arXiv subjects

V. O. Nesterenko

Publications and source records attributed to V. O. Nesterenko.

At least 37 records · Page 2Linked to original sources

arXiv2018

Individual low-energy toroidal dipole state in $^{24}$Mg

The low-energy dipole excitations in $^{24}$Mg are investigated within the Skyrme quasiparticle random-phase-approximation (QRPA) for axial nuclei. The calculations with the force SLy6 reveal a remarkable feature: the lowest $I^πK=1^-1$ excitation (E = 7.92 MeV) in $^{24}$Mg is a vortical toroidal state (TS) representing a specific vortex-antivortex realization of well-known spherical Hill's vortex in a strongly deformed axial confinement. This is a striking example of an {\it individual} TS which can be much easier discriminated in experiment than the toroidal dipole resonance embracing many states. The TS acquires the lowest energy due to the huge prolate axial deformation in $^{24}$Mg. The result persists for different Skyrme parameterizations (SLy6, SVbas, SkM*). We analyze spectroscopic properties of the TS and its relation with the cluster structure of $^{24}$Mg. Similar TS could exist in other highly prolate light nuclei. They could serve as promising tests for various reactions to probe a vortical (toroidal) nuclear flow.

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arXiv2018

Wavelet signatures of $K$-splitting of the Isoscalar Giant Quadrupole Resonance in deformed nuclei from high-resolution (p,p$'$) scattering off $^{146,148,150}$Nd

The phenomenon of fine structure of the Isoscalar Giant Quadrupole Resonance (ISGQR) has been studied with high energy-resolution proton inelastic scattering at iThemba LABS in the chain of stable even-mass Nd isotopes covering the transition from spherical to deformed ground states. A wavelet analysis of the background-subtracted spectra in the deformed 146,148,150Nd isotopes reveals characteristic scales in correspondence with scales obtained from a Skyrme RPA calculation using the SVmas10 parameterization. A semblance analysis shows that these scales arise from the energy shift between the main fragments of the K = 0, 1 and K = 2 components.

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arXiv2017

Deformation dependence of the isovector giant dipole resonance: The neodymium isotopic chain revisited

Proton inelastic scattering experiments at energy E_p = 200 MeV and a spectrometer scattering angle of 0 degree were performed on 144,146,148,150Nd and 152Sm exciting the IsoVector Giant Dipole Resonance (IVGDR). Comparison with results from photo-absorption experiments reveals a shift of resonance maxima towards higher energies for vibrational and transitional nuclei. The extracted photo-absorption cross sections in the most deformed nuclei, 150Nd and 152Sm, exhibit a pronounced asymmetry rather than a distinct double-hump structure expected as a signature of K-splitting. This behaviour can be related to the proximity of these nuclei to the critical point of the phase shape transition from vibrators to rotors with a soft quadrupole deformation potential. Self-consistent random-phase approximation (RPA) calculations using the SLy6 Skyrme force provide a relevant description of the IVGDR shapes deduced from the present data

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arXiv2017

Pairing and deformation effects in nuclear excitation spectra

We investigate effects of pairing and of quadrupole deformation on two sorts of nuclear excitations,$γ$-vibrational $K^π=2^+$ states and dipole resonances (isovector dipole, pygmy, compression, toroidal). The analysis is performed within the quasiparticle random-phase approximation (QRPA) based on the Skyrme energy functional using the Skyrme parametrization SLy6. Particular attention is paid to i) the role of the particle-particle (pp) channel in the residual interaction of QRPA, ii) comparison of volume pairing (VP) and surface pairing (SP), iii) peculiarities of deformation splitting in the various resonances. We find that the impact of the pp-channel on the considered excitations is negligible. This conclusion applies also to any other excitation except for the $K^π=0^+$ states. Furthermore, the difference between VP and SP is found small (with exception of peak height in the toroidal mode). In the low-energy isovector dipole (pygmy) and isoscalar toroidal modes, the branch $K^π=1^-$ is shown to dominate over $K^π=0^-$ one in the range of excitation energy $E <$ 8--10 MeV. The effect becomes impressive for the toroidal resonance whose low-energy part is concentrated in a high peak of almost pure $K^π=1^-$ nature. This peculiarity may be used as a fingerprint of the toroidal mode in future experiments. The interplay between pygmy, toroidal and compression resonances is discussed, the interpretation of the observed isoscalar giant dipole resonance is partly revised.

