Showing 1–46 of 46 results for author: Choi, B

Compositional gradient engineering for enhanced ferroelectricity in ultrathin AlScN

Authors: Zekun Hu, Haiwen Zhang, Rajeev Kumar Rai, Yuhong Cao, Xiaolei Tong, Pedram Yousefian, Hyunmin Cho, Bongjun Choi, Chao-Chuan Chen, Yunfei He, Kefei Bao, Chloe Leblanc, Eric A. Stach, Roy Olsson, Deep Jariwala

Abstract: Ferroelectric AlScN is promising for CMOS-compatible non-volatile memory, but thickness scaling is limited by leakage, premature breakdown, and defect-mediated failure. Here we show that compositional grading within a continuous wurtzite AlN-AlScN lattice mitigates these limitations by distributing structural and polarization discontinuities across the film thickness, reducing defect formation and… ▽ More Ferroelectric AlScN is promising for CMOS-compatible non-volatile memory, but thickness scaling is limited by leakage, premature breakdown, and defect-mediated failure. Here we show that compositional grading within a continuous wurtzite AlN-AlScN lattice mitigates these limitations by distributing structural and polarization discontinuities across the film thickness, reducing defect formation and local field concentration. In a 20 nm graded heterostructure, monotonic Sc incorporation and AlN-rich boundaries produce reversible ferroelectric switching, an as-grown metal-polar state, a 21% higher breakdown field, 10% enhanced remanent polarization, and 40x higher resistivity relative to homogeneous AlScN. Time-domain PUND measurements reveal strongly suppressed post-switching leakage, consistent with reduced defect-assisted and polarization-coupled conduction. This improved dielectric robustness enables ferroelectric functionality in 5 nm graded stacks containing only a 2 nm $\mathrm{Al}_{0.64}\mathrm{Sc}_{0.36}\mathrm{N}$ region, with measurable switching near 1 V. These results establish compositional grading as a defect- and field-management strategy for scalable ultrathin wurtzite ferroelectrics. △ Less

Submitted 10 June, 2026; originally announced June 2026.

Comments: 6 main figures

Plasmonic polaron in self-intercalated 1T-TiS2

Authors: Byoung Ki Choi, Woojin Choi, Zhiyu Tao, Ji-Eun Lee, Sae Hee Ryu, Seungrok Mun, Hyobeom Lee, Kyoungree Park, Seha Lee, Hayoon Im, Yong Zhong, Hyejin Ryu, Min Jae Kim, Sue Hyeon Hwang, Xuetao Zhu, Jiandong Guo, Jong Mok Ok, Jaekwang Lee, Haeyong Kang, Sungkyun Park, Jonathan D. Denlinger, Heung-Sik Kim, Aaron Bostwick, Zhi-Xun Shen, Choongyu Hwang , et al. (2 additional authors not shown)

Abstract: Electron-boson coupling is central to a comprehensive understanding of the diverse physical phenomena emerging from many-body interactions. Yet less attention has been paid to how plasmons, collective bosonic modes of electron density oscillation, interact with conduction electrons and how external parameters can tune this interaction. Here, we present a clear display of composite quasiparticles s… ▽ More Electron-boson coupling is central to a comprehensive understanding of the diverse physical phenomena emerging from many-body interactions. Yet less attention has been paid to how plasmons, collective bosonic modes of electron density oscillation, interact with conduction electrons and how external parameters can tune this interaction. Here, we present a clear display of composite quasiparticles stemming from electron-plasmon coupling, known as the plasmonic polaron, in self-intercalated 1T-TiS2, by using angle-resolved photoemission spectroscopy (ARPES), high-resolution electron energy loss spectroscopy (HR-EELS) and first-principles calculations. The single particle spectral function exhibits a distinctive plasmon-loss satellite with the same characteristic energy scale determined by HR-EELS measurements. The bosonic energy scale of plasmonic polaron is tunable by controlling charge carrier density and temperature, distinguishing itself from conventional polarons arising from electron-phonon interactions. Furthermore, we find that the dielectric screening strongly affects the formation of the plasmonic polaron states. Our findings provide direct spectroscopic evidence of plasmonic polarons and establish self-intercalated layered materials as a promising platform for studying, controlling, and harnessing plasmonic interactions in quantum materials. △ Less

Submitted 3 March, 2026; originally announced March 2026.

Photoinduced metastable cation disorder in metal halide double perovskites

Authors: Shunran Li, Burak Guzelturk, Conrad A. Kocoj, Donald A. Walko, Du Chen, Haidan Wen, Xian Xu, Xiaoming Wang, Bongjun Choi, Borui Li, Zhibo Kang, Cunming Liu, Suchismita Sarker, Benjamin T. Diroll, Xiaoyi Zhang, Yong Q. Cai, Yu He, Deep Jariwala, Yanfa Yan, Diana Y. Qiu, Peijun Guo

Abstract: Lead-free perovskites have emerged as environmentally benign alternatives to lead-halide counterparts for optoelectronics. Among them, the double perovskite Cs2AgInCl6 family exhibits remarkable white-light emission with proper composition engineering, enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photo- and… ▽ More Lead-free perovskites have emerged as environmentally benign alternatives to lead-halide counterparts for optoelectronics. Among them, the double perovskite Cs2AgInCl6 family exhibits remarkable white-light emission with proper composition engineering, enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photo- and structural dynamics governing their excited-state behavior remain poorly understood. Here, we report a long-lived metastable phase in the Cs2AgInCl6 double perovskite family and unravel this process and the concomitant electronic and structural evolution using a suite of tools including transient optical spectroscopy, time-resolved X-ray diffraction (TR-XRD) and X-ray absorption (TR-XAS). We show that the photoinduced, transient metastable phase is associated with B-site (Ag-In) disorder, which induces a dramatically reduced optical bandgap. Supported by TR-XRD and first-principles calculations, the Ag-In disorder drives the formation of Ag-rich and In-rich domains with millisecond lifetimes, with lifetimes increasing at lower temperatures. TR-XAS further reveals that photogenerated STEs oxidize Ag+ to Ag2+, facilitating this highly temporally asymmetric order-disorder transition. Our findings demonstrate a new mechanism, mediated by hole-localized STE formation, that enables prolongation of transient light-induced states to the multi-millisecond regime in double perovskites, opening possibilities to harvesting the functional properties of metastable phases of these materials. △ Less

Submitted 22 January, 2026; originally announced January 2026.

Inverted-Mode Scanning Tunneling Microscopy for Atomically Precise Fabrication

Authors: Eduardo Barrera, Bheeshmon Thanabalasingam, Rafik Addou, Damian Allis, Aly Asani, Jeremy Barton, Tomass Bernots, Brandon Blue, Adam Bottomley, Doreen Cheng, Byoung Choi, Megan Cowie, Chris Deimert, Michael Drew, Mathieu Durand, Tyler Enright, Robert A. Freitas Jr., Alan Godfrey, Ryan Groome, Si Yue Guo, Sheldon Haird, Aru Hill, Taleana Huff, Christian Imperiale, Alex Inayeh , et al. (35 additional authors not shown)

Abstract: Scanning Tunneling Microscopy (STM) enables fabrication of atomically precise structures with unique properties and growing technological potential. However, reproducible manipulation of covalently bonded atoms requires control over the atomic configuration of both sample and probe - a longstanding challenge in STM. Here, we introduce inverted-mode STM, an approach that enables mechanically contro… ▽ More Scanning Tunneling Microscopy (STM) enables fabrication of atomically precise structures with unique properties and growing technological potential. However, reproducible manipulation of covalently bonded atoms requires control over the atomic configuration of both sample and probe - a longstanding challenge in STM. Here, we introduce inverted-mode STM, an approach that enables mechanically controlled chemical reactions for atomically precise fabrication. Tailored molecules on a Si(100) surface image the probe apex, and the usual challenge of understanding the probe structure is effectively solved. The molecules can also react with the probe, with the two sides of the tunnel junction acting as reagents positioned with sub-angstrom precision. This allows abstraction or donation of atoms from or to the probe apex. We demonstrate this by using a novel alkynyl-terminated molecule to reproducibly abstract hydrogen atoms from the probe. The approach is expected to extend to other elements and moieties, opening a new avenue for scalable atomically precise fabrication. △ Less

Submitted 30 December, 2025; originally announced December 2025.

Evidence for electron localisation in a moiré-of-moiré superlattice

Authors: Hangyeol Park, Junhyeok Oh, Rasoul Ghadimi, Chiranjit Mondal, Yungi Jeong, Won Beom Choi, Kenji Watanabe, Takashi Taniguchi, Bohm-Jung Yang, Joonho Jang

Abstract: The localisation of electrons in a lattice potential is an quantum-mechanical phenomenon and is often associated with remarkable physical properties of solids involving electron spins, electric polarisations and topological effects. In particular, even a small amount of distortion of the lattice potential can localise otherwise-delocalised quantum states in low-dimensional electron systems, dramat… ▽ More The localisation of electrons in a lattice potential is an quantum-mechanical phenomenon and is often associated with remarkable physical properties of solids involving electron spins, electric polarisations and topological effects. In particular, even a small amount of distortion of the lattice potential can localise otherwise-delocalised quantum states in low-dimensional electron systems, dramatically influencing their thermodynamic properties and charge-transport behaviour. Study of such electron localisation induced by an aperiodic lattice potential remains exceptionally challenging in solid-state systems, since extrinsic disorders can trivially trap electrons in potential minima near disorders, obscuring the underlying quantum-mechanical origin of localisation phenomena. Van der Waals heterostructures can provide an alternative route for explorations of the phenomena via the emergence of superlattice potentials generated by rotating and stacking individual layers. Here, we report strong signatures of electron localisation in helical trilayer graphene, where the interplay of two moiré patterns gives rise to a moiré-of-moiré superlattice with distinct regions of moiré-periodic and moiré-aperiodic potentials. Remarkably, our measurements reveal the presence of double moiré-induced bands and high-order Brown-Zak oscillations, which are direct reflections of the periodic region with two constituent moiré patterns, and a superimposed anomalous hysteretic signal attributable to the aperiodic region. The data strongly suggest that electron wave functions are partially localised driven by the loss of a periodic lattice potential. Our work provides insight into the effects of spatially inhomogeneous lattice potentials on the low-dimensional electronic states and introduces a promising approach to control electron localisation for practical applications in solid-state devices. △ Less

Submitted 27 November, 2025; originally announced November 2025.

