arXiv subjects

Taekoo Oh

Publications and source records attributed to Taekoo Oh.

13 recordsLinked to original sources

arXiv2026

Spin nematic liquid crystal and scalar spin chirality in tetragonal lattice YbMnBi$_2$

A spin nematic order, analogous to the nematic liquid crystal, characterizes the spontaneous breaking of spin-space rotational symmetry while preserving time-reversal ($T$) symmetry. In contrast, scalar spin chirality (SSC), a composite three-spin order, breaks $T$ symmetry and is known to induce an anomalous Hall effect (AHE). Although a spin nematic phase has been suggested in frustrated magnets and the square-lattice iridate, how it might affect magnetotransport properties is unknown. Here we use polarized neutron scattering to show that tetragonal $A$MnBi$_2$ ($A$ = Ca, Yb) is a strictly $c$-axis-aligned collinear antiferromagnet (C-type), with $T_N \approx 270$ K and 290 K, respectively. On cooling from 450 K to $T_N$, low-energy spin excitations in YbMnBi$_2$ spontaneously change from isotropic to anisotropic in spin space within the tetragonal plane, forming a dynamic spin nematic phase around 400 K due to heavy Yb-induced spin-orbit coupling, before gapping out below $T_N$. Similar measurements on CaMnBi$_2$ reveal isotropic paramagnetic scattering without a spin nematic phase above $T_N$. Under an in-plane magnetic field, the Yb$^{3+}$ moments may interact with the dynamic spin nematic phase to induce nonzero SSC, giving rise to AHE and an anomalous Nernst effect (ANE) in YbMnBi$_2$ that are absent in CaMnBi$_2$ above $T_N$. A symmetry-based Ginzburg-Landau analysis shows that coupling terms between the nematic order and SSC are allowed under an external magnetic field, which could explain the rapid increase of AHE with field in YbMnBi$_2$. Our results provide compelling evidence for dynamic SSC-induced AHE and ANE in the paramagnetic phase of a compensated collinear antiferromagnet, opening a new avenue for the physics of composite spin orders and room-temperature spintronics without magnetic order.

cond-mat.str-el↗

arXiv2026

Microscopic Theory of the Phonon Thermal Hall Effect in Chiral Mott Insulators

The thermal Hall effect (THE) probes charge-neutral excitations in insulators, where the charge gap blocks electronic transport. Recently, phonons have been shown to induce a THE comparable in magnitude to the spin contribution, underscoring their critical role in thermal transport. Here, we develop a microscopic theory of the phonon thermal Hall effect (PTHE) in chiral Mott insulators. First, we derive the exact analytic form of the effective Raman interaction in half-filled Mott insulators, showing that its strength is directly proportional to the scalar spin chirality. Next, we demonstrate the intrinsic PTHE explicitly on the kagome lattice. Crucially, our formulation reveals a temperature-dependent crossover in the transport behavior under isotopic substitution. Using this result, we establish a scaling law that quantitatively separates the phonon contribution to the THE from other background signals. Our results not only provide the first fully microscopic derivation of the PTHE, but also establish a definitive experimental standard for isolating microscopic heat carriers in chiral Mott insulators.

cond-mat.str-el↗

arXiv2026

Magneto-Optical Detection of Anisotropic Spin Currents in Altermagnetic RuO2

Altermagnets are a recently identified class of collinear antiferromagnets that host large spin-split electronic bands, offering a promising platform for efficient spin-current generation. Among proposed candidates, the metallic oxide RuO2 is predicted to exhibit strong altermagnetic spin splitting; however, whether it sustains robust magnetic order beyond the ultrathin thickness limit remains unresolved. Here, we employ optical probes to investigate charge-to-spin conversion in a 12-nm-thick (101)-oriented RuO2 film grown on sapphire. Polarization-resolved second-harmonic generation reveals nonlinear optical responses consistent with the surface symmetry and N\'eel order of RuO2. Under an applied current, both second-harmonic generation and polar magneto-optical Kerr effect measurements detect a pronounced, directionally anisotropic spin polarization, exhibiting enhanced signals for current along [010] and strongly suppressed responses for current along [-101], in agreement with the symmetry of the altermagnetic spin-splitter effect. Non-magnetic or Rashba-type mechanisms cannot explain this symmetry-selective response. Scanning transmission electron microscopy further reveals that substantial strain persists even in relatively thick films, providing a possible explanation for the observed behavior. Therefore, these results establish RuO2 as an efficient spin source and demonstrate the potential of altermagnets for field-free spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Thermal Hall effect induced by phonon skew-scattering via orbital magnetization

Thermal transport acts as a powerful tool for studying the excitations and physical properties of insulators, where a charge gap suppresses electronic conduction. Recently, the thermal Hall effect has been observed across various materials, including insulators and semiconductors, but its fundamental origin remains unclear. Here, I propose a promising mechanism to explain the emergence of the thermal Hall effect in these systems: axial chiral phonon skew scattering mediated by orbital magnetization. Starting from basic principles, I derive the form and magnitude of the orbital magnetization-phonon coupling using the well-established Haldane model. Using this coupling, I calculate the thermal Hall conductivity and Hall angle as functions of temperature, achieving semi-quantitative agreement with experimental findings. This work enhances our understanding of the role of electron-phonon coupling in thermal transport and provides a pathway to tailor thermal properties in a broad range of materials.

