Unconventional pairing symmetry in iron-based layered superconductor LaO$_{0.9}$F$_{0.1-δ}$FeAs revealed by point-contact spectroscopy measurements

Abstract

Superconducting state has lower energy compared with its normal counterpart due to the condensation of paired electrons below $T_c$. For the electrons of each pair, there is a specific symmetry characterized by the angular momentum number $l$. For example, the cuprate superconductors are categorized by a d-wave symmetry with $l=2$ \cite{Tsuei2000RMP}, while many conventional superconductors have an s-wave symmetry with $l=0$. To carry out the pairing symmetry is the first step to resolve the mechanism of a new superconductor. Recently, superconductivity at 26 K \cite{Kamihara2008JACS} was discovered in LaO$_{1-x}$F$_{x}$FeAs (x=0.05-0.12). Here, for the first time, we report point-contact spectroscopy data for junctions between a normal metal and the superconductor LaO$_{0.9}$F$_{0.1-\delta}$FeAs (F-LaOFeAs). A zero-bias conductance peak was observed and its shape and magnitude suggests the presence of Andreev bound states at the surface of F-LaOFeAs, which gives a strong evidence of an unconventional pairing symmetry with a nodal gap function. By fitting the spectra to theoretical models for a p-wave or d-wave gap, we obtained the maximum gap value $\Delta_0\approx3.9\pm0.7$meV.