Low-energy enhancement of magnetic dipole radiation

Magnetic dipole strength functions have been deduced from averages of a large number of $M1$ transition strengths calculated within the shell model for the nuclides $^{90}$Zr, $^{94}$Mo, $^{95}$Mo, and $^{96}$Mo. An enhancement of $M1$ strength toward low transition energy has been found for all nuclides considered. Large $M1$ strengths appear for transitions between close-lying states with configurations including proton as well as neutron high-$j$ orbits that re-couple their spins and add up their magnetic moments coherently. The $M1$ strength function deduced from the calculated $M1$ transition strengths is compatible with the low-energy enhancement found in ($^3$He,$^3$He') and $(d,p)$ experiments. The present work presents for the first time an explanation of the experimental findings

Self-consistent tilted-axis-cranking study of triaxial strongly deformed bands in 158^{158}Er at ultrahigh spin

Stimulated by recent experimental discoveries, triaxial strongly deformed (TSD) states in $^{158}$Er at ultrahigh spins have been studied by means of the Skyrme-Hartree-Fock model and the tilted-axis-cranking method. Restricting the rotational axis to one of the principal axes -- as done in previous cranking calculations -- two well-defined TSD minima in the total Routhian surface are found for a given configuration: one with positive and another with negative triaxial deformation $\gamma$. By allowing the rotational axis to change direction, the higher-energy minimum is shown to be a saddle point. This resolves the long-standing question of the physical interpretation of the two triaxial minima at a very similar quadrupole shape obtained in the principal axis cranking approach. Several TSD configurations have been predicted, including a highly deformed band expected to cross lesser elongated TSD bands at the highest spins. Its transitional quadrupole moment $Q_t \approx 10.5$\,eb is close to the measured value of $\sim$11\,eb; hence, it is a candidate for the structure observed in experiment.Comment: 5 pages, 5 figure

Temperature-induced pair correlations in clusters and nuclei

The pair correlations in mesoscopic systems such as $nm$-size superconducting clusters and nuclei are studied at finite temperature for the canonical ensemble of fermions in model spaces with a fixed particle number: i) a degenerate spherical shell (strong coupling limit), ii) an equidistantly spaced deformed shell (weak coupling limit). It is shown that after the destruction of the pair correlations at T=0 by a strong magnetic field or rapid rotation, heating can bring them back. This phenomenon is a consequence of the fixed number of fermions in the canonical ensemble

A Composite Chiral Pair of Rotational Bands in the odd-A Nucleus 135Nd

High-spin states in 135Nd were populated with the 110Pd(30Si,5n)135Nd reaction at a 30Si bombarding energy of 133 MeV. Two Delta(I)=1 bands with close excitation energies and the same parity were observed. These bands are directly linked by Delta(I)=1 and Delta(I)=2 transitions. The chiral nature of these two bands is confirmed by comparison with three-dimensional tilted axis cranking calculations. This is the first observation of a three-quasiparticle chiral structure and established the primarily geometric nature of this phenomenon.Comment: 10 pages, 5 figures (1 in color), 1 table, submitted to Physics Review Letters, written in REVTEX4 forma

Second Backbend in the Mass A ~ 180 Region

Within the framework of selfconsistent cranked Hartree-Fock- Bogoliubov theory(one-dimensional) we predict second backbend in the yrast line of Os-182 at $I \approx 40$, which is even sharper than the first one observed experimentally at $I \approx 14$. Around such a high spin the structure becomes multi-quasiparticle type, but the main source of this strong discontinuity is a sudden large alignment of i_13/2 proton orbitals along the rotation axis followed soon by the alignment of j_15/2 neutron orbitals. This leads to drastic structural changes at such high spins. When experimentally confirmed, this will be observed for the first time in this mass region, and will be at the highest spin so far.Comment: 13 pages, 4 ps figure

Superdeformed Band in ^{36}Ar Described by Projected Shell Model

The projected shell model implements shell model configuration mixing in the projected deformed basis. Our analysis on the recently observed superdeformed band in $^{36}$Ar suggests that the neutron and proton 2-quasiparticle and the 4-quasiparticle bands cross the superdeformed ground band at the same angular momentum. This constitutes a picture of band disturbance in which the first and the second band-crossing, commonly seen at separate rotation frequencies in heavy nuclei, occur simultaneously. We also attempt to understand the assumptions of two previous theoretical calculations which interpreted this band. Electromagnetic properties of the band are predicted.Comment: 4 pages and 2 figures, accepted by Phys. Rev. C as a Rapid Communicatio

Classical Analysis of Phenomenological Potentials for Metallic Clusters

The classical trajectories of single particle motion in a Wodds-Saxon and a modified Nilsson potential are studied for axial quadrupole deformation. Both cases give rise to chaotic behaviour when the deformation in the Woods-Saxon and the l**2 term in the modified Nilsson potential are turned on. Important similarities, in particular with regard to the shortest periodic orbits, have been found.Comment: 9 pages LaTex + 4 figures available via e-mail requests from the authors, to appear in Phys.Rev.Let

Tri-axial Octupole Deformations and Shell Structure

Manifestations of pronounced shell effects are discovered when adding nonaxial octupole deformations to a harmonic oscillator model. The degeneracies of the quantum spectra are in a good agreement with the corresponding main periodic orbits and winding number ratios which are found by classical analysis.Comment: 10 pages, Latex, 4 postscript figures, to appear in JETP Letter

Measured g factors and the tidal-wave description of transitional nuclei near A = 100

The transient-field technique has been used in both conventional kinematics and inverse kinematics to measure the g factors of the 2+ states in the stable even isotopes of Ru, Pd and Cd. The statistical precision of the g(2+) values has been significantly improved, allowing a critical comparison with the tidal-wave version of the cranking model recently proposed for transitional nuclei in this region.Comment: Accepted for publication in Physical Review C, April 201

Tilted axis rotation, candidates for chiral bands, and wobbling motion in 138Nd

High-spin states in 138Nd were investigated using the reaction 94Zr(48Ca,4n), detecting coincident γ rays with the gasp spectrometer. A rich level scheme was constructed including four bands of negative parity at low spins, eight bands of dipole transitions, and eight bands of quadrupole transitions at medium spins. The cranked shell model and the tilted-axis cranking model are used to assign configurations to the observed bands, where zero pairing is assumed. For selected configurations the case of finite pairing is also considered. A consistent notation for configuration assignment that applies for both zero and finite pairing is introduced. The observed bands are interpreted as rotation around the short and long principal axes (quadrupole bands), as well as around a tilted axis (dipole bands). The dipole bands have an intermediate character, between magnetic and collective electric rotation. A pair of dipole bands is identified as candidates for chiral partners. The possible existence of the wobbling mode at low deformation and medium spins is discussed. The consistent interpretation of the multitude of observed bands strongly supports the existence of stable triaxial deformation at medium spins in 138Nd. ©2012 American Physical Societ