[Seminar] Topological Magnons from “nematicity”


2021年7月1日 (木) 10:00 11:00





Assistant Professor Yutaka Akagi - The University of Tokyo

Apr. 2005 – Mar. 2009: Bachelor Degree in Physics, Department of Physics, Tokyo Institute of Technology
Apr. 2009 – Mar. 2014: Ph.D. in Engineering, Department of Applied Physics, The University of Tokyo (Motome group)
Apr. 2014 – Oct. 2015: Postdoctoral Scholar, Theory of Quantum Matter Unit, OIST
Nov. 2015 – present: Assistant Professor, Katsura group, Department of Physics, The University of Tokyo


Topological Magnons from “nematicity”


The classification and characterization of different phases of matter based on the topology of band structures have recently attracted considerable attention [1]. The successful studies on the topological phenomena of electrons have been extended to bosonic systems [2]. However, in contrast to fermionic systems, bosonic Bogoliubov–de Gennes (BdG)-type systems, which contain pairing terms, possess non-Hermicity intrinsically resulting from Bose statistics. Therefore, the topological classification of Hermitian systems is not directly applicable to bosonic BdG systems [3]. In addition, the conventional time-reversal symmetry does not ensure the Kramers pairs which play an essential role to realize the Z2 topological insulators.
We here introduce the pseudo-time-reversal operator which ensures the existence of “Kramers pairs” of bosons by considering magnons from ordered “pair” spins that are aligned in opposite directions. Based on the magnons from such “nematicity,” we construct magnonic analogs of Z2 topological insulators in two and three dimensions (Kane-Mele and Fu-Kane-Mele models) [4,5]. The topological invariants are defined by using the bosonic Berry connection and curvature which are different from those of electrons. 
Developing the idea of magnons from “nematicity,” we also construct magnonic analogs of topological crystalline insulators [6]. The magnon surface states are topologically protected by the combined symmetry of time-reversal and half translation, which is naturally satisfied in antiferromagnets with a stacked structure. In addition, we show that the energy current flows through the system in response to an electric field. The counterpart of this phenomenon is absent in electronic systems. It implies that novel phenomena beyond mere analogy with electronic systems are expected to be discovered in such antiferromagnets. We also propose a realization of the model in a van der Waals material CrI3 [7] with a monoclinic structure. 
[1] A. P. Schnyder, S. Ryu, A. Furusaki, and A. W. W. Ludwig, Phys. Rev. B 78, 195125 (2008).
[2] H. Katsura, N. Nagaosa, and P. A. Lee, Phys. Rev. Lett. 104, 066403 (2010).
[3] H. Kondo, Y. Akagi, and H. Katsura, Prog. Theor. Exp. Phys. 2020, 12A104 (2020).
[4] H. Kondo, Y. Akagi, and H. Katsura, Phys. Rev. B 99, 041110(R) (2019).
[5] H. Kondo, Y. Akagi, and H. Katsura, Phys. Rev. B 100, 144401 (2019). 
[6] H. Kondo and Y. Akagi, Preprint arXiv:2012.02034.
[7] M. A. McGuire, H. Dixit, V. R. Cooper, and B. C. Sales, Chemistry of Materials 27, 612-620 (2015).

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