Showing 1–50 of 107 results for author: Yoon, J

Decoupling Josephson Coupling and Supercurrent Nonreciprocity in Twisted NbSe2/NbSe2 van der Waals Junctions

Authors: Seung-Gi Lee, Kwon-Neong Jung, Jae-Kuen Kim, Jae-Chun Jeon, Jiho Yoon, Stuart S. P. Parkin, Kun-Rok Jeon

Abstract: The microscopic origin of supercurrent nonreciprocity in van der Waals Josephson junctions remains under active debate, particularly regarding the role of twist-angle engineering in layered superconductors. Here, we investigate superconducting transport in twisted NbSe2/NbSe2 vertical Josephson junctions fabricated by dry transfer with controlled crystallographic alignment and chemically clean int… ▽ More The microscopic origin of supercurrent nonreciprocity in van der Waals Josephson junctions remains under active debate, particularly regarding the role of twist-angle engineering in layered superconductors. Here, we investigate superconducting transport in twisted NbSe2/NbSe2 vertical Josephson junctions fabricated by dry transfer with controlled crystallographic alignment and chemically clean interfaces. High-resolution transmission electron microscopy is employed to directly determine the twist angle and assess interface quality. While the Josephson coupling strength exhibits a pronounced dependence on twist angle, with characteristic voltages maximized near crystallographically equivalent orientations and suppressed at intermediate angles, the supercurrent diode efficiency remains negligibly small and shows no systematic twist-angle dependence. In contrast, enhanced diode-like responses emerge only in weakly coupled junctions exhibiting interfacial disorder and reduced transparency. Deliberate interface degradation further amplifies the apparent nonreciprocity, yielding diode efficiencies approaching 30% together with an irregular magnetic-field-strength dependence. These results establish a clear decoupling between Josephson coupling and supercurrent nonreciprocity in twisted NbSe2/NbSe2 junctions. Our findings identify interface disorder, rather than twist-angle-controlled momentum matching, as the dominant origin of the observed diode response and provide a critical benchmark for interpreting nonreciprocal superconducting transport in van der Waals Josephson devices. △ Less

Submitted 10 July, 2026; v1 submitted 7 July, 2026; originally announced July 2026.

Comments: 28 pages, 6 figures

Physics-Informed Discovery of Yield Functions in Plasticity via Convex Neural Representations

Authors: Hyeonbin Moon, Donghyuk Cho, Jecheon Yu, Jeong Whan Yoon, Seunghwa Ryu

Abstract: Identifying anisotropic yield functions remains challenging since yielding is not directly observed in full-field mechanical measurements, directional calibration can require many loading directions, and selecting an appropriate analytical form is nontrivial. This study proposes a physics-informed framework for discovering yield functions from full-field displacement data and reaction force data,… ▽ More Identifying anisotropic yield functions remains challenging since yielding is not directly observed in full-field mechanical measurements, directional calibration can require many loading directions, and selecting an appropriate analytical form is nontrivial. This study proposes a physics-informed framework for discovering yield functions from full-field displacement data and reaction force data, without stress observations, plastic strain measurements, direct yield surface data, or a prescribed parametric yield function. The framework identifies the yield function as a mechanically constrained constitutive component inside elastoplastic stress integration, rather than through direct stress-space supervision. The yield function is represented by a convex neural network that enforces convexity and positive homogeneity of degree one while imposing the assumed tension-compression symmetry, and this neural yield function is trained with a differentiable stress update and a physics-informed force equilibrium loss across multiple loading cases. The proposed framework is validated using finite element (FE) benchmark studies with von Mises, Hill 1948, and Yld2000-2d yield functions, assessing yield contour agreement, displacement-noise sensitivity, identifiability through plastically active stress states, epistemic uncertainty, and polynomial-surrogate deployment. This study provides a mechanics-constrained pathway for discovering anisotropic yield functions from displacement and force data while keeping the identified component within the structure of elastoplastic stress integration. △ Less

Submitted 12 June, 2026; originally announced June 2026.

Comments: 39 pages

Migration of gold atoms into a thiol-bonded molecular self-assembled monolayer, forming a cluster exhibiting a Coulomb staircase

Authors: Bingxin Li, Shanglong Ning, Chunyang Miao, Chenyang Guo, Gyu Don Kong, Xintai Wang, Victor I. Coldea, Yuqiao Li, Sam Harley, Oleg V. Kolosov, James Newson, Sam P. Jarvis, Ben J. Robinson, Mohammed Alzanbaqi, Ali Ismael, Colin J. Lambert, Hyo Jae Yoon, Jeremy J. Baumberg, Christopher J. B. Ford

Abstract: Thiol-based self-assembled monolayers (SAMs) on gold surfaces are one of the fundamental building blocks of molecular electronics. The strong chemical affinity of the gold and sulfur (Au-S) enables the formation of close-packed SAMs, but it also has recently been found to create a dynamic interface where surface reconstruction can occur under illumination, even with ambient light. This reconstruct… ▽ More Thiol-based self-assembled monolayers (SAMs) on gold surfaces are one of the fundamental building blocks of molecular electronics. The strong chemical affinity of the gold and sulfur (Au-S) enables the formation of close-packed SAMs, but it also has recently been found to create a dynamic interface where surface reconstruction can occur under illumination, even with ambient light. This reconstruction may facilitate migration of gold atoms, potentially leading to in-situ formation of gold clusters. However, research on this mechanism often centers on Au(111) crystalline surfaces and flicker-noise measurements. Electron transport in ensembles of molecules in lithographically defined junctions has remained largely unexplored at cryogenic temperatures. In this study, we observe single-electron phenomena characterized by reproducible Coulomb staircases across various long-chain alkanethiol SAMs, which fit the Coulomb-blockade theory of nm-sized metallic nanoparticles. We find no such current steps in samples with amine, rather than thiol, anchors. Additionally, we find that by adding a bipyridyl functional group, these phenomena can be harnessed for memristive switching and negative differential resistance. These findings indicate that the generally observed lack of reliability and reproducibility of molecular devices may be alleviated by using amine anchors instead of thiols to avoid nanoparticle effects. Conversely, the spontaneous formation of the nanoparticles could potentially be controlled and used to achieve useful functionalities, offering new pathways for designing multifunctional nanoelectronic components. △ Less

Submitted 21 December, 2025; originally announced December 2025.

Robust AC vector sensing at zero magnetic field with pentacene

Authors: Boning Li, Garrett Heller, Jungbae Yoon, Alexander Ungar, Hao Tang, Guoqing Wang, Patrick Hautle, Yifan Quan, Paola Cappellaro

Abstract: Quantum sensors based on electronic spins have emerged as powerful probes of microwave-frequency fields. Among other solid-state platforms, spins in molecular crystals offer a range of advantages, from high spin density to functionalization via chemical tunability. Here, we demonstrate microwave vector magnetometry using the photoexcited spin triplet of deuterated pentacene molecules, operating at… ▽ More Quantum sensors based on electronic spins have emerged as powerful probes of microwave-frequency fields. Among other solid-state platforms, spins in molecular crystals offer a range of advantages, from high spin density to functionalization via chemical tunability. Here, we demonstrate microwave vector magnetometry using the photoexcited spin triplet of deuterated pentacene molecules, operating at zero external magnetic field and room temperature. We achieve full three-dimensional microwave field reconstruction by detecting the Rabi frequencies of anisotropic spin-triplet transitions associated with two crystallographic orientations of pentacene in naphthalene crystals. We further introduce a phase alternated protocol that extends the rotating-frame coherence time by an order of magnitude and enables sensitivities of $1~μ\mathrm{T}/\sqrt{\mathrm{Hz}}$ with sub-micrometer spatial resolution. These results establish pentacene-based molecular spins as a practical and high-performance platform for microwave quantum sensing, and the control techniques are broadly applicable to other molecular and solid-state spin systems. △ Less

Submitted 22 December, 2025; v1 submitted 5 December, 2025; originally announced December 2025.

Cryogenic Magnetization Dynamics in Chemically Stabilized, Tensile-Strained Ultrathin Yttrium Iron Garnets with Tunable Magnetic Anisotropy

Authors: Jihyung Kim, Dongchang Kim, Seung-Gi Lee, Yung-Cheng Li, Jae-Chun Jeon, Jiho Yoon, Sachio Komori, Ryotaro Arakawa, Tomoyasu Taniyama, Stuart S. P. Parkin, Kun-Rok Jeon

Abstract: We report an interfacial chemical stability-driven reduction of low-temperature damping losses in tensile-strained, ultrathin Y3Fe5O12 (YIG) films grown by pulsed laser deposition, exhibiting ultralow damping constants and tunable magnetic anisotropy. Comparative broadband FMR measurements show that tensile-strained YIG films on Gd3Sc2Ga3O12 (GSGG) retain measurable damping even at nanometer thick… ▽ More We report an interfacial chemical stability-driven reduction of low-temperature damping losses in tensile-strained, ultrathin Y3Fe5O12 (YIG) films grown by pulsed laser deposition, exhibiting ultralow damping constants and tunable magnetic anisotropy. Comparative broadband FMR measurements show that tensile-strained YIG films on Gd3Sc2Ga3O12 (GSGG) retain measurable damping even at nanometer thicknesses and cryogenic temperatures down to 2 K, outperforming relaxed films on Gd3Ga5O12. Based on static magnetometry measurements along with microstructural and compositional analyses, we attribute these enhanced dynamic properties to the suppression of interdiffusion across the YIG/GSGG interface, resulting from enhanced chemical stability and favorable growth kinetics by the presence of Sc. Our findings highlight the importance of chemical and kinetic factors in achieving few-nanometer-thick YIG film with negligible low-temperature damping dissipation and perpendicular magnetic anisotropy for cryogenic spintronic applications. △ Less

Submitted 16 April, 2026; v1 submitted 17 July, 2025; originally announced July 2025.

