Magnetic behaviour of SO(5) superconductors

The distinction between type I and type II superconductivity is re-examined in the context of the SO(5) model recently put forth by Zhang. Whereas in conventional superconductivity only one parameter (the Ginzburg-Landau parameter kappa) characterizes the model, in the SO(5) model there are two essential parameters. These can be chosen to be kappa and another parameter, beta, related to the doping. There is a more complicated relation between kappa and the behaviour of a superconductor in a magnetic field. In particular, one can find type I superconductivity even when kappa is large, for appropriate values of beta.