ELECTRONIC AND MAGNETIC-STRUCTURE OF LACUO2.5

The recently discovered ''ladder'' compound LaCuO2.5 has been found to admit hole doping without altering its structure of coupled copper ox ide ladders. While susceptibility measurements on the parent compound suggest a spin gap and a spin-liquid state, NMR results indicate magne tic order at low temperatures. These seemingly contradictory results m ay be reconciled if in fact the magnetic slate is near the crossover f rom spin liquid to antiferromagnet, and we investigate this possibilit y. From a tight-binding fit to the valence band structure computed in the local density approximation, we deduce that the strength of the in terladder hopping term is approximately half that of intraladder hoppi ng, showing that the material is three-dimensional in character. A mea n-field treatment of the insulating magnetic state gives a spin-liquid phase whose spin gap decreases with increasing interladder coupling, vanishing (signaling a transition to the ordered phase) at a value som ewhat below that obtained for LaCuO2.5. The introduction of an on-site repulsion term, U, to the band scheme causes a transition to an antif erromagnetic insulator for rather small but finite values of U, reflec ting the predominance of (one-dimensional) ladder behavior, and an abs ence of any special nesting features.