Showing 1–50 of 95 results for author: Hong, H

Circular current induced by angular dynamics in swarmalator populations

Authors: Hyun Keun Lee, Hyunsuk Hong

Abstract: We propose a modified swarmalator model that generates collective rotational currents in phase synchronization. Our approach builds on the original swarmalator model [4], introducing a key modification: the phase-dependent terms in the spatial dynamics are replaced with a simpler driving term that depends on both the phase and a specified origin. We investigate the dynamics of this model through e… ▽ More We propose a modified swarmalator model that generates collective rotational currents in phase synchronization. Our approach builds on the original swarmalator model [4], introducing a key modification: the phase-dependent terms in the spatial dynamics are replaced with a simpler driving term that depends on both the phase and a specified origin. We investigate the dynamics of this model through extensive numerical simulations. When the origin is fixed, spiral motions of synchronized and clustered swarmalators emerge from a finite fraction of random initial conditions, resulting in collective currents. To prevent the unrealistic divergence of these spirals, we introduce a dynamic origin, defined as the center of the swarmalators' positions. With this dynamic origin, the system evolves into rotating collective currents, where synchronized swarmalators form stable circular patterns. In both the fixed and dynamic origin cases, we also observe no-current states, in which synchronized swarmalators aggregate near the origin. Finally, we find that the formation of collective currents can be facilitated by tuning the phase variables either at initialization or during the system's evolution. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Comments: 19 pages, 5 figures, to appear in J. Stat. Mech.: Theory Exp

Coherence enhanced by detrained oscillators: Breaking $π$-reflection symmetry

Authors: Hyunsuk Hong, Jae Sung Lee, Hyunggyu Park

Abstract: We study a generalized Kuramoto model in which each oscillator carries two coupled phase variables, representing a minimal swarmalator system. Assuming perfect correlation between the intrinsic frequencies associated with each phase variable, we identify a novel dynamic mode characterized by bounded oscillatory motion that breaks the $π$-reflection symmetry. This symmetry breaking enhances global… ▽ More We study a generalized Kuramoto model in which each oscillator carries two coupled phase variables, representing a minimal swarmalator system. Assuming perfect correlation between the intrinsic frequencies associated with each phase variable, we identify a novel dynamic mode characterized by bounded oscillatory motion that breaks the $π$-reflection symmetry. This symmetry breaking enhances global coherence and gives rise to a non-trivial mixed state, marked by distinct degrees of ordering in each variable. Numerical simulations confirm our analytic predictions for the full phase diagram, including the nature of transition. Our results reveal a fundamental mechanism through which detrained (dynamic) oscillators can promote global synchronization, offering broad insights into coupled dynamical systems beyond the classical Kuramoto paradigm. △ Less

Submitted 11 November, 2025; originally announced November 2025.

Comments: 15 pages, 5 figures. To appear in Chaos

Observation of tunable chiral spin textures with nonlinear optics

Authors: Youqiang Huang, Tiago V. C. Antao, Adolfo O. Fumega, Mikko Turunen, Yi Zhang, Hanlin Fang, Nianze Shang, Juan C. Arias-Munoz, Fedor Nigmatulin, Hao Hong, Andrew S. Kim, Faisal Ahmed, Hyunyong Choi, Sanshui Xiao, Kaihui Liu, Jose L. Lado, Zhipei Sun

Abstract: Chiral spin textures, such as spin spirals and skyrmions, are key to advancing spintronics by enabling ultrathin, energy-efficient memory, and high-density data storage and processing. However, their realization remains hindered by the scarcity of suitable host materials and the formidable experimental challenges associated with the characterization of these intricate chiral magnetic states. Here,… ▽ More Chiral spin textures, such as spin spirals and skyrmions, are key to advancing spintronics by enabling ultrathin, energy-efficient memory, and high-density data storage and processing. However, their realization remains hindered by the scarcity of suitable host materials and the formidable experimental challenges associated with the characterization of these intricate chiral magnetic states. Here, we report the observation of tunable chiral magnetic textures in van der Waals magnet CrPS$_4$ with nonlinear optics. These tunable textures exhibit strong chiral third-order nonlinear optical responses, driven by interlayer and intralayer spin couplings under varying magnetic fields and temperatures. These pronounced chiral nonlinear optical responses highlight the potency and high sensitivity of the nonlinear optical readout for probing non-collinear magnetic orders. Moreover, our findings position van der Waals magnets and their heterostructures as an exceptional platform for reconfigurable spin-photonics and spintronics, unifying optical, electrical, and magnetic properties through unique intralayer and interlayer spin coupling properties and effective spin interaction between photons and electrons. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Polymorphic spin ordering in a single-crystalline cobalt-doped Fe3GaTe2

Authors: Woohyun Cho, Jaehun Cha, Yoon-Gu Kang, Dong Hyun David Lee, Jaehwan Oh, Dohyun Kim, Sangsu Yer, Jaein Lee, Heemyoung Hong, Yongsoo Yang, Yeong Kwan Kim, Myung Joon Han, Heejun Yang

Abstract: A single crystalline system typically stabilizes a unique state for spin ordering below a critical temperature. Certain materials exhibit multiple magnetic states, driven by structural phase transitions under varying thermodynamic conditions. Recently, van der Waals magnets have demonstrated subtle interlayer exchange interactions, offering a promising approach to electrically control spin states… ▽ More A single crystalline system typically stabilizes a unique state for spin ordering below a critical temperature. Certain materials exhibit multiple magnetic states, driven by structural phase transitions under varying thermodynamic conditions. Recently, van der Waals magnets have demonstrated subtle interlayer exchange interactions, offering a promising approach to electrically control spin states without structural transformation. Here, we report the emergence of three distinct magnetic states, ferromagnetic ordering and both collinear and non-collinear antiferromagnetic orderings, in a layered single crystalline magnet, cobalt-doped Fe3GaTe2 ((Co, Fe)3GaTe2). These three magnetic phases occur without structural phase transitions, a phenomenon we designate as polymorphic spin ordering in the material. The introduction of 16% Co-doping in Fe3GaTe2 modulates the interlayer magnetic interaction, enabling multiple spin orderings within the same lattice system with three critical temperatures: a Curie temperature for a ferromagnetic state (Tc=210 K) and two Neel temperatures for the collinear (TN1=110 K) and non-collinear (TN2=30 K) antiferromagnetic states. Our findings are supported by magnetic force microscopy, first-principles calculations, and circular dichroism angular photoemission spectroscopy, which reveals varying spin ordering and changes in the topological band structure and Berry curvature at different temperatures within the single-crystalline (Co, Fe)3GaTe2. △ Less

Submitted 8 May, 2025; originally announced May 2025.

Advancing Antiferromagnetic Nitrides via Metal Alloy Nitridation

Authors: Qianying Wang, Zexu He, Lele Zhang, Qian Li, Haitao Hong, Ting Cui, Dongke Rong, Songhee Choi, Qiao Jin, Chen Ge, Can Wang, Qinghua Zhang, Liang Cheng, Jingbo Qi, Kui-juan Jin, Gang-Qin Liu, Er-Jia Guo

Abstract: Nitride materials, valued for their structural stability and exceptional physical properties, have garnered significant interest in both fundamental research and technological applications. The fabrication of high-quality nitride thin films is essential for advancing their use in microelectronics and spintronics. Yet, achieving single-crystal nitride thin films with excellent structural integrity… ▽ More Nitride materials, valued for their structural stability and exceptional physical properties, have garnered significant interest in both fundamental research and technological applications. The fabrication of high-quality nitride thin films is essential for advancing their use in microelectronics and spintronics. Yet, achieving single-crystal nitride thin films with excellent structural integrity remains a challenge. Here, we introduce a straightforward yet innovative metallic alloy nitridation technique for the synthesis of stable single-crystal nitride thin films. By subjecting metal alloy thin films to a controlled nitridation process, nitrogen atoms integrate into the lattice, driving structural transformations while preserving high epitaxial quality. Combining nanoscale magnetic imaging with a diamond nitrogen-vacancy (NV) probe, X-ray magnetic linear dichroism, and comprehensive transport measurements, we confirm that the nitridated films exhibit a robust antiferromagnetic character with a zero net magnetic moment. This work not only provides a refined and reproducible strategy for the fabrication of nitride thin films but also lays a robust foundation for exploring their burgeoning device applications. △ Less

Submitted 7 May, 2025; originally announced May 2025.

Comments: 22 pages; 4 figures

Charge transfer induced insulating state at antiperovskite/perovskite heterointerfaces

Authors: Ting Cui, Ying Zhou, Qianying Wang, Dongke Rong, Haitao Hong, Axin Xie, Jun-Jie Zhang, Qinghua Zhang, Can Wang, Chen Ge, Lin Gu, Shanmin Wang, Kuijuan Jin, Shuai Dong, Er-Jia Guo

Abstract: Heterointerfaces have been pivotal in unveiling extraordinary interfacial properties and enabling multifunctional material platforms. Despite extensive research on all-oxide interfaces, heterointerfaces between different material classes, such as oxides and nitrides, remain underexplored. Here we present the fabrication of a high-quality Dirac metal antiperovskite Ni3InN, characterized by an extre… ▽ More Heterointerfaces have been pivotal in unveiling extraordinary interfacial properties and enabling multifunctional material platforms. Despite extensive research on all-oxide interfaces, heterointerfaces between different material classes, such as oxides and nitrides, remain underexplored. Here we present the fabrication of a high-quality Dirac metal antiperovskite Ni3InN, characterized by an extremely low temperature coefficient of resistivity, approximately 1.8*10^-8 Ω*cm/K, over a broad temperature range. Atomically sharp heterointerfaces between Ni3InN and SrVO3 were constructed, revealing intriguing interfacial phenomena. Leveraging layer-resolved scanning transmission electron microscopy and electron energy loss spectroscopy, we identified pronounced charge transfer across the well-ordered interface. Remarkably, this interfacial electron transfer from Ni3InN to SrVO3 induces an insulating interfacial layer and an emergent magnetic moment within the Ni3InN layer, consistent with first-principles calculations. These findings pave the way for novel electronic and spintronic applications by enabling tunable interfacial properties in nitride/oxide systems. △ Less

Submitted 17 April, 2025; originally announced April 2025.