Comments: 24 pages, 6 figures

Interferometric evidence of non-volatile anomalous phase shifts in exchange-spin-split Josephson supercurrent diodes

Authors: Kun-Rok Jeon, Jae-Keun Kim, Jiho Yoon, Jae-Chun Jeon, Hyeon Han, Audrey Cottet, Takis Kontos, Stuart S. P. Parkin

Abstract: The recent realization of zero-field, polarity-reversible supercurrent rectification in proximity-magnetized Rashba-type Pt Josephson junctions (JJs) enables the development of superconducting logic circuits and cryogenic memory applications. Here, we demonstrate a non-volatile anomalous phase shift φ_0 directly probed via superconducting quantum interferometry, providing phase-sensitive evidence… ▽ More The recent realization of zero-field, polarity-reversible supercurrent rectification in proximity-magnetized Rashba-type Pt Josephson junctions (JJs) enables the development of superconducting logic circuits and cryogenic memory applications. Here, we demonstrate a non-volatile anomalous phase shift φ_0 directly probed via superconducting quantum interferometry, providing phase-sensitive evidence of spontaneous time-reversal symmetry breaking in these Rashba-type systems. By replacing the Pt barrier with 5d or 4d element layers exhibiting different (para-)magnetic susceptibilities, spin-orbit coupling strengths, and electronic band structures, we elucidate the role of proximity effects in governing zero-field diode behavior. Ta (W) JJs exhibit zero-field diode efficiencies of ~17% (~5%) at 2 K, which are slightly (significantly) lower than those of Pt JJs. Notably, the diode polarity in Ta and W JJs is reversed relative to Pt JJs. Combined with the large zero-field diode efficiency (~15% at 2 K) observed in highly magnetic-susceptible Pd JJs, these results show that non-volatile φ_0 and, consequently, zero-field diode performance can be tuned through proximity engineering of interfacial magnetic ordering and Rashba spin-orbit interaction. △ Less

Submitted 23 June, 2026; v1 submitted 26 May, 2025; originally announced May 2025.

Comments: 21 pages, 5 figures

Size dependence of the properties of synthetic-antiferromagnet-based stochastic magnetic tunnel junctions for probabilistic computing

Authors: Takuma Kinoshita, Ju-Young Yoon, Nuno Caçoilo, Ryota Mochizuki, Haruna Kaneko, Shun Kanai, Hideo Ohno, Shunsuke Fukami

Abstract: Stochastic magnetic tunnel junctions (s-MTJs) are core components for spintronics-based probabilistic computing (p-computing), a promising candidate for energy-efficient unconventional computing. To achieve reliable performance under practical conditions, the use of a synthetic antiferromagnetic (SAF) free-layer configuration was proposed due to its enhanced tolerance to magnetic field perturbatio… ▽ More Stochastic magnetic tunnel junctions (s-MTJs) are core components for spintronics-based probabilistic computing (p-computing), a promising candidate for energy-efficient unconventional computing. To achieve reliable performance under practical conditions, the use of a synthetic antiferromagnetic (SAF) free-layer configuration was proposed due to its enhanced tolerance to magnetic field perturbations. For engineering the SAF s-MTJs, we systematically investigate the properties of the SAF s-MTJs as a function of the junction size. We observe that decreasing junction size leads to shorter relaxation times, enhanced magnetic field robustness, and enhanced insensitivity to bias voltage. These findings provide key insights toward high-performance p-computers with reliable operation. △ Less

Submitted 8 May, 2025; originally announced May 2025.

Comments: 18 pages, 4 figures

Rapid low-temperature synthesis of graphene-coated SiC substrates for remote and van der Waals epitaxy

Authors: Se H. Kim, Hanjoo Lee, Dong Gwan Kim, Donghan Kim, Seugki Kim, Hyunho Yang, Yunsu Jang, Jangho Yoon, Hyunsoo Kim, Seoyong Ha, ByoungTak Lee, Jung-Hee Lee, Roy Byung Kyu Chung, Hongsik Park, Sungkyu Kim, Tae Hoon Lee, Hyun S. Kum

Abstract: Non-conventional epitaxial techniques, such as van der Waals epitaxy (vdWE) and remote epitaxy, have attracted substantial attention in the semiconductor research community for their capability to repeatedly produce high-quality free-standing films from a single mother wafer. Successful implementation of these epitaxial techniques depends on creating a robust, uniform two-dimensional (2D) material… ▽ More Non-conventional epitaxial techniques, such as van der Waals epitaxy (vdWE) and remote epitaxy, have attracted substantial attention in the semiconductor research community for their capability to repeatedly produce high-quality free-standing films from a single mother wafer. Successful implementation of these epitaxial techniques depends on creating a robust, uniform two-dimensional (2D) material surface. The conventional method for fabricating graphene on silicon carbide (SiC) is high-temperature graphitization. However, the extremely high temperature required for silicon sublimation (typically above 1500 °C) causes step-bunching of the SiC surface, forming non-uniform multilayer graphene stripes and an unfavorable surface morphology for epitaxial growth. Here, we developed a wafer-scale graphitization technique that allows fast synthesis of single-crystalline graphene at ultra-low temperatures by metal-assisted graphitization (MAG). We found annealing conditions that enable SiC dissociation while avoiding silicide formation, producing uniform single-crystalline graphene while maintaining the surface morphology of the substrate. The graphene thickness can be controlled by varying the metal thickness or annealing temperature, enabling remote epitaxy or vdWE. We successfully produced freestanding single-crystalline III-N (AlN, GaN) films on graphene/SiC via the 2D material-based layer transfer technique. Our results show that low-temperature graphene synthesis via MAG offers a promising route to producing large-scale ultra-wide bandgap free-standing crystalline membranes. △ Less

Submitted 20 May, 2025; v1 submitted 24 February, 2025; originally announced February 2025.

Magnetic Phase Diagram of Mn3+xSn1-x Epitaxial Thin Films: Extending the Anomalous Hall Effect to Low Temperatures via Intrinsic Alloying

Authors: K. Gas, J. -Y. Yoon, Y. Sato, H. Kubota, P. Dluzewski, S. Kret, J. Z. Domagala, Y. K. Edathumkandy, Y. Takeuchi, S. Kanai, H. Ohno, M. Sawicki, S. Fukami

Abstract: Antiferromagnets with broken time-reversal symmetry, such as Mn3Sn, have emerged as promising platforms for exploring topological and correlated electron physics. Mn3Sn is known to show two magnetic phase transitions: a non-collinear inverse triangular antiferromagnetic (IT-AFM) spin configuration is formed below its Neel temperature, whereas at T1 that usually locates below room temperature, it t… ▽ More Antiferromagnets with broken time-reversal symmetry, such as Mn3Sn, have emerged as promising platforms for exploring topological and correlated electron physics. Mn3Sn is known to show two magnetic phase transitions: a non-collinear inverse triangular antiferromagnetic (IT-AFM) spin configuration is formed below its Neel temperature, whereas at T1 that usually locates below room temperature, it transits to an incommensurate spin state. Accordingly, intriguing properties such as a strong anomalous Hall effect, observed from TN to T1, disappear below T1, limiting its utility at low temperatures. While bulk Mn3Sn has been extensively studied, the magnetic phase transitions and their tunability in thin films remain largely unexplored. Here, we investigate the magnetic and magneto-transport properties of Mn3+xSn1-x epitaxial thin films prepared by magnetron sputtering, systematically varying the Mn-Sn composition. Our results reveal that intrinsic alloying with Mn provides us with a handle to tune T1, with the IT-AFM phase stabilized down to liquid helium temperatures for x > 0.15. From a magnetic phase diagram for epitaxial thin films, we also find a consistent magnetic anomaly ~55 K below TN, accompanied by thermal hysteresis. Furthermore, the reduction of TN in thin films relative to bulk values is shown to correlate with lattice parameter changes. These findings extend the accessible temperature range for Mn3Sn's topological properties, paving the way for novel applications and further investigations into the interplay of spin, lattice, and electronic degrees of freedom in thin-film geometries. △ Less

Submitted 28 December, 2024; originally announced December 2024.

Comments: 16 pages, 5 figures, supplementary information

Journal ref: APL Mater. 13, 041105 (2025)

Momentum-Resolved Fingerprint of Mottness in Layer-Dimerized Nb$_3$Br$_8$

Authors: Mihir Date, Francesco Petocchi, Yun Yen, Jonas A. Krieger, Banabir Pal, Vicky Hasse, Emily C. McFarlane, Chris Körner, Jiho Yoon, Matthew D. Watson, Vladimir N. Strocov, Yuanfeng Xu, Ilya Kostanovski, Mazhar N. Ali, Sailong Ju, Nicholas C. Plumb, Michael A. Sentef, Georg Woltersdorf, Michael Schüler, Philipp Werner, Claudia Felser, Stuart S. P. Parkin, Niels B. M. Schröter

Abstract: In a well-ordered crystalline solid, insulating behaviour can arise from two mechanisms: electrons can either scatter off a periodic potential, thus forming band gaps that can lead to a band insulator, or they localize due to strong interactions, resulting in a Mott insulator. For an even number of electrons per unit cell, either band- or Mott-insulators can theoretically occur. However, unambiguo… ▽ More In a well-ordered crystalline solid, insulating behaviour can arise from two mechanisms: electrons can either scatter off a periodic potential, thus forming band gaps that can lead to a band insulator, or they localize due to strong interactions, resulting in a Mott insulator. For an even number of electrons per unit cell, either band- or Mott-insulators can theoretically occur. However, unambiguously identifying an unconventional Mott-insulator with an even number of electrons experimentally has remained a longstanding challenge due to the lack of a momentum-resolved fingerprint. This challenge has recently become pressing for the layer dimerized van der Waals compound Nb$_3$Br$_8$, which exhibits a puzzling magnetic field-free diode effect when used as a weak link in Josephson junctions, but has previously been considered to be a band-insulator. In this work, we present a unique momentum-resolved signature of a Mott-insulating phase in the spectral function of Nb$_3$Br$_8$: the top of the highest occupied band along the out-of-plane dimerization direction $k_z$ has a momentum space separation of $Δk_z=2π/d$, whereas the valence band maximum of a band insulator would be separated by less than $Δk_z=π/d$, where $d$ is the average spacing between the layers. As the strong electron correlations inherent in Mott insulators can lead to unconventional superconductivity, identifying Nb$_3$Br$_8$ as an unconventional Mott-insulator is crucial for understanding its apparent time-reversal symmetry breaking Josephson diode effect. Moreover, the momentum-resolved signature employed here could be used to detect quantum phase transition between band- and Mott-insulating phases in van der Waals heterostructures, where interlayer interactions and correlations can be easily tuned to drive such transition. △ Less

Submitted 21 October, 2024; originally announced October 2024.

