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Type-1.5 superconductivity in multiband systems: magnetic response, broken symmetries and microscopic theory. A brief overview

A conventional superconductor is described by a single complex order parameter field which has two fundamental length scales, the magnetic field penetration depth λand the coherence length ξ. Their ratio κdetermines the response of a superconductor to an external field, sorting them into two categories as follows; type-I when κ<1/\sqrt{2} and type-II when κ>1/\sqrt{2} . We overview here multicomponent systems which can possess three or more fundamental length scales and allow a separate &#34;type-1.5&#34; superconducting state when, e.g. in two-component case ξ_1<\sqrt{2}λ<ξ_2. In that state, as a consequence of the extra fundamental length scale, vortices attract one another at long range but repel at shorter ranges. As a consequence the system should form an additional Semi-Meissner state which properties we discuss below. In that state vortices form clusters in low magnetic fields. Inside the cluster one of the component is depleted and the superconductor-to-normal interface has negative energy. In contrast the current in second component is mostly concentrated on the cluster&#39;s boundary, making the energy of this interface positive. Here we briefly overview recent developments in Ginzburg-Landau and microscopic descriptions of this state.