A. C. Fonseca - 1. Instituto de Física, Universidade de São Paulo, USP.

A. C. Fonseca
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A. C. Fonseca
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1. Instituto de Física, Universidade de São Paulo, USP.
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Nuclear Theory (32)
 
Nuclear Experiment (14)
 
Physics - Materials Science (7)
 
Quantum Physics (3)
 
Physics - Mesoscopic Systems and Quantum Hall Effect (3)
 
Physics - Atomic Physics (3)
 
Physics - Biological Physics (2)
 
Computer Science - Programming Languages (2)
 
Computer Science - Distributed; Parallel; and Cluster Computing (2)
 
Instrumentation and Methods for Astrophysics (1)
 
Physics - Soft Condensed Matter (1)
 
Physics - Classical Physics (1)
 
Mathematical Physics (1)
 
Physics - Computational Physics (1)
 
Cosmology and Nongalactic Astrophysics (1)
 
Mathematics - Mathematical Physics (1)

Publications Authored By A. C. Fonseca

Graphyne (GY) and graphdiyne (GDY) are two-dimensional one-atom-thick carbon allotropes highly considered to substitute graphene in electronic applications because of the prediction of non null band-gap. There are seven types of GY structures not yet fully investigated in literature. In this work, by means of classical molecular dynamics simulations, the Young's modulus, Poisson's ratio and linear thermal expansion coefficient (TEC) of all originally proposed seven types of GYs and corresponding GDYs are calculated. Read More

Recently, a new two-dimensional carbon allotrope called pentagraphene (PG) was proposed. PG exhibits mechanical and electronic interesting properties, including typical band gap values of semiconducting materials. PG has a Cairo-tiling-like 2D lattice of non coplanar pentagons and its mechanical properties have not been yet fully investigated. Read More

Nucleon transfer reactions in deuteron-deuteron collisions at energies above the three- and four-body breakup threshold are described using exact four-body equations for transition operators that are solved in the momentum-space framework. Differential cross sections, analyzing powers, polarizations, and spin transfer coefficients are obtained using realistic two-nucleon potentials and including the Coulomb repulsion between protons. Overall good agreement between predictions and experimental data is found. Read More

p-3H and n-3He scattering in the energy range above the n-3He but below the d-d thresholds is studied by solving the 4-nucleon problem with a realistic nucleon-nucleon interaction. Three different methods -- Alt, Grassberger and Sandhas, Hyperspherical Harmonics, and Faddeev-Yakubovsky -- have been employed and their results for both elastic and charge-exchange processes are compared. We observe a good agreement between the three different methods, thus the obtained results may serve as a benchmark. Read More

3D printing presents an attractive alternative to visual representation of physical datasets such as astronomical images that can be used for research, outreach or teaching purposes, and is especially relevant to people with a visual disability. We here report the use of 3D printing technology to produce a representation of the all-sky Cosmic Microwave Background (CMB) intensity anisotropy maps produced by the Planck mission. The success of this work in representing key features of the CMB is discussed as is the potential of this approach for representing other astrophysical data sets. Read More

One way to write fast programs is to explore the potential parallelism and take advantage of the high number of cores available in microprocessors. This can be achieved by manually specifying which code executes on which thread, by using compiler parallelization hints (such as OpenMP or Cilk), or by using a parallel programming language (such as X10, Chapel or Aeminium. Regardless of the approach, all of these programs are compiled to an intermediate lower-level language that is sequential, thus preventing the backend compiler from optimizing the program and observing its parallel nature. Read More

There are billions of lines of sequential code inside nowadays' software which do not benefit from the parallelism available in modern multicore architectures. Automatically parallelizing sequential code, to promote an efficient use of the available parallelism, has been a research goal for some time now. This work proposes a new approach for achieving such goal. Read More

