THE SPIN-PEIERLS INSTABILITY IN A SPIN-1 2 XY CHAIN IN THE NONADIABATIC LIMIT/

The spin-Peierls instability in a spin- 1/2 XY chain coupled to disper sionless phonons of frequency omega has been studied in the non-adiaba tic limit. We have chosen the Lang-Firsov variational wavefunction for the phonon subsystem to obtain an effective spin Hamiltonian. The eff ective spin Hamiltonian is then solved in the framework of the mean-fi eld approximation and bond operator technique. We observed two types o f phase transitions, one is from the spin liquid phase to the dimerize d phase and the other is from the dimerized phase to the antiferromagn etic phase as we vary the spin-phonon coupling from a very low value. The variation of lattice distortion, dimerized order parameter and ene rgy gap with spin-phonon coupling parameter have also been investigate d here.