Linked Cluster Series Expansions for Two-Particle Bound States

Details Linked Cluster Series Expansions for Two-Particle Bound States Linked Cluster Series Expansions for Two-Particle Bound States

2000-10-23

arxiv.org/abs/cond-mat/0010354v1

Phys. Rev. B 63, 144410 (2001)

Physics

Condensed Matter

Strongly Correlated Electrons

10 figures, revtex

Scientific paper

10.1103/PhysRevB.63.144410

We develop strong-coupling series expansion methods to study two-particle spectra of quantum lattice models. At the heart of the method lies the calculation of an effective Hamiltonian in the two-particle subspace. We explicitly consider an orthogonality transformation to generate this block diagonalization, and find that maintaining orthogonality is crucial for systems where the ground state and the two-particle subspace are characterized by identical quantum numbers. We discuss the solution of the two-particle Schr\"odinger equation by using a finite lattice approach in coordinate space or by an integral equation in momentum space. These methods allow us to precisely determine the low-lying excitation spectra of the models at hand, including all two-particle bound/antibound states. Further, we discuss how to generate series expansions for the dispersions of the bound/antibound states. These allow us to employ series extrapolation techniques, whereby binding energies can be determined even when the expansion parameters are not small. We apply the method to the (1+1)D transverse Ising model and the two-leg spin-$\case 1/2$ Heisenberg ladder. For the latter model, we also calculate the coherence lengths and determine the critical properties where bound states merge with the two-particle continuum.

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