Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration

[摘要] We have studied the low-energy excitation spectrum of a dimerized and frustrated antiferromagnetic Heisenberg chain. We use an analytic approach, based on a description of the excitations as triplets above a strong-coupling singlet ground state. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. Additional singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of the elementary triplets. We have also calculated the contributions of the elementary and collective excitations into the spin structure factor. Our results are in excellent agreement with exact diagonalizations and dimer series expansions data as long as the dimerization parameter Sis not too small (delta > 0.1), i.e., while the elementary triplets can be treated as localized objects. [S0163-1829(99)02329-2].

[关键词] ANTI-FERROMAGNETIC CHAIN;SPIN-PEIERLS COMPOUND;BOUND-STATES;ANTIFERROMAGNETIC CHAINS;COMPETING INTERACTIONS;MAGNETIC EXCITATIONS;ALPHA'-NAV2O5;(VO)(2)P2O7;CUGEO3;LADDER