Longitudinal Quantum Fluctuations in Spin Transfer - An ECE Distinguished Seminar with Professor Kyung-Jin Lee (Seminar 1 of 2).

Please join us in the Electrical and Computer Engineering Department at the Technological Institute for an hour-long seminar with Professor Kyung-Jin Lee of the Korea Advanced Institute of Science and Technology.

Abstract:

Spintronics has transformed our ability to generate, transfer, and manipulate angular momentum in solids, enabling powerful strategies for current-driven control of magnetic states and devices. Despite its remarkable success, the field has traditionally been built upon three simplifying assumptions: a classical description of localized spins (magnetic moments), exchange interactions that act purely between spin degrees of freedom, and the predominance of spin dipole moments as the primary carriers of angular momentum. This talk explores what new physics emerges when these assumptions are relaxed and when additional internal degrees of freedom are taken seriously.

In particular, I will discuss the role of longitudinal quantum fluctuations in spin transfer processes [1], revealing regimes where angular momentum transport cannot be captured by conventional transverse spin dynamics. I will then introduce current-driven control of magnetism mediated by orbital exchange interactions [2], highlighting how orbital degrees of freedom provide a qualitatively new channel for coupling electric currents to magnetic order. If time permits, I will also present multipolar pathways for angular-momentum transport that go beyond conventional dipolar mechanisms [3], opening a broader framework for spintronics and orbitronics in which higher-rank moments play an active and controllable role.

[1] T. Lee et al., "Signatures of longitudinal spin pumping in a magnetic phase transition," Nature 638, 106-111 (2025).

[2] G.-H. Lee, K.-W. Kim, and K.-J. Lee, "Orbital exchange-mediated current control of magnetism," Nat. Commun. 17, 2236 (2026).

[3] H.-W. Ko and K.-J. Lee, "Magnetic octupole Hall effect in d-wave altermagnets," arXiv:2508.00794

Bio:

Kyung-Jin Lee received his B.S., M.S., and Ph.D. degrees from Korea Advanced Institute of Science and Technology (KAIST). He was a postdoctoral fellow at SPINTEC, France, from 2003 to 2005. In October 2020, he joined Department of Physics at KAIST, where he is currently a KAIST Endowed Chair Professor, following five years at Samsung Advance Institute of Technology and fifteen years at Korea University. He is an APS Fellow and the recipient of various awards including the Science and Technology Merit Metal from the Korean government and the Samil Prize (Natural Sciences) from the Samil Foundation. His current research interest includes orbital dynamics and quantum spin transfer.

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