arXiv subjects

E. Vigezzi

Publications and source records attributed to E. Vigezzi.

At least 55 records · Page 3Linked to original sources

arXiv2005

Pairing Matrix Elements and Pairing Gaps with Bare, Effective and Induced Interactions

The dependence on the single-particle states of the pairing matrix elements of the Gogny force and of the bare low-momentum nucleon-nucleon potential $v_{low-k}$ is studied in the semiclassical approximation for the case of a typical finite, superfluid nucleus ($^{120}$Sn). It is found that the matrix elements of $v_{low-k}$ follow closely those of $v_{Gogny}$ on a wide range of energy values around the Fermi energy $e_F$, those associated with $v_{low-k}$ being less attractive. This result explains the fact that around $e_F$ the pairing gap $Δ_{Gogny}$ associated with the Gogny interaction (and with a density of single-particle levels corresponding to an effective $k$-mass $m_k\approx 0.7 m$) is a factor of about 2 larger than $Δ_{low-k}$,being in agreement with $Δ_{exp}$= 1.4 MeV. The exchange of low-lying collective surface vibrations among pairs of nucleons moving in time-reversal states gives rise to an induced pairing interaction $v_{ind}$ peaked at $e_F$. The interaction $(v_{low-k}+ v_{ind})Z_ω$ arising from the renormalization of the bare nucleon-nucleon potential and of the single-particle motion ($ω-$mass and quasiparticle strength $Z_ω$) due to the particle-vibration coupling leads to a value of the pairing gap at the Fermi energy $Δ_{ren}$ which accounts for the experimental value.

nucl-th↗

arXiv2004

Quasi-particle properties and Cooper pairing in trapped Fermi gases

The possibility for the particles in a Fermi gas to emit and reabsorb density and spin fluctuations gives rise to an effective mass and to a lifetime of the quasi-particles, as well as to an effective pairing interaction which affect in an important way the BCS critical temperature. We calculate these effects for a spherically symmetric trapped Fermi gas of $\sim$ 1000 particles. The calculation provides insight on the many-body physics of finite Fermi gases and is closely related to similar problems recently considered in the case of atomic nuclei and neutron stars.

cond-mat.soft↗

arXiv2004

Correlation energy contribution to nuclear masses

The ground state correlation energies associated with collective surface and pairing vibrations are calculated for Pb- and Ca-isotopes. It is shown that this contribution, when added to those predicted by one of the most accurate modern nuclear mass formula (HFBCS MSk7 mass formula), reduces the associated rms error by an important factor, making mean field theory, once its time dependence is taken into account, a quantitative predictive tool for nuclear masses.

nucl-th↗

arXiv2004

Violation and persistence of the K-quantum number in warm rotating nuclei

The validity of the K-quantum number in rapidly rotating warm nuclei is investigated as a function of thermal excitation energy U and angular momentum I, for the rare-earth nucleus 163Er. The quantal eigenstates are described with a shell model which combines a cranked Nilsson mean-field and a residual two-body interaction, together with a term which takes into account the angular momentum carried by the K-quantum number in an approximate way. K-mixing is produced by the interplay of the Coriolis interaction and the residual interaction; it is weak in the region of the discrete rotational bands (U \lesim 1MeV), but it gradually increases until the limit of complete violation of the K-quantum number is approached around U \sim 2 - 2.5 MeV. The calculated matrix elements between bands having different K-quantum numbers decrease exponentially as a function of $ΔK$, in qualitative agreement with recent data.

nucl-th↗

arXiv2003

Many-body effects in nuclear structure

We calculate, for the first time, the state-dependent pairing gap of a finite nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v14) in a Hartree-Fock basis (with effective k-mass m_k eqult to 0.7 m), within the framework of the BCS approximation including scattering states up to 800 MeV above the Fermi energy to achieve convergence. The resulting gap accounts for about half of the experimental gap. We find that a consistent description of the low-energy nuclear spectrum requires, aside from the bare nucleon-nucleon interaction, not only the dressing of single-particle motion through the coupling to the nuclear surface, to give the right density of levels close to the Fermi energy (and thus an effective mass m* approximately equal to m), but also the renormalization of collective vibrational modes through vertex and self-energy processes, processes which are also found to play an essential role in the pairing channel, leading to a long range, state dependent component of the pairing interaction. The combined effect of the bare nucleon-nucleon potential and of the induced pairing interaction arising from the exchange of low-lying surface vibrations between nucleons moving in time reversal states close to the Fermi energy accounts for the experimental gap.

