Quantum Many-Body & Numerics

A collaboration between theoretical physicists Dr. Chengkang ZHOU and Professor Zi Yang MENG from the Department of Physics at The University of Hong Kong (HKU), along with experimentalists Mr. Zhenyuan ZENG and Professor Shiliang LI at the Institute of Physics (IOP), Chinese Academy of Sciences (CAS), and Professor Kenji NAKAJIMA from J-PARC Center, Japan, has led to a discovery in the realm of quantum physics. Their study, published in a recent issue of Nature Physics (Nat. Phys. (2024)), sheds light on the long-anticipated emergence of quasiparticles, akin to the famous Dirac particles obeying the relativistic Dirac equation. These quasiparticles, known as Dirac spinons, were theorised to exist within a novel quantum state called a quantum spin liquid state. This work captures the conic spin excitations arising from Dirac spinons, resulting in low-energy spin excitations with sharp energy-momentum characteristics. Their results provide strong evidence for the emergence of a Dirac quantum spin liquid state in YCu3-Br. Continue reading

How does matter acquire mass? It's a question of fundamental importance to physicists. Traditionally, we've understood that all matter in the universe gains mass through a process called spontaneous symmetry breaking. Yet, recent discoveries in both condensed matter and high-energy physics have unveiled a new mechanism of mass generation without any symmetry breaking, known as "symmetric mass generation" (SMG). In our current work, we set out to investigate whether SMG corresponds to a true continuous phase transition with universal behaviors. Using newly developed numerical probes of quantum entanglement, we delved into the heart of this phenomenon on interacting electrons on honeycomb lattice, which might find its relevance in quantum moire materials. What we discovered is that, the SMG transition is indeed a continuous phase transition, but it defies all traditional paradigms. This finding opens up exciting avenues for understanding the fundamentally new mechanism of the mass generation. This work is published on Phys. Rev. Lett. 132.156503 (2024).

Recent research demystifies the effects of long-range interactions on physical systems. Our quantum Monte Carlo simulations and field theoretical analysis on (2+1)-dimensional O(3) quantum critical points and two-dimensional Néel states with long-range interactions have uncovered the critical exponents of quantum critical points influenced by long-range interactions exhibit three distinct renormalization group fixed points [Phys. Rev. B 109, L081114 (2024) Editor's suggestion]. At finite temperature, our unbiased investigation demonstrated the spontaneous breaking of SU(2) symmetry in Heisenberg model with 1/r^α-form long-range interactions beyond the Mermin-Wagner theorem. Critical exponents have been determined for different ranges of α, revealing new critical behaviors [npj Quantum Mater. 8, 59 (2023)]. These works encourage further theoretical and experimental studies of quantum materials with long-range interactions, expanding our understanding beyond locality.

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A team of international researchers led by HKU (composed of PhD student Xu ZHANG and his advisor Dr Zi Yang MENG) and HKUST has made a significant discovery in the field of quantum materials, uncovering the controllable nonlinear Hall effect in twisted bilayer graphene. The findings, published as an Editors' Suggestion article in the prestigious physics journal Physical Review Letters, discovered that by adjusting the dispersion of the topological flat bands in twisted bilayer graphene, the Berry curvature dipole moments, which play a crucial role in the Hall effect, can be easily controlled and manipulated. Continue reading

August 18, 2023, is a significant day for Mr. Jiarui Zhao and Mr. Chengkang Zhou, as on this day they have successfully defended their Ph.D. thesis with the title “Disorder Operators, Entanglement Entropies, and Dynamic Spectra in Strongly Correlated Quantum Spin Systems：A Quantum Monte Carlo Study” and “Quantum Monte Carlo simulations of dimension tunability phenomena in the quantum spin system”. They are the first-generation HKU PhD graduates from Zi Yang Meng’s group. Congratulations to Dr. Jiarui Zhao and Dr. Chengkang Zhou! Continue reading