The Role of Ground State Correlations in the Single-Particle Strength of Odd Nuclei with Pairing

A method based on the consistent use of the Green function formalism has been developed to calculate the distribution of the single-particle strength in odd nuclei with pairing. The method takes into account the quasiparticle-phonon interaction, ground state correlations and a "refinement" of phenomenological single-particle energies and pairing gap values from the quasiparticle-phonon interaction under consideration. The calculations for 121Sn and 119Sn that were performed in the quasiparticle$\otimes$phonon approximation, have shown a reasonable agreement with experiment. The ground state correlations play a noticeable role and mostly improve the agreement with experiment or shift the results to the right direction.Comment: 11 page

On Cooper Pairing in Finite Fermi Systems

In order to analyse the role of the quasiparticle-phonon interaction in the origin of nuclear gap, we applied an approach which is similar to the Eliashberg theory for usual superconductors. We obtained that the averaged contribution of the quasiparticle-phonon mechanism to the observed value of the pairing gap for $^{120}$Sn is 26% and the BCS-type mechanism gives 74% . Thus, pairing is of a mixed nature at least in semi-magic nuclei -- it is due to the quasiparticle-phonon and BCS mechanisms, the first one being mainly a surface mechanism and the second one mainly a volume mechanism. The calculations of the strength distribution for the odd-mass nuclei $^{119}Sn$ and $^{121}Sn$ have shown that the quasiparticle-phonon mechanism mainly improves the description of the observed spectroscopic factors in these nuclei. For the case of nuclei with pairing in both proton and neutron systems it is necessary to go beyond the Eliashberg-Migdal approximations and include the vertex correction graphs in addition to the rainbow ones. The estimations for spectroscopic factors performed within a three-level model have shown that the contribution of the vertex correction graphs was rather noticeable.Comment: The 7-th International Spring Seminar on Nuclear Physics, "Challenges of Nuclear Structure",Maiori, May 27-31, 200

Impact of the phonon coupling on the dipole strength and radiative neutron capture

The E1 strength functions and radiative capture cross sections for several compound Sn isotopes, including unstable 132S n and 150S n, have been calculated using the self-consistent microscopic theory. In addition to the standard RPA or QRPA approaches, the method includes the quasiparticle-phonon coupling and the single-particle continuum. The results obtained show that the phonon contribution is very noticeable for the pygmy-dipole resonance, which, as it is known, is important for a description of the radiative neutron capture. The phonon contribution to the pygmy-dipole resonance and to the radiative neutron capture cross sections is increased with the (N-Z) difference growth. For example, in the (0-10) MeV interval the full theory gives 17% of EWSR for 150S n and 2.8% for 124S n, whereas within the continuum QRPA approach we have 5.1% and 1.7%, respectively. These facts indicate an important role of the self-consistent calculations that are of astrophysical interest for neutron-rich nuclei. The comparison with the phenomenological Generalized Lorentzian approach by Kopecky-Uhl has shown that the (Q)RPA approach gives a significant increase in the cross section by a factor of 2 for 132S n and a factor of 10 for 150S n and inclusion of the phonon coupling increases the cross sections for these nuclei even more, by a factor of 2-3.Comment: 4pages,3figures,International Conference on Nuclear Data for Science and Technology 201

