arXiv subjects

Young Jai Choi

Publications and source records attributed to Young Jai Choi.

18 recordsLinked to original sources

arXiv2026

High-resolution neutron diffraction determination of noncollinear antiferromagnetic order in the honeycomb magnetoelectric Fe$_{4}$Nb$_{2}$O$_{9}$

Magnetoelectric systems offer potential for device applications exploiting coupled states between electric and magnetic properties. Among magnetoelectric materials, \FNO has attracted special attention because of its pronounced dielectric signal at high magnetic transition temperatures. However, the magnetic ground state, which is essential information for understanding its unusual magnetoelectricity, remains unclarified. Here, we report a noncollinear magnetic ground state of Fe$_{4}$Nb$_{2}$O$_{9}$. To examine the magnetoelectric effect associated with sequential magnetic and structural transitions upon cooling, we conducted combined x-ray diffraction, magnetic susceptibility, magnetization, dielectric constant, and magnetodielectric experiments. Powder neutron diffraction experiments revealed a series of magnetic Bragg peaks and clear splitting of peaks via structural transition. Magnetic Rietveld refinements, combined with group theory analysis, determined a noncollinear antiferromagnetic structure including a significant $c$-axis moment component at 1.5 K. This study provides insights into the understanding of its magnetoelectric properties.

cond-mat.mtrl-sci↗

arXiv2025

Quantum simulation approach to ultra-weak magnetic anisotropy in a frustrated spin-1/2 antiferromagnet

The intrinsic equivalence between electron spin and qubit offers a natural foundation for quantum simulations of magnetic materials. However, incorporating magnetocrystalline anisotropy (MCA), a key feature of real magnets, remains a major challenge. Here, we develop a quantum simulation framework for MCA in CuSb2O6, a spin-1/2 antiferromagnet with alternating ferromagnetic chains arising from frustrated, anisotropic exchange interactions in a nearly square lattice. The $\mathrm{Cu}^{2+}$ spin network is modeled as a four-qubit square lattice, with four paired ancilla qubits introduced to encode angle-dependent MCA. This two-qubit representation per spin site resolves the limitation that squared Pauli operators yield only the identity, enabling MCA terms to be faithfully embedded into quantum circuits. Using the variational quantum eigensolver, we determine an exceptionally small easy-axis MCA constant, just 0.00022% of the nearest-neighbor exchange interaction, yet sufficient to drive a spin-flop transition with $90^{\circ}$ spin reorientation and strong angular variation in magnetic torque. Beyond this regime, the simulations uncover a half-saturated magnetic phase at ultra-high fields, stabilized by anisotropic next-nearest-neighbor interactions. Our findings demonstrate the feasibility of resource-efficient quantum simulations of complex magnetic phenomena in real materials.

quant-ph↗

arXiv2025

Dynamic Competition Between Hubbard and Superexchange Interactions Selectively Localizes Electrons and Holes Through Polarons

Controlling the effects of photoexcited polarons in transition metal oxides can enable the long timescale charge separation necessary for renewable energy applications as well as controlling new quantum phases through dynamically tunable electron-phonon coupling. In previously studied transition metal oxides, polaron formation is facilitated by a photoexcited ligand-to-metal charge transfer (LMCT). When the polaron is formed, oxygen atoms move away from iron centers, which increases carrier localization at the metal center and decreases charge hopping. Studies of yttrium iron garnet and erbium iron oxide have suggested that strong electron and spin correlations can modulate photoexcited polaron formation. To understand the interplay between strong spin and electronic correlations in highly polar materials, we studied gadolinium iron oxide (GdFeO3), which selectively forms photoexcited polarons through an Fe-O-Fe superexchange inter-action. Excitation-wavelength-dependent transient extreme ultraviolet (XUV) spectroscopy selectively excites LMCT and metal-to-metal charge transfer transitions (MMCT). The LMCT transition suppresses photoexcited polaron formation due to the balance between superexchange and Hubbard interactions, while MMCT transitions result in photoexcited polaron formation within 250+/-40 fs. Ab initio theory demonstrates that electron and hole polarons localize on iron centers following MMCT. In addition to understanding how strong electronic and spin correlations can control strong electron-phonon coupling, these experiments separately measure electron and hole polaron interactions on neighboring metal centers for the first time, providing insight into a large range of charge-transfer and Mott-Hubbard insulators.