Comments: 15 pages; 4 figures

Roadmap for Photonics with 2D Materials

Authors: F. Javier García de Abajo, D. N. Basov, Frank H. L. Koppens, Lorenzo Orsini, Matteo Ceccanti, Sebastián Castilla, Lorenzo Cavicchi, Marco Polini, P. A. D. Gonçalves, A. T. Costa, N. M. R. Peres, N. Asger Mortensen, Sathwik Bharadwaj, Zubin Jacob, P. J. Schuck, A. N. Pasupathy, Milan Delor, M. K. Liu, Aitor Mugarza, Pablo Merino, Marc G. Cuxart, Emigdio Chávez-Angel, Martin Svec, Luiz H. G. Tizei, Florian Dirnberger , et al. (123 additional authors not shown)

Abstract: Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combinat… ▽ More Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combination with layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary collection of those directions, where 2D materials contribute with polaritons of unique characteristics such as strong spatial confinement, large optical-field enhancement, long lifetimes, high sensitivity to external stimuli (e.g., electric and magnetic fields, heating, and strain), a broad spectral range from the far infrared to the ultraviolet, and hybridization with spin and momentum textures of electronic band structures. The explosion of photonics with 2D materials as a vibrant research area is producing breakthroughs, including the discovery and design of new materials and metasurfaces with unprecedented properties as well as applications in integrated photonics, light emission, optical sensing, and exciting prospects for applications in quantum information, and nanoscale thermal transport. This Roadmap summarizes the state of the art in the field, identifies challenges and opportunities, and discusses future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners. △ Less

Submitted 14 April, 2025; v1 submitted 6 April, 2025; originally announced April 2025.

Comments: 199 pages, 42 figures, 1154 references

Journal ref: ACS Photonics 12, 3961 (2025)

Phase-matching of high harmonic generation in twisted solids

Authors: Chenjun Ma, Chen Huang, Yilong You, Huazhan Liu, Zhitong Ding, Mingchao Ding, Jin Zhang, Guixin Li, Zhipei Sun, Shiwei Wu, Chaojie Ma, Enge Wang, Hao Hong, Kaihui Liu

Abstract: High harmonic generation (HHG) in solids could enable attosecond and ultraviolet light sources with high compactness, great controllability and rich functions. However, the HHG process is accompanied by a quite large wavevector mismatch that is uncompensated by any traditional phase-matching method, directly limiting its energy conversion efficiency. Here, we propose an effective strategy for phas… ▽ More High harmonic generation (HHG) in solids could enable attosecond and ultraviolet light sources with high compactness, great controllability and rich functions. However, the HHG process is accompanied by a quite large wavevector mismatch that is uncompensated by any traditional phase-matching method, directly limiting its energy conversion efficiency. Here, we propose an effective strategy for phase-matching of HHG with arbitrary harmonic orders in solids. Two flakes of solids with an interlayer twist induce a nonlinear optical phase that depends on the crystal symmetry, twist angle and harmonic order, which can be accurately designed to compensate for the phase mismatch in HHG. Guided by the twist-phase-matching theory, we achieved a record-high conversion efficiency of $~1.5\times10^{-5}$ for the fifth HHG in twisted hexagonal boron nitride crystals with a total thickness of only 1 $μm$. Our work establishes a foundation for developing ultrashort-wavelength and ultrafast-pulse laser sources in compact solid-state tabletop systems for fundamental and applied sciences. △ Less

Submitted 11 March, 2025; originally announced March 2025.

Confined Magnetization at the Sublattice-Matched Ruthenium Oxide Heterointerface

Authors: Yiyan Fan, Qinghua Zhang, Ting Lin, He Bai, Chuanrui Huo, Qiao Jin, Tielong Deng, Songhee Choi, Shengru Chen, Haitao Hong, Ting Cui, Qianying Wang, Dongke Rong, Chen Liu, Chen Ge, Tao Zhu, Lin Gu, Kuijuan Jin, Jun Chen, Er-Jia Guo

Abstract: Creating a heterostructure by combining two magnetically and structurally distinct ruthenium oxides is a crucial approach for investigating their emergent magnetic states and interactions. Previously, research has predominantly concentrated on the intrinsic properties of the ferromagnet SrRuO3 and recently discovered altermagnet RuO2 solely. Here, we engineered an ultrasharp sublattice-matched het… ▽ More Creating a heterostructure by combining two magnetically and structurally distinct ruthenium oxides is a crucial approach for investigating their emergent magnetic states and interactions. Previously, research has predominantly concentrated on the intrinsic properties of the ferromagnet SrRuO3 and recently discovered altermagnet RuO2 solely. Here, we engineered an ultrasharp sublattice-matched heterointerface using pseudo-cubic SrRuO3 and rutile RuO2, conducting an in-depth analysis of their spin interactions. Structurally, to accommodate the lattice symmetry mismatch, the inverted RuO2 layer undergoes an in-plane rotation of 18 degrees during epitaxial growth on SrRuO3 layer, resulting in an interesting and rotational interface with perfect crystallinity and negligible chemical intermixing. Performance-wise, the interfacial layer of 6 nm in RuO2 adjacent to SrRuO3 exhibits a nonzero magnetic moment, contributing to an enhanced anomalous Hall effect (AHE) at low temperatures. Furthermore, our observations indicate that, in contrast to SrRuO3 single layers, the AHE of [(RuO2)15/(SrRuO3)n] heterostructures shows nonlinear behavior and reaches its maximum when the SrRuO3 thickness reaches tens of nm. These results suggest that the interfacial magnetic interaction surpasses that of all-perovskite oxides (~5-unit cells). This study underscores the significance and potential applications of magnetic interactions based on the crystallographic asymmetric interfaces in the design of spintronic devices. △ Less

Submitted 4 December, 2024; originally announced December 2024.

Comments: 30 pages/5 figures

Deteriorated Interlayer Coupling in Twisted Bilayer Cobaltites

Authors: Dongke Rong, Xiuqi Chen, Shengru Chen, Jingfeng Zhang, Yue Xu, Yanxing Shang, Haitao Hong, Ting Cui, Qianying Wang, Chen Ge, Can Wang, Qiang Zheng, Qinghua Zhang, Lingfei Wang, Yu Deng, Kuijuan Jin, Gang-Qin Liu, Er-Jia Guo

Abstract: A wealth of remarkable behaviors is observed at the interfaces between magnetic oxides due to the coexistence of Coulomb repulsion and interatomic exchange interactions. While previous research has focused on bonded oxide heterointerfaces, studies on magnetism in van der Waals interfaces remain rare. In this study, we stacked two freestanding cobaltites with precisely controlled twist angles. Scan… ▽ More A wealth of remarkable behaviors is observed at the interfaces between magnetic oxides due to the coexistence of Coulomb repulsion and interatomic exchange interactions. While previous research has focused on bonded oxide heterointerfaces, studies on magnetism in van der Waals interfaces remain rare. In this study, we stacked two freestanding cobaltites with precisely controlled twist angles. Scanning transmission electron microscopy revealed clear and ordered moiré patterns, which exhibit an inverse relationship with the twist angle. We found that the Curie temperature in the twisted region is reduced by approximately 13 K compared to the single-layer region using nitrogen-vacancy (NV) magnetometry. This phenomenon may be related to the weakening of the orbital hybridization between oxygen ions and transition metal ions in the unbonded interfaces. Our findings suggest a potential avenue for modulating magnetic interactions in correlated systems through twist, providing opportunities for the discovery of unknown quantum states. △ Less

Submitted 22 April, 2025; v1 submitted 3 December, 2024; originally announced December 2024.

Comments: 42 pages,3 figures

A single-phase epitaxially grown ferroelectric perovskite nitride

Authors: Songhee Choi, Qiao Jin, Xian Zi, Dongke Rong, Jie Fang, Jinfeng Zhang, Qinghua Zhang, Wei Li, Shuai Xu, Shengru Chen, Haitao Hong, Cui Ting, Qianying Wang, Gang Tang, Chen Ge, Can Wang, Zhiguo Chen, Lin Gu, Qian Li, Lingfei Wang, Shanmin Wang, Jiawang Hong, Kuijuan Jin, Er-Jia Guo

Abstract: The integration of ferroelectrics with semiconductors is crucial for developing functional devices, such as field-effect transistors, tunnel junctions, and nonvolatile memories. However, the synthesis of high-quality single-crystalline ferroelectric nitride perovskites has been limited, hindering a comprehensive understanding of their switching dynamics and potential applications. Here we report t… ▽ More The integration of ferroelectrics with semiconductors is crucial for developing functional devices, such as field-effect transistors, tunnel junctions, and nonvolatile memories. However, the synthesis of high-quality single-crystalline ferroelectric nitride perovskites has been limited, hindering a comprehensive understanding of their switching dynamics and potential applications. Here we report the synthesis and characterizations of epitaxial single-phase ferroelectric cerium tantalum nitride (CeTaN3) on both oxides and semiconductors. The polar symmetry of CeTaN3 was confirmed by observing the atomic displacement of central ions relative to the center of the TaN6 octahedra, as well as through optical second harmonic generation. We observed switchable ferroelectric domains in CeTaN3 films using piezo-response force microscopy, complemented by the characterization of square-like polarization-electric field hysteresis loops. The remanent polarization of CeTaN3 reaches approximately 20 uC/cm2 at room temperature, consistent with theoretical calculations. This work establishes a vital link between ferroelectric nitride perovskites and their practical applications, paving the way for next-generation information and energy-storage devices with enhanced performance, scalability, and manufacturability. △ Less

Submitted 21 April, 2025; v1 submitted 22 October, 2024; originally announced October 2024.

Comments: 47 pages, 4 figures

Validity of annealed approximation in a high-dimensional system

Authors: Jaegon Um, Hyunsuk Hong, Hyunggyu Park

Abstract: This study investigates the suitability of the annealed approximation in high-dimensional systems characterized by dense networks with quenched link disorder, employing models of coupled oscillators. We demonstrate that dynamic equations governing dense-network systems converge to those of the complete-graph version in the thermodynamic limit, where link disorder fluctuations vanish entirely. Cons… ▽ More This study investigates the suitability of the annealed approximation in high-dimensional systems characterized by dense networks with quenched link disorder, employing models of coupled oscillators. We demonstrate that dynamic equations governing dense-network systems converge to those of the complete-graph version in the thermodynamic limit, where link disorder fluctuations vanish entirely. Consequently, the annealed-network systems, where fluctuations are attenuated, also exhibit the same dynamic behavior in the thermodynamic limit. However, a significant discrepancy arises in the incoherent (disordered) phase wherein the finite-size behavior becomes critical in determining the steady-state pattern. To explicitly elucidate this discrepancy, we focus on identical oscillators subject to competitive attractive and repulsive couplings. In the incoherent phase of dense networks, we observe the manifestation of random irregular states. In contrast, the annealed approximation yields a symmetric (regular) incoherent state where two oppositely coherent clusters of oscillators coexist, accompanied by the vanishing order parameter. Our findings imply that the annealed approximation should be employed with caution even in dense-network systems, particularly in the disordered phase. △ Less

Submitted 22 March, 2024; originally announced March 2024.

