Showing 1–32 of 32 results for author: Ko, K

Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

Authors: Jiarui Li, Christopher T. Parzyck, Eder G. Lomeli, Yidi Liu, Taehun Kim, Heemin Lee, Zengqing Zhuo, Eun Kyo Ko, Yaoju Tarn, Cheng-Tai Kuo, Ronny Sutarto, Chunjing Jia, Vivek Thampy, Jonathan Pelliciari, Wanli Yang, Brian Moritz, Yijun Yu, Jun-Sik Lee, Valentina Bisogni, Thomas P. Devereaux, Harold Y. Hwang, Wei-Sheng Lee

Abstract: The discovery of high-$T_c$ superconductivity in Ruddlesden-Popper nickelates has sparked substantial effort towards understanding unconventional electronic states beyond a traditional cuprate-like $d^9$ configurational ground state. An understanding of the interplay between magnetic ground states and multi-orbital physics is key for establishing a microscopic mechanism for superconductivity. In t… ▽ More The discovery of high-$T_c$ superconductivity in Ruddlesden-Popper nickelates has sparked substantial effort towards understanding unconventional electronic states beyond a traditional cuprate-like $d^9$ configurational ground state. An understanding of the interplay between magnetic ground states and multi-orbital physics is key for establishing a microscopic mechanism for superconductivity. In the bilayer nickelates, spin density wave (SDW) order is a prominent feature in the non-superconducting regime, yet its relation to superconducting pairing remains an open question. Here, we use resonant x-ray scattering to examine the existence of SDW order in superconducting bilayer nickelate thin films La$_2$PrNi$_2$O$_7$ (LPNO). Comparing superconducting and oxygen-deficient LPNO thin films, we find that superconductivity occurs in SDW-free, oxygen-stoichiometric regions, whereas oxygen-deficiency promotes SDW order, indicating phase segregation of SDW and superconductivity. Furthermore, Ni-$L_3$ and O-$K$ edge spectroscopy reveals distinct electronic structures - particularly along the $c$-axis - between the two regions. Our results identify oxygen stoichiometry as a key parameter controlling interlayer coupling and thus the electronic structure of bilayer nickelates. In concert with theory, we propose that a ligand hole primarily resides at the inter-bilayer apical oxygen, forming a robust interlayer five-spin polaron state, which serves as the ground state for superconducting bilayer nickelates. △ Less

Submitted 11 June, 2026; v1 submitted 4 May, 2026; originally announced May 2026.

Optical conductivity signatures of strong correlations and multiband superconductivity in infinite-layer nickelates

Authors: Woo Jin Kim, Kyuho Lee, Eun Kyo Ko, Jaeseok Son, Yonghun Lee, Yijun Yu, Soon Jae Moon, Tae Won Noh, Harold Y. Hwang

Abstract: Since the discovery of superconductivity in infinite-layer nickelates, there have been extensive efforts to unravel their electronic structure and pairing mechanism. In particular, understanding how the electronic structure evolves with doping is essential for clarifying theoretical models of superconductivity in nickelates. Here we present studies of the optical conductivity of Nd1-xSrxNiO2 thin… ▽ More Since the discovery of superconductivity in infinite-layer nickelates, there have been extensive efforts to unravel their electronic structure and pairing mechanism. In particular, understanding how the electronic structure evolves with doping is essential for clarifying theoretical models of superconductivity in nickelates. Here we present studies of the optical conductivity of Nd1-xSrxNiO2 thin films spanning the full phase diagram 0.025 < x < 0.30 using spectroscopic ellipsometry. The data are consistent with a two-band Drude model, which allows the decomposition of the intraband response into distinct contributions. One is from a "narrow" Drude term which we associate with electron bands, and the other a "broad" Drude term linked to the hole band with strong correlations. Increasing Sr doping leads to an expansion of the hole band spectral weight, and a corresponding reduction in the electron band, indicative of the multiband electronic structure and a doping-dependent reconstruction of the Fermi surface. Both doping and temperature-dependent optical spectra display significant spectral weight transfer from high to low energy, a hallmark of strong electronic correlations. In the superconducting state at optimal doping (x = 0.15), both electron and hole bands contribute to the superconducting condensate, signifying multiband superconductivity. △ Less

Submitted 10 February, 2026; originally announced February 2026.

Comments: 19 pages, 3 figures, 45 references

Electronic structure of compressively strained thin film La$_2$PrNi$_2$O$_7$

Authors: Bai Yang Wang, Yong Zhong, Sebastien Abadi, Yidi Liu, Yijun Yu, Xiaoliang Zhang, Yi-Ming Wu, Ruohan Wang, Jiarui Li, Yaoju Tarn, Eun Kyo Ko, Vivek Thampy, Makoto Hashimoto, Donghui Lu, Young S. Lee, Thomas P. Devereaux, Chunjing Jia, Harold Y. Hwang, Zhi-Xun Shen

Abstract: The discovery of superconductivity in the bulk nickelates under high pressure is a major advance in physics. The recent observation of superconductivity at ambient pressure in compressively strained bilayer nickelate thin films has now enabled direct characterization of the superconducting phase through angle resolved photoemission spectroscopy (ARPES). Here we present an in-situ ARPES study of co… ▽ More The discovery of superconductivity in the bulk nickelates under high pressure is a major advance in physics. The recent observation of superconductivity at ambient pressure in compressively strained bilayer nickelate thin films has now enabled direct characterization of the superconducting phase through angle resolved photoemission spectroscopy (ARPES). Here we present an in-situ ARPES study of compressively strained La$_2$PrNi$_2$O$_7$ films grown by oxide molecular beam epitaxy, and the ozone treated counterparts with an onset T$_c$ of 40 K, supplemented with results from pulsed laser deposition films with similar T$_c$. We resolve a systematic strain-driven electronic band shift with respect to that of bulk crystals, in qualitative agreement with density functional theory (DFT) calculations. However, the strongly renormalized flat 3$d_{z2}$ band shifts a factor of 5-10 smaller than anticipated by DFT. Furthermore, it stays ~70 meV below the Fermi level, contradicting the expectation that superconductivity results from the high density of states of this band at the Fermi level. We also observed a non-trivial k$_z$ dispersion of the cuprate-like 3$d_{x2-y2}$ band. Combined with results from both X-ray diffraction and DFT, we suggest that the strained films are under ~5 GPa effective pressure, considerably larger than the naïve expectation from the DFT relaxed structure. Finally, the ~70 meV energy position is intriguingly close to the collective mode coupling more prominently seen in thin films, in the energy range of both oxygen related phonons and the maximum of the spin excitation spectrum. △ Less

Submitted 23 April, 2025; v1 submitted 22 April, 2025; originally announced April 2025.