cond-mat.mtrl-sci↗

arXiv2024

Quantum-centric Supercomputing for Materials Science: A Perspective on Challenges and Future Directions

Computational models are an essential tool for the design, characterization, and discovery of novel materials. Hard computational tasks in materials science stretch the limits of existing high-performance supercomputing centers, consuming much of their simulation, analysis, and data resources. Quantum computing, on the other hand, is an emerging technology with the potential to accelerate many of the computational tasks needed for materials science. In order to do that, the quantum technology must interact with conventional high-performance computing in several ways: approximate results validation, identification of hard problems, and synergies in quantum-centric supercomputing. In this paper, we provide a perspective on how quantum-centric supercomputing can help address critical computational problems in materials science, the challenges to face in order to solve representative use cases, and new suggested directions.

quant-ph↗

arXiv2023

Type-II Red Phosphorus: Wavy Packing of Twisted Pentagonal Tubes

Elemental phosphorus exhibits fascinating structural varieties and versatile properties. The unique nature of phosphorus bonds can lead to the formation of extremely complex structures, and detailed structural information on some phosphorus polymorphs is yet to be investigated. In this study, we investigated an unidentified crystalline phase of phosphorus, type-II red phosphorus (RP), by combining state-of-the-art structural characterization techniques. Electron diffraction tomography, atomic-resolution scanning transmission electron microscopy (STEM), powder X-ray diffraction, and Raman spectroscopy were concurrently used to elucidate the hidden structural motifs and their packing in type-II RP. Electron diffraction tomography, performed using individual crystalline nanowires, was used to identify a triclinic unit cell with volume of 5330 Å^3, the largest unit cell for elemental phosphorus crystals up to now, which contains approximately 250 phosphorus atoms. Atomic-resolution STEM imaging, which was performed along different crystal-zone axes, confirmed that the twisted wavy tubular motif is the basic building block of type-II RP. Our study discovered and presented a new variation of building blocks in phosphorus, and it provides insights to clarify the complexities observed in phosphorus as well as other relevant systems.

cond-mat.mtrl-sci↗

arXiv2023

Antiadiabatic Small Polaron Formation in the Charge Transfer Insulator ErFeO3

Small polaron formation is dominant across a range of condensed matter systems. Small polarons are usually studied in terms of ground-state transport and thermal fluctuations, but small polarons can also be created impulsively by photoexcitation. The temporal response of the lattice and local electron correlations can then be separated, such as with transient XUV spectroscopy. To date, photoexcited small polaron formation has only been measured to be adiabatic. The reorganization energy of the polar lattice is large enough that the first electron-optical phonon scattering event creates a small polaron without significant carrier thermalization. Here, we use transient XUV spectroscopy to measure antiadiabatic polaron formation by frustrating the iron-centered octahedra in a rare-earth orthoferrite lattice. The small polaron is measured to take several picoseconds to form over multiple coherent charge hopping events between neighboring Fe3+-Fe2+ sites, a timescale that is more than an order of magnitude longer compared to previous materials. The measured interplay between optical phonons, electron correlations, and on-site lattice deformation give a clear picture of how antiadiabatic small polaron transport would occur in the material. The measurements also confirm the prediction of the Holstein and Hubbard-Holstein model that the electron hopping integral must be larger than the reorganization energy to achieve antiadiabaticity. Moreover, the measurements emphasize the importance of considering dynamical electron correlations, and not just changes in the lattice geometry, for controlling small polarons in transport or photoexcited applications.

cond-mat.mtrl-sci↗

arXiv2023

Electrical transport properties driven by unique bonding configuration in gamma-GeSe

Group-IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group-IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of gamma-GeSe, a recently identified polymorph of GeSe. gamma-GeSe exhibits high electrical conductivity (~106 S/m) and a relatively low Seebeck coefficient (9.4 uV/K at room temperature) owing to its high p-doping level (5x1021 cm-3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak-antilocalization because of spin-orbit coupling in the crystal. Our results demonstrate that gamma-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties.

