The low-energy properties of quantum materials are deeply connected to fundamental concepts in high-energy physics, including Weyl fermions, Majorana fermions, the chiral anomaly, and supersymmetry. With ongoing advances in synthesizing high-quality epitaxial atomic layers and heterostructures, vast opportunities have emerged for exploring new physics in topological quantum materials. In this talk, I will discuss two novel quantum states recently observed in epitaxial atomic layers. (1) Flat-band quantum states have attracted tremendous research interest, as they provide an ideal platform for exploring the rich physics of strongly correlated electrons. Recently, a series of flat bands with evenly spaced energies has been realized in hybrid Ag/Au quantum well systems. The flat-band ladder not only hosts electrons with a large effective mass, as in conventional flat-band systems, but also allows electrons to transition between energy levels with different topological characteristics. (2) A two-dimensional Weyl/half-Dirac semimetal has been observed in epitaxial Bi monolayer [1]. The spin-polarized massless bulk bands and the topological Fermi string edge states offer new insights into the topological properties of relativistic electrons in reduced dimensions [2]. Coupling a 2D Weyl semimetal to superconductors can give rise to a higher-order topological superconducting phase, accompanied by the emergence of Majorana corner states. [3]. I will discuss a braiding protocol of the Majorana states in the high-order Weyl superconductors.
[1] Q. Lu et al., Nature Communications 15, 6001 (2024)
[2] D. J. P. de Sousa et al., Physical Review Letters 133, 146605 (2024)
[3] C.-K. Chiu et al., arXiv:2412.06150