  Strong co-operative interactions occur between four different broken
symmetries involving charge-ordering and bond distortions in the quarter-filled
correlated zigzag electron ladder. The ground state is singlet, with spin gap
several times larger than in the spin-Peierls state of the one-dimensional
quarter-filled chain with the same parameters. We propose the quarter-filled
zigzag electron ladder model for several different organic charge-transfer
solids with coupled pairs of quasi-one-dimensional stacks, the spin-gap
transition temperatures in which are unusually high.
