Spin dynamics of dimerized Heisenberg chains

We study numerically the dimerized Heisenberg model with frustration appropriate for the quasi-1D spin-Peierls compound CuGeO$_3$. We present evidence for a bound state in the dynamical structure factor for any finite dimerization $δ$ and estimate the respective spectral weight. For the homogeneous case ($δ=0$) we show that the spin-wave velocity $v_s$ is renormalized by the n.n.n. frustration term $α$ as $v_s=π/2 J(1-bα)$ with $b\approx1.12$

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