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arXiv2017

Deformation-induced splitting of isoscalar E0 giant resonance: Skyrme random-phase-approximation analysis

The deformation-induced splitting of isoscalar giant monopole resonance (ISGMR) is systematically analyzed in a wide range of masses covering medium, rare-earth, actinide, and superheavy axial deformed nuclei. The study is performed within the fully self-consistent quasiparticle random-phase-approximation (QRPA) method based on the Skyrme functional. Two Skyrme forces, one with a large (SV-bas) and one with a small (SkP) nuclear incompressibility, are considered. The calculations confirm earlier results that, due to the deformation-induced E0-E2 coupling, the isoscalar E0 resonance attains a double-peak structure and significant energy upshift. Our results are compared with available analytic estimations. Unlike earlier studies, we get a smaller energy difference between the lower and upper peaks and thus a stronger E0-E2 coupling. This in turn results in more pumping of E0 strength into the lower peak and more pronounced splitting of ISGMR. We also discuss widths of the peaks and their negligible correlation with deformation.

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arXiv2016

Toroidal resonance: relation to pygmy mode, vortical properties and anomalous deformation splitting

We review a recent progress in investigation of the isoscalar toroidal dipole resonance (TDR). A possible relation of the TDR and low-energy dipole strength (also called a pygmy resonance) is analyzed. It is shown that the dipole strength in the pygmy region can by understood as a local manifestation of the collective vortical toroidal motion at the nuclear surface. Application of the TDR as a measure of the nuclear dipole vorticity is discussed. Finally, an anomalous splitting of the TDR in deformed nuclei is scrutinized.

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arXiv2016

Skyrme Random-Phase-Approximation description of lowest $K^π=2^+_γ$ states in axially deformed nuclei

The lowest quadrupole $γ$-vibrational $K^π=2^+$ states in axially deformed rare-earth (Nd, Sm, Gd, Dy, Er, Yb, Hf, W) and actinide (U) nuclei are systematically investigated within the separable random-phase-approximation (SRPA) based on the Skyrme functional. The energies $E_γ$ and reduced transition probabilities $B(E2)$ of $2^+_γ$-states are calculated with the Skyrme forces SV-bas and SkM$^*$. The energies of two-quasiparticle configurations forming the SRPA basis are corrected by using the pairing blocking effect. This results in a systematic downshift of $E_γ$ by 0.3-0.5 MeV and thus in a better agreement with the experiment, especially in Sm, Gd, Dy, Hf, and W regions. For other isotopic chains, a noticeable overestimation of $E_γ$ and too weak collectivity of $2^+_γ$-states still persist. It is shown that domains of nuclei with a low and high $2^+_γ$ -collectivity are related with the structure of the lowest 2-quasiparticle states and conservation of the Nilsson selection rules. The description of $2^+_γ$ states with SV-bas and SkM$^*$ is similar in light rare-earth nuclei but deviates in heavier nuclei. However SV-bas much better reproduces the quadrupole deformation and energy of the isoscalar giant quadrupole resonance. The accuracy of SRPA is justified by comparison with exact RPA. The calculations suggest that a further development of the self-consistent calculation schemes is needed for a systematic satisfactory description of the $2^+_γ$ states.

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arXiv2016

Magnetic dipole excitations of $^{50}$Cr

The low-lying $M1$-strength of the open-shell nucleus $^{50}$Cr has been studied with the method of nuclear resonance fluorescence up to 9.7 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC and Compton backscattered photons at the High Intensity $γ$-ray Source (HI$γ$S) facility between 6 and 9.7 MeV of the initial photon energy. Fifteen $1^{+}$ states have been observed between 3.6 and 9.7 MeV. Following our analysis, the lowest $1^{+}$ state at 3.6 MeV can be considered as an isovector orbital mode with some spin admixture. The obtained results generally match the estimations and trends typical for the scissors-like mode. Detailed calculations within the Skyrme Quasiparticle Random-Phase-Approximation method and the Large-Scale Shell Model justify our conclusions. The calculated distributions of the orbital current for the lowest $1^{+}$-state suggest the schematic view of Lipparini and Stringari (isovector rotation-like oscillations inside the rigid surface) rather than the scissors-like picture of Lo Iudice and Palumbo. The spin M1 resonance is shown to be mainly generated by spin-flip transitions between the orbitals of the $fp$-shell.

