Abstract
Fitting magnetic susceptibility data \ensuremath{\chi}(T) from the linear chain cuprate ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{3}$ for T\ensuremath{\le}800 K, Ami et al. [Phys. Rev. B 51, 5994 (1995)] have extracted an unexpectedly large in-chain exchange integral ${\mathit{J}}_{\mathrm{ch}}$=215\ifmmode\pm\else\textpm\fi{}25 meV. We reconsider qualitatively their fitting procedure [which is based on the rational polynomial Bonner-Fischer (BF) curve for the spin-1/2 antiferromagnetic Heisenberg chain (AFMHC)] by making use of an improved approximate AFMHC susceptibility expression. Our analytical \ensuremath{\chi}(T) expression covers in addition to the well-known high-T limit also the intermediate- and low-T regions described so far numerically (Bethe ansatz) and analytically (renormalizational group theory) by Eggert, Affleck, and Takahashi [Phys. Rev. Lett. 73, 332 (1994)]. Due to the presence of a characteristic low-T logarithmic feature (hump) in the region of experimental interest, 0.1${\mathit{k}}_{\mathit{BT}}$/${\mathit{J}}_{\mathrm{ch}}$0.2, the slope of the reduced susceptibility ${\mathit{J}}_{\mathrm{ch}}$\ensuremath{\chi}(T) is considerably smaller than the BF predictions, thus questioning the authors' statement that ${\mathit{J}}_{\mathrm{}\mathrm{ch}}$ is significantly larger than the corresponding ${\mathrm{CuO}}_{2}$-plane value of layered cuprates, ${\mathit{J}}_{\mathit{pl}}$=100 -- 130 meV. In addition, we report exact (pd-model) ${\mathrm{CuO}}_{3}$-chain cluster calculations with the aim to clarify whether or not, and under what changes in standard plane parameter sets, the suggested enhanced ${\mathit{J}}_{\mathrm{ch}}$ values and the large experimental charge gap ${\mathit{E}}_{\mathit{g}}$\ensuremath{\approxeq}2 eV might be described in a microscopically realistic fashion. The corresponding results suggest that ${\mathit{J}}_{\mathrm{ch}}$'s \ensuremath{\gtrsim} 200 meV are rather unlikely. \textcopyright{} 1996 The American Physical Society.
Published Version
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