Interaction of moire-induced quantum Hall channels in a locally gated graphene junction

Authors: Won Beom Choi, Myungjin Jeon, K. Watanabe, T. Taniguchi, Joonho Jang

Abstract: Manipulating electron quantum 1D channels is an important element in the field of quantum information due to their ballistic and phase coherence properties. In GaAs and graphene based two dimensional gas systems, these edge channels have been investigated with both integer and fractional quantum Hall effects, contributing to the realization of electron interferometer and anyon braiding. Often, at… ▽ More Manipulating electron quantum 1D channels is an important element in the field of quantum information due to their ballistic and phase coherence properties. In GaAs and graphene based two dimensional gas systems, these edge channels have been investigated with both integer and fractional quantum Hall effects, contributing to the realization of electron interferometer and anyon braiding. Often, at the p-n junction in the quantum Hall (QH) regime, the presence of a depletion region due to a band gap or the formation of gaps between the zeroth Landau levels (zLL) suppresses interaction between the co-propagating edge channels of opposing doping regimes and helps to preserve the phase coherence of the channels. Here, we observe a new type of p-n junction in hexagonal boron nitride aligned graphene that lacks both the zLL and band gap. In this system, a van Hove singularity (vHS) emerges at the p-n junctions under magnetic fields of several Tesla, owing to the doping inversion near the secondary Dirac point. By fabricating devices with independently tunable global bottom and local top gates, we enable the study of interactions between p-type and n-type QH edge channels through magnetic breakdown associated with the vHS. These findings provide valuable insights into the interactions of superlattice-induced QH edge channels in hBN-aligned graphene. △ Less

Submitted 2 November, 2025; originally announced November 2025.

Comments: 18 pages, 4 figures

Reduced Order Modeling of Energetic Materials Using Physics-Aware Recurrent Convolutional Neural Networks in a Latent Space (LatentPARC)

Authors: Zoë J. Gray, Joseph B. Choi, Youngsoo Choi, H. Keo Springer, H. S. Udaykumar, Stephen S. Baek

Abstract: Physics-aware deep learning (PADL) has gained popularity for use in complex spatiotemporal dynamics (field evolution) simulations, such as those that arise frequently in computational modeling of energetic materials (EM). Here, we show that the challenge PADL methods face while learning complex field evolution problems can be simplified and accelerated by decoupling it into two tasks: learning com… ▽ More Physics-aware deep learning (PADL) has gained popularity for use in complex spatiotemporal dynamics (field evolution) simulations, such as those that arise frequently in computational modeling of energetic materials (EM). Here, we show that the challenge PADL methods face while learning complex field evolution problems can be simplified and accelerated by decoupling it into two tasks: learning complex geometric features in evolving fields and modeling dynamics over these features in a lower dimensional feature space. To accomplish this, we build upon our previous work on physics-aware recurrent convolutions (PARC). PARC embeds knowledge of underlying physics into its neural network architecture for more robust and accurate prediction of evolving physical fields. PARC was shown to effectively learn complex nonlinear features such as the formation of hotspots and coupled shock fronts in various initiation scenarios of EMs, as a function of microstructures, serving effectively as a microstructure-aware burn model. In this work, we further accelerate PARC and reduce its computational cost by projecting the original dynamics onto a lower-dimensional invariant manifold, or 'latent space.' The projected latent representation encodes the complex geometry of evolving fields (e.g. temperature and pressure) in a set of data-driven features. The reduced dimension of this latent space allows us to learn the dynamics during the initiation of EM with a lighter and more efficient model. We observe a significant decrease in training and inference time while maintaining results comparable to PARC at inference. This work takes steps towards enabling rapid prediction of EM thermomechanics at larger scales and characterization of EM structure-property-performance linkages at a full application scale. △ Less

Submitted 15 December, 2025; v1 submitted 15 September, 2025; originally announced September 2025.

Reconfigurable, non-volatile control of optical anisotropy in ReS2 via ferroelectric gating

Authors: Mahfujur Rahaman, Seunguk Song, Aaliyah C. Khan, Bongjun Choi, Aaron M. Schankler, Kwan-Ho Kim, Wonchan Lee, Jason Lynch, Hyeon Suk Shin, Andrew M. Rappe, Deep Jariwala

Abstract: Electrically tunable linear dichroism (LD) with non-volatile properties represents a critical yet elusive feature for next-generation integrated photonic elements in practical device architectures. Here, we demonstrate record-breaking, non-volatile control of optical anisotropy in two-dimensional ReS2 via ferroelectric gating with aluminum scandium nitride (AlScN). Our ferroelectric field-effect t… ▽ More Electrically tunable linear dichroism (LD) with non-volatile properties represents a critical yet elusive feature for next-generation integrated photonic elements in practical device architectures. Here, we demonstrate record-breaking, non-volatile control of optical anisotropy in two-dimensional ReS2 via ferroelectric gating with aluminum scandium nitride (AlScN). Our ferroelectric field-effect transistors achieve near-unity (~95%) LD tunability of differential reflectance at room temperature--the highest reported for any electrically controlled 2D optical system. Crucially, the programmed optical states exhibit exceptional retention exceeding 12,000 seconds without applied bias, enabling true non-volatile optical memory. Through combined experimental characterization and ab initio calculations, we reveal that ferroelectric polarization switching induces substantial asymmetric charge transfer to ReS2, selectively populating conduction band states and triggering structural distortions that dramatically enhance optical anisotropy in the "up" polarization state while leaving the "down" state unperturbed. This ferroelectric-semiconductor coupling provides a universal platform for voltage-programmable, energy-efficient photonic devices with dynamic polarization control, addressing critical needs in integrated photonics as well as programmable far-field optics and telecommunications infrastructure. △ Less

Submitted 15 September, 2025; originally announced September 2025.

Comments: 5 main figures

Controlling structure and interfacial interaction of monolayer TaSe2 on bilayer graphene

Authors: Hyobeom Lee, Hayoon Im, Byoung Ki Choi, Kyoungree Park, Yi Chen, Wei Ruan, Yong Zhong, Ji-Eun Lee, Hyejin Ryu, Michael F. Crommie, Zhi-Xun Shen, Choongyu Hwang, Sung-Kwan Mo, Jinwoong Hwang

Abstract: Tunability of interfacial effects between two-dimensional (2D) crystals is crucial not only for understanding the intrinsic properties of each system, but also for designing electronic devices based on ultra-thin heterostructures. A prerequisite of such heterostructure engineering is the availability of 2D crystals with different degrees of interfacial interactions. In this work, we report a contr… ▽ More Tunability of interfacial effects between two-dimensional (2D) crystals is crucial not only for understanding the intrinsic properties of each system, but also for designing electronic devices based on ultra-thin heterostructures. A prerequisite of such heterostructure engineering is the availability of 2D crystals with different degrees of interfacial interactions. In this work, we report a controlled epitaxial growth of monolayer TaSe2 with different structural phases, 1H and 1T, on a bilayer graphene (BLG) substrate using molecular beam epitaxy, and its impact on the electronic properties of the heterostructures using angle-resolved photoemission spectroscopy. 1H-TaSe2 exhibits significant charge transfer and band hybridization at the interface, whereas 1T-TaSe2 shows weak interactions with the substrate. The distinct interfacial interactions are attributed to the dual effects from the differences of the work functions as well as the relative interlayer distance between TaSe2 films and BLG substrate. The method demonstrated here provides a viable route towards interface engineering in a variety of transition-metal dichalcogenides that can be applied to future nano-devices with designed electronic properties. △ Less

Submitted 27 July, 2024; originally announced July 2024.

Comments: 23 pages, 4 figures

Journal ref: Nano Convergence 11, 14 (2024)

Applying Machine Learning to Elucidate Ultrafast Demagnetization Dynamics in Ni and Ni80Fe20

Authors: Hasan Ahmadian Baghbaderani, Byoung-Chul Choi

Abstract: Understanding the correlation between fast and ultrafast demagnetization processes is crucial for elucidating the microscopic mechanisms underlying ultrafast demagnetization, which is pivotal for various applications in spintronics. Initial theoretical models attempted to establish this correlation but faced challenges due to the complex interplay of physical phenomena. To address this, we employe… ▽ More Understanding the correlation between fast and ultrafast demagnetization processes is crucial for elucidating the microscopic mechanisms underlying ultrafast demagnetization, which is pivotal for various applications in spintronics. Initial theoretical models attempted to establish this correlation but faced challenges due to the complex interplay of physical phenomena. To address this, we employed a variety of machine learning methods, including supervised learning regression algorithms and symbolic regression, to analyze limited experimental data and derive meaningful mathematical expressions between demagnetization time and the Gilbert damping factor. The results reveal that polynomial regression and K-nearest neighbors algorithms perform best in predicting demagnetization time. Additionally, sure-independence-screening-and-sparsifying-operator (SISSO) as a symbolic regression method suggested a direct correlation between demagnetization time and damping factor for Ni and Ni80Fe20, indicating spin-flip scattering predominantly influences the ultrafast demagnetization mechanism. The developed models demonstrate promising predictive capabilities, validated against independent experimental data. Comparative analysis between different materials underscores the significant impact of material properties on ultrafast demagnetization behavior. This study underscores the potential of machine learning in unraveling complex physical phenomena and offers valuable insights for future research in ultrafast magnetism. △ Less

Submitted 13 June, 2024; originally announced June 2024.

