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 $\beta$.