The spin–Peierls phase transition in an anisotropic XY quasi-one-dimensional system

The spin–Peierls phase transition in a quasi-one-dimensional XY chain of S=1/2 of spins interacting through an anisotropic exchange interaction is studied as a function of the degree of anisotropy γ. The spin–Peierls state is described using a mean field theory. As γ increases (small value of the anisotropy), the transition temperature Tc decreases. For intermediate values of the anisotropy parameter γ, a tricritical point separating the regions of first and second order phase transitions is obtained. For larger values of γ, a total suppression of the spin–Peierls state is predicted. Finally, a very unusual temperature dependent gap of the elementary spin excitations is predicted for the anisotropic XY system.