  It has been shown that interband mixing of gradients of two order parameters
(drag effect) in an isotropic bulk two-band superconductor plays important role
- such a quantity of the intergradients coupling exists that the two-band
superconductor is characterized with a single coherence length and a single
Ginzburg-Landau (GL) parameter. Other quantities or neglecting of the drag
effect lead to existence of two coherence lengths and dynamical instability due
to violation of the phase relations between the order parameters. Thus
so-called type-1.5 superconductors are impossible. An approximate method for
solving of set of GL equations for a multi-band superconductor has been
developed: using the result about the drag effect it has been shown that the
free-energy functional for a multi-band superconductor can be reduced to the GL
functional for an effective single-band superconductor.