Journal ref: Sci. Rep. vol 14, 6816 (2024)

Metal-to-insulator transition in oxide semimetals by anion doping

Authors: Haitao Hong, Huimin Zhang, Shan Lin, Jeffrey A. Dhas, Binod Paudel, Shuai Xu, Shengru Chen, Ting Cui, Yiyan Fan, Dongke Rong, Qiao Jin, Zihua Zhu, Yingge Du, Scott A. Chambers, Chen Ge, Can Wang, Qinghua Zhang, Le Wang, Kui-juan Jin, Shuai Dong, Er-Jia Guo

Abstract: Oxide semimetals exhibiting both nontrivial topological characteristics stand as exemplary parent compounds and multiple degrees of freedom, offering great promise for the realization of novel electronic states. In this study, we present compelling evidence of profound structural and transport phase shifts in a recently uncovered oxide semimetal, SrNbO3, achieved through effective in-situ anion do… ▽ More Oxide semimetals exhibiting both nontrivial topological characteristics stand as exemplary parent compounds and multiple degrees of freedom, offering great promise for the realization of novel electronic states. In this study, we present compelling evidence of profound structural and transport phase shifts in a recently uncovered oxide semimetal, SrNbO3, achieved through effective in-situ anion doping. Notably, a remarkable increase in resistivity of more than three orders of magnitude at room temperature is observed upon nitrogen-doping. The extent of electronic modulation in SrNbO3 is strongly correlated with the misfit strain, underscoring its phase instability to both chemical doping and crystallographic symmetry variations. Using first-principles calculations, we discern that elevating the level of nitrogen doping induces an upward shift in the conductive bands of SrNbO3-dNd. Consequently, a transition from a metallic state to an insulating state becomes apparent as the nitrogen concentration reaches a threshold of 1/3. This investigation sheds light on the potential of anion engineering in oxide semimetals, offering pathways for manipulating their physical properties. These insights hold promise for future applications that harness these materials for tailored functionalities. △ Less

Submitted 27 November, 2023; originally announced November 2023.

Comments: 18 pages, 4 figures

Strain mediated phase crossover in Ruddlesden Popper nickelates

Authors: Ting Cui, Songhee Choi, Ting Lin, Chen Liu, Gang Wang, Ningning Wang, Shengru Chen, Haitao Hong, Dongke Rong, Qianying Wang, Qiao Jin, Jia-Ou Wang, Lin Gu, Chen Ge, Can Wang, Jin Guang Cheng, Qinghua Zhang, Liang Si, Kui-juan Jin, Er-Jia Guo

Abstract: Recent progress on the signatures of pressure-induced high temperature superconductivity in Ruddlesden Popper (RP) nickelates (Lan+1NinO3n+1) has attracted growing interest in both theoretical calculations and experimental efforts. The fabrication of high-quality single crystalline RP nickelate thin films is critical for possible reducing the superconducting transition pressure and advancing appli… ▽ More Recent progress on the signatures of pressure-induced high temperature superconductivity in Ruddlesden Popper (RP) nickelates (Lan+1NinO3n+1) has attracted growing interest in both theoretical calculations and experimental efforts. The fabrication of high-quality single crystalline RP nickelate thin films is critical for possible reducing the superconducting transition pressure and advancing applications in microelectronics in the future. In this study, we report the observations of an active phase transition in RP nickelate films induced by misfit strain. We found that RP nickelate films favor the perovskite structure (n = infinite) under tensile strains, while compressive strains stabilize the La3Ni2O7 (n = 2) phase. The selection of distinct phases is governed by the strain dependent formation energy and electronic configuration. In compressively strained La3Ni2O7, we experimentally determined splitting energy is ~0.2 eV and electrons prefer to occupy in-plane orbitals. First principles calculations unveil a robust coupling between strain effects and the valence state of Ni ions in RP nickelates, suggesting a dual driving force for the inevitable phase co-existence transition in RP nickelates. Our work underscores the sensitivity of RP nickelate formation to epitaxial strain, presenting a significant challenge in fabricating pure-phase RP nickelate films. Therefore, special attention to stacking defects and grain boundaries between different RP phases is essential when discussing the pressure-induced superconductivity in RP nickelates. △ Less

Submitted 22 November, 2023; originally announced November 2023.

Comments: 29 pages, 5 figures, one supplementary materials

Suppression of water vapor condensation by glycerol droplets on hydrophobic surfaces

Authors: Zhan-Long Wang, Haonan Zhao, Zhen Xu, He Hong

Abstract: Vapor sink is an effective strategy to suppress the formation of water vapor condensation around hygroscopic materials, and consequently reduce frosting and icing. However, traditionally used materials, such as salt solutions, fibers, exhibit insufficient condensation inhibition or pose safety concerns, such as corrosiveness. In this study, we highlight the remarkable anti-condensation properties… ▽ More Vapor sink is an effective strategy to suppress the formation of water vapor condensation around hygroscopic materials, and consequently reduce frosting and icing. However, traditionally used materials, such as salt solutions, fibers, exhibit insufficient condensation inhibition or pose safety concerns, such as corrosiveness. In this study, we highlight the remarkable anti-condensation properties of glycerol droplets, attributed to their strong hygroscopicity. We compared the anti-condensation capabilities of glycerol droplets with commonly used salt solutions and hygroscopic alcohols. The results indicated that glycerol droplets establish a relatively expansive dry zone where the condensation is effectively inhibited, while also offering safety compared to other materials. Furthermore, we conducted a systematic study of the anti-condensation properties of glycerol droplets with experiments and theoretical analysis. We explored the varying trends of the dry zone ratio concerning temperature, cooling time, humidity, and droplet volume on hydrophobic surfaces. To provide a comprehensive understanding, we propose a straightforward yet robust theoretical model that elucidates the relationship between this ratio and temperature, aligning well with the experimental data. Our study not only sheds light on the superior anti-condensation qualities of glycerol but also offers insights and guidelines for the development of effective anti-icing and anti-frost materials. △ Less

Submitted 8 November, 2023; v1 submitted 6 November, 2023; originally announced November 2023.

Giant nonlinear optical wave mixing in van der Waals compound MnPSe3

Authors: Li Yue, Chang Liu, Shanshan Han, Hao Hong, Yijun Wang, Qiaomei Liu, Jiajie Qi, Yuan Li, Dong Wu, Kaihui Liu, Enge Wang, Tao Dong, Nanlin Wang

Abstract: Optical nonlinearities, one of the most fascinating properties of two-dimensional (2D) materials, are essential for exploring novel physics in 2D systems and developing next-generation nonlinear optical applications. While tremendous efforts have been made to discover and optimize second-order nonlinear optical responses in various 2D materials, higher odd-order nonlinear processes, which are in g… ▽ More Optical nonlinearities, one of the most fascinating properties of two-dimensional (2D) materials, are essential for exploring novel physics in 2D systems and developing next-generation nonlinear optical applications. While tremendous efforts have been made to discover and optimize second-order nonlinear optical responses in various 2D materials, higher odd-order nonlinear processes, which are in general much less efficient than second order ones, have been paid less attention despite their scientific and applicational significance. Here we report giant odd-order nonlinear optical wave mixing in a correlated van der Waals insulator MnPSe3 at room temperature. Illuminated by two near-infrared femtosecond lasers simultaneously, it generates a series of degenerate and non-degenerate four- and six-wave mixing outputs, with conversion efficiencies up to the order of $10^{-4}$ and $10^{-6}$ for the four- and six-wave mixing processes, respectively, far exceeding the efficiencies of several prototypical nonlinear optical materials (GaSe, LiNbO3). This work highlights the intriguing prospect of transition metal phosphorous trichalcogenides for future research of the nonlinear light matter interactions in 2D systems and for potential nonlinear photonic applications. △ Less

Submitted 28 September, 2023; originally announced October 2023.

Comments: 17 pages, 4 figures

In-situ scattering studies of superconducting vacancy-ordered monoclinic TiO thin films

Authors: Merve Baksi, Hawoong Hong, Divine P. Kumah

Abstract: We investigate the structural and transport properties of vacancy-ordered monoclinic superconducting $\mathrm{TiO}$ thin films grown by molecular beam epitaxy. The evolution of the crystal structure during growth is monitored by in-situ synchrotron X-ray diffraction. Long-range ordering of Ti and O vacancies in the disordered cubic phase stabilizes the vacancy-ordered monoclinic TiO phase. The red… ▽ More We investigate the structural and transport properties of vacancy-ordered monoclinic superconducting $\mathrm{TiO}$ thin films grown by molecular beam epitaxy. The evolution of the crystal structure during growth is monitored by in-situ synchrotron X-ray diffraction. Long-range ordering of Ti and O vacancies in the disordered cubic phase stabilizes the vacancy-ordered monoclinic TiO phase. The reduced structural disorder arising from vacancy-ordering is correlated with a superconductor-metal transition (SMT) in contrast to the superconductor-insulator transition (SIT) observed in cubic TiO, orthorhombic $Ti_2O_3$, and the Magneli $γ-Ti_3O_5$ and $γ-Ti_4O_7$ phase. Magnetoresistance measurements for the SIT phases indicate superconducting fluctuations persisting in the normal phase. These results confirm the role of disorder related to Ti and O vacancies and structural inhomogeneity in determining the electronic properties of the normal state of titanium oxide-based superconductors. △ Less

Submitted 18 September, 2023; v1 submitted 26 May, 2023; originally announced May 2023.

Journal ref: Adv. Physics Res. 2023, 2300086

Syntropic spin alignment at the interface between ferromagnetic and superconducting nitrides

Authors: Qiao Jin, Qinghua Zhang, Bai He, Yuting Zou, Yonglong Ga, Shengru Chen, Haitao Hong, Ting Cui, Dongke Rong, Jia-Ou Wang, Can Wang, Yanwei Cao, Lin Gu, Shanmin Wang, Kun Jiang, Zhi-Gang Cheng, Tao Zhu, Hongxin Yang, Kui-juan Jin, Er-Jia Guo

Abstract: The magnetic correlations at the superconductor/ferromagnet (S/F) interfaces play a crucial role in realizing dissipation-less spin-based logic and memory technologies, such as triplet-supercurrent spin-valves and "π" Josephson junctions. Here we report the coexistence of an induced large magnetic moment and a crypto ferromagnetic state at high-quality nitride S/F interfaces. Using polarized neutr… ▽ More The magnetic correlations at the superconductor/ferromagnet (S/F) interfaces play a crucial role in realizing dissipation-less spin-based logic and memory technologies, such as triplet-supercurrent spin-valves and "π" Josephson junctions. Here we report the coexistence of an induced large magnetic moment and a crypto ferromagnetic state at high-quality nitride S/F interfaces. Using polarized neutron reflectometry and d. c. SQUID measurements, we quantitatively determined the magnetization profile of S/F bilayer and confirmed the induced magnetic moment in the adjacent superconductor only exists below TC. Interestingly, the direction of the induced moment in the superconductors was unexpectedly parallel to that in the ferromagnet, which contrasts with earlier findings in S/F heterostructures based on metals or oxides. The first-principles calculations verify the observed unusual interfacial spin texture is caused by the Heisenberg direct exchange coupling through d orbital overlapping and severe charge transfer across the interfaces. Our work establishes an incisive experimental probe for understanding the magnetic proximity behavior at S/F interfaces and provides a prototype epitaxial building block for superconducting spintronics. △ Less

Submitted 11 April, 2023; originally announced April 2023.

