ABSTRACT
Several theories for the cuprates have been constructed in which we try to use Landau Fermi-liquid theory to understand the normal-state properties, and then set up a theory of the bcs type to explain superconductivity as a pair condensation of the weakly interacting Landau quasiparticles. To discuss cuprate superconductivity this chapter introduces a semion, an entity for which the phase changes by π2 on exchange. The semion is not necessarily to be associated with a real flux line. It appears, for instance, that a vacancy which moved by changing places with electrons whose spin did not flip might have semion character for certain ground states. A related difficulty concerns the behaviour of flux lines normal to the plane. When we considered the behaviour of flux lines in bulk material we ignored the entropy of arrangement of the flux lines.
