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1. 연사: 김세권 박사 (University of California at Los Angeles)
2. 일시: 2017. 8. 31(목), 오후 1시
3. 장소: 공학3동 201호 (세미나실)
4. 제목: Spin Superfluidity & Spin Quantum Hall Effects

In this talk, we discuss two emerging concepts for efficient spin transport, spin superfluidity and spin quantum Hall effects, which are spin analogs of mass superfluidity and charge quantum Hall effects, respectively. We first provide an introduction to superfluid spin transport in easy-plane magnets [1]. Then we theoretically investigate effects of quantum fluctuations on superfluid spin transport through easy-plane quantum antiferromagnetic spin chains in the large-spin limit [2]. Quantum fluctuations result in the decaying spin supercurrent by unwinding the magnetic order parameter within the easy plane, which is referred to as phase slips. We show that the topological term in the nonlinear sigma model for the spin chains qualitatively differentiates decaying rate of the spin supercurrent between integer and half-odd-integer spin chains.
In the second half, we discuss the coupled gyration modes of magnetic vortex disks arranged as a honeycomb lattice [3]. The lattice under suitable conditions is shown to support a chiral edge mode, the existence of which can be understood by mapping the system to the Haldane model for the zero-field quantum Hall effect [4]. The direction of the chiral edge mode is associated with the skyrmion number of the constituent vortices, which can be manipulated by an external field or by an electric current pulse. We conclude the talk with a few future research topics stemming from the discussed topics.
[1] E. B. Sonin, Adv. Pays. 59, 181 (2010)
[2] S. K. Kim and Y. Tserkovnyak, Phys. Rev. Lett. 116, 127201 (2016)
[3] S. K. Kim and Y. Tserkovnyak, Phys. Rev. Lett. 119, 077204 (2017)
[4] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988)