arXiv subjects

P. Buganu

Publications and source records attributed to P. Buganu.

16 recordsLinked to original sources

arXiv2026

Shape coexisting intruder states in even Sn isotopes and their influence on the $B_{4/2}<1$ anomaly

The normal and intruder configuration states of the even $^{114-122}$Sn nuclei are described in a geometric model using a double well collective potential. It is shown that subtle changes in the potential shapes can reproduce the mass dependence of the low-lying energy levels and the associated $E0$ and $E2$ transitions. The extracted shape dynamics reveals different instances of shape coexistence with and without mixing, in all considered nuclei. This information is used to track the origin of $B_{4/2}<1$ values reported for $^{114,122}$Sn isotopes, providing thus a phenomenological interpretation.

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arXiv2026

B$_{4/2}$ anomaly related to the shape coexistence in $^{94}$Zr

Solutions of the Bohr-Mottelson Hamiltonian with a sixth-order anharmonic oscillator potential, for both axially-symmetric and $γ$-unstable deformations, are applied to investigate the lowest states of the $^{94}$Zr nucleus. The experimentally observed $B_{4/2}$ anomaly is explained, within the present approach, by the presence of a coexistence between an approximately spherical shape ($2_{1}^{+}$) and an axially-symmetric deformed one ($4_{1}^{+}$). On the other hand, the energies of the $0_{2}^{+}$ and $2_{2}^{+}$ states and their associated electromagnetic transitions fit better a dynamical triaxial deformation configuration ($γ$-unstable). This overall picture of the lowest states in $^{94}$Zr corresponds to a triple shape coexistence scenario.

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arXiv2026

Study of the shape coexistence in the 96Zr, 96Mo, 96Ru isobars

Three stable isobars, $^{96}_{40}$Zr$_{56}$, $^{96}_{42}$Mo$_{54}$ and $^{96}_{44}$Ru$_{52}$, which are in the vicinity of the harmonic oscillator proton shell closure Z=40 and the spin-orbit neutron shell closure N=50, are investigated for the presence of the shape coexistence and mixing phenomena. The ground state deformation of these isobars is extracted from the potential energy surface determined with the Covariant Density Functional Theory using a density-dependent point-coupling interaction, while the excited states are described involving the Bohr-Mottelson Hamiltonian with octic potential for both axially symmetric and $γ$-unstable quadrupole deformations. Within the broader view of the two approaches, the obtained results clearly highlight the significant contribution of these phenomena to the structure of the states of these nuclei.

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arXiv2026

Shape phase transition, coexistence and mixing in the $^{98-106}$Ru isotopes

The deformation properties within the $^{98-106}$Ru even-even isotopic chain, are investigated by means of the Covariant Density Functional Theory with a Density-Dependent Point-Coupling X parametrization. The considered nuclei are found to exhibit very shallow prolate and triaxial ground state deformation. This information is used to ascertain their dynamical behavior within prolate $γ$-stable and $γ$-unstable instances of a phenomenological Bohr-Mottelson Hamiltonian with an octic potential in the axial deformation variable. The comparative study of the low-lying collective states, revealed the presence of a shape phase transition from low to high deformation, as well as evidence of shape coexistence and mixing between spherical vibrator, $γ$-unstable or prolate configurations in ground and excited states. It is also shown that the effect of shape coexistence and mixing on the $γ$-band states can account to some extent for the typical $γ$-unstable staggering even in prolate $γ$-stable conditions.

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arXiv2026

Bohr-Mottelson Hamiltonian with octic potential applied to the $^{106-116}$Cd isotopes

The Bohr-Mottelson Hamiltonian, with an octic potential in the $β$-deformation variable, is numerically solved for a $γ$-unstable symmetry of the nuclear system. The analytical structure of the model allows the description of multiple phenomena of great interest for the nuclear structure such as ground-state shape phase transitions and their critical points, dynamical shape phase transitions, shape coexistence with and without mixing, anomalous in-band $E2$ transitions, large $E2$ intra-band transitions and large monopole transition between the first excited $0^+$ state and the ground state, respectively. As a first application of the present model is selected the $^{106-116}$Cd isotope chain known in literature to manifest shape phase transition, respectively shape coexistence and mixing.

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arXiv2022

Shape Coexistence in 74Ge, 74Se and 74Kr Investigated by Phenomenological and Microscopic Models

The deformation properties of 74Ge, 74Se and 74Kr are studied within the phenomenological Bohr-Mottelson model, having as input the experimental collective energy states, as well with Covariant Density Functional theories based on microscopic structural information. The results of these approaches are shown to be compatible in what concerns the presence of coexisting shapes in the considered nuclei, while the emergence of shape mixing is deduced from the phenomenological calculated collective states.

