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Physical Review C

Gauge-invariant periodic quantized good-parity states in Lipkin-Meshkov-Glick model

Kit-Keung Kan

Published Nov 1, 1980• 10 citations
Publisher

Abstract

The gauge-invariant periodic quantization method is applied to time-dependent states with definite parities in the Lipkin-Meshkov-Glick model. It is shown that the resulting discrete energy spectrum is free of discontinuity and predicts the correct number of states for all interaction strengths. It offers therefore a qualitative improvement over the gauge-invariant periodic quantized time-dependent Hartree-Fock states of this model.NUCLEAR STRUCTURE Time-dependent description of many-body bound states, gauge-invariant periodic quantization method, Lipkin-Meshkov-Glick model, good-parity time-dependent states.

DOI
10.1103/physrevc.22.2228

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Time-dependent Hartree-Fock dynamics and phase transition in Lipkin-Meshkov-Glick model

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The time-dependent Hartree-Fock solutions of the two-level Lipkin-Meshkov-Glick model are studied by transforming the time-dependent Hartree-Fock equations into Hamilton's canonical form and analyzing the qualitative structure of the Hartree-Fock energy surface in the phase space. It is shown that as the interaction strength increases these time-dependent Hartree-Fock solutions undergo a qualitative change associated with the ground state phase transition previously studied in terms of coherent states. For twobody interactions stronger than the critical value, two types of time-dependent Hartree-Fock solutions (the "librations" and "rotations" in Hamilton's mechanics) exist simultaneously, while for weaker interactions only the rotations persist. It is also shown that the coherent states with the maximum total pseudospin value are determinants, so that time-dependent Hartree-Fock analysis is equivalent to the coherent state method.NUCLEAR STRUCTURE Lipkin-Meshkov-Glick model, TDHF, transformation of TDHF equations to Hamilton's canonical form, coherent states, ground state phase transition.

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The well known and analytically solvable Lipkin–Meshkov–Glick model was devised to test several many-body approximation approaches used in nuclear physics. Here, we use this model to illustrate a method that sets lower bounds for the ground state energy of a quantum Hamiltonian. The idea of the method consists of dividing the Hamiltonian into identical groups of terms that represent subdivisions in the particle population of the system, and then to make an appropriate use of the ground energy of one of these groups. The bounds clearly improve as the size of these groups increases.

