Abstract
Andreev reflection spectra have been measured in a new superconductor EuAsFeO0.85F0.15 with an unexpectedly low superconducting transition temperature Tc ≈ 11.3 K compared to related FeAs compounds based on Sm and Gd surrounding Eu in lanthanide series. The nearly fivefold lower Tc, compared to the expected value, is attributed to the divalent properties of Eu ions; the weakly magnetic Eu3+ ions may be present and the highly magnetic Eu2+ ions are strong suppressors of superconductivity. Most of the measured spectra have features corresponding to two energy gaps whose values varied, from contact to contact, over 2Δs/kTc = 2.2 −4.7 and 2Δ1/kTc = 5.1−11.7 for small and large gaps, respectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6−6.4. The relatively large crystallite sizes (at least ∼25 μm) and the large number of measured contacts (several tens) suggest, with a high degree of probability, that these spectra account quite fully for the gap distribution in essentially all the crystallographic directions. These data and the absence of zero gaps in the measured spectra indicate anisotropic s− or s±-symmetry of the order parameter in EuAsFeO0.85F0.15 that has been found in other, similar compounds with higher Tc. Thus, the character of the gap function Δ(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tc pnictides.
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