MIXED-SPIN LADDERS AND PLAQUETTE SPIN CHAINS

We investigate low-energy properties of a generalized spin ladder mode l with both of the spin alternation and the bond alternation, which al lows us to systematically study not only ladder systems but also alter nating spin chains. By exploiting non-linear a model techniques we stu dy the model with particular emphasis on the competition between gapfu l and gapless states. Our approach turns out to provide a more consist ent semi-classical description of alternating spin chains than that in the previous work. We also study a closely related model, i.e., a spi n chain with plaquette structure, and show that frustration causes lit tle effect on its low-energy properties so far as the strength of frus tration is weaker than a certain critical value.