  The magnetic vortices in superconductors usually repel each other. Several
cases are discussed when the vortex interaction has an attractive tail and thus
a minimum, leading to vortex clusters and chains. Decoration pictures then
typically look like in the intermediate state of type-I superconductors,
showing lamellae or islands of Meissner state or surrounded by Meissner state,
but with the normal regions filled with Abrikosov vortices that are typical for
type-II superconductors in the mixed state. Such intermediate-mixed state was
observed and investigated in detail in pure Nb, TaN and other materials 40
years ago; last year it was possibly also observed in MgB$_2$, where it was
called "a totally new state" and ascribed to the existence of two
superconducting electron bands, one of type-I and one of type-II. The
complicated electronic structure of MgB$_2$ and its consequences for
superconductivity and vortices are discussed. It is shown that for the real
superconductor MgB$_2$ which possesses a single transition temperature, the
assumption of two independent order parameters with separate penetration depths
and separate coherence lengths is unphysical.

Superconductivity