Comments: 21 pages, 5 figures, supplementary file with 14 figures

Swarmalators with thermal noise

Authors: Hyunsuk Hong, Kevin P. O'Keeffe, Jae Sung Lee, Hyunggyu Park

Abstract: We investigate a population of swarmalators, a mobile version of phase oscillators that both sync in time and swarm through space. We focus on a XY-type model of identical swarmalators running on a one-dimensional ring and subject to thermal noise. We uncover four distinct collective states, some of which capture the behavior of real-world swarmalators such as vinegar eels and sperm. Among these,… ▽ More We investigate a population of swarmalators, a mobile version of phase oscillators that both sync in time and swarm through space. We focus on a XY-type model of identical swarmalators running on a one-dimensional ring and subject to thermal noise. We uncover four distinct collective states, some of which capture the behavior of real-world swarmalators such as vinegar eels and sperm. Among these, the most intriguing is the `mixed state', which blends two of the other states. We present a comprehensive phase diagram from the Fourier mode analysis with a high accuracy, which is in excellent agreement with numerical simulation results. Our model serves as a tractable toy model for thermal systems that both self-synchronize and self-assemble interdependently. △ Less

Submitted 23 February, 2023; originally announced February 2023.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. Research 5, 023105 (2023)

Miniature Magnetic Nano islands in a Morphotropic Cobaltite Matrix

Authors: Shengru Chen, Dongke Rong, Yue Xu, Miming Cai, Xinyan Li, Qinghua Zhang, Shuai Xu, Yan-Xing Shang, Haitao Hong, Ting Cui, Qiao Jin, Jia-Ou Wang, Haizhong Guo, Lin Gu, Qiang Zheng, Can Wang, Jinxing Zhang, Gang-Qin Liu, Kui-juan Jin, Er-Jia Guo

Abstract: High-density magnetic memories are key components in spintronics, quantum computing, and energy-efficient electronics. Reduced dimensionality and magnetic domain stability at the nanoscale are essential for the miniaturization of magnetic storage units. Yet, inducing magnetic order, and selectively tuning spin-orbital coupling at specific locations have remained challenging. Here we demonstrate th… ▽ More High-density magnetic memories are key components in spintronics, quantum computing, and energy-efficient electronics. Reduced dimensionality and magnetic domain stability at the nanoscale are essential for the miniaturization of magnetic storage units. Yet, inducing magnetic order, and selectively tuning spin-orbital coupling at specific locations have remained challenging. Here we demonstrate the construction of switchable magnetic nano-islands in a nonmagnetic matrix based on cobaltite homo-structures. The magnetic and electronic states are laterally modified by epitaxial strain, which is regionally controlled by freestanding membranes. Atomically sharp grain boundaries isolate the crosstalk between magnetically distinct regions. The minimal size of magnetic nano-islands reaches 35 nm in diameter, enabling an areal density of 400 Gbit per inch square. Besides providing an ideal platform for precisely controlled read and write schemes, this methodology can enable scalable and patterned memories on silicon and flexible substrates for various applications. △ Less

Submitted 14 January, 2023; originally announced January 2023.

Comments: 20 pages,4 figures

Synthesis of functional nitride membranes using sacrificial water-soluble BaO layers

Authors: Shengru Chen, Qiao Jin, Shan Lin, Haitao Hong, Ting Cui, Dongke Rong, Guozhu Song, Shanmin Wang, Kuijuan Jin, Qiang Zheng, Er-Jia Guo

Abstract: Transition metal nitrides (TMNs) exhibit fascinating physical properties that hold great potential in future device applications. To stack two-dimensional TMNs with other functional materials that have dissimilar orientations and symmetries requires to separate epitaxial TMNs from the growth substrates. However, the lattice constants of TMNs are not compatible with those of most sacrificial layers… ▽ More Transition metal nitrides (TMNs) exhibit fascinating physical properties that hold great potential in future device applications. To stack two-dimensional TMNs with other functional materials that have dissimilar orientations and symmetries requires to separate epitaxial TMNs from the growth substrates. However, the lattice constants of TMNs are not compatible with those of most sacrificial layers, leading to a great challenge to fabricate high-quality single crystalline TMN membranes. In this letter, we report the application of a water-soluble BaO sacrificial layer as a general approach to create freestanding TMN membranes. Taken CrN as an example, the relatively small lattice mismatch and identical cubic structure between BaO and CrN ensure the growth of heterostructures. Millimeter-size CrN membrane allows us to directly observe the planar-view of atomic structure and to correlate its electronic state with intrinsic transport properties. Our work provides the opportunity to fabricate freestanding TMN membranes and the ability to transfer them to arbitrary substrates. The integration of TMN membranes with other materials will stimulate further studies in the emergent phenomena at heterointerfaces. △ Less

Submitted 17 December, 2022; v1 submitted 27 November, 2022; originally announced November 2022.

Comments: 16 pages; 3 figures

Emergent magnetic states and tunable exchange bias at all 3d nitride heterointerfaces

Authors: Qiao Jin, Qinghua Zhang, He Bai, Amanda Huon, Timothy Charlton, Shengru Chen, Shan Lin, Haitao Hong, Ting Cui, Can Wang, Haizhong Guo, Lin Gu, Tao Zhu, Michael R. Fitzsimmons, Kui-juan Jin, Shanmin Wang, Er-Jia Guo

Abstract: Interfacial magnetism stimulates the discovery of giant magnetoresistance and spin-orbital coupling across the heterointerfaces, facilitating the intimate correlation between spin transport and complex magnetic structures. Over decades, functional heterointerfaces composed of nitrides are seldomly explored due to the difficulty in synthesizing high-quality and correct composition nitride films. He… ▽ More Interfacial magnetism stimulates the discovery of giant magnetoresistance and spin-orbital coupling across the heterointerfaces, facilitating the intimate correlation between spin transport and complex magnetic structures. Over decades, functional heterointerfaces composed of nitrides are seldomly explored due to the difficulty in synthesizing high-quality and correct composition nitride films. Here we report the fabrication of single-crystalline ferromagnetic Fe3N thin films with precisely controlled thickness. As film thickness decreasing, the magnetization deteriorates dramatically, and electronic state transits from metallic to insulating. Strikingly, the high-temperature ferromagnetism maintains in a Fe3N layer with a thickness down to 2 u. c. (~ 8 Å). The magnetoresistance exhibits a strong in-plane anisotropy and meanwhile the anomalous Hall resistance reserves its sign when Fe3N layer thickness exceeds 5 u. c. Furthermore, we observe a sizable exchange bias at the interfaces between a ferromagnetic Fe3N and an antiferromagnetic CrN. The exchange bias field and saturation moment strongly depend on the controllable bending curvature using cylinder diameter engineering (CDE) technique, implying the tunable magnetic states under lattice deformation. This work provides a guideline for exploring functional nitride films and applying their interfacial phenomena for innovative perspectives towards the practical applications. △ Less

Submitted 12 September, 2022; originally announced September 2022.

Improved Numerical Scheme for the Generalized Kuramoto Model

Authors: Hyun Keun Lee, Hyunsuk Hong, Joonhyun Yeo

Abstract: We present an improved and more accurate numerical scheme for a generalization of the Kuramoto model of coupled phase oscillators to the three-dimensional space. The present numerical scheme relies crucially on our observation that the generalized Kuramoto model corresponds to particles on the unit sphere undergoing rigid body rotations with position-dependent angular velocities. We demonstrate th… ▽ More We present an improved and more accurate numerical scheme for a generalization of the Kuramoto model of coupled phase oscillators to the three-dimensional space. The present numerical scheme relies crucially on our observation that the generalized Kuramoto model corresponds to particles on the unit sphere undergoing rigid body rotations with position-dependent angular velocities. We demonstrate that our improved scheme is able to reproduce known analytic results and capture the expected behavior of the three-dimensional oscillators in various cases. On the other hand, we find that the conventional numerical method, which amounts to a direct numerical integration with the constraint that forces the particles to be on the unit sphere at each time step, may result in inaccurate and misleading behavior especially in the long time limit. We analyze in detail the origin of the discrepancy between the two methods and present the effectiveness of our method in studying the limit cycle of the Kuramoto oscillators. △ Less

Submitted 10 May, 2023; v1 submitted 15 August, 2022; originally announced August 2022.

Comments: 9 pages, 6 figures, to appear in J. Stat. Mech.: Theory Exp

Braiding lateral morphotropic grain boundary in homogeneitic oxides

Authors: Shengru Chen, Qinghua Zhang, Dongke Rong, Yue Xu, Jinfeng Zhang, Fangfang Pei, He Bai, Yan-Xing Shang, Shan Lin, Qiao Jin, Haitao Hong, Can Wang, Wensheng Yan, Haizhong Guo, Tao Zhu, Lin Gu, Yu Gong, Qian Li, Lingfei Wang, Gang-Qin Liu, Kui-juan Jin, Er-Jia Guo

Abstract: Interfaces formed by correlated oxides offer a critical avenue for discovering emergent phenomena and quantum states. However, the fabrication of oxide interfaces with variable crystallographic orientations and strain states integrated along a film plane is extremely challenge by conventional layer-by-layer stacking or self-assembling. Here, we report the creation of morphotropic grain boundaries… ▽ More Interfaces formed by correlated oxides offer a critical avenue for discovering emergent phenomena and quantum states. However, the fabrication of oxide interfaces with variable crystallographic orientations and strain states integrated along a film plane is extremely challenge by conventional layer-by-layer stacking or self-assembling. Here, we report the creation of morphotropic grain boundaries (GBs) in laterally interconnected cobaltite homostructures. Single-crystalline substrates and suspended ultrathin freestanding membranes provide independent templates for coherent epitaxy and constraint on the growth orientation, resulting in seamless and atomically sharp GBs. Electronic states and magnetic behavior in hybrid structures are laterally modulated and isolated by GBs, enabling artificially engineered functionalities in the planar matrix. Our work offers a simple and scalable method for fabricating unprecedented innovative interfaces through controlled synthesis routes as well as provides a platform for exploring potential applications in neuromorphics, solid state batteries, and catalysis. △ Less

Submitted 13 July, 2022; originally announced July 2022.