We present results for the three-cluster breakup in deuteron-deuteron collisions at 130 and 270 MeV deuteron beam energy. The breakup amplitude is calculated using the first term in the Neumann series expansion of the corresponding exact four-nucleon equations. In analogy with nucleon-deuteron breakup where an equivalent approximation is compared with exact calculations, we expect this single-scattering approximation to provide a rough estimation of three-body breakup observables in quasifree configurations. Read More

Distributed data processing platforms such as MapReduce and Pregel have substantially simplified the design and deployment of certain classes of distributed graph analytics algorithms. However, these platforms do not represent a good match for distributed graph mining problems, as for example finding frequent subgraphs in a graph. Given an input graph, these problems require exploring a very large number of subgraphs and finding patterns that match some "interestingness" criteria desired by the user. Read More

Fully converged calculations of deuteron-deuteron elastic scattering observables are performed at energies above three- and four-body breakup threshold. Differential cross sections and analyzing powers are obtained using realistic nucleon-nucleon force models together with the Coulomb repulsion between protons. For all observables we find a very reasonable agreement with the available experimental data limited to deuteron beam energies up to 25. Read More

We report new measurements of the doubly-polarized photodisintegration of $^3$He at an incident photon energy of 16.5 MeV, carried out at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility located at Triangle Universities Nuclear Laboratory (TUNL). The spin-dependent double-differential cross sections and the contribution from the three--body channel to the Gerasimov-Drell-Hearn (GDH) integrand were extracted and compared with the state-of-the-art three--body calculations. Read More

Proton-${}^3$H elastic scattering and charge-exchange reaction ${}^3$H$(p,n){}^3$He in the energy regime above four-nucleon breakup threshold are described in the momentum-space transition operator framework. Fully converged results are obtained using realistic two-nucleon potentials and two-proton Coulomb force as dynamic input. Differential cross section, proton analyzing power, outgoing neutron polarization, and proton-to-neutron polarization transfer coefficients are calculated between 6 and 30 MeV proton beam energy. Read More

Deuteron-deuteron elastic scattering and transfer reactions in the energy regime above four-nucleon breakup threshold are described by solving exact four-particle equations for transition operators. Several realistic nuclear interaction models are used, including the one with effective many-nucleon forces generated by the explicit $\Delta$-isobar excitation; the Coulomb force between protons is taken into account as well. Differential cross sections, deuteron analyzing powers, outgoing nucleon polarization, and deuteron-to-neutron polarization transfer coefficients are calculated at 10 MeV deuteron energy. Read More

Microscopic calculations of four-body collisions become very challenging in the energy regime above the threshold for four free particles. The neutron-${}^3$He scattering is an example of such process with elastic, rearrangement, and breakup channels. We aim to calculate observables for elastic and inelastic neutron-${}^3$He reactions up to 30 MeV neutron energy using realistic nuclear force models. Read More

Exact four-body equations of Alt, Grassberger and Sandhas are solved for neutron-${}^3\mathrm{He}$ and proton-${}^3\mathrm{H}$ scattering in the energy regime above the four-nucleon breakup threshold. Cross sections and spin observables for elastic, transfer, charge-exchange, and breakup reactions are calculated using realistic nucleon-nucleon interaction models, including the one with effective many-nucleon forces due to explicit $\Delta$-isobar excitation. The experimental data are described reasonably well with only few exceptions such as vector analyzing powers. Read More

Recently, it was proposed based on classical elasticity theory and experiments at macroscale, that the conformations of sheets inside cylindrical tubes present a universal behavior. A natural question is whether this behavior still holds at nanoscale. Based on molecular dynamics simulations and analytical modeling for graphene and boron nitride membranes confined inside carbon nanotubes, we show that the class of universality observed at macroscale is violated at nanoscale. Read More

We present a new analysis of the connection between the classical conservation theorems and the role played by the Dirac matrices in order to obtain a four spinor version of the Dirac equation for the two electrons bound problem. The separation of the set of equations in its angular and radial parts is done and an asymptotic solution for the radial system of equations is discussed for the ground state of the atom. Read More

