Maze-like surface reconstruction on pure SrFe$_2$As$_2$ observed by STM/STS*

We measured undoped SrFe$_2$As$_2$ samples using a low temperature scanning tunneling microscope. Similar samples showed superconductivity in up to 15\% of its volume in measurements of the diamagnetic screening although being undoped as confirmed by energy dispersive x-ray spectroscopy. The samples where cleaved at room temperature at a pressure $P<10^{-8}$ mbar before introducing them into the low temperature scanning tunneling microscope (LTSTM) operating at a temperature of $T=4.2$ K. Beside the usual striped surface reconstruction and disordered regions we observed areas showing a maze like reconstruction. Atomically resolved images strongly suggest that the maze structure is formed by adatoms and thus, by inference, is the striped phase. Scanning tunneling spectroscopy (STS) measurements showed a superconducting gap on both the striped and maze phase of the sample, similar to the spectra on a cobalt doped sample. The results will be discussed in detail.