arXiv subjects

Insu Baek

Publications and source records attributed to Insu Baek.

6 recordsLinked to original sources

arXiv2026

Modern Approach to Orbital Hall Effect Based on Wannier Picture of Solids

The orbital Hall effect (OHE) and its key figure of merit, the orbital Hall conductivity (OHC), are central to the emerging field of orbitronics. The OHC is conventionally computed via Kubo linear response theory, where the orbital angular momentum (OAM) operator is approximated through atom-centered projections that retain only local contributions, discarding physically important itinerant components. Here, we present a rigorous framework for the OAM operator grounded in the modern theory of orbital magnetization, formulated in a Wannier function basis. Critically, this naturally introduces a hierarchy of non-local contributions to the OAM operator. In turn, this gives rise to an additional non-Kubo contribution to the OHC of purely geometric origin, absent in all atom-centered treatments. First-principles calculations across a diverse set of materials demonstrate that this term can constitute a significant fraction of the total OHC, with its omission leading to qualitatively incorrect predictions. Our framework provides a unified and numerically tractable approach to orbital transport and improves our understanding of the OAM in solids, allowing for a more precise estimation of various orbital effects in complex materials.

cond-mat.mes-hall↗

arXiv2026

Anatomy of the modern theory of orbital magnetism from first-principles: term-by-term analysis in the gauge-covariant formalism

We present an in-depth analysis of the orbital magnetism by means of the so-called modern theory based on the Berry phase across distinct classes of materials-d transition metals, sp metals, and transition metal dichalcogenides-highlighting the microscopic nature of band structure characteristics. We adopt a gauge-covariant formulation of the modern theory proposed in [Lopez et al. Phys. Rev. B 85, 014435 (2012)], which enables the calculation of orbital magnetism in a controlled manner in any chosen gauge of Wannier functions and gives the total contribution as a gauge-invariant measurable. This captures consistently the contributions due to the anomalous position, velocity, and orbital angular momentum of Wannier basis, as well as the contributions due to Hamiltonian such that their sum is gauge-invariant. For d transition metals, we find that the atom-centered approximation captures the majority of the total contribution given by modern theory, which we attribute to localized nature of d electrons. However, 5d metals tend to exhibit larger deviation between the two methods than 3d metals do, as 5d electrons are more delocalized than 3d electrons. On the other hand, sp metals exhibit a strong deviation between the two methods, where large kinetic energy of sp electrons is important. Finally, in 1H-MoS2, we find that the valley orbital moment far exceeds the atomic limit of d electrons due to coherent hybridization between valence and conduction bands in direct band gaps. Our work elucidates the interplay of the chemical nature of electronic orbitals and the effect of band structures in a consistent manner and highlights the role of Berry phase in orbital magnetism. The results suggest a promising direction of orbitronics beyond controlling atomic orbitals, in which the orbital magnetism can be greatly enhanced by exploiting Berry phase.

cond-mat.mes-hall↗

arXiv2025

Deterministic N\'eel vector switching of altermagnets via magnetic octupole torque

Altermagnets have recently emerged as promising materials for next-generation spintronic devices. For their device applications, realizing a single-domain configuration is essential but remains challenging. We theoretically consider injecting magnetic multipoles into altermagnets, which can be achieved by applying an in-plane current to an altermagnet/normal metal bilayer. We demonstrate for $d$-wave altermagnets that the torque generated by the magnetic octupole injection can achieve magnetic-field-free deterministic switching of the altermagnets' N\'eel vector and transform their multidomain configurations into a single domain. This method allows the switching in diverse altermagnets, thereby facilitating their device applications and fundamental studies. This work also exemplifies the usefulness of magnetic multipole currents.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic octupole Hall effect in heavy transition metals

d-wave altermagnets have the magnetic octupole as their primary order parameter. A recent study [Han et al. arXiv 2409.14423 (2024)] demonstrated that magnetic octupole current can induce N\'eel vector dynamics. Therefore, identifying materials that can efficiently generate a magnetic octupole current is essential. In this paper, we investigate the magnetic octupole Hall effect in 4d and 5d transition metals. By employing atomic magnetic octupole operators, we calculate the magnetic octupole Hall conductivity using first-principles calculations. We also explore the microscopic origin of the magnetic octupole Hall effect and find that it results from the combined effect of orbital texture and spin-orbit coupling. Additionally, we analyze the ratio of spin Hall conductivity to magnetic octupole Hall conductivity across various materials and identify those that are optimal for observing magnetic octupole physics. We also discuss potential applications arising from the magnetic octupole Hall effect. Our work serves as a valuable reference for identifying materials suitable for studying magnetic octupole physics.

cond-mat.mtrl-sci↗

arXiv2024

Harnessing magnetic octupole Hall effect to induce torque in altermagnets

d-wave altermagnets have magnetic octupoles as their order parameters [Phys. Rev. X 14, 011019 (2024)]. We theoretically show that magnetic octupoles injected from outside generate torque on the d-wave altermagnets. The injection can be achieved by the magnetic octupole Hall effect in an adjacent layer. We calculate the magnetic octupole Hall conductivity of the heavy metal Pt and find a sizable value comparable to its spin Hall conductivity. Our work generalizes the spin Hall phenomenology (generation by heavy metals and detection by torque in ferromagnets) to the magnetic octupole Hall phenomenology (generation by heavy metals and detection by torque in altermagnets), which can be utilized to electrically control magnetic configurations of altermagnets.

cond-mat.mes-hall↗

arXiv2024

Nonlinear Orbital and Spin Edelstein Effect in Centrosymmetric Metals

Nonlinear spintronics combines nonlinear dynamics with spintronics, opening up new possibilities beyond linear responses. A recent theoretical work [Xiao et al., Phys. Rev. Lett. 130, 166302 (2023)] predicts the nonlinear generation of spin density [nonlinear spin Edelstein effect (NSEE)] in centrosymmetric metals based on symmetry analysis combined with first principle calculation. However, its microscopic mechanism is limited to a specific set of materials with local inversion symmetry breaking and is not applicable to general materials. This paper focuses on the fundamental role of orbital degrees of freedom for the nonlinear generation in centrosymmetric systems. Using a combination of tight-binding model and density functional theory calculations, we demonstrate that nonlinear orbital density can arise independently of spin-orbit coupling. In contrast, spin density follows through spin-orbit coupling. We further elucidate the microscopic mechanism responsible for this phenomenon, which involves the NSEE induced by electric-field-induced orbital Rashba texture. In addition, we also explore the potential applications of the nonlinear orbital and spin Edelstein effect for field-free switching of magnetization.

cond-mat.mtrl-sci↗