Exchange integral and the charge gap of the linear-chain cuprate Sr2CuO3

Fitting magnetic susceptibility data chi(T) from the linear chain cuprate Sr2CuO3 for T less than or equal to 800 K, Ami et al. [Phys. Rev. B 51, 5994 (1995)] have extracted an unexpectedly large in-chain exchange integral J(ch)= 215+/-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 chi(T) expression covers in addition to the well-known high-ir 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<k(B)T/J(ch)<0.2, the slope of the reduced susceptibility J(ch)chi(T) is considerably smaller than the BF predictions, thus questioning the authors' statement that J(ch) is significantly larger than the corresponding CuO2-plane value of layered cuprates, J(pl)=100 - 130 meV. In addition, we report exact (pd-model) CuO3-chain cluster calculations with the aim to clarify whether or not, and under what changes in standard plane parameter sets, the suggested enhanced J(ch) values and the large experimental charge gap E(g) approximate to 2 eV might be described in a microscopically realistic fashion. The corresponding results suggest that J(ch))'s greater than or similar to 200 meV are rather unlikely.