Showing 1–50 of 102 results for author: Kim, H J

Sublattice-resolved coherent phonon dynamics in charge density waves

Authors: Kyoung Hun Oh, Honglie Ning, Zongqi Shen, Yifan Su, Jack Maier, Gyeongbo Kang, Hyeongi Choi, Dong Wu, Qiaomei Liu, Hyun-Woo J. Kim, Seunghyeok Ha, Jaehwon Kim, Byungjune Lee, B. J. Kim, N. L. Wang, Yao Wang, Hoyoung Jang, Nuh Gedik

Abstract: Phonons govern fundamental material properties and play a central role in various electronic phase transitions. Coherent driving of specific phonon modes enables on-demand phase control, motivating sublattice-resolved identification of real-space phonon motions. Yet experimentally resolving these motions remains challenging, limiting precise phonon-based control. Here, we introduce a dynamical pro… ▽ More Phonons govern fundamental material properties and play a central role in various electronic phase transitions. Coherent driving of specific phonon modes enables on-demand phase control, motivating sublattice-resolved identification of real-space phonon motions. Yet experimentally resolving these motions remains challenging, limiting precise phonon-based control. Here, we introduce a dynamical protocol to track element-resolved phonon dynamics in the charge density wave material EuTe4, in which the dominant Te-sublattice charge order is accompanied by a previously unreported Eu-sublattice component. We leverage the elemental selectivity of time-resolved resonant X-ray scattering to reveal three coherent phonon modes with distinct sublattice character, thereby disentangling Eu- and Te-dominated lattice dynamics, in good agreement with theoretical calculations of the phonon eigenvectors. This time-domain approach, which surpasses the energy-resolution limits of conventional frequency-domain inelastic scattering, provides a broadly applicable framework for decomposing coherent phonons in multi-element materials, which is crucial for the targeted control of phases of matter. △ Less

Submitted 6 August, 2026; originally announced August 2026.

Comments: 7 pages, 4 figures

Non-Perturbative Renormalization Group for Ising-Nematic Criticality: A Closed-Form Nonlocal Ansatz

Authors: Hyeon Jung Kim, Kyoung-Min Kim, Ki-Seok Kim

Abstract: The two-dimensional metallic quantum critical problem is a long-standing puzzle that is widely believed to hold the key to resolving ubiquitous non-Fermi liquid behavior in strongly correlated electronic systems. In this study, we present a non-perturbative renormalization group (RG) analysis of the metallic Ising-nematic quantum critical point in two dimensions, formulated directly around an intr… ▽ More The two-dimensional metallic quantum critical problem is a long-standing puzzle that is widely believed to hold the key to resolving ubiquitous non-Fermi liquid behavior in strongly correlated electronic systems. In this study, we present a non-perturbative renormalization group (RG) analysis of the metallic Ising-nematic quantum critical point in two dimensions, formulated directly around an intrinsically nonlocal infrared (IR) boson propagator. Rather than treating the anomalous dynamical critical exponent $a$ as a fixed phenomenological parameter, we regard it as an intrinsic component of the fixed-point data to be determined from the internal consistency of the low-energy patch field theory under highly anisotropic scaling dimensions ($[k_0]=a+1$, $[k_x]=2$, $[k_y]=1$). While the leading two-loop diagrammatics vanish identically due to kinematic pole configurations, our three-loop evaluation reveals a profound structural asymmetry between the sectors: the fermion self-energy and Yukawa vertex receive non-vanishing logarithmic corrections, whereas the corresponding bosonic counter-term remains strictly zero. Consequently, we find that no self-consistent, intersecting fixed-point solution for the exponent $a$ exists within the three-loop truncation, failing to reproduce the physical value of $a \approx 1.85$ observed in quantum Monte Carlo simulations. We conjecture that the cross-linked topology of the four-loop boson self-energy diagrams is exactly marginal and yields the minimal, mandatory bosonic counter-term required to restore multi-sector self-consistency. Our framework establishes a rigid multi-loop matching scheme necessary to uniquely pin down the critical exponent, and uncovers a stable phase space for field anomalous dimensions. △ Less

Submitted 28 May, 2026; originally announced May 2026.

