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Yoon Seok Oh

Publications and source records attributed to Yoon Seok Oh.

At least 19 recordsLinked to original sources
arXiv2026

$p$-wave magnet and hedgehog-type Berry curvature in helimagnetic MnAu$_2$

Recently discovered altermagnetism in collinear compensated magnets shows even-parity spin texture in momentum space. Beyond the collinear spin ordering, unique odd-parity spin textures emerge in noncollinear compensated magnets. The noncollinear candidates, however, remain unexplored toward a room-temperature metallic altermagnet. Here, we demonstrate that MnAu2 exhibits metallic p-wave magnetism and large spin splitting induced by helical spin ordering. By adapting the band-unfolding scheme based on the translation operators combined with spin rotations, first-principles calculations reveal an unconventional Fermi surface around the $\tilde{M}$-point composed of a single electron pocket with $p$-wave spin texture. Moreover, the spin twist in the helimagnet triggers topologically non-trivial hedgehog Berry curvature, which links to the nonlinear Hall effect and spin Hall effect. Considering the experimental T$_C$ = 335 to 370 K, MnAu$_2$ could establish itself as an ideal candidate for a room-temperature metallic $p$-wave magnet, promising for versatile spintronic applications.

cond-mat.mtrl-sci↗
arXiv2025

Ferroelectric amplitude switching and continuous memory

Although ferroelectric systems inherently exhibit binary switching behavior, recent advances in analog memory device have spurred growing interest in achieving continuous memory states. In this work, we demonstrate ferroelectric amplitude switching at the mesoscopic scale in compositionally graded Ba1-xSrxTiO3 heterostructures, enabling continuous modulation of polarization magnitude without altering its direction, which we defined as amplitude switching. Using switching current measurement, piezoresponse force microscopy and Landau-Ginzburg-Devonshire simulations, we reveal that compositionally graded ferroelectric heterostructure can possess amplitude switching behavior through a double well potential with flattened minima. This behavior supports stable, continuous polarization states and establishes a new platform for analog memory applications. These findings introduce amplitude switching as a new dynamic of the order parameter, paving the way for energy-efficient and reliable analog memory systems.

cond-mat.mtrl-sci↗
arXiv2025

Tunneling magnetoresistance in altermagnetic RuO$_2$-based magnetic tunnel junctions

Altermagnets exhibit characteristics akin to antiferromagnets, with spin-split anisotropic bands in momentum space. RuO$_2$ has been considered as a prototype altermagnet; however, recent reports have questioned altermagnetic ground state in this material. In this study, we provide direct experimental evidence of altermagnetic characteristics in RuO$_2$ films by demonstrating spin-dependent tunneling magnetoresistance (TMR) in RuO$_2$-based magnetic tunnel junctions. Our results show the spin-splitted anisotropic band structure of RuO$_2$, with the observed TMR determined by the direction of the Néel vector of RuO$_2$. These results reflect the altermagnetic nature of RuO$_2$ and highlight its potential for spintronic applications, leveraging the combined strengths of ferromagnetic and antiferromagnetic systems.

cond-mat.mtrl-sci↗
arXiv2023

Thermal Hall effects due to topological spin fluctuations in YMnO$_3$

The thermal Hall effect in magnetic insulators has been considered a powerful method for examining the topological nature of charge-neutral quasiparticles such as magnons. Yet, unlike the kagome system, the triangular lattice has received less attention for studying the thermal Hall effect because the scalar spin chirality cancels out between adjacent triangles. However, such cancellation cannot be perfect if the triangular lattice is distorted, which could open the possibility of a non-zero thermal Hall effect. Here, we report that the trimerized triangular lattice of multiferroic hexagonal manganite YMnO$_3$ produces a highly unusual thermal Hall effect due to topological spin fluctuations with the additional intricacy of a Dzyaloshinskii-Moriya interaction under an applied magnetic field. We conclude the thermal Hall conductivity arises from the system's topological nature of spin fluctuations. Our theoretical calculations demonstrate that the thermal Hall conductivity is also related in this material to the splitting of the otherwise degenerate two chiralities, left and right, of its 120$^{\circ}$ magnetic structure. Our result is one of the most unusual cases of topological physics due to this broken $Z_2$ symmetry of the chirality in the supposedly paramagnetic state of YMnO$_3$, with strong topological spin fluctuations. These new mechanisms in this important class of materials are crucial in exploring new thermal Hall physics and exotic excitations.

