1. Introduction
Magnetic moment, as one of the most important observables of atomic nucleus, provides rich information about nuclear structure, has attracted a lot of attentions for decades [1–4] and also challenges the various nuclear models. Among them, the description of the odd-A nuclei is one of the greatest successes of the nuclear shell model by Mayer and Jensen. In this extreme single-particle picture, the even–even core of the odd-A nuclei is regarded as an inert object and the corresponding magnetic moment is from the unpaired valence nucleon (valence-nucleon approximation), which leads to the well-known Schmidt values. Thus the magnetic moment of odd–even nuclei around doubly magic ones can be reproduced well, and the still existing deviations can be further explained by the meson exchange current (MEC, i.e. the exchange of a charged meson) and configuration mixing (CM, or core polarization, i.e. the correlation not included in the mean-field approximation) [2, 3, 5–9]. Recently, by considering the configuration mixing and meson exchange current corrections, the newly measured magnetic moment of 133Sb [10], 67Ni, 69Cu [11], 49Sc [12] and other odd nuclei near magic ones [13] have been well reproduced.
In spherical odd-mass nuclei, the addition of an odd nucleon to an even core generates the configuration mixing, which can be treated in the perturbation theory. For deformed odd-A nuclei, the even–even core is not inert, and the magnetic moment can not be fully understood by the valence-nucleon approximation. Although the spin of the deformed odd-A nuclei is in some way determined by the orbit of the last odd nucleon, the contribution from the core can not be ignored. Therefore the odd nucleon should be coupled to a collective nuclear droplet (the core), i.e. a strong coupling between the collective rotation and intrinsic single particle motion in the well-deformed nuclei [14]. In such a strong coupling approximation, the magnetic moment is determined by the final spin and intrinsic nucleonic motion. Up to now, the magnetic moments of odd-mass nuclei such as carbon and neon isotopes have been investigated by using the deformed Skyrme Hartree–Fock model [15] and deformed axially symmetric Woods–Saxon potential [16].
On the theoretical side, many successful nuclear structure models have been developed in the past few decades. However, the application for nuclear magnetic moments is still not satisfactory and the theoretical description of nuclear magnetic moments has been a long-standing problem [3, 6, 7]. The covariant density functional theory (CDFT), taking Lorentz symmetry into account in a self-consistent way, has received wide attention due to its successful description of many nuclear properties in a large number of stable and exotic nuclei [17–23] and the successful application of its predictions to r-process simulations [24–26]. It includes naturally the nuclear spin–orbit potential in a covariant way. It can well reproduce the isotopic shifts of Pb isotopes [27] and explain naturally the origin of the pseudospin symmetry as the relativistic symmetry [28] and the spin symmetry in the anti-nucleon spectrum [29]. Moreover, it can include the nuclear magnetism self-consistently [30], and provide a consistent description of currents and time-odd fields, which play an important role in nuclear magnetic moments [8, 31–35] and nuclear rotations [36–41].
In particular, by considering the time-odd fields, one-pion exchange current, first-order and second-order configuration mixing effects, the magnetic moments of spherical odd-A nuclei with doubly closed shell core plus or minus one nucleon have been well reproduced [9, 32–35]. However, the present covariant description of nuclear magnetic moment is mainly restricted to the spherical odd-A nuclei near magic shell. Although the magnetic moment of deformed odd-A nucleus 33Mg has also been described by the CDFT [42], the ground-state spin should be investigated further.
Rotational phenomenon in sd shell of light nuclei has attracted a lot of attentions, including the rotational bands in alpha-cluster nuclei [43]. In the middle of the sd shell, odd-mass nuclei with mass number 19 ≤ A ≤ 25 are strongly deformed [14], and their low-lying states form rotational bands. In particular, the 5/2[202] orbit, the only nilsson orbit with ${K}^{\pi }=5/{2}^{+}$ in the sd shell region, forms the ground-state band in the A = 25 mirror nuclei 25Mg and 25Al. The facts that ground state spin of odd-A aluminum isotopes, i.e. 23,25,27,29,31,33Al, is $5/{2}^{+}$, and the ab initio shell-model calculations [44] indicate the odd proton occupying the d5/2 orbit in the ground state together with corresponding nuclear magnetic moments provides a good opportunity to investigate the rotational structure in those deformed nuclei.
