Exploring quantum dynamics in an open many-body system: Transition to superradiance

We study the dynamics of a complex open quantum many-body system. The coupling to external degrees of freedom can be viewed as a coupling to a radiation field, to continuum states or to a measuring apparatus. This perturbation is treated in terms of an effective non-Hermitian Hamiltonian. The influence of such coupling on the properties of the many-body dynamics is discussed, with emphasis on new effects related to dynamical segregation of fast and slow decays and the phase transition to Dicke superradiance. Relations to quantum optics, continuum shell model, theory of measurement, quantum chaos, percolation theory, and to quantum reactions are stressed.Comment: 15 pages, 7 figure

Super-Radiance: From Nuclear Physics to Pentaquarks

The phenomenon of super-radiance in quantum optics predicted by Dicke 50 years ago and observed experimentally has its counterparts in many-body systems on the borderline between discrete spectrum and continuum. The interaction of overlapping resonances through the continuum leads to the redistribution of widths and creation of broad super-radiant states and long-lived compound states. We explain the physics of super-radiance and discuss applications to weakly bound nuclei, giant resonances and widths of exotic baryons.Comment: 10 pages, 4 figure

Nuclear Structure, Random Interactions and Mesoscopic Physics

Standard concepts of nuclear physics explaining the systematics of ground state spins in nuclei by the presence of specific coherent terms in the nucleon-nucleon interaction were put in doubt by the observation that these systematics can be reproduced with high probability by randomly chosen rotationally invariant interactions. We review the recent development in this area, along with new original results of the authors. The self-organizing role of geometry in a finite mesoscopic system explains the main observed features in terms of the created mean field and correlations that are considered in analogy to the random phase approximation.Comment: review paper; 54 pages with 16 figure

High-lying single-particle modes, chaos, correlational entropy, and doubling phase transition

Highly-excited single-particle states in nuclei are coupled with the excitations of a more complex character, first of all with collective phonon-like modes of the core. In the framework of the quasiparticle-phonon model we consider the structure of resulting complex configurations using the $1k_{17/2}$ orbital in $^{209}$Pb as an example. Although, on the level of one- and two-phonon admixtures, the fully chaotic GOE regime is not reached, the eigenstates of the model carry significant degree of complexity that can be quantified with the aid of correlational invariant entropy. With artificially enhanced particle-core coupling, the system undergoes the doubling phase transition with the quasiparticle strength concentrated in two repelling peaks. This phase transition is clearly detected by correlational entropy.Comment: 8 pages, 6 figure

Many-Body Physics on the Border of Nuclear Stability

A brief overview is given of the Continuum Shell Model, a novel approach that extends the traditional nuclear shell model into the domain of unstable nuclei and nuclear reactions. While some of the theoretical aspects, such as role and treatment of one- and two-nucleon continuum states, are discussed more in detail, a special emphasis is made on relation to observed nuclear properties, including definitions of the decay widths and their relation to the cross sections, especially in the cases of non-exponential decay. For the chain of He isotopes we demonstrate the agreement of theoretical results with recent experimental data. We show how the interplay of internal collectivity and coherent coupling to continuum gives rise to the universal mechanism of creating pigmy giant resonances.Comment: 6 pages, 3 figure