Improved Dilution Refrigerator for Scanning Tunneling Microscopy

Following the milestone “A 10 mK Scanning Probe Microscopy Facility” that JanisULT developed for the National Institute of Standards and Technology (NIST) [REVIEW OF SCIENTIFIC INSTRUMENTS, 81, 121101 (2010)], JanisULT has successfully developed another top-loading mK STM dilution refrigerator (DR) for Samsung Advanced Institute of Technology (SAIT) in Korea with superior design and performance over the NIST system. The SAIT system includes a 15.5 Tesla superconducting magnet with compensation coil and has a 1.5” diameter central access into the UHV space. Using a top manipulator, the large bore central access allows an STM with in-situ prepared sample and probe tip to be translated into the magnet center without warming up the system above 4 K. Other novel features incorporated in the SAIT system include:
  • Comprehensive UHV compatible wires and coaxial cables are installed to fit the user’s special applications. User-friendly mechanical heat switch is installed for fast pre-cooling of the DR stage. Multiple shutters are assembled to block the infra-rad radiation heat load. Based on the experience from the NIST system, the SAIT system used a Joule-Thomson (JT) stage for mixture condensation along with the 1K pot. JanisULT automated gas handling system (GHS2) with oil-free Roots pumping station.
The SAIT system has been successfully installed on site and reached the following performance specifications:
  • The mixing chamber has reached a base temperature of 6.6 mK per the CMN sensor calibrated with the fixed superconducting point device (FPD). More than 300 µW @ 100 mK cooling power has been achieved. The solenoid has reached maximum magnetic field of 15.5 T @ 4.2K with DR at base temperature. To prove ULT STM performance and atomic resolution, the user has been studying graphite and graphene only so far. A “standard” superconducting sample will be used to measure the BCS gap and determine the electronic temperature.
The user has disclosed, “We now have a UHV ultra-low temperature scanning probe microscope working with a base temperature of 6.6 mK and the maximum magnetic field of 15.5 T.”