Observation of Current-Induced Orbital Quadrupole Accumulation - An ECE Distinguished Seminar with Professor Kyung-Jin Lee (Seminar 2 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 and orbitronics rely on current-induced accumulations of magnetic dipoles: spin [1, 2] and orbital angular momentum [3-5]. However, electronic orbitals inherently carry multipoles beyond the dipole, with the rank-2 orbital quadrupole as the leading term. Here we use polarization-resolved Kerr microscopy to observe current-induced orbital-quadrupole accumulation at the surfaces of Ti and Pt, metals with markedly different spin-orbit-coupling strengths. By separating the symmetric and antisymmetric components of the off-diagonal optical conductivity, we isolate the time-reversal-even quadrupolar response from the conventional time-reversal-odd magnetic-dipolar one, and find that the quadrupolar optical response exceeds the dipolar one in both metals. First-principles analysis of the measured responses indicates that the quadrupole accumulations are of the same order of magnitude in the two metals despite their widely different spin-orbit-coupling strengths, consistent with a previously unidentified channel of charge-to-orbital conversion that does not require spin-orbit coupling. Our findings establish that current-induced orbital polarization is fundamentally multipolar, expanding current-induced phenomena from the dipolar to the multipolar regime and opening a route to electrical control of orbital-ordered phases.

[1] Y. K. Kato, R. C. Myers, A. C. Gossard & D. D. Awschalom, Observation of the spin Hall effect in semiconductors. Science 306, 1910-1913 (2004).

[2] C. Stamm et al. Magneto-optical detection of the spin Hall effect in Pt and W thin films. Phys. Rev.

Lett. 119, 087203 (2017).

[3] Y.-G. Choi et al. Observation of the orbital Hall effect in a light metal Ti. Nature 619, 52-56 (2023).

[4] I. Lyalin, S. Alikhah, M. Berritta, P. M. Oppeneer & R. K. Kawakami, Magneto-optical detection of the orbital Hall effect in chromium. Phys. Rev. Lett. 131, 156702 (2023).

[5] Y. Marui et al. Spin and orbital Hall currents detected via current-induced magneto-optical Kerr effect in V and Pt. Phys. Rev. B 108, 144436 (2023).

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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