Resolving Structural Origins for Superconductivity in Strain-Engineered La$_3$Ni$_2$O$_7$ Thin Films

Authors: Lopa Bhatt, Abigail Y. Jiang, Eun Kyo Ko, Noah Schnitzer, Grace A. Pan, Dan Ferenc Segedin, Yidi Liu, Yijun Yu, Yi-Feng Zhao, Edgar Abarca Morales, Charles M. Brooks, Antia S. Botana, Harold Y. Hwang, Julia A. Mundy, David A. Muller, Berit H. Goodge

Abstract: The discovery of high-temperature superconductivity in bulk La$_3$Ni$_2$O$_7$ under high hydrostatic pressure and, more recently, biaxial compression in epitaxial thin films has ignited significant interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel-oxygen bonding environment are thought to be key drivers for stabilizing… ▽ More The discovery of high-temperature superconductivity in bulk La$_3$Ni$_2$O$_7$ under high hydrostatic pressure and, more recently, biaxial compression in epitaxial thin films has ignited significant interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel-oxygen bonding environment are thought to be key drivers for stabilizing superconductivity, but specific details of which bonds and which modifications are most relevant remains so far unresolved. While direct, atomic-scale structural characterization under hydrostatic pressure is beyond current experimental capabilities, static stabilization of strained La$_3$Ni$_2$O$_7$ films provides a platform well-suited to investigation with new picometer-resolution electron microscopy methods. Here, we use multislice electron ptychography to directly measure the atomic-scale structural evolution of La$_3$Ni$_2$O$_7$ thin films across a wide range of biaxial strains tuned via substrate. By resolving both the cation and oxygen sublattices, we study strain-dependent evolution of atomic bonds, providing the opportunity to isolate and disentangle the effects of specific structural motifs for stabilizing superconductivity. We identify the lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity. Rather than previously supposed $c$-axis compression, our results highlight the importance of in-plane biaxial compression in superconducting thin films, which suggests an alternative -- possibly cuprate-like -- understanding of the electronic structure. Identifying local regions of inhomogeneous oxygen stoichiometry and high internal strain near crystalline defects, we suggest potential pathways for improving the sharpness and temperature of the superconducting transition. △ Less

Submitted 14 January, 2025; originally announced January 2025.

Comments: 4 figures, 3 tables, 15 supplemental figures

Superconductivity and normal-state transport in compressively strained La$_2$PrNi$_2$O$_7$ thin films

Authors: Yidi Liu, Eun Kyo Ko, Yaoju Tarn, Lopa Bhatt, Jiarui Li, Vivek Thampy, Berit H. Goodge, David A. Muller, Srinivas Raghu, Yijun Yu, Harold Y. Hwang

Abstract: The discovery of superconductivity under high pressure in Ruddlesden-Popper phases of bulk nickelates has sparked great interest in stabilizing ambient pressure superconductivity in thin-film form using epitaxial strain. Recently, signs of superconductivity have been observed in compressively strained bilayer nickelate thin films with an onset temperature exceeding 40 K, albeit with broad and two-… ▽ More The discovery of superconductivity under high pressure in Ruddlesden-Popper phases of bulk nickelates has sparked great interest in stabilizing ambient pressure superconductivity in thin-film form using epitaxial strain. Recently, signs of superconductivity have been observed in compressively strained bilayer nickelate thin films with an onset temperature exceeding 40 K, albeit with broad and two-step-like transitions. Here, we report intrinsic superconductivity and normal-state transport properties in compressively strained La$_2$PrNi$_2$O$_7$ thin films, achieved through a combination of isovalent Pr substitution, growth optimization, and precision ozone annealing. The superconducting onset occurs above 48 K, with zero resistance reached above 30 K, and the critical current density at 1.4 K is 100-fold larger than previous reports. The normal-state resistivity exhibits quadratic temperature dependence indicative of Fermi liquid behaviour, and other phenomenological similarities to transport in overdoped cuprates suggest parallels in their emergent properties. △ Less

Submitted 3 September, 2025; v1 submitted 14 January, 2025; originally announced January 2025.

Comments: 18 pages, 4 figures. The updated manuscript includes an Author Correction for the sign error in the Hall coefficient

Unstable Retention Behavior in MIFIS FEFET: Accurate Analysis of the Origin by Absolute Polarization Measurement

Authors: Song-Hyeon Kuk, Kyul Ko, Bong Ho Kim, Jae-Hoon Han, Sang-Hyeon Kim

Abstract: Ferroelectric field-effect-transistor (FEFET) has emerged as a scalable solution for 3D NAND and embedded flash (eFlash), with recent progress in achieving large memory window (MW) using metal-insulator-ferroelectric-insulator-semiconductor (MIFIS) gate stacks. Although the physical origin of the large MW in the MIFIS stack has already been discussed, its retention characteristics have not been ex… ▽ More Ferroelectric field-effect-transistor (FEFET) has emerged as a scalable solution for 3D NAND and embedded flash (eFlash), with recent progress in achieving large memory window (MW) using metal-insulator-ferroelectric-insulator-semiconductor (MIFIS) gate stacks. Although the physical origin of the large MW in the MIFIS stack has already been discussed, its retention characteristics have not been explored yet. Here, we demonstrate MIFIS FEFET with a maximum MW of 9.7 V, and show that MIFIS FEFET has unstable retention characteristics, especially after erase. We discover the origin of the unstable retention characteristics and prove our hypothesis with absolute polarization measurement and different operation modes, showing that the unstable retention characteristics is a fundamental issue. Based on the understanding, we discuss a novel charge compensation model and promising engineering methodologies to achieve stable retention in MIFIS FEFET. △ Less

Submitted 27 June, 2024; originally announced June 2024.

Comments: We are submitting this to an IEEE journal but because of delays, we would like to share the information

Operando Analysis of Adsorption-Limited Hydrogen Oxidation Reaction at Palladium Surfaces

Authors: Yukun Liu, Kunmo Koo, Zugang Mao, Xianbiao Fu, Xiaobing Hu, Vinayak P. Dravid

Abstract: Palladium (Pd) catalysts have been extensively studied for the direct synthesis of H2O through the hydrogen oxidation reaction at ambient conditions. This heterogeneous catalytic reaction not only holds considerable practical significance but also serves as a classical model for investigating fundamental mechanisms, including adsorption and reactions between adsorbates. Nonetheless, the governing… ▽ More Palladium (Pd) catalysts have been extensively studied for the direct synthesis of H2O through the hydrogen oxidation reaction at ambient conditions. This heterogeneous catalytic reaction not only holds considerable practical significance but also serves as a classical model for investigating fundamental mechanisms, including adsorption and reactions between adsorbates. Nonetheless, the governing mechanisms and kinetics of its intermediate reaction stages under varying gas conditions remains elusive. This is attributed to the intricate interplay between adsorption, atomic diffusion, and concurrent phase transformation of catalyst. Herein, the Pd-catalyzed, water-forming hydrogen oxidation is studied, in situ, to investigate intermediate reaction stages via fluid cell transmission electron microscopy. The dynamic behaviors of water generation, associated with reversible palladium hydride formation, are captured in real time with a nanoscale spatial resolution. Our findings suggest that the hydrogen oxidation rate catalyzed by Pd is significantly affected by the sequence in which gases are introduced. Through direct evidence of electron diffraction and density functional theory calculation, we demonstrate that the hydrogen oxidation rate is limited by adsorption processes of gas precursors. These nanoscale insights help identify the optimal reaction conditions for Pd-catalyzed hydrogen oxidation, which has substantial implications for water production technologies. The developed understanding also advocates a broader exploration of analogous mechanisms in other metal-catalyzed reactions. △ Less

Submitted 22 April, 2024; originally announced April 2024.

