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Progress of Theoretical Physics

Quantum Theory of Collective Motion: Quantized Self-Consistent Collective-Coordinate Method for the Large-Amplitude Nuclear Collective Motion

Toshio Marumori, Fumihiko Sakata, Tsutomu Une, Yukio Hashimoto, Toshihide Maskawa

Published Nov 1, 1981• 6 citations
Publisher

Abstract

This is the third in a series of papers which intends to develop a new microscopic theory capable by itself to determine the “maximaldecoupled” collective subspace in the many-particle Hilbert space as well as the collective Hamiltonian specifying the subspace. The fundamental principle to formulate the theory is called the invariance principle of the Schrödinger equation. The main purpose of this paper is to show that, when we positively utilize the invariance principle, we can directly obtain a quantum theory of collective motion going beyond the random phase approximation.

DOI
10.1143/ptp.66.1651

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Progress of Theoretical Physics•Oct 1, 1988•2 citations
This is the third in a series of papers which intends to develop a new microscopic theory capable by itself to determine the “maximaldecoupled” collective subspace in the many-particle Hilbert space as well as the collective Hamiltonian specifying the subspace. The fundamental principle to formulate the theory is called the invariance principle of the Schrödinger equation. The main purpose of this paper is to show that, when we positively utilize the invariance principle, we can directly obtain a quantum theory of collective motion going beyond the random phase approximation.
The aim of this series of papers is to propose a microscopic theory to go beyond the situations where collective motions are described by the random phase approximation, i.e., by small amplitude harmonic oscillations about equilibrium. The theory is thus ap­ propriate for the microscopic description of the large amplitude collective motion of soft nuclei. The essential idea is to develop a method to determine the collective subspace (or submanifold) in the many-particle Hilbert space in an optimal way, on the basis of a fundamental princi­ ple called the invariance principle of the Schrodinger equation. By using the principle with­ in the framework of the Hartree-Fock theory, it is shown that the theory can clarify the structure of the so-called phonon-bands by self-consistently deriving the collective Hamil­ tonian where the number of the physical phonon is conserved. The purpose of this paper is not to go into detailed quantitative discussion, but rather to develop the basic idea.
A theory of large amplitude collective motion of a many-particle system is presented, which is relevant, for example, to nuclear fission. The theory is a combination of techniques used in many areas of physics and mathematics. The starting point is the application of the time-dependent Schrödinger equation to generate invariant subspaces of the Hamiltonian in the Hartree–Fock approximation. This is a generalization of the group-theoretical device of generating orbits of a group in the construction of reduced representations. It is shown how solutions of the time-dependent Schrödinger equation can be expressed as instantaneous stationary states of a constrained static Hamiltonian. Thus contact is made with the traditional cranking models and constrained Hartree–Fock theories of large amplitude collective motion. The collective motion is quantized using the Hill–Wheeler–Griffin method of generator coordinates in a basis of generalized coherent states. One is thereby able to exploit much of the theory of harmonic oscillator coherent states, which have been so successfully used in the quantum theory of the laser. The resulting Schrödinger equation for the collective dynamics is expressed both in the Bargmann representation and in the more familiar Schrödinger representation. It is shown that solution of the Schrödinger equation in the small amplitude harmonic approximation reproduces the well-known RPA result. A pilot calculation for 28Si shows that application in large amplitude is also feasible.
A procedure is developed for treating a nuclear collective motion via the path integral formalism by defining a collective propagator in the "phase space" of the collective subspace. This collective subspace is projected out of the full many-body Hilbert space by an approach of the Generator Coordinate Method. Under the approximation of quadratic "momentum" dependence we write the propagator in the coordinate representation from which we derive the collective Schrödinger equation in terms of a canonical pair and .
By placing emphasis on conceptual correspondence to the “classical” theory which has been developed within the framework of the time-dependent Hartree-Fock theory, a full quantum theory appropriate for describing large-amplitude collective motion is proposed. A central problem of the quantum theory is how to determine an optimal representation called a dynamical representation; the representation is specific for the collective subspace where the large-amplitue collective motion is replicated as satisfactoryly as possible. As an extension of the classical theory where the concept of an approximate integral surface plays an important role, the dynamical representation is properly characterized by introducing a concept of an approximate invariant subspace of the Hamiltonian.
It is the purpose of the present paper to illustrate the quantum theory of dynamical collective subspace, which has been proposed in a previous paper to describe the large-amplitude collective motion, by using a simple four-level model with the pairing interactions. The quantum theory provides us a prescription to dynamically specify a set of "global" collective operators as well as the "optimum" dynamical collective subspace spanned by them. This prescription is applied to the simple model, and the results thus obtained are compared with exact numerical solutions. It is shown that the results by our theory are in excellent agreement with the exact solutions, not only in the low-lying collective states in the vicinity of the "vacuum" with the stable mean-field but also in the higher excited collective states far from it. Through the investigation, it is demonstrated that our theory is quite powerful in disclosing microscopic structure of transfiguration of collectivity near the critical point of the "phase transition". It is the essential statement derived by our theory that the conventional (local) collective operators based on the Tamm-Dancoff approximation or the random phase approximation should be replaced by relevant (global) collective operators with changeable microscopic structure depending on the amplitude of collective motion, in order to properly specify a set of collective excited states.