Evidence of multiple nodeless energy gaps in superconductingBa0.6K0.4Fe2As2single crystals from scanning tunneling spectroscopy

Abstract

We report on low-temperature scanning tunneling microscopy and spectroscopy (STM) studies of the electronic structure of single-crystalline ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Multiple superconducting gaps are observed in the density of states (DOS) and the sizes of the two dominant gaps ${\ensuremath{\Delta}}_{L}$ and ${\ensuremath{\Delta}}_{S}$ are 7.6 and 3.3 meV, respectively. The flat bottom of the DOS spectra near zero bias indicates the nodeless feature of the gaps, while the global fitting to the spectra definitely requires the anisotropy. The nodeless gaps with finite anisotropy revealed in our STM data agree well with the expectations of an extended $s$-wave superconductivity.