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Progress of Theoretical Physics Supplement

Rocky Paths, Dead Ends and Smooth Paths towards a Theory of Nuclear Collective Motion

Felix M. H. Villars

Published Jan 1, 1983• 2 citations
Publisher

Abstract

This report contains two main parts.The first part summarizes the well known construction based on TDHF of a collective Hamiltonian .!J((p, q) from an A-particle Hamiltonian fl.This construction is stated as a prescription, that may also be applied to any one particle operator 'Y, and its time derivative [iH, 'Y].The operators Q} (p, q) and ciJ (p, q) so obtained are required to obey ci}(p, q)=[i.!J((p, q), Q}(p, q)].It is shown that this consistency condition on collective dynamics can indeed be enforced, and leads to the same conditions as the TDHF-variational principle.The second part presents an alternative to TDHF, which is quantum mechanical.from the outset.Eigenfunctions of the collective "position" operator Q are constructed by projection.In the adiabatic limit, the results obtained by variationally optimizing the trial function for the system are in substantial agreement with quantized TDHF.Zero-point energy corrections are automatically included.

DOI
10.1143/ptps.74.184

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This report contains two main parts.The first part summarizes the well known construction based on TDHF of a collective Hamiltonian .!J((p, q) from an A-particle Hamiltonian fl.This construction is stated as a prescription, that may also be applied to any one particle operator 'Y, and its time derivative [iH, 'Y].The operators Q} (p, q) and ciJ (p, q) so obtained are required to obey ci}(p, q)=[i.!J((p, q), Q}(p, q)].It is shown that this consistency condition on collective dynamics can indeed be enforced, and leads to the same conditions as the TDHF-variational principle.The second part presents an alternative to TDHF, which is quantum mechanical.from the outset.Eigenfunctions of the collective "position" operator Q are constructed by projection.In the adiabatic limit, the results obtained by variationally optimizing the trial function for the system are in substantial agreement with quantized TDHF.Zero-point energy corrections are automatically included.
The title originally planned for this series of lectures was “Time-dependent Hartree Fock Theory of Nuclear Motions and Collisions”. In fact, in what follows, the TDHF method will be considered for collective motion only. This is because a new, more practical theory of collisions has recently emerged. These lectures thus contain two main and distinct chapters, the first dedicated to this new theory of collisions and the second to collective motion. As a distraction to the reader, a last and small chapter recalls elementary ideas about some conjectured mechanisms for the dynamics of a very special system with collective motions, namely a living organ in the nervous system. This Summer School being dedicated to the subject of order and chaos, it is hoped that the considerations which follow never go too far from that fascinating subject indeed. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Aiming at exploring microscopic dynamics responsible for the dissipative large-amplitude collective motion, the dynamic response and correlation functions are introduced within the general theory of nuclear coupled-master equations. The theory is based on the microscopic theory of nuclear collective dynamics which has been developed within the time-dependent Hartree-Fock (TDHF) theory for disclosing the complex structure of the TDHF manifold. A systematic numerical method for calculating the dynamic response and correlation functions is proposed. By performing numerical calculation for a simple model Hamiltonian, it is pointed out that the dynamic response function gives important information in understanding the large-amplitude dissipative collective motion which is described by an ensemble of trajectories within the TDHF manifold.
The time-dependent variational principle of the Schr\"odinger equation is applied to a formally exact solution of the Schr\"odinger equation whose variational elements are operators which define a collective subspace of the many-body system under study. This generalizes the procedure employed to derive time-dependent Hartree-Fock theory. Four distinct formally exact time-independent variational principles, including several familiar forms, are derived. Application to a class of exactly soluble models, studied in the vibrational regime, illustrates the different modes of implementation of two of the principles. The general theory of large amplitude collective motion is derived. It is shown that the principles can be applied equally well, using either boson or fermion pair degrees of freedom. Some aspects of the relation to the semiclassical limit are discussed.[NUCLEAR STRUCTURE Time-dependent variational principle for collective motion. Invariance principle of Schr\"odinger equation. Generalized coherent state trial function. Exact time-independent variational principles. Variational principle for intrinsic state. Cranking variational principle. Large amplitude collective motion. Semiclassical limit.]
A tractable method is presented for the study of the adiabaticity of a given time-dependent Hartree-Fock solution. In terms of the Baranger-V\'en\'eroni approach, this amounts to extracting the two operators ${\ensuremath{\rho}}_{0}$ and $\ensuremath{\chi}$, for which rather simple expressions are derived. These explicit formulas are exploited to illustrate some implications of the adiabatic approximation itself and several related approximations. In particular, the incompatibility of two current hypotheses for the $\ensuremath{\chi}$ operator (locality and absence of particle-particle, hole-hole matrix elements) is pointed out. Finally, the whole discussion is exemplified by the simple case of uniform translational motion.NUCLEAR STRUCTURE Discussion of adiabatic limit of TDHF approximation, derivation of practical method of study adiabaticity of collective motion from TDHF solutions.
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 starting point for most studies of large amplitude collective motion in nuclear physics, the time-dependent Hartree-Fock equations, can be mapped to a problem in classical Hamiltonian mechanics, which is the form of the problem studied in this work. For a system with N degrees of freedom, collective motion is identified with motion completely confined to a surface in K<N dimensions. Conditions for the existence of such decoupled surfaces are worked out and a procedure for constructing them is formulated. For most practical problems, where such surfaces do not strictly exist, a concept of approximate decoupling is developed and a test of its accuracy is described. Our results generalize the method of maximum global decoupling found in the earlier literature.