Decisive proofs of the s(+/-) -> s(++) transition in the temperature dependence of the magnetic penetration depth
One of the features of the unconventional 
state in iron-based superconductors is possibility to transform to the s++ state with the increase of the nonmagnetic disorder. Detection of such a transition would prove the existence of the 
state. Here we study the temperature dependence of the London magnetic penetration depth within the two-band model for the 
and s++ superconductors. By solving Eliashberg equations accounting for the spin-fluctuation mediated pairing and nonmagnetic impurities in the T-matrix approximation, we have derived a set of specific signatures of the 
transition: (1) sharp change in the behavior of the penetration depth λL as a function of the impurity scattering rate at low temperatures; (2) before the transition, the slope of 
increases as a function of temperature, and after the transition this value decreases; (3) the sharp jump in the inverse square of the penetration depth as a function of the impurity scattering rate, 
, at the transition; (4) change from the single-gap behavior in the vicinity of the transition to the two-gap behavior upon increase of the impurity scattering rate in the superfluid density ρs(T).