2026-10-15 東京大学
絶対零度では閉じているループが、有限温度では磁気モノポールによって切断される
＜関連情報＞
- https://www.t.u-tokyo.ac.jp/press/pr2026-10-15-002
- https://journals.aps.org/prx/abstract/10.1103/bkkq-q1v9
任意の位相におけるトポロジカル相転移とその熱力学的運命古典的なパイロクロアスピンアイス Topological Phase Transitions and Their Thermodynamic Fate in Arbitrary- Classical Pyrochlore Spin Ice
Sena Watanabe, Yukitoshi Motome, and Haruki Watanabe
Physical Review X Published: 14 October, 2026
DOI: https://doi.org/10.1103/bkkq-q1v9
Abstract
We develop a self-contained theoretical framework that classifies the topological phases and critical phenomena of classical pyrochlore magnets with arbitrary spin S, subject to competing exchange and single-ion anisotropies. We work first in the monopole-free (ice-rule) limit. In the small-ω regime, where the single-ion term favors low spin amplitudes, dualities reveal a dichotomy: Integer spins exhibit a continuous 3D XY deconfinement transition, whereas half-integer spins remain in a U(1) Coulomb liquid without any transition. In the large-ω regime, where the local spin amplitudes are maximized (|Sz|=S), the macroscopic flux is quantized to multiples of 2S. By mapping the defect structure to topological loop gases, we prove that the compatibility between the physical ice rule and the modular ℤ2 flux conservation holds at the vertex level if and only if S≤3/2. For =3/2, this maps the system to the three-state Potts model, whose symmetry-allowed cubic invariant drives a first-order transition. For S≥2, no mapping to a discrete clock model exists. Using a decomposition of the partition function, we show that the only processes sensitive to the discrete ℤ2 structure carry exponentially small statistical weight and correspond to irrelevant operators at the 3D XY fixed point, so the deconfinement transition is of the 3D XY type. Finally, incorporating thermal monopoles, we show that they act as a symmetry-breaking effective magnetic field that severs defect strings. Consequently, the continuous transitions are rounded into crossovers, whereas the first-order S=3/2 transition is predicted to survive at finite temperatures, terminating at a critical end point. Classical Monte Carlo simulations for S up to 7/2 corroborate the predicted crossover behavior at the accessible temperatures; these temperatures lie above the estimated critical end point, so the predicted survival of the =3/2 first-order transition remains to be tested directly.
