ИСТИНА

A detailed investigation of multiband superconductivity and low-frequency Leggett's plasma resonances in Mg(1-x)Al(x)B2 system (0 < x < 0.4) was carried out with a help of Josephson and Andreev spectroscopy of individual ScS (superconductor-constriction-superconductor) Sharvin-type contacts, intrinsic Andreev spectroscopy (intrinsic multiple Andreev reflections effect that usually exists due to presence of contact stacks at cryogenic cleaves) and intrinsic Josephson effect spectroscopy. The contacts were made by break-junction technique. Tunneling spectroscopy and Multiple Andreev Reflections effect (MARE) spectroscopy was used to obtain values of two distinct bulk superconducting gaps of MgB2 (Тс ≈ 41 K) Δ_σ = (11 ± 1) meV, Δ_π = (2 ± 0.5) meV. Natural stacks of Josephson junctions in c-direction show existence of intrinsic Josephson effect that evidences for quasi-two-dimensional superconductivity in Mg(1-x)Al(x)B2. A fine structure corresponding to the low frequency plasma modes was observed on CVC of Josephson (SIS) and Andreev (SnS) Mg(1-x)Al(x)B2 - contacts and investigated. It was found that at the low T = 4.2 K for the contacts with the local Tc in range of (30 ÷ 40) K energy value of Leggett plasma mode ω_0 = (4 ÷ 5) meV and ω_0^2 is proportional to the (Δ_σ Δ_π). These facts are in good agreement with the theoretical predictions of Iskin and de Melo and other works.