cond-mat.str-el↗
arXiv2022

Diagonal and off-diagonal thermal conduction with resonant phonon scattering in $Ni_3TeO_6$

The coupling between phonon and magnon is ubiquitous in magnetic materials and plays a crucial role in many aspects of magnetic properties, most notably in spintronics. Yet, this academically and technologically interesting problem still poses a severe challenge to a general understanding of the issue in certain materials. We report that Ni3TeO6 exhibits clear evidence of significant magnon-phonon coupling in both longitudinal thermal conductivity ($κ_{xx}$) and thermal Hall coefficient ($κ_{xy}$). The Debye-Callaway model, a phenomenological description for phonon heat conduction, can explain the measured magnetic field dependence of $κ_{xx}(H)$: phonon scattering from spin fluctuation in the paramagnetic phase and additional scattering due to magnon-phonon coupling in the collinear antiferromagnetic phase. We further suggest that a similar approach could be applied to understand the finite $κ_{xy}$ values in $Ni_3TeO_6$.

cond-mat.mtrl-sci↗
arXiv2022

Reversibly controlled ternary polar states and ferroelectric bias promoted by boosting square-tensile-strain

Interaction between dipoles often emerges intriguing physical phenomena, such as exchange bias in the magnetic heterostructures and magnetoelectric effect in multiferroics, which lead to advances in multifunctional heterostructures. However, the defect-dipole tends to be considered the undesired to deteriorate the electronic functionality. Here, we report deterministic switching between the ferroelectric and the pinched states by exploiting a new substrate of cubic perovskite, BaZrO$_{3}$, which boosts square-tensile-strain to BaTiO$_{3}$ and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation. First-principles calculations propose a complex of an oxygen vacancy and two Ti$^{3+}$ ions coins a charge-neutral defect-dipole. Cooperative control of the defect-dipole and the spontaneous polarization reveals ternary in-plane polar states characterized by biased/pinched hysteresis loops. Furthermore, we experimentally demonstrate that three electrically controlled polar-ordering states lead to switchable and non-volatile dielectric states for application of non-destructive electro-dielectric memory. This discovery opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates.

cond-mat.mtrl-sci↗
arXiv2022

Significant thermal Hall effect in the 3$d$ cobalt Kitaev system $Na_2Co_2TeO_6$

Kitaev physics has recently attracted attention in condensed matter for its anticipated novel quantum spin liquid state. The thermal transport measurement is crucial for probing the novel features of charge-neutral quasiparticles. In this letter, we report a significant thermal Hall effect in $Na_2Co_2TeO_6$ (NCTO), a Kitaev quantum spin liquid candidate, when the magnetic field is applied along the out-of-plane direction of the honeycomb plane. The thermal conductivity ($κ_{xx}$) and thermal Hall conductivity ($κ_{xy}$) in NCTO reveal distinct magnetic field dependences below and above the Neel temperature ($T_N$) of 27 K. For $T>T_N$, $κ_{xx}$ has a monotonic decrease in the field dependence, while $κ_{xy}$ persists up to $T^*$ = 150 K. On the other hand, both $κ_{xx}$ and $κ_{xy}$ exhibit complex field dependence for $T<T_N$.

cond-mat.str-el↗
arXiv2022

Pressure-dependent structure of BaZrO$_3$ crystals as determined by Raman Spectroscopy

The structure of dielectric perovskite BaZrO$_3$, long known to be cubic at room temperature without any structural phase transition with variation of temperature, has been recently disputed to have different ground state structures with lower symmetries involving octahedra rotation. The pressure-dependent Raman scattering measurements can identify the hierarchy of energetically-adjacent polymorphs, helping in turn understand its ground state structure at atmospheric pressure. Here, Raman scattering spectra of high-quality BaZrO$_3$ single crystals grown by the optical floating zone method are investigated in a pressure range from 1 atm to 42 GPa. First, based on the analyses of the infrared and Raman spectra measured at the atmosphere, it is found that all observed vibrational modes can be assigned according to the cubic $Pm\bar{3}m$ structure. In addition, by applying pressure, two structural phase transitions are found at 8.4 and 19.2 GPa, one from the cubic to the rhombohedral $R\bar{3}c$ phase and the other from the rhombohedral to the tetragonal $I4/mcm$ phase. Based on the two pressure-induced structural phase transitions, the true ground state structure of BaZrO$_3$ at room temperature and ambient pressure is corroborated to be cubic while the rhombohedral phase is the closest second.

