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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 nonmagnetic with a finite energy gap to the
excited states [20,21]; if it is odd,
the energy gap collapses [21]. For the 2leg ladder system,
the nonmagnetic 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 3leg 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 Nleg spin ladder structure and (b) a schematic ground state
of the 2leg spin ladder system.

Next: Haldane systems
Up: 1.1 Antiferromagnetic spin systems
Previous: Spin Peierls system