nucl-th↗

arXiv2003

Parity inversion, breakdown of shell closure and particle-vibration coupling in Be-isotopes

The coupling of single-particle motion and of vibrations in $^{11}_4$Be produces dressed neutrons which spend only a fraction of the time in pure single-particle states, and which weighting differently from the bare neutrons, lead to parity inversion. The interaction of the two least bound neutrons in the ground state of $^{12}_4$Be mediated by the $v_{14}$ Argonne nucleon-nucleon potential and by the exchange of the phonon cloud, give rise to a strongly correlated state, where the neutrons spend more than half of the time in (s$^2$+d$^2$)-configurations, resulting in a breaking of the N=8 shell closure.

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arXiv2002

Rotational Damping and Compound Formation in Warm Rotating Nuclei

The rotational damping width Γ_{rot} and the compound damping width Γ_{comp} are two fundamental quantities that characterize rapidly rotating compound nuclei having finite thermal excitation energy. A two-component structure in the strength function of consecutive E2 transitions reflects the two widths, and it causes characteristic features in the double and triple gamma-ray spectra. We discuss a new method to extract experimentally values of Γ_{rot} and Γ_{comp}. The first preliminary result of this method is presented.

nucl-th↗

arXiv2002

Stability properties of |Psi|^2 in Bohmian dynamics

According to Bohmian dynamics, the particles of a quantum system move along trajectories, following a velocity field determined by the wave-function Psi(x,t). We show that for simple one-dimensional systems any initial probability distribution of a statistical ensemble approaches asymptotically |Psi(x,t)}|^2 if the system is subject to a random noise of arbitrarily small intensity.

quant-ph↗

arXiv2001

Effect of particle-phonon coupling on pairing correlations in nuclei

The influence of particle-phonon coupling on pairing correlations in nuclei is studied by solving the Dyson equation including the anomalous (pairing) Green function. We develop the formalism for solving the equation with the minimum of approximations. The solution of the Dyson equation is compared with the diagonalization of particle-phonon coupled Hamiltonian in a small space. This comparison reveals that the effect of many-phonon states is incorporated in the Dyson equation. We calculate the pairing gap of the neutron in 120Sn. We compare analytically the present method with a simpler treatment based on Bloch-Horowitz perturbation theory.

nucl-th↗

arXiv2001

Solution of the Dyson equation for nucleons in the superfluid phase

We investigate the role the interweaving of surface vibrations and nucleon motion has on Cooper pair formation in spherical superfluid nuclei. A quantitative calculation of the state-dependent pairing gap requires to go beyond the quasiparticle approximation, treating in detail the breaking of the single-particle strength and of the associated poles. This is done solving self-consistently the Dyson equation, including both a bare nucleon-nucleon interaction (which for simplicity we choose as a monopole-pairing force of constant matrix elements g) and an induced interaction arizing from the exchange of vibrations (calculated microscopically in QRPA) between pairs of nucleons moving in time reversal states. Both the normal and anomalous density Green functions are included, thus treating self-energy and pairing processes on equal footing. We apply the formalism to the superfluid nucleus 120Sn. Adjusting the value of g so as to reproduce, for levels close to the Fermi level, the empirical odd-even mass difference (Delta approx 1.4 MeV), it is found that the pairing gap receives about equal contributions from the monopole-pairing force and from the induced interaction. [More sentences continue. See the main body.]

nucl-th↗

arXiv2001

Gamma-ray strength functions in thermally excited rotating nuclei

A general discussion and illustration is given of strength functions for rotational transitions in two-dimensional E(gamma_1) x E(gamma_2) spectra. Especially, a narrow component should be proportional to the compound damping width, related to the mixing of basis rotational bands into compound bands with fragmented transition strength. Three E(gamma_1) x E(gamma_2) spectra are made by setting gates on triple coincidences, selecting cascades which feed into specific low-lying bands in the nucleus 168Hf. In each of the gated spectra, we find a ridge, carrying about 100 decay paths. This ridge is ascribed to rotational transitions in the excitation energy range of 1.0 to 1.5 MeV above the yrast line. The FWHM of the ridges are around 40 keV, about a factor of two smaller than calculated on the basis of mixed cranked mean field bands.

