A low noise cryogen-free scanning tunneling microscope-superconducting magnet system with vacuum sample transfer.

Abstract

Imaging atomically resolved surfaces and performing spectroscopy of exotic surfaces at cryogenic temperature in the presence of the magnetic field is an engineering challenge. Additionally, performing these measurements in an all-cryogen-free environment compounds the above complexity due to the associated vibration and acoustic noise generated by the running of cryogenic cold heads. We here report successful integration of a cryogen-free scanning tunneling microscope (STM) with a cryogen-free superconducting vector-magnet, connected to an ultra-high vacuum cluster assembly for in situ sample transfer. We present details of the integration involving vibration and electrical noise isolation procedures allowing for operation of the STM at extremely low noise levels below 30 fA/Hz during normal operations of the complete vacuum-line assembly with multiple turbomolecular pumps. We demonstrate the above STM capability at cryogenic temperature and in the presence of the magnetic field through atomic resolution imaging of graphite and thin films of gold on the mica substrate transferred in situ to the STM chamber. We also demonstrate spectroscopy signatures of the superconducting gap in MgB2 thin films. The design of our in-house customized cluster-vacuum-line assembly provides unsought opportunities in continuous uninterrupted imaging of ultra-clean in-vacuum grown surfaces without the need for cryogenic refills in either the STM or the magnet.