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Progress of Theoretical Physics

A Note on the Two-Level Pairing Model Obeying the su(2) su(2)-Algebra: -- Re-formation in Terms of the su(1,1) su(1,1)-Algebra --

Constança Providência, João da Providência, Y. Tsue, M. Yamamura

Published Apr 1, 2006• 0 citations
Publisher

Abstract

The two-level pairing model obeying the su(2) ⊗ su(2)-algebra, which was discussed in a previous paper, is re-formed in the framework of the su(1,1) ⊗ su(1,1)-algebra in the Schwinger boson representation. With the aid of the MYT mapping method, the Schwinger boson representation is reduced to the Holstein-Primakoff boson representation, which is expressed in terms of two kinds of bosons. Further, with the aid of the squeezed coherent state, the classical counterpart is obtained.

DOI
10.1143/ptp.115.759

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The two-level pairing model obeying the su(2) ⊗ su(2)-algebra, which was discussed in a previous paper, is re-formed in the framework of the su(1,1) ⊗ su(1,1)-algebra in the Schwinger boson representation. With the aid of the MYT mapping method, the Schwinger boson representation is reduced to the Holstein-Primakoff boson representation, which is expressed in terms of two kinds of bosons. Further, with the aid of the squeezed coherent state, the classical counterpart is obtained.
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A new method for describing the two-level pairing model in a many-fermion system is presented. The basic idea comes from the Schwinger boson representation for the su(2) ⊗ su(2)-algebra, in which four kinds of boson operators govern the dynamics. It is well known that in the original fermion space, it is almost impossible to introduce any trial state for the variation except the BCS state in the framework of the mean field approximation. However, in the Schwinger boson representation, various trial states can be defined on the condition that the mean field approximation is useful. In these states, the su(2) ⊗ su(1,1)-coherent state is mainly discussed in this paper.
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Considering the present situation of Lie-algebraic studies of high-temperature superconductivity, a Schwinger-type boson realization of the su(4) algebra is investigated. The basic framework comes from a possible form of the su(n) and the su(n,1) algebras in the Schwinger representation, which was recently proposed by two of the present authors (M. Y. & A. K.), together with J. da Providência. The form investigated in this paper is, in some sense, a natural extension of the Schwinger boson representation for the so(5), so(4) and su(2)⊗su(2) algebras, which was also proposed by the present authors. The orthogonal set for the irreducible representation of the su(4) algebra is completely analogous to that expressed in terms of fermion operators for pairing correlation.