Based on the above considerations, it is necessary to study the ground-state magnetic moment of odd-A aluminum isotopes in deformed CDFT. In section 2 , we will briefly introduce the theoretical framework of CDFT and corresponding formulas for calculating the magnetic moments in the strong coupling approximation. The calculations are described and the results are discussed in section 3 . Finally, section 4 contains a brief summary and a perspective.
2. Framework
Following the finite-range meson-exchange version of the covariant density functional theory in [22, 45, 46], where nucleons are described as Dirac particles interacting via the exchange of the isoscalar meson σ, isoscalar-vector meson ω and isovector-vector meson ρ as well as photon, the lagrangian density is adopted as follows
$\begin{eqnarray}\begin{array}{rcl}{ \mathcal L } & = & \overline{\psi }\left[{\rm{i}}{\gamma }^{\mu }{\partial }_{\mu }-M-{g}_{\sigma }\sigma -{g}_{\omega }{\gamma }^{\mu }{\omega }_{\mu }-{g}_{\rho }{\gamma }^{\mu }\vec{\tau }\cdot {\vec{\rho }}_{\mu }\right.\\ & & \left.-e{\gamma }^{\mu }\displaystyle \frac{1-{\tau }_{3}}{2}{A}_{\mu }\right]\psi +\displaystyle \frac{1}{2}{\partial }^{\mu }\sigma {\partial }_{\mu }\sigma -\displaystyle \frac{1}{2}{m}_{\sigma }^{2}{\sigma }^{2}-\displaystyle \frac{1}{3}{g}_{2}{\sigma }^{3}\\ & & -\displaystyle \frac{1}{4}{g}_{3}{\sigma }^{4}-\displaystyle \frac{1}{4}{{\rm{\Omega }}}^{\mu \nu }{{\rm{\Omega }}}_{\mu \nu }+\displaystyle \frac{1}{2}{m}_{\omega }^{2}{\omega }^{\mu }{\omega }_{\mu }+\displaystyle \frac{1}{4}{c}_{3}{\left({\omega }^{\mu }{\omega }_{\mu }\right)}^{2}\\ & & -\displaystyle \frac{1}{4}{\vec{R}}^{\mu \nu }\cdot {\vec{R}}_{\mu \nu }+\displaystyle \frac{1}{2}{m}_{\rho }^{2}{\vec{\rho }}^{\mu }\cdot {\vec{\rho }}_{\mu }-\displaystyle \frac{1}{4}{F}^{\mu \nu }{F}_{\mu \nu }.\end{array}\end{eqnarray}$
It should be noted that for the even–even nuclei with time-reversal symmetry, the space-like components of vector meson and photon fields, i.e. time-odd fields, vanish and do not contribute to the energy functional. However, in odd-A nuclei, the unpaired odd nucleon breaks the time-reversal invariance, and time-odd fields ${\boldsymbol{V}}({\boldsymbol{r}})$ exist. Then the Dirac equation for nucleon becomes
$\begin{eqnarray}\{{\boldsymbol{\alpha }}\cdot [-{\rm{i}}{\boldsymbol{\nabla }}-{\boldsymbol{V}}({\boldsymbol{r}})]+{V}_{0}({\boldsymbol{r}})+\beta [M+S({\boldsymbol{r}})]\}{\psi }_{i}={\varepsilon }_{i}{\psi }_{i},\end{eqnarray}$
with $S({\boldsymbol{r}})$ the scalar potential $\begin{eqnarray}S({\boldsymbol{r}})={g}_{\sigma }\sigma ({\boldsymbol{r}}),\end{eqnarray}$
${V}_{0}({\boldsymbol{r}})$ the usual vector potential, i.e. the time-like component of vector potential, $\begin{eqnarray}{V}_{0}({\boldsymbol{r}})={g}_{\omega }{\omega }_{0}({\boldsymbol{r}})+{g}_{\rho }{\tau }_{3}{\rho }_{0}({\boldsymbol{r}})+e(1-{\tau }_{3}){A}_{0}({\boldsymbol{r}})/2,\end{eqnarray}$
and ${\boldsymbol{V}}({\boldsymbol{r}})$ the space-like component of vector potential $\begin{eqnarray}{\boldsymbol{V}}({\boldsymbol{r}})={g}_{\omega }{\boldsymbol{\omega }}({\boldsymbol{r}}).\end{eqnarray}$