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arXiv2015

Deformation-induced splitting of the monopole giant resonance in 24Mg

The strong deformation splitting of the isoscalar giant monopole resonance (ISGMR), recently observed in ($α,α'$) reaction in prolate $^{24}$Mg, is analyzed in the framework of the Skyrme quasiparticle random-phase-approximation (QRPA) approach with the Skyrme forces SkM*, SVbas and SkP$^δ$. The calculations with these forces give close results and confirm that the low-energy E0-peak is caused by the deformation-induced coupling of ISGMR with the K=0 branch of the isoscalar giant quadrupole resonance.

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arXiv2015

Monopole giant resonance in $^{100-132}$Sn, $^{144}$Sm and $^{208}$Pb

The isoscalar giant monopole resonance (GMR) in spherical nuclei $^{100-132}$Sn, $^{144}$Sm, and $^{208}$Pb is investigated within the Skyrme random-phase-approximation (RPA) for a variety of Skyrme forces and different pairing options. The calculated GMR strength functions are directly compared to the available experimental distributions. It is shown that, in accordance to results of other groups, description of GMR in Sn and heavier Sm/Pb nuclei needs different values of the nuclear incompressibilty, $K \approx$ 200 or 230 MeV, respectively. Thus none from the used Skyrme forces is able to describe GMR in these nuclei simultaneously. The GMR peak energy in open-shell $^{120}$Sn is found to depend on the isoscalar effective mass, which might be partly used for a solution of the above problem. Some important aspects of the problem (discrepancies of available experimental data, proper treatment of the volume and surface compression in finite nuclei, etc) are briefly discussed.

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arXiv2015

Deformation effects in Giant Monopole Resonance

The isoscalar giant monopole resonance (GMR) in Samarium isotopes (from spherical $^{144}$Sm to deformed $^{148-154}$Sm) is investigated within the Skyrme random-phase-approximation (RPA) for a variety of Skyrme forces. The exact RPA and its separable version (SRPA) are used for spherical and deformed nuclei, respectively. The quadrupole deformation is shown to yield two effects: the GMR broadens and attains a two-peak structure due to the coupling with the quadrupole giant resonance.

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arXiv2014

Relation of E1 pygmy and toroidal resonances

A possible relation of the low-lying E1 (pygmy resonance) and toroidal strengths is analyzed by using Skyrme-RPA results for the strength functions, transition densities and current fields in $^{208}$Pb. It is shown that the irrotational pygmy motion can appear as a local manifestation of the collective vortical toroidal dipole resonance (TDR) at the nuclear surface. The RPA results are compared to unperturbed (1ph) ones.

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arXiv2014

Transport of the repulsive Bose-Einstein condensate in a double-well trap: interaction impact and relation to Josephson effect

Two aspects of the transport of the repulsive Bose-Einstein condensate (BEC) in a double-well trap are inspected: impact of the interatomic interaction and analogy to the Josephson effect. The analysis employs a numerical solution of 3D time-dependent Gross-Pitaevskii equation for a total order parameter covering all the trap. The population transfer is driven by a time-dependent shift of a barrier separating the left and right wells. Sharp and soft profiles of the barrier velocity are tested. Evolution of the relevant characteristics, involving phase differences and currents, is inspected. It is shown that the repulsive interaction substantially supports the transfer making it possible i) in a wide velocity interval and ii) three orders of magnitude faster than in the ideal BEC. The transport can be approximately treated as the d.c. Josephson effect. A dual origin of the critical barrier velocity (break of adiabatic following and d.c.-a.c. transition) is discussed. Following the calculations, robustness of the transport (d.c.) crucially depends on the interaction and barrier velocity profile. Only soft profiles which minimize undesirable dipole oscillations are acceptable.