Comments: 30 pages, 4 figures

Electron-Induced Radiation Chemistry in Environmental Transmission Electron Microscopy

Authors: Kunmo Koo, Nikhil S. Chellam, Sangyoon Shim, Chad A. Mirkin, George C. Schatz, Xiaobing Hu, Vinayak P. Dravid

Abstract: Environmental transmission electron microscopy (E-TEM) enables direct observation of nanoscale chemical processes crucial for catalysis and materials design. However, the high-energy electron probe can dramatically alter reaction pathways through radiolysis - the dissociation of molecules under electron beam irradiation. While extensively studied in liquid-cell TEM, the impact of radiolysis in gas… ▽ More Environmental transmission electron microscopy (E-TEM) enables direct observation of nanoscale chemical processes crucial for catalysis and materials design. However, the high-energy electron probe can dramatically alter reaction pathways through radiolysis - the dissociation of molecules under electron beam irradiation. While extensively studied in liquid-cell TEM, the impact of radiolysis in gas-phase reactions remains unexplored. Here, we present a numerical model elucidating radiation chemistry in both gas and liquid E-TEM environments. Our findings reveal that while gas-phase E-TEM generates radiolytic species with lower reactivity than liquid-phase systems, these species can accumulate to reaction-altering concentrations, particularly at elevated pressures. We validate our model through two case studies: the radiation-promoted oxidation of aluminum nanocubes and disproportionation of carbon monoxide. In both cases, increasing the electron beam dose rate directly accelerates their reaction kinetics, as demonstrated by enhanced AlOx growth and carbon deposition. Based on these insights, we establish practical guidelines for controlling radiolysis in closed-cell nanoreactors. This work not only resolves a fundamental challenge in electron microscopy but also advances our ability to rationally design materials with sub-Angstrom resolution. △ Less

Submitted 20 December, 2024; v1 submitted 27 February, 2024; originally announced February 2024.

Journal ref: ACS Nano 19 (10), 10369-10380 (2025)

Unlocking the Structural Mystery of Vaterite CaCO3

Authors: Xingyuan San, Junwei Hu, Mingyi Chen, Haiyang Niu, Paul J. M. Smeets, Jie Deng, Kunmo Koo, Roberto dos Reis, Vinayak P. Dravid, Xiaobing Hu

Abstract: Calcium carbonate (CaCO3), the most abundant biogenic mineral on earth, plays a crucial role in various fields. Of the four polymorphs, calcite, aragonite, vaterite, and amorphous CaCO3, vaterite is the most enigmatic one due to an ongoing debate regarding its structure that has persisted for nearly a century. In this work, based on systematic transmission electron microscopy characterizations, el… ▽ More Calcium carbonate (CaCO3), the most abundant biogenic mineral on earth, plays a crucial role in various fields. Of the four polymorphs, calcite, aragonite, vaterite, and amorphous CaCO3, vaterite is the most enigmatic one due to an ongoing debate regarding its structure that has persisted for nearly a century. In this work, based on systematic transmission electron microscopy characterizations, elaborate crystallographic analysis and machine learning aided molecular dynamics simulations with ab initio accuracy, we reveal that vaterite can be regarded as a polytypic structure. The basic phase is a monoclinic lattice possessing pseudohexagonal symmetry. Direct imaging and atomic-scale simulations provide evidence that a single grain of vaterite can have three orientation variants. Additionally, we find that vaterite undergoes a second-order phase transition. These atomic scale insights provide a comprehensive understanding of the structure of vaterite and offer new perspectives on the biomineralization process of calcium carbonate. △ Less

Submitted 13 June, 2023; originally announced June 2023.

Honeycomb oxide heterostructure: a new platform for Kitaev quantum spin liquid

Authors: Baekjune Kang, Miju Park, Sehwan Song, Seunghyun Noh, Daeseong Choe, Minsik Kong, Minjae Kim, Choongwon Seo, Eun Kyo Ko, Gangsan Yi, Jung-woo Yoo, Sungkyun Park, Jong Mok Ok, Changhee Sohn

Abstract: Kitaev quantum spin liquid, massively quantum entangled states, is so scarce in nature that searching for new candidate systems remains a great challenge. Honeycomb heterostructure could be a promising route to realize and utilize such an exotic quantum phase by providing additional controllability of Hamiltonian and device compatibility, respectively. Here, we provide epitaxial honeycomb oxide th… ▽ More Kitaev quantum spin liquid, massively quantum entangled states, is so scarce in nature that searching for new candidate systems remains a great challenge. Honeycomb heterostructure could be a promising route to realize and utilize such an exotic quantum phase by providing additional controllability of Hamiltonian and device compatibility, respectively. Here, we provide epitaxial honeycomb oxide thin film Na3Co2SbO6, a candidate of Kitaev quantum spin liquid proposed recently. We found a spin glass and antiferromagnetic ground states depending on Na stoichiometry, signifying not only the importance of Na vacancy control but also strong frustration in Na3Co2SbO6. Despite its classical ground state, the field-dependent magnetic susceptibility shows remarkable scaling collapse with a single critical exponent, which can be interpreted as evidence of quantum criticality. Its electronic ground state and derived spin Hamiltonian from spectroscopies are consistent with the predicted Kitaev model. Our work provides a unique route to the realization and utilization of Kitaev quantum spin liquid. △ Less

Submitted 8 February, 2023; v1 submitted 10 November, 2022; originally announced November 2022.

