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 antiferromagneti c Heisenberg chains (AFMHC), we draw attention to a hump in the temper ature region of measurements; The slope of the reduced susceptibility J chi(T) is considerably smaller than the frequently used Bonner-Fisch er predictions, implying an enhanced exchange integral J(ch). We repor t also exact (pd-model) cluster calculations according to them under r easonable changes in standard CuO2-plane parameter sets; the recently suggested enhanced J(ch)s greater than or similar to 200 meV and the e xperimental charge gap E(g) approximate to 2eV cannot be described in a realistic fashion. Assuming classical lattice variables, the consequ ences of sizable electron-lattice interactions as weak charge density waves (CDW) of spin-Peierls type and specific lattice distortions occu rring for open-shell systems are briefly discussed on the basis of tot al energy minimizations.