Comments: 12 pages, 3 figures

Characterization of a graphene-hBN superlattice field effect transistor

Authors: Won Beom Choi, Youngoh Son, Hangyeol Park, Yungi Jeong, Junhyeok Oh, K. Watanabe, T. Taniguchi, Joonho Jang

Abstract: Graphene provides a unique platform for hosting high quality 2D electron systems. Encapsulating graphene with hexagonal boron nitride (hBN) to shield it from noisy environments offers the potential to achieve ultrahigh performance nanodevices, such as photodiodes and transistors. However, the absence of a bandgap at the Dirac point presents challenges for using this system as a useful transistor.… ▽ More Graphene provides a unique platform for hosting high quality 2D electron systems. Encapsulating graphene with hexagonal boron nitride (hBN) to shield it from noisy environments offers the potential to achieve ultrahigh performance nanodevices, such as photodiodes and transistors. However, the absence of a bandgap at the Dirac point presents challenges for using this system as a useful transistor. In this study, we investigated the functionality of hBN-aligned monolayer graphene as a field effect transistor (FET). By precisely aligning the hBN and graphene, bandgaps open at the first Dirac point and at the hole-doped induced Dirac point via an interfacial moiré potential. To characterize this as a submicrometer scale FET, we fabricated a global bottom gate to tune the density of a conducting channel and a local top gate to switch off this channel. This demonstrated that the system could be tuned to an optimal on/off ratio regime by separately controlling the gates. These findings provide a valuable reference point for the further development of FETs based on graphene heterostructures. △ Less

Submitted 12 July, 2024; v1 submitted 10 May, 2024; originally announced May 2024.

Journal ref: Appl. Phys. Lett. 125, 033503 (2024)

Atomic arrangement of van der Waals heterostructures using X-ray scattering and crystal truncation rod analysis

Authors: Ryung Kim, Byoung Ki Choi, Kyeong Jun Lee, Hyuk Jin Kim, Hyun Hwi Lee, Tae Gyu Rhee, Yeong Gwang Khim, Young Jun Chang, Seo Hyoung Chang

Abstract: Vanadium diselenide (VSe2) has intriguing physical properties such as unexpected ferromagnetism at the two-dimensional limit. However, the experimental results for room temperature ferromagnetism are still controversial and depend on the detailed crystal structure and stoichiometry. Here we introduce crystal truncation rod (CTR) analysis to investigate the atomic arrangement of bilayer VSe2 and bi… ▽ More Vanadium diselenide (VSe2) has intriguing physical properties such as unexpected ferromagnetism at the two-dimensional limit. However, the experimental results for room temperature ferromagnetism are still controversial and depend on the detailed crystal structure and stoichiometry. Here we introduce crystal truncation rod (CTR) analysis to investigate the atomic arrangement of bilayer VSe2 and bilayer graphene (BLG) hetero-structures grown on a 6H-SiC(0001) substrate. Using non-destructive CTR analysis, we were able to obtain electron density profiles and detailed crystal structure of the VSe2/BLG heterostructures. Specifically, the out-of-plane lattice parameters of each VSe2 layer were modulated by the interface compared to that of the bulk VSe2 1T phase. The atomic arrangement of the VSe2/BLG heterostructure provides deeper understanding and insight for elucidating the magnetic properties of the van der Waals heterostructure. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Comments: 17 pages, 4 figures

Journal ref: Current Applied Physics 46, 70 (2023)

Machine-learning-assisted analysis of transition metal dichalcogenide thin-film growth

Authors: Hyuk Jin Kim, Minsu Chong, Tae Gyu Rhee, Yeong Gwang Khim, Min-Hyoung Jung, Young-Min Kim, Hu Young Jeong, Byoung Ki Choi, Young Jun Chang

Abstract: In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data al… ▽ More In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data allows us to characterize and categorize the growth modes step by step, and extract hidden knowledge of the epitaxial film growth process. In this study, we employed the ML-assisted RHEED analysis method to investigate the growth of 2D thin films of transition metal dichalcogenides (ReSe2) on graphene substrates by MBE. Principal component analysis (PCA) and K-means clustering were used to separate statistically important patterns and visualize the trend of pattern evolution without any notable loss of information. Using the modified PCA, we could monitor the diffraction intensity of solely the ReSe2 layers by filtering out the substrate contribution. These findings demonstrate that ML analysis can be successfully employed to examine and understand the film-growth dynamics of 2D materials. Further, the ML-based method can pave the way for the development of advanced real-time monitoring and autonomous material synthesis techniques. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Comments: 21 pages, 4 figures

Journal ref: Nano Convergence 10, 10 (2023)

Controlling spin-orbit coupling to tailor type-II Dirac bands

Authors: Nguyen Huu Lam, Phuong Lien Nguyen, Byoung Ki Choi, Trinh Thi Ly, Ganbat Duvjir, Tae Gyu Rhee, Yong Jin Jo, Tae Heon Kim, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Younghun Hwang, Young Jun Chang, Jaekwang Lee, Jungdae Kim

Abstract: NiTe2, a type-II Dirac semimetal with strongly tilted Dirac band, has been explored extensively to understand its intriguing topological properties. Here, using density-functional theory (DFT) calculations, we report that the strength of spin-orbit coupling (SOC) in NiTe2 can be tuned by Se substitution. This results in negative shifts of the bulk Dirac point (BDP) while preserving the type-II Dir… ▽ More NiTe2, a type-II Dirac semimetal with strongly tilted Dirac band, has been explored extensively to understand its intriguing topological properties. Here, using density-functional theory (DFT) calculations, we report that the strength of spin-orbit coupling (SOC) in NiTe2 can be tuned by Se substitution. This results in negative shifts of the bulk Dirac point (BDP) while preserving the type-II Dirac band. Indeed, combined studies using scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) confirm that the BDP in the NiTe2-xSex alloy moves from +0.1 eV (NiTe2) to -0.3 eV (NiTeSe) depending on the Se concentrations, indicating the effective tunability of type-II Dirac fermions. Our results demonstrate an approach to tailor the type-II Dirac band in NiTe2 by controlling the SOC strength via chalcogen substitution. This approach can be applicable to different types of topological materials. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Comments: 25 pages, 4 figures

Journal ref: ACS Nano 16, 11227 (2022)

Ultrastrong Light-Matter Coupling in 2D Metal-Chalcogenates

Authors: Surendra B. Anantharaman, Jason Lynch, Mariya Aleksich, Christopher E. Stevens, Christopher Munley, Bongjun Choi, Sridhar Shenoy, Thomas Darlington, Arka Majumdar, P. James Shuck, Joshua Hendrickson, J. Nathan Hohman, Deep Jariwala

Abstract: Hybridization of excitons with photons to form hybrid quasiparticles, exciton-polaritons (EPs), has been widely investigated in a range of semiconductor material systems coupled to photonic cavities. Self-hybridization occurs when the semiconductor itself can serve as the photonic cavity medium resulting in strongly-coupled EPs with Rabi splitting energies > 200 meV at room temperatures which rece… ▽ More Hybridization of excitons with photons to form hybrid quasiparticles, exciton-polaritons (EPs), has been widely investigated in a range of semiconductor material systems coupled to photonic cavities. Self-hybridization occurs when the semiconductor itself can serve as the photonic cavity medium resulting in strongly-coupled EPs with Rabi splitting energies > 200 meV at room temperatures which recently were observed in layered two-dimensional (2D) excitonic materials. Here, we report an extreme version of this phenomenon, an ultrastrong EP coupling, in a nascent, 2D excitonic system, the metal organic chalcogenate (MOCHA) compound named mithrene. The resulting self-hybridized EPs in mithrene crystals placed on Au substrates show Rabi Splitting in the ultrastrong coupling range (> 600 meV) due to the strong oscillator strength of the excitons concurrent with the large refractive indices of mithrene. We further show bright EP emission at room temperature as well as EP dispersions at low-temperatures. Importantly, we find lower EP emission linewidth narrowing to ~1 nm when mithrene crystals are placed in closed Fabry-Perot cavities. Our results suggest that MOCHA materials are ideal for polaritonics in the deep green-blue part of the spectrum where strong excitonic materials with large optical constants are notably scarce. △ Less

Submitted 21 August, 2023; originally announced August 2023.

Robustness of the intrinsic anomalous Hall effect in Fe3GeTe2 to a uniaxial strain

Authors: Mijin Lim, Byeonghyeon Choi, Minjae Ghim, Je-Geun Park, Hyun-Woo Lee

Abstract: Fe3GeTe2 (FGT), a ferromagnetic van der Waals topological nodal line semimetal, has recently been studied. Using first-principles calculations and symmetry analysis, we investigate the effect of a uniaxial tensile strain on the nodal line and the resultant intrinsic anomalous Hall effect (AHE). Our results reveal their robustness to the in-plane strain. Moreover, the intrinsic AHE remains robust e… ▽ More Fe3GeTe2 (FGT), a ferromagnetic van der Waals topological nodal line semimetal, has recently been studied. Using first-principles calculations and symmetry analysis, we investigate the effect of a uniaxial tensile strain on the nodal line and the resultant intrinsic anomalous Hall effect (AHE). Our results reveal their robustness to the in-plane strain. Moreover, the intrinsic AHE remains robust even for artificial adjustment of the atomic positions introduced to break the crystalline symmetries of FGT. When the spin-orbit coupling is absent, the nodal line degeneracy remains intact as long as the inversion symmetry or the two-fold screw symmetry is maintained, which reveal that the nodal line may emerge much more easily than previously predicted. This strong robustness is surprising and disagrees with the previous experimental report [Y. Wang et al., Adv. Mater. 32, 2004533 (2020)], which reports that a uniaxial strain of less than 1 % of the in-plane lattice constant can double the anomalous Hall resistance. This discrepancy implies that the present understanding of the AHE in FGT is incomplete. The possible origins of this discrepancy are discussed. △ Less

Submitted 26 April, 2023; originally announced April 2023.