Comments: 9 pages, 3 figures

Switching of magnetic domains in a noncollinear antiferromagnet at the nanoscale

Authors: Atul Pandey, Prajwal Rigvedi, Edouard Lesne, Jitul Deka, Jiho Yoon, Wolfgang Hoppe, Chris Koerner, Banabir Pal, James M. Taylor, Stuart S. P. Parkin, Georg Woltersdorf

Abstract: Antiferromagnets that display very small stray magnetic field are ideal for spintronic applications. Of particular interest are non-collinear, chiral antiferromagnets of the type Mn3X (X=Sn, Ge), which display a large magnetotransport response that is correlated with their antiferromagnetic ordering. The ability to read out and manipulate this ordering is crucial for their integration into spintro… ▽ More Antiferromagnets that display very small stray magnetic field are ideal for spintronic applications. Of particular interest are non-collinear, chiral antiferromagnets of the type Mn3X (X=Sn, Ge), which display a large magnetotransport response that is correlated with their antiferromagnetic ordering. The ability to read out and manipulate this ordering is crucial for their integration into spintronic devices. These materials exhibit a tiny unbalanced magnetic moment such that a large external magnetic field can, in principle, be used to set the material into a single antiferromagnetic domain. However, in thin films of Mn3Sn, we find that such fields induce only a partial magnetic ordering. By detecting two orthogonal in-plane components of the magnetic order vector, we find that the non-switchable fraction has a unidirectional anisotropy. This also enables us to visualize switching along multiple easy axes in Mn3Sn. Studying the switching at the nanoscale allows us to correlate the pining behavior to crystal grain boundaries in the Mn3Sn nanowire structures. △ Less

Submitted 23 September, 2024; originally announced September 2024.

Anomalous Nernst effect based near field imaging of magnetic nanostructures

Authors: Atul Pandey, Jitul Deka, Jiho Yoon, Chris Koerner, Rouven Dreyer, James M. Taylor, Stuart S. P. Parkin, Georg Woltersdorf

Abstract: The anomalous Nernst effect (ANE) gives rise to an electrical response transverse to the magnetization and an applied temperature gradient in a magnetic metal. A nanoscale temperature gradient can be generated by the use of a laser beam applied to the apex of an atomic force microscope tip, thereby allowing for spatially-resolved ANE measurements beyond the optical diffraction limit. Such a method… ▽ More The anomalous Nernst effect (ANE) gives rise to an electrical response transverse to the magnetization and an applied temperature gradient in a magnetic metal. A nanoscale temperature gradient can be generated by the use of a laser beam applied to the apex of an atomic force microscope tip, thereby allowing for spatially-resolved ANE measurements beyond the optical diffraction limit. Such a method has been used previously to map in-plane magnetized magnetic textures. However, the spatial distribution of the out-of-plane temperature gradient, which is needed to fully interpret such an ANE-based imaging, was not studied. We therefore use a well-known magnetic texture, a magnetic vortex core, to demonstrate the reliability of the ANE method for the imaging of magnetic domains with nanoscale resolution. Moreover, since the ANE signal is directly proportional to the temperature gradient, we can also consider the inverse problem and deduce information about the nanoscale temperature distribution. Our results together with finite element modeling indicate that besides the out-of-plane temperature gradients, there are even larger in-plane temperature gradients. Thus we extend the ANE imaging to study out-of-plane magnetization in a racetrack nano-wire by detecting the ANE signal generated by in-plane temperature gradients. In all cases, a spatial resolution of about 80 nm is obtained. These results are significant for the rapidly growing field of thermo-electric imaging of antiferromagnetic spintronic device structures. △ Less

Submitted 17 July, 2024; originally announced July 2024.

SG-NNP: Species-separated Gaussian Neural Network Potential with Linear Elemental Scaling and Optimized Dimensions for Multi-component Materials

Authors: Ji Wei Yoon, Bangjian Zhou, J Senthilnath

Abstract: Accurate simulations of materials at long-time and large-length scales have increasingly been enabled by Machine-learned Interatomic Potentials (MLIPs). There have been increasing interest on improving the robustness of such models. To this end, we engineer a novel set of Gaussian-type descriptors that scale linearly with the number of atoms, reduce informational degeneracy for multi-component ato… ▽ More Accurate simulations of materials at long-time and large-length scales have increasingly been enabled by Machine-learned Interatomic Potentials (MLIPs). There have been increasing interest on improving the robustness of such models. To this end, we engineer a novel set of Gaussian-type descriptors that scale linearly with the number of atoms, reduce informational degeneracy for multi-component atomic environments and apply them in Species-separated Gaussian Neural Network Potentials (SG-NNPs). The robustness of our method was tested by analyzing the impact of various design choices and hyperparameters on Molybdenum (Mo) SG-NNP performance during training and inference/simulation. With less dimensions, SG-NNPs are shown to have superior atomic forces and total energy predictions than other traditional and ML descriptor-based interatomic potentials on diverse set of materials - Ni, Cu, Li, Mo, Si, Ge, NiMo, Li3N and NbMoTaW. From the obtained results we can observe that the proposed method improves the performance of atomic descriptors of complex environments with multiple species. △ Less

Submitted 9 July, 2024; originally announced July 2024.

Generating Nanoporous Graphene from Point and Stone-Wales Defects: A Study with Dimensionally Restricted Molecular Dynamics (DR-MD)

Authors: Ji Wei Yoon

Abstract: Defects in graphene are both a boon and a bane for applications - they can induce uncontrollable effects but can also provide novel ways to manipulate the properties of pristine graphene. Nanoporous Graphene, which contains nanoscopic holes, has found impactful applications in sustainability domains, e.g. gas separation, water filtration membranes and battery technologies. For this report, we inve… ▽ More Defects in graphene are both a boon and a bane for applications - they can induce uncontrollable effects but can also provide novel ways to manipulate the properties of pristine graphene. Nanoporous Graphene, which contains nanoscopic holes, has found impactful applications in sustainability domains, e.g. gas separation, water filtration membranes and battery technologies. For this report, we investigate pore formation in graphene with no defect, one and two mono-vacancies, and two di-vacancies using bespoke Dimensionally Restricted Molecular Dynamics (DR-MD) designed for the purpose. We show DR-MD to be superior to free-standing or substrate suspended configurations for simulating stable defected structures. Applying DR-MD, stable pore configurations are identified, and their formation mechanisms elucidated. We also investigated formation mechanisms due to two Stone-Wales 55-77 defects, and the formation energies of their linearly extended structures, along the zigzag and armchair directions, and when they are placed in different relative orientations. This study offers a way to identify stable porous defect structures in graphene and insights into atomistic pore formation mechanisms for an environmentally important material. △ Less

Submitted 6 October, 2023; v1 submitted 26 August, 2023; originally announced August 2023.

Comments: 20 pages, 10 figures

MSC Class: -

Explainable machine learning to enable high-throughput electrical conductivity optimization and discovery of doped conjugated polymers

Authors: Ji Wei Yoon, Adithya Kumar, Pawan Kumar, Kedar Hippalgaonkar, J Senthilnath, Vijila Chellappan

Abstract: The combination of high-throughput experimentation techniques and machine learning (ML) has recently ushered in a new era of accelerated material discovery, enabling the identification of materials with cutting-edge properties. However, the measurement of certain physical quantities remains challenging to automate. Specifically, meticulous process control, experimentation and laborious measurement… ▽ More The combination of high-throughput experimentation techniques and machine learning (ML) has recently ushered in a new era of accelerated material discovery, enabling the identification of materials with cutting-edge properties. However, the measurement of certain physical quantities remains challenging to automate. Specifically, meticulous process control, experimentation and laborious measurements are required to achieve optimal electrical conductivity in doped polymer materials. We propose a ML approach, which relies on readily measured absorbance spectra, to accelerate the workflow associated with measuring electrical conductivity. The classification model accurately classifies samples with a conductivity > 25 to 100 S/cm, achieving a maximum of 100 % accuracy rate. For the subset of highly conductive samples, we employed a regression model to predict their conductivities, yielding an impressive test R2 value of 0.984. We tested the models with samples of the two highest conductivities (498 and 506 S/cm) and showed that they were able to correctly classify and predict the two extrapolative conductivities at satisfactory levels of errors. The proposed ML-assisted workflow results in an improvement in the efficiency of the conductivity measurements by 89 % of the maximum achievable using our experimental techniques. Furthermore, our approach addressed the common challenge of the lack of explainability in ML models by exploiting bespoke mathematical properties of the descriptors and ML model, allowing us to gain corroborated insights into the spectral influences on conductivity. Through this study, we offer an accelerated pathway for optimizing the properties of doped polymer materials while showcasing the valuable insights that can be derived from purposeful utilization of ML in experimental science. △ Less

Submitted 27 April, 2024; v1 submitted 8 August, 2023; originally announced August 2023.

Comments: 33 Pages, 17 figures

Journal ref: Knowledge-Based Systems 295C (2024) 111812

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

Authors: Jun-Yeong Yoon, Yangjin Lee, Dong-Gyu Kim, Dong Gun Oh, Jin Kyun Kim, Linshuo Guo, Jungcheol Kim, Jeongheon Choe, Kihyun Lee, Hyeonsik Cheong, Chae Un Kim, Young Jai Choi, Yanhang Ma, Kwanpyo Kim

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

Submitted 2 August, 2023; originally announced August 2023.

Comments: 16 pages, 5 figures, 1 table

Journal ref: Angew. Chem. Int. Ed. e202307102 (2023)

Gravitational Edge Mode in Asymptotically AdS$_2$: JT Gravity Revisited

Authors: Euihun Joung, Prithvi Narayan, Junggi Yoon

Abstract: We study the gravitational edge mode of the Jackiw-Teitelboim (JT) gravity and its $sl(2,\mathbb{R})$ BF theory description with the asymptotic AdS$_2$ boundary condition. We revisit the derivation of the Schwarzian theory from the wiggling boundary as an action for the gravitational edge mode. We present an alternative description for the gravitational edge mode from the metric fluctuation with t… ▽ More We study the gravitational edge mode of the Jackiw-Teitelboim (JT) gravity and its $sl(2,\mathbb{R})$ BF theory description with the asymptotic AdS$_2$ boundary condition. We revisit the derivation of the Schwarzian theory from the wiggling boundary as an action for the gravitational edge mode. We present an alternative description for the gravitational edge mode from the metric fluctuation with the fixed boundary, which is often referred as "would-be gauge mode". We clarify the relation between the wiggling boundary and the would-be gauge mode. We demonstrate a natural top-down derivation of $PSL(2,\mathbb{R})$ gauging and the path integral measure of the Schwarzian theory. In the $sl(2,\mathbb{R})$ BF theory, we incorporate the gravitational edge mode and derive the Schwarzian theory with $PSL(2,\mathbb{R})$ gauging. We also discuss the path integral measure from the Haar measure in the Iwasawa decomposition of $PSL(2,\mathbb{R})$. △ Less

Submitted 8 March, 2024; v1 submitted 12 April, 2023; originally announced April 2023.