On microscopic theory of radiative nuclear reaction characteristics

A survey of some results in the modern microscopic theory of properties of nuclear reactions with gamma-rays is given. First of all, we discuss the impact of phonon coupling (PC) on the photon strength function (PSF) because it represents the most natural physical source of additional strength found for Sn isotopes in recent experiments that could not be explained within the stan- dard HFB+QRPA approach. The self-consistent version of the Extended Theory of Finite Fermi Systems in the Quasiparticle Time Blocking Approximation, or simply QTBA, is applied. It uses the HFB mean field and includes both the QRPA and PC effects on the basis of the SLy4 Skyrme force. With our microscopic E1 PSFs, the following properties have been calculated for many stable and unstable even-even semi-magic Sn and Ni isotopes as well as for double-magic 132Sn and 208Pb using the reaction codes EMPIRE and TALYS with several nuclear level density (NLD) models: 1) the neutron capture cross sections, 2) the corresponding neutron capture gamma spectra, 3) the av- erage radiative widths of neutron resonances. In all the properties considered, the PC contribution turned out to be significant, as compared with the standard QRPA one, and necessary to explain the available experimental data. The results with the phenomenological so-called generalized super- fluid NLD model turned out to be worse, on the whole, than those obtained with the microscopic HFB+combinatorial NLD model. Finally, we also discuss the modern microscopic NLD models based on the self-consistent HFB method and show their relevance to explain experimental data as compared with the phenomeno- logical models. The use of these self-consistent microscopic approaches is of particular relevance for nuclear astrophysics, but also for the study of double-magic nuclei.Comment: 13 pages, 14 figures, a survey given as a plenary talk to the Intern. Conference "NUCLEUS 2015" (June 29 - July 3, 2015, Saint-Petersburg, Russia). To be published in Phys. Atom. Nuc

Close-coupling calculations of rotational energy transfer in p-H2+HD

We report quantum-mechanical close-coupling calculations for rotational state resolved cross sections for p-H2+HD collisions. The low temperature limit of p-H2+HD is investigated, which is of significant astrophysical interest in regard to the cooling of primordial gas and the interstellar media. Sharp resonances have been reproduced in the cross sections of some transition states at very low kinetic energies, E< 0.00001 eV.Comment: 10 pages, 6 figure

Pair Wave Functions in Atomic Fermi Condensates

Recent experiments have observed condensation behavior in a strongly interacting system of fermionic atoms. We interpret these observations in terms of a mean-field version of resonance superfluidity theory. We find that the objects condensed are not bosonic molecules composed of bound fermion pairs, but are rather spatially correlated Cooper pairs whose coherence length is comparable to the mean spacing between atoms. We propose experiments that will help to further probe these novel pairs

Quantum Bose liquids with logarithmic nonlinearity: Self-sustainability and emergence of spatial extent

The Gross-Pitaevskii (GP) equation is a long-wavelength approach widely used to describe the dilute Bose-Einstein condensates (BEC). However, in many physical situations, such as higher densities, this approximation unlikely suffices hence one might need models which would account for long-range correlations and multi-body interactions. We show that the Bose liquid described by the logarithmic wave equation has a number of drastic differences from the GP one. It possesses the self-sustainability property: while the free GP condensate tends to spill all over the available volume the logarithmic one tends to form a Gaussian-type droplet - even in the absence of an external trapping potential. The quasi-particle modes of the logarithmic BEC are shown to acquire a finite size despite the bare particles being assumed point-like, i.e., the spatial extent emerges here as a result of quantum many-body correlations. Finally, we study the elementary excitations and demonstrate that the background density changes the topological structure of their momentum space which, in turn, affects their dispersion relations. Depending on the density the latter can be of the massive relativistic, massless relativistic, tachyonic and quaternionic type.Comment: 14 pages, 5 figures. Updates: v2: minor corrections (published version

Microscopic nature of the photon strength function: stable and unstable Ni and Sn isotopes

The pygmy-dipole resonances and photon strength functions in stable and unstable Ni and Sn isotopes are calculated within the microscopic self-consistent version of the extended theory of finite fermi systems which includes the QRPA and phonon coupling effects and uses the known Skyrme forces SLy4. The pygmy dipole resonance in $^{72}Ni$ is predicted with the mean energy of 12.4 MeV and the energy-weighted sum rule exhausting 25.6\% of the total strength. The microscopically obtained photon E1 strength functions are used to calculate nuclear reaction properties, i.e the radiative neutron capture cross section, gamma-ray spectra, and average radiative widths. Our main conclusion is that in all these quantities it is necessary to take the phonon coupling effects into account.Comment: 4 pages, 5 figures, 2 tables. Talk at 15-th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics (CGS15), Dresden, August 2o1