Superexchange, charge gap, and lattice distortions of Sr(Ca)(2)CuO3 - The interplay of e-e and e-ph interactions

Magnetic susceptibility data chi(T) of the linear cuprate Sr2CuO3 are discussed. If its CuO3 chains are modeled by spin-1/2 antiferromagnetic Heisenberg chains (AFMHC), we draw attention to a hump in the temperature region of measurements; The slope of the reduced susceptibility J chi(T) is considerably smaller than the frequently used Bonner-Fischer predictions, implying an enhanced exchange integral J(ch). We report also exact (pd-model) cluster calculations according to them under reasonable changes in standard CuO2-plane parameter sets; the recently suggested enhanced J(ch)s greater than or similar to 200 meV and the experimental charge gap E(g) approximate to 2eV cannot be described in a realistic fashion. Assuming classical lattice variables, the consequences of sizable electron-lattice interactions as weak charge density waves (CDW) of spin-Peierls type and specific lattice distortions occurring for open-shell systems are briefly discussed on the basis of total energy minimizations.