LATTICE INSTABILITY IN THE SPIN LADDER SYSTEM UNDER MAGNETIC-FIELD

We theoretically study the lattice instability in the spin-gap systems under it magnetic field. When the magnetic field is larger than a cri tical value h(c1), the spin gap collapses and the magnetization phenom enon occurs. We found that lattice distortion occurs in the spin ladde r at an incommensurate wave vector corresponding to the magnetization, while it does not occur in the Haldane system. At low temperatures th e magnetization curve shows a first order phase transition with this l attice distortion.