Research Interests

My research interests are centered around theoretical low-energy nuclear structure physics described in the framework of self-consistent mean-field models.

The topics of my research are related to three challenging problems which appear in all disciplines of physics dealing with finite many-particle (mesoscopic) systems, namely how their complexity emerges from their simpler building blocks; how, complementary, patterns, regularities, and symmetries arise in complex systems; and finally how the interaction between composite objects can be derived from the properties of and the interactions between their building blocks.

Among the known finite quantum mechanical systems, atomic nuclei exhibit the richest variety of phenomena. Some of them are linked to individual nucleons, others to the collective behavior of the nucleus as a whole, or correlations among a small number of nucleons. Nuclei can be set into such rapid rotation that the rotational frequency is comparable to the frequency of the internal motion of the individual nucleons, which then is substantially modified by inertial forces. The angular momentum behaves similar to an external magnetic field: it tries to align the angular momenta of the individual particles along the axis of rotation, which changes the structure of the rotating states with angular momentum. In most cases rotational states can be grouped into rotational bands, many of which might coexist in a heavy nucleus. As it are the nucleons themselves which generate the moment of inertia, a rotating nucleus behaves in many respects quite different from most other rotating quantum systems for which a fixed molecular skeleton can be assumed. For many nuclei, collective rotational motion is mixed with various modes of shape vibrations or excitations of individual nucleons.

The extrapolation of any nuclear property based on smooth trends with proton and neutron numbers, angular momentum and excitation energy does rarely work on a quantitative level. The strong interaction in the nucleus cannot be treated in a perturbative way, and nuclei are too small for statistical methods to be valid, which makes the nuclear many-body problem a challenging one.

My research has three major directions:

A large part of my recent research activities consists in the development and optimization of a fully microscopic method to describe long-range correlations, low-energy excitations, and large-amplitude motion within a unified beyond-mean-field approach. The idea is to perform variational configuration mixing of symmetry-restored axial or triaxial self-consistent HFB states within the Generator Coordinate Method for even and odd nuclei. The same Skyrme-type effective interaction is used for the configuration mixing and to generate the mean-field states. The method allows for a unified description of collective vibrational and rotational motion together with the motion of individual nucleons. Exact particle-number and angular momentum projection allow for the calculation of moments of the nuclear density in the laboratory frame without any further approximations. It also delivers in-band as well as out-of-band transition matrix elements in the lab frame. As the full model space of occupied single-particle states is used, no effective charge needs to be introduced. The method has the advantage that its results still can be interpreted in the intuitive picture of shapes of a nuclear fluid and shells of single-particle states provided by mean-field models. An implementation of the method with some symmetry restrictions on the mean-field states delivers results that demonstrate its descriptive power throughout the chart of nuclei from 16O up to 240Pu. In many cases, this beyond-mean-field method offers a powerful alternative to the nuclear shell model as no assumptions about the evolution of the single-particle states nor any restrictions of the model space of single-particle states have to be put into the calculations. An efficient and very precise approximation scheme for the calculation of ground-state properties has recently been set up and will be used for a large-scale analysis of quadrupole correlation energies beyond the mean-field level.

One focus of the application of these methods is the structure of exotic nuclei, in particular nuclei with a neutron-to-proton ratio far off stability and very heavy nuclei at the upper end of the chart of nuclei. For those nuclei, the least bound nucleons have a very small separation energy and therefore occupy single-particle states close to the continuum of unbound states. With that, These loosely-bound exotic nuclei are likely to exhibit some entirely different quantum many-body effects than their well-bound stable counterparts, which will provide a magnifying glass for particular features of the nuclear many-body problem. The phenomena under investigation include the evolution of single-particle spectra, as for example the quenching of known shells and the possibility of new shells appearing for exotic nuclei, and its consequences for nuclear properties, as for example the systematics of excitation spectra and shape coexistence phenomena. New experimental facilities employing rare-isotope beams as well as modern spectrometers utilizing multi-detector arrays which are planned, under construction or already deliver first data will extend substantially our knowledge about these phenomena in the next decade. Shell quenching addresses also the fundamental question about the origin of the heavy elements in the universe. Recent studies suggest that the quenching of neutron shells is necessary to explain the observed abundances of heavy elements in the universe: the shell structure of very neutron-rich nuclei beyond our current experimental reach is imprinted into the pattern of abundances.

Another issue of my research is the construction of effective interactions. The recent data for nuclei far off stability call for a refinement of the isovector channel of the effective energy functional and the interaction at low density in general. Steps in that direction are the investigation of more involved density-dependencies of the coupling constants and a refinement of the fitting protocols.

Another focus of my research is the proper description of odd-mass nuclei in the framework of mean-field and beyond-mean-field approaches. One goal is the understanding the origin of the odd-even staggering of nuclear properties by disentangling the contributions from the rearrangement of the mean field from those of pairing and other correlations. In odd-mass nuclei, the unpaired nucleon polarizes the core of paired nucleons, which requires a careful treatment of pairing correlations on the level of HFB. When it will be available for odd-mass nuclei in the near future, the method described above will give a unique access to the complex excitation spectra of odd-mass nuclei. One aspect of this research program is the improved modeling of effective pairing interaction in nuclei, as well as the modeling of the so-called ``time-odd'' parts of the effective nucleon-nucleon interaction, that govern for example the spin-spin and spin-isospin interaction. The goal is the consistent description of odd-mass nuclei, of rapidly-rotating nuclei, spin and spin-isospin excitations and beta decay within a unified approach.

I work on these projects with a number of international collaborators including (present and past)
Karim Bennaceur IPN Lyon, France
George F. Bertsch Institute for Nuclear Theory, Seattle, Washington, U.S.A.
Paul Bonche Service de Physique Théorique, CEA/Saclay, France
Jacek Dobaczewski Institute for Theoretical Physics, Warsaw University, Poland
Thomas Duguet Service de Physique Nucléaire, Institut de Recherche sur les Lois Fondamentales de l'Univers, CEA Saclay, France
Jonathan Engel Department of Physics, The University of North Carolina at Chapel Hill, U.S.A.
Hubert Flocard Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, Orsay, France
Stéphane Goriely Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, Belgique
Paul-Henri Heenen Service de Physique Nucléaire Théorique, Université Libre de Bruxelles, Belgique
Jacques Meyer IPN Lyon, France
Joachim A. Maruhn Institut für Theoretische Physik, Universität Frankfurt am Main, Deutschland
Witek Nazarewicz University of Tennessee, Knoxville, and Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
J. Mike Pearson Departement de Physique, Université de Montréal, Québec, Canada
Paul-Gerhard Reinhard Institut für Theoretische Physik II, Universität Erlangen, Deutschland
Cédric Simenel Service de Physique Nucléaire, Institut de Recherche sur les Lois Fondamentales de l'Univers, CEA Saclay, France

and in the framework of the following projects

Letter of Intent for SPIRAL 2 "Improving effective forces, mean-field-based methods, and predictions: dedicated measurements"
UNEDF (Universal Nuclear Energy Density Functional) project
MATS collaboration (Measurements with with an Advanced Trapping System)

MB July 29, 2008.