Topological phase transitions of Dirac magnons in honeycomb ferromagnets
蜂巢鐵磁中狄拉克磁振子的拓樸相變

Yu-Shan Lu, Jian-Lin Li, and Chien-Te Wu*
Physical Review Letters, 127, Vol. 21, 217202 (2021)
DOI: https://doi.org/10.1103/PhysRevLett.127.217202

Aside from fermionic systems, the topological properties of bosonic systems have attracted extensive attention in recent years, especially the study of the thermal Hall effect in ferromagnetic materials with Dzyaloshinskii-Moriya interaction (DMI). Topological phase transitions may arise when there exist two or more distinct topological phases, and they are often revealed by a gap-closing phenomenon. In this work, we consider the magnons in honeycomb ferromagnets described by a Heisenberg Hamiltonian containing a DMI and Zeeman interaction. We demonstrate that this bosonic system has an unusual type of topological phase transition: the system exhibits a temperature-dependent topological phase transition due to boson-boson interaction. Specifically, when the temperature rises, the magnonic energy gap at Dirac points closes and reopens at a critical temperature Tc identified as topological phase transition temperature. Furthermore, our theory indicates that the thermal Hall conductivity in the magnonic system exhibits a sign reversal at Tc, which can serve as an experimental probe of its topological nature.

近年來，除了費米子系統外，玻色子系統的拓撲性質也引起了廣泛的關注，特別是具有Dzyaloshinskii-Moriya相互作用(DMI)的鐵磁材料中的熱霍爾效應的研究。當存在兩個或多個不同的拓撲相時，系統可能會展現拓撲相變，這些相變通常與能隙閉合現象有關。在此研究中，我們使用包含了DMI和賽曼相互作用的海森堡哈密頓量來描述蜂巢鐵磁體中的磁振子。我們證明了此一玻色子系統具有特異的拓樸相變：玻色子的交互作用導致此系統可藉由控制溫度來達到拓撲相變。更具體地說，當溫度升高時，狄拉克點的能隙逐漸關閉並在與拓撲相變臨界溫度 Tc 處重新打開。此外，我們的理論表明，此磁振子系統中的熱霍爾電導率在 Tc 處表現出符號反轉，這個重要拓樸性質可以簡單地在實驗中驗證。