Comments: 7 pages, 3 figures

External screening and lifetime of exciton population in single-layer ReSe$_2$ probed by time- and angle-resolved photoemission spectroscopy

Authors: Klara Volckaert, Byoung Ki Choi, Hyuk Jin Kim, Deepnarayan Biswas, Denny Puntel, Simone Peli, Fulvio Parmigiani, Federico Cilento, Young Jun Chang, Søren Ulstrup

Abstract: The semiconductor ReSe$_2$ is characterized by a strongly anisotropic optical absorption and is therefore promising as an optically active component in two-dimensional heterostructures. However, the underlying femtosecond dynamics of photoinduced excitations in such materials has not been sufficiently explored. Here, we apply an infrared optical excitation to single-layer ReSe$_2$ grown on a bilay… ▽ More The semiconductor ReSe$_2$ is characterized by a strongly anisotropic optical absorption and is therefore promising as an optically active component in two-dimensional heterostructures. However, the underlying femtosecond dynamics of photoinduced excitations in such materials has not been sufficiently explored. Here, we apply an infrared optical excitation to single-layer ReSe$_2$ grown on a bilayer graphene substrate and monitor the temporal evolution of the excited state signal using time- and angle-resolved photoemission spectroscopy. We measure an optical gap of $(1.53 \pm 0.02)$ eV, consistent with resonant excitation of the lowest exciton state. The exciton distribution is tunable via the linear polarization of the pump pulse and exhibits a biexponential decay with time constants given by $τ_1 = (110 \pm 10)$ fs and $τ_2 = (650 \pm 70)$ fs, facilitated by recombination via an in-gap state that is pinned at the Fermi level. By extracting the momentum-resolved exciton distribution we estimate its real-space radial extent to be greater than 17.1 Å, implying significant exciton delocalization due to screening from the bilayer graphene substrate. △ Less

Submitted 10 January, 2023; originally announced January 2023.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Materials 7, L041001 (2023)

Artificial intelligence approaches for materials-by-design of energetic materials: state-of-the-art, challenges, and future directions

Authors: Joseph B. Choi, Phong C. H. Nguyen, Oishik Sen, H. S. Udaykumar, Stephen Baek

Abstract: Artificial intelligence (AI) is rapidly emerging as an enabling tool for solving various complex materials design problems. This paper aims to review recent advances in AI-driven materials-by-design and their applications to energetic materials (EM). Trained with data from numerical simulations and/or physical experiments, AI models can assimilate trends and patterns within the design parameter sp… ▽ More Artificial intelligence (AI) is rapidly emerging as an enabling tool for solving various complex materials design problems. This paper aims to review recent advances in AI-driven materials-by-design and their applications to energetic materials (EM). Trained with data from numerical simulations and/or physical experiments, AI models can assimilate trends and patterns within the design parameter space, identify optimal material designs (micro-morphologies, combinations of materials in composites, etc.), and point to designs with superior/targeted property and performance metrics. We review approaches focusing on such capabilities with respect to the three main stages of materials-by-design, namely representation learning of microstructure morphology (i.e., shape descriptors), structure-property-performance (S-P-P) linkage estimation, and optimization/design exploration. We provide a perspective view of these methods in terms of their potential, practicality, and efficacy towards the realization of materials-by-design. Specifically, methods in the literature are evaluated in terms of their capacity to learn from a small/limited number of data, computational complexity, generalizability/scalability to other material species and operating conditions, interpretability of the model predictions, and the burden of supervision/data annotation. Finally, we suggest a few promising future research directions for EM materials-by-design, such as meta-learning, active learning, Bayesian learning, and semi-/weakly-supervised learning, to bridge the gap between machine learning research and EM research. △ Less

Submitted 26 March, 2023; v1 submitted 15 November, 2022; originally announced November 2022.

A physics-aware deep learning model for energy localization in multiscale shock-to-detonation simulations of heterogeneous energetic materials

Authors: Phong C. H. Nguyen, Yen-Thi Nguyen, Pradeep K. Seshadri, Joseph B. Choi, H. S. Udaykumar, Stephen Baek

Abstract: Predictive simulations of the shock-to-detonation transition (SDT) in heterogeneous energetic materials (EM) are vital to the design and control of their energy release and sensitivity. Due to the complexity of the thermo-mechanics of EM during the SDT, both macro-scale response and sub-grid mesoscale energy localization must be captured accurately. This work proposes an efficient and accurate mul… ▽ More Predictive simulations of the shock-to-detonation transition (SDT) in heterogeneous energetic materials (EM) are vital to the design and control of their energy release and sensitivity. Due to the complexity of the thermo-mechanics of EM during the SDT, both macro-scale response and sub-grid mesoscale energy localization must be captured accurately. This work proposes an efficient and accurate multiscale framework for SDT simulations of EM. We introduce a new approach for SDT simulation by using deep learning to model the mesoscale energy localization of shock-initiated EM microstructures. The proposed multiscale modeling framework is divided into two stages. First, a physics-aware recurrent convolutional neural network (PARC) is used to model the mesoscale energy localization of shock-initiated heterogeneous EM microstructures. PARC is trained using direct numerical simulations (DNS) of hotspot ignition and growth within microstructures of pressed HMX material subjected to different input shock strengths. After training, PARC is employed to supply hotspot ignition and growth rates for macroscale SDT simulations. We show that PARC can play the role of a surrogate model in a multiscale simulation framework, while drastically reducing the computation cost and providing improved representations of the sub-grid physics. The proposed multiscale modeling approach will provide a new tool for material scientists in designing high-performance and safer energetic materials. △ Less

Submitted 21 March, 2023; v1 submitted 8 November, 2022; originally announced November 2022.

Journal ref: Pyrotech. 2023, e202200268

Advanced interfacial phase change material: structurally confined and interfacially extended superlattice

Authors: Hyeon wook Lim, Young sam Kim, Kyu-jin Jo, Seok-Choi, Chang Woo Lee, Dasol Kim, Ki hyeon Kwon, Hoe don Kwon, Soo bin Hwang, Byung-Joon Choi, Cheol-Woong Yang, Eun Ji Sim, Mann-Ho Cho

Abstract: Interfacial Phase Change Memory (iPCM) retrench unnecessary power consumption due to wasted heat generated during phase change by reducing unnecessary entropic loss. In this study, an advanced iPCM (GeTe/Ti-Sb2Te3 Superlattice) is synthesized by doping Ti into Sb2Te3. Structural analysis and density functional theory (DFT) calculations confirm that bonding distortion and structurally well-confined… ▽ More Interfacial Phase Change Memory (iPCM) retrench unnecessary power consumption due to wasted heat generated during phase change by reducing unnecessary entropic loss. In this study, an advanced iPCM (GeTe/Ti-Sb2Te3 Superlattice) is synthesized by doping Ti into Sb2Te3. Structural analysis and density functional theory (DFT) calculations confirm that bonding distortion and structurally well-confined layers contribute to improve phase change properties in iPCM. Ti-Sb2Te3 acts as an effective thermal barrier to localize the generated heat inside active region, which leads to reduction of switching energy. Since Ge-Te bonds adjacent to short and strong Ti-Te bonds are more elongated than the bonds near Sb-Te, it is easier for Ge atoms to break the bond with Te due to strengthened Peierls distortions (Rlong/Rshort) during phase change process. Properties of advanced iPCM (cycling endurance, write speed/energy) exceed previous records. Moreover, well-confined multi-level states are obtained with advanced iPCM, showing potential as a neuromorphic memory. Our work paves the way for designing superlattice based PCM by controlling confinement layers. △ Less

Submitted 3 October, 2022; v1 submitted 30 September, 2022; originally announced September 2022.

Comments: 5 Figures and 2 table, 5 Supplementary figures, 27 pages There are problems with typos and quality degradation of figures during PDF conversion process, so we resubmit the updated one in which those issues are resolved. Under mutual agreement between our group(Prof.m.h.cho) and Prof. Robert E.Simpson group, each research content is submitted separately to arXiv.org on the same day. (2022. 9/30)

PARC: Physics-Aware Recurrent Convolutional Neural Networks to Assimilate Meso-scale Reactive Mechanics of Energetic Materials

Authors: Phong C. H. Nguyen, Yen-Thi Nguyen, Joseph B. Choi, Pradeep K. Seshadri, H. S. Udaykumar, Stephen Baek

Abstract: The thermo-mechanical response of shock-initiated energetic materials (EM) is highly influenced by their microstructures, presenting an opportunity to engineer EM microstructure in a "materials-by-design" framework. However, the current design practice is limited, as a large ensemble of simulations is required to construct the complex EM structure-property-performance linkages. We present the Phys… ▽ More The thermo-mechanical response of shock-initiated energetic materials (EM) is highly influenced by their microstructures, presenting an opportunity to engineer EM microstructure in a "materials-by-design" framework. However, the current design practice is limited, as a large ensemble of simulations is required to construct the complex EM structure-property-performance linkages. We present the Physics-Aware Recurrent Convolutional (PARC) Neural Network, a deep-learning algorithm capable of learning the mesoscale thermo-mechanics of EM from a modest number of high-resolution direct numerical simulations (DNS). Validation results demonstrated that PARC could predict the themo-mechanical response of shocked EM with a comparable accuracy to DNS but with notably less computation time. The physics awareness of PARC enhances its modeling capabilities and generalizability, especially when challenged in unseen prediction scenarios. We also demonstrate that visualizing the artificial neurons at PARC can shed light on important aspects of EM thermos-mechanics and provide an additional lens for conceptualizing EM. △ Less

Submitted 24 March, 2023; v1 submitted 4 April, 2022; originally announced April 2022.

