Excitation spectra and thermodynamic response of segmented Heisenberg spin chains

The spectral and thermodynamic response of segmented quantum spin chains is analyzed using a combination of numerical techniques and finite-size scaling arguments. Various distributions of segment lengths are considered, including the two extreme cases of quenched and annealed averages. As the impurity concentration is increased, it is found that (i) the integrated spectral weight is rapidly reduced, (ii) a pseudogap feature opens up at small frequencies, and (iii) at larger frequencies a discrete peak structure emerges, dominated by the contributions of the smallest cluster segments. The corresponding low-temperature thermodynamic response has a divergent contribution due to the odd-site clusters and a subdominant exponentially activated component due to the even-site segments whose finite-size gap is responsible for the spectral weight suppression at small frequencies. Based on simple scaling arguments, approximate low-temperature expressions are derived for the uniform susceptibility and the heat capacity. These are shown to be in good agreement with numerical solutions of the Bethe ansatz equations for ensembles of open-end chains.