Comments: 38 pages, 4 main figures

Atomically engineered cobaltite layers for robust ferromagnetism

Authors: Shengru Chen, Qinghua Zhang, Xujing Li, Jiali Zhao, Shan Lin, Qiao Jin, Haitao Hong, Amanda Huon, Timothy Charlton, Qian Li, Wensheng Yan, Jiaou Wang, Chen Ge, Can Wang, Baotian Wang, Michael R. Fitzsimmons, Haizhong Guo, Lin Gu, Wen Yin, Kuijuan Jin, Er Jia Guo

Abstract: Emergent phenomena at heterointerfaces are directly associated with the bonding geometry of adjacent layers. Effective control of accessible parameters, such as the bond length and bonding angles, offers an elegant method to tailor competing energies of the electronic and magnetic ground states. In this study, we construct unit thick syntactic layers of cobaltites within a strongly tilted octahedr… ▽ More Emergent phenomena at heterointerfaces are directly associated with the bonding geometry of adjacent layers. Effective control of accessible parameters, such as the bond length and bonding angles, offers an elegant method to tailor competing energies of the electronic and magnetic ground states. In this study, we construct unit thick syntactic layers of cobaltites within a strongly tilted octahedral matrix via atomically precise synthesis. The octahedral tilt patterns of adjacent layers propagate into cobaltites, leading to a continuation of octahedral tilting while maintaining significant misfit tensile strain. These effects induce severe rumpling within an atomic plane of neighboring layers triggers the electronic reconstruction between the splitting orbitals. First-principles calculations reveal that the cobalt ions transits to a higher spin state level upon octahedral tilting, resulting in robust ferromagnetism in ultrathin cobaltites. This work demonstrates a design methodology for fine-tuning the lattice and spin degrees of freedom in correlated quantum heterostructures by exploiting epitaxial geometric engineering. △ Less

Submitted 7 July, 2022; originally announced July 2022.

Comments: 18 pages, 5 figures

Asymmetric Ground States in La$_{0.67}$Sr$_{0.33}$MnO$_3$/BaTiO$_3$ heterostructures Induced by Flexoelectric Bending

Authors: Mingqun Qi, Zhen Yang, Shengru Chen, Shan Lin, Qiao Jin, Haitao Hong, Dongke Rong, Haizhong Guo, Can Wang, Kui-juan Jin, Zhenping Wu, Er-Jia Guo

Abstract: Misfit strain delivered from single-crystal substrates typically modifies the ground states of transition metal oxides, generating increasing interests in designing modern transducers and sensors. Here, we demonstrate that magnetotransport properties of La$_{0.67}$Sr$_{0.33}$MnO$_3$ (LSMO) films were continuously tuned by uniaxial strain produced by a home-designed bending jig. The electrical cond… ▽ More Misfit strain delivered from single-crystal substrates typically modifies the ground states of transition metal oxides, generating increasing interests in designing modern transducers and sensors. Here, we demonstrate that magnetotransport properties of La$_{0.67}$Sr$_{0.33}$MnO$_3$ (LSMO) films were continuously tuned by uniaxial strain produced by a home-designed bending jig. The electrical conductivity and Curie temperature of LSMO films are enhanced by bending stresses. The resistivity of a u-shape bended LSMO decays three times faster than that of a n-shape bended LSMO as a response to the same magnitude of strain. The asymmetric magnetic states in uniaxially strained LSMO are attributed to the dual actions of Jahn-Teller distortion and strain gradient mediated flexoelectric fields in an adjacent ferroelectric layer. These findings of multi-field regulation in a single material provide a feasible means for developing flexible electronic and spintronic devices. △ Less

Submitted 7 July, 2022; originally announced July 2022.

Comments: 17 pages, 4 figures

Swarmalators on a ring with distributed couplings

Authors: Kevin O'Keeffe, Hyunsuk Hong

Abstract: We study a simple model of identical swarmalators, generalizations of phases oscillators that swarm through space. We confine the movements to a one-dimensional (1D) ring and consider distributed (non-identical) couplings; the combination of these two effects captures an aspect of the more realistic 2D swarmalator model \cite{o2017oscillators}. We find new collective states as well as generalizati… ▽ More We study a simple model of identical swarmalators, generalizations of phases oscillators that swarm through space. We confine the movements to a one-dimensional (1D) ring and consider distributed (non-identical) couplings; the combination of these two effects captures an aspect of the more realistic 2D swarmalator model \cite{o2017oscillators}. We find new collective states as well as generalizations of previously reported ones which we describe analytically. These states imitate the behavior of vinegar eels, catalytic microswimmers, and other swarmalators which move on quasi-1D rings. △ Less

Submitted 12 September, 2022; v1 submitted 18 April, 2022; originally announced April 2022.

Free energy landscape of two-state protein Acylphosphatase with large contact order revealed by force-dependent folding and unfolding dynamics

Authors: Xuening Ma, Hao Sun, Haiyan Hong, Zilong Guo, Huanhuan Su, Hu Chen

Abstract: Acylphosphatase (AcP) is a small protein with 98 amino acid residues that catalyzes the hydrolysis of carboxyl-phosphate bonds. AcP is a typical two-state protein with slow folding rate due to its relatively large contact order in the native structure. The mechanical properties and unfolding behavior of AcP has been studied by atomic force microscope. But the folding and unfolding dynamics at low… ▽ More Acylphosphatase (AcP) is a small protein with 98 amino acid residues that catalyzes the hydrolysis of carboxyl-phosphate bonds. AcP is a typical two-state protein with slow folding rate due to its relatively large contact order in the native structure. The mechanical properties and unfolding behavior of AcP has been studied by atomic force microscope. But the folding and unfolding dynamics at low forces has not been reported. Here using stable magnetic tweezers, we measured the force-dependent folding rates within a force range from 1 pN to 3 pN, and unfolding rates from 15 pN to 40 pN. The obtained unfolding rates show different force sensitivities at forces below and above ~27 pN, which determines a free energy landscape with two energy barriers. Our results indicate that the free energy landscape of small globule proteins have general Bactrian camel shape, and large contact order of the native state produces a high barrier dominate at low forces. △ Less

Submitted 11 March, 2022; originally announced March 2022.

Comments: 21 pages, 9 figures

Anisotropic electronic phase transition in CrN epitaxial thin films

Authors: Qiao Jin, Jiali Zhao, Manuel Roldan, Shan Lin, Shengru Chen, Haitao Hong, Yiyan Fan, Dongke Rong, Haizhong Guo, Chen Ge, Can Wang, Jia-Ou Wang, Shanmin Wang, Kui-juan Jin, Er-Jia Guo

Abstract: Electronic phase transition in strongly correlated materials is extremely sensitive to the dimensionality and crystallographic orientations. Transition metal nitrides (TMNs) are seldom investigated due to the difficulty in fabricating the high-quality and stoichiometric single crystals. In this letter, we report the epitaxial growth and electronic properties of CrN films on different-oriented NdGa… ▽ More Electronic phase transition in strongly correlated materials is extremely sensitive to the dimensionality and crystallographic orientations. Transition metal nitrides (TMNs) are seldom investigated due to the difficulty in fabricating the high-quality and stoichiometric single crystals. In this letter, we report the epitaxial growth and electronic properties of CrN films on different-oriented NdGaO3 (NGO) substrates. Astonishingly, the CrN films grown on (110)-oriented NGO substrates maintain a metallic phase, whereas the CrN films grown on (010)-oriented NGO substrates are semiconducting. We attribute the unconventional electronic transition in the CrN films to the strongly correlation with epitaxial strain. The effective modulation of bandgap by the anisotropic strain triggers the metal-to-insulator transition consequently. This work provides a convenient approach to modify the electronic ground states of functional materials using anisotropic strain and further stimulates the investigations of TMNs. △ Less

Submitted 20 November, 2021; originally announced November 2021.

Coupling disorder in a population of swarmalators

Authors: Hyunsuk Hong, Kangmo Yeo, Hyun Keun Lee

Abstract: We consider a population of two-dimensional oscillators with random couplings, and explore the collective states. The coupling strength between oscillators is randomly quenched with two values one of which is positive while the other is negative, and the oscillators can spatially {\it{move}} depending on the state variables for phase and position. We find that the system shows the phase transition… ▽ More We consider a population of two-dimensional oscillators with random couplings, and explore the collective states. The coupling strength between oscillators is randomly quenched with two values one of which is positive while the other is negative, and the oscillators can spatially {\it{move}} depending on the state variables for phase and position. We find that the system shows the phase transition from the incoherent state to the fully synchronized one at a proper ratio of the number of positive couplings to the total. The threshold is numerically measured, and analytically predicted by the linear stability analysis of the fully synchronized state. It is found that the random couplings induces the long-term state patterns appearing for constant strength. The oscillators move to the places where the randomly quenched couplings work as if annealed. We further observe that the system with mixed randomnesses for quenched couplings shows the combination of the deformed patterns understandable with each annealed averages. △ Less

Submitted 29 October, 2021; v1 submitted 31 August, 2021; originally announced August 2021.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. E 104, 044214 (2021)

Absence of moment fragmentation in the mixed $B$-site pyrochlore Nd$_{2}$GaSbO$_{7}$

Authors: S. J. Gomez, P. M. Sarte, M. Zelensky, A. M. Hallas, B. A. Gonzalez, K. H. Hong, E. J. Pace, S. Calder, M. B. Stone, Y. Su, E. Feng, M. D. Le, C. Stock, J. P. Attfield, S. D. Wilson, C. R. Wiebe, A. A. Aczel

Abstract: Nd-based pyrochlore oxides of the form Nd$_{2}B_{2}$O$_{7}$ have garnered a significant amount of interest owing to the moment fragmentation physics observed in Nd$_{2}$Zr$_{2}$O$_{7}$ and speculated in Nd$_{2}$Hf$_{2}$O$_{7}$. Notably this phenomenon is not ubiquitous in this family, as it is absent in Nd$_{2}$Sn$_{2}$O$_{7}$, which features a smaller ionic radius on the $B$-site. Here, we explor… ▽ More Nd-based pyrochlore oxides of the form Nd$_{2}B_{2}$O$_{7}$ have garnered a significant amount of interest owing to the moment fragmentation physics observed in Nd$_{2}$Zr$_{2}$O$_{7}$ and speculated in Nd$_{2}$Hf$_{2}$O$_{7}$. Notably this phenomenon is not ubiquitous in this family, as it is absent in Nd$_{2}$Sn$_{2}$O$_{7}$, which features a smaller ionic radius on the $B$-site. Here, we explore the necessary conditions for moment fragmentation in the Nd pyrochlore family through a detailed study of the mixed $B$-site pyrochlore Nd$_{2}$GaSbO$_{7}$. The $B$-site of this system is characterized by significant disorder and an extremely small average ionic radius. Similarly to Nd$_{2}$Sn$_{2}$O$_{7}$, we find no evidence for moment fragmentation through our bulk characterization and neutron scattering experiments, indicating that chemical pressure (and not necessarily the $B$-site disorder) plays a key role in the presence or absence of this phenomenon in this material family. Surprisingly, the presence of significant $B$-site disorder in Nd$_{2}$GaSbO$_{7}$ does not generate a spin glass ground state and instead the same all-in-all-out magnetic order identified in other Nd pyrochlores is found here. △ Less

Submitted 1 April, 2021; originally announced April 2021.