${\boldsymbol{\rho }}({\boldsymbol{r}})$ and ${\boldsymbol{A}}({\boldsymbol{r}})$ are often not taken into account as they turn out to be small compared with ${\boldsymbol{\omega }}({\boldsymbol{r}})$ field in light nuclei [47]. In the present paper, the bold types are adopted to indicate space vectors and arrows for the vectors in isospin space.The Klein–Gordon equations for scalar meson field σ, time-like components of vector mesons fields ω0, ρ0 and electromagnetic fields A0 are the same as in the [45]. The space-like component of vector meson field ${\boldsymbol{\omega }}$ is determined by
$\begin{eqnarray}\{-{\rm{\Delta }}+{m}_{\omega }^{2}\}{\boldsymbol{\omega }}={g}_{\omega }{{\boldsymbol{j}}}_{{\rm{B}}}-{c}_{3}{\omega }^{\nu }{\omega }_{\nu }{\boldsymbol{\omega }},\end{eqnarray}$
with the baryon current ${{\boldsymbol{j}}}_{{\rm{B}}}={\sum }_{i}{n}_{i}{\bar{\psi }}_{i}{\boldsymbol{\gamma }}{\psi }_{i}$. As the pair correlation is not considered here, the occupation numbers ni take the value one (zero) for the states below (above) the Fermi surface. For more details, such as the total energy of the nucleus after considering the time-odd fields, can be found in [42] and references therein.The electromagnetic current operator used to describe the nuclear magnetic moment is written as [31, 42, 48–50]
$\begin{eqnarray}{\hat{J}}^{\mu }(x)=Q\bar{\psi }(x){\gamma }^{\mu }\psi (x)+\displaystyle \frac{\kappa }{2M}{\partial }_{\nu }[\bar{\psi }(x){\sigma }^{\mu \nu }\psi (x)],\end{eqnarray}$
where the nucleon charge is $Q\equiv \displaystyle \frac{e}{2}(1-{\tau }_{3})$, the antisymmetric tensor operator is ${\sigma }^{\mu \nu }=\displaystyle \frac{{\rm{i}}}{2}[{\gamma }^{\mu },{\gamma }^{\nu }]$, and κ is the free anomalous gyromagnetic ratio of the nucleon with κp = 1.793 and κn = −1.913 for proton and neutron respectively.In equation (7 ), the first term gives the Dirac current and the second term is the so-called anomalous current. The nuclear dipole magnetic moment, in unit of the nuclear magneton ${\mu }_{{\rm{N}}}=e{\hslash }/2{Mc}$, is given by [35]7 ). The first term in above equation gives the Dirac magnetic moment, and the second term gives the anomalous magnetic moment.
$\begin{eqnarray}{\boldsymbol{\mu }}=\displaystyle \frac{1}{2{\mu }_{{\rm{N}}}}\int {\rm{d}}{\boldsymbol{r}}{\boldsymbol{r}}\times \langle g.s.| \hat{{\boldsymbol{j}}}({\boldsymbol{r}})| g.s.\rangle \end{eqnarray}$
$\begin{eqnarray}=\int {\rm{d}}{\boldsymbol{r}}\,\left[\displaystyle \frac{{{Mc}}^{2}}{{\hslash }c}Q{\psi }^{+}({\boldsymbol{r}}){\boldsymbol{r}}\times {\boldsymbol{\alpha }}\psi ({\boldsymbol{r}})+\kappa {\psi }^{+}({\boldsymbol{r}})\beta {\boldsymbol{\Sigma }}\psi ({\boldsymbol{r}})\right],\end{eqnarray}$
where $\hat{{\boldsymbol{j}}}({\boldsymbol{r}})$ is the operator of space-like components of the electromagnetic current in equation (Therefore, the nuclear magnetic moment vector operator in the covariant density functional theory, in unit of μN, is given by
$\begin{eqnarray}\hat{{\boldsymbol{\mu }}}=\displaystyle \frac{{{Mc}}^{2}}{{\hslash }c}Q{\boldsymbol{r}}\times {\boldsymbol{\alpha }}+\kappa \beta {\boldsymbol{\Sigma }}.\end{eqnarray}$
For deformed odd-A nuclei, the valence nucleon approximation is invalid and there is a strong coupling between the core and the valence nucleon. Therefore, the total magnetic moment consists of two parts, i.e. the intrinsic nucleonic motion and the collective rotational motion. In the axially deformed case with the projection of the momentum on the nuclear symmetry axis K (contribution from the intrinsic nucleonic motion) and spin I, the nuclear magnetic moment can be written as [14]8a ).
