Charge ordering and spin gap transitions in quarter-filled ladders

Several new families of charge transfer solids with ladder-like structure show unusually high spin gap transition temperatures. We argue that these materials are best described by a 1/4-filled ladder with zigzag interstack bonds. We find that in the 1/4-filled zigzag ladder, a cooperative bond-charge ordering leads to the formation of local singlets and a spin gap. The spin gap for the zigzag ladder is an order of magnitude larger than that found in equivalent one dimensional systems.