113 research outputs found
Fermionic Shadow Wavefunction Variational calculations of the vacancy formation energy in He
We present a novel technique well suited to study the ground state of
inhomogeneous fermionic matter in a wide range of different systems. The system
is described using a Fermionic Shadow wavefunction (FSWF) and the energy is
computed by means of the Variational Monte Carlo technique. The general form of
FSWF is useful to describe many--body systems with the coexistence of different
phases as well in the presence of defects or impurities, but it requires
overcoming a significant sign problem. As an application, we studied the energy
to activate vacancies in solid He.Comment: 4 pages, 2 figure
Variational Monte Carlo for spin-orbit interacting systems
Recently, a diffusion Monte Carlo algorithm was applied to the study of spin
dependent interactions in condensed matter. Following some of the ideas
presented therein, and applied to a Hamiltonian containing a Rashba-like
interaction, a general variational Monte Carlo approach is here introduced that
treats in an efficient and very accurate way the spin degrees of freedom in
atoms when spin orbit effects are included in the Hamiltonian describing the
electronic structure. We illustrate the algorithm on the evaluation of the
spin-orbit splittings of isolated carbon and lead atoms. In the case of the
carbon atom, we investigate the differences between the inclusion of spin-orbit
in its realistic and effective spherically symmetrized forms. The method
exhibits a very good accuracy in describing the small energy splittings,
opening the way for a systematic quantum Monte Carlo studies of spin-orbit
effects in atomic systems.Comment: 7 pages, 0 figure
Variational Monte Carlo study of the ground state properties and vacancy formation energy of solid para-H2 using a shadow wave function
A Shadow Wave Function (SWF) is employed along with Variational Monte Carlo
techniques to describe the ground state properties of solid molecular
para-hydrogen. The study has been extended to densities below the equilibrium
value, to obtain a parameterization of the SWF useful for the description of
inhomogeneous phases. We also present an estimate of the vacancy formation
energy as a function of the density, and discuss the importance of relaxation
effects near the vacant site
A tutorial review on time-frequency analysis of non-stationary vibration signals with nonlinear dynamics applications
Time-frequency analysis (TFA) for mechanical vibrations in non-stationary operations is the main subject of this article, concisely written to be an introducing tutorial comparing different time-frequency techniques for non-stationary signals. The theory was carefully exposed and complemented with sample applications on mechanical vibrations and nonlinear dynamics. A particular phenomenon that is also observed in non-stationary systems is the Sommerfeld effect, which occurs due to the interaction between a non-ideal energy source and a mechanical system. An application through TFA for the characterization of the Sommerfeld effect is presented. The techniques presented in this article are applied in synthetic and experimental signals of mechanical systems, but the techniques presented can also be used in the most diverse applications and also in the numerical solution of differential equation
Equation of state of low--density neutron matter and the pairing gap
We report results of the equation of state of neutron matter in the
low--density regime, where the Fermi wave vector ranges from . Neutron matter in this regime is superfluid because of
the strong and attractive interaction in the channel. The properties of
this superfluid matter are calculated starting from a realistic Hamiltonian
that contains modern two-- and three--body interactions. The ground state
energy and the superfluid energy gap are calculated using the Auxiliary
Field Diffusion Monte Carlo method. We study the structure of the ground state
by looking at pair distribution functions as well as the Cooper-pair wave
function used in the calculations.Comment: 12 pages, 7 figure
The Fermion Monte Carlo revisited
In this work we present a detailed study of the Fermion Monte Carlo algorithm
(FMC), a recently proposed stochastic method for calculating fermionic
ground-state energies [M.H. Kalos and F. Pederiva, Phys. Rev. Lett. vol. 85,
3547 (2000)]. A proof that the FMC method is an exact method is given. In this
work the stability of the method is related to the difference between the
lowest (bosonic-type) eigenvalue of the FMC diffusion operator and the exact
fermi energy. It is shown that within a FMC framework the lowest eigenvalue of
the new diffusion operator is no longer the bosonic ground-state eigenvalue as
in standard exact Diffusion Monte Carlo (DMC) schemes but a modified value
which is strictly greater. Accordingly, FMC can be viewed as an exact DMC
method built from a correlated diffusion process having a reduced Bose-Fermi
gap. As a consequence, the FMC method is more stable than any transient method
(or nodal release-type approaches). We illustrate the various ideas presented
in this work with calculations performed on a very simple model having only
nine states but a full sign problem. Already for this toy model it is clearly
seen that FMC calculations are inherently uncontrolled.Comment: 49 pages with 4 postscript figure
Suscetibilidade à corrosão por pites do alumínio por cloreto em meio de EDTA e álcool propargílico
06
Enhancing Qubit Readout with Autoencoders
In addition to the need for stable and precisely controllable qubits, quantum
computers take advantage of good readout schemes. Superconducting qubit states
can be inferred from the readout signal transmitted through a dispersively
coupled resonator. This work proposes a novel readout classification method for
superconducting qubits based on a neural network pre-trained with an
autoencoder approach. A neural network is pre-trained with qubit readout
signals as autoencoders in order to extract relevant features from the data
set. Afterwards, the pre-trained network inner layer values are used to perform
a classification of the inputs in a supervised manner. We demonstrate that this
method can enhance classification performance, particularly for short and long
time measurements where more traditional methods present lower performance.Comment: 16 pages, 23 figure
Study of solid 4He in two dimensions. The issue of zero-point defects and study of confined crystal
Defects are believed to play a fundamental role in the supersolid state of
4He. We report on studies by exact Quantum Monte Carlo (QMC) simulations at
zero temperature of the properties of solid 4He in presence of many vacancies,
up to 30 in two dimensions (2D). In all studied cases the crystalline order is
stable at least as long as the concentration of vacancies is below 2.5%. In the
2D system for a small number, n_v, of vacancies such defects can be identified
in the crystalline lattice and are strongly correlated with an attractive
interaction. On the contrary when n_v~10 vacancies in the relaxed system
disappear and in their place one finds dislocations and a revival of the
Bose-Einstein condensation. Thus, should zero-point motion defects be present
in solid 4He, such defects would be dislocations and not vacancies, at least in
2D. In order to avoid using periodic boundary conditions we have studied the
exact ground state of solid 4He confined in a circular region by an external
potential. We find that defects tend to be localized in an interfacial region
of width of about 15 A. Our computation allows to put as upper bound limit to
zero--point defects the concentration 0.003 in the 2D system close to melting
density.Comment: 17 pages, accepted for publication in J. Low Temp. Phys., Special
Issue on Supersolid
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