CSIC - Institute for the Structure of Matter

Madrid, Spain

CSIC - Institute for the Structure of Matter

Madrid, Spain
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Sarriguren P.,CSIC - Institute for the Structure of Matter
Physical Review C - Nuclear Physics | Year: 2013

Electron-capture rates at different density and temperature conditions are evaluated for a set of pf-shell nuclei representative of the constituents in presupernova formations. The nuclear structure part of the problem is described within a quasiparticle random-phase approximation based on a deformed Skyrme Hartree-Fock self-consistent mean field with pairing correlations and residual interactions in particle-hole and particle-particle channels. The energy distributions of the Gamow-Teller strength are evaluated and compared to benchmark shell-model calculations and experimental data extracted from charge-exchange reactions. The model dependence of the weak rates are discussed and the various sensitivities to both density and temperature are analyzed. © 2013 American Physical Society.


Gonzalez J.,CSIC - Institute for the Structure of Matter
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We investigate the many-body instabilities of electrons interacting near Van Hove singularities arising in monolayer and twisted bilayer graphene. We show that a pairing instability must be dominant over the tendency to magnetic order as the Fermi level is tuned to the Van Hove singularity in the conduction band of graphene. As a result of the extended character of the saddle points in the dispersion, we find that the pairing of the electrons takes place preferentially in a channel of f-wave symmetry, with an order parameter vanishing at the position of the saddle points along the Fermi line. In the case of the twisted bilayers, the dispersion has instead its symmetry reduced down to the C3v group and, most importantly, it leads to susceptibilities that diverge at the saddle points but are integrable along the Fermi line. This implies that a ferromagnetic instability becomes dominant in the twisted graphene bilayers near the Van Hove singularity, with a strength which is amplified as the lowest subband of the electron system becomes flatter for decreasing twist angle. © 2013 American Physical Society.


Gonzalez J.,CSIC - Institute for the Structure of Matter
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We investigate the dynamical breakdown of the chiral symmetry in the theory of Dirac fermions in graphene with long-range Coulomb interaction. We analyze the electron-hole vertex relevant for the dynamical gap generation in the ladder approximation, showing that it blows up at a critical value α c in the graphene fine structure constant, which is quite sensitive to many-body corrections. Under static random phase approximation (RPA) screening of the interaction potential, we find that taking into account electron self-energy corrections to the vertex increases the critical coupling to α c 4.9, for a number N=4 of two-component Dirac fermions. When dynamical screening of the interaction is instead considered, the effect of Fermi velocity renormalization in the electron and hole states leads to the value α c1.75 for N=4, substantially larger than that obtained without electron self-energy corrections (-0.99), but still below the nominal value of the interaction coupling in isolated free-standing graphene. © 2012 American Physical Society.


Gonzalez J.,CSIC - Institute for the Structure of Matter
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We investigate the development of a gapped phase in the field theory of Dirac fermions in graphene with long-range Coulomb interaction. In the large- N approximation, we show that the chiral symmetry is only broken below a critical number of two-component Dirac fermions Nc =32/ π2, that is exactly half the value found in quantum electrodynamics in 2+1 dimensions. Adopting otherwise a ladder approximation, we give evidence of the existence of a critical coupling at which the anomalous dimension of the order parameter of the transition diverges. This result is consistent with the observation that chiral symmetry breaking may be driven by the long-range Coulomb interaction in the Dirac field theory, despite the divergent scaling of the Fermi velocity in the low-energy limit. © 2010 The American Physical Society.


Sarriguren P.,CSIC - Institute for the Structure of Matter
Physical Review C - Nuclear Physics | Year: 2012

A theoretical approach based on a deformed quasiparticle random phase approximation built on a Skyrme self-consistent mean field is used to describe the recent measurements of the Gamow-Teller GT - strength distribution extracted from the charge-exchange reaction 76Ge(3He ,t )76As with high energy resolution. The same analysis is made to describe the Gamow-Teller GT + strength distribution measured in the 76Se(d ,2He)76As reaction. Combining these two branches, the nuclear matrix element for the two-neutrino double-β decay process is evaluated and compared to experiment. The role of the nuclear deformation on those processes is emphasized and analyzed. © 2012 American Physical Society.


