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We investigate classical limit of the geometric collective model and interacting boson model at zero angular momentum. Both models exhibit complex interplay between regular and chaotic modes of motions. Types of classical orbits are shown to affect properties of quantal spectra.

We explore classical dynamics of quadrupole deformations within the space of control parameters of the interacting boson model at L = 0. Poincare sections are used to visualize regions of regular and chaotic motion in the phase space. Changes of stability of certain trajectories are observed while crossing the Alhassid‐Whelan arc of regularity in the parameter space. Periodic orbits are related to the observed fluctuations of level density.

We investigate the relationship between ground-state (zero-temperature) quantum phase transitions in systems with variable Hamiltonian parameters and classical (temperature-driven) phase transitions in standard thermodynamics. An analogy is found between (i) phase-transitional distributions of the ground-state related branch points of quantum Hamiltonians in the complex parameter plane and (ii) distributions of zeros of classical partition functions in complex temperatures. Our approach properly describes the first- and second-order quantum phase transitions in the interacting boson model and can be generalized to finite temperatures., to be published by AIP in Proc. of the Workshop "Nuclei and Mesoscopic Physics" (Michigan State Univ., Oct 2004); 10 pages, 3 figures

We show that O(6) limit is simultaneously a dynamical symmetry of the U(6) group of the Interacting Boson Model and a critical point of a prolate‐oblate phase transition. This observation leads to a new form of the Casten triangle of which the phase transitional behavior is studied. Empirical evidence for such a transition in the Pt region is discussed.

The method of two-step γ cascades, following the thermal neutron capture, has proved to be a promising tool for investigation of photon strength functions in the region of transition energies near 3 MeV. Using this method we studied behavior of photon strength functions in 108Ag, 116In, 125Te, 144,146Nd, 150Sm, 156,158Gd, 163Dy and 168Er. Besides the detailed information for individual nuclei a large amount of experimental data allows us to look for general trends in the mass region incorporating spherical, transitional and deformed nuclei. Despite continuing systematic study on other nuclei, some general conclusions could already be done.