arXiv subjects

Hye-Won Ko

Publications and source records attributed to Hye-Won Ko.

5 recordsLinked to original sources

arXiv2026

Observation of current-induced orbital quadrupole accumulation

Spintronics and orbitronics rely on current-induced accumulations of magnetic dipoles: spin and orbital angular momentum. 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.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic Octupole Hall Effect in d-Wave Altermagnets

Order parameters not only characterize symmetry-broken equilibrium phases but also govern transport phenomena in the nonequilibrium regime. Altermagnets, a class of magnetic systems integrating ferromagnetic and antiferromagnetic features, host multipolar orders in addition to dipolar Neel order. In this work, we demonstrate the multipole Hall effect in d-wave altermagnets--a transverse flow of multipole moments induced by an electric field. Using symmetry analysis and linear response theory, we show that the magnetic octupole Hall effect persists even in symmetries where the spin-splitter effect is forbidden and thus provides a robust experimental signature. In addition, we identify a sizable electric quadrupole Hall effect, originating from quadrupole splittings in the band structure. Our results expand the family of Hall effects to include higher-order multipolar responses and establish altermagnets as a versatile platform for exploring multipole transport beyond spin and orbital degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2025

Orbital Pumping Incorporating Both Orbital Angular Momentum and Position

We develop a theory of adiabatic orbital pumping, highlighting qualitative differences from spin pumping. An oscillating magnetic field pumps not only orbital angular momentum current but also orbital angular position current. The latter, which has no spin counterpart, underscores the incompleteness of existing orbital torque theories. Importantly, both types of orbital currents can be detected as transverse electric voltages, which contain considerable second harmonic components unlike in spin pumping. Moreover, orbital currents can be pumped by lattice dynamics that carry phonon angular momentum, implying that orbital currents can, in turn, induce phonon angular momentum. Our work open up new possibilities for generating orbital currents and provides a broader understanding of the interplay between spin, orbital, and phonon dynamics.

cond-mat.mes-hall↗

arXiv2021

Identifying intrinsic and extrinsic mechanisms of anisotropic magnetoresistance with terahertz probes

Identifying the intrinsic and extrinsic origins of magneto-transport in spin-orbit coupled systems has long been a central theme in condensed matter physics. However, it has been elusive owing to the lack of an appropriate experimental tool. In this work, using terahertz time-domain spectroscopy, we unambiguously disentangle the intrinsic and extrinsic contributions to the anisotropic magnetoresistance (AMR) of a permalloy film. We find that the scattering-independent intrinsic contribution to AMR is sizable and is as large as the scattering-dependent extrinsic contribution to AMR. Moreover, the portion of intrinsic contribution to total AMR increases with increasing temperature due to the reduction of extrinsic contribution. Further investigation reveals that the reduction of extrinsic contribution is caused by the phonon/magnon-induced negative AMR. Our result will stimulate further researches on other spin-orbit-interaction-induced phenomena for which identifying the intrinsic and extrinsic contributions is important.

cond-mat.mtrl-sci↗

arXiv2020

Role of orbital hybridization in anisotropic magnetoresistance

We theoretically and numerically show that longitudinal orbital currents in ferromagnets depend on the magnetization direction, which contribute to the anisotropic magnetoresistance (AMR). This orbital contribution to AMR arises from the momentum-dependent orbital splitting, which is generally present in multi-orbital systems through the orbital anisotropy and the orbital hybridization. We highlight the latter orbital hybridization as an unrecognized origin of AMR and also as a common origin of AMR and orbital Hall effect.

cond-mat.mes-hall↗