Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO0.85F0.15: evidence for the strong order parameter anisotropy

• Main Authors: Dmitriev, V. M, Khlybov, E. P, Kondrashov, D. S, Terekhov, A. V, Rybaltchenko, L. F, Khristenko, E. V, Ishchenko, L. A, Kostyleva, I. E, Zaleski, A. J
• Format: Journal Article
• Language: English
• Published: 16-12-2010
• Subjects: Physics - Superconductivity
• DOI: 10.48550/arxiv.1012.2010

Andreev reflection spectra have been measured in a new superconductor
EuAsFeO0.85F0.15 having an unexpectedly low superconducting transition
temperature Tc~11.3 K among related FeAs compounds on a base Sm and Gd
surrounding Eu in the series of lanthanides. The nearly fivefold lower Tc, as
against the expected value, is attributed to the divalent properties of Eu ions
when in the compound investigated along with the weakly magnetic Eu3+ ions may
be present and the strongly magnetic Eu2+ ones that is a strong destructive
factor for superconductivity. Most of the spectra measured showed features that
corresponds to two energy gaps whose values varied from contact to contact
within 2{\Delta}s/kTc=2.2{\pm}4.7 and 2{\Delta}1/kTc=5.1{\pm}11.7 for small and
large gap, respectively. The corresponding variations for single-gap spectra
are 2{\Delta}/kTc = 2.6{\pm}6.4. The relatively large size of crystallites (no
less than ~25 \mu{m}) and the large number of contacts measured (several tens)
suggest with a high degree of probability that the spectra obtained account
quite fully for the gap distribution practically in all crystallographic
directions. The data obtained and the absence of zero gaps in the measured
spectra evidence in favor of the anisotropic s- or s\pm-symmetry of the order
parameter in EuAsFeO0.85F0.15 that was revealed in other similar compounds with
higher Tc. Thus, the character of the gap function {\Delta}(k) in this compound
is inconsistent with the d-wave superconductivity observed in some low-Tc
pnictides.