cond-mat.mes-hall↗

arXiv2025

Spin-phonon coupling and thermal Hall effect in the Kitaev model

The Kitaev model, which involves bond-direction-dependent spin interactions on a honeycomb lattice, has attracted significant interest due to its exact solvability and potential uses in quantum computing. A key feature of this model is the half-quantized thermal Hall conductivity (HQTHC) under a magnetic field perpendicular to the lattice; however, HQTHC only appears at low temperatures. Here, in the higher temperature range beyond the HQTHC regime, we theoretically suggest an extrinsic phonon contribution to the thermal Hall effect in the Kitaev model through skew-scattering of chiral phonons by scalar spin chirality, previously examined in Mott insulators. We demonstrate the emergence of scalar spin chirality from fluctuating spins, estimate the resulting field strength and its symmetric form applied to chiral phonons, and obtain the associated thermal Hall conductivity in semi-quantitative agreement with existing experiments. This work offers a fundamental understanding of how spin-phonon interactions influence strongly correlated systems.

cond-mat.str-el↗

arXiv2024

Phonon thermal Hall effect in Mott insulators via skew-scattering by the scalar spin chirality

Thermal transport is a crucial probe for studying excitations in insulators. In Mott insulators, the primary candidates for heat carriers are spins and phonons, and which dominates the thermal conductivity is a persistent issue. Typically, phonons dominate the longitudinal thermal conductivity while the thermal Hall effect (THE) is primarily associated with spins, which requires time-reversal symmetry breaking. The coupling between phonons and spins usually depends on spin-orbit interaction and is relatively weak. Here, we propose a new mechanism for this coupling and the associated THE: the skew scattering of phonons via spin fluctuations by the scalar spin chirality. This coupling does not require spin-orbit interaction and is ubiquitous in Mott insulators, leading to a thermal Hall angle on the order of $10^{-3}$ to $10^{-2}$. Based on this mechanism, we investigate the THE in YMnO$_3$ with a trimerized triangular lattice where the THE beyond spins was recognized, and predict the THE in the Kagome and square lattices.

cond-mat.str-el↗

arXiv2024

Unraveling the dynamics of magnetization in topological insulator-ferromagnet heterostructures via spin-orbit torque

Spin-orbit coupling stands as a pivotal determinant in the realm of condensed matter physics. In recent, its profound influence on spin dynamics opens up a captivating arena with promising applications. Notably, the topological insulator-ferromagnet heterostructure has been recognized for inducing spin dynamics through applied current, driven by spin-orbit torque. Building upon recent observations revealing spin flip signals within this heterostructure, our study elucidates the conditions governing spin flips by studying the magnetization dynamics. We establish that the interplay between spin-anisotropy and spin-orbit torque plays a crucial role in shaping the physics of magnetization dynamics within the heterostructure. Furthermore, we categorize various modes of magnetization dynamics, constructing a comprehensive phase diagram across distinct energy scales, damping constants, and applied frequencies. This research not only offers insights into controlling spin direction but also charts a new pathway to the practical application of spin-orbit coupled systems.

cond-mat.mes-hall↗

arXiv2024

Quantum geometric bound for saturated ferromagnetism

Despite its abundance in nature, predicting the occurrence of ferromagnetism in the ground state is possible only under very limited conditions such as in a flat band system with repulsive interaction or in a band with a single hole under infinitely large Coulomb repulsion, etc. Here, we propose a general condition to achieve saturated ferromagnetism based on the quantum geometry of electronic wave functions in itinerant electron systems. By analyzing the spin excitations of multi-band repulsive Hubbard models with an integer band filling, relevant to either ferromagnetic insulators or semimetals, we show that quantum geometry stabilizes the Goldstone mode in the strongly correlated limit. Our theory indicates the stability of ferromagnetism in a large class of insulators and semimetals other than the previously studied flat band systems and their variants. Moreover, we rigorously prove that saturated ferromagnetism is forbidden in any system with trivial quantum geometry, which includes every half-filled system. We believe that our findings reveal a profound connection between quantum geometry and ferromagnetism, which can be extended to various symmetry-broken ground states in itinerant electronic systems.

cond-mat.str-el↗

arXiv2023

Nonreciprocal transport in U(1) gauge theory of high-Tc cuprates

The nature of the charge carriers in high-Tc cuprates is an essential issue to reveal their novel physical properties and the mechanism of their superconductivity. However, the experimental probes and the theoretical analysis have been mostly restricted to the linear responses. On the other hand, recent observations of Rashbatype spin-orbit coupling (SOC) on the surface of high-Tc cuprates imply the possible nonlinear and nonreciprocal transport phenomena under in-plane magnetic fields. In this paper, we study the nonreciprocal transport properties on the surface of cuprates by employing a U(1) gauge theory framework, where the electrons are considered to be fractionalized. Our investigation highlights the intricate variations in nonreciprocal transport with respect to temperatures and dopings. First, it reveals contrasting behavior of nonreciprocity in each normal phase. Second, it discerns the tendencies between underdoped and overdoped superconducting states and their paraconductivity. The complex behaviors of nonreciprocal transport originate from the spinful and spinless nature of the charge carriers and their kinetic energy scales. These findings pave a new avenue to explore the electronic states in high-Tc cuprates in terms of nonreciprocal transport phenomena.