Comments: 32 pages, 6 figures; v2: typos corrected, references added; v3: reference added, typo corrected

Exploring the magnetic properties of individual barcode nanowires using wide-field diamond microscopy

Authors: Jungbae Yoon, Jun Hwan Moon, Jugyeong Jeong, Yu Jin Kim, Kihwan Kim, Hee Seong Kang, Yoo Sang Jeon, Eunsoo Oh, Sun Hwa Lee, Kihoon Han, Dongmin Lee, Chul-Ho Lee, Young Keun Kim, Donghun Lee

Abstract: Barcode magnetic nanowires typically comprise a multilayer magnetic structure in a single body with more than one segment type. Interestingly, owing to selective functionalization and novel interactions between the layers, barcode magnetic nanowires have attracted significant attention, particularly in the field of bioengineering. However, an analysis of their magnetic properties at the individual… ▽ More Barcode magnetic nanowires typically comprise a multilayer magnetic structure in a single body with more than one segment type. Interestingly, owing to selective functionalization and novel interactions between the layers, barcode magnetic nanowires have attracted significant attention, particularly in the field of bioengineering. However, an analysis of their magnetic properties at the individual nanowire level remains challenging. With this background, herein, we investigated the characterization of magnetic nanowires at room temperature under ambient conditions based on magnetic images obtained via wide-field quantum microscopy with nitrogen-vacancy centers in diamond. Consequently, we could extract critical magnetic properties, such as the saturation magnetization and coercivity, of single nanowires by comparing the experimental results with those of micromagnetic simulations. This study opens up the possibility for a versatile characterization method suited to individual magnetic nanowires. △ Less

Submitted 21 February, 2023; originally announced February 2023.

Comments: main text (20 pages, 5 figures), supplementary materials (9 pages, 8 figures)

Alternative understanding of the skyrmion Hall effect based on one-dimensional domain wall motion

Authors: Kyoung-Woong Moon, Jungbum Yoon, Changsoo Kim, Jae-Hun Sim, Se Kwon Kim, Soong-Geun Je, Chanyong Hwang

Abstract: A moving magnetic skyrmion exhibits transverse deflection. This so-called skyrmion Hall effect has been explained by the Thiele equation. Here, we provide an alternative interpretation of the skyrmion Hall effect based on the dynamics of domain walls enclosing the skyrmion. We relate the spin-torque-induced local rotation of the domain wall segments to the shift of the skyrmion core, explaining th… ▽ More A moving magnetic skyrmion exhibits transverse deflection. This so-called skyrmion Hall effect has been explained by the Thiele equation. Here, we provide an alternative interpretation of the skyrmion Hall effect based on the dynamics of domain walls enclosing the skyrmion. We relate the spin-torque-induced local rotation of the domain wall segments to the shift of the skyrmion core, explaining the skyrmion Hall effect at the micromagnetic level. Bases on our intuitive interpretation, we also show that the skyrmion Hall effect can be suppressed by combining the spin-transfer and spin-orbit torques, whereby removing the major obstacle to utilizing skyrmions in devices. △ Less

Submitted 9 November, 2022; v1 submitted 9 November, 2022; originally announced November 2022.

Journal ref: Appl. Phys. Express 15, 123001 (2022)

Unitarity of Symplectic Fermion in $α$-vacua with Negative Central Charge

Authors: Shinsei Ryu, Junggi Yoon

Abstract: We study the two-dimensional free symplectic fermion with anti-periodic boundary condition. This model has negative norm states with naive inner product. This negative norm problem can be cured by introducing a new inner product. We demonstrate that this new inner product follows from the connection between the path integral formalism and the operator formalism. This model has negative central cha… ▽ More We study the two-dimensional free symplectic fermion with anti-periodic boundary condition. This model has negative norm states with naive inner product. This negative norm problem can be cured by introducing a new inner product. We demonstrate that this new inner product follows from the connection between the path integral formalism and the operator formalism. This model has negative central charge, $c=-2$, and we clarify how CFT$_2$ with negative central charge can have the non-negative norm. We introduce $α$-vacua in which the Hamiltonian is seemingly non-Hermitian. In spite of non-Hermiticity we find that the energy spectrum is real. We also compare a correlation function with respect to the $α$-vacua with that of the de Sitter space. △ Less

Submitted 6 February, 2023; v1 submitted 25 August, 2022; originally announced August 2022.

Comments: 5 pages + appendices; v2: typos corrected, 2 figures added

Sign-Problem-Free Variant of Complex Sachdev-Ye-Kitaev Model

Authors: Byungmin Kang, Junggi Yoon

Abstract: We construct a sign-problem free variant of the complex Sachdev-Ye-Kitaev (SYK) model which keeps all the essential properties of the SYK model, including the analytic solvability in the large-$N$ limit and being maximally chaotic. In addition to the number of complex fermions $N$, our model has an additional parameter $M$ controlling the number of terms in the Hamiltonian which we take… ▽ More We construct a sign-problem free variant of the complex Sachdev-Ye-Kitaev (SYK) model which keeps all the essential properties of the SYK model, including the analytic solvability in the large-$N$ limit and being maximally chaotic. In addition to the number of complex fermions $N$, our model has an additional parameter $M$ controlling the number of terms in the Hamiltonian which we take $M \to \infty$ with keeping $M/N$ constant in the large-$N$ limit. While our model respects global $U(1)$ symmetry associated with the fermion number conservation, both the large-$N$ limit and the sign-problem free nature become explicit in the Majorana representation. We present a detailed analysis on our model, i.e., the random matrix classification based on the symmetry analysis, analytic approach, and the quantum Monte Carlo simulations. All these analysis show that our model exhibit a non-Fermi liquid (NFL) physics, a gapless fermionic system lying beyond the conventional Fermi liquid picture. △ Less

Submitted 28 July, 2021; originally announced July 2021.

Comments: 17 pages, 8 figures

Role of two-dimensional Ising superconductivity in the non-equilibrium quasiparticle spin-to-charge conversion efficiency

Authors: Kun-Rok Jeon, Kyungjune Cho, Anirban Chakraborty, Jae-Chun Jeon, Jiho Yoon, Hyeon Han, Jae-Keun Kim, Stuart S. P. Parkin

Abstract: Non-equilibrium studies of two-dimensional (2D) superconductors (SCs) with Ising spin-orbit coupling are prerequisite for their successful application to equilibrium spin-triplet Cooper pairs and, potentially, Majorana fermions. By taking advantage of the recent discoveries of 2D SCs and their compatibility with any other materials, we fabricate here non-local magnon devices to examine how such 2D… ▽ More Non-equilibrium studies of two-dimensional (2D) superconductors (SCs) with Ising spin-orbit coupling are prerequisite for their successful application to equilibrium spin-triplet Cooper pairs and, potentially, Majorana fermions. By taking advantage of the recent discoveries of 2D SCs and their compatibility with any other materials, we fabricate here non-local magnon devices to examine how such 2D Ising superconductivity affects the conversion efficiency of magnon spin to quasiparticle charge in superconducting flakes of 2H-NbSe2 transferred onto ferrimagnetic insulating Y3Fe5O12. Comparison with a reference device based on a conventionally paired superconductor shows that the Y3Fe5O12-induced in-plane (IP) exchange spin-splitting in the NbSe2 flake is hindered by its inherent out-of-plane (OOP) spin-orbit-field, which, in turn, limits the transition-state enhancement of the spin-to-charge conversion efficiency. Our out-of-equilibrium study highlights the significance of symmetry matching between underlying Cooper pairs and exchange-induced spin-splitting for the giant transition-state spin-to-charge conversion and may have implications towards proximity-engineered spin-polarized triplet pairing via tuning the relative strength of IP exchange and OOP spin-orbit fields in ferromagnetic insulator/2D Ising SC bilayers. △ Less

Submitted 26 October, 2021; v1 submitted 24 May, 2021; originally announced May 2021.

Comments: 25 pages, 4 figures, Supplementary Information

Journal ref: ACS Nano 15, 16819 (2021)

Angstrom-wide conductive channels in black phosphorus by Cu intercalation

Authors: Suk Woo Lee, Lu Qiu, Jong Chan Yoon, Yohan Kim, Da Li, Inseon Oh, Gil-Ho Lee, Jung-Woo Yoo, Hyung-Joon Shin, Feng Ding, Zonghoon Lee

Abstract: Intercalation is an effective method to improve and modulate properties of two-dimensional materials. Even so, spatially controlled intercalation at atomic scale, which is important to introduce and modulated properties, has not been successful due to difficulties in controlling the diffusion of intercalants. Here, we show formation of angstrom-wide conductive channels (~4.3 A) in black phosphorus… ▽ More Intercalation is an effective method to improve and modulate properties of two-dimensional materials. Even so, spatially controlled intercalation at atomic scale, which is important to introduce and modulated properties, has not been successful due to difficulties in controlling the diffusion of intercalants. Here, we show formation of angstrom-wide conductive channels (~4.3 A) in black phosphorus by Cu intercalation. The atomic structure, resultant microstructural effects, intercalation mechanism, and local variations of electronic properties modulated in black phosphorus by Cu intercalation were investigated extensively by transmission electron microscopy including in situ observation, DFT calculation, and conductive atomic force microscopy. △ Less

Submitted 21 January, 2021; originally announced January 2021.

The Open Catalyst 2020 (OC20) Dataset and Community Challenges

Authors: Lowik Chanussot, Abhishek Das, Siddharth Goyal, Thibaut Lavril, Muhammed Shuaibi, Morgane Riviere, Kevin Tran, Javier Heras-Domingo, Caleb Ho, Weihua Hu, Aini Palizhati, Anuroop Sriram, Brandon Wood, Junwoong Yoon, Devi Parikh, C. Lawrence Zitnick, Zachary Ulissi

Abstract: Catalyst discovery and optimization is key to solving many societal and energy challenges including solar fuels synthesis, long-term energy storage, and renewable fertilizer production. Despite considerable effort by the catalysis community to apply machine learning models to the computational catalyst discovery process, it remains an open challenge to build models that can generalize across both… ▽ More Catalyst discovery and optimization is key to solving many societal and energy challenges including solar fuels synthesis, long-term energy storage, and renewable fertilizer production. Despite considerable effort by the catalysis community to apply machine learning models to the computational catalyst discovery process, it remains an open challenge to build models that can generalize across both elemental compositions of surfaces and adsorbate identity/configurations, perhaps because datasets have been smaller in catalysis than related fields. To address this we developed the OC20 dataset, consisting of 1,281,040 Density Functional Theory (DFT) relaxations (~264,890,000 single point evaluations) across a wide swath of materials, surfaces, and adsorbates (nitrogen, carbon, and oxygen chemistries). We supplemented this dataset with randomly perturbed structures, short timescale molecular dynamics, and electronic structure analyses. The dataset comprises three central tasks indicative of day-to-day catalyst modeling and comes with pre-defined train/validation/test splits to facilitate direct comparisons with future model development efforts. We applied three state-of-the-art graph neural network models (CGCNN, SchNet, Dimenet++) to each of these tasks as baseline demonstrations for the community to build on. In almost every task, no upper limit on model size was identified, suggesting that even larger models are likely to improve on initial results. The dataset and baseline models are both provided as open resources, as well as a public leader board to encourage community contributions to solve these important tasks. △ Less

Submitted 24 September, 2021; v1 submitted 19 October, 2020; originally announced October 2020.