nucl-th↗

arXiv2000

The halo of the exotic nucleus 11Li: a single Cooper pair

If neutrons are progressively added to a normal nucleus, the Pauli principle forces them into states of higher momentum. When the core becomes neutron-saturated, the nucleus expels most of the wavefunction of the last neutrons outside to form a halo, which because of its large size can have lower momentum. It is an open question how nature stabilizes such a fragile system and provides the glue needed to bind the halo neutrons to the core. Here we show that this problem is similar to that of the instability of the normal state of an electron system at zero temperature solved by Cooper, solution which is at the basis of BCS theory of superconductivity. By mimicking this approach using, aside from the bare nucleon-nucleon interaction, the long wavelength vibrations of the nucleus $^{11}$Li, the paradigm of halo nuclei, as tailored glues of the least bound neutrons, we are able to obtain a unified and quantitative picture of the observed properties of $^{11}$Li.

nucl-th↗

arXiv2000

Room Temperature Organic Superconductor?

The electron--phonon coupling in fullerene C28 has been calculated from first principles. The value of the associated coupling constant lambda/N(0) is found to be a factor three larger than that associated with C60. Assuming similar values of the density of levels at the Fermi surface N(0) and of the Coulomb pseudopotential for C28-based solids as those associated with alkali-doped fullerides A3C60, one obtains Tc(C28) \approx 8 Tc(C60).

cond-mat.supr-con↗

arXiv1999

Probing the width of compound states with rotational gamma rays

The intrinsic width of (multiparticle-multihole) compound states is an elusive quantity, of difficult direct access, as it is masked by damping mechanisms which control the collective response of nuclei. Through microscopic cranked shell model calculations, it is found that the strength function associated with two-dimensional gamma-coincidence spectra arising from rotational transitions between states lying at energies >1 MeV above the yrast line, exhibits a two-component structure controlled by the rotational (wide component) and compound (narrow component) damping width. This last component is found to be directly related to the width of the multiparticle-multihole autocorrelation function.

nucl-th↗

arXiv1998

Microscopic Structure of Rotational Damping

The damping of collective rotational motion is studied microscopically, making use of shell model calculations based on the cranked Nilsson deformed mean-field and on residual two-body interactions, and focusing on the shape of the gamma-gamma correlation spectra and on its systematic behavior. It is shown that the spectral shape is directly related to the damping width of collective rotation, \Gammarot, and to the spreading width of many-particle many-hole configurations, \Gammamu. The rotational damping width is affected by the shell structure, and is very sensitive to the position of the Fermi surface, besides mass number, spin and deformation. This produces a rich variety of features in the rotational damping phenomena.

nucl-th↗

arXiv1998

Stability of Designed Proteins Against Mutations

The stability of model proteins with designed sequences is assessed in terms of the number of sequences (obtained from the designed sequence through mutations), which fold into 5the ``native'' conformation. By a complete enumeration of the total number of sequences obtained by introducing up to 4 point mutations and up to 7 composition--conserving mutations (swapping of amino acids) in a 36mers chain, it is found that there are $10^8-10^9$ sequences which in the folding process target onto the ``native'' conformation. Consequently, proteins with designed sequences display a remarkable degree of stability and, to a large extent, of designability.

cond-mat.soft↗

arXiv1998

On the Origin of Nuclear Superfluidity

The induced pairing interaction arising from the exchange of collective surface vibrations among nucleons moving in time reversal states close to the Fermi energy is found to lead to values of the pairing gap which are similar to those experimentally observed.

nucl-th↗

arXiv1998

Semiclassical Approximation to Neutron Star Superfluidity Corrected for Proximity Effects

The inner crust of a neutron star is a superfluid and inhomogeneous system, consisting of a lattice of nuclei immersed in a sea of neutrons. We perform a quantum calculation of the associated pairing gap and compare it to the results one obtains in the Local Density Approximation (LDA). It is found that the LDA overestimates the spatial dependence of the gap, and leads to a specific heat of the system which is too large at low temperatures, as compared with the quantal result. This is caused by the neglect of proximity effects and the delocalized character of the single-particle wavefunctions close to the Fermi energy. It is possible to introduce an alternative, simple semiclassical approximation of the pairing gap which leads to a specific heat that is in good agreement with the quantum calculation.

nucl-th↗