1. J.R. Schrieffer, Theory of Superconductivity (Addison-Wesley, Reading, MA, USA, 1964)
2. R.B. Laughlin, Phys. Rev. Lett. 50, 1395 (1983)
3. P.W. Anderson, Mater. Res. Bull. 8, 153 (1973)
4. P. Fazekas, P.W. Anderson, Philos. Mag. 30, 423 (1974)
5. For a review, see for instance, G. Misguich, C. Lhuillier, in Frustrated Spin Models, ed. by H.T. Diep (World Scientific, New Jersey, 2004)