cond-mat.mtrl-sci↗

arXiv2022

Large anomalous Hall effect and anisotropic magnetoresistance in intrinsic nanoscale spin-valve-type structure of an antiferromagnet

A spin valve is a prototype of spin-based electronic devices found on ferromagnets, in which an antiferromagnet plays a supporting role. Recent findings in antiferromagnetic spintronics show that an antiferromagnetic order in single-phase materials solely governs dynamic transport, and antiferromagnets are considered promising candidates for spintronic technology. In this work, we demonstrated antiferromagnet-based spintronic functionality on an itinerant Ising antiferromagnet of Ca0.9Sr0.1Co2As2 by integrating nanoscale spin-valve-type structure and investigating anisotropic magnetic properties driven by spin-flips. Multiple stacks of 1 nm thick spin-valve-like unit are intrinsically embedded in the antiferromagnetic spin structure. In the presence of a rotating magnetic field, a new type of the spin-valve-like operation was observed for large anomalous Hall conductivity and anisotropic magnetoresistance, whose effects are maximized above the spin-flip transition. In addition, a joint experimental and theoretical study provides an efficient tool to read out various spin states, which scheme can be useful for implementing extensive spintronic applications.

cond-mat.str-el↗

arXiv2022

Sign-tunable anisotropic magnetoresistance and electrically detectable dual magnetic phases in a helical antiferromagnet

The helimagnetic order describes a non-collinear spin texture of antiferromagnets, arising from competing exchange interactions. Although collinear antiferromagnets are elemental building blocks of antiferromagnetic (AFM) spintronics, the potential of implementing spintronic functionality in non-collinear antiferromagnets has not been clarified thus far. Here, we propose an AFM helimagnet of EuCo2As2 as a novel single-phase spintronic material that exhibits a remarkable sign reversal of anisotropic magnetoresistance (AMR). The contrast in the AMR arises from two electrically distinctive magnetic phases with spin reorientation driven by magnetic field lying on the easy-plane, which switches the sign of the AMR from positive to negative. Further, various AFM memory states associated with the evolution of the spin structure under magnetic fields were identified theoretically, based on an easy-plane anisotropic spin model. These results reveal that non-collinear antiferromagnets hold potential for developing spintronic devices.

cond-mat.str-el↗

arXiv2022

Kondo interaction in FeTe and its potential role in the magnetic order

Finding d-electron heavy fermion (HF) states has been an important topic as the diversity in d-electron materials can lead to many exotic Kondo effect-related phenomena or new states of matter such as correlation-driven topological Kondo insulator or cooperation between long-range magnetism and Kondo lattice behavior. Yet, obtaining direct spectroscopic evidence for a d-electron HF system has been elusive to date. Here, we report the observation of Kondo lattice behavior in an antiferromagnetic metal, FeTe, via angle-resolved photoemission spectroscopy (ARPES) and transport properties measurements. The Kondo lattice behavior is represented by the emergence of a sharp quasiparticle at low temperatures. The transport property measurements confirm the low-temperature Fermi liquid behavior and reveal successive coherent-incoherent crossover upon increasing temperature. We interpret the Kondo lattice behavior as a result of hybridization between localized Fe 3dxy and itinerant Te 5pz orbitals. Our interpretation is further evidenced by Fano-type tunneling spectra which accompany a hybridization gap. Our observations strongly suggest unusual cooperation between Kondo lattice behavior and long-range magnetic order.

cond-mat.str-el↗

arXiv2021

Evolution of electronic structure of Ru-doped single-crystal iridiates, Sr$_2$Ir$_{1-x}$Ru$_x$O$_4$

We investigated Ru-doped single-crystal 5$d$ iridiates, Sr$_2$Ir$_{1-x}$Ru$_x$O$_{4}$, at three different doping concentrations ($x =$ 0.01, 0.07 and 0.10) using optical spectroscopy. The undoped pristine compound (Sr$_2$IrO$_{4}$) is known as a novel $J_{eff}$ = 1/2 Mott insulator. Remarkably, the optical conductivity spectra of all three samples exhibited the insulating behavior, although we observed weak Drude components in the optical conductivity spectra down to the lowest temperature of 30 K. The charge-carrier densities of the Ru-doped iridiates estimated from the Drude components are significantly smaller than the expected values estimated from the nominal Ru-doping concentrations. Herein, we provide temperature- and doping-dependent electronic structure evolution of Ru-doped iridiates. We expect that our results will be useful for understanding the intriguing Ru-doping-dependent properties of 5$d$ iridiate Sr$_2$IrO$_{4}$.