Comments: 37 pages, 11 figures, submitted to ACS Catalysis

An Introduction to Electrocatalyst Design using Machine Learning for Renewable Energy Storage

Authors: C. Lawrence Zitnick, Lowik Chanussot, Abhishek Das, Siddharth Goyal, Javier Heras-Domingo, Caleb Ho, Weihua Hu, Thibaut Lavril, Aini Palizhati, Morgane Riviere, Muhammed Shuaibi, Anuroop Sriram, Kevin Tran, Brandon Wood, Junwoong Yoon, Devi Parikh, Zachary Ulissi

Abstract: Scalable and cost-effective solutions to renewable energy storage are essential to addressing the world's rising energy needs while reducing climate change. As we increase our reliance on renewable energy sources such as wind and solar, which produce intermittent power, storage is needed to transfer power from times of peak generation to peak demand. This may require the storage of power for hours… ▽ More Scalable and cost-effective solutions to renewable energy storage are essential to addressing the world's rising energy needs while reducing climate change. As we increase our reliance on renewable energy sources such as wind and solar, which produce intermittent power, storage is needed to transfer power from times of peak generation to peak demand. This may require the storage of power for hours, days, or months. One solution that offers the potential of scaling to nation-sized grids is the conversion of renewable energy to other fuels, such as hydrogen or methane. To be widely adopted, this process requires cost-effective solutions to running electrochemical reactions. An open challenge is finding low-cost electrocatalysts to drive these reactions at high rates. Through the use of quantum mechanical simulations (density functional theory), new catalyst structures can be tested and evaluated. Unfortunately, the high computational cost of these simulations limits the number of structures that may be tested. The use of machine learning may provide a method to efficiently approximate these calculations, leading to new approaches in finding effective electrocatalysts. In this paper, we provide an introduction to the challenges in finding suitable electrocatalysts, how machine learning may be applied to the problem, and the use of the Open Catalyst Project OC20 dataset for model training. △ Less

Submitted 14 October, 2020; originally announced October 2020.

Comments: 27 pages

ACM Class: I.2.6; J.2

Giant transition-state enhancement of quasiparticle spin-Hall effect in an exchange-spin-split superconductor detected by non-local magnon spin-transport

Authors: Kun-Rok Jeon, Jae-Chun Jeon, Xilin Zhou, Andrea Migliorini, Jiho Yoon, Stuart S. P. Parkin

Abstract: Although recent experiments and theories have shown a variety of exotic transport properties of non-equilibrium quasiparticles (QPs) in superconductor (SC)-based devices with either Zeeman or exchange spin-splitting, how QP interplays with magnon spin currents remains elusive. Here, using non-local magnon spin-transport devices where a singlet SC (Nb) on top of a ferrimagnetic insulator (Y3Fe5O12)… ▽ More Although recent experiments and theories have shown a variety of exotic transport properties of non-equilibrium quasiparticles (QPs) in superconductor (SC)-based devices with either Zeeman or exchange spin-splitting, how QP interplays with magnon spin currents remains elusive. Here, using non-local magnon spin-transport devices where a singlet SC (Nb) on top of a ferrimagnetic insulator (Y3Fe5O12) serves as a magnon spin detector, we demonstrate that the conversion efficiency of magnon spin to QP charge via inverse spin-Hall effect (iSHE) in such an exchange-spin-split SC can be greatly enhanced by up to 3 orders of magnitude compared with that in the normal state, particularly when its interface superconducting gap matches the magnon spin accumulation. Through systematic measurements with varying the current density and SC thickness, we identify that superconducting coherence peaks and exchange spin-splitting of the QP density-of-states, yielding a larger spin excitation while retaining a modest QP charge-imbalance relaxation, are responsible for the giant QP iSHE. The latter exchange-field-modified QP relaxation is experimentally proved by spatially resolved measurements with varying the separation of electrical contacts on the spin-split Nb. △ Less

Submitted 28 August, 2020; v1 submitted 20 April, 2020; originally announced April 2020.

Comments: 30 pages, 6 figures

Journal ref: ACS Nano 14, 15874 (2020)

Methods for comparing uncertainty quantifications for material property predictions

Authors: Kevin Tran, Willie Neiswanger, Junwoong Yoon, Qingyang Zhang, Eric Xing, Zachary W. Ulissi

Abstract: Data science and informatics tools have been proliferating recently within the computational materials science and catalysis fields. This proliferation has spurned the creation of various frameworks for automated materials screening, discovery, and design. Underpinning these frameworks are surrogate models with uncertainty estimates on their predictions. These uncertainty estimates are instrumenta… ▽ More Data science and informatics tools have been proliferating recently within the computational materials science and catalysis fields. This proliferation has spurned the creation of various frameworks for automated materials screening, discovery, and design. Underpinning these frameworks are surrogate models with uncertainty estimates on their predictions. These uncertainty estimates are instrumental for determining which materials to screen next, but the computational catalysis field does not yet have a standard procedure for judging the quality of such uncertainty estimates. Here we present a suite of figures and performance metrics derived from the machine learning community that can be used to judge the quality of such uncertainty estimates. This suite probes the accuracy, calibration, and sharpness of a model quantitatively. We then show a case study where we judge various methods for predicting density-functional-theory-calculated adsorption energies. Of the methods studied here, we find that the best performer is a model where a convolutional neural network is used to supply features to a Gaussian process regressor, which then makes predictions of adsorption energies along with corresponding uncertainty estimates. △ Less

Submitted 20 February, 2020; v1 submitted 20 December, 2019; originally announced December 2019.

Comments: 24 pages, 7 figures Submitted to Machine Learning: Science & Technology journal of IOP

Fabrication and Imaging of Monolayer Phosphorene with Preferred Edge Configurations via Graphene-Assisted Layer-by-Layer Thinning

Authors: Yangjin Lee, Sol Lee, Jun-Yoeong Yoon, Jinwoo Cheon, Hu Young Jeong, Kwanpyo Kim

Abstract: Phosphorene, a monolayer of black phosphorus (BP), is an elemental two-dimensional material with interesting physical properties, such as high charge carrier mobility and exotic anisotropic in-plane properties. To fundamentally understand these various physical properties, it is critically important to conduct an atomic-scale structural investigation of phosphorene, particularly regarding various… ▽ More Phosphorene, a monolayer of black phosphorus (BP), is an elemental two-dimensional material with interesting physical properties, such as high charge carrier mobility and exotic anisotropic in-plane properties. To fundamentally understand these various physical properties, it is critically important to conduct an atomic-scale structural investigation of phosphorene, particularly regarding various defects and preferred edge configurations. However, it has been challenging to investigate mono- and few-layer phosphorene because of technical difficulties arising in the preparation of a high-quality sample and damages induced during the characterization process. Here, we successfully fabricate high-quality monolayer phosphorene using a controlled thinning process with transmission electron microscopy, and subsequently perform atomic-resolution imaging. Graphene protection suppresses the e-beam-induced damage to multi-layer BP and one-side graphene protection facilitates the layer-by-layer thinning of the samples, rendering high-quality monolayer and bilayer regions. We also observe the formation of atomic-scale crystalline edges predominantly aligned along the zigzag and (101) terminations, which is originated from edge kinetics under e-beam-induced sputtering process. Our study demonstrates a new method to image and precisely manipulate the thickness and edge configurations of air-sensitive two-dimensional materials. △ Less

Submitted 3 December, 2019; originally announced December 2019.

Comments: 21 pages, 5 figures

Journal ref: Nano Letters 2019

Anomalous thickness-dependent electrical conductivity in van der Waals layered transition metal halide, Nb_3Cl_8

Authors: Jiho Yoon, Edouard Lesne, Kornelia Sklarek, John Sheckelton, Chris Pasco, Stuart S. P. Parkin, Tyrel M. McQueen, Mazhar N. Ali

Abstract: Understanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb3Cl8, over a… ▽ More Understanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb3Cl8, over a wide range of thicknesses both with and without hexagonal boron nitride (hBN) encapsulation. The conductivity is found to increase by more than three orders of magnitude when the thickness is decreased from 280 μm to 5 nm, at 300 K. At low temperatures and below ~50 nm, the conductance becomes thickness independent, implying surface conduction is dominating. Temperature dependent conductivity measurements indicate Nb3Cl8 is an insulator, however the effective activation energy decreases from a bulk value of 310 meV to 140 meV by 5nm. X-ray photoelectron spectroscopy (XPS) shows mild surface oxidation in devices without hBN capping, however, no significant difference in transport is observed when compared to the capped devices, implying the thickness dependent transport behavior is intrinsic to the material. A conduction mechanism comprised of a higher conductivity surface channel in parallel with a lower conductivity interlayer channel is discussed. △ Less

Submitted 13 November, 2019; originally announced November 2019.

