Low temperature scanning tunneling microscopy and spectroscopy investigation of FeSe$_{1-\delta}$ and FeSe$_{1-x}$S$_{x}$ single crystals*

Due to its relatively simple crystallographic structure, investigations into FeSe$_{1-\delta}$ have held the promise to provide an avenue towards a better understanding of the mechanism of superconductivity in the iron-pnictides/chalcogenides and the relationship between nematicity and superconducting state. Here, we present low-temperature scanning tunneling microscopy and spectroscopy investigations of high purity FeSe$_{1-\delta}$ and sulfur substituted FeSe$_{1-x}$S$_x$ single crystals. Vortex core anisotropy and vortex matter in these systems will be discussed.