Scalar chiral order in spin ladders with four-spin exchanges

We show that scalar chiral order can be induced by four-spin exchanges in the two-leg spin ladder. By using the spin-chirality duality transformation, it is found that scalar-chiral-ordered states are exact ground states in a family of spin ladder models. In the scalar chiral phase, there is a finite energy gap above the doubly degenerate ground states. It is also shown that the SU(4)-symmetric model is on a multicritical point surrounded by the staggered dimer, staggered scalar chiral, and gapless phases.