Usual superconductors are classified into two categories as follows: type-1 when the ratio of the magnetic field penetration length (lambda) to coherence length (xi) with Ginzburg-Landau parameter kappa=lambda/xi <1/sqrt{2} and type-2 when kappa >1/s
qrt{2}. The boundary case kappa =1/sqrt{2} is also considered to be a special situation, frequently termed as Bogomolnyi limit. Here we discuss multicomponent systems which can possess three or more fundamental length scales and allow a separate superconducting state, which was recently termed type-1.5. In that state a system has the following hierarchy of coherence and penetration lengths xi_1<sqrt{2}lambda<xi_2. We also briefly overview the works on single-component regime $kappa approx 1/sqrt{2}$ and comment on recent discussion by Brandt and Das in the proceedings of the previous conference in this series.
The recent paper by V. G. Kogan and J. Schmalian Phys. Rev. B 83, 054515 (2011) argues that the widely used two-component Ginzburg-Landau (GL) models are not correct, and further concludes that in the regime which is described by a GL theory there co
uld be no disparity in the coherence lengths of two superconducting components. This would in particular imply that (in contrast to $U(1)times U(1)$ superconductors), there could be no type-1.5 superconducting regime in U(1) multiband systems for any finite interband coupling strength. We point out that these claims are incorrect and based on an erroneous scheme of reduction of a two-component GL theory. We also attach a separate rejoinder on reply by Kogan and Schmalian. In their reply Phys. Rev. B 86, 016502 (2012) to our comment Phys. Rev. B 86, 016501 (2012) Kogan and Schmalian did not refute or, indeed, discuss the main points of criticism in the comment. Unfortunately they instead advance new incorrect claims regarding two-band and type-1.5 superconductivity. The main purpose of the attached rejoinder is to discuss these new incorrect claims.
