Microscopic analysis of shape-phase transitions in even-evenN~90rotating nuclei

Abstract

We study in cranked Nilsson plus random phase approximation shape transitions in fast rotating nuclei undergoing backbending, more specifically 156 Dy and 162 Yb. We found that a backbending in 156 Dy is correlated with the disappearance of the collective, positive signature γ-vibrational mode in the rotating frame, and, a shape transition (axial→ nonaxial) is accompanied with a large acquiring of the γ deformation. We show that such a shape transition can be considered as a phase transition of the first order. In 162 Yb the quasiparticle alignment dominates in the backbending and the shape transition (axial→ nonaxial) is accompanied with a smooth transition from zero to nonzero values of the γ deformation. We extend the classical Landau theory for rotating nuclei and show that the backbending in 162 Y b is identified with the second order phase transition. A description of spectral and decay properties of the yrast states and low-lying excitations demonstrates a good agreement between our results and experimental data.

Effects of symmetry breaking in finite quantum systems

“…In order to elucidate this aspect the experimental data in 156 Dy and 162 Yb (see Fig. 20) have been analyzed [455,554] with the aid of the cranking Hamiltonian…”

“…The interaction includes separable monopole pairing, double stretched quadrupolequadrupole (QQ) and monopole-monopole terms. The details on this model Hamiltonian can be found in [554]. At each rotational frequency, the total mean-field energy R = R is minimized on the mesh β = 0.0 − 0.6, γ = −π/3 − +π/3.…”

“…To elucidate the different character of the shape transition from axially symmetric to the triaxial shape, one can choose the deformation parameter γ as the order parameter that reflects the broken axial symmetry [455,554]. Such a choice is well justified, since the deformation parameter β preserves its value before and after the shape transition in both nuclei: β t ≈ 0.2 for 162 Yb and β t ≈ 0.31 for 156 Dy.…”

“…The energy is counted relatively to the value E min = E Ω (β t , γ) at Ω = 0.255, 0.302 MeV for 162 Yb and 156 Dy, respectively. From [554].…”

“…Therefore, for Ω near the transition point Ω c , one may expand the functional F (Ω, γ) = E Ω (γ, β t ) − E min (see Fig. 23) in the form [554]:…”