Comments: 11 pages, 8 figures

Journal ref: Phys. Rev. B 103, 214419 (2021)

Collective steady-state patterns of swarmalators with finite-cutoff interaction distance

Authors: Hyun Keun Lee, Kangmo Yeo, Hyunsuk Hong

Abstract: We study the steady-state patterns of population of the coupled oscillators that sync and swarm, where the interaction distances among oscillators have finite-cutoff in interaction distance. We examine how the static patterns known in the infinite-cutoff are reproduced or deformed, and explore a new static pattern that does not appear until a finite-cutoff is considered. All steady-state patterns… ▽ More We study the steady-state patterns of population of the coupled oscillators that sync and swarm, where the interaction distances among oscillators have finite-cutoff in interaction distance. We examine how the static patterns known in the infinite-cutoff are reproduced or deformed, and explore a new static pattern that does not appear until a finite-cutoff is considered. All steady-state patterns of the infinite-cutoff, static sync, static async, and static phase wave are respectively repeated in space for proper finite-cutoff ranges. Their deformation in shape and density takes place for the other finite-cutoff ranges. Bar-like phase wave states are observed, which has not been the case for the infinite-cutoff. All the patterns are investigated via numerical and theoretical analysis. △ Less

Submitted 22 March, 2021; originally announced March 2021.

Comments: 9 pages, 8 figures

Journal ref: Chaos 31, 033134 (2021)

Van Vleck excitons in Ca$_{2}$RuO$_{4}$

Authors: P. M. Sarte, C. Stock, B. R. Ortiz, K. H. Hong, S. D. Wilson

Abstract: A framework is presented for modeling and understanding magnetic excitations in localized, intermediate coupling magnets where the interplay between spin-orbit coupling, magnetic exchange, and crystal field effects are known to create a complex landscape of unconventional magnetic behaviors and ground states. A spin-orbit exciton approach for modeling these excitations is developed based upon a Ha… ▽ More A framework is presented for modeling and understanding magnetic excitations in localized, intermediate coupling magnets where the interplay between spin-orbit coupling, magnetic exchange, and crystal field effects are known to create a complex landscape of unconventional magnetic behaviors and ground states. A spin-orbit exciton approach for modeling these excitations is developed based upon a Hamiltonian which explicitly incorporates single-ion crystalline electric field and spin exchange terms. This framework is then leveraged to understand a canonical Van Vleck $j\rm{_{eff}}=0$ singlet ground state whose excitations are coupled spin and crystalline electric field levels. Specifically, the anomalous Higgs mode [Jain et al. Nat. Phys. 13, 633 (2017)], spin-waves [S. Kunkemöller et al. Phys. Rev. Lett. 115, 247201 (2015)], and orbital excitations [L. Das et al. Phys. Rev. X 8, 011048 (2018)] in the multiorbital Mott insulator Ca$_2$RuO$_4$ are captured and good agreement is found with previous neutron and inelastic x-ray spectroscopic measurements. Furthermore, our results illustrate how a crystalline electric field-induced singlet ground state can support coherent longitudinal, or amplitude excitations, and transverse wavelike dynamics. We use this description to discuss mechanisms for accessing a nearby critical point. △ Less

Submitted 14 November, 2020; originally announced November 2020.

Comments: 18 pages, 8 figures

Journal ref: Phys. Rev. B 102, 245119 (2020)

Unravelling a Zigzag Pathway for Hot-Carrier Collection at CH3NH3PbI3/Graphene Interfaces

Authors: Jin Zhang, Hao Hong, Jincan Zhang, Chunchun Wu, Hailin Peng, Kaihui Liu, Sheng Meng

Abstract: The capture of photoexcited deep-band hot carriers, excited by photons with energies far above the bandgap, is of significant importance for photovoltaic and photoelectronic applications since it is directly related to the quantum efficiency of photon-to-electron conversion. By employing time-resolved photoluminescence and state-of-the-art time-domain density functional theory, we reveal that phot… ▽ More The capture of photoexcited deep-band hot carriers, excited by photons with energies far above the bandgap, is of significant importance for photovoltaic and photoelectronic applications since it is directly related to the quantum efficiency of photon-to-electron conversion. By employing time-resolved photoluminescence and state-of-the-art time-domain density functional theory, we reveal that photoexcited hot carriers in organic-inorganic hybrid perovskites prefer a zigzag interfacial charge-transfer pathway, i.e., the hot carriers transfer back and forth between CH3NH3PbI3 and graphene, before they reach a charge separated state. Driven by quantum coherence and interlayer vibrational modes, this pathway at the semiconductor-graphene interface takes about 400 femtoseconds, much faster than the relaxation process within CH3NH3PbI3 (in several picoseconds). We further demonstrate that the transfer rate of the pathway can be further enhanced by interfacial defects. Our work provides a new insight for the fundamental understanding and precise manipulation of hot-carrier dynamics at the complex semiconductor-graphene interfaces, paving the way for highly efficient photovoltaic and photoelectric device optimization. △ Less

Submitted 27 July, 2020; originally announced July 2020.

Thermodynamic cost of synchronizing a population of beating cilia

Authors: Hyunsuk Hong, Junghyo Jo, Changbong Hyeon, Hyunggyu Park

Abstract: Synchronization among arrays of beating cilia is one of the emergent phenomena in biological processes at meso-scopic scales. Strong inter-ciliary couplings modify the natural beating frequencies, $ω$, of individual cilia to produce a collective motion that moves around a group frequency $ω_m$. Here we study the thermodynamic cost of synchronizing cilia arrays by mapping their dynamics onto a gene… ▽ More Synchronization among arrays of beating cilia is one of the emergent phenomena in biological processes at meso-scopic scales. Strong inter-ciliary couplings modify the natural beating frequencies, $ω$, of individual cilia to produce a collective motion that moves around a group frequency $ω_m$. Here we study the thermodynamic cost of synchronizing cilia arrays by mapping their dynamics onto a generic phase oscillator model. The model suggests that upon synchronization the mean heat dissipation rate is decomposed into two contributions, dissipation from each cilium's own natural driving force and dissipation arising from the interaction with other cilia, the latter of which can be interpreted as the one produced by a potential with a time-dependent protocol in the framework of our model. The spontaneous phase-synchronization of beating dynamics of cilia induced by strong inter-ciliary coupling is always accompanied with a significant reduction of dissipation for the cilia population, suggesting that organisms as a whole expend less energy by attaining a temporal order. At the level of individual cilia, however, a population of cilia with $|ω|< ω_m$ expend more amount of energy upon synchronization. △ Less

Submitted 23 March, 2020; originally announced March 2020.

Comments: 6 pages, 3 figures

Journal ref: J. Stat. Mech. (2020) 074001

Spin-Orbit Excitons in CoO

Authors: P. M. Sarte, M. Songvilay, E. Pachoud, R. A. Ewings, C. D. Frost, D. Prabhakaran, K. H. Hong, A. J. Browne, Z. Yamani, J. P. Attfield, E. E. Rodriguez, S. D. Wilson, C. Stock

Abstract: CoO has an odd number of electrons in its unit cell, and therefore is expected to be metallic. Yet, CoO is strongly insulating owing to significant electronic correlations, thus classifying it as a Mott insulator. We investigate the magnetic fluctuations in CoO using neutron spectroscopy. The strong and spatially far-reaching exchange constants reported in [Sarte et al. Phys. Rev. B 98 024415 (201… ▽ More CoO has an odd number of electrons in its unit cell, and therefore is expected to be metallic. Yet, CoO is strongly insulating owing to significant electronic correlations, thus classifying it as a Mott insulator. We investigate the magnetic fluctuations in CoO using neutron spectroscopy. The strong and spatially far-reaching exchange constants reported in [Sarte et al. Phys. Rev. B 98 024415 (2018)], combined with the single-ion spin-orbit coupling of similar magnitude [Cowley et al. Phys. Rev. B 88, 205117 (2013)] results in significant mixing between $j_{eff}$ spin-orbit levels in the low temperature magnetically ordered phase. The high degree of entanglement, combined with the structural domains originating from the Jahn-Teller structural distortion at $\sim$ 300 K, make the magnetic excitation spectrum highly structured in both energy and momentum. We extend previous theoretical work on PrTl$_{3}$ [Buyers et al. Phys. Rev. B 11, 266 (1975)] to construct a mean-field and multi-level spin exciton model employing the aforementioned spin exchange and spin-orbit coupling parameters for coupled Co$^{2+}$ ions on a rocksalt lattice. This parameterization, based on a tetragonally distorted type-II antiferromagnetic unit cell, captures both the sharp low energy excitations at the magnetic zone center, and the energy broadened peaks at the zone boundary. However, the model fails to describe the momentum dependence of the excitations at high energy transfers, where the neutron response decays faster with momentum than the Co$^{2+}$ form factor. We discuss such a failure in terms of a possible breakdown of localized spin-orbit excitons at high energy transfers. △ Less

Submitted 1 August, 2019; v1 submitted 1 August, 2019; originally announced August 2019.

Comments: (main text - 21 pages, 12 figures; supplementary information - 15 pages, 3 figures, to be published in Phys. Rev. B)

Journal ref: Phys. Rev. B 100, 075143 (2019)

Ordered magnetism in the intrinsically decorated $j\rm{_{eff}}$ = $\frac{1}{2}$ $α$-CoV$_{3}$O$_{8}$

Authors: P. M. Sarte, A. A. Arévalo-López, M. Songvilay, D. Le, T. Guidi, V. García-Sakai, S. Mukhopadhyay, S. C. Capelli, W. D. Ratcliff, K. H. Hong, G. M. McNally, E. Pachoud, J. P. Attfield, C. Stock

Abstract: The antiferromagnetic mixed valence ternary oxide $α$-CoV$_{3}$O$_{8}$ displays disorder on the Co$^{2+}$ site that is inherent to the $Ibam$ space group. The zero field structural and dynamic properties of $α$-CoV$_{3}$O$_{8}$~have been investigated using a combination of neutron and x-ray diffraction, DC susceptibility, and neutron spectroscopy. The low temperature magnetic and structural proper… ▽ More The antiferromagnetic mixed valence ternary oxide $α$-CoV$_{3}$O$_{8}$ displays disorder on the Co$^{2+}$ site that is inherent to the $Ibam$ space group. The zero field structural and dynamic properties of $α$-CoV$_{3}$O$_{8}$~have been investigated using a combination of neutron and x-ray diffraction, DC susceptibility, and neutron spectroscopy. The low temperature magnetic and structural properties are consistent with a random macroscopic distribution of Co$^{2+}$ over the 16$k$ metal sites. However, by applying the sum rules of neutron scattering we observe the collective magnetic excitations are parameterized with an ordered Co$^{2+}$ arrangement and critical scattering consistent with a three dimensional Ising universality class. The low energy spectrum is well-described by Co$^{2+}$ cations coupled $via$ a three dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the $ab$ plane and along $c$, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long range antiferromagnetic order at $T\rm{_{N}}\sim$ 19 K. A crystal field analysis finds two bands of excitations separated in energy at $\hbar ω$ $\sim$ 5 meV and 25 meV, consistent with a $j\rm{_{eff}}=\frac{1}{2}$ ground state with little mixing between spin-orbit split Kramers doublets. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated $j\rm{_{eff}}$ manifolds may play a key role in making the dynamics largely insensitive to disorder. △ Less

Submitted 6 December, 2018; v1 submitted 1 November, 2018; originally announced November 2018.