Comments: 17 pages, 3 figures

Evidence of Higher Order Topology in Multilayer WTe$_2$ from Josephson Coupling through Anisotropic Hinge States

Authors: Yong-Bin Choi, Yingming Xie, Chui-Zhen Chen, Jin-Ho Park, Su-Beom Song, Jiho Yoon, Bum Joon Kim, Takashi Taniguchi, Kenji Watanabe, Hu-Jong Lee, Jong-Hwan Kim, Kin Chung Fong, Mazhar N. Ali, Kam Tuen Law, Gil-Ho Lee

Abstract: The noncentrosymmetric Td-WTe$_2$, previously known as a type-II Weyl semimetal, is expected to have higher order topological phases with topologically protected, helical one-dimensional (1D) hinge states when their scarcely separated Weyl points get annihilated. However, the detection of these hinge states is difficult in the presence of the semimetallic behaviour of the bulk. Here, we spatially… ▽ More The noncentrosymmetric Td-WTe$_2$, previously known as a type-II Weyl semimetal, is expected to have higher order topological phases with topologically protected, helical one-dimensional (1D) hinge states when their scarcely separated Weyl points get annihilated. However, the detection of these hinge states is difficult in the presence of the semimetallic behaviour of the bulk. Here, we spatially resolved the hinge states by analysing the magnetic field interference of supercurrent in Nb-WTe$_2$-Nb proximity Josephson junctions. The Josephson current along the a-axis of the WTe$_2$ crystal, but not along the b-axis, showed sharp enhancements at the edges of the junction; the amount of enhanced Josephson current was comparable to the upper limits of a single 1D conduction channel. Our experimental observations provide evidence of the higher order topological phase in WTe$_2$ and its corresponding anisotropic topological hinge states, in good agreement with theoretical calculations. Our work paves the way for hinge transport studies on topological semimetals in superconducting heterostructures, including their topological superconductivity. △ Less

Submitted 6 September, 2019; v1 submitted 5 September, 2019; originally announced September 2019.

Comments: 29 pages, 11 figures

Universality, scaling and collapse in supercritical fluids

Authors: Min Young Ha, Tae Jun Yoon, Tsvi Tlusty, YongSeok Jho, Won Bo Lee

Abstract: The Supercritical Fluid (SCF) is known to exhibit salient dynamic and thermodynamic crossovers and inhomogeneous molecular distribution. But the question as to what basic physics underlies these microscopic and macroscopic anomalies remains open. Here, using an order parameter extracted by machine learning, the fraction of gas-like (or liquid-like) molecules, we find simplicity and universality in… ▽ More The Supercritical Fluid (SCF) is known to exhibit salient dynamic and thermodynamic crossovers and inhomogeneous molecular distribution. But the question as to what basic physics underlies these microscopic and macroscopic anomalies remains open. Here, using an order parameter extracted by machine learning, the fraction of gas-like (or liquid-like) molecules, we find simplicity and universality in SCF: First, all isotherms of a given fluid collapse onto a single master curve described by a scaling relation. The observed power law holds from the high-temperature and pressure regime down to the critical point where it diverges. Second, phase diagrams of different compounds collapse onto their master curves by the same scaling exponent, thereby demonstrating a putative law of corresponding supercritical states in simple fluids. The reported results support a model of the SCF as a mixture of two interchangeable microstates, whose spatiotemporal dynamics gives rise to unique macroscopic properties. △ Less

Submitted 21 February, 2019; originally announced February 2019.

Comments: 10 pages, 2 figures

Topological extension of the isomorph theory based on the Shannon entropy

Authors: Tae Jun Yoon, Min Young Ha, Emanuel A. Lazar, Won Bo Lee, Youn-Woo Lee

Abstract: Isomorph theory is one of the promising theories to understand the quasi-universal relationship between thermodynamic, dynamic and structural characteristics. Based on the hidden scale invariance of the inverse power law potentials, it rationalizes the excess entropy scaling law of dynamic properties. This work aims to show that this basic idea of isomorph theory can be extended by examining the m… ▽ More Isomorph theory is one of the promising theories to understand the quasi-universal relationship between thermodynamic, dynamic and structural characteristics. Based on the hidden scale invariance of the inverse power law potentials, it rationalizes the excess entropy scaling law of dynamic properties. This work aims to show that this basic idea of isomorph theory can be extended by examining the microstructural features of the system. Using the topological framework in conjunction with the entropy calculation algorithm, we demonstrate that Voronoi entropy, a measure of the topological diversity of single atoms, provides a scaling law for the transport properties of soft-sphere fluids, which is comparable to the frequently used excess entropy scaling. By examining the relationship between the Voronoi entropy and the solid-like fraction of simple fluids, we suggest that the Frenkel line, a rigid-nonrigid crossover line, {be} a topological isomorphic line where the scaling relation qualitatively changes. △ Less

Submitted 30 July, 2019; v1 submitted 9 January, 2019; originally announced January 2019.

Comments: 21 pages, 7 figures

Journal ref: Phys. Rev. E 100, 012118 (2019)

Interplay of Inhomogeneous Electrochemical Reactions with Mechanical Responses in Silicon-Graphite Anode and its Impacts on Degradation

Authors: Junhyuk Moon, Shinya Wakita, Heechul Jung, Sungnim Cho, Jaegu Yoon, Joowook Lee, Sihyung Lee, Kimihiko Ito, Yoshimi Kubo, Heung Chan Lee, Young-Gyoon Ryu

Abstract: Enhanced EV market penetration requires durability of the battery with high energy throughput. For long-term cycle stability of silicon-graphite anode capable of high energy density, the reversible redox reactions are crucial. Here, we unveil intriguing electrochemical phenomena such as crosstalk of lithium ion ($Li^{+}$) between silicon and graphite, $Li^{+}$ accumulation in silicon, and capacity… ▽ More Enhanced EV market penetration requires durability of the battery with high energy throughput. For long-term cycle stability of silicon-graphite anode capable of high energy density, the reversible redox reactions are crucial. Here, we unveil intriguing electrochemical phenomena such as crosstalk of lithium ion ($Li^{+}$) between silicon and graphite, $Li^{+}$ accumulation in silicon, and capacity depression of graphite under high pressure, which engender the irreversible redox reactions. Active material properties, i.e. the size of silicon and the hardness of graphite, silicon-graphite anode, are modified based on the unveiled results to enhance the reaction homogeneity and reduce subsequent degradation. Owing to the property change of the anode active materials, silicon-graphite anode paired with high nickel cathode allows the prismatic cell with 8.7 Ah to reach cycling performance over 750 cycles with volumetric energy density of 665 $Whl^{-1}$, which is corresponding to 800 $Whl^{-1}$ in the prismatic cell with 87 Ah. Finally, the cycling performance can be tailored by the design of electrode regulating $Li^{+}$ crosstalk. Our findings provide electrochemical insights into degradation mechanisms and a promising direction on the progressive improvement of materials and the design of electrodes in silicon-graphite anode. △ Less

Submitted 10 June, 2019; v1 submitted 5 January, 2019; originally announced January 2019.

Comments: 14 pages, 5 figures, 1 table (11 pages and 7 figures for supplementary materials)

A corresponding-state framework for the structural transition of supercritical fluids across the Widom delta

Authors: Tae Jun Yoon, Min Young Ha, Won Bo Lee, Youn-Woo Lee

Abstract: This work proposes a classification algorithm based on the radical Voronoi tessellation to define the Widom delta, supercritical gas-liquid coexistence region, of polyatomic molecules. In specific, we use a weighted mean-field classification method to classify a molecule into either gas-like or liquid-like. Classical percolation theory methods are adopted to understand the generality of the struct… ▽ More This work proposes a classification algorithm based on the radical Voronoi tessellation to define the Widom delta, supercritical gas-liquid coexistence region, of polyatomic molecules. In specific, we use a weighted mean-field classification method to classify a molecule into either gas-like or liquid-like. Classical percolation theory methods are adopted to understand the generality of the structural transition and to locate the Widom delta. A structural analysis on various supercritical fluids shows that the proposed method detects the influence of the attractive interaction on the structural transition of supercritical fluids. Moreover, we demonstrate that the supercritical gas-liquid coexistence region of water overlap with the ridges of the response function maxima. From the pressure-temperature relation, a three-parameter corresponding state theorem is derived, which states that the fraction of gas-like molecules of a substance is equal to that of another if their reduced pressure, reduced temperature and the critical compressibility factor are the same. △ Less

Submitted 22 April, 2019; v1 submitted 22 December, 2018; originally announced December 2018.

Comments: 24 pages, 7 figures

Journal ref: J. Chem. Phys. 150, 154503 (2019)

Double bit in-plane magnetic skyrmions on a track

Authors: Kyoung-Woong Moon, Jungbum Yoon, Changsoo Kim, Chanyong Hwang

Abstract: A magnetic skyrmion isusually refers to a twisted spin texture surrounded by uniformly aligned out-of-plane spinsin the background of a uniformly magnetized state. The invariance of the magnetic skyrmion conserves its topological charge under any continuous transformations of the spin textures, leads to which represents the robustness of a magnetic skyrmion their texture against external perturbat… ▽ More A magnetic skyrmion isusually refers to a twisted spin texture surrounded by uniformly aligned out-of-plane spinsin the background of a uniformly magnetized state. The invariance of the magnetic skyrmion conserves its topological charge under any continuous transformations of the spin textures, leads to which represents the robustness of a magnetic skyrmion their texture against external perturbations, making it ideal to use skyrmions as . Such a behaviour is required for an ideal information carriers. To date, most magnetic skyrmion studies have been performedfocused on in perpendicularly magnetized systems, where the skyrmion topological number is determined by the relative orientation between the core and /outer perpendicular magnetization directions of the skyrmion is either up/down or down/up. Here we show that there also exists a new type of magnetic skyrmion with surrounding spins to be uniformly aligned to the in-plane direction. By continuous transformation and relaxation of the spin textures of out-of-plane skyrmions, we showed that an in-plane skyrmion, where the background magnetization is in the in-plane direction, is also possible. Different from the conventional perpendicular magnetic skyrmionsContrary to the skyrmions in a perpendicularly magnetized state, the in-plane magnetic skyrmions with opposite signs of topological charges can inherentlycharges can inherently coexist in the in-plane magnetization system coexist. Moreover, the iIn-plane skyrmions of opposite charge can move together by an electric current and exhibit with an opposite spin Hall effect. These findings showdemonstrate the inherent possibility of a double-bit transfer in a single magnetic wire that is not possible in a perpendicularly magnetized system. △ Less

Submitted 29 November, 2018; originally announced November 2018.

