BIPOLARON PICTURE OF SPIN GAP-STATE IN DOUBLE-CHAIN SYSTEMS

Slightly doped spin gap state in double-chain system is studied in terms of Abelian bosonization method. It is found that (i) The spin gap formation in terms of the interchain exchange coupling J(perpendicular to) persists only when the two chains are doped with equal number of carriers. (ii) Interchain hopping t(perpendicular to) is irrelevant in the spin gap state as long as max(t(perpendicular to),t)delta << \J(perpendicular to)\ (t: intrachain hopping, delta: carrier concentration) is satisfied. (iii) Only the interchain singlet superconductivity and the 4k(F) charge density wave show power-law (critical) correlations while all. the others decay exponentially. This means that the low energy physics is described in terms of the bipolaron model.