Comments: 25 pages, 8 figures, 1 figure in Appendix

Journal ref: Phys. Rev. B, 98 224410 (2018)

In-situ strain tuning of the Dirac surface states in Bi2Se3 films

Authors: David Floetotto, Yang Bai, Yang-Hao Chan, Peng Chen, Xiaoxiong Wang, Paul Rossi, Cai-Zhi Xu, Can Zhang, Joe A. Hlevyack, Jonathan D. Denlinger, Hawoong Hong, Mei-Yin Chou, Eric J. Mittemeijer, James N. Eckstein, Tai-Chang Chiang

Abstract: Elastic strain has the potential for a controlled manipulation of the band gap and spin-polarized Dirac states of topological materials, which can lead to pseudo-magnetic-field effects, helical flat bands and topological phase transitions. However, practical realization of these exotic phenomena is challenging and yet to be achieved. Here, we show that the Dirac surface states of the topological i… ▽ More Elastic strain has the potential for a controlled manipulation of the band gap and spin-polarized Dirac states of topological materials, which can lead to pseudo-magnetic-field effects, helical flat bands and topological phase transitions. However, practical realization of these exotic phenomena is challenging and yet to be achieved. Here, we show that the Dirac surface states of the topological insulator Bi2Se3 can be reversibly tuned by an externally applied elastic strain. Performing in-situ x-ray diffraction and in-situ angle-resolved photoemission spectroscopy measurements during tensile testing of epitaxial Bi2Se3 films bonded onto a flexible substrate, we demonstrate elastic strains of up to 2.1% and quantify the resulting reversible changes in the topological surface state. Our study establishes the functional relationship between the lattice and electronic structures of Bi2Se3 and, more generally, demonstrates a new route toward momentum-resolved mapping of strain-induced band structure changes. △ Less

Submitted 1 July, 2018; originally announced July 2018.

Journal ref: Nano Lett. 18 (2018) 5628-5632

Photoemission Study of the Electronic Structure of Valence Band Convergent SnSe

Authors: C. W. Wang, Y. Y. Y. Xia, Z. Tian, J. Jiang, B. H. Li, S. T. Cui, H. F. Yang, A. J. Liang, X. Y. Zhan, G. H. Hong, S. Liu, C. Chen, M. X. Wang, L. X. Yang, Z. Liu, Q. X. Mi, G. Li, J. M. Xue, Z. K. Liu, Y. L. Chen

Abstract: IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance.The multiple close-to-degenerate valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure of merit in the SnSe single crystal. To date, there have been primarily theoretical calculations of this particular electronic band st… ▽ More IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance.The multiple close-to-degenerate valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure of merit in the SnSe single crystal. To date, there have been primarily theoretical calculations of this particular electronic band structure. In this paper, however, using angle-resolved photoemission spectroscopy, we perform a systematic investigation of the electronic structure of SnSe. We directly observe three predicted hole bands with small energy differences between their band tops and relatively small in-plane effective masses, in good agreement with the ab initio calculations and critical for the enhancement of the Seebeck coefficient while keeping high electrical conductivity. Our results reveal the complete band structure of SnSe and help to provide a deeper understanding of the electronic origin of the excellent thermoelectric performances in SnSe. △ Less

Submitted 12 April, 2018; originally announced April 2018.

Comments: 14 pages, 5 figures

Journal ref: Physical Review B 96, 165118 (2017)

Microstructure and elevated-temperature mechanical properties of refractory AlMo0.5NbTa0.5TiZr High Entropy Alloy fabricated by powder metallurgy

Authors: Yuqiao Li, Junho Lee, Byungchul Kang, Soon Hyung Hong

Abstract: New approaches for the design of alloy systems with multiprincipal elements is recently researched in refractory materials field. However, most research aimed at arc melting process with weakness of coarsening of grains and inhomogeneous microstructure of segregation of elements during the cooling. This study aims to design and fabricate high-entropy alloy with powder metallurgy. In this study, a… ▽ More New approaches for the design of alloy systems with multiprincipal elements is recently researched in refractory materials field. However, most research aimed at arc melting process with weakness of coarsening of grains and inhomogeneous microstructure of segregation of elements during the cooling. This study aims to design and fabricate high-entropy alloy with powder metallurgy. In this study, a refractory high entropy alloys with composition near AlMo0.5NbTa0.5TiZr were produced by powder metallurgy. The alloy consists of two body-centered cubic (BCC) phases. One phase was disordered BCC enriched with Mo, Nb and Ta and the other phase was ordered BCC enriched with Al and Zr. The AlMo0.5NbTa0.5TiZr alloy had a density of 7.46g/cm3 and Vickers microhardness of 678HV. Its compressive yield strength was 2466MPa at 298K and 964MPa at 1273K. The properties of the alloy and the beneficial effects from powder metallurgy on the microstructure and properties were outlined. △ Less

Submitted 31 December, 2017; originally announced January 2018.

Ultrafast, highly-sensitive infrared photodetectors based on two-dimensional oxyselenide crystals

Authors: Jianbo Yin, Zhenjun Tan, Hao Hong, Jinxiong Wu, Hongtao Yuan, Yujing Liu, Cheng Chen, Congwei Tan, Fengrui Yao, Yulin Chen, Zhongfan Liu, Kaihui Liu, Hailin Peng

Abstract: Infrared detection and sensing is deeply embedded in modern technology and human society and its development has always been benefitting from the discovery of new photoelectric response materials. The rise of two-dimensional (2D) materials, thanks to their distinct electronic structure, extreme dimensional confinement and strong light-matter interactions, provides new material platform for next-ge… ▽ More Infrared detection and sensing is deeply embedded in modern technology and human society and its development has always been benefitting from the discovery of new photoelectric response materials. The rise of two-dimensional (2D) materials, thanks to their distinct electronic structure, extreme dimensional confinement and strong light-matter interactions, provides new material platform for next-generation infrared photodetection. Ideal infrared detectors should have fast respond, high sensitivity and air-stability, which is rare to meet at the same time for all existing 2D materials, either graphene, transition metal dichalcogenide or black phosphorous. Herein we demonstrate a new infrared photodetector based on 2D Bi2O2Se crystals, whose main characteristics are superb in the whole 2D family: high sensitivity of ~65 A/W at 1200 nm and ultrafast intrinsic photoresponse of ~1 ps at room temperature. Such great performance is attributed to the suitable electronic bandgap and high carrier mobility of 2D oxyselenide. With additional merits of mass production, excellent stability and flexibility, 2D oxyselenide detectors should open new avenues in highly-sensitive, high-speed, low-cost, flexible infrared photodetection and imaging. △ Less

Submitted 16 December, 2017; originally announced December 2017.

Graphene quantum dots prevent alpha-synucleinopathy in Parkinson's disease

Authors: Donghoon Kim, Je Min Yoo, Heehong Hwang, Junghee Lee, Su Hyun Lee, Seung Pil Yun, Myung Jin Park, MinJun Lee, Seulah Choi, Sang Ho Kwon, Saebom Lee, Seung-Hwan Kwon, Sangjune Kim, Yong Joo Park, Misaki Kinoshita, Young-Ho Lee, Seokmin Shin, Seung R. Paik, Sung Joong Lee, Seulki Lee, Byung Hee Hong, Han Seok Ko

Abstract: While the emerging evidence indicates that the pathogenesis of Parkinson's disease (PD) is strongly correlated to the accumulation of alpha-synuclein (α-syn) aggregates, there has been no clinical success in anti-aggregation agents for the disease to date. Here we show that graphene quantum dots (GQDs) exhibit anti-amyloid activity via direct interaction with α-syn. Employing biophysical, biochemi… ▽ More While the emerging evidence indicates that the pathogenesis of Parkinson's disease (PD) is strongly correlated to the accumulation of alpha-synuclein (α-syn) aggregates, there has been no clinical success in anti-aggregation agents for the disease to date. Here we show that graphene quantum dots (GQDs) exhibit anti-amyloid activity via direct interaction with α-syn. Employing biophysical, biochemical, and cell-based assays as well as molecular dynamics (MD) simulation, we find that GQDs have notable potency in not only inhibiting fibrillization of α-syn but also disaggregating mature fibrils in a time-dependent manner. Remarkably, GQDs rescue neuronal death and synaptic loss, reduce Lewy body (LB)/Lewy neurite (LN) formation, ameliorate mitochondrial dysfunctions, and prevent neuron-to-neuron transmission of α-syn pathology induced by α-syn preformed fibrils (PFFs) in neurons. In addition, in vivo administration of GQDs protects against α-syn PFFs-induced loss of dopamine neurons, LB/LN pathology, and behavioural deficits through the penetration of the blood-brain barrier (BBB). The finding that GQDs function as an anti-aggregation agent provides a promising novel therapeutic target for the treatment of PD and related α-synucleinopathies. △ Less

Submitted 9 July, 2018; v1 submitted 17 October, 2017; originally announced October 2017.

Correlated disorder in the Kuramoto model: Effects on phase coherence, finite-size scaling, and dynamic fluctuations

Authors: Hyunsuk Hong, Kevin P. O'Keeffe, Steven H. Strogatz

Abstract: We consider a mean-field model of coupled phase oscillators with quenched disorder in the natural frequencies and coupling strengths. A fraction $p$ of oscillators are positively coupled, attracting all others, while the remaining fraction $1-p$ are negatively coupled, repelling all others. The frequencies and couplings are deterministically chosen in a manner which correlates them, thereby correl… ▽ More We consider a mean-field model of coupled phase oscillators with quenched disorder in the natural frequencies and coupling strengths. A fraction $p$ of oscillators are positively coupled, attracting all others, while the remaining fraction $1-p$ are negatively coupled, repelling all others. The frequencies and couplings are deterministically chosen in a manner which correlates them, thereby correlating the two types of disorder in the model. We first explore the effect of this correlation on the system's phase coherence. We find that there is a a critical width $γ_c$ in the frequency distribution below which the system spontaneously synchronizes. Moreover, this $γ_c$ is independent of $p$. Hence, our model and the traditional Kuramoto model (recovered when $p=1$) have the same critical width $γ_c$. We next explore the critical behavior of the system by examining the finite-size scaling and the dynamic fluctuation of the traditional order parameter. We find that the model belongs to the same universality class as the Kuramoto model with deterministically (not randomly) chosen natural frequencies for the case of $p<1$. △ Less

Submitted 25 May, 2016; originally announced May 2016.