cond-mat.mtrl-sci↗
arXiv2021

Persistence of Ising-like easy-axis spin correlations in the paramagnetic state of the spin-1 chain compound NiTe$_2$O$_5$

A $^{125}$Te nuclear magnetic resonance (NMR) study was carried out in the paramagnetic state of the recently discovered quasi-one-dimensional spin-1 chain compound NiTe$_2$O$_5$. We observed that the $^{125}$Te NMR spectrum splits into two in a magnetic field applied along the $c$ axis. Based on the strong temperature variation of the relative intensity ratio of the split lines, we infer that the line splitting arises from the two sublattice susceptibilities induced in opposite directions along the chains. In great support of this interpretation, a quantitative analysis of the spin-lattice relaxation rate $T_1^{-1}$ and the Knight shift data unravels dominant transverse spin fluctuations. We conclude that Ising-like uniaxial spin correlations persist up to surprisingly high temperatures compared to the exchange energy scales. Spin-charge coupling mechanism via a self-doping effect may be important.

cond-mat.str-el↗
arXiv2019

Unconventional critical behavior in quasi-one-dimensional $S$ = 1 chain NiTe$_{2}$O$_{5}$

Here we report a new quasi-one-dimensional $S$ = 1 chain compound NiTe$_{2}$O$_{5}$. From the comprehensive study of the structure and magnetic properties on high quality single crystalline NiTe$_{2}$O$_{5}$, it's revealed that NiTe$_{2}$O$_{5}$ undergoes a transition into an intriguing long-range antiferromagnetic order at $T_{\rm N}=30.5 {\rm K}$, in which longitudinal magnetic moments along the chain direction are ferromagnetically ordered, while their transverse components have an alternating ferromagnetic-antiferromagnetic coupling. Even though the temperature dependence of magnetic susceptibility represents an archetypal anisotropic antiferromagnetic order, we found that critical behavior of unconventional nature with $α'\sim0.25$ and $β\sim0.18$ is accompanied by the temperature evolution of the antiferromagnetic order parameter.

cond-mat.str-el↗
arXiv2019

Room Temperature Ferroelectric Ferromagnet in 1D Tetrahedral Chain Network

Ferroelectricity occurs in crystals with broken spatial inversion symmetry. In conventional perovskite oxides, concerted ionic displacements within a three-dimensional network of transition metal-oxygen polyhedra (MOx) manifest spontaneous polarization. Meanwhile, some two-dimensional networks of MOx can foster geometric ferroelectricity with magnetism, owing to the distortion of the polyhedra. Because of the fundamentally different mechanism of ferroelectricity in a two-dimensional network, one can further challenge an uncharted mechanism of ferroelectricity in a one-dimensional channel of MOx and estimate its feasibility. This communication presents ferroelectricity and coupled ferromagnetism in a one-dimensional FeO4 tetrahedral chain network of a brownmillerite SrFeO2.5 epitaxial thin film. The result provides a new paradigm for designing low-dimensional MOx networks, which is expected to benefit the realization of macroscopic ferro-ordering materials including ferroelectric ferromagnets.

cond-mat.mtrl-sci↗
arXiv2014

Experimental demonstration of hybrid improper ferroelectricity and presence of abundant charged walls in (Ca,Sr)$_{3}$Ti$_{2}$O$_{7}$ crystals

Standing on successful first principles predictions for new functional ferroelectric materials, a number of new ferroelectrics have been experimentally discovered. Utilizing trilinear coupling of two types of octahedron rotations, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds (Ca$_{3}$Ti$_{2}$O$_{7}$, Ca$_{3}$Mn$_{2}$O$_{7}$, and (Ca/Sr/Ba)$_{3}$(Sn/Zr/Ge)$_{2}$O$_{7}$). However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. Here we provide the first experimental demonstration of room-temperature switchable polarization in the bulk crystals of Ca$_{3}$Ti$_{2}$O$_{7}$ as well as Sr-doped Ca$_{3}$Ti$_{2}$O$_{7}$. In addition, (Ca,Sr)$_{3}$Ti$_{2}$O$_{7}$ is found to exhibit an intriguing ferroelectric domain structure resulting from orthorhombic twins and (switchable) planar polarization. The planar domain structure accompanies abundant charged domain walls with conducting head-to-head and insulating tail-to-tail configurations, which exhibit two-order-of-magnitude conduction difference. These discoveries provide new research opportunities not only on new stable ferroelectrics of Ruddlesden-Popper compounds, but also on meandering conducting domain walls formed by planar polarization.

