The well-defined delocalization modes of $J_{\mathrm{AF}}$ and ${2J}_{\mathrm{AF}}$, where $J_{\mathrm{AF}}$ is the antiferromagnetic exchange integral in a spin pair, are obtained in the $S=1/2\mathrm{}$ alternating chains for $(\alpha {/J}_{\mathrm{AF}}{)}^2\ll 1$ at $T=\infty$ in terms of the continued fraction formalism with the recurrence relations method. These modes correspond to the single and the double singlet-to-triplet local excitations, respectively. Dynamically these very short-ranged and very weak correlations survive at $T=\infty$. We raise the possibility of observing these modes in neutron and Raman scattering at high temperatures, $k_{\mathrm{B}}T\gg J_{\mathrm{AF}}$.

• Received 28 December 1998

DOI:https://doi.org/10.1103/PhysRevLett.83.1668