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Register a new userSupersolidity of the $alpha$ cluster structure in the nucleus $^{12}$C
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For more than half a century, the structure of $^{12}$C, such as the ground band, has been understood to be well described by the three $alpha$ cluster model based on a geometrical crystalline picture. On the contrary, recently it has been claimed that the ground state of $^{12}$C is also well described by a nonlocalized cluster model without any of the geometrical configurations originally proposed to explain the dilute gas-like Hoyle state, which is now considered to be a Bose-Einstein condensate of $alpha$ clusters. The challenging unsolved problem is how we can reconcile the two exclusive $alpha$ cluster pictures of $^{12}$C, crystalline vs nonlocalized structure. We show that the crystalline cluster picture and the nonlocalized cluster picture can be reconciled by noticing that they are a manifestation of supersolidity with properties of both crystallinity and superfluidity. This is achieved through a superfluid $alpha$ cluster model based on effective field theory, which treats the Nambu-Goldstone zero mode rigorously. For several decades, scientists have been searching for a supersolid in nature.Nuclear $alpha$ cluster structure is considered to be the first confirmed example of a stable supersolid.
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Densities and transition densities are computed in an equilateral triangular alpha-cluster model for $^{12}$C, in which each $alpha$ particle is taken as a gaussian density distribution. The ground-state, the symmetric vibration (Hoyle state) and the asymmetric bend vibration are analyzed in a molecular approach and dissected into their components in a series of harmonic functions, revealing their intrinsic structures. The transition densities in the laboratory frame are then used to construct form-factors and to compute DWBA inelastic cross-sections for the $^{12}$C$(alpha, alpha)$ reaction. The comparison with experimental data indicates that the simple geometrical model with rotations and vibrations gives a reliable description of reactions where $alpha$-cluster degrees of freedom are involved.
Background: Recent theoretical and experimental researches using proton-induced $alpha$-knockout reactions provide direct manifestation of $alpha$-cluster formation in nuclei. In recent and future experiments, $alpha$-knockout data are available for neutron-rich beryllium isotopes. In $^{12}$Be , rich phenomena are induced by the formation of $alpha$-clusters surrounded by neutrons, for instance, breaking of the neutron magic number $N=8$. Purpose: Our objective is to provide direct probing of the $alpha$-cluster formation in the $^{12}$Be target through associating the structure information obtained by a microscopic theory with the experimental observables of $alpha$-knockout reactions. Method: We formulate a new wave function of the Tohsaki-Horiuchi-Schuck-R{o}pke (THSR) type for the structure calculation of ${}^{12}$Be nucleus and integrate it with the distorted wave impulse approximation framework for the $alpha$-knockout reaction calculation of $^{12}$Be$(p,palpha)^{8}$He. Results: We reproduce the low-lying spectrum of the $^{12}$Be nucleus using the THSR wave function and discuss the cluster structure of the ground state. Based on the microscopic wave function, the optical potentials and $alpha$-cluster wave function are determined and utilized in the calculation of ${}^{12}$Be($p,palpha$)${}^{8}$He reaction at 250 MeV. The possibility of probing the clustering state of $^{12}$Be through this reaction is demonstrated by analysis of the triple differential cross sections that are sensitively dependent on the $alpha$-cluster amplitude at the nuclear surface. Conclusions: This study provides a feasible approach to validate directly the theoretical predictions of clustering features in the $^{12}$Be nucleus through the $alpha$-knockout reaction.
Lowest energy spectrum of the $^{12}$C nucleus is analyzed in the 3$alpha$ cluster model with a deep $alphaalpha$-potential of Buck, Friedrich and Wheatley with Pauli forbidden states in the $S$ and $D$ waves. The direct orthogonalization method is applied for the elimination of the 3$alpha$-Pauli forbidden states. The effects of possible first order quantum phase transition are shown in the lowest $^{12}$C($0_1^+)$ and $^{12}$C($2_1^+)$ states from weakly bound phase to a deep phase. The ground and lowest $2^+$ states of the $^{12}$C nucleus in the deep phase are created by the critical eigen states of the Pauli projector for the $0^+$ and $2^+$ three-alpha functional spaces, respectively.
Geometric configurations of three-$alpha$ particles in the ground- and first-excited $J^pi=0^+$ states of $^{12}$C are discussed within two types of $alpha$-cluster models which treat the Pauli principle differently. Though there are some quantitative differences especially in the internal region of the wave functions, equilateral triangle configurations are dominant in the ground state, while in the first excited $0^+$ state isosceles triangle configurations dominate, originating from $^8{rm Be}+alpha$ configurations.
The molecular algebraic model based on three and four alpha clusters is used to describe the inelastic scattering of alpha particles populating low-lying states in $^{12}$C and $^{16}$O. Optical potentials and inelastic formfactors are obtained by folding densities and transition densities obtained within the molecular model. One-step and multi-step processes can be included in the reaction mechanism calculation. In spite of the simplicity of the approach the molecular model with rotations and vibrations provides a reliable description of reactions where $alpha$-cluster degrees of freedom are involved and good results are obtained for the excitation of several low-lying states. Within the same model we briefly discuss the expected selection rules for the $alpha$-transfer reactions from $^{12}$C and $^{16}$O.
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