Tuning orbital-selective phase transitions in a two-dimensional Hund's correlated system

Authors: Eun Kyo Ko, Sungsoo Hahn, Changhee Sohn, Sangmin Lee, Seung-Sup B. Lee, Byungmin Sohn, Jeong Rae Kim, Jaeseok Son, Jeongkeun Song, Youngdo Kim, Donghan Kim, Miyoung Kim, Choong H. Kim, Changyoung Kim, Tae Won Noh

Abstract: Hund's rule coupling ($\textit{J}$) has attracted much attention recently for its role in the description of the novel quantum phases of multi orbital materials. Depending on the orbital occupancy, $\textit{J}$ can lead to various intriguing phases. However, experimental confirmation of the orbital occupancy dependency has been difficult as controlling the orbital degrees of freedom normally accom… ▽ More Hund's rule coupling ($\textit{J}$) has attracted much attention recently for its role in the description of the novel quantum phases of multi orbital materials. Depending on the orbital occupancy, $\textit{J}$ can lead to various intriguing phases. However, experimental confirmation of the orbital occupancy dependency has been difficult as controlling the orbital degrees of freedom normally accompanies chemical inhomogeneities. Here, we demonstrate a method to investigate the role of orbital occupancy in $\textit{J}$ related phenomena without inducing inhomogeneities. By growing SrRuO$_3$ monolayers on various substrates with symmetry-preserving interlayers, we gradually tune the crystal field splitting and thus the orbital degeneracy of the Ru $\textit{t$_2$$_g$}$ orbitals. It effectively varies the orbital occupancies of two-dimensional (2D) ruthenates. Via in-situ angle-resolved photoemission spectroscopy, we observe a progressive metal-insulator transition (MIT). It is found that the MIT occurs with orbital differentiation: concurrent opening of a band insulating gap in the $\textit{d$_x$$_y$}$ band and a Mott gap in the $\textit{d$_x$$_z$$_/$$_y$$_z$}$ bands. Our study provides an effective experimental method for investigation of orbital-selective phenomena in multi-orbital materials. △ Less

Submitted 18 October, 2022; v1 submitted 11 October, 2022; originally announced October 2022.

Heteroepitaxial control of Fermi liquid, Hund metal, and Mott insulator phases in the single-atomic-layer limit

Authors: Jeong Rae Kim, Byungmin Sohn, Hyeong Jun Lee, Sangmin Lee, Eun Kyo Ko, Sungsoo Hahn, Sangjae Lee, Younsik Kim, Donghan Kim, Hong Joon Kim, Youngdo Kim, Jaeseok Son, Charles H. Ahn, Frederick J. Walker, Ara Go, Miyoung Kim, Choong H. Kim, Changyoung Kim, Tae Won Noh

Abstract: Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities. In that respect, manipulating and measuring novel physical properties of oxide heterointerfaces are highly desired. Yet, despite extensive studies, obtaining direct information on their momentum-resolved electronic structure remains a great challenge. This is because most correlated interfacial phenom… ▽ More Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities. In that respect, manipulating and measuring novel physical properties of oxide heterointerfaces are highly desired. Yet, despite extensive studies, obtaining direct information on their momentum-resolved electronic structure remains a great challenge. This is because most correlated interfacial phenomena appear within a few atomic layers from the interface, thus limiting the application of available experimental probes. Here, we utilize atomic-scale epitaxy and photoemission spectroscopy to demonstrate the interface control of correlated electronic phases in atomic-scale ruthenate--titanate heterostructures. While bulk SrRuO$_3$ is a ferromagnetic metal, the heterointerfaces exclusively realize three distinct correlated phases in the single-atomic-layer limit. Our theory reveals that atomic-scale structural proximity effects lead to the emergence of Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO$_3$. These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure. △ Less

Submitted 8 March, 2022; originally announced March 2022.

Comments: 25 pages, 5 main figures, 16 supplementary figures

Flexoelectric control of a ferromagnetic metal

Authors: Wei Peng, Se Young Park, Chang Jae Roh, Junsik Mun, Hwiin Ju, Jinkwon Kim, Eun Kyo Ko, Zhengguo Liang, Sungsoo Hahn, Jinfeng Zhang, Liang Si, Yong Jin Jo, Tae Heon Kim, Changyoung Kim, Lingfei Wang, Miyoung Kim, Jong Seok Lee, Tae Won Noh, Daesu Lee

Abstract: Electric fields have played a key role in discovering and controlling exotic electronic states of condensed matter. However, electric fields usually do not work in metals as free carriers tend to screen electrostatic fields. While a pseudo-electric field generated by inhomogeneous lattice strain, namely a flexoelectric field, can in principle work in all classes of materials, it remains experiment… ▽ More Electric fields have played a key role in discovering and controlling exotic electronic states of condensed matter. However, electric fields usually do not work in metals as free carriers tend to screen electrostatic fields. While a pseudo-electric field generated by inhomogeneous lattice strain, namely a flexoelectric field, can in principle work in all classes of materials, it remains experimentally unexplored in metals. Here, using heteroepitaxy and atomic-scale imaging, we show that flexoelectric fields can polarize a metallic oxide SrRuO3 with unexpectedly large Ru off-center displacements. We also observe that the flexoelectrically induced polar state of SrRuO3 leads to sizable lattice expansion, similar to the electrostrictive expansion caused by ionic displacements in dielectrics under an external electric field. We further suggest that flexoelectrically driven Ru off-centering promotes strong coupling between lattice and electronic degrees of freedom, possibly enhancing the ferromagnetism of SrRuO3. Beyond conventional electric fields, flexoelectric fields may universally engender novel electronic states and their control via pure atomic displacements in a nondestructive and fast manner. △ Less

Submitted 7 March, 2022; originally announced March 2022.

Comments: 33 pages, 13 figures

CaCu$_3$Ru$_4$O$_{12}$: a high Kondo-temperature transition metal oxide

Authors: D. Takegami, C. Y. Kuo, K. Kasebayashi, J. -G. Kim, C. F. Chang, C. E. Liu, C. N. Wu, D. Kasinathan, S. G. Altendorf, K. Hoefer, F. Meneghin, A. Marino, Y. F. Liao, K. D. Tsuei, C. T. Chen, K. -T. Ko, A. Günther, S. G. Ebbinghaus, J. W. Seo, D. H. Lee, G. Ryu, A. C. Komarek, S. Sugano, Y. Shimakawa, A. Tanaka , et al. (4 additional authors not shown)

Abstract: We present a comprehensive study of CaCu$_3$Ru$_4$O$_{12}$ using bulk sensitive hard and soft x-ray spectroscopy combined with local-density approximation (LDA) + dynamical mean-field theory (DMFT) calculations. Correlation effects on both the Cu and Ru ions can be observed. From the Cu $2p$ core level spectra we deduce the presence of magnetic Cu$^{2+}$ ions hybridized with a reservoir of itinera… ▽ More We present a comprehensive study of CaCu$_3$Ru$_4$O$_{12}$ using bulk sensitive hard and soft x-ray spectroscopy combined with local-density approximation (LDA) + dynamical mean-field theory (DMFT) calculations. Correlation effects on both the Cu and Ru ions can be observed. From the Cu $2p$ core level spectra we deduce the presence of magnetic Cu$^{2+}$ ions hybridized with a reservoir of itinerant electrons. The strong photon energy dependence of the valence band allows us to disentangle the Ru, Cu, and O contributions and thus to optimize the DMFT calculations. The calculated spin and charge susceptibilities show that the transition metal oxide CaCu$_3$Ru$_4$O$_{12}$ must be classified as a Kondo system and that the Kondo temperature is in the range of 500-1000 K. △ Less

Submitted 6 December, 2021; originally announced December 2021.