Journal ref: Sci. Adv. 2023, eadd6868

Enhanced passive thermal stealth properties of VO$_2$ thin films via gradient W doping

Authors: Hyuk Jin Kim, Young Hwan Choi, Dong Kyu Lee, In Hak Lee, Byoung Ki Choi, Soo-Hyun Phark, Young Jun Chang

Abstract: Thermal stealth and camouflage have been intensively studied for blending objects with their surroundings against remote thermal image detection. Adaptive control of infrared emissivity has been explored extensively as a promising way of thermal stealth, but it still requires an additional feedback control. Passive modulation of emissivity, however, has been remained as a great challenge which req… ▽ More Thermal stealth and camouflage have been intensively studied for blending objects with their surroundings against remote thermal image detection. Adaptive control of infrared emissivity has been explored extensively as a promising way of thermal stealth, but it still requires an additional feedback control. Passive modulation of emissivity, however, has been remained as a great challenge which requires a precise engineering of emissivity over wide temperature range. Here, we report a drastic improvement of passive camouflage thin films capable of concealing thermal objects at near room temperature without any feedback control, which consists of a vanadium dioxide (VO2) layer with gradient tungsten (W) concentration. The gradient W-doping widens the metal-insulator transition width, accomplishing self-adaptive thermal stealth with a smooth change of emissivity. Our simple approach, applicable to other similar thermal camouflage materials for improving their passive cloaking, will find wide applications, such as passive thermal camouflage, urban energy-saving smart windows, and improved infrared sensors. △ Less

Submitted 12 May, 2021; originally announced May 2021.

Comments: 20 pages, 4 figures

Interlayer coupling and ultrafast hot electron transfer dynamics in metallic VSe2/graphene van der Waals heterostructures

Authors: Tae Gwan Park, Byoung Ki Choi, Junho Park, Jungdae Kim, Young Jun Chang, Fabian Rotermund

Abstract: Atomically thin vanadium diselenide (VSe2 ) is a two-dimensional transition metal dichalcogenide exhibiting attractive properties due to its metallic 1T-phase. With the recent development of methods to manufacture high-quality monolayer VSe 2 on van der Waals materials, the outstanding properties of VSe2 -based heterostructures have been widely studied for diverse applications. Dimensional reducti… ▽ More Atomically thin vanadium diselenide (VSe2 ) is a two-dimensional transition metal dichalcogenide exhibiting attractive properties due to its metallic 1T-phase. With the recent development of methods to manufacture high-quality monolayer VSe 2 on van der Waals materials, the outstanding properties of VSe2 -based heterostructures have been widely studied for diverse applications. Dimensional reduction and interlayer coupling with a van der Waals substrate lead to its distinguishable characteristics from its bulk counterparts. However, only a few fundamental studies have investigated the interlayer coupling effects and hot electron transfer dynamics in VSe2 heterostructures. In this work, we reveal ultrafast and efficient interlayer hot electron transfer and interlayer coupling effects in VSe2 /graphene heterostructures. Femtosecond time-resolved reflectivity measurements showed that hot electrons in VSe 2 were transferred to graphene within a 100-fs timescale with high efficiency. Besides, coherent acoustic phonon dynamics indicated interlayer coupling in VSe2 /graphene heterostructures and efficient thermal energy transfer to three-dimensional substrates. Our results provide valuable insights into the intriguing properties of metallic transition metal dichalcogenide heterostructures and motivate designing optoelectronic and photonic devices with tailored properties. △ Less

Submitted 12 May, 2021; originally announced May 2021.

Comments: 30 pages, 5 figures, supplementary information(7 pages, 5 figures)

Journal ref: ACS Nano 2021, 15, 4, 7756

Modulating Curie Temperature and Magnetic Anisotropy in Nanoscale Layered Cr_{2}Te_{3} Films: Implications for Room-Temperature Spintronics

Authors: In Hak Lee, Byoung Ki Choi, Hyuk Jin Kim, Min Jay Kim, Hu Young Jeong, Jong Hoon Lee, Seung-Young Park, Younghun Jo, Chanki Lee, Jun Woo Choi, Seong Won Cho, Suyuon Lee, Younghak Kim, Beom Hyun Kim, Kyeong Jun Lee, Jin Eun Heo, Seo Hyoung Chang, Fengping Li, Bheema Lingam Chittari, Jeil Jung, Young Jun Chang

Abstract: Nanoscale layered ferromagnets have demonstrated fascinating two-dimensional magnetism down to atomic layers, providing a peculiar playground of spin orders for investigating fundamental physics and spintronic applications. However, strategy for growing films with designed magnetic properties is not well established yet. Herein, we present a versatile method to control the Curie temperature (T_{C}… ▽ More Nanoscale layered ferromagnets have demonstrated fascinating two-dimensional magnetism down to atomic layers, providing a peculiar playground of spin orders for investigating fundamental physics and spintronic applications. However, strategy for growing films with designed magnetic properties is not well established yet. Herein, we present a versatile method to control the Curie temperature (T_{C}) and magnetic anisotropy during growth of ultrathin Cr_{2}Te_{3} films. We demonstrate increase of the TC from 165 K to 310 K in sync with magnetic anisotropy switching from an out-of-plane orientation to an in-plane one, respectively, via controlling the Te source flux during film growth, leading to different c-lattice parameters while preserving the stoichiometries and thicknesses of the films. We attributed this modulation of magnetic anisotropy to the switching of the orbital magnetic moment, using X-ray magnetic circular dichroism analysis. We also inferred that different c-lattice constants might be responsible for the magnetic anisotropy change, supported by theoretical calculations. These findings emphasize the potential of ultrathin Cr_{2}Te_{3} films as candidates for developing room-temperature spintronics applications and similar growth strategies could be applicable to fabricate other nanoscale layered magnetic compounds. △ Less

Submitted 5 April, 2021; originally announced April 2021.

Comments: 30 pages, 6 figures, accepted in ACS Applied Nano Materials

Electronic structure and charge-density wave transition in monolayer VS_{2}

Authors: Hyuk Jin Kim, Byoung Ki Choi, In Hak Lee, Min Jay Kim, Seung-Hyun Chun, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Young Jun Chang

Abstract: Vanadium disulfide (VS_{2}) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS_{2} monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measure… ▽ More Vanadium disulfide (VS_{2}) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS_{2} monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs). △ Less

Submitted 5 April, 2021; originally announced April 2021.

Comments: 25 pages, 4 figures, accepted in Current Applied Physics

Synthesis of Three-Dimensionally Interconnected Hexagonal Boron Nitride Networked Cu-Ni Composite

Authors: Zahid Hussain, Hye-Won Yang, Byang-Sang Choi

Abstract: A three-dimensionally interconnected hexagonal boron nitride (3Di-hBN) networked Cu-Ni (3Di-hBN-Cu-Ni) composite was successfully synthesized in situ using a simple two-step process which involved the compaction of mixed Cu-Ni powders (70 wt.% Cu and 30 wt.% Ni) into a disc followed by metal-organic chemical vapor deposition (MOCVD) process at 1000°C. During MOCVD, the Cu-Ni alloy grains acted as… ▽ More A three-dimensionally interconnected hexagonal boron nitride (3Di-hBN) networked Cu-Ni (3Di-hBN-Cu-Ni) composite was successfully synthesized in situ using a simple two-step process which involved the compaction of mixed Cu-Ni powders (70 wt.% Cu and 30 wt.% Ni) into a disc followed by metal-organic chemical vapor deposition (MOCVD) process at 1000°C. During MOCVD, the Cu-Ni alloy grains acted as a template for the growth of hexagonal boron nitride (hBN) while decaborane and ammonia were used as precursors for boron and nitrogen, respectively. Boron and nitrogen atoms diffused into the Cu-Ni solution during the MOCVD process, precipitated out and grew along the Cu-Ni interfaces upon cooling. It was demonstrated that pores were generated during the sintering process and then filled by bulk hBN during the MOCVD process (indicated by energy dispersive spectroscopy) as the pores also served as catalytic sites for the nucleation and growth of hBN. Optical microscopy examination indicated that there was a minimum amount of bulk hBN at certain compaction pressure (280 MPa) and sintering time (30 min). Scanning electron microscopy and transmission electron microscopy revealed that the interconnected network of hBN layers surrounding the Cu-Ni grains was developed in the 3Di-hBN-Cu-Ni composite. This 3Di-hBN network is expected to enhance the resistance of the 3Di-hBN-Cu-Ni composite against mechanical, thermal and chemical attacks. Moreover, foam-like 3Di-hBN was extracted from 3Di-hBN-Cu-Ni composite which could be further applied in the fields of biomedicine and energy storage. △ Less

Submitted 24 February, 2026; v1 submitted 29 March, 2021; originally announced March 2021.

