HKU Physicists Draw on Pride and Prejudice to Break Down Barriers in Quantum Magnetism

Professor Zi Yang MENG from the HKU Department of Physics and the State Key Laboratory of Optical Quantum Materials, together with Professor Cristian D. BATISTA (University of Tennessee) and Professor Shiliang LI (Institute of Physics, Chinese Academy of Sciences), have recently published an invited Perspective in Nature Physics. Titled “An integrated theoretical and numerical approach to understand modern experiments on quantum magnetism”, the article advocates closer and more equal collaboration among three major pillars of quantum many-body physics: analytical theory, numerical simulation and experimental materials science.

Drawing inspiration from Jane Austen’s Pride and Prejudice, the authors examine how disciplinary divisions may hinder scientific progress. In the quest to decipher extreme states of matter, such as quantum spin liquids and complex phase transitions, the field has traditionally treated its three core pillars as fragmented entities. Over the years, the authors observed a prevailing attitude among theorists and computational physicists—particularly pronounced within the theoretical community—where one’s own sub-discipline is viewed as the primary driver of truth, while the others are diminished as mere auxiliary tools to validate one's own cleverness. Researchers can sometimes regard their own specialism as the main source of scientific insight while treating other approaches primarily as supporting or verification tools. The authors argue that overcoming these attitudes is essential for addressing increasingly complex problems in quantum magnetism.

“This reflects a very human flaw, perfectly captured by Jane Austen,” Professor Meng noted. “Theorists often harbour an elitist ‘pride’, believing that analytical field theories dictate the entire scientific narrative. Meanwhile, numerical and materials experts hold defensive ‘prejudices’, dismissing theoretical constructs as castles in the air. This traditional paradigm, which treats related disciplines as subservient tools, severely hinders progress in our field. These three pillars must operate on equal footing as an inseparable trinity. Only by embracing this integrated approach can we truly advance our understanding of quantum magnetism.”

Bringing Different Approaches Together

The Perspective draws on several studies to demonstrate how this integrated approach works in practice. Quantum magnetic materials are often too complex to be understood through any single method. Analytical theory proposes possible explanations, numerical simulations test them under different conditions, while experiments determine whether the predictions reflect the behaviour of real materials.

To champion this holistic framework, the authors showcase three major core systems where they have actively implemented this integrated approach in recent years, chronicling the field's historical progress and the leap from low to high dimensions:

• The Chasm from 1D to 2D (Luttinger Liquids to Triangular Quantum Ising Magnets): The neutron scattering spectra of one-dimensional magnetic materials can be perfectly reproduced by fusing analytical Luttinger theory with numerical Density Matrix Renormalisation Group (DMRG) simulations, both yielding flawless agreement with experimental spectra. Operating under their proposed collective paradigm, the team utilised sign-problem-free, large-scale Quantum Monte Carlo (QMC) and thermal Tensor Renormalisation Group (TRG) calculations to precisely map the microscopic parameters of the 2D magnet TmMgGaO₄. This successfully predicted and experimentally verified the elusive Berezinskii-Kosterlitz-Thouless (BKT) phase in a challenging 2D quantum Ising model. [See the HKU press release]

• Phase Diagrams of Triangular Lattice Heisenberg Magnets: This section highlights breakthrough quantum spin liquid (QSL) materials. The article tracks the domain's evolution from the 120° magnetically ordered Ba₃CoSb₂O₉ to the genuine J₁-J₂ quantum spin liquid behaviours observed in KYbSe₂ and NaYbSe₂. This milestone was reached by seamlessly combining advanced many-body numerical modelling with the Schwinger boson theoretical framework pioneered by Professor Batista and collaborators, as well as material synthesis and characterisation, exemplifying an egalitarian alliance between theory and materials.

• The Historical Evolution of Kagome Lattice Heisenberg Magnets: Offering an expanded and detailed retrospective, the article reviews the domain's journey from early models like Herbertsmithite (described by a homogeneous antiferromagnetic Heisenberg model) to the recent breakthroughs in the 3J model calculation and experiment represented by YCu_3(OH)_6Br_2[Br_{1-x}(OH)_x]. Building upon HKU's 2024 breakthrough on this material—where the experimental team painstakingly co-aligned over 5,000 single crystals to secure neutron scattering data and observed the computed Dirac quantum spin liquid spectrum. Comparing the resulting data with numerical calculations provided evidence consistent with the predicted behaviour of a Dirac quantum spin liquid.

Looking Ahead

The authors believe this integrated approach will be essential for investigating other challenging systems, including Kitaev materials and novel quantum spin liquids. Combining theoretical models, advanced simulations, machine learning and experiments could help researchers discover and understand new quantum states and materials.

Developed through extensive discussion and editorial refinement over nearly two years, the Perspective retains its central message that analytical theory and numerical simulation should stand as equal partners alongside experiments in addressing the complex challenges of quantum magnetism. Professor Meng hopes that this vision will resonate with researchers and students across the quantum many-body physics community and encourage closer, more integrated collaboration in the field.

This work was supported by the high-performance computing platforms of HKU Information Technology Services, the “Blackbody” supercomputer cluster in the HKU Department of Physics, and computational resources from Beijing PARATERA Tech CO., Ltd., with funding from the Research Grants Council (RGC) of Hong Kong.