Comments: Accepted for publication in Phys. Rev. X

Journal ref: Phys. Rev. X 12, 011017 (2022)

Observation of the orbital Hall effect in a light metal Ti

Authors: Young-Gwan Choi, Daegeun Jo, Kyung-Hun Ko, Dongwook Go, Kyung-Han Kim, Hee Gyum Park, Changyoung Kim, Byoung-Chul Min, Gyung-Min Choi, Hyun-Woo Lee

Abstract: The orbital angular momentum is a core ingredient of orbital magnetism, spin Hall effect, giant Rashba spin splitting, orbital Edelstein effect, and spin-orbit torque. However, its experimental detection is tricky. In particular, direct detection of the orbital Hall effect remains elusive despite its importance for electrical control of magnetic nanodevices. Here we report the direct observation o… ▽ More The orbital angular momentum is a core ingredient of orbital magnetism, spin Hall effect, giant Rashba spin splitting, orbital Edelstein effect, and spin-orbit torque. However, its experimental detection is tricky. In particular, direct detection of the orbital Hall effect remains elusive despite its importance for electrical control of magnetic nanodevices. Here we report the direct observation of the orbital Hall effect in a light metal Ti. The Kerr rotation by the accumulated orbital magnetic moment is measured at Ti surfaces, whose result agrees with theoretical calculations semiquantitatively and is supported by the orbital torque measurement in Ti-based magnetic heterostructures. The results confirm the electron orbital angular momentum as an essential dynamic degree of freedom, which may provide a novel mechanism for the electric control of magnetism. The results may also deepen the understanding of spin, valley, phonon, and magnon dynamics coupled with orbital dynamics. △ Less

Submitted 30 September, 2021; originally announced September 2021.

Journal ref: Nature 619, 52-56 (2023)

Direct observation of orbital driven strong interlayer coupling in puckered two-dimensional PdSe2

Authors: Jung Hyun Ryu, Jeong-Gyu Kim, Bongjae Kim, Kyoo Kim, Sooran Kim, Byeong-Gyu Park, Younghak Kim, Kyung-Tae Ko, Kimoon Lee

Abstract: Interlayer coupling between individual unit layers has played a critical role for layer-dependent properties in two-dimensional (2D) materials. While recent studies have revealed the significant degrees of interlayer interactions, the overall electronic structure of the 2D material has been mostly addressed by the intralayer interactions. Here, we report the direct observation of a highly dispersi… ▽ More Interlayer coupling between individual unit layers has played a critical role for layer-dependent properties in two-dimensional (2D) materials. While recent studies have revealed the significant degrees of interlayer interactions, the overall electronic structure of the 2D material has been mostly addressed by the intralayer interactions. Here, we report the direct observation of a highly dispersive single electronic band along the interlayer direction in puckered 2D PdSe2 as an experimental hallmark of strong interlayer couplings. Remarkably large band dispersion along kz-direction near Fermi level, which is even wider than the in-plane one, is observed by the angle-resolved photoemission spectroscopy measurement. Employing the X-ray absorption spectroscopy and density functional theory calculations, we reveal that the strong interlayer coupling in 2D PdSe2 originates from the unique directional bonding of Pd d orbitals associated with unexpected Pd 4d9 configuration, which consequently gives rise to the strong layer-dependency of the band gap. △ Less

Submitted 19 July, 2021; originally announced July 2021.

Journal ref: Small, 2022, Online Published

Two-channel anomalous Hall effect in SrRuO3

Authors: Graham Kimbell, Paul M. Sass, Bart Woltjes, Eun Kyo Ko, Tae Won Noh, Weida Wu, Jason W. A. Robinson

Abstract: The Hall effect in SrRuO$_3$ thin-films near the thickness limit for ferromagnetism shows an extra peak in addition to the ordinary and anomalous Hall effects. This extra peak has been attributed to a topological Hall effect due to two-dimensional skyrmions in the film around the coercive field; however, the sign of the anomalous Hall effect in SrRuO$_3$ can change as a function of saturation magn… ▽ More The Hall effect in SrRuO$_3$ thin-films near the thickness limit for ferromagnetism shows an extra peak in addition to the ordinary and anomalous Hall effects. This extra peak has been attributed to a topological Hall effect due to two-dimensional skyrmions in the film around the coercive field; however, the sign of the anomalous Hall effect in SrRuO$_3$ can change as a function of saturation magnetization. Here we report Hall peaks in SrRuO$_3$ in which volumetric magnetometry measurements and magnetic force microscopy indicate that the peaks result from the superposition of two anomalous Hall channels with opposite sign. These channels likely form due to thickness variations in SrRuO$_3$, creating two spatially separated magnetic regions with different saturation magnetizations and coercive fields. The results are central to the development of strongly correlated materials for spintronics. △ Less

Submitted 11 April, 2020; v1 submitted 14 February, 2020; originally announced February 2020.

Comments: Main: 7 pages, 6 figures. Supplementary: 10 pages, 12 figures

Journal ref: Phys. Rev. Materials 4, 054414 (2020)

Controllable thickness inhomogeneity and Berry-curvature-engineering of anomalous Hall effect in SrRuO3 ultrathin films

Authors: Lingfei Wang, Qiyuan Feng, Han Gyeol Lee, Eun Kyo Ko, Qingyou Lu, Tae Won Noh

Abstract: In quantum matters hosting electron-electron correlation and spin-orbit coupling, spatial inhomogeneities, arising from competing ground states, can be essential for determining and understanding topological properties. A prominent example is Hall anomalies observed in SrRuO3 films, which were interpreted in terms of either magnetic skyrmion-induced topological Hall effect (THE) or inhomogeneous a… ▽ More In quantum matters hosting electron-electron correlation and spin-orbit coupling, spatial inhomogeneities, arising from competing ground states, can be essential for determining and understanding topological properties. A prominent example is Hall anomalies observed in SrRuO3 films, which were interpreted in terms of either magnetic skyrmion-induced topological Hall effect (THE) or inhomogeneous anomalous Hall effect (AHE). To clarify this ambiguity, we systematically investigated the AHE of SrRuO3 ultrathin films with controllable inhomogeneities in film thickness (tSRO). By harnessing the step-flow growth of SrRuO3 films, we induced microscopically-ordered stripes with one-unit-cell differences in tSRO. The resultant spatial distribution of momentum-space Berry curvatures enables a two-channel AHE, which shows hump-like anomalies similar to the THE and can be continuously engineered via sub-unit-cell control of tSRO. In these inhomogeneous SRO films, we microscopically identified a two-step magnetic switching and stripe-like ferromagnetic domains. These features are fingerprints for distinguishing the two-channel AHE from the skyrmion-induced THE. △ Less

Submitted 22 August, 2019; originally announced August 2019.