Accelerated Discovery of Nitrogen-Coordinated Dual-Atom Hydrogen Evolution Reaction Electrocatalysts via Machine Learning Potentials

Authors: Yanmei Zang, Hyun Gyu Park, Gi Beom Sim, Tae Hyeon Park, Ho Jin Lee, Xiaorong Zou, D. ChangMo Yang, Soohaeng Yoo Willow, Hye Jung Kim, Chang Woo Myung

Abstract: The hydrogen evolution reaction (HER) is central to sustainable hydrogen production, and nitrogen coordinated dual atom catalysts (DACs) offer a promising route to noble metal activity at low cost. Yet their vast compositional and coordination design space remains underexplored, as density functional theory (DFT) screening at scale is prohibitive. Here, we map the HER landscape of graphene support… ▽ More The hydrogen evolution reaction (HER) is central to sustainable hydrogen production, and nitrogen coordinated dual atom catalysts (DACs) offer a promising route to noble metal activity at low cost. Yet their vast compositional and coordination design space remains underexplored, as density functional theory (DFT) screening at scale is prohibitive. Here, we map the HER landscape of graphene supported TM2@Nx-Gr DACs, screening 23 transition metals across 20 nitrogen coordination motifs using a machine learning potential (MLP) benchmarked against DFT. Intermediate coordination (2N to 4N) consistently yields near-optimal ΔGH*, with Ti2@2Na, Mn2@2Na, Fe2@2Na, Cu2@2Na, Rh2@2Na, Zr2@2Na, Zr2@2Nb, Zr2@2Nc, Nb2@2Nc, Zr2@2Nd, Mn2@2Ne, Mn2@2Nf, Ti2@3Na, Au2@3Na, Fe2@3Na, Pd2@3Nb, Rh2@3Nc, Rh2@3Nd, Au2@3Nd, V2@4Na, Ti2@4Nb, Pd2@4Nb, Ti2@4Nc, Cr2@4Nd, Ni2@4Nd, Cu2@4Nd emerging as standout, synthesizable candidates, most exhibiting metallic or narrow gap (<0.25 eV) character. The MLP reaches near-DFT accuracy, with a mean absolute error of 80 meV for Gibbs binding free energies at orders of magnitude lower computational cost, establishing MLP driven screening as a practical engine for next-generation catalyst discovery. △ Less

Submitted 28 May, 2026; originally announced May 2026.

Comments: Research article, 32 pages, 18 figures

Taming the 3D Wilson-Fisher Fixed Point via Nonlocal Effective Action

Authors: Hyeon Jung Kim, Seung-Jong Yoo, Jinmo Bok, Lemuel John Sese, Semin Park, Ki-Seok Kim

Abstract: We present a Renormalization Group (RG) framework based on a nonlocal effective action ansatz to analyze the strong coupling dynamics of the three-dimensional relativistic $φ^{4}$ theory. By implementing a Hubbard-Stratonovich transformation, we decouple the quartic interaction into the primary field $φ$ and an auxiliary field $\varphi \sim φ^2$, allowing both exponents $Δ_φ$ and $Δ_{\varphi}$ to… ▽ More We present a Renormalization Group (RG) framework based on a nonlocal effective action ansatz to analyze the strong coupling dynamics of the three-dimensional relativistic $φ^{4}$ theory. By implementing a Hubbard-Stratonovich transformation, we decouple the quartic interaction into the primary field $φ$ and an auxiliary field $\varphi \sim φ^2$, allowing both exponents $Δ_φ$ and $Δ_{\varphi}$ to act as independent, unconstrained variables rather than fixed scaling dimensions. Within this nonlocal propagator framework, both the field self-energies and vertex corrections are evaluated at the one-loop order. The resulting one-loop logarithmic derivatives determine the renormalization group flows of the couplings and the scaling exponents. For $d=3$ and $ε\approx-0.198$, the self-consistent equations yield a representative fixed point at $Δ_φ\approx0.97714$, $Δ_{\varphi}\approx-0.65260$, and $Δ_{φ^2}\approx1.04573$, corresponding to $η_φ\approx0.04572$ and $ν\approx0.51170$. Relative to the high-precision conformal-bootstrap benchmarks, the deviations are approximately $0.48\%$ for $Δ_φ$, $25.97\%$ for $Δ_{φ^2}$, and $18.77\%$ for $ν$, demonstrating sub-percent agreement in the fundamental-field sector while revealing substantially larger deviations in the composite and correlation-length sectors within the leading-order truncation. △ Less

Submitted 5 August, 2026; v1 submitted 18 May, 2026; originally announced May 2026.

