THE STEPPED SPIN-PEIERLS PHASE-TRANSITION IN THE QUASI-ONE-DIMENSIONAL SPIN-1 2 QUANTUM XY-MODEL/

Using an extension of the Jordan-Wigner transformation (JWT) in two di mensions, the effects of weak interchain coupling on the spin-Peierls (SP) system is studied. The magnetic interaction is considered to be a quantum XY-interaction, and the spin-lattice distortion is treated qu asiclassically. On the analogy of the stepped Peierls transition theor y, we propose a stepped SP transition theory. We find that, when any f inite transverse coupling J perpendicular to is introduced between nea rest chains, the usual SP transition will be modified, and the dimeriz ation and the opening of the gap will no longer occur simultaneously. This leads to the appearance of a gapless SP phase over a certain temp erature region.