Solution of the scattering problem turns to be very difficult task both from the formal as well as from the computational point of view. If the last two decades have witnessed decisive progress in ab initio bound state calculations, rigorous solution of the scattering problem remains limited to A$\leq$4 case. Therefore there is a rising interest to apply bound-state-like methods to handle non-relativistic scattering problems. Read More

Background: Theoretical calculations of the four-particle scattering above the four-cluster breakup threshold are technically very difficult due to nontrivial singularities or boundary conditions. Further complications arise when the long-range Coulomb force is present. Purpose: We aim at calculating proton-${}^3$He elastic scattering observables above three- and four-cluster breakup threshold. Read More

The first measurement of the three-body photodisintegration of longitudinally-polarized ^3He with a circularly-polarized \gamma-ray beam was carried out at the High Intensity \gamma-ray Source (HI\gamma S) facility located at Triangle Universities Nuclear Laboratory (TUNL). The spin-dependent double-differential cross sections and the contributions from the three-body photodisintegration to the ^3He GDH integrand are presented and compared with state-of-the-art three-body calculations at the incident photon energies of 12.8 and 14. Read More

We study the recombination of two neutrons and deuteron into neutron and ${}^3$H using realistic nucleon-nucleon potential models. Exact Alt, Grassberger, and Sandhas equations for the four-nucleon transition operators are solved in the momentum-space framework using the complex-energy method with special integration weights. We find that at astrophysical or laboratory neutron densities the production of ${}^3$H via the neutron-neutron-deuteron recombination is much slower as compared to the radiative neutron-deuteron capture. Read More

The four-body equations of Alt, Grassberger and Sandhas are solved for the neutron-${}^3$H scattering at energies above the four-nucleon breakup threshold. The accuracy and practical applicability of the employed complex energy method is significantly improved by the use of integration with the special weights. This allows to obtain fully converged results with realistic nuclear interactions. Read More

In this lecture we aim to present a formalism based on Faddeev-like equations for describing nuclear three-cluster reactions that include elastic, transfer and breakup channels. Two different techniques based on momentum-space and configuration-space representations are explained in detail. An important new feature of these methods is the possibility to account for the repulsive Coulomb interaction between two of the three clusters in all channels. Read More

The n-3H and p-3He elastic phase-shifts below the trinucleon disintegration thresholds are calculated by solving the 4-nucleon problem with three different realistic nucleon-nucleon interactions (the I-N3LO model by Entem and Machleidt, the Argonne v18 potential model, and a low-k model derived from the CD-Bonn potential). Three different methods -- Alt, Grassberger and Sandhas, Hyperspherical Harmonics, and Faddeev-Yakubovsky -- have been used and their respective results are compared. For both n-3H and p-3He we observe a rather good agreement between the three different theoretical methods. Read More

The thermal conductivity (TC) of isolated graphene with different concentrations of isotopes (C13) is studied with equilibrium molecular dynamics method at 300K. In the limit of pure C12 or C13 graphene, TC of graphene in zigzag and armchair directions are ~630 W/mK and ~1000W/mK, respectively. We find that the TC of graphene can be maximally reduced by ~80%, in both armchair and zigzag directions, when a random distribution of C12 and C13 is assumed at different doping concentrations. Read More

Nucleon transfer reactions in low-energy deuteron-deuteron scattering are described by solving exact four-particle equations in momentum space. The Coulomb interaction between the protons is included using the screening and renormalization method. Various realistic potentials are used between nucleon pairs. Read More

Proton-deuteron radiative capture and two- and three-body photodisintegration of 3He at low energy are described using realistic hadronic dynamics and including the Coulomb force. The sensitivity of the observables to the relativistic corrections of one-nucleon electromagnetic current operator is studied. Significant effects of the relativistic spin-orbit charge are found for the vector analyzing powers in the proton-deuteron radiative capture and for the beam-target parallel-antiparallel spin asymmetry in the three-body photodisintegration of 3He. Read More

We show that using parameters consistent with the charge symmetry violating difference between the strong nn and pp scattering lengths provides significant constraints on the amplitude for the dd -> alpha pi0 reaction. Read More