Comments: Completely rewritten and fine-tuned

Magnetic Brightening and Nanoscale Imaging of Spin-Polarized Helical Edge Modes

Authors: Samuel Haeuser, Richard H. J. Kim, Lin-Lin Wang, Thomas Koschny, Pedro M. Lozano, Genda Gu, Randall K. Chan, Joong-Mok Park, Martin Mootz, Liang Luo, Qiang Li, Jigang Wang

Abstract: Efficient sub-10 nm electric transport remains a major challenge for nanoelectronics due to high losses and impedance mismatches in conventional Drude metals. Despite their promise of dissipationless, reflection-free conduction, topologically protected chiral edge modes remain little explored in their nanoscale spin polarized transport-particularly regarding real-space visualization, magnetic fiel… ▽ More Efficient sub-10 nm electric transport remains a major challenge for nanoelectronics due to high losses and impedance mismatches in conventional Drude metals. Despite their promise of dissipationless, reflection-free conduction, topologically protected chiral edge modes remain little explored in their nanoscale spin polarized transport-particularly regarding real-space visualization, magnetic field tunability, and high-frequency edge conductivity. Here, we report magnetic brightening and nanoscale visualization of highly spin-polarizable infrared helical edge states using cryogenic magneto-infrared scattering-type scanning near-field optical microscopy (cm-IR-sSNOM). Our measurements reveal magnetic field-induced near-field conductivity at step edges, uncovering quantum spin Hall spin-splitting modes with enhanced infrared polarizability and slightly narrowed near-field profiles. In addition, the infrared edge electrodynamic response scales nearly linearly with atomic layer number, providing compelling evidence that magnetic-field-induced gaps do not disrupt individual-layer edge states at energies of around 100 meV. These results sharply contrast with microwave and DC transport, where even small magnetically induced gaps decrease edge conduction. Magnetically tunable, topologically robust high-frequency edge modes open a pathway toward ultralow-loss nanoscale interconnects and quantum logic architectures for next-generation microelectronics, spintronics and quantum information science. △ Less

Submitted 6 May, 2026; originally announced May 2026.

Journal ref: Nature Nanotechnology (2026)

Terahertz magneto-nanoscopy of encapsulated monolayer graphene

Authors: Richard H. J. Kim, Sunwoong Yang, Taehoon Kim, Samuel J. Haeuser, Joong-Mok Park, Randall K. Chan, Thomas Koschny, Young-Mi Bahk, Sung Ju Hong, Jigang Wang

Abstract: This study investigates the nanoscale conductivity of encapsulated monolayer graphene at temperatures down to 5 K and magnetic fields of up to 1 T. We use the scattering-type scanning near-field optical microscopy (s-SNOM) technique to probe magnetic-field-dependent responses from graphene close to charge neutrality in the terahertz spectral region. We observe the near-perfect high-$q$ reflector b… ▽ More This study investigates the nanoscale conductivity of encapsulated monolayer graphene at temperatures down to 5 K and magnetic fields of up to 1 T. We use the scattering-type scanning near-field optical microscopy (s-SNOM) technique to probe magnetic-field-dependent responses from graphene close to charge neutrality in the terahertz spectral region. We observe the near-perfect high-$q$ reflector behavior of graphene but with subtle changes by the presence of magnetic fields. Measurements align with calculations of the magneto-optical conductivity and the near-field spectroscopic contrast that describes the field-tunable cyclotron resonance of Dirac fermions. Our result provides an initial step toward understanding temperature and magnetic-field effects on nanoscale terahertz transport in two-dimensional quantum materials. △ Less

Submitted 24 April, 2026; originally announced April 2026.

Journal ref: Journal of Applied Physics, 139, 164303 (2026)

Stochastic Loop Corrections to Belief Propagation for Tensor Network Contraction

Authors: Gi Beom Sim, Tae Hyeon Park, Kwang S. Kim, Yanmei Zang, Xiaorong Zou, Hye Jung Kim, D. ChangMo Yang, Soohaeng Yoo Willow, Chang Woo Myung

Abstract: Tensor network contraction is a fundamental computational challenge underlying quantum many-body physics, statistical mechanics, and machine learning. Belief propagation (BP) provides an efficient approximate solution, but introduces systematic errors on graphs with loops. Here, we introduce a hybrid method that achieves accurate results by stochastically sampling loop corrections to BP and showca… ▽ More Tensor network contraction is a fundamental computational challenge underlying quantum many-body physics, statistical mechanics, and machine learning. Belief propagation (BP) provides an efficient approximate solution, but introduces systematic errors on graphs with loops. Here, we introduce a hybrid method that achieves accurate results by stochastically sampling loop corrections to BP and showcase our method by applying it to the two-dimensional ferromagnetic Ising model. For any pairwise Markov random field with symmetric edge potentials, our approach exploits an exact factorization of the partition function into the BP contribution and a loop correction factor summing over all valid loop configurations, weighted by edge weights derived directly from the potentials. We sample this sum using Markov chain Monte Carlo with moves that preserve the loop constraint, combined with umbrella sampling to ensure efficient exploration across all correlation strengths. Our stochastic approach provides unbiased estimates with controllable statistical error in any parameter regime. △ Less

Submitted 7 April, 2026; v1 submitted 9 March, 2026; originally announced March 2026.

