Quantum

The world smallest discrete unit of a phenomenon

“Representation Theory for Massless Quasiparticles in BdG Systems” by Prof Zheng-Xin LIU on Tuesday, July 2, 2024, 4:00 pm | CYMP 522

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.

“A morphism between two QFTs” by Prof Liang KONG on Friday, May 31, 2024, 3:00 pm | MW103

Over the past few years, synthetic dimensions in photonics have been rapidly developing and generating significant interest. By utilizing different degrees of freedom of light, synthetic dimensions can be constructed. This seminar will focus on the creation of a synthetic frequency dimension in dynamically modulated ring resonator systems.

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