What’s New

HKIQST member, Prof. Yi Yang 楊易, has been awarded the prestigious Croucher Innovation Award (2024) for his groundbreaking research on nonrelativistic free-electron-light interaction with nanophotonics. Prof. Yang and his research team aim to realize strong free-electron-light interaction by interfacing slow nonrelativistic electrons with judiciously designed photonic environments, which has the potential to revolutionize electron microscopy and spectroscopy, and generate entangled electrons and photons for quantum light generation and manipulation. The synergy between free electron optics and nanophotonics has already led to new discoveries and applications, such as the stimulated inverse Cherenkov effect, entanglement between free electrons and photons, tunable integrated radiation sources and particle accelerators, and even biomedical imaging. Prof. Yang’s innovative approach to investigating the interplay between nonrelativistic electrons, Van der Waals materials, and nanostructures has the potential to explore quantum recoil, spin polarization, and multi-photon emission processes.

Linearly dispersive gapless quasiparticles can appear at general momentum points of superconductors due to topological reasons like K theory or symmetry indicators theory. However, the zero modes associated with these quasiparticles are generally ‘accidental’ from symmetry point of view. In this talk, we apply projective representation (rep) theory to analyze the bulk gapless quasiparticles in BdG systems. Different from the description of semimetals, we need to specially treat the particle-hole ‘symmetry’ since it is anti-comm-uting with the BdG Hamiltonian. Hence the notion of ‘simple irreducible reps (irreps)’ and ‘composite irreps’ are introduced to label the energy modes. We show that without charge conjugation symmetry (unitary symmetry that commutes with the Hamiltonian), no robust bulk zero modes exist at any fixed momentum point in the bulk. However, robust zero modes at certain high symmetry momentum points can be protected by (effective) charge conjugation symmetries, resulting in gapless quasiparticles with linear, quadratic, or higher-order dispersions determined by the effective k · p theory. The low energy physical properties of the system are determined by the rep carried by the zero modes. This theory provides a framework to classify nodal Superconductors/Superfluids/Quantum Spin Liquids with specified (projective) symmetry group and sheds light on the realization of Majorana-type massless quasiparticles in condensed matter physics.

The United Nations has declared 2025 as the International Year of Quantum Science and Technology (IYQ), recognizing the importance and potential of quantum technologies. The year-long initiative will celebrate the impacts of quantum science.