$\begin{eqnarray}\mu ={g}_{R}I+({g}_{K}-{g}_{R})\displaystyle \frac{{K}^{2}}{I+1},\quad \quad ({\rm{for}}\ K\gt 1/2).\end{eqnarray}$
gR is the corresponding effective rotational gyromagnetic factor, with ${g}_{R}\sim Z/A$. The intrinsic gyromagnetic factor reads ${g}_{K}={\mu }_{\mathrm{intri}.}/K$ and intrinsic magnetic moment ${\mu }_{\mathrm{intri}.}$ is obtained from self-consistent CDFT calculations in equation (3. Results and discussion
In the present CDFT calculations, both the Dirac equation for nucleons and the Klein–Gordon equations for mesons are solved in a isotropic harmonic oscillator basis [51, 52] and a basis of 14 major oscillator shells is adopted. The oscillator frequency is given by ${\hslash }{\omega }_{0}=41{A}^{-1/3}$ MeV. The effective meson-exchange interaction parameter PK1 [46] is used throughout the calculation.
The energy curve, i.e. the total energies as a function of quadrupole deformation, is obtained through the quadrupole deformation constrained calculation by constraining the mass quadrupole moment $\langle {\hat{Q}}_{2}\rangle $ to a given value q [53],
$\begin{eqnarray}\langle H^{\prime} \rangle =\langle H\rangle +\displaystyle \frac{1}{2}C{\left(\langle {\hat{Q}}_{2}\rangle -q\right)}^{2},\end{eqnarray}$
where $\langle H\rangle $ is the total energy and C is the stiffness constant. The quadrupole deformation parameter β2 is obtained from the calculated $\langle {\hat{Q}}_{2}\rangle $ through $\begin{eqnarray}\langle {\hat{Q}}_{2}\rangle =\langle {\hat{Q}}_{2}{\rangle }_{p}+\langle {\hat{Q}}_{2}{\rangle }_{n}=\displaystyle \frac{3}{\sqrt{5\pi }}{{AR}}_{0}^{2}{\beta }_{2},\end{eqnarray}$
with ${R}_{0}=1.2{A}^{1/3}$. Both the adiabatic and the configuration-fixed deformation constrained calculations [45, 54, 55] will be adopted in the following.In figure 1(a), the energy curves for 23Al, i.e. the total energies as a function of the quadrupole deformation parameter β2 calculated by adiabatic (shown as open circles) and configuration-fixed (shown as solid lines) deformation constrained CDFT approach with time-odd fields, are presented. The local minima in the energy curves for each configuration are represented by stars and labeled as A, B, C, D and E. A is the ground state and found to be prolate deformed, β2 = 0.40, with the total energy of −167.89 MeV, in comparison with the corresponding data of −168.72 MeV [56]. The deviation between experimental and theoretical energy is 0.83 MeV, which could be caused by rotational energy correction [58]. It should be noted that the ${\beta }_{2}$ values obtained by Hartree–Fock–Bogoliubov calculations based on the D1S Gongy effective nucleon–nucleon interaction is also around 0.4 [59], further supporting the large deformation in 23Al.
Figure 1. The total energies (a) and magnetic moments (b) for 23Al as functions of quadrupole deformation β2 by adiabatic and configuration-fixed (indicated by open circles and solid lines respectively) deformation constrained CDFT approach with time-odd fields using PK1 parameter set. The minima in the energy curves for different configurations are indicated by stars and marked as A, B, C, D, and E respectively. In panel (a), the experimental energy −168.72 MeV [56] (dotted line) is shown for comparison with A. In panel (b), the experimental magnetic moment μ = 3.889μN [57] (dotted line in (b)) and the Schmidt magnetic moments of $\pi 2{s}_{1/2},\pi 1{d}_{3/2}$, and $\pi 1{d}_{5/2}$ orbitals (dashed lines) are also shown for comparison. |
Using equation (8a ), the effective electromagnetic current gives the intrinsic magnetic moment of valence nucleons with assigned configurations in figure 1(b). The calculated magnetic moment for the ground state is 4.96 μN, larger than the data μ = 3.889 μN [57]. It could be seen that the intrinsic magnetic moment is sensitive to configuration, but to β2, it is not sensitive for the configurations A and E, and sensitive for B, C and D. This can be understood by the configuration mixing of single particle components. For example, the magnetic moment of configuration A does not change much with β2, as the odd proton occupies the orbital $\pi 5/2$[202+] with third component K = 5/2 and the configuration mixing is very weak. In comparison, the third component in configuration B, i.e. π1/2[211+], is K = 1/2, which has a strong mixing for configurations πd3/2, s1/2 and d5/2, thus the magnetic moment varies quickly with β2.
