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

TL;DR

This study investigates the superconducting energy gaps in NdFeAs(O,F) using point-contact Andreev-reflection spectroscopy, revealing evidence for two-gap superconductivity and pseudogap phenomena with temperature-dependent features.

Contribution

It provides the first detailed analysis of the two-gap structure in NdFeAs(O,F) through PCAR spectroscopy and discusses unusual temperature effects including pseudogap presence.

Findings

Identification of two superconducting gaps at 4-7 mV

Observation of a zero-bias conductance peak emerging at 15-20 K

Detection of pseudogap features persisting above Tc

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.