Development of an ultra-low temperature scanning tunneling microscope and applications for low temperature physics

In the past several years, we have been constructing an 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 ultra high vacuum (UHV). One can prepare clean sample surfaces in an UHV chamber, and load/unload samples and tips to an STM head keeping the low temperature and UHV conditions. One can also cool them back to the base temperature within 3 h. As one of examples for application of this ULT-STM, we show scanning tunneling spectroscopy observations of the Landau levels (LLs) at graphite surfaces. We observed not only a series of LLs, which have normal field dependence but also almost field-independent ones near the Fermi energy. Although the existence of such levels has been known indirectly from previous magneto-optical measurements, the present experiment verifies such field-independent LLs unambiguously.