Ortigoso J.,CSIC - Institute for the Structure of Matter
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2012

A consensus that questions the perfunctory use of the quantum adiabatic theorem has emerged since Marzlin and Sanders showed the existence of an inconsistency in the applicability of the theorem. Further analysis proved that the inconsistency may arise from the existence of resonant terms in the Hamiltonian, but recent work indicates that the debate about the full extent of the problem remains open. Here, we first show that key premises required in the standard demonstration of the theorem do not hold for a dual Hamiltonian involved in the Marzlin-Sanders inconsistency. Also, we show that two simple conditions can identify systems for which the adiabatic approximation fails, in spite of satisfying traditional quantitative conditions that were believed to guarantee its validity. Finally, we prove that the inconsistency only arises for Hamiltonians that contain resonant terms whose amplitudes go asymptotically to zero. © 2012 American Physical Society.


Calcagni G.,CSIC - Institute for the Structure of Matter
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute. © 2014 American Physical Society.


Sarriguren P.,CSIC - Institute for the Structure of Matter
Physical Review C - Nuclear Physics | Year: 2015

β-decay properties of even- and odd-A neutron-rich Ge, Se, Kr, Sr, Ru, and Pd isotopes involved in the astrophysical rapid neutron capture process are studied within a deformed proton-neutron quasiparticle random-phase approximation. The underlying mean field is described self-consistently from deformed Skyrme-Hartree-Fock calculations with pairing correlations. Residual interactions in the particle-hole and particle-particle channels are also included in the formalism. The isotopic evolution of the various nuclear equilibrium shapes and the corresponding charge radii are investigated in all the isotopic chains. The energy distributions of the Gamow-Teller strength as well as the β-decay half-lives are discussed and compared with the available experimental information. It is shown that nuclear deformation plays a significant role in the description of the decay properties in this mass region. Reliable predictions of the strength distributions are essential to evaluate decay rates in astrophysical scenarios. © 2015 American Physical Society.


Rodriguez-Oliveros R.,CSIC - Institute for the Structure of Matter | Sanchez-Gil J.A.,CSIC - Institute for the Structure of Matter
Optics Express | Year: 2012

Gold nanostars are theoretically studied as efficient thermal heaters at their corresponding localized surface-plasmon resonances (LSPRs). Numerical calculations are performed through the 3D Green's Theorem method to obtain the absorption and scattering cross sections for Au nanoparticles with star-like shape of varying symmetry and tip number. Their unique thermoplasmonic properties, with regard to their (red-shifted) LSPR wavelentgh, (∼ 30-fold increase) steady-state temperature, and scattering/absorption cross section ratios, make them specially suitable for optical heating and in turn for cancer thermal therapy. © 2011 Optical Society of America.


Sarriguren P.,CSIC - Institute for the Structure of Matter
Physical Review C - Nuclear Physics | Year: 2011

Weak decay rates under stellar density and temperature conditions holding at the rapid proton-capture process are studied in neutron-deficient medium-mass waiting-point nuclei extending from Ni up to Sn. Neighboring isotopes to these waiting-point nuclei are also included in the analysis. The nuclear structure part of the problem is described within a deformed Skyrme Hartree- Fock+BCS+quasiparticle random-phase-approximation approach, which reproduces not only the β-decay half-lives but also the available Gamow-Teller strength distributions, measured under terrestrial conditions. The various sensitivities of the decay rates to both density and temperature are discussed. In particular, we study the impact of contributions coming from thermally populated excited states in the parent nucleus and the competition between β decays and continuum electron captures. © 2011 American Physical Society.

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