Spectral evidence for Dirac spinons in a kagome lattice antiferromagnet

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

New paradigm of quantum phase transitions discovered from entanglement measurements

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).

旧锦新样 | 寂静春天里的动力学

在这样混乱嘈杂的春天里,哪里能找到寂静,哪里能找到新的生机酝酿时静默却不可阻挡的力量,是考验从业人员实力、修为和气魄的大问题。春天当然是动态的,但是动态要有一个静的表面,新发现孕育的时候如林间的野花幼苗,阳光雨露之下静却决绝。一开始就熙熙攘攘热热闹闹,混淆了心智,耗尽了力气,怎么可能看到成熟和收获呢? 就是在这样的考虑之下,笔者和朋友们决定在凝聚态物理学量子多体计算的领地中,在这个春天里培育一株幼苗,小心地浇灌它,期望它成长为凝聚态物理学量子多体计算领地中的大树。这株幼苗,就是量子多体系统的动力学性质计算。 Continue reading

旧锦新样 | 百年孤独和一级相变

编者按:最近某次关于分数陈绝缘体相变问题的讨论,涉及到多体系统作用量三阶项会导致一级相变的故事,勾起笔者对一段往事的追怀,想起八年前这篇游戏文字。文字背后的物理现象是在阻挫三角晶格横场 Ising model 量子临界点上,作用量三阶项消失,QCP 展现 emergent continuous symmetry;而在阻挫六角晶格横场 Ising mode 的量子临界点上,作用量三阶项不消失,QCP 为一级相变。这个故事背后的研究论文是 [Phys. Rev. B 96, 115160 (2017)]。类似的故事,笔者和合作者最近研究三角晶格量子环路模型相图的工作中,亦有所体现 [arXiv:2205.04472情愿不自由,便是自由了 | 量子多体中的呐喊与彷徨之十二]。不想如今这样的现象又在分数陈绝缘体相变问题中改头换面地出现。可见马尔克斯百年孤独的经典开头诚不我欺,“多年之后面对 ……” 这样的事情,在个人的生命历程中总是反复地出现,一如身处的环境总在重复其文化历史基因中的错误一样。值得记取的教训,除了马贡多小镇无法摆脱的悲剧宿命和多体系统作用量三阶项会导致一级相变,恐怕还是“学好蒙特卡洛,不会被忽悠”。 Continue reading

Unearth the myth in long-range interacting systems

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.

Unlock Tunable Nonlinear Hall Effect in Twisted Bilayer Graphene

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

Successful PhD defenses for the first-generation HKU PhD graduates

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


本文是为中国物理学会 Chinese Physics B 30周年刊庆邀请撰写的综述文章:A sport and a pastime: Model design and computation in quantum many-body systems 所做的创作谈。从文章发表到今天,不过两个月时间,但这两个月内世界变化之快如同梦境。穿行在分裂的世界中,我们几近不能分辨现实是否只是梦境的替身,抑或是这世上太多人都因为失去了记忆而变得只是随着风向空转。趁着我们还没有失去记忆,在这里记录下综述文章中量子多体问题模型设计与计算求解的物理学内容,和在完成这篇30多页的科学论文时,我们的内心寄托。希望能够在这个没有信用的世界中,留下一些信用的记录。 Continue reading

Advanced computing system ‘Blackbody’ for solving the challenging physics problems

To pursue advances in astronomy, quantum physics, and interdisciplinary science, our research teams worked together to deploy a new supercomputing system, named ‘Blackbody’ supercomputer -- the name stems from their respective research topics of ‘Black hole’ and ‘Quantum many-body‘ physics. The system has been set up in fall 2022 and is currently in full operation. Thank you for the hard work and Congratulations for all the participants! Continue reading