235 research outputs found
Trends in condensed matter physics: is research going faster and faster?
In this paper we study research trends in condensed matter physics. Trends
are analyzed by means of the the number of publications in the different
sub-fields as function of the years. We found that many research topics have a
similar behavior with an initial fast growth and a next slower exponential
decay. We derived a simple model to describe this behavior and built up some
predictions for future trends
Correlated geminal wave function for molecules: an efficient resonating valence bond approach
We show that a simple correlated wave function, obtained by applying a
Jastrow correlation term to an Antisymmetrized Geminal Power (AGP), based upon
singlet pairs between electrons, is particularly suited for describing the
electronic structure of molecules, yielding a large amount of the correlation
energy. The remarkable feature of this approach is that, in principle, several
Resonating Valence Bonds (RVB) can be dealt simultaneously with a single
determinant, at a computational cost growing with the number of electrons
similarly to more conventional methods, such as Hartree-Fock (HF) or Density
Functional Theory (DFT). Moreover we describe an extension of the Stochastic
Reconfiguration (SR) method, that was recently introduced for the energy
minimization of simple atomic wave functions. Within this extension the atomic
positions can be considered as further variational parameters, that can be
optimized together with the remaining ones. The method is applied to several
molecules from Li_2 to benzene by obtaining total energies, bond lengths and
binding energies comparable with much more demanding multi configuration
schemes.Comment: 20 pages, 5 figures, to be published in the Journal of Chemical
Physic
First-principles GW calculations for fullerenes, porphyrins, phtalocyanine, and other molecules of interest for organic photovoltaic applications
We evaluate the performances of ab initio GW calculations for the ionization
energies and HOMO-LUMO gaps of thirteen gas phase molecules of interest for
organic electronic and photovoltaic applications, including the C60 fullerene,
pentacene, free-base porphyrins and phtalocyanine, PTCDA, and standard monomers
such as thiophene, fluorene, benzothiazole or thiadiazole. Standard G0W0
calculations, that is starting from eigenstates obtained with local or
semilocal functionals, significantly improve the ionization energy and band gap
as compared to density functional theory Kohn-Sham results, but the calculated
quasiparticle values remain too small as a result of overscreening. Starting
from Hartree-Fock-like eigenvalues provides much better results and is
equivalent to performing self-consistency on the eigenvalues, with a resulting
accuracy of 2~4% as compared to experiment. Our calculations are based on an
efficient gaussian-basis implementation of GW with explicit treatment of the
dynamical screening through contour deformation techniques.Comment: 10 pages, 3 figure
Resonating Valence Bond wave function: from lattice models to realistic systems
Although mean field theories have been very successful to predict a wide
range of properties for solids, the discovery of high temperature
superconductivity in cuprates supported the idea that strongly correlated
materials cannot be qualitatively described by a mean field approach. After the
original proposal by Anderson, there is now a large amount of numerical
evidence that the simple but general resonating valence bond (RVB) wave
function contains just those ingredients missing in uncorrelated theories, so
that the main features of electron correlation can be captured by the
variational RVB approach. Strongly correlated antiferromagnetic (AFM) systems,
like Cs2CuCl4, displaying unconventional features of spin fractionalization,
are also understood within this variational scheme. From the computational
point of view the remarkable feature of this approach is that several
resonating valence bonds can be dealt simultaneously with a single determinant,
at a computational cost growing with the number of electrons similarly to more
conventional methods, such as Hartree-Fock or Density Functional Theory.
Recently several molecules have been studied by using the RVB wave function; we
have always obtained total energies, bonding lengths and binding energies
comparable with more demanding multi configurational methods, and in some cases
much better than single determinantal schemes. Here we present the paradigmatic
case of benzene.Comment: 14 pages, 4 figures. Proceedings of the Conference on Computational
Physics CCP2004. To appear in Computer Physics Communication
How strong is the Second Harmonic Generation in single-layer monochalcogenides? A response from first-principles real-time simulations
Second Harmonic Generation (SHG) of single-layer monochalcogenides, such as
GaSe and InSe, has been recently reported [2D Mater. 5 (2018) 025019; J. Am.
Chem. Soc. 2015, 137, 79947997] to be extremely strong with respect to bulk and
multilayer forms. To clarify the origin of this strong SHG signal, we perform
first-principles real-time simulations of linear and non-linear optical
properties of these two-dimensional semiconducting materials. The simulations,
based on ab-initio many-body theory, accurately treat the electron-hole
correlation and capture excitonic effects that are deemed important to
correctly predict the optical properties of such systems. We find indeed that,
as observed for other 2D systems, the SHG intensity is redistributed at
excitonic resonances. The obtained theoretical SHG intensity is an order of
magnitude smaller than that reported at the experimental level. This result is
in substantial agreement with previously published simulations which neglected
the electron-hole correlation, demonstrating that many-body interactions are
not at the origin of the strong SHG measured. We then show that the
experimental data can be reconciled with the theoretical prediction when a
single layer model, rather than a bulk one, is used to extract the SHG
coefficient from the experimental data.Comment: 8 pages, 4 figure
RVB phase of hydrogen at high pressure: towards the first ab-initio Molecular Dynamics by Quantum Monte Carlo
Optical properties of periodic systems within the current-current response framework: pitfalls and remedies
We compare the optical absorption of extended systems using the
density-density and current-current linear response functions calculated within
many-body perturbation theory. The two approaches are formally equivalent for a
finite momentum of the external perturbation. At
, however, the equivalence is maintained only if a small
expansion of the density-density response function is used. Moreover, in
practical calculations this equivalence can be lost if one naively extends the
strategies usually employed in the density-based approach to the current-based
approach. Specifically we discuss the use of a smearing parameter or of the
quasiparticle lifetimes to describe the finite width of the spectral peaks and
the inclusion of electron-hole interaction. In those instances we show that the
incorrect definition of the velocity operator and the violation of the
conductivity sum rule introduce unphysical features in the optical absorption
spectra of three paradigmatic systems: silicon (semiconductor), copper (metal)
and lithium fluoride (insulator). We then demonstrate how to correctly
introduce lifetime effects and electron-hole interactions within the
current-based approach.Comment: 17 pages, 6 figure
Exciton interference in hexagonal boron nitride
In this letter we report a thorough analysis of the exciton dispersion in
bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation
at finite , and we compare our results with
recent high-accuracy electron energy loss data. Simulations reproduce the
measured dispersion and the variation of the peak intensity. We focus on the
evolution of the intensity, and we demonstrate that the excitonic peak is
formed by the superposition of two groups of transitions that we call and
from the k-points involved in the transitions. These two groups
contribute to the peak intensity with opposite signs, each damping the
contributions of the other. The variations in number and amplitude of these
transitions determine the changes in intensity of the peak. Our results
contribute to the understanding of electronic excitations in this systems along
the direction, which is the relevant direction for spectroscopic
measurements. They also unveil the non-trivial relation between valley physics
and excitonic dispersion in h--BN, opening the possibility to tune excitonic
effects by playing with the interference between transitions. Furthermore, this
study introduces analysis tools and a methodology that are completely general.
They suggest a way to regroup independent-particle transitions which could
permit a deeper understanding of excitonic properties in any system
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