cond-mat.str-el↗

arXiv2021

Stable hump-like Hall effect and non-coplanar spin textures in SrRuO$_3$ ultrathin film

We observed a hump-like feature in Hall effects of SrRuO$_3$ ultrathin films, and systematically investigated it with controlling thicknesses, temperatures and magnetic fields. The hump-like feature is extremely stable, even surviving as a magnetic field is tilted by as much as 85$^\circ$. Based on the atomic-level structural analysis of a SrRuO$_3$ ultrathin film with a theoretical calculation, we reveal that atomic rumplings at the thin-film surface enhance Dzyaloshinskii-Moriya interaction, which can generate stable chiral spin textures and a hump-like Hall effect. Moreover, temperature dependent resonant X-ray measurements at Ru L-edge under a magnetic field showed that the intensity modulation of unexpected peaks was correlated with the hump region in the Hall effect. We verify that the two-dimensional property of ultrathin films generates stable non-coplanar spin textures having a magnetic order in a ferromagnetic oxide material.

cond-mat.str-el↗

arXiv2021

Capping and gate control of anomalous Hall effect and hump structure in ultra-thin SrRuO$_3$ films

Ferromagnetism and exotic topological structures in SrRuO$_3$ (SRO) induce sign-changing anomalous Hall effect (AHE). Recently, hump structures have been reported in the Hall resistivity of SRO thin films, especially in the ultra-thin regime. We investigate the AHE and hump structure in the Hall resistivity of SRO ultra-thin films with an SrTiO$_3$ (STO) capping layer and ionic liquid gating. STO capping results in sign changes in the AHE and modulation of the hump structure. In particular, the hump structure in the Hall resistivity is strongly modulated and even vanishes in STO-capped 4 unit cell (uc) films. In addition, the conductivity of STO-capped SRO ultra-thin films is greatly enhanced with restored ferromagnetism. We also performed ionic liquid gating to modulate the electric field at SRO/STO interface. Drastic changes in the AHE and hump structure are observed with different gate voltages. Our study shows that the hump structure as well as the AHE can be controlled by tuning inversion symmetry and the electric field at the interface.

cond-mat.mtrl-sci↗

arXiv2021

Noncollinear antiferromagnetic order in the buckled honeycomb lattice of magnetoelectric Co4Ta2O9 determined by single-crystal neutron diffraction

Co4Ta2O9 exhibits a three-dimensional magnetic lattice based on the buckled honeycomb motif. It shows unusual magnetoelectric effects, including the sign change and non-linearity. These effects cannot be understood without the detailed knowledge of the magnetic structure. Herein, we report neutron diffraction and direction-dependent magnetic susceptibility measurements on Co4Ta2O9 single crystals. Below 20.3 K, we find a long-range antiferromagnetic order in the alternating buckled and flat honeycomb layers of Co2+ ions stacked along the c axis. Within experimental accuracy, the magnetic moments lie in the ab plane. They form a canted antiferromagnetic structure with a tilt angle of ~ 14 degrees at 15 K in the buckled layers, while the magnetic moments in each flat layer are collinear. This is directly evidenced by a finite (0, 0, 3) magnetic Bragg peak intensity, which would be absent in the collinear magnetic order. The magnetic space group is C2'/c. It is different from the previously reported C2/c' group, also found in the isostructural Co4Nb2O9. The revised magnetic structure successfully explains the major features of the magnetoelectric tensor of Co4Ta2O9 within the framework of the spin-flop model.

cond-mat.str-el↗

arXiv2020

Evidence for the Alternating Next-Nearest Neighbor model in the dynamic behavior of a frustrated antiferromagnet