Comments: This version corresponds to the accepted manuscript. The published version is available at http://kjmm.org/journal/view.php?viewtype=pubreader&number=769

Journal ref: Korean J. Met. Mater.. 2021;59(7):505-513

Ultrafast triggering of insulator-metal transition in two-dimensional VSe$_2$

Authors: Deepnarayan Biswas, Alfred J. H. Jones, Paulina Majchrzak, Byoung Ki Choi, Tsung-Han Lee, Klara Volckaert, Jiagui Feng, Igor Marković, Federico Andreatta, Chang-Jong Kang, Hyuk Jin Kim, In Hak Lee, Chris Jozwiak, Eli Rotenberg, Aaron Bostwick, Charlotte E. Sanders, Yu Zhang, Gabriel Karras, Richard T. Chapman, Adam S. Wyatt, Emma Springate, Jill A. Miwa, Philip Hofmann, Phil D. C. King, Young Jun Chang , et al. (2 additional authors not shown)

Abstract: Assembling transition metal dichalcogenides (TMDCs) at the two-dimensional (2D) limit is a promising approach for tailoring emerging states of matter such as superconductivity or charge density waves (CDWs). Single-layer (SL) VSe$_2$ stands out in this regard because it exhibits a strongly enhanced CDW transition with a higher transition temperature compared to the bulk in addition to an insulatin… ▽ More Assembling transition metal dichalcogenides (TMDCs) at the two-dimensional (2D) limit is a promising approach for tailoring emerging states of matter such as superconductivity or charge density waves (CDWs). Single-layer (SL) VSe$_2$ stands out in this regard because it exhibits a strongly enhanced CDW transition with a higher transition temperature compared to the bulk in addition to an insulating phase with an anisotropic gap at the Fermi level, causing a suppression of anticipated 2D ferromagnetism in the material. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these electronic phases in SL VSe$_2$ using ultrafast pump-probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap on a timescale of 480 fs, which we disentangle from the ensuing hot carrier dynamics. Our work thereby reveals that the phase transition in SL VSe$_2$ is driven by electron-lattice coupling and demonstrates the potential for controlling electronic phases in 2D materials with light. △ Less

Submitted 27 July, 2020; originally announced July 2020.

Comments: 23 pages including supplementary information, 3 figures in the main text and 6 figures in the supplemental text

Journal ref: Nano Lett. 21, 1968-1975 (2021)

Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe$_{2}$

Authors: B. K. Choi, S. Ulstrup, S. M. Gunasekera, J. Kim, S. Y. Lim, L. Moreschini, J. S. Oh, S. -H. Chun, C. Jozwiak, A. Bostwick, E. Rotenberg, H. Cheong, I. -W. Lyo, M. Mucha-Kruczynski, Y. J. Chang

Abstract: Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemissio… ▽ More Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe$_{2}$, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe$_{2}$, in contrast to the 2H materials, the out-of-plane transition metal $d_{z^{2}}$ and chalcogen $p_{z}$ orbitals do not contribute significantly to the top of the valence band which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe$_{2}$ using scanning tunneling spectroscopy and explore the implications on the gap following surface-doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe$_{2}$. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place amongst two-dimensional crystals. △ Less

Submitted 29 May, 2020; originally announced May 2020.

Comments: 37 pages (including Supporting Information), 7 figures in the main text

Journal ref: ACS Nano 14, 7880 (2020)

Deep learning for synthetic microstructure generation in a materials-by-design framework for heterogeneous energetic materials

Authors: Sehyun Chun, Sidhartha Roy, Yen Thi Nguyen, Joseph B. Choi, H. S. Udaykumar, Stephen S. Baek

Abstract: The sensitivity of heterogeneous energetic (HE) materials (propellants, explosives, and pyrotechnics) is critically dependent on their microstructure. Initiation of chemical reactions occurs at hot spots due to energy localization at sites of porosities and other defects. Emerging multi-scale predictive models of HE response to loads account for the physics at the meso-scale, i.e. at the scale of… ▽ More The sensitivity of heterogeneous energetic (HE) materials (propellants, explosives, and pyrotechnics) is critically dependent on their microstructure. Initiation of chemical reactions occurs at hot spots due to energy localization at sites of porosities and other defects. Emerging multi-scale predictive models of HE response to loads account for the physics at the meso-scale, i.e. at the scale of statistically representative clusters of particles and other features in the microstructure. Meso-scale physics is infused in machine-learned closure models informed by resolved meso-scale simulations. Since microstructures are stochastic, ensembles of meso-scale simulations are required to quantify hot spot ignition and growth and to develop models for microstructure-dependent energy deposition rates. We propose utilizing generative adversarial networks (GAN) to spawn ensembles of synthetic heterogeneous energetic material microstructures. The method generates qualitatively and quantitatively realistic microstructures by learning from images of HE microstructures. We show that the proposed GAN method also permits the generation of new morphologies, where the porosity distribution can be controlled and spatially manipulated. Such control paves the way for the design of novel microstructures to engineer HE materials for targeted performance in a materials-by-design framework. △ Less

Submitted 5 April, 2020; originally announced April 2020.

Emergence of a Metal-Insulator Transition and High Temperature Charge Density Waves in VSe2 at the Monolayer Limit

Authors: Ganbat Duvjir, Byoung Ki Choi, Iksu Jang, Søren Ulstrup, Soonmin Kang, Trinh Thi Ly, Sanghwa Kim, Young Hwan Choi, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Je-Geun Park, Raman Sankar, Ki-Seok Kim, Jungdae Kim, Young Jun Chang

Abstract: Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe2 on graphe… ▽ More Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe2 on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe2. Renormalization group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge density wave transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe2. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K, driven by lattice distortions. The heterointerface plays an important role in driving this novel metal-insulator transition in the family of monolayered transition metal dichalcogenides. △ Less

Submitted 9 August, 2018; originally announced August 2018.

Comments: 21 pages, 4 figures

Journal ref: published in Nano Letters 2018

Universal renormalization group flow toward perfect Fermi-surface nesting driven by enhanced electron-electron correlations in monolayer vanadium diselenide

Authors: Iksu Jang, Ganbat Duvjir, Byoung Ki Choi, Jungdae Kim, Young Jun Chang, Ki-Seok Kim

Abstract: In the present study we examine nature of a charge ordering transition in monolayer vanadium diselenide ($VSe_{2}$), which would be distinguished from that of $VSe_{2}$ bulk samples, driven by more enhanced electron-electron correlations. Recently, angle resolved photoemission spectroscopy measurements uncovered that the Fermi surface nesting becomes perfect, where the dynamics of hot electrons is… ▽ More In the present study we examine nature of a charge ordering transition in monolayer vanadium diselenide ($VSe_{2}$), which would be distinguished from that of $VSe_{2}$ bulk samples, driven by more enhanced electron-electron correlations. Recently, angle resolved photoemission spectroscopy measurements uncovered that the Fermi surface nesting becomes perfect, where the dynamics of hot electrons is dispersionless along the orthogonal direction of the nesting wave-vector. In addition, scanning tunneling microscopy measurements confirmed that the resulting CDW state shows essentially the same modulation pattern as the three dimensional system of $VSe_{2}$. Here, we perform the renormalization group analysis based on an effective field theory in terms of critical CDW fluctuations and hot electrons of imperfect Fermi-surface nesting. As a result, we reveal that the imperfect nesting universally flows into perfect nesting in two dimensions, where the Fermi velocity along the orthogonal direction of the nesting vector vanishes generically. We argue that this electronic reconstruction is responsible for the observation that the CDW transition temperature is much more enhanced to be around $T_{c} > 300$ $K$ than that of the bulk sample. △ Less

Submitted 22 October, 2018; v1 submitted 10 April, 2018; originally announced April 2018.

Journal ref: Phys. Rev. B 99, 014106 (2019)

Nature of self-diffusion in two-dimensional fluids

Authors: Bongsik Choi, Kyeong Hwan Han, Changho Kim, Peter Talkner, Akinori Kidera, Eok Kyun Lee

Abstract: Self-diffusion in a two-dimensional simple fluid is investigated by both analytical and numerical means. We investigate the anomalous aspects of self-diffusion in two-dimensional fluids with regards to the mean square displacement, the time-dependent diffusion coefficient, and the velocity autocorrelation function using a consistency equation relating these quantities. We numerically confirm the c… ▽ More Self-diffusion in a two-dimensional simple fluid is investigated by both analytical and numerical means. We investigate the anomalous aspects of self-diffusion in two-dimensional fluids with regards to the mean square displacement, the time-dependent diffusion coefficient, and the velocity autocorrelation function using a consistency equation relating these quantities. We numerically confirm the consistency equation by extensive molecular dynamics simulations for finite systems, corroborate earlier results indicating that the kinematic viscosity approaches a finite, non-vanishing value in the thermodynamic limit, and establish the finite size behavior of the diffusion coefficient. We obtain the exact solution of the consistency equation in the thermodynamic limit and use this solution to determine the large time asymptotics of the mean square displacement, the diffusion coefficient, and the velocity autocorrelation function. An asymptotic decay law of the velocity autocorrelation function resembles the previously known self-consistent form, $1/(t\sqrt{\ln t})$, however with a rescaled time. △ Less

Submitted 6 November, 2017; v1 submitted 19 September, 2017; originally announced September 2017.

Comments: 10 pages, to appear in New Journal of Physics

Journal ref: New J. Phys. 19, 123038 (2017)

A Thickness Dependent Enhancement of Optical Resolution in the Vicinity of an Epsilon-near-zero Slab

Authors: Young-Rok Jang, Soo Bong Choi, Doo Jae Park, Jisoo Kyoung

Abstract: Recent studies reports that an epsilon-near-zero (ENZ) thin slab between a specimen and a substrate contributes in enhancing the spatial resolution of the optical system. Here, we investigate the ENZ thickness dependence of the resolution enhancement. By employing the edge response function, the resolution of the optical system is directly measured when imaging a sharp edge of a metal film. We fou… ▽ More Recent studies reports that an epsilon-near-zero (ENZ) thin slab between a specimen and a substrate contributes in enhancing the spatial resolution of the optical system. Here, we investigate the ENZ thickness dependence of the resolution enhancement. By employing the edge response function, the resolution of the optical system is directly measured when imaging a sharp edge of a metal film. We found that the optimum ENZ slab thickness was 700 nm and the achieved resolution was 11 μm at the wavelength of 8 μm. Owing to the enhanced resolution by ENZ slab, we successfully imaged the subwavelength slit arrays. △ Less

Submitted 23 May, 2016; originally announced May 2016.