Comments: 7 pages, 6 figures

Phase coherence induced by correlated disorder

Authors: Hyunsuk Hong, Kevin P. O'Keeffe, Steven H. Strogatz

Abstract: We consider a mean-field model of coupled phase oscillators with quenched disorder in the coupling strengths and natural frequencies. When these two kinds of disorder are uncorrelated (and when the positive and negative couplings are equal in number and strength), it is known that phase coherence cannot occur and synchronization is absent. Here we explore the effects of correlating the disorder. S… ▽ More We consider a mean-field model of coupled phase oscillators with quenched disorder in the coupling strengths and natural frequencies. When these two kinds of disorder are uncorrelated (and when the positive and negative couplings are equal in number and strength), it is known that phase coherence cannot occur and synchronization is absent. Here we explore the effects of correlating the disorder. Specifically, we assume that any given oscillator either attracts or repels all the others, and that the sign of the interaction is deterministically correlated with the given oscillator's natural frequency. For symmetrically correlated disorder with zero mean, we find that the system spontaneously synchronizes, once the width of the frequency distribution falls below a critical value. For asymmetrically correlated disorder, the model displays coherent traveling waves: the complex order parameter becomes nonzero and rotates with constant frequency different from the system's mean natural frequency. Thus, in both cases, correlated disorder can trigger phase coherence. △ Less

Submitted 15 January, 2016; originally announced January 2016.

Comments: 6 pages, 6 figures

Journal ref: Phys. Rev. E 93, 022219 (2016)

Winding number excitation detects phase transition in one-dimensional XY model with variable interaction range

Authors: Hyunsuk Hong, Beom Jun Kim

Abstract: We numerically study the critical behavior of the one-dimensional XY model of the size N with variable interaction range L. As expected, the standard local order parameter of the magnetization is shown to well detect the mean-field type transition which occurs at any nonzero value of L/N. The system is particularly interesting since the underlying one-dimensional structure allows us to study the t… ▽ More We numerically study the critical behavior of the one-dimensional XY model of the size N with variable interaction range L. As expected, the standard local order parameter of the magnetization is shown to well detect the mean-field type transition which occurs at any nonzero value of L/N. The system is particularly interesting since the underlying one-dimensional structure allows us to study the topological excitation of the winding number across the whole system even though the system shares the mean-field transition with the globally-coupled system. We propose a novel nonlocal order parameter based on the width of the winding number distribution which exhibits a clear signature of the transition nature of the system. △ Less

Submitted 16 May, 2015; originally announced May 2015.

Comments: 6 pages, 6 figures

Journal ref: Phys. Rev. E 91, 052120 (2015)

Finite-size scaling, dynamic fluctuations, and hyperscaling relation in the Kuramoto model

Authors: Hyunsuk Hong, Hugues Chaté, Lei-Han Tang, Hyunggyu Park

Abstract: We revisit the Kuramoto model to explore the finite-size scaling (FSS) of the order parameter and its dynamic fluctuations near the onset of the synchronization transition, paying particular attention to effects induced by the randomness of the intrinsic frequencies of oscillators. For a population of size $N$, we study two ways of sampling the intrinsic frequencies according to the {\it same} giv… ▽ More We revisit the Kuramoto model to explore the finite-size scaling (FSS) of the order parameter and its dynamic fluctuations near the onset of the synchronization transition, paying particular attention to effects induced by the randomness of the intrinsic frequencies of oscillators. For a population of size $N$, we study two ways of sampling the intrinsic frequencies according to the {\it same} given unimodal distribution $g(ω)$. In the `{\em random}' case, frequencies are generated independently in accordance with $g(ω)$, which gives rise to oscillator number fluctuation within any given frequency interval. In the `{\em regular}' case, the $N$ frequencies are generated in a deterministic manner that minimizes the oscillator number fluctuations, leading to quasi-uniformly spaced frequencies in the population. We find that the two samplings yield substantially different finite-size properties with clearly distinct scaling exponents. Moreover, the hyperscaling relation between the order parameter and its fluctuations is valid in the regular case, but is violated in the random case. In this last case, a self-consistent mean-field theory that completely ignores dynamic fluctuations correctly predicts the FSS exponent of the order parameter but not its critical amplitude. △ Less

Submitted 25 August, 2015; v1 submitted 22 March, 2015; originally announced March 2015.

Journal ref: Phys. Rev. E 92, 022122 (2015)

Non-destructive electron microscopy imaging and analysis of biological samples with graphene coating

Authors: Jong Bo Park, Yong-Jin Kim, Je Min Yoo, Youngsoo Kim, Seong-Min Kim, Sang Jin Kim, Roman Gorbachev, I. I. Barbolina, Myung-Han Yoon, Byung Hee Hong, Konstantin S. Novoselov

Abstract: In electron microscopy, charging of non-conductive biological samples by focused electron beams hinders their high-resolution imaging. Gold or platinum coatings have been commonly used to prevent such sample charging, but it disables further quantitative and qualitative chemical analyses by energy dispersive spectroscopy (EDS). Here we report that graphene-coating on biological samples enables non… ▽ More In electron microscopy, charging of non-conductive biological samples by focused electron beams hinders their high-resolution imaging. Gold or platinum coatings have been commonly used to prevent such sample charging, but it disables further quantitative and qualitative chemical analyses by energy dispersive spectroscopy (EDS). Here we report that graphene-coating on biological samples enables non-destructive high-resolution imaging by scanning electron microscopy (SEM) as well as chemical analysis by EDS, utilizing graphene's transparency to electron beams, high conductivity, outstanding mechanical strength, and flexibility. We believe that the graphene-coated imaging and analysis would provide us a new opportunity to explore various biological phenomena unseen before due to the limitation in sample preparation and image resolution, which will broaden our understanding on the life mechanism of various living organisms. △ Less

Submitted 8 July, 2014; originally announced July 2014.

Growth Dynamics and Gas Transport Mechanism of Nanobubbles in Graphene Liquid Cells

Authors: Dongha Shin, Jong Bo Park, Yong-Jin Kim, Sang Jin Kim, Jin Hyoun Kang, Bora Lee, Sung-Pyo Cho, Byung Hee Hong, Konstantin S. Novoselov

Abstract: Formation, evolution, and vanishing of bubbles are common phenomena in our nature, which can be easily observed in boiling or falling waters, carbonated drinks, gas-forming electrochemical reactions, etc. However, the morphology and the growth dynamics of the bubbles at nanoscale have not been fully investigated owing to the lack of proper imaging tools that can visualize nanoscale objects in liqu… ▽ More Formation, evolution, and vanishing of bubbles are common phenomena in our nature, which can be easily observed in boiling or falling waters, carbonated drinks, gas-forming electrochemical reactions, etc. However, the morphology and the growth dynamics of the bubbles at nanoscale have not been fully investigated owing to the lack of proper imaging tools that can visualize nanoscale objects in liquid phase. Here we demonstrate, for the first time, that the nanobubbles in water encapsulated by graphene membrane can be visualized by in situ ultrahigh vacuum transmission electron microscopy (UHV-TEM), showing the critical radius of nanobubbles determining its unusual long-term stability as well as two distinct growth mechanisms of merging nanobubbles (Ostwald ripening and coalescing) depending on their relative sizes. Interestingly, the gas transport through ultrathin water membranes at nanobubble interface is free from dissolution, which is clearly different from conventional gas transport that includes condensation, transmission and evaporation. Our finding is expected to provide a deeper insight to understand unusual chemical, biological and environmental phenomena where nanoscale gas-state is involved. △ Less

Submitted 7 June, 2014; originally announced June 2014.

Comments: 10 text pages, 4 figure pages, and 4 page Supplementary Information. Supplementary movies will be provided on request

Length-dependent thermal conductivity in suspended single-layer graphene

Authors: Xiangfan Xu, Luiz F. C. Pereira, Yu Wang, Jing Wu, Kaiwen Zhang, Xiangming Zhao, Sukang Bae, Cong Tinh Bui, Rongguo Xie, John T. L. Thong, Byung Hee Hong, Kian Ping Loh, Davide Donadio, Baowen Li, Barbaros Özyilmaz

Abstract: Graphene exhibits extraordinary electronic and mechanical properties, and extremely high thermal conductivity. Being a very stable atomically thick membrane that can be suspended between two leads, graphene provides a perfect test platform for studying thermal conductivity in two-dimensional systems, which is of primary importance for phonon transport in low-dimensional materials. Here we report e… ▽ More Graphene exhibits extraordinary electronic and mechanical properties, and extremely high thermal conductivity. Being a very stable atomically thick membrane that can be suspended between two leads, graphene provides a perfect test platform for studying thermal conductivity in two-dimensional systems, which is of primary importance for phonon transport in low-dimensional materials. Here we report experimental measurements and non-equilibrium molecular dynamics simulations of thermal conduction in suspended single layer graphene as a function of both temperature and sample length. Interestingly and in contrast to bulk materials, when temperature at 300K, thermal conductivity keeps increasing and remains logarithmic divergence with sample length even for sample lengths much larger than the average phonon mean free path. This result is a consequence of the two-dimensional nature of phonons in graphene and provides fundamental understanding into thermal transport in two-dimensional materials. △ Less

Submitted 22 April, 2014; originally announced April 2014.

Comments: 36 pages,11 figures

Journal ref: Nat. Commun. 5, 3689 (2014)

Optical Probing of Electronic Interaction between Graphene and Hexagonal Boron Nitride

Authors: Gwanghyun Ahn, Hye Ri Kim, Taeg Yeoung Ko, Kyoungjun Choi, Kenji Watanabe, Takashi Taniguchi, Byung Hee Hong, Sunmin Ryu

Abstract: Even weak van der Waals (vdW) adhesion between two-dimensional solids may perturb their various materials properties owing to their low dimensionality. Although the electronic structure of graphene has been predicted to be modified by the vdW interaction with other materials, its optical characterization has not been successful. In this report, we demonstrate that Raman spectroscopy can be utilize… ▽ More Even weak van der Waals (vdW) adhesion between two-dimensional solids may perturb their various materials properties owing to their low dimensionality. Although the electronic structure of graphene has been predicted to be modified by the vdW interaction with other materials, its optical characterization has not been successful. In this report, we demonstrate that Raman spectroscopy can be utilized to detect a few % decrease in the Fermi velocity (vF) of graphene caused by the vdW interaction with underlying hexagonal boron nitride (hBN). Our study also establishes Raman spectroscopic analysis which enables separation of the effects by the vdW interaction from those by mechanical strain or extra charge carriers. The analysis reveals that spectral features of graphene on hBN are mainly affected by change in vF and mechanical strain, but not by charge doping unlike graphene supported on SiO2 substrates. Graphene on hBN was also found to be less susceptible to thermally induced hole doping. △ Less

Submitted 30 October, 2013; originally announced October 2013.

Comments: 19 pages, 4 figures

Journal ref: ACS Nano 7, 1533 (2013)