Construction of a Dilution Refrigerator Based Ultra-Low Temperature Scanning Tunneling Microscope

We constructed a dilution refrigerator based ultra-low temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 20 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). One can load samples/tips, which are prepared in a UHV chamber, to an STM head maintaining the low temperature and UHV conditions. After then they can be cooled back to the base temperature in several hours. We report results of a test measurement on a superconducting NbSe 2 sample as well as recent STM/STS studies on graphite samples such as observations of the Landau quantization and visualization of the possible localized states in magnetic fields.