Comments: 24 pages, 6 figures, 4 figures in Supplementary Materials

Increases of a Diamagnetic Property by Flux-Pinning in Volume Defect-Dominating Superconductors

Authors: H. B. Lee, G. C. Kim, Y. C. Kim, R. K. Ko, D. Y. Jeong

Abstract: Whereas there are two critical fields that are H$_{c1}$ and H$_{c2}$ in the ideal type II superconductor, there is another critical field H$_{c1}'$ defined as the field showing the maximum diamagnetic property in the real type II superconductor. We would present that H$_{c1}'$ is able to be proved theoretically and experimentally. We have derived an equation based on flux-pinning effect of volume… ▽ More Whereas there are two critical fields that are H$_{c1}$ and H$_{c2}$ in the ideal type II superconductor, there is another critical field H$_{c1}'$ defined as the field showing the maximum diamagnetic property in the real type II superconductor. We would present that H$_{c1}'$ is able to be proved theoretically and experimentally. We have derived an equation based on flux-pinning effect of volume defects. MgB$_2$ bulks which were synthesized by Mg and B are similar to this model. The number of quantum fluxes pinned at a defect of radius r, a pinning penetration depth, magnetic flux penetration method, and a magnetization at H$_{c1}'$ in the static state are suggested through the equation of the model. It was speculated that pinned fluxes at a volume defect in the superconductor have to be pick-out depinned from the defect and move an inside of the superconductor when pick-out forces of pinned fluxes is larger than pinning force of the defect (F$_{pickout}$ $>$ F$_{pinning}$) or when the shortest distance between pinned fluxes at a volume defect is the same as that of H$_{c2}$. In reality, $Δ$G$_{dynamic}$ which is sum of fluxes movement energy and fluxes vibration energy is involved in movement of pinned fluxes. When volume defects are small and many, the number of pinned fluxes at a volume defect calculated by experimental results was closer to that of ideally calculated ones because of a small $Δ$G$_{dynamic}$. However, when volume defects are large and a few, the number of pinned fluxes at a volume defect calculated by experimental results were much fewer than that of ideally calculated ones because of a large $Δ$G$_{dynamic}$. △ Less

Submitted 5 November, 2020; v1 submitted 12 April, 2019; originally announced April 2019.

Comments: 15 pages 6 figures

Large-Scale Conformal Growth of Atomic-Thick MoS2 for Highly Efficient Photocurrent Generation

Authors: Tri Khoa Nguyen, Anh Duc Nguyen, Chinh Tam Le, Farman Ullah, Kyo-in Koo, Eunah Kim, Dong-Wook Kim, Joon I. Jang, Yong Soo Kim

Abstract: Controlling the interconnection of neighboring seeds (nanoflakes) to full coverage of the textured substrate is the main challenge for the large-scale conformal growth of atomic-thick transition metal dichalcogenides by chemical vapor deposition. Herein, we report on a controllable method for the conformal growth of monolayer MoS2 on not only planar but also micro- and nano-rugged SiO2/Si substrat… ▽ More Controlling the interconnection of neighboring seeds (nanoflakes) to full coverage of the textured substrate is the main challenge for the large-scale conformal growth of atomic-thick transition metal dichalcogenides by chemical vapor deposition. Herein, we report on a controllable method for the conformal growth of monolayer MoS2 on not only planar but also micro- and nano-rugged SiO2/Si substrates via metal-organic chemical vapor deposition. The continuity of monolayer MoS2 on the rugged surface is evidenced by scanning electron microscopy, cross-section high-resolution transmission electron microscopy, photoluminescence (PL) mapping, and Raman mapping. Interestingly, the photo-responsivity (~254.5 mA/W) of as-grown MoS2 on the nano-rugged substrate exhibits 59 times higher than that of the planar sample (4.3 mA/W) under a small applied bias of 0.1 V. This value is record high when compared with all previous MoS2-based photocurrent generation under low or zero bias. Such a large enhancement in the photo-responsivity arises from a large active area for light-matter interaction and local strain for PL quenching, where the latter effect is the key factor and unique in the conformally grown monolayer on the nano-rugged surface. The result is a step toward the batch fabrication of modern atomic-thick optoelectronic devices. △ Less

Submitted 27 July, 2018; originally announced July 2018.

Comments: 25 pages, 5 figures

Restoration of Topological Surface State by Vacuum Annealing in Magnetically Doped Topological Insulator

Authors: Jinsu Kim, Eun-Ha Shin, Manoj K. Sharma, Kyuwook Ihm, Otgonbayar Dugerjav, Chanyong Hwang, Hwangho Lee, Kyung-Tae Ko, Jae-Hoon Park, Miyoung Kim, Hanchul Kim, Myung-Hwa Jung

Abstract: The introduction of magnetic order on the surface of topological insulators in general breaks the two-dimensional character of topological surface state (TSS). Once the TSS disappears, it is improbable to restore the topological surface properties. In this report, we demonstrate that it is possible to restore the inherent TSS by ultra-high vacuum annealing. Starting from an antiferromagnetic Gd-do… ▽ More The introduction of magnetic order on the surface of topological insulators in general breaks the two-dimensional character of topological surface state (TSS). Once the TSS disappears, it is improbable to restore the topological surface properties. In this report, we demonstrate that it is possible to restore the inherent TSS by ultra-high vacuum annealing. Starting from an antiferromagnetic Gd-doped Bi2Te3, that has surface state gap without TSS properties, after annealing we observed the gap closing as well as typical TSS features in physical properties. The microscopic mechanism of atomic migration and TSS restoration by annealing process is unraveled by the combination of scanning tunneling microscopy measurements and density functional theory calculations. This approach to control the surface of topological insulators and stabilize the TSS simply by vacuum annealing provides a new platform towards the exploitation of their topological properties. △ Less

Submitted 14 September, 2017; originally announced September 2017.

Comments: 21 pages, 4 figures, Supplementary information

Bulk and surface electronic properties of SmB6: a hard x-ray photoelectron spectroscopy study

Authors: Y. Utsumi, D. Kasinathan, K-T. Ko, S. Agrestini, M. W. Haverkort, S. Wirth, Y-H. Wu, K-D. Tsuei, D-J. Kim, Z. Fisk, A. Tanaka, P. Thalmeier, L. H. Tjeng

Abstract: We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) measurements on in-situ cleaved and ex-situ polished SmB6 single crystals. Using the multiplet-structure in the Sm 3d core level spectra, we determined reliably that the valence of Sm in bulk SmB6 is close to 2.55 at ~5 K. Temperature dependent measurements revealed that the Sm valence gradually increases to 2.64 at… ▽ More We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) measurements on in-situ cleaved and ex-situ polished SmB6 single crystals. Using the multiplet-structure in the Sm 3d core level spectra, we determined reliably that the valence of Sm in bulk SmB6 is close to 2.55 at ~5 K. Temperature dependent measurements revealed that the Sm valence gradually increases to 2.64 at 300 K. From a detailed line shape analysis we can clearly observe that not only the J=0 but also the J=1 state of the Sm 4f 6 configuration becomes occupied at elevated temperatures. Making use of the polarization dependence, we were able to identify and extract the Sm 4f spectral weight of the bulk material. Finally, we revealed that the oxidized or chemically damaged surface region of the ex-situ polished SmB6 single crystal is surprisingly thin, about 1 nm only. △ Less

Submitted 2 October, 2017; v1 submitted 9 May, 2017; originally announced May 2017.

