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We find a {Delta}I=4 bifurcation phenomenon in many ground state rotational bands by performing a staggering parameter analysis, which has been employed often for superdeformed bands. We find also that a small modification of the intrinsic state of the interacting boson model wave function in the {ital sd}-boson space, which has been used extensively for ground bands, is able to produce the regular {Delta}I=4 bifurcation pattern. Adding a {ital g} boson, we are able to generate even an irregular pattern in the {Delta}I=4 bifurcation phenomenon, which seems to be observed in some ground bands. {copyright} {ital 1997} {ital The American Physical Society}

We develop a relativistic density-dependent Hartree approach for finite nuclei, where the coupling constants of the relativistic Hartree Lagrangian are made density dependent and are obtained from the relativistic Brueckner-Hartree-Fock results of nuclear matter. The calculated results on binding energies and root mean square radii of {sup 16}O and {sup 40}Ca agree very well with experiment. The charge densities from electron scattering are also calculated and their dependence on the nucleon-nucleon interaction is discussed in relation with nuclear matter properties.

The authors calculate effects of direct meson-..delta../sub 33/ interactions on pion double charge exchange to isobaric analog states in /sup 18/O. Virtual ..delta../sub 33/'s in the ground state are shown to contribute negligibly, contrary to recent expectations. On-shell ..delta../sub 33/ interactions can have a large effect through ..pi..+rho exchange, and inclusion of these terms improves agreement with experiment below resonance. Data at the higher energies cannot be explained by a simple combination of sequential pion-nucleon scattering plus direct meson-..delta../sub 33/ terms.

At certain combinations of temperature and density, nuclear matter may exist as a liquid-gas mixture exhibiting phase instabilities, a characteristic signature of which may be found in the emission of intermediate-mass fragments in nuclear collisions. The present analysis of fragment distributions from proton- and heavy-ion-induced reactions, in the framework of a theory of condensation, is suggestive of the occurrence of such phase transitions with a critical exponent $k\ensuremath{\sim}1.7$ and a critical temperature ${T}_{c}\ensuremath{\sim}12$ MeV.

Anomalies in the experimental spectra for the positive-parity states of the Pt and Hg isotopes are explained in the ..gamma..-deformed-rotor model which takes into account the coupling of the two quasiparticle excitations with the ground band. It is found that the 0--2-quasiparticle coupling is very important. Moreover, it is recognized that the linear increase in the moment of inertia with the square of the angular velocity can be described by this coupling.

Critical conditions for instability in the isovector unnatural-parity channel in nuclei are investigated in terms of the random-phase approximation. The particle-hole residual interaction consists of the ..pi.. and rho(2..pi..) meson-exchange attractive components in addition to the short-range repulsive Landau term. It is found that the nonresonant part of the iterated two-pion exchange moves the characteristics of the nuclear phase transition closer to those for rho meson condensation. This mode may dominate at high density.

Relative cross sections for the ($p$, ${\ensuremath{\pi}}^{\ensuremath{-}}$) reaction for nuclei in the region $40lAl56$ are calculated with use of ${(1{f}_{\frac{7}{2}})}^{n}$ wave functions and a zero-range plane-wave approximation for this three-nucleon process. The results of the calculation for the low-lying discrete states are in excellent agreement with recent data and many detailed aspects of the spectra are understood for the first time.