The calculated total energies (Etot), excitation energies (Ex), quadrupole deformation (βcal), valence nucleon configuration and intrinsic magnetic moments (μintri.) of 23Al for different configurations are listed in table 1. For ground state A, it has prolate deformation, and main component of corresponding valence nucleon wave function $\pi \tfrac{5}{2}$[202+] belongs to $\pi \,1{d}_{5/2}$, while the intrinsic magnetic moment $\,4.96\,{\mu }_{{\rm{N}}}$ is close to the Schmidt value $\,4.79\,{\mu }_{{\rm{N}}}$ of $\pi \,1{d}_{5/2}$. For state B, the unpaired valence nucleon configuration is $\pi \tfrac{1}{2}$[211+] with the main component belonging to $\pi \,2{s}_{1/2}$, and the calculated excitation energy 1.12 MeV is also in reasonable agreement with the corresponding experimental data 0.550 MeV of $1/{2}^{+}$ in [60]. For states C, and E, they have negative quadrupole deformation, and corresponding configurations are $\pi \tfrac{1}{2}$[220+] and $\pi \tfrac{3}{2}$[211+]. It could be seen one pair of neutrons have been broken in state D. In fact, Both the main components of state C and E are belonging to $\pi \,1{d}_{5/2}$ but with different third components, and the corresponding excitation energies of states C and E are 2.39 and 4.20 MeV, close to the experimental values 2.575 and 3.197 MeV.
Table 1. The calculated total energies (Etot), the excitation energies (Ex), the quadrupole deformation parameters (β2), the valence nucleon configuration, and the intrinsic magnetic moments (μintri.) of 23Al for states A, B, C, D and E, in comparison with the experimental energies Ex(exp.) [60], and ground-state magnetic moment (μexp.) [57]. The energy is in unit of MeV and the magnetic moment is in μN. |
| State | $\,\,{E}_{x}({E}_{{\rm{tot}}})$ | β2 | Valence nucleon configuration | μintri.(μexp.) | Ex(exp.) |
|---|---|---|---|---|---|
| A | (−167.89) | 0.40 | $\pi \tfrac{5}{2}$[202+] $\left(1{d}_{5/2}\tfrac{5}{2}\right)$ | 4.96 (3.889) | |
| B | 1.12(−166.77) | 0.52 | $\pi \tfrac{1}{2}$[211+] $\left(2{s}_{1/2}\tfrac{1}{2}\right)$ | −0.23 | 0.550 |
| C | 2.39(−165.50) | −0.26 | $\pi \tfrac{1}{2}$[220+] $\left(1{d}_{5/2}\tfrac{1}{2}\right)$ | 1.52 | (2.575) |
| D | 2.35(−165.54) | 0.28 | $\pi \tfrac{5}{2}[202+]\otimes \nu \left\{\tfrac{1}{2}[220+]\tfrac{3}{2}[211+]\right\}$ | 2.11 | |
| $\left(\pi \ 1{d}_{5/2}\tfrac{5}{2}\otimes \nu \ 1{d}_{5/2}\tfrac{1}{2}\ 1{d}_{5/2}\tfrac{3}{2}\right)$ | |||||
| E | 4.20(−163.69) | −0.18 | $\pi \tfrac{3}{2}$[211+] $\left(1{d}_{5/2}\tfrac{3}{2}\right)$ | −0.55 | (3.197) |
In order to inspect the evolution of the single-proton level and corresponding configuration for 23Al, proton single-particle energies as a function of β2 for the configuration A and C are shown in figure 2. The states with positive (negative) parity are marked by solid (dashed) lines, and the occupied orbitals are labeled by filled circles. The self-consistent CDFT calculation indicates the odd proton is in 5/2[202] orbital with β2 = 0.40 for the ground state.
