magnetic
Feshbach resonance spectra, analyze their statistical properties, and compare
to predictions from a random-matrix-theory inspired model. Furthermore,
theoretical coupled-channels simulations of the anisotropic molecular
Hamiltonian at zero magnetic field show that weakly-bound, near threshold
diatomic levels form overlapping, uncoupled chaotic series that when combined
are randomly distributed. The Zeeman interaction shifts and couples these
levels, leading to a Feshbach spectrum of zero-energy bound states with
nearest-neighbor spacings that changes from randomly to chaotically distributed
for increasing magnetic field. Finally, we show that the extreme temperature
sensitivity of a small, but sizeable fraction of the resonances in the Dy and
Er atom-loss spectra is due to resonant non-zero partial-wave collisions. Our
threshold analysis for these resonances indicates a large collision-energy
dependence of the three-body recombination rate
Multiscaling in passive scalar advection as stochastic shape dynamics
The Kraichnan rapid advection model is recast as the stochastic dynamics of
tracer trajectories. This framework replaces the random fields with a small set
of stochastic ordinary differential equations. Multiscaling of correlation
functions arises naturally as a consequence of the geometry described by the
evolution of N trajectories. Scaling exponents and scaling structures are
interpreted as excited states of the evolution operator. The trajectories
become nearly deterministic in high dimensions allowing for perturbation theory
in this limit. We calculate perturbatively the anomalous exponent of the third
and fourth order correlation functions. The fourth order result agrees with
previous calculations.Comment: 14 pages, LaTe
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