Atomically resolved probe-type scanning tunneling microscope for use in harsh vibrational cryogen-free superconducting magnet

TL;DR

This paper introduces a robust probe-type STM capable of atomic resolution in harsh cryogen-free superconducting magnets, achieving low temperatures and stable operation for detailed surface and superconducting gap studies.

Contribution

The development of an atomically resolved STM system that operates effectively in cryogen-free superconducting magnets with high vibrational noise and low temperatures.

Findings

Achieved atomic resolution imaging of graphite and NiSe2.

Successfully measured superconducting gap evolution near critical temperature.

Maintained low drift rates and noise levels in a challenging environment.

Abstract

We present a probe-type scanning tunneling microscope (STM) with atomic resolution that is designed to be directly inserted and work in a harsh vibrational cryogen-free superconducting magnet system. When a commercial variable temperature insert (VTI) is installed in the magnet and the STM is in turn housed in the VTI, a lowest temperature of 1.6 K can be achieved, where the STM still operates well. We have tested it in an 8 T superconducting magnet cooled with the pulse-tube cryocooler (PTC) and obtained atomically revolved graphite and NiSe2 images as well as the scanning tunneling spectrum (STS, i.e. dI/dV spectrum) data of the latter near its critical temperature, which show the formation process of the superconducting gap as a function of temperature. The drifting rates of the STM at 1.6 K in X-Y plane and Z direction are 1.15 and 1.71 pm/min respectively. Noise analysis for the tunneling current shows that the amplitudes of the dominant peaks (6.84 and 10.25Hz) are low. This is important as a cryogen-free magnet system has long been considered too harsh for any atomic resolution measurement.