Two-band superconductors: Hidden criticality deep in the superconducting   state
  We show that two-band superconductors harbor hidden criticality deep in the superconducting state, stemming from the critical temperature of the weaker band taken as an independent system. For sufficiently small interband coupling $\gamma$ the coherence length of the weaker band exhibits a remarkable deviation from the conventional monotonic increase with temperature, namely, a pronounced peak close to the hidden critical point. The magnitude of the peak scales proportionally to \gamma^(-\mu), with the Landau critical exponent \mu = 1/3, the same as found for the mean-field critical behavior with respect to the source field in ferromagnets and ferroelectrics. Here reported hidden criticality of multi-band superconductors can be experimentally observed by, e.g., imaging of the variations of the vortex core in a broader temperature range. Similar effects are expected for the superconducting multilayers. 
L. Komendov\'a, Yajiang Chen, A. A. Shanenko, M. V. Milo\v{s}evi\'c   and F. M. Peeters
cond-mat.supr-con cond-mat.stat-mech hep-ph