Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field of a single crystal under static magnetic field
s parallel to the axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic -wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound, with the same Tc. We estimated the range of the large gap value 6-8 meV (depending on small variation of and its a space anisotropy of about 30%, and the small gap 1.7 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps.
Terahertz phonon spectroscopy studies of bismuth cuprates are overviewed.
Detailed temperature dependence of both superconducting gaps was obtained directly by means of SnS-Andreev spectroscopy. The Delta sigma,pi(T) -curves were shown to be deviated from standard BCS-like behavior, due to k-space proximity effect between
sigma - and pi - condensates, which could give a key to experimental determination of interband electron-phonon coupling constants. For the first time, an excellent qualitative agreement with theoretical predictions of Nicol and Carbotte, and Moskalenko and Suhl was shown. dI(V)/dV-spectra of SnS-Andreev contacts based on MgB2 samples (with defects of crystal structure), and Mg(Al)B2 polycrystalline samples (with the local critical temperatures Tc variation 10 K < Tc < 37 K) were studied by means of the break-junction technique within the temperature range 4.2 K < T < Tc.
