Specific heat of defects in Haldane systemsY2BaNiO5and NENP: Absence of free spin-1/2 excitations

Abstract

We have measured the specific heat of the antiferromagnetic spin-1 chain compounds NENP and ${\mathrm{Y}}_2$ ${\mathrm{BaNiO}}_5$ diluted with Zn, at temperatures between 0.1 and 15 K and magnetic fields up to 9 T. We find that the defect-related Schottky anomaly in ${\mathrm{Y}}_2$ ${\mathrm{BaNi}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$ ${\mathrm{O}}_5$ is better described by assuming the defects are S=1, rather than free S=1/2, the latter proposed to exist within the valence-bond-solid model of finite S=1 chains. The relationship of these results to electron-spin-resonance data for NENP, which imply free S=1/2 defects is discussed.