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arXiv2013

Nuclear vorticity in isoscalar E1 modes: Skyrme-RPA analysis

Two basic concepts of nuclear vorticity, hydrodynamical (HD) and Rawenthall-Wambach (RW), are critically inspected. As a test case, we consider the interplay of irrotational and vortical motion in isoscalar electric dipole E1(T=0) modes in $^{208}$Pb, namely the toroidal and compression modes. The modes are described in a self-consistent random-phase-approximation (RPA) with the Skyrme force SLy6. They are examined in terms of strength functions, transition densities, current fields, and formfactors. It is shown that the RW conception (suggesting the upper component of the nuclear current as the vorticity indicator) is not robust. The HD vorticity is not easily applicable either because the definition of a velocity field is too involved in nuclear systems. Instead, the vorticity is better characterized by the toroidal strength which closely corresponds to HD treatment and is approximately decoupled from the continuity equation.

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arXiv2013

Inverse population transfer of the repulsive Bose-Einstein condensate in a double-well trap: strong interaction-induced support

An inverse population transfer of the repulsive Bose-Einstein condensate (BEC) in a weakly bound double-well trap is explored within the 3D time-dependent Gross-Pitaevskii equation. The model avoids numerous common approximations (two-mode treatment, time-space factorization, etc) and closely follows the conditions of Heidelberg experiments, thus providing a realistic description of BEC dynamics. The transfer is driven by a time-dependent shift of a barrier separating the left and right wells. It is shown that completeness and robustness of the process considerably depend on the amplitude and time profile of the shift velocity. Soft profiles provide the most robust inversion. The repulsive interaction substantially supports the transfer making it possible i) in a wide velocity interval and ii) three orders of magnitude faster than in the ideal BEC.

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arXiv2013

Deformation effects in toroidal and compression dipole excitations of $^{170}$Yb: Skyrme-RPA analysis

The effect of nuclear deformation on the isoscalar toroidal and compression dipole modes in prolate $^{170}$Yb is studied in the framework of the random-phase-approximation method with a representative set of Skyrme forces (SV-bas, SLy6, SkM$^*$ and SkI3). It is shown that the deformation crucially redistributes the strength of both modes. The compression mode has the same sequence of $μ$=0 and 1 branches as the isovector giant dipole resonance where for prolate nuclei the $μ=0$ mode is lower in energy ($μ$ being the projection of the axial momentum of the mode). Instead, the toroidal mode exhibits an anomalous (opposite) sequence where the $μ$=1 branch precedes the $μ$=0 one.

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arXiv2013

Toroidal, compression, and vortical dipole strengths in $^{144-154}$Sm: Skyrme-RPA exploration of deformation effect

A comparative analysis of toroidal, compressional and vortical dipole strengths in the spherical $^{144}$Sm and the deformed $^{154}$Sm is performed within the random-phase-approximation using a set of different Skyrme forces. Isoscalar (T=0), isovector (T=1), and electromagnetic excitation channels are considered. The role of the nuclear convection $j_{\text{con}}$ and magnetization $j_{\text{mag}}$ currents is inspected. It is shown that the deformation leads to an appreciable redistribution of the strengths and causes a spectacular deformation splitting (exceeding 5 MeV) of the isoscalar compressional mode. In $^{154}$Sm, the $μ$=0 and $μ$=1 branches of the mode form well separated resonances. When stepping from $^{144}$Sm to $^{154}$Sm, we observe an increase of the toroidal, compression and vortical contributions in the low-energy region (often called pygmy resonance). The strength in this region seems to be an overlap of various excitation modes. The energy centroids of the strengths depend significantly on the isoscalar effective mass $m_0$. Skyrme forces with a large $m_0$ (typically $m_0/m \approx 0.8 - 1$) seem to be more suitable for description of experimental data for the isoscalar giant dipole resonance.

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arXiv2012

On the toroidal nature of the low-energy E1 mode

The nature of E1 low-energy strength (LES), often denoted as a "pygmy dipole resonance", is analyzed within the random-phase-approximation (RPA) in $^{208}$Pb using Skyrme forces in fully self-consistent manner. A first overview is given by the strength functions for the dipole, compressional and toroidal operators. More detailed insight is gained by averaged transition densities and currents where the latter provide very illustrative flow pattern. The analysis reveals clear isoscalar toroidal flow in the low-energy bin 6.0-8.8 MeV of the LES and a mixed isoscalar/isovector toroidal/compression flow in the higher bin 8.8-10.5 MeV. Thus the modes covered by LES embrace both vortical and irrotational motion. The simple collective picture of the LES as a "pygmy" mode (oscillations of the neutron excess against the nuclear core) is not confirmed.

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