Figure 2. Single-proton energy levels of the ground-state configuration A and C in 23Al obtained from configuration-fixed deformation constrained calculations. Each pair of time reversal conjugate states are split up into two levels with the opposite third component of angular momentum Ω > 0 and $\bar{{\rm{\Omega }}}\lt 0$, denoted by black and red lines respectively. The filled circles indicate the corresponding occupations in the ground state A and state C. |
As presented in figure 2, each pair of time reversal conjugate states is split up into two levels with opposite third component of angular momentum Ω > 0 and $\bar{{\rm{\Omega }}}\lt 0$, due to the broken time reversal invariance by the time-odd fields. The energy splitting for time reversal conjugate states ranges from 0.01 to 0.6 MeV, and the larger splitting occurs at the orbital with larger third component. At β2 ≈ 0.48, the level crossing happens between the 5/2[202] and 1/2[211] orbital.
In table 2, the calculated energy (Ecal), quadrupole deformation (β2), valence nucleon configuration, intrinsic magnetic moment (μintri.), and the final magnetic moment (μtot.) of odd-A Al isotopes in CDFT approach are presented, in comparison with the corresponding experimental spin, parity and magnetic moment. Generally speaking, the quadrupole deformation of Al isotopes is decreasing as the neutron number increases except for 29Al. The quadrupole deformation β2 of 33Al in CDFT calculation is 0.06 and indicates a good magic shell for N = 20. The obtained β2 is also in agreement with the Hartree–Fock–Bogoliubov calculations based on the D1S Gongy effective nucleon–nucleon interaction [59]. In fact, the region of deformation around the classic magic number N = 20 is a hot topic, while 33Al located at the edge of the island of inversion has a transitional character and is thought to be a key isotope as the transition into the island of inversion [61] is particularly rapid in the N = 20 isotones. Recently, the measurement of electric quadrupole moment and corresponding shell model calculation show that a component of intruder configuration, i.e. two-particle-two-hole (2p–2h) neutron excitation across N = 20, from the sd orbitals to the fp orbitals, exists in the ground state wave function [62]. However, the ab initio shell-model calculations together with phenomenological USDB interaction [63] present that the sd model space is able to reproduce correctly the electromagnetic moments of Al isotopes, including 33Al [44]. Thus the shell structure in 33Al needs more investigation. In addition, the ab initio shell-model calculations also support the occupancy of d5/2 proton orbital is approaching 5 [44], in agreement with the present CDFT calculation as well as the last unpaired proton occupying the $5/{2}^{+}[202]$ orbital. It should be pointed out that the calculated total energies within CDFT in 23,25,27,29,31Al are close and also slightly larger than the experimental values except for 33Al. The case that the CDFT energies of Al isotopes with N ≥ 20 are smaller than the data is also observed in relativistic continuum Hartree-Bogoliubov (RCHB) calculations [64].
Table 2. The calculated energy $\left({E}_{{\rm{cal}}}\right)$, the quadrupole deformation (β2), valence nucleon configuration, intrinsic magnetic moment (μintri.), and final magnetic moments (μtot.) of odd-A Al isotopes in CDFT approach, in comparison with the corresponding experimental spin, parity, energy and magnetic moment. |
| CDFT | Exp. | |||||||
|---|---|---|---|---|---|---|---|---|
| Nuclei | β2 | Etot(MeV) | Configuration | μintri.(μN) | μtot.(μN) | Iπ | Eexp. | μexp. |
| 23Al | 0.40 | −167.89 | $\pi 5/{2}^{+}[202]$ | 4.96 | 3.92 | 5/2+ | −168.72 | 3.889(5) |
| 25Al | 0.39 | −198.01 | $\pi 5/{2}^{+}[202]$ | 5.00 | 3.91 | 5/2+ | −200.52 | 3.6455(12) |
| 27Al | 0.18 | −221.26 | $\pi 5/{2}^{+}[202]$ | 5.10 | 3.96 | 5/2+ | −224.95 | 3.6415069(7) |
| 29Al | 0.27 | −239.65 | $\pi 5/{2}^{+}[202]$ | 5.12 | 3.96 | 5/2+ | −242.10 | |
| 31Al | 0.17 | −253.63 | $\pi 5/{2}^{+}[202]$ | 5.16 | 3.96 | 5/2+ | −255.00 | 3.830(5) |
| 33Al | 0.06 | −267.43 | $\pi 5/{2}^{+}[202]$ | 5.22 | 3.99 | 5/2+ | −264.65 | 4.088(5) |
