ИСТИНА

We present the study of the Leggett collective oscillations with energy ω_0 of a superconductor plasma in two-gap superconductors MgB2, MgB2+MgO and Mg(1-x)Al(x)B2 with various doping level. We revisit our previous tunneling spectroscopy data to find the ω_0^2 dependence on Δ_σ(0) Δ_π(0): (i) observed experimentally, and (ii) simulated using the reduced coupling constants λ_{ij} estimated from the experimental Δ_{σ,π}(T) dependences. The comparison of both results at T << Tc shows the monotonic ω_0^2(Δ_σ Δ_π) behaviour, and the simulated ω_0 values tending to the Sharapov et al. predictions, those well exceed the experimental ω_0 values. We conclude on the substantial softening of the Leggett mode energy ω_0 --> 2Δ_π(0) in Mg(1-x)Al(x)B2 compounds. In the dI(V)/dV-spectra of SnS-junctions formed in Gd(O,F)FeAs and (Sm,Th)OFeAs oxypnictides (the 1111 family) with various doping degree and maximum Tc ≈ 50 K, as well as in (K,Na){x}Fe{2-y}Se2 selenides with Tc ≈ 26 K, the observed fine structure was attributed to an interaction with a spin exciton. In particular, multiple emission of up to 4 bosons was detected in (Sm,Th)OFeAs. At T << Tc, the spin exciton energy \epsilon_0 was not exceed the indirect SC gap Δ_L(0) + Δ_S(0) in the oxypnictides, and 2Δ(0) in single-gap selenides. In the 1111-compounds, under doping variation, \epsilon_0 was scaled with Tc together with Δ_L(0) and Δ_S(0). The experimental temperature dependence \epsilon_0(T) agrees well with the ω_R(T) dependence calculated within the s± model.