Determining the surface ground state in YbB6 and developing ultra-sensitive nanoscale dissipation measurements in scanning probe microscopy

Abstract

My Ph.D. research comprised two main focuses: (1) the analysis of surface states in YbB6 using scanning tunneling microscopy (STM) and spectroscopy (STS) to determine its ground state and resolve discrepancies in angle-resolved photoemission spectroscopy (ARPES) measurements, and (2) the design and construction of a novel scanning probe microscope (SPM) to expand the lab’s capabilities from STM into ultra-sensitive atomic force microscopy (AFM) for both force and dissipation studies. The first chapter provides an introduction and overview of the STM and AFM techniques and their implementation. Chapter 2 presents my work on YbB6, which is published in its entirety as: Aaron Coe, Zhi-Huai Zhu, Yang He,Dae-Jeong Kim, Zachary Fisk, Jason D. Hoffman, andJennifer E. Hoffman, “Nanoscale Conducting and Insulating Domains on YbB6,” Physical Review Letters 134, 236205 (2025). ARPES measurements have disagreed on the ground state of YbB6 on the (001) surface. This discrepancy arises largely from the complex surface structure, caused by the absence of a natural cleavage plane and the presence of polarized terminations. Together these effects produce a disordered surface with nanoscale variations in band bending. As a result, the band structure appears smeared in spatially averaging techniques, and spectral features vary across the surface and over time. Anatomically resolved technique is therefore required to identify pristine terminations and determine their elemental identity. Using STM/STS, we achieved this and proposed a ground state incorporating Rashba spin-splitting that reconciles the conflicting ARPES results. The third chapter describes the development of a novel millikelvin SPM implementing STM and two AFM modalities. The most intriguing emergent behaviors of quantum materials are determined not only by their static band structure, but by the dynamics of their quasiparticle interactions. For example, various quantum critical fluctuations may drive unconventional superconductivity, while spin fluctuations may drive exotic topological phases. Such dynamics are largely invisible to conventional scanning tunneling microscopy (STM), while ultrafast optics typically average over the spatial variations that are common to strongly correlated materials. We have developed a unique scanning probe microscope (SPM) combining STM and pendulum atomic force microscopy (pAFM) operating below 100 mK in magnetic fields up to 14 Tesla. Atomic-scale fluctuation-dissipation dynamics are quantified by local shifts in resonance frequency (reflecting tip-sample force) and quality factor (indicating dissipation) of a scanned cantilever oscillating like a tiny pendulum above the sample. Our pAFM flexibly employs a qPlus sensor with custom cryogenic preamplifier, or an optically detected soft-silicon cantilever for improved force and power resolution. This manuscript is in preparation.