Momentum space three-body Faddeev-like equations are used to calculate elastic, transfer and charge exchange reactions resulting from the scattering of deuterons on 12C and 16O or protons on 13C and 17O; 12C and 16O are treated as inert cores. All possible reactions are calculated in the framework of the same model space. Comparison with previous calculations based on approximate methods used in nuclear reaction theory is discussed. Read More

Full Faddeev-type calculations are performed for one-neutron knockout reaction of $^{14}$Be on proton target at 69 MeV/u incident energy. Inclusive transverse momentum distributions for the outgoing ($^{12}$Be + n) system and semi-inclusive cross sections are presented. A significant proton-core single scattering contribution emerges where the valence neutron has non zero angular momentum relative to the core. Read More

Low momentum two-nucleon interactions obtained with the renormalization group method and the similarity renormalization group method are used to study the cutoff dependence of low energy 3N and 4N scattering observables. The residual cutoff dependence arises from omitted short-ranged 3N (and higher) forces that are induced by the renormalization group transformations, and may help to estimate the sensitivity of various 3N and 4N scattering observables to short-ranged many-body forces. Read More

Full Faddeev-type calculations are performed for $^{11}$Be breakup on proton target at 38.4, 100, and 200 MeV/u incident energies. The convergence of the multiple scattering expansion is investigated. Read More

The four-nucleon bound state and scattering below three-body breakup threshold are described based on the realistic coupled-channel potential CD Bonn + $\Delta$ which allows the excitation of a single nucleon to a $\Delta$ isobar. The Coulomb repulsion between protons is included. In the four-nucleon system the two-baryon coupled-channel potential yields effective two-, three- and four-nucleon forces, mediated by the $\Delta$ isobar and consistent with each other and with the underlying two-nucleon force. Read More

Two-body scattering is studied by solving the Lippmann-Schwinger equation in momentum space without angular-momentum decomposition for a local spin dependent short range interaction plus Coulomb. The screening and renormalization approach is employed to treat the Coulomb interaction. Benchmark calculations are performed by comparing our procedure with partial-wave calculations in configuration space for p-10Be, p-16O and 12C-10Be elastic scattering, using a simple optical potential model. Read More

The continuum discretized coupled channels (CDCC) method is compared to the exact solution of the three-body Faddeev equations in momentum space. We present results for: i) elastic and breakup observables of d-12C at E_d=56 MeV, ii) elastic scattering of d-58Ni at E_d=80 MeV, and iii) elastic, breakup and transfer observables for 11Be+p at E_{11Be}/A=38.4 MeV. Read More

Helical or coiled nanostructures have been object of intense experimental and theoretical studies due to their special electronic and mechanical properties. Recently, it was experimentally reported that the dynamical response of foamlike forest of coiled carbon nanotubes under mechanical impact exhibits a nonlinear, non-Hertzian behavior, with no trace of plastic deformation. The physical origin of this unusual behavior is not yet fully understood. Read More

The four-body equations of Alt, Grassberger and Sandhas are solved, for the first time, for proton-${}^3\mathrm{He}$ scattering including the Coulomb interaction between the three protons using the method of screening and renormalization as it was done recently for proton-deuteron scattering. Various realistic two-nucleon potentials are used. Large Coulomb effects are seen on all observables. Read More

Four-body equations in momentum space are solved for neutron-$\He$, proton-$\Hh$, and deuteron-deuteron scattering; all three reactions are coupled. The Coulomb interaction between the protons is included using the screening and renormalization approach as it was recently done for proton-deuteron and proton-$\He$ scattering. Realistic interactions are used between nucleon pairs. Read More

The Coulomb interaction between the two protons is included in the calculation of three-nucleon hadronic and electromagnetic reactions using screening and renormalization approach. Calculations are done using integral equations in momentum space. The reliability of the method is demonstrated. Read More