cond-mat.mtrl-sci↗
arXiv2014

55~Tesla coercive magnetic field in frustrated Sr$_3$NiIrO$_6$

We have measured extremely large coercive magnetic fields of up to 55~T in Sr$_3$NiIrO$_6$, with a switched magnetic moment $\approx 0.8~μ_{\rm B}$ per formula unit. As far as we are aware, this is the largest coercive field observed thus far. This extraordinarily hard magnetism has a completely different origin from that found in conventional ferromagnets. Instead, it is due to the evolution of a frustrated antiferromagnetic state in the presence of strong magnetocrystalline anisotropy due to the overlap of spatially-extended Ir$^{4+}$ 5$d$ orbitals with oxygen 2$p$ and Ni$^{2+}$ 3$d$ orbitals. This work highlights the unusual physics that can result from combining the extended $5d$ orbitals in Ir$^{4+}$ with the frustrated behaviour of triangular lattice antiferromagnets.

cond-mat.mtrl-sci↗
arXiv2014

Magnetic Structure of Hexagonal Mn-doped LuFeO$_{3}$

Neutron scattering techniques are used to investigate the crystalline and magnetic structure of LuFe$_{0.75}$Mn$_{0.25}$O$_{3}$ in bulk polycrystalline form. We find that the crystalline structure is described by the hexagonal P6$_{3}$cm space group similar to that of thin-film LuFeO$_{3}$, and that the system orders antiferromagnetically below T$_{N}$ = 134 K. Inelastic neutron scattering reveals nearest-neighbor superexchange parameters that are enhanced relative to LuMnO$_{3}$. The observation of significant diffuse scattering above T$_{N}$ demonstrates the frustrated nature of the system; comparisons with similar materials suggest the ground state magnetic configuration is sensitive to local crystallographic distortions.

cond-mat.mtrl-sci↗
arXiv2014

Color theorems, chiral domain topology and magnetic properties of FexTaS2

Common mathematical theory can have profound applications in understanding real materials. The intrinsic connection between aperiodic orders observed in the Fibonacci sequence, Penrose tiling, and quasicrystals is a well-known example. Another example is the self-similarity in fractals and dendrites. From transmission electron microscopy experiments, we found that FexTaS2 crystals with x=1/4 and 1/3 exhibit complicated antiphase and chiral domain structures related to ordering of intercalated Fe ions with 2a*2a and sqrt3a*sqrt3a superstructures, respectively. These complex domain patterns are found to be deeply related with the four color theorem, stating that four colors are sufficient to identify the countries on a planar map with proper coloring, and its variations for two-step proper coloring. Furthermore, the domain topology is closely relevant to their magnetic properties. Our discovery unveils the importance of understanding the global topology of domain configurations in functional materials.

cond-mat.mtrl-sci↗
arXiv2013

Hysteretic melting transition of a soliton lattice in a commensurate charge modulation

We report on the observation of the hysteretic transition of a commensurate charge modulation in IrTe$_2$ from transport and scanning tunneling microscopy (STM) studies. Below the transition ($T_{\rm C} \approx 275$ K on cooling) a $q = 1/5$ charge modulation was observed, which is consistent with previous studies. Additional modulations [$q_n = (3n+2)^{-1}$] appear below a second transition at $T_{\rm S}\approx 180$ K on cooling. The coexistence of various modulations persist up to $T_{\rm C}$ on warming. The atomic structures of charge modulations and the temperature dependent STM studies suggest that 1/5 modulation is a periodic soliton lattice which partially melts below $T_{\rm S}$ on cooling. Our results provide compelling evidence that the ground state of IrTe$_2$ is a commensurate 1/6 charge modulation, which originates from periodic dimerization of Te atoms visualized by atomically resolved STM images.

cond-mat.str-el↗
arXiv2013

Anionic depolymerization transition in IrTe2

Selenium (Se) substitution drastically increases the transition temperature of iridium ditelluride (IrTe$_{2}$) to a diamagnetic superstructure from 278 K to 560 K. Transmission electron microscopy experiments revealed that this enhancement is accompanied by the evolution of non-sinusoidal structure modulations from $q = 1/5(10\bar{1})$- to $q = 1/6(10\bar{1})$-types. These comprehensive results are consistent with the concept of the destabilization of polymeric Te-Te bonds at the transition, the temperature of which is increased by chemical and hydrostatic pressure and by the substitution of Te with the more electronegative Se. This temperature-induced depolymerization transition in IrTe$_{2}$ is unique in crystalline inorganic solids.

cond-mat.str-el↗