Comments: 11 pages, 8 figures

Journal ref: Phys. Rev. B 96, 155130 (2017)

Strain-gradient-induced magnetic anisotropy in straight-stripe mixed-phase bismuth ferrites: An insight into flexomagnetic phenomenon

Authors: Jin Hong Lee, Kwang-Eun Kim, Byung-Kweon Jang, Ahmet A. Ünal, Sergio Valencia, Florian Kronast, Kyung-Tae Ko, Stefan Kowarik, Jan Seidel, Chan-Ho Yang

Abstract: Implementation of antiferromagnetic compounds as active elements in spintronics has been hindered by their insensitive nature against external perturbations which causes difficulties in switching among different antiferromagnetic spin configurations. Electrically-controllable strain gradient can become a key parameter to tune the antiferromagnetic states of multiferroic materials. We have discover… ▽ More Implementation of antiferromagnetic compounds as active elements in spintronics has been hindered by their insensitive nature against external perturbations which causes difficulties in switching among different antiferromagnetic spin configurations. Electrically-controllable strain gradient can become a key parameter to tune the antiferromagnetic states of multiferroic materials. We have discovered a correlation between an electrically-written straight-stripe mixed-phase boundary and an in-plane antiferromagnetic spin axis in highly-elongated La-5%-doped BiFeO$_{3}$ thin films by performing polarization-dependent photoemission electron microscopy in conjunction with cluster model calculations. Model Hamiltonian calculation for the single-ion anisotropy including the spin-orbit interaction has been performed to figure out the physical origin of the link between the strain gradient present in the mixed phase area and its antiferromagnetic spin axis. Our findings enable estimation of the strain-gradient-induced magnetic anisotropy energy per Fe ion at around 5$\times$10$^{-12}$ eV m, and provide a new pathway towards an electric-field-induced 90$^{\circ}$ rotation of antiferromagnetic spin axis at room temperature by flexomagnetism. △ Less

Submitted 21 April, 2017; originally announced April 2017.

Comments: 32 pages, 5 figures

Journal ref: Phys. Rev. B 96, 064402 (2017)

Electronically highly cubic conditions for Ru in alpha-RuCl3

Authors: S. Agrestini, C. -Y. Kuo, K. -T. Ko, Z. Hu, D. Kasinathan, H. Babu Vasili, J. Herrero-Martin, S. M. Valvidares, E. Pellegrin, L. -Y. Jang, A. Henschel, M. Schmidt, A. Tanaka, L. H. Tjeng

Abstract: We studied the local Ru 4d electronic structure of alpha-RuCl3 by means of polarization dependent x-ray absorption spectroscopy at the Ru-L2,3 edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru 4d local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal spl… ▽ More We studied the local Ru 4d electronic structure of alpha-RuCl3 by means of polarization dependent x-ray absorption spectroscopy at the Ru-L2,3 edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru 4d local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal splitting of the t2g orbitals is -12 $\pm10$ meV, i.e. negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent with our magnetic circular dichroism measurements, we found that the ratio of the orbital and spin moments is 2.0, the value expected for a Jeff = 1/2 ground state. We have thus shown that as far as the Ru 4d local properties are concerned, alpha-RuCl3 is an ideal candidate for the realization of Kitaev physics. △ Less

Submitted 17 April, 2017; originally announced April 2017.

Journal ref: Phys. Rev. B 96, 161107 (2017)

Long-range interactions in the effective low energy Hamiltonian of Sr2IrO4: a core level resonant inelastic x-ray scattering study

Authors: S. Agrestini, C. -Y. Kuo, M. Moretti Sala, Z. Hu, D. Kasinathan, K. -T. Ko, P. Glatzel, M. Rossi, J. -D. Cafun, K. O. Kvashnina, A. Matsumoto, T. Takayama, H. Takagi, L. H. Tjeng, M. W. Haverkort

Abstract: We have investigated the electronic structure of Sr2IrO4 using core level resonant inelastic x-ray scattering. The experimental spectra can be well reproduced using ab initio density functional theory based multiplet ligand field theory calculations, thereby validating these calculations. We found that the low-energy, effective Ir t2g orbitals are practically degenerate in energy. We uncovered tha… ▽ More We have investigated the electronic structure of Sr2IrO4 using core level resonant inelastic x-ray scattering. The experimental spectra can be well reproduced using ab initio density functional theory based multiplet ligand field theory calculations, thereby validating these calculations. We found that the low-energy, effective Ir t2g orbitals are practically degenerate in energy. We uncovered that covalency in Sr2IrO4, and generally in iridates, is very large with substantial oxygen ligand hole character in the Ir t2g Wannier orbitals. This has far reaching consequences, as not only the onsite crystal-field energies are determined by the long range crystal-structure, but, more significantly, magnetic exchange interactions will have long range distance dependent anisotropies in the spin direction. These findings set constraints and show pathways for the design of d^5 materials that can host compass-like magnetic interactions. △ Less

Submitted 30 November, 2016; originally announced December 2016.

Journal ref: Phys. Rev. B 95, 205123 (2017)

Electronic and Spin States of SrRuO3 Thin Films: an X-ray Magnetic Circular Dichroism Study

Authors: S. Agrestini, Z. Hu, C. -Y. Kuo, 1 M. W. Haverkort, K. -T. Ko, N. Hollmann, Q. Liu, E. Pellegrin, M. Valvidares, J. Herrero-Martin, P. Gargiani, P. Gegenwart, M. Schneider, S. Esser, A. Tanaka, A. C. Komarek, L. H. Tjeng

Abstract: We report a study of the local magnetism in thin films of SrRuO3 grown on (111) and (001) oriented SrTiO3 substrates using x-ray magnetic circular dichroism spectroscopy (XMCD) at the Ru-L2,3 edges. The application of the sum rules to the XMCD data gives an almost quenched orbital moment and a spin moment close to the value expected for the low spin state S = 1 . Full-multiplet cluster calculation… ▽ More We report a study of the local magnetism in thin films of SrRuO3 grown on (111) and (001) oriented SrTiO3 substrates using x-ray magnetic circular dichroism spectroscopy (XMCD) at the Ru-L2,3 edges. The application of the sum rules to the XMCD data gives an almost quenched orbital moment and a spin moment close to the value expected for the low spin state S = 1 . Full-multiplet cluster calculations indicate that the low spin state is quite stable and suggest that the occurrence of a transition to the high spin state S = 2 in strained thin films of SrRuO3 is unlikely as it would be too expensive in energy. △ Less

Submitted 27 January, 2015; originally announced January 2015.