cond-mat.supr-con↗

arXiv2023

Emergent inductance from spin fluctuations in strongly correlated magnets

Recently, the intriguing phenomenon of emergent inductance has been theoretically proposed and experimentally observed in nanoscale spiral spin systems subjected to oscillating currents. Building upon these recent developments, we put forward the concept of emergent inductance in strongly correlated magnets in the normal state with spin fluctuations. It is argued that the inductance shows a positive peak at temperatures above the ordering temperature. As for the frequency dependence, in systems featuring a single-band structure or a gapped multi-band, we observe a Drude-type, while in gapless multi-band systems, a non-Drude inductance with a sharp dip near zero frequency. These results offer valuable insights into the behavior of strongly correlated magnets and open up new possibilities for harnessing emergent inductance in practical applications.

cond-mat.str-el↗

arXiv2022

Theory of transverse magnetization in spin-orbit coupled antiferromagnets

Some antiferromagnets under a magnetic field develop magnetization perpendicular to the field as well as more conventional ones parallel to the field. So far, the transverse magnetization (TM) has been attributed to either spin canting effect or the presence of cluster magnetic multipolar ordering. However, a general theory of TM based on microscopic understanding is still missing. Here, we construct a general microscopic theory of TM in antiferromagnets with cluster magnetic multipolar ordering by considering classical spin Hamiltonians with spin anisotropy that arises from the spin-orbit coupling. First, from general symmetry analysis, we show that TM can appear only when all crystalline symmetries are broken other than the antiunitary mirror, antiunitary two-fold rotation, and inversion symmetries. Moreover, by analyzing spin Hamiltonians, we show that TM always appears when the degenerate ground state manifold of the spin Hamiltonian is discrete. On the other hand, when the degenerate ground state manifold is continuous, TM generally does not appear except when the magnetic field direction and the spin configuration satisfy specific geometric conditions under single-ion anisotropy. Finally, we show that TM can induce anomalous planar Hall Effect, a unique transport phenomenon that can be used to probe multipolar antiferromagnetic structures. We believe that our theory provides a useful guideline for understanding the anomalous magnetic responses of the antiferromagnets with complex magnetic structures.

cond-mat.mes-hall↗

arXiv2018

Magnetic field induced topological semimetals near a quantum critical point of pyrochlore iridates

Motivated by the recent experimental observation of anomalous magneto-transport properties near the Mott quantum critical point (QCP) of pyrochlore iridates, we study the generic topological band structure near QCP in the presence of magnetic field. We have found that the competition between different energy scales can generate various topological semi-metal phases near QCP. Here the central role is played by the presence of a quadratic band crossing (QBC) with four-fold degeneracy in the paramagnetic band structure. Due to the large band degeneracy and strong spin-orbit coupling, the degenerate states at QBC can show an anisotropic Zeeman effect as well as the conventional isotropic Zeeman effect. Through the competition between three different magnetic energy scales including the exchange energy between Ir electrons and two Zeeman energies, various topological semimetals can be generated near QCP. Moreover, we have shown that these three magnetic energy scales can be controlled by modulating the magnetic multipole moment (MMM) of the cluster of spins in a unit cell, which can couple to the intrinsic MMM of the degenerate states at QBC. We propose the general topological band structure under magnetic field achievable near QCP, which would facilitate the experimental discovery of novel topological semimetal states in pyrochlore iridates.

cond-mat.mes-hall↗

arXiv2018

Unconventional anomalous Hall effect from antiferromagnetic domain walls of Nd2Ir2O7 thin films

Ferroic domain walls (DWs) create different symmetries and ordered states compared with those in single-domain bulk materials. In particular, the DWs of an antiferromagnet (AFM) with non-coplanar spin structure have a distinct symmetry that cannot be realized in those of their ferromagnet counterparts. In this paper, we show that an unconventional anomalous Hall effect (AHE) can arise from the DWs of a non-coplanar AFM, Nd2Ir2O7. Bulk Nd2Ir2O7 has a cubic symmetry; thus, its Hall signal should be zero without an applied magnetic field. The DWs generated in this material break the two-fold rotational symmetry, which allows for finite anomalous Hall conductivity. A strong f-d exchange interaction between the Nd and Ir magnetic moments significantly influences antiferromagnetic domain switching. Our epitaxial Nd2Ir2O7 thin film showed a large enhancement of the AHE signal when the AFM domains switched, indicating that the AHE is mainly due to DWs. Our study highlights the symmetry broken interface of AFM materials as a new means of exploring topological effects and their relevant applications.

cond-mat.str-el↗