arXiv subjects

S. Frauendorf

Publications and source records attributed to S. Frauendorf.

At least 55 records · Page 3Linked to original sources

arXiv2013

Transverse wobbling-a collective mode in odd-A triaxial nuclei

The wobbling motion of a triaxial rotor coupled to a high-j quasiparticle is treated semiclassi- cally. Longitudinal and transverse coupling regimes can be distinguished depending on, respectively, whether the quasiparticle angular momentum is oriented parallel or perpendicular to the rotor axis with the largest moment of inertia. Simple analytical expressions for the wobbling frequency and the electromagnetic E2 and M1 transition probabilities are derived assuming rigid alignment of the quasiparticle with one of the rotor axes and harmonic oscillations (HFA). Transverse wobbling is characterized by a decrease of the wobbling frequency with increasing angular momentum. Two examples for transverse wobbling, 163Lu and 135Pr, are studied in the framework of the full triax- ial particle-rotor model and the HFA. The signature of transverse wobbling, decreasing wobbling frequency and enhanced E2 inter-band transitions, is found in agreement with experiment.

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arXiv2013

Nuclear deformation and neutron excess as competing effects for pygmy dipole strength

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A = 124, 128, 132, and 134 in nuclear resonance fluorescence experiments using the ELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIgS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides.

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arXiv2013

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.

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arXiv2013

Relation between Wigner energy and proton-neutron pairing

The linear term proportional to $|N-Z|$ in the nuclear symmetry energy (Wigner energy)is obtained in a model that uses isovector pairing on single particle levels from a deformed potential combined with a $\vec T^2$ interaction. The pairing correlations are calculated by numerical diagonalization of the pairing Hamiltonian acting on the six or seven levels nearest the $N=Z$ Fermi surface. The experimental binding energies of nuclei with $N\approx Z$ are well reproduced. The Wigner energy emerges as a consequence of restoring isospin symmetry. We have found the Wigner energy to be insensitive to the presence of moderate isoscalar pair correlations.

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arXiv2013

Evidence for Multiple Chiral Doublet Bands in $^{133}$Ce

Two distinct sets of chiral-partner bands have been identified in the nucleus $^{133}$Ce. They constitute a multiple chiral doublet (M$χ$D), a phenomenon predicted by relativistic mean field (RMF) calculations and observed experimentally here for the first time. The properties of these chiral bands are in good agreement with results of calculations based on a combination of the constrained triaxial RMF theory and the particle-rotor model.

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arXiv2013

Tidal Waves in $^{102}$Pd: A Rotating Condensate of Multiple $d$ bosons

Low-lying collective excitations in even-even vibrational and transitional nuclei may be described semi-classically as quadrupole running waves on the surface of the nucleus ("tidal waves"), and the observed vibrational-rotational behavior can be thought of as resulting from a rotating condensate of interacting $d$ bosons. These concepts have been investigated by measuring lifetimes of the levels in the yrast band of the $^{102}$Pd nucleus with the Doppler Shift Attenuation Method. The extracted $B(E2)$ reduced transition probabilities for the yrast band display a monotonic increase with spin, in agreement with the interpretation based on rotation-induced condensation of aligned $d$ bosons.

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arXiv2013

Cross-over between different symmetries

The yrast states of even even vibrational and transitional nuclei are interpreted as a rotating condensate of interacting d-bosons. The corresponding semi-classical tidal wave concept is used for microscopic calculations of energies and E2 transition probabilities. The strong octupole correlations in the light rare earth and actinide nuclides are interpreted as rotation-induced condensation of interacting f-bosons.

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arXiv2012

Pairing at High Spin

Pair correlations are described in the framework of the HFB approximation applied to a uniformly rotating system (Cranking model). The reduction of the moments of inertia, the classification of rotational bands as multi quasiparticle configurations, and the signatures of the rotation induced transition to the unpaired state are discussed.

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arXiv2012

Tidal wave in 102Pd: Rotating condensate of up to seven d-bosons

The yrast states of even even vibrational and transitional nuclei are inter- preted as a rotating condensate of interacting d-bosons and the corresponding semi-classical tidal wave concept. A simple experimental manifestation of the anharmonicity caused by the boson interaction is found. The interpretation is substantiated by calculations based on the Collective Model and the Cranking Model.

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arXiv2011

Self-consistent tilted-axis-cranking study of triaxial strongly deformed bands in $^{158}$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 $γ$. 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.

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arXiv2011

Tidal Waves -- a non-adiabatic microscopic description of the yrast states in near-spherical nuclei

The yrast states of nuclei that are spherical or weakly deformed in their ground states are described as quadrupole waves running over the nuclear surface, which we call "tidal waves". The energies and E2 transition probabilities of the yrast states in nuclides with $Z$= 44, 46, 48 and $N=56, ~58,..., 66$ are calculated by means of the cranking model in a microscopic way. The nonlinear response of the nucleonic orbitals results in a strong coupling between shape and single particle degrees of freedom.

