Some quantum antiferromagnetic models are described which have spin-disordered states as ground states. The models include frustrated antiferromagnets and quantum spin-1 chains. The disordered states can be characterised by various order parameters. Links with classical lattice statistical...

A spin-12 antiferromagnetic generalized ladder model is constructed which consists of four-spin plaquettes, coupled through weaker exchange interactions, to two-spin rungs. In an extended parameter regime, the exact ground state of the ladder is determined. In this state, the four-spin...

Recently, Lenski et al. have carried out an experiment on bacterial evolution. Their findings support the theory of punctuated equilibrium in biological evolution. We show that the M=2 Bak–Sneppen model can explain some of the experimental results in a qualitative manner.