International Journal of Modern Physics E•Jun 1, 2008•0 citations
The gauge-invariant periodic quantization method is applied to time-dependent states with definite parities in the Lipkin-Meshkov-Glick model. It is shown that the resulting discrete energy spectrum is free of discontinuity and predicts the correct number of states for all interaction strengths. It offers therefore a qualitative improvement over the gauge-invariant periodic quantized time-dependent Hartree-Fock states of this model.NUCLEAR STRUCTURE Time-dependent description of many-body bound states, gauge-invariant periodic quantization method, Lipkin-Meshkov-Glick model, good-parity time-dependent states.
The time-dependent Hartree-Fock solutions of the two-level Lipkin-Meshkov-Glick model are studied by transforming the time-dependent Hartree-Fock equations into Hamilton's canonical form and analyzing the qualitative structure of the Hartree-Fock energy surface in the phase space. It is shown that as the interaction strength increases these time-dependent Hartree-Fock solutions undergo a qualitative change associated with the ground state phase transition previously studied in terms of coherent states. For twobody interactions stronger than the critical value, two types of time-dependent Hartree-Fock solutions (the "librations" and "rotations" in Hamilton's mechanics) exist simultaneously, while for weaker interactions only the rotations persist. It is also shown that the coherent states with the maximum total pseudospin value are determinants, so that time-dependent Hartree-Fock analysis is equivalent to the coherent state method.NUCLEAR STRUCTURE Lipkin-Meshkov-Glick model, TDHF, transformation of TDHF equations to Hamilton's canonical form, coherent states, ground state phase transition.
Based on the variational principle in a general manifold in the Hilbert space and the principle of gauge invariance, the general gauge invariant periodic quantization method is formulated. This method, which is a direct generalization of the one previously obtained in the case of time-dependent Hartree-Fock, leads to a quantization rule which takes the Bohr-Sommerfeld form when the variational parameters are transformed to canonical variables. The method is illustrated by applying it to the manifold of minimum wave packets for a simple harmonic oscillator. It leads to the exact energy spectrum and a discrete set of wave packets which have maximum overlap with the exact eigenstates. The method is also applied to the exact Schr\"odinger dynamics where it exposes some open questions of the method.[NUCLEAR STRUCTURE Time-dependent description of many-body bound states. Gauge invariant periodic quantization method, time-dependent variational principle and their canonical formulation.]
A theory is presented in which approximations to quantum observables are obtained by applying the stationary-phase approximation to an exact functional integral representation of the many-body evolution operator. The requirement that the leading correction to the stationary-phase approximation vanish leads to a time-dependent Hartree-Fock mean field. Application of the theory to the Fourier transform of the trace of the evolution operator and the study of its poles yields quantized bound states with large amplitude. The theory is shown to reproduce the familiar static Hartree-Fock and random-phase approximations in the appropriate limits and yields an excellent approximation to the entire spectrum of the exactly solvable Lipkin model.NUCLEAR STRUCTURE Derivation of time-dependent mean-field theory from functional integral representation. Quantum theory of large amplitude collective motion. Application to Lipkin model.
A time-dependent variational procedure is proposed that possesses the same constants of the motion as the exact many-body Schr\"odinger dynamics. The class of trial wave functions is larger than the manifold of Slater determinants that supports the time-dependent Hartree-Fock dynamics. These wave functions can be regarded as superpositions of the eigenfunctions of the conserved observable of interest and the variational equations display the usual parametric structure, with properly admixed energy gradients and the symplectic metric tensor. In an application to the Lipkin-Meshkov-Glick model, significant improvements over the usual mean-field or determinantal dynamics can be achieved.NUCLEAR STRUCTURE Mean field symmetry breaking; symmetry restoration; nondeterminantal wave function; time-dependent variational principle; parametric equations of motion; mean energy metric tensor; canonical coordinates; quasispin models; comparison with time-dependent Hartree-Fock dynamics.
The Lipkin-Meshkov-Glick (LMG) model originally describes a Fermionic many-body system in nuclear physics. However, in recent years, it has been widely found in condensed matter physics, quantum information systems, and quantum optics, and it is of wider and wider interest. Previous studies on this model mainly focused on the physics under the thermal dynamical limit, such as quantum phase transitions and quantum entanglement. There are also some researches about LMG model with finite size in some special limits, but the finite-size effect on energy spectrum is not very clear yet. This is the main motivation of this work. In this paper, the exact diagonalization method and the quantum perturbation theory are used to calculate and analyze the energy-level structure of the LMG model at a finite N. To solve it, we first map this model into the angular-momentum space to obtain a reduced LMG model. By this mapping, the dimension of Hilbert space is reduced to N+1 from 2N. The exact solution of its energy levels can be obtained easily in the U(1) limit where the total spin is conserved. We find that the levels are woven into a fishing-net structure in the U(1) limit. While away from the U(1) limit, the crossings between even and odd levels will open a gap, and the system's energy levels will be grouped into pairs with an odd and an even level, forming some bound states, called doublet states, and the parity of each doublet state will oscillate as the Zeeman field increases. This work gives the values of the critical Zeeman field for the parity crossings. These critical values shift as the interacting parameters and disappear at zero in the Z2 limit. In the Z2 limit, the system energy levels form splittings near the zero Zeeman field. In this article, we analytically calculate the relationship between these energy gaps and the Zeeman field. For odd and even number N, the parity of each state has a different behavior. Specifically, the ground state and the doublet excited states of the system with odd N will suffer a parity reversion at zero Zeeman field, while the states with even N will not. By tuning the interacting parameters, we also study the crossover from the U(1) limit to the Z2 limit. The parity oscillation we find in this system is a very important physical phenomenon, which also exists in some other systems like optical cavity quantum electrodynamics and magnetic molecule system.
The well known and analytically solvable Lipkin–Meshkov–Glick model was devised to test several many-body approximation approaches used in nuclear physics. Here, we use this model to illustrate a method that sets lower bounds for the ground state energy of a quantum Hamiltonian. The idea of the method consists of dividing the Hamiltonian into identical groups of terms that represent subdivisions in the particle population of the system, and then to make an appropriate use of the ground energy of one of these groups. The bounds clearly improve as the size of these groups increases.