A rigid commensurate host and a defect that switches: vacancies in the frustrated alternating spin-(1, 1/2) Heisenberg chain
The study investigated how a frustrating coupling ($J_2$) affects an alternating spin chain, finding it transitions directly from a partially polarized state to a singlet phase, not via a partially polarized window. The resulting singlet phase is critical and gapless, exhibiting specific modulation characteristics. Removing an S=1 site (a defect) switches the doped chain's ground state between triplet and singlet at $J_2^*$, demonstrating how defects interact with the host structure.