Extending STM Capabilities: Multi-Frequency Lock-In Amplification for Closed-Cycle STM and Tip-Controlled Lattice Distortions in WTe₂

Abstract

Scanning tunneling microscopy (STM) is a versatile tool that provides sub-nanometer topographic information, gives access to the electronic structure by measuring the local density of states, and enables local modification of the atomic structure. This thesis contains research results on two distinct topics related to STM's abilities. The first part illustrates STM results on the local modification of the lattice structure in the ferroelectric and Weyl semimetal WTe$_2$. Lattice distortions are created using current pulses by an STM tip, leading to in-plane shifts of surface atoms similar in magnitude to previously reported ferroelectric switching in WTe$_2$. In addition, we observe out-of-plane rearrangements of Te atoms, potentially suggesting local perturbations of the Jahn-Teller distortion. These distortions are accompanied by changes in the local density of states, indicating modifications to the electronic structure. The lattice distortions extend over nanometer-scale regions and can be repositioned or erased. This work demonstrates the potential for reversibly tuning the electronic structure of WTe$_2$ on the nanometer scale. The second part focuses on improvements to a closed-cycle cryostat STM system using an emerging measurement technique called multi-channel lock-in amplification (MCLA). The scarcity of helium and the increasing cost of liquid helium production motivate the adoption of closed-cycle cryostats. However, the elevated vibrational noise levels of such cooling systems limit their usability with STM. This work illustrates the adoption of the MCLA technique to improve the performance of a closed-cycle STM system. We developed an open-source MCLA that acquires 72 harmonics. Our implementation is built on an off-the-shelf field-programmable gate array (FPGA) development board, making it affordable and customizable. The use of MCLA enables a twelve-fold reduction in measurement time, allowing to perform quasiparticle interference mapping, which has not previously been reported for STM systems operated with closed-cycle cryostats.