I will discuss that in contrast to single-component superconductors, which are described at the level of Ginzburg-Landau theory by a single Ginzburg-Landau parameter and are divided in type-I and type-II classes, two-component systems (in particular multiband supercondiuctors) in general possess three fundamental length scales and can possess a separate regime which falls outside this dichotomy (the term "type-1.5" superconductivity was coined for it recently by Moshchalkov et al.). In that state, as a consequence of the extra fundamental length scale, two vortices attract one another at long range but repel at shorter ranges, and therefore N vortices should form clusters in low magnetic fields. In such clusters one can define a negative interface energy inside a cluster and at the same there one can define a positive interface energy associated with the cluster's boundary. Moreover it is important for the physics of the vortex clusters that the interaction forces between N vortices are non-pairwise at intermediate length scales which strongly affects structure formation in the system (i.e. in the cluster interaction energy of N vortices is not a sum of pairwise interaction). I will also overview the current experimental efforts in the field.

E. Babaev & J.M. Speight Semi-Meissner state and neither type-I nor type-II superconductivity in multicomponent systems Phys.Rev. B 72 180502 (2005).

E. Babaev, J. Carlstrom, J. M. Speight Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors Phys. Rev. Lett. 105, 067003 (2010).

J. Carlstrom, E. Babaev, M. Speight Type-1.5 superconductivity in multiband systems: the effects of interband couplings arXiv:1009.2196 J. Carlstrom, J. Garaud, E. Babaev Non-pairwise intervortex interaction forces, arXiv:1101.4599.