14,556 research outputs found
The Problem with the Linpack Benchmark Matrix Generator
We characterize the matrix sizes for which the Linpack Benchmark matrix
generator constructs a matrix with identical columns
Ab-initio Gutzwiller method: first application to Plutonium
Except for small molecules, it is impossible to solve many electrons systems
without imposing severe approximations. If the configuration interaction
approaches (CI) or Coupled Clusters techniques \cite{FuldeBook} are applicable
for molecules, their generalization for solids is difficult. For materials with
a kinetic energy greater than the Coulomb interaction, calculations based on
the density functional theory (DFT), associated with the local density
approximation (LDA) \cite{Hohenberg64, Kohn65} give satisfying qualitative and
quantitative results to describe ground state properties. These solids have
weakly correlated electrons presenting extended states, like materials or
covalent solids. The application of this approximation to systems where the
wave functions are more localized ( or -states) as transition metals
oxides, heavy fermions, rare earths or actinides is more questionable and can
even lead to unphysical results : for example, insulating FeO and CoO are
predicted to be metalic by the DFT-LDA..
Dynamics of Quantum Noise in a Tunnel Junction under ac Excitation
We report the first measurement of the \emph{dynamical response} of shot
noise (measured at frequency ) of a tunnel junction to an ac excitation
at frequency . The experiment is performed in the quantum regime,
at very low temperature T=35mK and high
frequency GHz. We observe that the noise responds in phase
with the excitation, but not adiabatically. The results are in very good
agreement with a prediction based on a new current-current correlator.Comment: Theory removed. More experimental details. One extra figur
Electronic transport in AlMn(Si) and AlCuFe quasicrystals: Break-down of the semiclassical model
The semi-classical Bloch-Boltzmann theory is at the heart of our
understanding of conduction in solids, ranging from metals to semi-conductors.
Physical systems that are beyond the range of applicability of this theory are
thus of fundamental interest. It appears that in quasicrystals and related
complex metallic alloys, a new type of break-down of this theory operates. This
phenomenon is related to the specific propagation of electrons. We develop a
theory of quantum transport that applies to a normal ballistic law but also to
these specific diffusion laws. As we show phenomenological models based on this
theory describe correctly the anomalous conductivity in quasicrystals.
Ab-initio calculations performed on approximants confirm also the validity of
this anomalous quantum diffusion scheme. This provides us with an ab-initio
model of transport in approximants such as alpha-AlMnSi and AlCuFe 1/1 cubic
approximant.Comment: 11 pages, 5 figure
Improving the modelling of redshift-space distortions - II. A pairwise velocity model covering large and small scales
We develop a model for the redshift-space correlation function, valid for
both dark matter particles and halos on scales Mpc. In its simplest
formulation, the model requires the knowledge of the first three moments of the
line-of-sight pairwise velocity distribution plus two well-defined
dimensionless parameters. The model is obtained by extending the
Gaussian-Gaussianity prescription for the velocity distribution, developed in a
previous paper, to a more general concept allowing for local skewness, which is
required to match simulations. We compare the model with the well known
Gaussian streaming model and the more recent Edgeworth streaming model. Using
N-body simulations as a reference, we show that our model gives a precise
description of the redshift-space clustering over a wider range of scales. We
do not discuss the theoretical prescription for the evaluation of the velocity
moments, leaving this topic to further investigation.Comment: 18 pages, 10 figures, published in MNRA
Constraining Dark Matter-Neutrino Interactions using the CMB and Large-Scale Structure
We present a new study on the elastic scattering cross section of dark matter
(DM) and neutrinos using the latest cosmological data from Planck and
large-scale structure experiments. We find that the strongest constraints are
set by the Lyman-alpha forest, giving sigma_{DM-neutrino} < 10^{-33} (m_DM/GeV)
cm^2 if the cross section is constant and a present-day value of
sigma_{DM-neutrino} < 10^{-45} (m_DM/GeV) cm^2 if it scales as the temperature
squared. These are the most robust limits on DM-neutrino interactions to date,
demonstrating that one can use the distribution of matter in the Universe to
probe dark ("invisible") interactions. Additionally, we show that scenarios
involving thermal MeV DM and a constant elastic scattering cross section
naturally predict (i) a cut-off in the matter power spectrum at the Lyman-alpha
scale, (ii) N_eff ~ 3.5 +/- 0.4, (iii) H_0 ~ 71 +/- 3 km/s/Mpc and (iv) the
possible generation of neutrino masses.Comment: 12 pages, 5 figure
Optimization of quantum Monte Carlo wave functions by energy minimization
We study three wave function optimization methods based on energy
minimization in a variational Monte Carlo framework: the Newton, linear and
perturbative methods. In the Newton method, the parameter variations are
calculated from the energy gradient and Hessian, using a reduced variance
statistical estimator for the latter. In the linear method, the parameter
variations are found by diagonalizing a non-symmetric estimator of the
Hamiltonian matrix in the space spanned by the wave function and its
derivatives with respect to the parameters, making use of a strong
zero-variance principle. In the less computationally expensive perturbative
method, the parameter variations are calculated by approximately solving the
generalized eigenvalue equation of the linear method by a nonorthogonal
perturbation theory. These general methods are illustrated here by the
optimization of wave functions consisting of a Jastrow factor multiplied by an
expansion in configuration state functions (CSFs) for the C molecule,
including both valence and core electrons in the calculation. The Newton and
linear methods are very efficient for the optimization of the Jastrow, CSF and
orbital parameters. The perturbative method is a good alternative for the
optimization of just the CSF and orbital parameters. Although the optimization
is performed at the variational Monte Carlo level, we observe for the C
molecule studied here, and for other systems we have studied, that as more
parameters in the trial wave functions are optimized, the diffusion Monte Carlo
total energy improves monotonically, implying that the nodal hypersurface also
improves monotonically.Comment: 18 pages, 8 figures, final versio
Zero-variance zero-bias quantum Monte Carlo estimators of the spherically and system-averaged pair density
We construct improved quantum Monte Carlo estimators for the spherically- and
system-averaged electron pair density (i.e. the probability density of finding
two electrons separated by a relative distance u), also known as the
spherically-averaged electron position intracule density I(u), using the
general zero-variance zero-bias principle for observables, introduced by
Assaraf and Caffarel. The calculation of I(u) is made vastly more efficient by
replacing the average of the local delta-function operator by the average of a
smooth non-local operator that has several orders of magnitude smaller
variance. These new estimators also reduce the systematic error (or bias) of
the intracule density due to the approximate trial wave function. Used in
combination with the optimization of an increasing number of parameters in
trial Jastrow-Slater wave functions, they allow one to obtain well converged
correlated intracule densities for atoms and molecules. These ideas can be
applied to calculating any pair-correlation function in classical or quantum
Monte Carlo calculations.Comment: 13 pages, 9 figures, published versio
- …