Characteristics of superconducting subsystems in magnesium diborides and iron oxypnictides from data on spectroscopy of multiple Andreev reflections

Abstract

Physical parameters of condensates in multiple-gap superconductors are determined by coupling both within and between bands. The simplest description of the physics of the system in case of two superconducting order parameters is offered by the Moskalenko-Suhl two-band model, in which strength of coupling is determined by four constants of electron–boson interaction. The characteristic ratio 2Δ0/kBTc usually exceeds the limit of the BCS theory equal to 3.53, thus requiring renormalization to be introduced for Δ0 or Tc in both BCS integrals. This implies that at least six parameters are to be handled in the Moskalenko–Suhl model to describe a two-gap superconducting system. The quantities observed using various techniques are superposition of contributions from each band and interband interaction, and thus usually cannot be separated in the experiment. Moreover, it is not possible to explore in the experiment individual properties of each of the superconducting subsystems, i.e. to study them in the absence of crossband interaction. In contrast to the Eliashberg model extended for the two-band case, the Moskalenko-Suhl model provides the simplest technique to describe the superconducting state using a minimal set of quantities, a feature that is undoubtedly attractive for experimentalists. The factor that is required for such an estimate to be reliable is direct, simultaneous, and accurate measurement of both order parameters as a function of temperature, a task that is very challenging for the experiment. Multiple Andreev reflection effect (MARE) spectroscopy may be used to determine dependences of order parameters Δ1,2(T) directly without involving additional approximation of experimental spectra of the dynamic conductance of Andreev contacts. We have fitted the experimental dependences Δ1,2(T) obtained within the extended Moskalenko–Suhl model to estimate parameters of superconducting systems such as MgB2 + MgO, Mg1-xAlxB2, and iron-containing oxypnictides ReO1-xFyFeAs (Re = Gd, Sm:Th, La). The intraband coupling was shown to be stronger than the crossband coupling by a factor of 15 for magnesium diborides and 10 for ferrous arsenides with maximal Tc, this ratio decreases together with Tc. The estimated eigen characteristic ratios for “strong” bands are ≈5.5 and 4.6, respectively, are almost independent on chemical composition in the explored range Tc > 20 K. This ratio for “weak” bands is close to the weak-coupling BCS limit 3.5.