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arXiv:nucl-th/0503071 · Journal of Physics G Nuclear and Particle Physics

Quantum phase transitions in the U(5)–O(6) large-N limit

Feng Pan, Yu Zhang, Jerry Paul Draayer

Published Aug 2, 2005• 28 citations
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Abstract

The U (5)– O (6) transitional behaviour of the interacting Boson model in the large- N limit is revisited. Some low-lying energy levels, overlaps of the ground state wavefunctions, B ( E 2) transition rate for the decay of the first excited energy level to the ground state, and the order parameters are calculated for different total numbers of bosons. The results show that critical behaviours of these quantities are greatly enhanced with an increase in the total number of bosons N ; in particular, the fractional occupation probability for d bosons in the ground state, the difference between the expectation value of n d in the first excited 0 + state and the ground state, and another quantity related to the isomer shift behave similarly in both the O (6)– U (5) large- N and U (5)– SU (3) phase transitions.

DOI
10.1088/0954-3899/31/9/006

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The U (5)– O (6) transitional behaviour of the interacting Boson model in the large- N limit is revisited. Some low-lying energy levels, overlaps of the ground state wavefunctions, B ( E 2) transition rate for the decay of the first excited energy level to the ground state, and the order parameters are calculated for different total numbers of bosons. The results show that critical behaviours of these quantities are greatly enhanced with an increase in the total number of bosons N ; in particular, the fractional occupation probability for d bosons in the ground state, the difference between the expectation value of n d in the first excited 0 + state and the ground state, and another quantity related to the isomer shift behave similarly in both the O (6)– U (5) large- N and U (5)– SU (3) phase transitions.
The spectral degeneracies emerging in the critical region of the interacting boson model are systematically investigated. The results indicate that the level crossings in the transitions from the U(5) to the SU(3) quasidynamical symmetry can induce the approximate spectral degeneracies along the first-order transitional line for large boson number NNN. The remnants of these types of degeneracies in finite-NNN systems have been preliminarily identified from the low-lying spectra of 150Nd^{150}\mathrm{Nd}150Nd. In addition, the similar approximate degeneracies emerging in the phase transition between the U(5) and SO(6) quasidynamical symmetries are parallel emphasized and those in other critical models are also mentioned.
We investigate the finite-size scaling exponents for the critical point at the shape-phase transition from U(5) (spherical) to O(6) (deformed \ensuremath{\gamma}-unstable) dynamical symmetries of the interacting boson model, making use of the Holstein-Primakoff boson expansion and the continuous unitary transformation technique. We compute exactly the leading-order correction to the ground-state energy, the gap, the expectation value of the d-boson number in the ground state and the E2E2E2 transition probability from the ground state to the first excited state and determine the corresponding finite-size scaling exponents.
The quantum phase transitional behavior of an alternative characterization of the spherical to axially deformed shape-phase transition in the interacting boson model is explored. Specifically, the usual SU(3) quadrupole-quadrupole interaction is replaced by an O(6) cubic interaction, and the transitional behaviors of the low-lying energy levels, eigenstates, isomer shifts, $E$2 transition rates, and expectation values of shape variables across the entire transitional region are all examined within this context. A comparison with outcomes of the usual U(5)-SU(3) transitional description shows that the spherical to axially deformed shape-phase transition can also be described within this alternative context, especially near the critical point. However, with the O(6) cubic interaction the transition is considerably smoother than for the U(5)-SU(3) case, but with the nuclear shape less well defined, even in the axially deformed limit. It is also shown that the new scheme seems better than the usual U(5)-SU(3) scheme in describing the low-lying spectrum of 152{}^{152}152Sm with X(5) critical point symmetry.
We investigate the evolution of quantal spectra and the corresponding wave functions along the [O(6)-U(5)] \ensuremath{\supset} O(5) transition of the interacting boson model. The model is integrable in this regime, and its ground state passes through a second-order structural phase transition. We show that the whole spectrum as a function of the Hamiltonian control parameter as well as structures of all excited states exhibit rather organized and correlated behaviors, which provide deeper insight into the nature of this transitional path.
We study the energy level statistics of the states in U(5) and O(6) dynamical symmetries of the interacting boson model and the high spin states with backbending in U(5) symmetry. In the calculations, the degeneracy resulting from the additional quantum number is eliminated manually. The calculated results indicate that the finite boson number N effect is prominent. When N has a value close to a realistic one, increasing the interaction strength of subgroup O(5) makes the statistics vary from Poisson type to Gaussian orthogonal ensemble type and further recover to Poisson type. However, in the case of $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty},$ they all tend to be Poisson type. The fluctuation property of the energy levels with backbending in high spin states in U(5) symmetry involves a signal of shape-phase transition between spherical vibration and axial rotation.