Comments: 6 pages, 6 figures, submitted to Phys Rev B

Journal ref: Phys. Rev. B 91, 075127 (2015)

Contiguous 3d and 4f magnetism: towards strongly correlated 3d electrons in YbFe2Al10

Authors: P. Khuntia, P. Peratheepan, A. Strydom, Y. Utsumi, K. -T. Ko, K. -D. Tsuei, L. H. Tjeng, F. Steglich, M. Baenitz

Abstract: We present magnetization, specific heat, and 27Al NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T) divergence of the low-T specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard X-ray phot… ▽ More We present magnetization, specific heat, and 27Al NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T) divergence of the low-T specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard X-ray photo emission spectroscopy (HAXPES) study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, Yb3+ carries a full and stable moment, and Fe carries a moment of about 3.1 mB. The enhanced value of the Sommerfeld Wilson ratio and the dynamic scaling of spin-lattice relaxation rate divided by T [27(1/T1T)] with static susceptibility suggests admixed ferromagnetic correlations. 27(1/T1T) simultaneously tracks the valence fluctuations from the 4f -Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3d moments at low temperature, the latter evolve out of an Yb 4f admixed conduction band. △ Less

Submitted 15 October, 2014; v1 submitted 19 February, 2014; originally announced February 2014.

Comments: To appear in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 113, 216403 (2014)

Realization of giant magnetoelectricity in helimagnets

Authors: Sae Hwan Chun, Yi Sheng Chai, Yoon Seok Oh, Deepshikha Jaiswal-Nagar, So Young Haam, Ingyu Kim, Bumsung Lee, Dong Hak Nam, Kyung-Tae Ko, Jae-Hoon Park, Jae-Ho Chung, Kee Hoon Kim

Abstract: We show that low field magnetoelectric (ME) properties of helimagnets Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22 can be efficiently tailored by Al-substitution level. As x increases, the critical magnetic field for switching electric polarization is systematically reduced from ~1 T down to ~1 mT, and the ME susceptibility is greatly enhanced to reach a giant value of 2.0 x 10^4 ps/m at an optimum x = 0.08. We… ▽ More We show that low field magnetoelectric (ME) properties of helimagnets Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22 can be efficiently tailored by Al-substitution level. As x increases, the critical magnetic field for switching electric polarization is systematically reduced from ~1 T down to ~1 mT, and the ME susceptibility is greatly enhanced to reach a giant value of 2.0 x 10^4 ps/m at an optimum x = 0.08. We find that control of nontrivial orbital moment in the octahedral Fe sites through the Al-substitution is crucial for fine tuning of magnetic anisotropy and obtaining the conspicuously improved ME characteristics. △ Less

Submitted 21 January, 2010; v1 submitted 8 January, 2010; originally announced January 2010.

Journal ref: Phys. Rev. Lett. 104, 037204 (2010)

Coarsening Dynamics in a Two-dimensional XY model with Hamiltonian Dynamics

Authors: Kyo-Joon Koo, Woon-Bo Baek, Bongsoo Kim, Sung Jong Lee

Abstract: We investigate the coarsening dynamics in the two-dimensional Hamiltonian XY model on a square lattice, beginning with a random state with a specified potential energy and zero kinetic energy. Coarsening of the system proceeds via an increase in the kinetic energy and a decrease in the potential energy, with the total energy being conserved. We find that the coarsening dynamics exhibits a consis… ▽ More We investigate the coarsening dynamics in the two-dimensional Hamiltonian XY model on a square lattice, beginning with a random state with a specified potential energy and zero kinetic energy. Coarsening of the system proceeds via an increase in the kinetic energy and a decrease in the potential energy, with the total energy being conserved. We find that the coarsening dynamics exhibits a consistently superdiffusive growth of a characteristic length scale with 1/z > 1/2 (ranging from 0.54 to 0.57). Also, the number of point defects (vortices and antivortices) decreases with exponents ranging between 1.0 and 1.1. On the other hand, the excess potential energy decays with a typical exponent of 0.88, which shows deviations from the energy-scaling relation. The spin autocorrelation function exhibits a peculiar time dependence with non-power law behavior that can be fitted well by an exponential of logarithmic power in time. We argue that the conservation of the total Josephson (angular) momentum plays a crucial role for these novel features of coarsening in the Hamiltonian XY model. △ Less

Submitted 21 October, 2006; originally announced October 2006.

Comments: 7 pages, 4 figures

Effects of Al Addition on the Native Defects in Hafnia

Authors: Quan Li, K. M. Koo, W. M. Lau, P. F. Lee, J. Y. Dai, Z. F. Hou, X. G. Gong

Abstract: Two occupied native defect bands are detected in pure HfO2, with one located in the middle of the band gap and the other slightly above the valence band maximum. The investigation on the electronic structures of hafnium aluminate thin films as a function of Al concentration discloses the evolution of such bands while the density of states of the former one reduces drastically with the Al additio… ▽ More Two occupied native defect bands are detected in pure HfO2, with one located in the middle of the band gap and the other slightly above the valence band maximum. The investigation on the electronic structures of hafnium aluminate thin films as a function of Al concentration discloses the evolution of such bands while the density of states of the former one reduces drastically with the Al addition, that of the later one remains rather unaffected. Our first principles studies of the system attribute the two bands to the charged oxygen vacancy, and the oxygen interstitial related defect states of the HfO2, respectively. We further demonstrate that the observed evolution of the defect bands originates from the interaction in-between the added Al and the native defects of pure HfO2, which effectively passivates the VO+ induced mid-gap states but has little effect on other aspects of the electronic structure of the material. △ Less

Submitted 26 June, 2005; originally announced June 2005.

Comments: 14 pages

Jacobian Elliptic Wave Solutions for the Wadati-Segur-Ablowitz Equation

Authors: Chooi-Gim Rosy Teh, W. K. Koo, B. S. Lee

Abstract: Jacobian elliptic travelling wave solutions for a new Hamiltonian amplitude equation determining some instabilities of modulated wave train are obtained. By mere variation of the Jacobian elliptic parameter $k^2$ from zero to one, these solutions are transformed from a trivial one to the known solitary solutions. Jacobian elliptic travelling wave solutions for a new Hamiltonian amplitude equation determining some instabilities of modulated wave train are obtained. By mere variation of the Jacobian elliptic parameter $k^2$ from zero to one, these solutions are transformed from a trivial one to the known solitary solutions. △ Less

Submitted 18 July, 1996; originally announced July 1996.

Comments: 6 pages, no figure, latex

Simulations of deposition growth models in various dimensions. Are overhangs important?

Authors: David Y. K. Ko, Flavio Seno

Abstract: We present simulation results of deposition growth of surfaces in 2, 3 and 4 dimensions for ballistic deposition where overhangs are present, and for restricted solid on solid deposition where there are no overhangs. The values of the scaling exponents for the two models are found to be different, suggesting that they belong to different universality classes. We present simulation results of deposition growth of surfaces in 2, 3 and 4 dimensions for ballistic deposition where overhangs are present, and for restricted solid on solid deposition where there are no overhangs. The values of the scaling exponents for the two models are found to be different, suggesting that they belong to different universality classes. △ Less

Submitted 14 December, 1993; originally announced December 1993.

Comments: figures available from authors