Force-noise spectroscopy by tunnelling current deflection sensing

TL;DR

This paper introduces a highly sensitive electro-mechanical setup for measuring AC force noise in a low-temperature tunneling microscope, revealing inelastic transport contributions to force fluctuations.

Contribution

It presents a novel force-noise spectroscopy method using tunneling current deflection sensing with high force resolution at low temperatures.

Findings

Force noise peaks at phonon density of states energies.

Resolution in force noise measurement reaches femtonewtons per root Hz.

Inelastic transport processes influence force fluctuations.

Abstract

An electro-mechanical setup for the measurement of AC-forces in a low-temperature tunnelling microscope has been developed, which enables extremely high force resolution. The crosstalk of vibrations onto the tunnelling current is used to measure the deflection of a force-sensing cantilever beam. We demonstrate its capability to measure the noise of the force at a tunnelling contact using polycrystalline Iridium. Depending on temperature, spring constant and current, a resolution in the range of $\rm {fN}/\sqrt{\rm Hz}$ is possible. We observe peak levels of the force-noise at the energy of the expected phonon maximal density of states, which suggests that inelastic transport processes contribute to force fluctuations.