What’s New

Shaping and controlling electromagnetic waves have wide-ranging scientific and practical implications. In this talk, I will present a few theoretical proposals to generate novel topological structures in light. In particular, I show that the transmission nodal lines are general topological responses from non-local metasurfaces and can be used to generate spatiotemporal optical vortices. Additionally, I discuss the possible topological structures that can occur in electromagnetic waves. I show that three-dimensional topological structures, known as hopfions and Shankar skyrmions can be created in free-space electromagnetic waves. Such complex shaping of light may be used for optical emulation of new topological physics, or be used in optical trapping and manipulation applications.

Discoveries of new superconductors with high transition temperatures have been a perpetual drive of condensed matter physics. In this talk, I attempt to give an overview on recent advances in this vibrant area, with some of own stories squeezed in. We start from predictive designs of freestanding or supported superconducting monolayers that may exhibit high-Tc superconductivity, as well as low-dimensional systems that display exotic Ising, chiral, or p-wave superconductivity. On the mechanistic side, I will demonstrate how plasmonic excitations or pronounced correlation effects can enhance the superconductivity of iron-based superconductors, and predict designer substrates that may optimize the strain in La3Ni2O7 thin films for maximally enhanced Tc.

Significant advances have been made in fundamental research of topological insulators (TIs), yet their device applications remain elusive. We propose an approach towards seamless integration of two-dimensional (2D) TIs into semiconductor devices. Using first-principles calculations, we show that heteroepitaxially grown III-V semiconductor ultrathin films can self-convert into 2D TIs. Remarkably, on GaSb(111) monolayer GaAs1-xBix becomes universally a 2D TI at any alloy concentration, x, enabled by natural formation of semiconductor heterojunctions. For the GaAs-rich monolayer, having type-II (III) band alignment with GaSb, an intriguing interfacial band offset inversion emerges between surface Ga-s and substrate Sb-p bands; for the GaBi-rich monolayer, with type-I (I’) alignment, the conventional intra-surface band gap inversion arises between Ga-s and Bi-p bands. The lattice-matching epitaxy of GaAs0.25Bi0.75 alloy enables growth of thin-film 2D TIs with a gap up to ~330 meV. Our findings pave the way to engineering wafer-scale large-gap 2D TIs to potentially operate at room temperature.