Spin Peierls system

Historically, the `spin Peierls' system was the first macroscopic system proposed to have a spin gap [14,15,16]. It consists of an S=1/2 antiferromagnetic spin chain with a spin-lattice coupling. If the lattice is soft in the chain direction, and the chains are magnetically well separated from each other [17,18], a periodic deformation (dimerization) of the lattice takes place at a finite temperature $T_{\rm SP}$ (Fig.3a$\rightarrow$b). The lattice dimerization alternatively enhances [$J(1+\delta)$] or reduces [$J(1-\delta)$] the antiferromagnetic interactions, and brings about singlet pair formations on the enhanced exchange links (Fig.3b).

  

Figure 3: (a) An S=1/2 spin chain with an uniform antiferromagnetic interaction J. (b) The lattice dimerized state below the spin Peierls transition temperature $T_{\rm SP}$.

The ground state structure of the spin-Peierls system is a stacking of singlet pairs along the chain. The ground state is non-magnetic, because each singlet pair produces no magnetic field. Since the singlet pairs are well localized, there is a finite energy gap between the ground state and the excited states. This system is studied in Chapter 6.