In figure 3, the magnetic moments of odd-A Al isotopes in CDFT approach with and without considering the collective coupling are presented, in comparison with the corresponding experimental data [57]. The intrinsic magnetic moments are obtained from the CDFT calculations based on the valence nucleon configuration $\pi 1{d}_{5/2}$ for 23,25,27,29,31,33Al. It is easy to see that the intrinsic magnetic moments for Al isotopes are around 5μN, close to the Schmidt value 4.79 μN of $\pi 1{d}_{5/2}$. After including the coupling of collective rotation and intrinsic single particle motion within equation (10 ), the calculation is greatly improved and the magnetic moments of Al isotopes are well reproduced, with relative deviation from the available data of five nuclei is about 5%. Taking 23Al as an example, the intrinsic magnetic moment is 4.96 μN and the final magnetic moment with collective motion is 3.92 μN, excellently reproducing the experimental value 3.889 μN. The calculated magnetic moments of 25Al and 27Al are 3.91 and 3.96 μN, close to the data 3.646 and 3.642 μN, respectively. For 31Al and 33Al, the calculated magnetic moment in CDFT are 3.96 and 3.99 μN, also well reproducing the corresponding data 3.830 and 4.088 μN, respectively.
Figure 3. Magnetic moments of odd-A Al isotopes in CDFT approaches with and without considering the collective motion, in comparison with the corresponding experimental data [57]. The results with (red) and without (blue) time-odd fields (TOF) are also shown. |
Comparing the CDFT results with and without considering the time-odd fields shown in figure 3, it is found that the theoretical descriptions are improved after including the time-odd fields, i.e. the relative deviation from the data of five nuclei is reduced from 7.6% to 5%. It can be understood that the self-consistent polarization currents in core will slightly reduce the intrinsic magnetic moment here and finally bring the total magnetic moment closer to the data, while the importance of time-odd fields on nuclear magnetic moments has been discussed in [31, 47].
In the CDFT calculations, the empirical value ${g}_{R}=Z/A$ of rigid rotor is adopted and it varies from 0.39 to 0.57 for 23–33Al. Moreover, the collective gyromagnetic factors gR for 25Al and 27Al are 0.52 and 0.48, respectively, close to the available experimental gyromagnetic factor for even–even core 24Mg (0.54) and 26Mg (0.50). In [65], the self-consistent CDFT calculations for the gyromagnetic factor of low-lying excited states in 24Mg were carried out, and the available experimental gyromagnetic factor has been reproduced quite well. Furthermore, the calculated gyromagnetic factors have been found to be almost the same for the low-lying excited states with different angular momenta and close to the empirical value. In fact, even when the collective gyromagnetic factor gR is increased or decreased by 20%, the final magnetic moments change slightly. It can be understood that the collective gyromagnetic factor Z/A is far less than the intrinsic magnetic moment μintri. as shown in equation (10 ).
4. Summary and prospective
In summary, the ground-state properties, especially the magnetic moments of 23,25,27,29,31,33Al have been studied in CDFT. At first, using the configuration-fixed deformation constrained calculation, the potential energy, intrinsic magnetic moment and single proton energy levels as a function of quadrupole deformation for 23Al are given. The ground state of 23Al has been found to be prolate deformed, β2 = 0.40, with the odd proton in 5/2[202] orbital and intrinsic magnetic moment 4.96 μN, which is close to the Schmidt value of πd5/2. After including the strong coupling of collective rotation and intrinsic single particle motion, the ground-state spin and parity $5/{2}^{+}$ of odd-A Al isotopes can be understood and the corresponding magnetic moments are well reproduced, with relative deviation from the data of five nuclei about 5%. In addition, it is found that the theoretical descriptions are improved after considering the time-odd fields, i.e. the relative deviation from the data of five nuclei is reduced from 7.6% to 5%. The above calculations support that the ground state $5/{2}^{+}$ is built on the intrinsic state 5/2[202] and a rotational structure exists in the ground state of the odd mass Al isotopes. A further investigation with angular momentum projection based on our deformed CDFT solution is to be done in the future, and other forms of energy functionals such as PC-PK1 [66], one of the most accurate density functionals at present, should be adopted to check whether the present conclusion depends on the functionals.