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arXiv2021

A $γ$-rigid solution of the Bohr Hamiltonian with deformation-dependent mass term for Kratzer potential and $γ= 30^\circ$

In this work, the Davydov-Chaban Hamiltonian, describing the collective motion of $γ$-rigid atomic nuclei, is amended by allowing the mass parameter to depend on the nuclear deformation. Further, Z(4)-DDM (Deformation-Dependent Mass) model is proposed by considering the Kratzer potential for the $β$ variable, and solving the problem by techniques of asymptotic iteration method (AIM). The results of the calculated spectra and $B(E2)$ transition rates for series of $^{192-196}$Pt isotopes are compared with the corresponding experimental data as well as with other theoretical models. Exact analytical expressions are derived for spectra and normalized wave functions of the Kratzer potential. The obtained results show an overall good agreement with the experimental data and an important improvement in respect to other models

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arXiv2018

Isovector and isoscalar proton-neutron pairing in $N>Z$ nuclei

We propose a particle number conserving formalism for the treatment of isovector-isoscalar pairing in nuclei with $N>Z$. The ground state of the pairing Hamiltonian is described by a quartet condensate to which is appended a pair condensate formed by the neutrons in excess. The quartets are built by two isovector pairs coupled to the total isospin $T=0$ and two collective isoscalar proton-neutron pairs. To probe this ansatz for the ground state we performed calculations for $N>Z$ nuclei with the valence nucleons moving above the cores $^{16}$O, $^{40}$Ca and $^{100}$Sn. The calculations are done with two pairing interactions, one state-independent and the other of zero range, which are supposed to scatter pairs in time-revered orbits. It is proven that the ground state correlation energies calculated within this approach are very close to the exact results provided by the diagonalization of the pairing Hamiltonian. Based on this formalism we have shown that moving away of N=Z line, both the isoscalar and the isovector proton-neutron pairing correlations remain significant and that they cannot be treated accurately by models based on a proton-neutron pair condensate.

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arXiv2018

Davydov-Chaban Hamiltonian within the formalism of deformation-dependent effective mass for Davidson potential

In this work, we modify the Davydov-Chaban Hamiltonian describing the collective motion of a $γ$-rigid atomic nucleus by allowing the mass to depend on nuclear deformation. Exact analytical expressions are derived for energy spectra as well as normalized wave functions for Davidson potential. The model, called Z(4)-DDMD (Deformation Dependent Mass with Davidson potential), is achieved by using the Asymptotic Iteration Method (AIM). The numerical calculations for energy spectra and B(E2) transition probabilities are compared to the experimental data of $^{192-196}$Pt isotopes. The obtained results show an overall agreement with the experiment and an important improvement in respect to other models.

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arXiv2017

Davydov-Chaban Hamiltonian with deformation-dependent mass term for γ = 30°

Motivation : Several theoretical comparisons with experimental data have recently pointed out that the mass tensor of the collective Bohr Hamiltonian cannot be considered as a constant and should be taken as a function of the collective coordinates. Method : The Davydov-Chaban Hamiltonian, describing the collective motion of γ-rigid atomic nuclei, is modified by allowing the mass to depend on the nuclear deformation. Moreover, the eigenvalue problem for this Hamiltonian is solved for Davidson potential and γ = 30° involving an Asymptotic Iteration Method (AIM). The present model is conventionally called Z(4)-DDM-D (Deformation Dependent Mass with Davidson potential), in respect to the so called Z(4) model. Results : Exact analytical expressions are derived for energy spectra and normalized wave functions, for the present model. The obtained results show an overall agreement with the experimental data for 108-116Pd, 128-132Xe, 136;138Ce and 190-198Pt and an important improvement in respect to other models. Prediction of a new candidate nucleus for triaxial symmetry is made. Conclusion : The dependence of the mass on the deformation reduces the increase rate of the moment of inertia with deformation, removing a main drawback of the model and leading to an improved agreement with the corresponding experimental data.

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arXiv2015

Sextic potential for $γ$-rigid prolate nuclei

The equation of the Bohr-Mottelson Hamiltonian with a sextic oscillator potential is solved for $γ$-rigid prolate nuclei. The associated shape phase space is reduced to three variables which are exactly separated. The angular equation has the spherical harmonic functions as solutions, while the $β$ equation is brought to the quasi-exactly solvable case of the sextic oscillator potential with a centrifugal barrier. The energies and the corresponding wave functions are given in closed form and depend, up to a scaling factor, on a single parameter. The $0^{+}$ and $2^{+}$ states are exactly determined, having an important role in the assignment of some ambiguous states for the experimental $β$ bands. Due to the special properties of the sextic potential, the model can simulate, by varying the free parameter, a shape phase transition from a harmonic to an anharmonic prolate $β$-soft rotor crossing through a critical point. Numerical applications are performed for 39 nuclei: $^{98-108}$Ru, $^{100,102}$Mo, $^{116-130}$Xe, $^{132,134}$Ce, $^{146-150}$Nd, $^{150,152}$Sm, $^{152,154}$Gd, $^{154,156}$Dy, $^{172}$Os, $^{180-196}$Pt, $^{190}$Hg and $^{222}$Ra. The best candidates for the critical point are found to be $^{104}$Ru and $^{120,126}$Xe, followed closely by $^{128}$Xe, $^{172}$Os, $^{196}$Pt and $^{148}$Nd.

