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Register a new userNew algorithm in the variation after projection calculations for non-yrast nuclear states
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We present a novel and simple algorithm in the variation after projection (VAP) approach for the non-yrast nuclear states. It is for the first time that the yrast state and non-yrast states can be varied on the same footing. The orthogonality among the calculated states is automatically fulfilled by solving the Hill-Wheeler equation. This avoids the complexity of the frequently used Gram-Schmidt orthogonalization, as adopted by the excited VAMPIR method. Thanks to the Cauchys interlacing theorem in the matrix theory, the sum of the calculated lowest projected energies with the same quantum numbers can be safely minimized. Once such minimization is converged, all the calculated energies and the corresponding states can be obtained, simultaneously. The present VAP calculations are performed with time-odd Hartree-Fock Slater determinants. It is shown that the calculated VAP energies (both yrast and non-yrast) are very close to the corresponding ones from the full shell model calculations. It looks the present algorithm is not limited to the VAP, but should be universal, i.e., one can do the variation with different forms of the many-body wavefunctions to calculate the excited states in different quantum many-body systems.
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Projection is noninvertible. This means two different vectors may have the same projected components. In nuclear case, one may take the intrinsic state as a vector, and take the nuclear wave function as the projected component obtained by projecting the former onto good quantum numbers. This immediately comes to the conclusion that, for a given nuclear state in the laboratory frame of reference, the corresponding intrinsic state in the intrinsic frame of reference can not be uniquely determined. In this letter, I will show this interesting phenomenon explicitly based on the improved variation after projection(VAP) method. First of all, it is found that, the form of the trial VAP wavefunction with spin $J$ can be greatly simplified by adopting just one projected state rather than previously adopting all $(2J+1)$ spin-projected states for each selected Slater determinant. This is crucial in the calculations of high-spin states with arbitrary intrinsic Slater determinants. Based on this simplified VAP, the present calculations show that orthogonal intrinsic states (differed by $K$) may have almost the same projected wavefunctions, indicating the uncertainty of the nuclear intrinsic states. This is quite different from the traditional concept of intrinsic state which is expected to be unique.
We implemented a variation after projection (VAP) algorithm based on a triaxially deformed Hartree-Fock-Bogoliubov vacuum state. This is the first projected mean field study that includes all the quantum numbers (except parity), i.e., spin ($J$), isospin ($T$) and mass number ($A$). Systematic VAP calculations with $JTA$-projection have been performed for the even-even $sd$-shell nuclei with the USDB Hamiltonian. All the VAP ground state energies are within 500 keV above the exact shell model values. Our VAP calculations show that the spin projection has two important effects: (1) the spin projection is crucial in achieving good approximation of the full shell model calculation. (2) the intrinsic shapes of the VAP wavefunctions with spin projection are always triaxial, while the Hartree-Fock-Bogoliubov methods likely provide axial intrinsic shapes. Finally, our analysis suggests that one may not be possible to associate an intrinsic shape to an exact shell model wave function.
The backbending phenomenon in $^{48}$Cr has been investigated using the recently developed Projected Configuration Interaction (PCI) method, in which the deformed intrinsic states are directly associated with shell model (SM) wavefunctions. Two previous explanations, (i) $K=0$ band crossing, and (ii) $K=2$ band crossing have been reinvestigated using PCI, and it was found that both explanations can successfully reproduce the experimental backbending. The PCI wavefunctions in the pictures of $K=0$ band crossing and $K=2$ band crossing are highly overlapped. We conclude that there are no unique intrinsic states associated with the yrast states after backbending in $^{48}$Cr.
We have recently developed an efficient method of performing the full quantum number projection from the most general mean-field (HFB type) wave functions including the angular momentum, parity as well as the proton and neutron particle numbers. With this method, we have been investigating several nuclear structure mechanisms. In this report, we discuss the obtained quantum rotational spectra of the tetrahedral nuclear states formulating certain experimentally verifiable criteria, of the high-spin states, focussing on the wobbling- and chiral-bands, and of the drip-line nuclei as illustrative examples.
We have developed an efficient method for quantum number projection from most general HFB type mean-field states, where all the symmetries like axial symmetry, number conservation, parity and time-reversal invariance are broken. Applying the method, we have microscopically calculated, for the first time, the energy spectra based on the exotic tetrahedral deformation in $^{108,110}$Zr. The nice low-lying rotational spectra, which have all characteristic features of the molecular tetrahedral rotor, are obtained for large tetrahedral deformation, $alpha_{32} gtsim 0.25$, while the spectra are of transitional nature between vibrational and rotational with rather high excitation energies for $alpha_{32}approx 0.1-0.2$
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