Short-Channel Field Effect Transistors with 9-Atom and 13-Atom wide Graphene Nanoribbons

Authors: Juan Pablo Llinas, Andrew Fairbrother, Gabriela Borin Barin, Wu Shi, Kyunghoon Lee, Shuang Wu, Byung Yong Choi, Rohit Braganza, Jordan Lear, Nicholas Kau, Wonwoo Choi, Chen Chen, Zahra Pedramrazi, Tim Dumslaff, Akimitsu Narita, Xinliang Feng, Klaus Müllen, Felix Fischer, Alex Zettl, Pascal Ruffieux, Eli Yablonovitch, Michael Crommie, Roman Fasel, Jeffrey Bokor

Abstract: Bottom-up synthesized GNRs and GNR heterostructures have promising electronic properties for high performance field effect transistors (FETs) and ultra-low power devices such as tunnelling FETs. However, the short length and wide band gap of these GNRs have prevented the fabrication of devices with the desired performance and switching behaviour. Here, by fabricating short channel (Lch ~20 nm) dev… ▽ More Bottom-up synthesized GNRs and GNR heterostructures have promising electronic properties for high performance field effect transistors (FETs) and ultra-low power devices such as tunnelling FETs. However, the short length and wide band gap of these GNRs have prevented the fabrication of devices with the desired performance and switching behaviour. Here, by fabricating short channel (Lch ~20 nm) devices with a thin, high-k gate dielectric and a 9-atom wide (0.95 nm) armchair GNR as the channel material, we demonstrate FETs with high on-current (Ion >1 uA at Vd = -1 V) and high Ion/Ioff ~10^5 at room temperature. We find that the performance of these devices is limited by tunnelling through the Schottky barrier (SB) at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high performance short-channel FETs with bottom-up synthesized armchair GNRs. △ Less

Submitted 27 January, 2017; v1 submitted 21 May, 2016; originally announced May 2016.

Comments: v3: added local back gate data

Journal ref: Nature Communications 8, Article number: 633 (2017)

Normal versus anomalous self-diffusion in two-dimensional fluids: Memory function approach and generalized asymptotic Einstein relation

Authors: Hyun Kyung Shin, Bongsik Choi, Peter Talkner, Eok Kyun Lee

Abstract: Based on the generalized Langevin equation for the momentum of a Brownian particle a generalized asymptotic Einstein relation is derived. It agrees with the well-known Einstein relation in the case of normal diffusion but continues to hold for sub- and super-diffusive spreading of the Brownian particle's mean square displacement. The generalized asymptotic Einstein relation is used to analyze data… ▽ More Based on the generalized Langevin equation for the momentum of a Brownian particle a generalized asymptotic Einstein relation is derived. It agrees with the well-known Einstein relation in the case of normal diffusion but continues to hold for sub- and super-diffusive spreading of the Brownian particle's mean square displacement. The generalized asymptotic Einstein relation is used to analyze data obtained from molecular dynamics simulations of a two-dimensional soft disk fluid. We mainly concentrated on medium densities for which we found super-diffusive behavior of a tagged fluid particle. At higher densities a range of normal diffusion can be identified. The motion presumably changes to sub-diffusion for even higher densities. △ Less

Submitted 4 October, 2014; originally announced October 2014.

Room-Temperature Charge Stability Modulated by Quantum Effects in a Nanoscale Silicon Island

Authors: S. J. Shin, J. J. Lee, H. J. Kang, J. B. Choi, S. -R. Eric Yang, Y. Takahashi, D. G. Hasko

Abstract: We report on transport measurement performed on a room-temperature-operating ultra-small Coulomb blockade devices with a silicon island of sub-5nm. The charge stability at 300K exhibits a substantial change in slopes and diagonal size of each successive Coulomb diamond, but remarkably its main feature persists even at low temperature down to 5.3K except for additional Coulomb peak splitting. This… ▽ More We report on transport measurement performed on a room-temperature-operating ultra-small Coulomb blockade devices with a silicon island of sub-5nm. The charge stability at 300K exhibits a substantial change in slopes and diagonal size of each successive Coulomb diamond, but remarkably its main feature persists even at low temperature down to 5.3K except for additional Coulomb peak splitting. This key feature of charge stability with additional fine structures of Coulomb peaks are successfully modeled by including the interplay between Coulomb interaction, valley splitting, and strong quantum confinement, which leads to several low-energy many-body excited states for each dot occupancy. These excited states become enhanced in the sub-5nm ultra-small scale and persist even at 300K in the form of cluster, leading to the substantial modulation of charge stability. △ Less

Submitted 18 January, 2012; originally announced January 2012.

Comments: 12 pages, 5 figures; updated version of NanoLett. 11, 1591 (2011); addition/corrections included

Journal ref: NanoLett. 2011, vol.11, pp 1591-1597

Electric field-induced quantum interference control in a semiconductor: A new manifestation of the Franz-Keldysh effect

Authors: J. K. Wahlstrand, H. Zhang, S. B. Choi, S. Kannan, D. S. Dessau, J. E. Sipe, S. T. Cundiff

Abstract: In (100)-oriented GaAs illuminated at normal incidence by a laser and its second harmonic, interference between one- and two-photon absorption results in ballistic current injection, but not modulation of the overall carrier injection rate. Results from a pump-probe experiment on a transversely biased sample show that a constant electric field enables coherent control of the carrier injection rate… ▽ More In (100)-oriented GaAs illuminated at normal incidence by a laser and its second harmonic, interference between one- and two-photon absorption results in ballistic current injection, but not modulation of the overall carrier injection rate. Results from a pump-probe experiment on a transversely biased sample show that a constant electric field enables coherent control of the carrier injection rate. We ascribe this to the nonlinear optical Franz-Keldysh effect and calculate it for a two-band parabolic model. The mechanism is relevant to centrosymmetric semiconductors as well. △ Less

Submitted 29 March, 2011; v1 submitted 11 August, 2010; originally announced August 2010.

Comments: 4 pages, 2 figures. New data, less theory

Enhanced Quantum Effects in an Ultra-Small Coulomb Blockaded Device Operating at Room-Temperature

Authors: S. J. Shin, C. S. Jeong, B. J. Park, T. K. Yoon, J. J. Lee, S. J. Kim, J. B. Choi, Y. Takahashi, D. G. Hasko

Abstract: An ultra-small Coulomb blockade device can be regarded as a mesoscopic artificial atom system and provides a rich experimental environment for studying quantum transport phenomena[1]. Previously, these quantum effects have been investigated using relatively large devices at ultra-low temperatures, where they give rise to a fine additional structure on the Coulomb oscillations [2-13]. Here, we re… ▽ More An ultra-small Coulomb blockade device can be regarded as a mesoscopic artificial atom system and provides a rich experimental environment for studying quantum transport phenomena[1]. Previously, these quantum effects have been investigated using relatively large devices at ultra-low temperatures, where they give rise to a fine additional structure on the Coulomb oscillations [2-13]. Here, we report transport measurements carried out on a sub-2nm single-electron device; this size is sufficiently small that Coulomb blockade, and other quantum effects, persist up to room temperature (RT). These devices were made by scaling the size of a FinFET structure down to an ultimate limiting form, resulting in the reliable formation of a sub-2nm silicon Coulomb island. Four clear Coulomb diamonds can be observed at RT and the 2nd Coulomb diamond is unusually large, due to quantum confinement. The observed characteristics are successfully modeled on the basis of a very low electron number on the island, combined with Pauli spin exclusion. These effects offer additional functionality for future RT-operating single-electron device applications △ Less

Submitted 21 January, 2010; originally announced January 2010.

Comments: 7 pages, 4 figures

Dynamics of Vortex Core Switching in Ferromagnetic Nanodisks

Authors: Q. F. Xiao, J. Rudge, B. C. Choi, Y. K. Hong, G. Donohoe

Abstract: Dynamics of magnetic vortex core switching in nanometer-scale permalloy disk, having a single vortex ground state, was investigated by micromagnetic modeling. When an in-plane magnetic field pulse with an appropriate strength and duration is applied to the vortex structure, additional two vortices, i.e., a circular- and an anti-vortex, are created near the original vortex core. Sequentially, the… ▽ More Dynamics of magnetic vortex core switching in nanometer-scale permalloy disk, having a single vortex ground state, was investigated by micromagnetic modeling. When an in-plane magnetic field pulse with an appropriate strength and duration is applied to the vortex structure, additional two vortices, i.e., a circular- and an anti-vortex, are created near the original vortex core. Sequentially, the vortex-antivortex pair annihilates. A spin wave is created at the annihilation point and propagated through the entire element; the relaxed state for the system is the single vortex state with a switched vortex core. △ Less

Submitted 25 November, 2006; originally announced November 2006.

Comments: to appear in Appl. Phys. Lett

Shot-noise and conductance measurements of transparent superconductor / two-dimensional electron gas junctions

Authors: B. -R. Choi, A. E. Hansen, T. Kontos, C. Hoffmann, S. Oberholzer, W. Belzig, C. Schoenenberger, T. Akazaki, H. Takayanagi

Abstract: We have measured the conductance and shot-noise of superconductor-normal metal (S-N) junctions between a Niobium (Nb) film and a 2-dimensional electron gas (2DEG), formed in an InAs-based semiconductor heterostructure. Adjacent to the junction, the 2DEG is shaped into a submicrometer beam-splitter. The current shot-noise measured through one arm of the beam-splitter is found to be enhanced due t… ▽ More We have measured the conductance and shot-noise of superconductor-normal metal (S-N) junctions between a Niobium (Nb) film and a 2-dimensional electron gas (2DEG), formed in an InAs-based semiconductor heterostructure. Adjacent to the junction, the 2DEG is shaped into a submicrometer beam-splitter. The current shot-noise measured through one arm of the beam-splitter is found to be enhanced due to Andreev reflection. Both noise and conductance measurements indicate that the Nb-2DEG interface is of high quality with a transparency approaching approx. 60-70 %. The present device can be seen as a quasi-ballistic S-N beam-splitter junction. △ Less

Submitted 25 October, 2004; originally announced October 2004.

