EXCHANGE INTEGRAL AND THE CHARGE GAP OF THE LINEAR-CHAIN CUPRATE SR2CUO3

Fitting magnetic susceptibility data chi(T) from the linear chain cupr ate 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 excha nge integral J(ch)= 215+/-25 meV. We reconsider qualitatively their fi tting procedure [which is based on the rational polynomial Bonner-Fisc her (BF) curve for the spin-1/2 antiferromagnetic Heisenberg chain (AF MHC)] by making use of an improved approximate AFMHC susceptibility ex pression. Our analytical chi(T) expression covers in addition to the w ell-known high-ir limit also the intermediate- and low-T regions descr ibed so far numerically (Bethe ansatz) and analytically (renormalizati onal group theory) by Eggert, Affleck, and Takahashi [Phys. Rev. Lett. 73, 332 (1994)]. Due to the presence of a characteristic low-T logari thmic 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 con siderably smaller than the BF predictions, thus questioning the author s' statement that J(ch) is significantly larger than the corresponding CuO2-plane value of layered cuprates, J(pl)=100 - 130 meV. In additio n, we report exact (pd-model) CuO3-chain cluster calculations with the aim to clarify whether or not, and under what changes in standard pla ne parameter sets, the suggested enhanced J(ch) values and the large e xperimental 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.