Quantum Many-Body & Numerics

Our team was one of the winners of the 2024 Top China Cited Paper Award (Physics) for "Momentum Space Quantum Monte Carlo on Twisted Bilayer Graphene", published in Chinese Physics Letters. This award for China-based authors is given to the top ten papers in the top 1% of the most cited articles published in IOP Publishing journals over the past three years (2021 – 2023). This data is from the citations recorded in Web of Science. This article (Chin. Phys. Lett. 38, 077305 (2021)) has developed a momentum space quantum Monte Carlo algorithm to deal with the long-range Coulomb interaction problem that emerges in two-dimensional quantum moire materials such as twisted graphene, which is one of the universal challenges in correlated electron systems. Three years ago, Zi Yang quoted the famous poetry of ancient Chinese poet Jiang Kui（姜夔) to explain how the “Coulomb force is endless, momentum Monte Carlo sends longing thoughts”（库伦作用无尽期，动量蒙卡寄相思) inspire them to pursue the research of long range interaction in quantum moire systems. The quantum many-body paradigm of the moire system is becoming a beautiful stream that carries them to the heartland of scientific creativity.

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 Dr. Zeliang Sun, Professors Kai Sun and Liuyan Zhao from the Department of Physics at the University of Michigan, and Professor Rui He from the Department of Electrical and Computer Engineering at Texas Tech University, has led to a significant discovery in quantum physics. Their study was published in a recent issue of Nature Physics (Nat. Phys. (2024)). The research team was the first to experimentally observe a transition from a 3-dimensional (3D) long-range order state to a 2-dimensional (2D) flat pattern vestigial order state in NiPS3. The vestigial order can be seen as the retention during the process of symmetry breaking. This occurs when the primary magnetic long-range order state breaks down into a simpler form - in this case, a 2D vestigial order state (known as Z3 Potts-nematicity) - as the NiPS3 material is thinned. The collaboration between the large-scale Monte Carlo simulation and the experiments led to this discovery during the dimension crossover process. Continue reading.

Congratulations from Zi Yang:

August 6, 2024, is a significant day for Mr. Xu Zhang, as on this day he has successfully defended his Ph.D. thesis with the title “Electromagnetic response in two dimensional flat band systems”. He has greatly impressed the Examining Committee comprised of Profs. Xi Dai and Ning Wang from HKUST and Profs. Wang Yao and Dong-Keun Ki from HKU, in clearly presenting the major research outputs during his Phd years in HKU, as well as addressing the interesting questions spontaneously appeared during the defense. Congratulations to Dr. Xu Zhang! And hope him a successful next step and becomes the next rising star in theoretical condensed matter physics. Continue reading

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. 20, 1097–1102 (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.