Low Temperature Scanning Tunneling Microscope and Its Application to Material Characterization

Abstract

A low temperature and high vacuum compatible fiber optic interferometer was designed and constructed to debug a malfunctioning low temperature scanning tunneling microscope (LT-STM). With its help, the temperature dependent behavior of a Pan-style piezoelectric actuator was studied. The scanning tunneling microscope (STM) was modified accordingly and worked reliably below 10 K.

Other properties of the STM were also improved. The electronic noise was reduced from hundreds of picoamps to approximately 10 picoamps by improving the shielding and avoiding ground loops. An eddy current damper was implemented to reduce the vibrational noise. A new mechanical sample stage was introduced to allow manipulation of the sample during experiments. As a result, beautiful atomic resolution images of graphite and self-assembled dodecanethiol monolayer on gold were obtained.

Scanning tunneling spectroscopy (STS) measurements were carried out on flux-grown HfNiSn single crystals. Instead of a semiconductor gap, a square root zero bias anomaly (ZBA) which typically presents in disordered systems was observed. Both the temperature dependent resistivity and the magnetoresistance of HfNiSn show characteristic features of disordered systems as well. Below 200 K, the resistivity saturates or obeys a 3 dimensional variable-range hopping (VRH) behavior. The magnetoresistance can be well explained by the Fukuyama-Hoshino (F-H) model for 3D weak anti-localization (WAL). These results indicate that the intrinsic anti-site disorder in HfNiSn may cause Anderson localization. In addition, theories developed and refined in the 1980's for the electron-electron interaction in disordered systems can be used to understand the physical properties and to guide the modifications of half Heusler materials.