Novel excitations in van der Waals heterostructures
Emergent phases in quantum materials give rise to novel excitations, which provide crucial insights into the nature of these phases and may inspire new design concepts for quantum devices with unprecedented functionalities, or even more exotic emergent phases. Some of these excitations, or quasiparticles, are predicted to follow laws different from those governing electrons and may hold potential for quantum computation. How can these unusual quasiparticles be experimentally detected? Could entirely new phases of matter be constructed using these excitations as fundamental building blocks? And what kinds of excitations might emerge from these unknown phases of matter? Recent advances in isolating two-dimensional materials and stacking them with atomic precision have enabled highly controlled, tunable platforms to to address these questions.
Reference: Y. Xie et al, Nature, 502, 101-105, (2019); A. Pierce, Y. Xie et al, Nature Physics, 18, 37-41 (2022); Y. Xie et al, Nature, 600, 439-443, (2021).
Correlation and topology in incommensurate heterostructures
In conventional crystals, electrons follow a well-defined energy-momentum relationship, governedby the Bloch theorem. This paradigm, however, breaks down in incommensurate structures, where two or more periodic patterns coexist. How should we understand electron correlations and topology in these systems? Could weak incommensurate structures serve as perturbations to explore quantum phases in the parent structure and test their robustness against disorder? Can incommensurate patterns be harnessed as control parameters to engineer novel quantum phases? A promising platform for these investigations is multilayer (three or more layers) moiré heterostructures of 2D materials, where multiple length scales can be intentionally designed and created. These systems also host a range of correlated and topological phases, providing a rich landscape to study the effects of incommensurability with unprecedented tunability.
Reference: Y. Xie et al, arxiv: 2404.01372 (2024)
Scanning single-electron-transistor microscopy of quantum materials
A single-electron transistor (SET) is an electronic device capable of detecting electrostatic potential signals with high sensitivity. Integrating an SET into a scanning microscope enables high-resolution measurements of the local electronic chemical potential of a material on the sub-micron scale. These capabilities have led to the discovery of numerous topological and correlated phases in quantum materials, such as fractional Chern insulators, which are lattice analogues of the fractional quantum Hall effect. Currently, our group is most excited about applying this technique to moiré systems, where fractional quantum anomalous Hall insulators and other competing phases have recently been observed. We aim to gain insights into the following questions: 1) What are the optimal conditions for the emergence of these observed phases? 2) Can these systems support phases that are fundamentally distinct from the fractional quantum Hall effect? 3) How can we understand the competition between various phases? 4) What is the relationship between their transport and thermodynamic properties?
Reference: A. Pierce, Y. Xie et al, Nature Physics, 17, 1210-1215, (2021); Y. Xie et al, Nature, 600, 439-443, (2021). A. Pierce, Y. Xie et al, arxiv: 2401.12284 (2023).
Next-generation scanning probe modalities
The growing arrays of exotic and unexpected phenomena in quantum materials often challenge the conventional probing methods, necessitating the development of more sophisticated measurement tools. In the past, we we have developed spin-polarized scanning tunneling spectroscopy technique and established spin-polarization as a key trait of Majorana zero modes. We have also invented an approach for extracting the thermodynamic properties of neutral excitations that leverages their influence on the bulk charge gap. We are interested in addressing the following questions: 1) Can we achieve measurement speeds fast enough to probe the system’s dynamics? 2) Can we approach the fundamental limit of measurement sensitivity? 3) What is the maximum number of physical quantities that can be measured at the same time? 4) What other sensors can be integrated into a scanning microscope?”
Reference: S. Jeon , Y. Xie et al, Science, 358, 772-776 (2017). A. Pierce, Y. Xie et al, Nature Physics, 18, 37-41 (2022).