X-ray photon correlation spectroscopy (XPCS) enables us to study dynamics of antiferromagnets. Using coherent soft X-ray diffraction, we resonantly probe Mn and Co Bragg peaks in the frustrated magnetic chain compound Lu2CoMnO6 significantly below the Neel temperature. Bragg peaks of incommensurate order slide towards commensurate 'up up down down' order with decreasing temperature. Antiferromagnetic inhomogeneities produce speckle within the Bragg peaks, whose dynamics are probed by XPCS and compared to the classic Axial Next-Nearest Neighbor Interaction model of frustration. The data supports a novel model prediction: with decreasing temperature the dynamics become faster.

cond-mat.str-el↗

arXiv2019

Highly nonlinear magnetoelectric effect in antiferromagnetic Co4Ta2O9 single crystals

Strongly correlated materials with multiple order parameters provide unique insights into the fundamental interactions in condensed matter systems and present opportunities for innovative technological applications. A class of antiferromagnetic honeycomb lattices compounds, A4B2O9 (A = Co, Fe, Mn; B = Nb, Ta), have been explored owing to the occurrence of linear magnetoelectricity. We observe a highly nonlinear magnetoelectric effect on single crystals of Co4Ta2O9 (CTO), distinctive from the linear behavior in the isostructural Co4Nb2O9. Ferroelectricity emerges primarily along the [110] direction under magnetic fields, with the onset of antiferromagnetic order at TN = 20.5 K. For in-plane magnetic field, a spin-flop occurs at HC ~ 0.3 T, above which the ferroelectric polarization gradually becomes negative and reaches a broad minimum. Upon increasing magnetic field further, the polarization crosses zero and increases continuously to ~60 uC/m2 at 9 T. In contrast, the polarization for a magnetic field perpendicular to the hexagonal plane increases monotonously and reaches ~80 uC/m2 at 9 T. This observation of a strongly nonlinear magnetoelectricity suggests that two types of inequivalent Co2+ sublattices generate magnetic field-dependent ferroelectric polarization with opposite signs. These results motivate fundamental and applied research on the intriguing magnetoelectric characteristics of these honeycomb lattice materials.

cond-mat.str-el↗

arXiv2018

Antiferromagnet-based spintronic functionality by controlling isospin domains in a layered perovskite iridate

The novel electronic state of the canted antiferromagnetic (AFM) insulator, strontium iridate (Sr2IrO4) has been well described by the spin-orbit-entangled isospin Jeff = 1/2, but the role of isospin in transport phenomena remains poorly understood. In this study, antiferromagnet-based spintronic functionality is demonstrated by combining unique characteristics of the isospin state in Sr2IrO4. Based on magnetic and transport measurements, large and highly anisotropic magnetoresistance (AMR) is obtained by manipulating the antiferromagnetic isospin domains. First-principles calculations suggest that electrons whose isospin directions are strongly coupled to in-plane net magnetic moment encounter the isospin mismatch when moving across antiferromagnetic domain boundaries, which generates a high resistance state. By rotating a magnetic field that aligns in-plane net moments and removes domain boundaries, the macroscopically-ordered isospins govern dynamic transport through the system, which leads to the extremely angle-sensitive AMR. As with this work that establishes a link between isospins and magnetotransport in strongly spin-orbit-coupled AFM Sr2IrO4, the peculiar AMR effect provides a beneficial foundation for fundamental and applied research on AFM spintronics.

cond-mat.str-el↗

arXiv2014

Magnetic Control of Ferroelectric Polarization in a Self-formed Single Magnetoelectric Domain of Multiferroic Ba3NbFe3Si2O14

We have discovered strong magnetoelectric (ME) effects in the single chiral-helical magnetic state of single-crystalline langasite Ba3NbFe3Si2O14 that is crystallographically chiral. The ferroelectric polarization, predominantly aligned along the a axis below the Neel temperature of ~27 K, changes in a highly non-linear fashion in the presence of in-plane magnetic fields (H) perpendicular to the a axis (H//b*). This ME effect as well as smaller ME effects in other directions exhibit no poling dependence, suggesting the presence of a self-formed single ME domain. In addition, these ME effects accompany no-measurable hysteresis, which is crucial for many technological applications.

cond-mat.str-el↗