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arXiv2015

Analytical solution for the Davydov-Chaban Hamiltonian with sextic potential for $γ=30^{\circ}$

An analytical solution for the Davydov-Chaban Hamiltonian with a sextic oscillator potential for the variable $β$ and $γ$ fixed to $30^{\circ}$, is proposed. The model is conventionally called Z(4)-Sextic. For the considered potential shapes the solution is exact for the ground and $β$ bands, while for the $γ$ band an approximation is adopted. Due to the scaling property of the problem the energy and $B(E2)$ transition ratios depend on a single parameter apart from an integer number which limits the number of allowed states. For certain constraints imposed on the free parameter, which lead to simpler special potentials, the energy and $B(E2)$ transition ratios are parameter independent. The energy spectra of the ground and first $β$ and $γ$ bands as well as the corresponding $B(E2)$ transitions, determined with Z(4)-Sextic, are studied as function of the free parameter and presented in detail for the special cases. Numerical applications are done for the $^{128,130,132}$Xe and $^{192,194,196}$Pt isotopes, revealing a qualitative agreement with experiment and a phase transition in Xe isotopes.

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arXiv2013

Description of the isotope chain $^{180-196}$Pt within some solvable approaches

Energies of the ground, $β$ and $γ$ bands as well as the associated B(E2) values are determined for each even-even isotope of the $^{180-196}$Pt chain by the exact solutions of some differential equations which approximate the generalized Bohr-Mottelson Hamiltonian. The emerging approaches are called the Sextic and Spheroidal Approach (SSA), the Sextic and Mathieu Approach (SMA), the Infinite Square Well and Spheroidal Approach (ISWSA) and the Infinite Square Well and Mathieu Approach (ISWMA), respectively. While the first three methods were formulated in some earlier papers of the present authors, ISWMA is an inedited approach of this work. Numerical results are compared with those obtained with the so called X(5) and Z(5) models. A contour plot for the probability density as function of the intrinsic dynamic deformations is given for a few states of the three considered bands with the aim of evidencing the shape evolution along the isotope chain and pointing out possible shape coexistence.

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arXiv2013

Application of the sextic oscillator with centrifugal barrier and the spheroidal equation for some X(5) candidate nuclei

The eigenvalue equation associated to the Bohr-Mottelson Hamiltonian is considered in the intrinsic reference frame and amended by replacing the harmonic oscillator potential in the $β$ variable with a sextic oscillator potential with centrifugal barrier plus a periodic potential for the $γ$ variable. After the separation of variables, the $β$ equation is quasi-exactly solved, while the solutions for the $γ$ equation are just the angular spheroidal functions. An anharmonic transition operator is used to determine the reduced E2 transition probabilities. The formalism is conventionally called the Sextic and Spheroidal Approach (SSA) and applied for several X(5) candidate nuclei: $^{176,178,180,188,190}$Os, $^{150}$Nd, $^{170}$W, $^{156}$Dy, $^{166,168}$Hf. The SSA predictions are in good agreement with the experimental data of the mentioned nuclei. The comparison of the SSA results with those yielded by other models, such as X(5) \cite{Iache9}, Infinite Square Well (ISW) \cite{Raduta}, and Davidson (D) like potential \cite{Raduta} for the $β$, otherwise keeping the spheroidal functions for the $γ$, and the Coherent State Model (CSM) \cite{Rad1,Rad2,Rad3,Rad4,RaSa,Rad5} respectively, suggests that SSA represents a good approach to describe nuclei achieving the critical point of the U(5)$\rightarrow$SU(3) shape phase transition.

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arXiv2011

New features of the triaxial nuclei described with a coherent state model

Supplementing the Liquid Drop Model (LDM) Hamiltonian, written in the intrinsic reference frame, with a sextic oscillator plus a centrifugal term in the variable $β$ and a potential in $γ$ with a minimum in $\fracπ{6}$, the Schödinger equation is separated for the two variables which results in having a new description for the triaxial nuclei, called Sextic and Mathieu Approach (SMA). SMA is applied for two non-axial nuclei, $^{180}$Hf and $^{182}$W and results are compared with those yielded by the Coherent State Model (CSM). As the main result of this paper we derive analytically the equations characterizing SMA from a semi-classical treatment of the CSM Hamiltonian. In this manner the potentials in $β$ and $γ$ variables respectively, show up in a quite natural way which contrasts their ad-hoc choice when SMA emerges from LDM.

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arXiv2008

A solvable model which has X(5) as a limiting symmetry and removes some inherent drawbacks

Solvable Hamiltonians for the $β$ and $γ$ intrinsic shape coordinates are proposed. The eigenfunctions of the $γ$ Hamiltonian are spheroidal periodic functions, while the Hamiltonian for the $β$ degree of freedom involves the Davidson's potential and admits eigenfunctions which can be expressed in terms of the generalized Legendre polynomials. The proposed model goes to X(5) in the limit of $|γ|$-small. Some drawbacks of the X(5) model, as are the eigenfunction periodicity and the $γ$ Hamiltonian hermiticity, are absent in the present approach. Results of numerical applications to $^{150}$Nd, $^{154}$Gd and $^{192}$Os are in good agreement to the experimental data. Comparison with X(5) calculations suggests that the present approach provides a quantitative better description of the data. This is especially true for the excitation energies in the gamma band.

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