Right panel: Defect strings on the pyrochlore lattice form a closed loop in the absence of monopoles; two thermally excited monopoles, shown in red and blue, open the loop.
A team led by Prof. Haruki WATANABE, in collaboration with Prof. Yukitoshi MOTOME and graduate student Sena WATANABE from the University of Tokyo, has developed a unified framework for classifying phases in classical pyrochlore spin ice with arbitrary spin values. Published in Physical Review X, the study reveals distinct behavior for integer and half-integer spins and explains how string-like defects and magnetic monopoles determine whether phase boundaries survive at finite temperatures. In these models, most transitions become smooth crossovers, while a first-order transition for spin 3/2 is predicted to persist and end at a critical point without an external magnetic field. Together with the team’s related work on high-pressure water ice, the findings clarify when apparently different states belong to the same thermodynamic phase.
Publication:
Sena Watanabe, Yukitoshi Motome, and Haruki Watanabe
Topological phase transitions and their thermodynamic fate in arbitrary-S classical pyrochlore spin ice
Physical Review X (2026)
https://doi.org/10.1103/bkkq-q1v9
Related publication:
Continuous crossover between high-pressure ice phases VII and X driven by monopole screening: A model study
Physical Review B (2026)
https://doi.org/10.1103/6m5r-8k5m
Press releases and media coverage:
HKUST — English
https://hkust.edu.hk/news/hkust-researchers-redraw-boundary-between-phases-matter
HKUST — Traditional Chinese
https://hkust.edu.hk/zh-hant/news/hkust-researchers-redraw-boundary-between-phases-matter
EurekAlert!
https://www.eurekalert.org/news-releases/1147712
Phys.org
https://phys.org/news/2026-10-framework-links-magnetic-phase-boundaries.html