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Gross F.,College of William and Mary | Gross F.,Jefferson Lab | Stadler A.,University of Lisbon | Stadler A.,University Of Ivora
Physical Review C - Nuclear Physics | Year: 2010

We present the effective range expansions for the 1S0 and 3S1 scattering phase shifts, and the relativistic deuteron wave functions that accompany our recent high precision fits (with χ2/N data1) to the 2007 world np data below 350 MeV. The wave functions are expanded in a series of analytical functions (with the correct asymptotic behavior at both large and small arguments) that can be Fourier-transformed from momentum to coordinate space and are convenient to use in any application. A fortran subroutine to compute these wave functions can be obtained from the authors. © 2010 The American Physical Society.

Carmelo J.M.P.,University of Minho | Carmelo J.M.P.,University of Stuttgart | Araujo M.A.N.,University Of Ivora | Araujo M.A.N.,University of Lisbon | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The spectral-weight distribution in recent neutron scattering experiments on the parent compound La 2CuO 4 (LCO), which are limited in energy range to about 450 meV, is studied in the framework of the Hubbard model on the square lattice with effective nearest-neighbor transfer integral t and on-site repulsion U. Our study combines a number of numerical and theoretical approaches, including, in addition to standard treatments, density matrix renormalization group calculations for Hubbard cylinders and a suitable spinon approach for the spin excitations. The latter spin-12 spinons are the spins of the rotated electrons that singly occupy sites. These rotated electrons are mapped from the electrons by a uniquely defined unitary transformation, in which rotated-electron single and double occupancy are good quantum numbers for finite interaction values. Our results confirm that the U/8t magnitude suitable to LCO corresponds to intermediate U values smaller than the bandwidth 8t, which we estimate to be 8t2.36 eV for U/8t0.76. This confirms the unsuitability of the conventional linear spin-wave theory. Our theoretical studies provide evidence for the occurrence of ground-state d-wave spinon pairing in the half-filled Hubbard model on the square lattice. This pairing applies only to the rotated-electron spin degrees of freedom, but it could play a role in a possible electron d-wave pairing formation upon hole doping. We find that the higher-energy spin spectral weight extends to about 566 meV and is located at and near the momentum [π,π]. The continuum weight energy-integrated intensity vanishes or is extremely small at momentum [π,0]. This behavior of this intensity is consistent with that of the spin waves observed in recent high-energy neutron scattering experiments, which are damped at the momentum [π,0]. We suggest that future LCO neutron scattering experiments scan the energies between 450 and 566 meV and momenta around [π,π]. © 2012 American Physical Society.

Ramalho G.,University of Lisbon | Pena M.T.,University of Lisbon | Stadler A.,University Of Ivora | Stadler A.,University of Lisbon
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

Using a covariant spectator quark model that describes the recent lattice QCD data for the Δ electromagnetic form factors and all available experimental data on γN→Δ transitions, we analyze the charge and magnetic dipole distributions of the Δ baryon and discuss its shape. We conclude that the quadrupole moment of the Δ is a good indicator of the deformation and that the Δ + charge distribution has an oblate shape. We also calculate transverse moments and find that they do not lead to unambiguous conclusions about the underlying shape. © 2012 American Physical Society.

Lopes I.,University Of Ivora | Lopes I.,University of Lisbon | Casanellas J.,University of Lisbon | Eugenio D.,University of Lisbon
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We studied the rate at which stars capture dark matter (DM) particles, considering different assumptions regarding the DM characteristics and, in particular, investigating how the stellar physics influences the capture rate. Two scenarios were considered: first, we assumed the maximal values for the spin-dependent and spin-independent DM particle-nucleon scattering cross sections allowed by the limits from direct detection experiments. Second, we considered that both scattering cross sections are of the same order, with the aim of studying the dependencies of the capture rate on stellar elements other than hydrogen. We found that the characteristics of the capture rate are very different in the two scenarios. Furthermore, we quantified the uncertainties on the computed capture rate (Cχ) and on the ratio between the luminosities from DM annihilations and thermonuclear reactions (L χ/Lnuc) derived from an imprecise knowledge of the stellar structure and DM parameters. For instance, while an uncertainty of 10% on the typical DM velocity leads to similar errors on the computed C χ and Lχ/Lnuc, the same uncertainty on the stellar mass becomes more relevant and duplicates the errors. Our results may be used to evaluate the reliability of the computed capture rate for the hypothetical use of stars other than the Sun as DM probes. © 2011 American Physical Society.

Rodriguez J.P.,California State University, Los Angeles | Araujo M.A.N.,University Of Ivora | Araujo M.A.N.,University of Lisbon | Sacramento P.D.,University of Lisbon
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

A two-orbital t-J model over the square lattice that describes low-energy electronic excitations in iron-pnictide high-T c superconductors is analyzed with Schwinger-boson-slave-fermion mean-field theory and by exact numerical diagonalization on a finite system. When interorbital hole hopping is suppressed, a quantum critical point (QCP) is identified that separates a commensurate spin-density wave (cSDW) state at strong Hund's rule coupling from a hidden half metal state at weak Hund's rule coupling. Low-energy spin waves that disperse anisotropically from cSDW momenta are predicted at the QCP. Nested Fermi surfaces similar to those observed experimentally in iron-pnictide materials are also predicted in such case. © 2011 American Physical Society.

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