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 [Commun Phys 7, 207 (2024) 和 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

送会稽严生序

不知道为什么，最近几年来，不佞总是遇到这些有独特、决绝、悲愤、戏谑，又充满了才华的会稽人。并从他们的身上，收获了很多的乐趣和眼下龌龊环境中难得的慰藉。严生不拘于时从不佞游，尔来已经四年矣。而正是因为他的才华、决绝、戏谑和努力，工作得到了多方的认可，很快就要回到西湖边的一所学校开创新的量子多体计算研究事业。不佞一面为他高兴，一面也觉得是时候给严生写一篇赠序了。Continue reading

当 H-index 追上年龄的时候，该做些什么？ | 量子多体中的坟

H-index 做为衡量研究人员——也就是当代的读书人——而设计出的一个量化标志，本身是一个客观的指标，没有好恶的分别。但是不佞日前注意到自己的 H-index 也赶上了年龄，却莫名地产生出浓重的焦虑，焦虑之下遂开始琢磨此事背后的历史、文化和心理内涵，不琢磨则已，越琢磨越发现此事大有问题，以至于几乎引发了不佞的中年危机，用了好多天才走出来，遂决定把这番自我解剖的痛苦历程，拿出来与同好同行分享，以慰往者，以飨来者。 Continue reading