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

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Point contact spectroscopy (PCS) is a technique which can reveal the size and symmetry of a superconducting gap (Δ) 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 Δ 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 Δ. We report our Z-dependent, a-b plane PCS measurements on single crystals of the iron-based superconductor BaFe₂₋ₓCoₓAs₂. Our measurements show that BaFe₂₋ₓCoₓAs₂ (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.