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arXiv2011

Microscopic Calculation of IBM Parameters by Potential Energy Surface Mapping

A coherent state technique is used to generate an Interacting Boson Model (IBM) Hamiltonian energy surface that simulates a mean field energy surface. The method presented here has some significant advantages over previous work. Specifically, that this can be a completely predictive requiring no a priori knowledge of the IBM parameters. The technique allows for the prediction of the low lying energy spectra and electromagnetic transition rates which are of astrophysical interest. Results and comparison with experiment are included for krypton, molybdenum, palladium, cadmium, gadolinium, dysprosium and erbium nuclei.

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arXiv2011

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.

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arXiv2010

Instantaneous Shape Sampling - a model for the $γ$-absorption cross section of transitional nuclei

The influence of the quadrupole shape fluctuations on the dipole vibrations in transitional nuclei is investigated in the framework of the Instantaneous Shape Sampling Model, which combines the Interacting Boson Model for the slow collective quadrupole motion with the Random Phase Approximation for the rapid dipole vibrations. Coupling to the complex background configurations is taken into account by folding the results with a Lorentzian with an energy dependent width. The low-energy energy portion of the $γ$- absorption cross section, which is important for photo-nuclear processes, is studied for the isotopic series of Kr, Xe, Ba, and Sm. The experimental cross sections are well reproduced. The low-energy cross section is determined by the Landau fragmentation of the dipole strength and its redistribution caused by the shape fluctuations. Collisional damping only wipes out fluctuations of the absorption cross section, generating the smooth energy dependence observed in experiment. In the case of semi-magic nuclei, shallow pygmy resonances are found in agreement with experiment.

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arXiv2010

Tidal waves as yrast states in transitional nuclei

The yrast states of transitional nuclei are described as quadrupole waves running over the nuclear surface, which we call tidal waves. In contrast to a rotor, which generates angular momentum by increasing the angular velocity at approximately constant deformation, a tidal wave generates angular momentum by increasing the deformation at approximately constant angular velocity. The properties of the tidal waves are calculated by means of the cranking model in a microscopic way. The calculated energies and E2 transition probabilities of the yrast states in the transitional nuclides with $Z$= 44, 46, 48 and $N=56, 58, ..., 66$ reproduce the experiment in detail. The nonlinear response of the nucleonic orbitals results in a strong coupling between shape and single particle degrees of freedom.

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arXiv2009

Structure of $^{240}$Pu: Evidence for Octupole Phonon Condensation?

The expanded level structure of $^{240}$Pu available from the present study highlights the role of strong octupole correlations in this nucleus. Besides a delayed alignment in the yrast band, the observations include the presence of both $I^{+}{\to}(I-1)^{-}$ and $I^{-}{\to}(I-1)^{+}$ E1 transitions linking states of the yrast and negative-parity bands at high spin and the presence of an additional even-spin, positive-parity band deexciting exclusively to the negative parity sequence. The observations appear to be consistent with expectations based on the recently proposed concept of octupole phonon condensation.

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arXiv2008

Chirality in odd-$A$ nucleus $^{135}$Nd in particle rotor model

A particle rotor model is developed which couples several valence protons and neutrons to a rigid triaxial rotor core. It is applied to investigating the chirality in odd-$A$ nucleus $^{135}$Nd with $πh_{11/2}^2\otimesνh^{-1}_{11/2}$ configuration for the first time in a fully quantal approach. For the two chiral sister bands, the observed energies and the $B(M1)$ and $B(E2)$ values for the in-band as well as interband transitions are reproduced excellently. Root mean square values of the angular momentum components and their probability distributions are used for discussing in detail the chiral geometry of the aplanar rotation and its evolution with angular momentum. Chirality is found to change from a soft chiral vibration to nearly static chirality at spin $I=39/2$ and back to another type of chiral vibration at higher spin.

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arXiv2008

Skyrme-Random-Phase-Approximation description of E1 strength in 92-100Mo

The isovector dipole E1 strength in Mo isotopes with A=92,94,96,98,100 is analyzed within the self-consistent separable random-phase approximation (SRPA) model with Skyrme forces SkT6, SkM*, SLy6, and SkI3. The special attention is paid to the low-energy region near the particle thresholds (4-12 MeV), which is important for understanding of astrophysical processes. We show that, due to a compensation effect, the influence of nuclear deformation on E1 strength below 10-12 MeV is quite modest. At the same time, in agreement with previous predictions, the deformation increases the strength at higher energy. At 4-8 MeV the strength is mainly determined by the tail of E1 giant resonance. The four Skyrme forces differ in description of the whole giant resonance but give rather similar results below 12 MeV.

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