  We report the design and performance of a cryogen-free, pulse-tube
refrigerator (PTR)-based scanning probe microscopy (SPM) system capable of
operating at the base temperature of near 5K. We achieve this by combining a
home-made interface design between the PTR cold head and the SPM head, with an
automatic gas-handling system. The interface design isolates the PTR vibrations
by a combination of polytetrafluoroethylene and stainless-steel bellows, and by
placing the SPM head on a passive vibration isolation table via two cold stages
that are connected to thermal radiation shields using copper heat links. The
gas-handling system regulates the helium heat-exchange gas pressures,
facilitating both the cool down to and the maintenance of the base temperature.
We discuss the effects of each component using measured vibration,
current-noise, temperature, and pressure data. We demonstrate that our SPM
system performance is comparable to known liquid-helium-based systems with the
measurements of the superconducting gap spectrum of Pb, atomic-resolution
scanning tunneling microscopy image and quasiparticle interference pattern of
Au(111) surface, and non-contact atomic force microscopy image of NaCl(100)
surface. Without the need for cryogen refills, the current SPM system enables
uninterrupted low-temperature measurements.
