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Register a new userExamining the wobbling interpretation of low-lying bands in $gamma$-soft nuclei within the interacting boson-fermion model
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The interpretation of the recently reported low-lying excited bands in $gamma$-soft odd-mass nuclei as wobbling bands is examined in terms of the interacting boson-fermion model that is based on the universal nuclear energy density functional. The predicted mixing ratios of the $Delta{I}=1$ electric quadrupole ($E2$) to magnetic dipole ($M1$) transition rates between yrast bands and those yrare bands previously interpreted as wobbling bands in $^{135}$Pr, $^{133}$La, $^{127}$Xe, and $^{105}$Pd nuclei are consistently smaller in magnitude than the experimental values on which the wobbling interpretation is based. These calculated mixing ratios indicate the predominant $M1$ character of the transitions from the yrare bands under consideration to the yrast bands, being in agreement with the new experimental data, which involve both the angular distribution and linear polarization measurements. The earlier wobbling assignments are severely questioned.
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It is argued that the experimental criteria recently used to assign wobbling nature to low-spin bands in several nuclei are insufficient and risky. New experimental data involving angular distribution and linear polarization measurements on an excited band in 187Au, previously interpreted as longitudinal wobbling, are presented. The new data show that the linking transitions have dominant magnetic nature and exclude the wobbling interpretation.
A systematic investigation of the nuclear observables related to the triaxial degree of freedom is presented using the multi-quasiparticle triaxial projected shell model (TPSM) approach. These properties correspond to the observation of $gamma$-bands, chiral doublet bands and the wobbling mode. In the TPSM approach, $gamma$-bands are built on each quasiparticle configuration and it is demonstrated that some observations in high-spin spectroscopy that have remained unresolved for quite some time could be explained by considering $gamma$-bands based on two-quasiparticle configurations. It is shown in some Ce-, Nd- and Ge-isotopes that the two observed aligned or s-bands originate from the same intrinsic configuration with one of them as the $gamma$-band based on a two-quasiparticle configuration. In the present work, we have also performed a detailed study of $gamma$-bands observed up to the highest spin in Dysposium, Hafnium, Mercury and Uranium isotopes. Furthermore, several measurements related to chiral symmetry breaking and wobbling motion have been reported recently. These phenomena, which are possible only for triaxial nuclei, have been investigated using the TPSM approach. It is shown that doublet bands observed in lighter odd-odd Cs-isotopes can be considered as candidates for chiral symmetry breaking. Transverse wobbling motion recently observed in $^{135}$Pr has also been investigated and it is shown that TPSM approach provides a reasonable description of the measured properties.
Inspired by the recent experimental data (Phys. Lett. B {bf 675} (2009) 420), we extend the triaxial projected shell model approach to study the $gamma$-band structure in odd-mass nuclei. As a first application of the new development, the $gamma$-vibrational structure of $^{103}$Nb is investigated. It is demonstrated that the model describes the ground-state band and multi-phonon $gamma$-vibrations quite satisfactorily, supporting the interpretation of the data as one of the few experimentally-known examples of simultaneous occurrence of one- and two-$gamma$-phonon vibrational bands. This generalizes the well-known concept of the surface $gamma$-oscillation in deformed nuclei built on the ground-state in even-even systems to $gamma$-bands based on quasiparticle configurations in odd-mass systems.
We propose a method to incorporate the coupling between shape and pairing collective degrees of freedom in the framework of the interacting boson model (IBM), based on the nuclear density functional theory. To account for pairing vibrations, a boson-number non-conserving IBM Hamiltonian is introduced. The Hamiltonian is constructed by using solutions of self-consistent mean-field calculations based on a universal energy density functional and pairing force, with constraints on the axially-symmetric quadrupole and pairing intrinsic deformations. By mapping the resulting quadrupole-pairing potential energy surface onto the expectation value of the bosonic Hamiltonian in the boson condensate state, the strength parameters of the boson Hamiltonian are determined. An illustrative calculation is performed for $^{122}$Xe, and the method is further explored in a more systematic study of rare-earth $N=92$ isotones. The inclusion of the dynamical pairing degree of freedom significantly lowers the energies of bands based on excited $0^+$ states. The results are in quantitative agreement with spectroscopic data, and are consistent with those obtained using the collective Hamiltonian approach.
A triaxial core rotating around the middle axis, i.e. 2-axis, is cranked around the 1-axis, due to the coupling of an odd proton from a high j orbital. Using the Bargmann representation of a new and complex boson expansion of the angular momentum components, the eigenvalue equation of the model Hamiltonian acquires a Schr{o}dinger form with a fully separated kinetic energy. From a critical angular momentum, the potential energy term exhibits three minima, two of them being degenerate. Spectra of the deepest wells reflects a chiral-like structure. Energies corresponding to the deepest and local minima respectively, are analytically expressed within a harmonic approximation. Based on a classical analysis, a phase diagram is constructed. It is also shown that the transverse wobbling mode is unstable. The wobbling frequencies corresponding to the deepest minimum are used to quantitatively describe the wobbling properties in $^{135}$Pr. Both energies and e.m. transition probabilities are described.
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