$a-b$ Plane Point Contact Spectroscopy measurements of optimally Cobalt doped Ba-122 iron-pnictide superconductors

Abstract

Point contact spectroscopy (PCS) is a technique which can reveal the size and symmetry of a superconducting gap ($\Delta$) and is especially useful for new materials such as the iron-based superconductors. PCS is usually employed in conjunction with the extended Blonder-Tinkham-Klapwijk (BTK) model which is used to extract information such as the existence of nodes in $\Delta$ from PCS data obtained on unconventional superconductors. The BTK model uses a dimensionless parameter $Z$ to quantify the barrier strength across a normal metal - superconductor junction. We have used a unique feature of PCS which allows variation of $Z$ to obtain crucial information about $\Delta$. We report our $Z$-dependent, $a-b$ plane PCS measurements on single crystals of the iron-based superconductor BaFe$_{2-x}$Co$_{x}$As$_{2}$. Our measurements show that BaFe$_{2-x}$Co$_{x}$As$_{2}$ ($x$=0.148) is a superconductor with two gaps which does not contain any nodes. The $Z$ dependent point contact spectra rule out a pure $d$ symmetry and the gaps at optimal doping have negligible anisotropy.