Possible two-gap superconductivity in NdFeAs(O,F) probed by point-contact Andreev-reflection spectroscopy

Abstract

Systematic studies of the NdFeAsOF superconducting energy gap via the point-contact Andreev-reflection (PCAR) spectroscopy are presented. The PCAR conductance spectra show at low temperatures a pair of gap-like peaks at about 4 - 7 mV indicating the superconducting energy gap and in most cases also a pair of humps at around 10 mV. Fits to the s-wave two-gap model of the PCAR conductance allowed to determine two superconducting energy gaps in the system. The energy-gap features however disappear already at T* = 15 to 20 K, much below the particular Tc of the junction under study. At T* a zero-bias conductance (ZBC) peak emerges, which at higher temperatures usually overwhelms the spectrum with intensity significantly higher than the conductance signal at lower temperatures. Possible causes of this unexpected temperature effect are discussed. In some cases the conductance spectra show just a reduced conductance around the zero-bias voltage, the effect persisting well above the bulk transition temperature. This indicates a presence of the pseudogap in the system.

Revised paper, 6 pages, 5 figures, accepted for Supercond. Sci. & Technol. Special issues: Anisotropic and multiband pairing: from borides to multicomponent superconductivity. Expected online publication: December 2008 (print: January 2009)

References in corpus (3)

• Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2
• The BCS-like gap in superconductor SmFeAsO_0.85F_0.15
• Point contact Andreev reflection spectroscopy of NdFeAsO_0.85

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• Coexistence of two order parameters and a pseudogaplike feature in the iron-based superconductor LaFeAsO_(1-x)F_x
• Point-contact Andreev-reflection spectroscopy in ReFeAsO_{1-x}F_x (Re = La, Sm): Possible evidence for two nodeless gaps
• Investigation of Superconducting Gap Structure in TbFeAsO${}_{0.9}$F${}_{0.1}$ using Point Contact Andreev Reflection