Comments: full paper including figures can be retrieved from: http://www.unibas.ch/phys-meso/Research/Papers/2004/Shot-Noise-Yjunction.pdf

Spin singlet-triplet transition in a Si-based two-electron double quantum dot molecule

Authors: S. D. Lee, S. J. Kim, J. S. Kang, Y. B. Cho, J. B. Choi, Sooa Park, S. -R. Eric Yang, S. J. Lee, T. H. Zyung

Abstract: We report a successful measurement of the magnetic field-induced spin singlet-triplet transition in silicon-based coupled dot systems. Our specific experimental scheme incorporates a lateral gate-controlled Coulomb-blockaded structure in Si to meet the proposed scheme of Loss and DiVincenzo [1], and a non-equilibrium single-electron tunneling technique to probe the fine energy splitting between… ▽ More We report a successful measurement of the magnetic field-induced spin singlet-triplet transition in silicon-based coupled dot systems. Our specific experimental scheme incorporates a lateral gate-controlled Coulomb-blockaded structure in Si to meet the proposed scheme of Loss and DiVincenzo [1], and a non-equilibrium single-electron tunneling technique to probe the fine energy splitting between the spin singlet and triplet, which varies as a function of applying magnetic fields and interdot coupling constant. Our results, exhibiting the singlet-triplet crossing at a magnetic field for various interdot coupling constants, are in agreement with the theoretical predictions, and give the first experimental demonstration of the possible spin swapping occurring in the coupled double dot systems with magnetic field. *Electronic address: jungchoi@chungbuk.ac.kr [1] D. Loss and D. P. DiVincenzo, Phys. Rev. A 57, 120 (1998). △ Less

Submitted 2 October, 2004; originally announced October 2004.

Comments: 4 pages, 6 figures

Nonequilibrium Extension of the Landau-Lifshitz-Gilbert Equation for Magnetic Systems

Authors: Jeongwon Ho, B. C. Choi, F. C. Khanna, Sang Pyo Kim

Abstract: Using the invariant operator method for an effective Hamiltonian including the radiation-spin interaction, we describe the quantum theory for magnetization dynamics when the spin system evolves nonadiabatically and out of equilibrium, $d \hatρ/dt \neq 0$. It is shown that the vector parameter of the invariant operator and the magnetization defined with respect to the density operator, both satis… ▽ More Using the invariant operator method for an effective Hamiltonian including the radiation-spin interaction, we describe the quantum theory for magnetization dynamics when the spin system evolves nonadiabatically and out of equilibrium, $d \hatρ/dt \neq 0$. It is shown that the vector parameter of the invariant operator and the magnetization defined with respect to the density operator, both satisfying the quantum Liouville equation, still obey the Landau-Lifshitz-Gilbert equation. △ Less

Submitted 26 May, 2004; originally announced May 2004.

Comments: RevTex 3 Pages in double column; no figure

Auger Effect in the High-Resolution Ce 3d-edge Resonant Photoemission

Authors: E. -J. Cho, R. -J. Jung, B. -H. Choi, S. -J. Oh, T. Iwasaki, A. Sekiyama, S. Imada, S. Suga, T. Muro, J. -G. Park, Y. S. Kwon

Abstract: The bulk-sensitive Ce 4$f$ spectral weights of various Ce compounds including CeFe$_2$, CeNi$_2$, and CeSi$_2$ were obtained with the resonant photoemission technique at the Ce 3d-edge. We found the lineshapes change significantly with the small change of the incident photon energy. Detailed analysis showed that this phenomenon results primarily from the Auger transition between different multip… ▽ More The bulk-sensitive Ce 4$f$ spectral weights of various Ce compounds including CeFe$_2$, CeNi$_2$, and CeSi$_2$ were obtained with the resonant photoemission technique at the Ce 3d-edge. We found the lineshapes change significantly with the small change of the incident photon energy. Detailed analysis showed that this phenomenon results primarily from the Auger transition between different multiplet states of the Ce $\underline{3d_{5/2}}4f^2$ (bar denotes a hole) electronic configuration in the intermediate state of the resonant process. This tells us that extra care should be taken for the choice of the resonant photon energy when extracting Ce 4$f$ spectral weights from the Ce 3$d$-edge resonant photoemission spectra. The absorption energy corresponding to the lowest multiplet structure of the Ce $\underline{3d_{5/2}}4f^2$ configuration seems to be the logical choice. △ Less

Submitted 14 January, 2003; originally announced January 2003.

Comments: 13 pages, 5 figures, submitted to Phys. Rev. B

Localized Character of 4f Electrons in CeRh$_x$(x=2,3) and CeNi$_x$(x=2,5)

Authors: Ran-Ju Jung, Byung-Hee Choi, Hyeong-Do Kim, S. -J. Oh, En-Jin Cho, T. Iwasaki, A. Sekiyama, S. Imada, S. Suga, J. -G. Park

Abstract: We have measured Ce 4f spectral weights of extremely $α$-like Ce-transition metal intermetallic compounds CeRh$_x$ (x=2,3) and CeNi$_x$ (x=2,5) by using the {\it bulk-sensitive} resonant photoemission technique at the Ce $M_5$($3d_{5/2}\to4f$)-edge. Unprecedentedly high energy resolution and longer escape depth of photoemitted electron at this photon energy enabled us to distinguish the sharp Ko… ▽ More We have measured Ce 4f spectral weights of extremely $α$-like Ce-transition metal intermetallic compounds CeRh$_x$ (x=2,3) and CeNi$_x$ (x=2,5) by using the {\it bulk-sensitive} resonant photoemission technique at the Ce $M_5$($3d_{5/2}\to4f$)-edge. Unprecedentedly high energy resolution and longer escape depth of photoemitted electron at this photon energy enabled us to distinguish the sharp Kondo resonance tails at the Fermi level, which can be well described by the Gunnarsson-Schönhammer(GS) calculation based on the Anderson Impurity Hamiltonian. On the other hand, the itinerant 4f band description shows big discrepancies, which implies that Ce 4f electrons retain localized characters even in extremely $α$-like compounds. △ Less

Submitted 13 January, 2003; originally announced January 2003.

Comments: 4 pages, 3 figures, submitted to Phys. Rev. Lett

Photoemission Study of Rare-Earth Ditelluride Compounds (ReTe_2 : Re = La, Pr, Sm, and Gd)

Authors: Jaegwan Chung, Junghwan Park, J. G. Park, Byung-Hee Choi, S. J. Oh, E. J. Cho, H. D. Kim, Y. S. Kwon

Abstract: We studied the electronic structure of rare-earth ditelluride (ReTe_2 : Re = La, Pr, Sm, and Gd) using photoemission spectroscopy. From the x-ray photoelectron spectroscopy (XPS) study of the 3d core levels of rare-earth elements, we found that all the rare earth elements are trivalent. We have also made theoretical calculations using the Gunnarsson and Schoenhammer approximation and multiplet c… ▽ More We studied the electronic structure of rare-earth ditelluride (ReTe_2 : Re = La, Pr, Sm, and Gd) using photoemission spectroscopy. From the x-ray photoelectron spectroscopy (XPS) study of the 3d core levels of rare-earth elements, we found that all the rare earth elements are trivalent. We have also made theoretical calculations using the Gunnarsson and Schoenhammer approximation and multiplet calculations for the rare earth elements to find that the La and Gd~3d peaks are well explained using our calculations. There is no considerable change in the line-shape of the Te~3d peaks depending on different rare earth elements. On ther other hand, valence band spectra studied with the ultraviolet photoelectron spectroscopy (UPS) show a small change in the Te p band depending on rare-earth elements. According to the UPS data, LaTe_2 has very low carrier density at the Fermi level while SmTe_2 and PrTe_2 show strongly metallic band structure effects near the Fermi level. △ Less

Submitted 27 October, 1999; originally announced October 1999.

Comments: 1 tex file and 3 ps files

Geometric and magnetic properties of Co/Pd system

Authors: S. -J. Oh, Wookje Kim, Wondong Kim, B. -H. Choi, Jae-Young Kim, Hoon Koh, H. -J. Kim, J. -H. Park

Abstract: We measured geometric and magnetic properties of Co films on the Pd(111) surface by x-ray photoelectron diffraction (XPD), x-ray magnetic circular dichroism (MCD) at the Co L_{2,3} edge, and the surface magneto-optical Kerr effect (SMOKE) measurements. Co thin films are found to grow incoherently with fcc island structure on the smooth Pd(111) substrate. Comparison of MCD and SMOKE measurements… ▽ More We measured geometric and magnetic properties of Co films on the Pd(111) surface by x-ray photoelectron diffraction (XPD), x-ray magnetic circular dichroism (MCD) at the Co L_{2,3} edge, and the surface magneto-optical Kerr effect (SMOKE) measurements. Co thin films are found to grow incoherently with fcc island structure on the smooth Pd(111) substrate. Comparison of MCD and SMOKE measurements of Co thin films grown on rough and smooth Pd(111) surfaces suggests that perpendicular remanent magnetization and Co orbital moment are enhanced by the rough interface. Pd capping layer also induces perpendicular orbital moment enhancement. These observations indicate the influence of hybridization between Co 3d and Pd 4d at the interface on the magnetic anisotropy. △ Less

Submitted 26 October, 1999; originally announced October 1999.

Comments: 6 ps files