Spin ladder system

If one changes the stacking orientation of the singlet pairs from that of the spin Peierls state, the ground state of the smallest `spin ladder' system is obtained [19]. The spin ladder system is an S=1/2 antiferromagnetic square lattice with finite width and infinite length (Fig.4a). The ground state of this system depends on the lattice width, namely, the number of `legs' in the ladder. If the number of the legs is even, the ground state becomes non-magnetic with a finite energy gap to the excited states [20,21]; if it is odd, the energy gap collapses [21]. For the 2-leg ladder system, the non-magnetic ground state can be visualized as shown in Fig.4b; it is a stacking of singlet pairs on the rungs. The finite energy gap of this system originates from the localization of these singlet pairs. In the 3-leg ladder system, the localized pair formation on the rungs becomes impossible, and hence the energy gap collapses. These systems are discussed in Chapter 4.

  

Figure 4: (a) The N-leg spin ladder structure and (b) a schematic ground state of the 2-leg spin ladder system.