1,023 research outputs found
Si3N4 single-crystal nanowires grown from silicon micro and nanoparticles near the threshold of passive oxidation
A simple and most promising oxide-assisted catalyst-free method is used to
prepare silicon nitride nanowires that give rise to high yield in a short time.
After a brief analysis of the state of the art, we reveal the crucial role
played by the oxygen partial pressure: when oxygen partial pressure is slightly
below the threshold of passive oxidation, a high yield inhibiting the formation
of any silica layer covering the nanowires occurs and thanks to the synthesis
temperature one can control nanowire dimensions
Selfconsistent hybridization expansions for static properties of the Anderson impurity model
By means of a projector-operator formalism we derive an approximation based
on a self consistent hybridization expansion to study the ground state
properties of the Anderson Impurity model. We applied the approximation to the
general case of finite Coulomb repulsion , extending previous work with the
same formalism in the infinite- case. The treatment provides a very accurate
calculation of the ground state energy and their related zero temperature
properties in the case in which is large enough, but still finite, as
compared with the rest of energy scales involved in the model. The results for
the valence of the impurity are compared with exact results that we obtain from
equations derived using the Bethe ansatz and with a perturbative approach. The
magnetization and magnetic susceptibility is also compared with Bethe ansatz
results. In order to do this comparison, we also show how to regularize the
Bethe ansatz integral equations necessary to calculate the impurity valence,
for arbitrary values of the parameters.Comment: 8 pages, 5 figure
A non-perturbative analysis of symmetry breaking in two-dimensional phi^4 theory using periodic field methods
We describe the generalization of spherical field theory to other modal
expansion methods. The main approach remains the same, to reduce a
d-dimensional field theory into a set of coupled one-dimensional systems. The
method we discuss here uses an expansion with respect to periodic-box modes. We
apply the method to phi^4 theory in two dimensions and compute the critical
coupling and critical exponents. We compare with lattice results and
predictions via universality and the two-dimensional Ising model.Comment: 12 pages, 4 figures, version to appear in Physics Letters
Relaxation and derelaxation of pure and hydrogenated amorphous silicon during thermal annealing experiments
The structural relaxation of pure amorphous silicon (a-Si) and hydrogenated
amorphous silicon (a-Si:H) materials, that occurs during thermal annealing
experiments, has been analysed by Raman spectroscopy and differential scanning
calorimetry. Unlike a-Si, the heat evolved from a-Si:H cannot be explained by
relaxation of the Si-Si network strain, but it reveals a derelaxation of the
bond angle strain. Since the state of relaxation after annealing is very
similar for pure and hydrogenated materials, our results give strong
experimental support to the predicted configurational gap between a-Si and
crystalline silicon.Comment: 15 pages, 3 figures, 1 table to be published in Applied Physics
Letter
Applying the Hilbert--Huang Decomposition to Horizontal Light Propagation C_n^2 data
The Hilbert Huang Transform is a new technique for the analysis of
non--stationary signals. It comprises two distinct parts: Empirical Mode
Decomposition (EMD) and the Hilbert Transform of each of the modes found from
the first step to produce a Hilbert Spectrum. The EMD is an adaptive
decomposition of the data, which results in the extraction of Intrinsic Mode
Functions (IMFs). We discuss the application of the EMD to the calibration of
two optical scintillometers that have been used to measure C_n^2 over
horizontal paths on a building rooftop, and discuss the advantage of using the
Marginal Hilbert Spectrum over the traditional Fourier Power Spectrum.Comment: 9 pages, 11 figures, proc. SPIE 626
Initial state preparation with dynamically generated system-environment correlations
The dependence of the dynamics of open quantum systems upon initial
correlations between the system and environment is an utterly important yet
poorly understood subject. For technical convenience most prior studies assume
factorizable initial states where the system and its environments are
uncorrelated, but these conditions are not very realistic and give rise to
peculiar behaviors. One distinct feature is the rapid build up or a sudden jolt
of physical quantities immediately after the system is brought in contact with
its environments. The ultimate cause of this is an initial imbalance between
system-environment correlations and coupling. In this note we demonstrate
explicitly how to avoid these unphysical behaviors by proper adjustments of
correlations and/or the coupling, for setups of both theoretical and
experimental interest. We provide simple analytical results in terms of
quantities that appear in linear (as opposed to affine) master equations
derived for factorized initial states.Comment: 6 pages, 2 figure
A Brillouin torus decomposition for Chern insulators
Two-band Chern insulators are topologically classified by the Chern number,
, which is given by the integral of the Berry curvature of the occupied band
over the Brillouin torus. The curvature itself comes from the imaginary part of
a more basic object, the quantum geometric tensor, . On the other hand, the
integral over the Brillouin torus of the real part of gives rise to another
magnitude, the quantum volume, , that like also changes abruptly
when the system undergoes a topological phase transition. Recently, the
information about the topology of the system contained in the quantum volume
has been investigated. In this paper we present new results regarding the
underlying geometric structure of two-band Chern insulators. Since a generic
model describing the system can be characterized by a map, the classifying map,
from the Brillouin torus to the two-sphere, we study its properties at the
geometric level. We present a procedure for splitting the Brillouin torus into
different sectors in such a way that the classifying map when restricted to
each of them is an injective immersion. By doing so, we show that and
have a very rich inner structure, meaning that they are composed by
different contributions that behave differently at the topological phase
transition. In particular, some specific regions are found to be ones
responsible for the non vanishing value of the topological invariant . In
addition, the present work makes contact with, and clarifies, some
interpretations of the quantum volume in terms of the Euler characteristic
number that were done in the recent literature. We illustrate our findings by a
careful analysis of some selected models for Chern insulators corresponding to
tight-binding Hamiltonians
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