HKIQST Colloquium: “Quantum Simulation based on Atoms and Ions” by Prof. PU Han on Friday, November 29, 2024, 14:30 LG1 – CYCP1, Chong Yuet Ming Physics Building, HKU

Quantum simulation is one of the pillars of Quantum Revolution 2.0. Its essence is to simulate a complicated and hard-to-control quantum system using a simple and controllable one. Ultracold atoms and ions, due to their unprecedented controllability, have become an important platform for quantum simulation. In this talk, I will introduce two recent works in collaboration with my experimental colleagues at Rice. The first work concerns the simulation of electron transfer — an important issue relevant to many biochemical processes and material science — using trapped ions; and the second concerns the simulation of spin-charge separation — a unique phenomenon in one dimensional quantum many-body system — using two-component ultracold Fermi gas. These two works clearly demonstrated the advantages of performing quantum simulation using cold atoms and ions. I will also discuss some future directions based on these works.

CTCP Seminar : “Anomaly pattern of point groups and high-order topological states” by Prof Yang QI Wednesday, October 30, 2024, 4:00pm KKLG101, LG1/F, K.K. Leung Building, HKU

An essential feature of topological crystalline states (TCSs), which are short-range-entanged topological states protected by crystalline symmetries, is they generally have high-order gapless boundary states, such as one-dimensional hinge states and zero-dimensional corner states on a two-dimensional surface. Therefore, such TCSs are also called high-order topological states. In this work, we design a systematic method to compute possible high-order boundary states of a TCS for all possible surface geometries. We show that the location of surface gapless region, dubbed the anomaly pattern, can be symmetrically and continuously deformed without changing the topologically-protected gapless states, and such deformation defines a homotopy equivalence between anomaly patterns. The list of equivalent classes of anomaly patterns are completely determined by the point-group symmetry, and it is universal for all types of TCSs, including bosonic, free-fermion and interacting-fermion states. We also describe how to compute the anomaly pattern of a bulk topological state, for all types of TCSs.

Joint Colloquium “Neutron Scattering Studies of Protein Dynamics and its Associated Quantum Effects” by Prof Xiang-qiang CHU on Wednesday, 13 Nov, 2024, 5-6pm MWT4, HKU

The roles of quantum effects in biological systems have long fascinated biophysicists. Meanwhile, proteins
undergo sophisticated motions in space and time, which are believed to ultimately govern the biological function
and activities of the proteins. Quasi-elastic neutron scattering (QENS) provides exceptional tools for studying the
dynamics of proteins in the time range of picosecond to nanosecond at the molecular level. In this talk, based
on our recent work on various biological systems studied by QENS and other techniques, such as inelastic
neutron scattering (INS), small angle neutron scattering (SANS), and neutron spin echo (NSE), I will discuss the
possibility of using neutron scattering techniques to reveal the quantum mechanical effects, such as tunneling
effect in the dynamics of proteins and connect them with protein activities or functions.

HKU Centennial Distinguished Chinese Scholars Public Lecture “Century of Superconductors” by Professor Donglai FENG on Wednesday, December 11, 2024 3:30-4:30 pm RHT, HKU

100+ years after its discovery in 1911, superconductivity remains one of the most fascinating and challenging topics in physics. In this talk, the history of superconductor, its physics and applications as well as the recent progress made by Chinese will be reviewed. Several inspiring anecdotes of scientists working in this field will also be highlighted.

Joint Seminar “Neutron Spectroscopy Studies on Kitaev Quantum Magnets” by Prof Jinsheng Wen on Monday, Dec 9, 2024 11am MW103, HKU

The Kitaev model is an exactly solvable quantum-spin-liquid model defined on a honeycomb lattice with S = 1/2. The key element underlying this model is the bond-anisotropic Kitaev interaction. However, in a spin-only system, it is unrealistic to have such anisotropic interactions. In this talk, I will show that the Kitaev interaction can be realized in a Mott insulator α-RuCl3, which has an effective spin of 1/2 by entangling the spin and orbital degrees of freedom. I will also show that by applying an in-plane magnetic field, the zigzag magnetic order ground state in α-RuCl3 can be completely suppressed, and a quantum-spin-liquid state can be induced. More recently, we extend the Kitaev physics to higher-spin system, where we find in a honeycomb-lattice antiferromagnet Na3Ni2BiO6 that there is a profound 1/3 magnetization plateau, which is stabilized by the Kitaev interaction, as evidenced from the neutron spectroscopy. This will also be discussed in the presentation.

HKIQST Colloquium: “More is different: the beauty of multiband in iron-based superconductors” by Prof. DING Hong on Friday, December 13, 2024, 10:30 am LG1 – CYCP3, Chong Yuet Ming Physics Building, HKU

During this colloquium, the unnoticed beauty of multiband of iron-based superconductors will be highlighted, including i) emerge of topological band which coexists with superconductivity, leading to the discovery of Majorana zero mode; ii) Interplay of multicomponent superconductivity, leading to the discovery of exotic paring with time-reversal symmetry breaking and fractional vortex; iii) Contribution of Hund coupling towards pairing in this unique class of superconductors.

CTCP Seminar : “Noninvertible Gauge Symmetry in (2+1)d Topological Orders: A String-Net Model Realization” by Prof. Yidun WAN on Friday, Sept 20, 2024, 4:00 pm CYMP 522, HKU

We develop a systematic framework for understanding symmetries in topological phases in 2+1 dimensions using the string-net model, encompassing both gauge symmetries that preserve anyon species and global symmetries permuting anyon species, including both invertible symmetries describable by groups and noninvertible symmetries described by categories. As an archetypal example, we reveal the first noninvertible categorical gauge symmetry of topological orders in 2+1 dimensions: the Fibonacci gauge symmetry of the doubled Fibonacci topological order, described by the Fibonacci fusion 2-category. Our approach involves two steps: first, establishing duality between different string-net models with Morita equivalent input fusion categories that describe the same topological order; and second, constructing symmetry transformations within the same string-net model when the dual models have isomorphic input fusion categories, achieved by composing duality maps with isomorphisms of degrees of freedom between the dual models. If time permits, I will also talk about a subsequent work on anyon condensation.

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

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.

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