The four-body equations of Alt, Grassberger and Sandhas are solved for $\nH$ scattering at energies below three-body breakup threshold using various realistic interactions including one derived from chiral perturbation theory. After partial wave decomposition the equations are three-variable integral equations that are solved numerically without any approximations beyond the usual discretization of continuum variables on a finite momentum mesh. Large number of two-, three- and four-nucleon partial waves are considered until the convergence of the observables is obtained. Read More

The AGS equations are solved for $n{}^3\mathrm{H}$ and $p{}^3\mathrm{He}$ scattering including the Coulomb interaction. Comparison with previous work confirms the accuracy of the calculation and helps clarify a number of issues related to the $n{}^3\mathrm{H}$ total cross section at the peak of the resonance region, as well as an $A_y$ deficiency in $p{}^3\mathrm{He}$. Calculations are fully converged in terms of $NN$ partial waves and involve no uncontrolled approximations. Read More

Real filaments are not perfectly homogeneous. Most of them have various materials composition and shapes making their stiffnesses not constant along the arclength. We investigate the existence of circular and helical equilibrium solutions of an intrinsically straight rod with varying bending and twisting stiffnesses, within the framework of the Kirchhoff model. Read More

Helical amorphous nanosprings have attracted particular interest due to their special mechanical properties. In this work we present a simple model, within the framework of the Kirchhoff rod model, for investigating the structural properties of nanosprings having asymmetric cross section. We have derived expressions that can be used to obtain the Young's modulus and Poisson's ratio of the nanospring material composite. Read More

High precision cross-section data of the deuteron-proton breakup reaction at 130 MeV deuteron energy are compared with the theoretical predictions obtained with a coupled-channel extension of the CD Bonn potential with virtual Delta-isobar excitation, without and with inclusion of the long-range Coulomb force. The Coulomb effect is studied on the basis of the cross-section data set, extended in this work to about 1500 data points by including breakup geometries characterized by small polar angles of the two protons. The experimental data clearly prefer predictions obtained with the Coulomb interaction included. Read More

The Coulomb interaction between the protons is included in the description of proton-deuteron scattering using the screening and renormalization approach in the framework of momentum-space integral equations. Two new calculational schemes are presented that confirm the reliability of the perturbative approach for treating the screened Coulomb interaction in high partial waves, used by us in earlier works. Read More

We use realistic two- and three-nucleon interactions in a hybrid chiral-perturbation-theory calculation of the charge-symmetry-breaking reaction $dd\to\alpha\pi^0$ to show that a cross section of the experimentally measured size can be obtained using LO and NNLO pion-production operators. This result supports the validity of our power counting scheme and demonstrates the necessity of using an accurate treatment of ISI and FSI. Read More

The Coulomb interaction between the two protons is included in the calculation of proton-deuteron breakup and of three-body electromagnetic disintegration of ${}^3\mathrm{He}$. The hadron dynamics is based on the purely nucleonic charge-dependent (CD) Bonn potential and its realistic extension CD Bonn + $\Delta$ to a coupled-channel two-baryon potential, allowing for single virtual $\Delta$-isobar excitation. Calculations are done using integral equations in momentum space. Read More

Nanosprings have been object of intense investigations in recent years. They can be classified as normal or binormal depending on the geometry of their cross-section. Normal amorphous nanosprings have not been observed experimentally up to now, and only recently the synthesis of a crystalline ZnO normal nanohelix has been reported. Read More

We present a model to study Young's modulus and Poisson's ratio of the composite material of amorphous nanowires. It is an extension of the model derived by two of us [Da Fonseca and Galvao, Phys. Rev. Read More

Helical configurations of inhomogeneous symmetric rods with non-constant bending and twisting stiffness are studied within the framework of the Kirchhoff rod model. From the static Kirchhoff equations, we obtain a set of differential equations for the curvature and torsion of the centerline of the rod and the Lancret's theorem is used to find helical solutions. We obtain a free standing helical solution for an inhomogeneous rod whose curvature and torsion depend on the form of variation of the bending coefficient along the rod. Read More