Comments: 22 pages, 4 figures

Journal ref: Phys. Rev. Applied 12, 064054 (2019)

Topological generalization of the rigid-nonrigid transition in soft-sphere and hard-sphere fluids

Authors: Tae Jun Yoon, Emanuel A. Lazar, Min Young Ha, Won Bo Lee, Youn-Woo Lee

Abstract: A fluid particle changes its dynamics from diffusive to oscillatory as the system density increases up to the melting density. Hence, the notion of the Frenkel line was introduced to demarcate the fluid region into rigid and nonrigid liquid subregions based on the collective particle dynamics. In this work, we apply a topological framework to locate the Frenkel lines of the soft-sphere and the har… ▽ More A fluid particle changes its dynamics from diffusive to oscillatory as the system density increases up to the melting density. Hence, the notion of the Frenkel line was introduced to demarcate the fluid region into rigid and nonrigid liquid subregions based on the collective particle dynamics. In this work, we apply a topological framework to locate the Frenkel lines of the soft-sphere and the hard-sphere models relying on the system configurations. The topological characteristics of the ideal gas and the maximally random jammed state are first analyzed, then the classification scheme designed in our earlier work is applied. The classification result shows that the fraction of solid-like atoms increases from zero to one in the rigid liquid region. The dependence of the solid-like fraction on the bulk density is understood based on the theory of fluid polyamorphism. The percolation behavior of solid-like clusters is described based on the fraction of solid-like molecules in an integrated manner. The crossover densities are obtained by examining the percolation of solid-like clusters. The resultant crossover densities of soft-sphere fluids converge to that of hard-sphere fluid. Hence, the topological method successfully highlights the generality of the Frenkel line. △ Less

Submitted 10 November, 2018; originally announced November 2018.

Comments: 9 pages, 5 figures, 2 tables

Journal ref: Phys. Rev. E 99, 052603 (2019)

Topological Characterization of Rigid-Nonrigid Transition across the Frenkel Line

Authors: Tae Jun Yoon, Min Young Ha, Emanuel A. Lazar, Won Bo Lee, Youn-Woo Lee

Abstract: The dynamics of supercritical fluids, a state of matter beyond the gas-liquid critical point, changes from diffusive to oscillatory motions at high pressure. This transition is believed to occur across a locus of thermodynamic states called the Frenkel line. The Frenkel line has been extensively investigated from the viewpoint of the dynamics, but its structural meaning is not still well understoo… ▽ More The dynamics of supercritical fluids, a state of matter beyond the gas-liquid critical point, changes from diffusive to oscillatory motions at high pressure. This transition is believed to occur across a locus of thermodynamic states called the Frenkel line. The Frenkel line has been extensively investigated from the viewpoint of the dynamics, but its structural meaning is not still well understood. This letter interprets the mesoscopic picture of the Frenkel line entirely based on a topological and geometrical framework. This discovery makes it possible to understand the mechanism of rigid/non-rigid transition based not on the dynamics of individual atoms, but on their instantaneous configurations. The topological classification method reveals that the percolation of solid-like structures occurs above the rigid-nonrigid crossover densities. △ Less

Submitted 14 October, 2018; v1 submitted 8 July, 2018; originally announced July 2018.

Comments: 15 pages, 4 figures, and the Supporting Information

"Two-phase" thermodynamics of the Frenkel line

Authors: Tae Jun Yoon, Min Young Ha, Won Bo Lee, Youn-Woo Lee

Abstract: The Frenkel line, a crossover line between rigid and nonrigid dynamics of fluid particles, has recently been the subject of intense debate regarding its relevance as a partitioning line of supercritical phase, where the main criticism comes from the theoretical treatment of collective particle dynamics. From an independent point of view, this Letter suggests that the two-phase thermodynamics model… ▽ More The Frenkel line, a crossover line between rigid and nonrigid dynamics of fluid particles, has recently been the subject of intense debate regarding its relevance as a partitioning line of supercritical phase, where the main criticism comes from the theoretical treatment of collective particle dynamics. From an independent point of view, this Letter suggests that the two-phase thermodynamics model may alleviate this contentious situation. The model offers new criteria for defining the Frenkel line in the supercritical region and builds a robust connection among the preexisting, seemingly inconsistent definitions. In addition, one of the dynamic criteria locates the rigid-nonrigid transition of the soft-sphere and the hard-sphere models. Hence, we suggest the Frenkel line be considered as a dynamic rigid-nonrigid fluid boundary, without any relation to gas-liquid transition. These findings provide an integrative viewpoint combining fragmentized definitions of the Frenkel line, allowing future studies to be carried out in a more reliable manner. △ Less

Submitted 1 August, 2018; v1 submitted 20 June, 2018; originally announced June 2018.

Comments: 15 pages, 4 figures, 1 Supporting Information (PDF file)

Journal ref: J. Phys. Chem. Lett., 2018, 9, 4550-4554

Thouless time for mass-deformed SYK

Authors: Tomoki Nosaka, Dario Rosa, Junggi Yoon

Abstract: We study the onset of RMT dynamics in the mass-deformed SYK model (i.e. an SYK model deformed by a quadratic random interaction) in terms of the strength of the quadratic deformation. We use as chaos probes both the connected unfolded Spectral Form Factor (SFF) as well as the Gaussian-filtered SFF, which has been recently introduced in the literature. We show that they detect the chaotic/integrabl… ▽ More We study the onset of RMT dynamics in the mass-deformed SYK model (i.e. an SYK model deformed by a quadratic random interaction) in terms of the strength of the quadratic deformation. We use as chaos probes both the connected unfolded Spectral Form Factor (SFF) as well as the Gaussian-filtered SFF, which has been recently introduced in the literature. We show that they detect the chaotic/integrable transition of the mass-deformed SYK model at different values of the mass deformation: the Gaussian-filtered SFF sees the transition for large values of the mass deformation; the connected unfolded SFF sees the transition at small values. The latter is in qualitative agreement with the transition as seen by the OTOCs. We argue that the chaotic/integrable deformation affect the energy levels inhomogeneously: for small values of the mass deformation only the low-lying states are modified while for large values of the mass deformation also the states in the bulk of the spectrum move to the integrable behavior. △ Less

Submitted 26 April, 2018; originally announced April 2018.

Comments: 44 pages, many figures

Report number: KIAS-P18037

Purely electronic nanometallic ReRAM

Authors: Yang Lu, Jung Ho Yoon, Yanhao Dong, I-Wei Chen

Abstract: Resistance switching random access memory (ReRAM), with the ability to repeatedly modulate electrical resistance, has been highlighted as a feasible high-density memory with the potential to replace negative-AND (NAND) flash memory. Such resistance modulation usually involves ion migration and filament formation, which usually lead to relatively low device reliability and yield. Resistance switchi… ▽ More Resistance switching random access memory (ReRAM), with the ability to repeatedly modulate electrical resistance, has been highlighted as a feasible high-density memory with the potential to replace negative-AND (NAND) flash memory. Such resistance modulation usually involves ion migration and filament formation, which usually lead to relatively low device reliability and yield. Resistance switching can also come from an entirely electronic origin, as in nanometallic memory, by electron trapping and detrapping. Recent research has revealed additional merits of its mechanism, which entails smart, atomic-sized floating gates that can be easily engineered in amorphous Si, oxides, and nitrides. This article addresses the basic ideas of nanometallic ReRAM, which may also be a contender for analogue computing and non-von Neumann-type computation. △ Less

Submitted 9 April, 2018; originally announced April 2018.

Widom delta of supercritical gas-liquid coexistence

Authors: Min Young Ha, Tae Jun Yoon, Tsvi Tlusty, Yongseok Jho, Won Bo Lee

Abstract: We report on the coexistence of liquid-like and gas-like structures in supercritical fluid (SCF). The deltoid coexistence region encloses the Widom line, and may therefore be termed the "Widom delta". Machine learning analysis of simulation data shows continuous transition across the delta, from liquid-like to gas-like states, with fractions following a simplified two-state model. This suggests a… ▽ More We report on the coexistence of liquid-like and gas-like structures in supercritical fluid (SCF). The deltoid coexistence region encloses the Widom line, and may therefore be termed the "Widom delta". Machine learning analysis of simulation data shows continuous transition across the delta, from liquid-like to gas-like states, with fractions following a simplified two-state model. This suggests a microscopic view of the SCF as a mixture of liquid-like and gas-like structures, where the anomalous behavior near the critical point originates from fluctuations between the two types. △ Less

Submitted 17 January, 2018; originally announced January 2018.

Supersymmetric SYK Model with Global Symmetry

Authors: Prithvi Narayan, Junggi Yoon

Abstract: In this paper, we introduce an $\mathcal{N}=1$ supersymmetric SYK model with $SO(q)$ global symmetry. We study the large $N$ expansion of the bi-local collective action of our model. At strong coupling limit, this model exhibits a super-reparametrization symmetry, and the $SO(q)$ global symmetry is enhanced to a $\widehat{SO}(q)$ local symmetry. The corresponding symmetry algebra is the semi-direc… ▽ More In this paper, we introduce an $\mathcal{N}=1$ supersymmetric SYK model with $SO(q)$ global symmetry. We study the large $N$ expansion of the bi-local collective action of our model. At strong coupling limit, this model exhibits a super-reparametrization symmetry, and the $SO(q)$ global symmetry is enhanced to a $\widehat{SO}(q)$ local symmetry. The corresponding symmetry algebra is the semi-direct product of the super-Virasoro and the super-Kac-Moody algebras. These emergent symmetries are spontaneously and explicitly broken, which leads to a low energy effective action: super-Schwarzian action plus an action of a super-particle on the $SO(q)$ group manifold. We analyze the zero mode contributions to the chaotic behavior of four point functions in various $SO(q)$ channels. In singlet channel, we show that the out-of-time-ordered correlators related to bosonic bi-locals exhibit the saturation of the chaos bound as in the non-SUSY SYK model. On the other hand, we find that the ones with fermionic bi-locals in the singlet channel have ${π\overβ}$ Lyapunov exponent. In the anti-symmetric channel, we demonstrate that the out-of-time-ordered correlator related to a $SO(q)$ generator grows linearly in time. We also compute the non-zero mode contributions which give consistent corrections to the leading Lyapunov exponents from the zero modes. △ Less

Submitted 7 December, 2017; originally announced December 2017.

Comments: 48 pages + Appendix, 1 figure and 2 tables

Oscillatory spin-orbit torque switching induced by field-like torques

Authors: Jong Min Lee, Jae Hyun Kwon, Rajagopalan Ramaswamy, Jung Bum Yoon, Jaesung Son, Xuepeng Qiu, Rahul Mishra, Shalabh Srivastava, Kaiming Cai, Hyunsoo Yang

Abstract: Deterministic magnetization switching using spin-orbit torque (SOT) has recently emerged as an efficient means to electrically control the magnetic state of ultrathin magnets. The SOT switching still lacks in oscillatory switching characteristics over time, therefore, it is limited to bipolar operation where a change in polarity of the applied current or field is required for bistable switching. T… ▽ More Deterministic magnetization switching using spin-orbit torque (SOT) has recently emerged as an efficient means to electrically control the magnetic state of ultrathin magnets. The SOT switching still lacks in oscillatory switching characteristics over time, therefore, it is limited to bipolar operation where a change in polarity of the applied current or field is required for bistable switching. The coherent rotation based oscillatory switching schemes cannot be applied to SOT because the SOT switching occurs through expansion of magnetic domains. Here, we experimentally achieve oscillatory switching in incoherent SOT process by controlling domain wall dynamics. We find that a large field-like component can dynamically influence the domain wall chirality which determines the direction of SOT switching. Consequently, under nanosecond current pulses, the magnetization switches alternatively between the two stable states. By utilizing this oscillatory switching behavior we demonstrate a unipolar deterministic SOT switching scheme by controlling the current pulse duration. △ Less

Submitted 14 November, 2017; originally announced November 2017.

