Fizika Nizkikh Temperatur: Volume 45, Number 11 (November 2019), p. 1366-1380    ( to contents , go back )

Characteristics of the superconducting subsystems in magnesium diborides and iron oxypnictides revealed by multiple Andreev reflection spectroscopy

T.E. Kuzmicheva1 and S.A. Kuzmichev1,2

1P.N. Lebedev Physical Institute, Moscow 119991, Russia
E-mail: kuzmichevate@lebedev.ru

2M.V. Lomonosov Moscow State University, Moscow 119991, Russia

Received July 2, 2019, published online September 27, 2019

Abstract

In multiple-gap superconductors, the physical parameters of the condensates are determined by both, intraband and interband coupling. In case the two superconducting order parameters are in a weak-coupling limit, a model by Moskalenko and Suhl provides the simplest outline of such system, with four electron-boson coupling constants describing the pairing strength. Commonly, the characteristic ratio 2Δ0/kBTc exceeds the BCS-limit 3.53, thus requiring a Δ0 or Tc renormalization in both BCS-integrals. If so, in the framework of Moskalenko and Suhl model one needs to operate at least six parameters in order to describe the two-gap superconducting system. On the other hand, the quantities observed in various experiments appear as a combination of each band contributions and the cross-pairing, to be hardly separated in the experiment. Moreover, it is impossible to study in the experiment each superconducting subsystem separately, i.e. in ab-sence of interband interaction. Unlike Eliashberg theory extended for two-band case, the Moskalenko and Suhl model facilitates an easy and experimenter-friendly way to study the superconducting state using a minimum set of parameters. Anyway, the most important factor ensuring the reliability of such estimate, is direct, simultaneous and precise measurement of both order parameters versus temperature, which in itself seems a challenging experimental problem. Multiple Andreev reflection spectroscopy provides direct measurement of gap temperature dependences Δ1,2(T) and requires no fit of the dynamic conductance spectrum of the Andreev contact. In such superconducting systems as MgB2 + MgO, Mg1−xAlxB2, and Fe-based oxypnictides ReO1−xFy FeAs (Re = Gd, Sm:Th, La), we fit the experimentally obtained Δ1,2(T) dependences with the extended Moskalenko and Suhl model and estimate the parameters of both condensates. We show for magnesium diborides and iron arsenides with maximum Tc, the intraband interaction to be about 15 and 10 times, correspondingly, stronger than interband one. This ratio decreases along with Tc. The estimated eigen characteristic ratios for the “driving” bands are ≈ 5.5 and 4.6, respectively, remaining almost constant under the variation in chemical composition within the studied range Tc > 20 K. For “driven” bands, this ratio is close to the weak-coupling BCS-limit 3.5.

Key words: two-gap superconductivity, magnesium diborides, iron-based superconductors, Andreev spectroscopy, break-junction.

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