ELFT HÍRADÓ

[Karlo Penc <penc.karlo AT wigner.mta.hu>]

Kedves Kollégák!

Szeretném a figyelmükbe ajánlani a BME Fizika Tanszékének szemináriumát.

2018. február 26., hétfő 10
BME F épület I. lh. I. em. Schay terem

prof. Shuichi Murakami
Tokyo Institute of Technology

Topological bands and topological phase transitions in magnonic systems

In electronic systems, various interesting phenomena such as spin Hall effect 
and topological insulators originate from Berry curvature of Bloch 
wavefunctions. We theoretically study analogous phenomena for magnons (spin 
waves). We propose that the dipolar interaction gives rise to nonzero Berry 
curvature [1-3]. In a thin-film ferromagnet in a long-wavelength regime, we 
can calculate the Berry curvature for each magnonic band, and only when the 
magnetic field is out-of-plane, the Berry curvature is nonzero. When the 
exchange coupling is included, the magnonic bands are modified, and there 
appear a number of band anticrossing points. Around such an anticrossing 
point, the Berry curvature is enhanced. This Berry curvature gives rise to 
thermal Hall effect of magnons [1,2], and it also gives rise to a shift of 
wavepackets in reflection or refraction [3]. Furthermore, in analogy to the 
quantum Hall effect for electrons, we can design topological magnon band 
structure. By introducing artificial spatial periodicity into the magnet, for 
example by fabricating nanostructures with two different magnets in a 
periodic structure or by making a periodic array of nanomagnets, we 
theoretically propose emergence of topological edge modes, analogous to those 
in electronic quantum Hall effect. The edge modes are chiral, and propagate 
along the edge of the magnet in one way. We call this a topological magnonic 
crystal [4,5].

References
[1] R. Matsumoto and S. Murakami, Phys. Rev. Lett. 106, 197202 (2011); Phys. 
Rev. B 84, 184406 (2011).
[2] R. Matsumoto, R. Shindou, and S. Murakami, Phys. Rev. B 89, 054420 (2014).
[3] A. Okamoto, S. Murakami, Phys. Rev. B 96, 174437 (2017); S. Murakami and 
A. Okamoto, J. Phys. Soc. Jpn. 86, 011010 (2017).
[4] R. Shindou, R. Matsumoto, S. Murakami, and J. Ohe, Phys. Rev. B 87, 
174427 (2013).
[5] R. Shindou, J. Ohe, R. Matsumoto, S. Murakami, and E. Saitoh, Phys. Rev. 
B 87, 174402 (2013).

Üdvözlettel,
Bordács Sándor