Anomalous spin-orbit torque switching due to field-like torque-assisted domain wall reflection

Authors: Jungbum Yoon, Seo-Won Lee, Jae Hyun Kwon, Jong Min Lee, Jaesung Son, Xuepeng Qiu, Kyung-Jin Lee, Hyunsoo Yang

Abstract: Spin-orbit torques (SOT) allow the electrical control of magnetic states. Current-induced SOT switching of the perpendicular magnetization is of particular technological importance. The SOT consists of damping-like and field-like torques so that the efficient SOT switching requires to understand combined effects of the two torque-components. Previous quasi-static measurements have reported an incr… ▽ More Spin-orbit torques (SOT) allow the electrical control of magnetic states. Current-induced SOT switching of the perpendicular magnetization is of particular technological importance. The SOT consists of damping-like and field-like torques so that the efficient SOT switching requires to understand combined effects of the two torque-components. Previous quasi-static measurements have reported an increased switching probability with the width of current pulses, as predicted with considering the damping-like torque only. Here we report a decreased switching probability at longer pulse-widths, based on time-resolved measurements. Micromagnetic analysis reveals that this anomalous SOT switching results from domain wall reflections at sample edges. The domain wall reflection is found to strongly depend on the field-like torque and its relative sign to the damping-like torque. Our result demonstrates a key role of the field-like torque in the deterministic SOT switching and notifies the importance of sign correlation of the two torque-components, which may shed light on the SOT switching mechanism. △ Less

Submitted 14 November, 2017; originally announced November 2017.

Journal ref: Science Advances 3, e1603099 (2017)

Oxygen Partial Pressure during Pulsed Laser Deposition: Deterministic Role on Thermodynamic Stability of Atomic Termination Sequence at SrRuO3/BaTiO3 Interface

Authors: Yeong Jae Shin, Lingfei Wang, Yoonkoo Kim, Ho-Hyun Nahm Daesu Lee, Jeong Rae Kim, Sang Mo Yang, Jong-Gul Yoon, Jin-Seok Chung, Miyoung Kim, Seo Hyoung Chang, Tae Won Noh

Abstract: With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface at… ▽ More With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO2 or TiO2-SrO) with oxygen partial pressure (PO2) during PLD. We found that a uniform SrO-TiO2 termination sequence at the SRO/BTO interface can be achieved by lowering the PO2 to 5 mTorr, regardless of the total background gas pressure (Ptotal), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures. △ Less

Submitted 10 August, 2017; originally announced August 2017.

Comments: 27 pages, 6 figures, Supporting Information

Overcoming the Fundamental Barrier Thickness Limits of Ferroelectric Tunnel Junctions through BaTiO3/SrTiO3 Composite Barriers

Authors: Lingfei Wang, Myung Rae Cho, Yeong Jae Shin, Jeong Rae Kim, Saikat Das, Jong-Gul Yoon, Jin-Seok Chung, Tae Won Noh

Abstract: Ferroelectric tunnel junctions (FTJs) have attracted increasing research interest as a promising candidate for non-volatile memories. Recently, significant enhancements of tunneling electroresistance (TER) have been realized through modifications of electrode materials. However, direct control of the FTJ performance through modifying the tunneling barrier has not been adequately explored. Here, ad… ▽ More Ferroelectric tunnel junctions (FTJs) have attracted increasing research interest as a promising candidate for non-volatile memories. Recently, significant enhancements of tunneling electroresistance (TER) have been realized through modifications of electrode materials. However, direct control of the FTJ performance through modifying the tunneling barrier has not been adequately explored. Here, adding a new direction to FTJ research, we fabricated FTJs with BaTiO3 single barriers (SB-FTJs) and BaTiO3/SrTiO3 composite barriers (CB-FTJs), and reported a systematic study of FTJ performances by varying the barrier thicknesses and compositions. For the SB-FTJs, the TER is limited by pronounced leakage current for ultrathin barriers and extremely small tunneling current for thick barriers. For the CB-FTJs, the extra SrTiO3 barrier provides an additional degree of freedom to modulate the barrier potential and tunneling behavior. The resultant high tunability can be utilized to overcome the barrier thickness limits and enhance the overall CB-FTJ performances beyond those of SB-FTJ. Our results reveal a new paradigm to manipulate the FTJs through designing multilayer tunneling barriers with hybrid functionalities. △ Less

Submitted 21 July, 2017; originally announced July 2017.

Comments: 5 figures

Journal ref: Nano Letters 16 (6) 3911 (2016)

SYK Models and SYK-like Tensor Models with Global Symmetry

Authors: Junggi Yoon

Abstract: In this paper, we study an SYK model and an SYK-like tensor model with global symmetry. First, we study the large $N$ expansion of the bi-local collective action for the SYK model with manifest global symmetry. We show that the global symmetry is enhanced to a local symmetry at strong coupling limit, and the corresponding symmetry algebra is the Kac-Moody algebra. The emergent local symmetry toget… ▽ More In this paper, we study an SYK model and an SYK-like tensor model with global symmetry. First, we study the large $N$ expansion of the bi-local collective action for the SYK model with manifest global symmetry. We show that the global symmetry is enhanced to a local symmetry at strong coupling limit, and the corresponding symmetry algebra is the Kac-Moody algebra. The emergent local symmetry together with the emergent reparametrization is spontaneously and explicit broken. This leads to a low energy effective action. We evaluate four point functions, and obtain spectrum of our model. We derive the low energy effective action and analyze the chaotic behavior of the four point functions. We also consider the recent 3D gravity conjecture for our model. We also introduce an SYK-like tensor model with global symmetry. We first study chaotic behavior of four point functions in various channels for the rank-3 case, and generalize this into a rank-$(q-1)$ tensor model. △ Less

Submitted 7 August, 2017; v1 submitted 6 July, 2017; originally announced July 2017.

Comments: 61 pages, 8 figures; v2: typos corrected, references added, appendix D revised; v3: typos corrected, 1 figure added for clarification, substantial revisions and clarifications (conclusions unchanged)

Supersymmetric SYK Model: Bi-local Collective Superfield/Supermatrix Formulation

Authors: Junggi Yoon

Abstract: We discuss the bi-local collective theory for the $\mathcal{N}=1,2$ supersymmetric Sachdev-Ye-Kitaev (SUSY SYK) models. We construct a bi-local superspace, and formulate the bi-local collective superfield theory of the one-dimensional SUSY vector model. The bi-local collective theory provides systematic analysis of the SUSY SYK models. We find that this bi-local collective theory naturally leads t… ▽ More We discuss the bi-local collective theory for the $\mathcal{N}=1,2$ supersymmetric Sachdev-Ye-Kitaev (SUSY SYK) models. We construct a bi-local superspace, and formulate the bi-local collective superfield theory of the one-dimensional SUSY vector model. The bi-local collective theory provides systematic analysis of the SUSY SYK models. We find that this bi-local collective theory naturally leads to supermatrix formulation in the bi-local superspace. This supermatrix formulation drastically simplifies the analysis of the SUSY SYK models. We also study $\mathcal{N}=1$ bi-local superconformal generators in the supermatrix formulation, and find the eigenvectors of teh superconformal Casimir. We diagonalize the quadratic action in large $N$ expansion. △ Less

Submitted 27 July, 2017; v1 submitted 19 June, 2017; originally announced June 2017.

Comments: 48 page; v2:typo corrected, reference added, notation for the eigenfunctions changed

Abelian Tensor Models on the Lattice

Authors: Soumyadeep Chaudhuri, Victor I. Giraldo-Rivera, Anosh Joseph, R. Loganayagam, Junggi Yoon

Abstract: We consider a chain of Abelian Klebanov-Tarnopolsky fermionic tensor models coupled through quartic nearest-neighbor interactions. We characterize the gauge-singlet spectrum for small chains ($L=2,3,4,5$) and observe that the spectral statistics exhibits strong evidences in favor of quasi-many body localization. We consider a chain of Abelian Klebanov-Tarnopolsky fermionic tensor models coupled through quartic nearest-neighbor interactions. We characterize the gauge-singlet spectrum for small chains ($L=2,3,4,5$) and observe that the spectral statistics exhibits strong evidences in favor of quasi-many body localization. △ Less

Submitted 7 May, 2018; v1 submitted 4 May, 2017; originally announced May 2017.

Comments: v3: 43 pages + appendices, 19 figures, minor changes, references updated, version matches with published version

Journal ref: Phys. Rev. D 97, 086007 (2018)

SYK-like Tensor Models on the Lattice

Authors: Prithvi Narayan, Junggi Yoon

Abstract: We study large $N$ tensor models on the lattice without disorder. We introduce techniques which can be applied to a wide class of models, and illustrate it by studying some specific rank-3 tensor models. In particular, we study Klebanov-Tarnopolsky model on lattice, Gurau-Witten model (by treating it as a tensor model on four sites) and also a new model which interpolates between these two models.… ▽ More We study large $N$ tensor models on the lattice without disorder. We introduce techniques which can be applied to a wide class of models, and illustrate it by studying some specific rank-3 tensor models. In particular, we study Klebanov-Tarnopolsky model on lattice, Gurau-Witten model (by treating it as a tensor model on four sites) and also a new model which interpolates between these two models. In each model, we evaluate various four point functions at large $N$ and strong coupling, and discuss their spectrum and long time behaviors. We find similarities as well as differences from SYK model. We also generalize our analysis to rank-$D$ tensor models where we obtain analogous results as $D=3$ case for the four point functions which we computed. For $D>5$, we are able to compute the next-to-subleading ${1 \over N}$ corrections for a specific four point function. △ Less

Submitted 11 July, 2017; v1 submitted 3 May, 2017; originally announced May 2017.

Comments: 46 pages, 29 figures; v2:typos corrected, reference added; v3:minor revisions, to be published in JHEP