Comments: 12+4 pages, 5+1 figures

Terahertz-nanoscale visualization of the microscopic spin-charge architecture of colossal magnetoresistive switching

Authors: Samuel Haeuser, Randall K. Chan, Richard H. J. Kim, Joong-Mok Park, Martin Mootz, Thomas Koschny, Jigang Wang

Abstract: Resolving sub-10 nm spin switching and the associated terahertz (THz) electrodynamics during the colossal magnetoresistance (CMR) transition is a definitive frontier in reaching the fundamental spatial, temporal, and energy-dissipation limits of spin-based microelectronics and quantum logic architectures. Yet, the requirement of simultaneous control of high magnetic field, cryogenic environment, a… ▽ More Resolving sub-10 nm spin switching and the associated terahertz (THz) electrodynamics during the colossal magnetoresistance (CMR) transition is a definitive frontier in reaching the fundamental spatial, temporal, and energy-dissipation limits of spin-based microelectronics and quantum logic architectures. Yet, the requirement of simultaneous control of high magnetic field, cryogenic environment, and nanometer-scale resolution has remained an elusive benchmark for terahertz nanoscopy, leaving the obscured nano-scale high-frequency dynamics of these transitions largely unexplored. Here, we overcome these limitations by utilizing a custom-built cryogenic magneto-THz scattering-type scanning near-field optical microscopy (cm-THz-sSNOM) platform to resolve the nanoscale, THz spectroscopic evolution of the magnetic field-driven CMR transition in a manganite single crystal $\text{Pr}_{2/3}\text{Ca}_{1/3}\text{MnO}_{3}$. Our measurements provide a real-space visualization of the local THz conductivity, capturing the moment that magnetic-field-induced spin switching triggers the phase transition from an antiferromagnetic insulator to a ferromagnetic metal. THz nano-imaging, together with an ellipsoidal near-field model, reveals a multi-scale transition initiated by 1-2 nm isolated spin-flip sites at low magnetic fields, which coalesce into $\sim$15~nm conducting regions as the threshold field is approached. These results provide an in situ, previously inaccessible THz real-space view of CMR switching, establishing a general analysis framework for mapping spin-charge-lattice-orbit-coupled dynamics at spatial scales that transcend the nominal sSNOM resolution. △ Less

Submitted 9 March, 2026; originally announced March 2026.

Journal ref: Advanced Science, 10.1002/advs.76799 (2026)

Vortex-Controlled Quasiparticle Multiplication and Self-Growth Dynamics in Superconducting Resonators

Authors: Joong M. Park, Martin Mootz, Richard H. J. Kim, Zhixiang Chong, Samuel Haeuser, Randall K. Chan, Liang Luo, Dominic P. Goronzy, Mark C. Hersam, Ilias E. Perakis, Akshay A Murthy, Alexander Romanenko, Anna Grassellino, Jigang Wang

Abstract: Even in the quantum limit, non-equilibrium quasiparticle (QP) populations induce QP poisoning that irreversibly relaxes the quantum state and significantly degrades the coherence of transmon qubits. A particularly detrimental yet previously unexplored mechanism arises from QP multiplication facilitated by vortex trapping in superconducting quantum circuits, where a high-energy QP relaxes by breaki… ▽ More Even in the quantum limit, non-equilibrium quasiparticle (QP) populations induce QP poisoning that irreversibly relaxes the quantum state and significantly degrades the coherence of transmon qubits. A particularly detrimental yet previously unexplored mechanism arises from QP multiplication facilitated by vortex trapping in superconducting quantum circuits, where a high-energy QP relaxes by breaking additional Cooper pairs and amplifying the QP population due to the locally reduced excitation gap and enhanced quantum confinement within the vortex core. Here we directly resolve this elusive QP multiplication process by revealing vortex-controlled QP self-generation in a highly nonequilibrium regime preceding the phonon bottleneck of QP relaxation. At sufficiently low fluence, femtosecond-resolved magneto-reflection spectroscopy directly reveals a continuously increasing QP population that is strongly dependent on magnetic-field-tuned vortex density and absent at higher excitation fluences. Quantitative analysis of the emergent QP pre-bottleneck dynamics further reveals that, although the phonon population saturates within $\simeq$10~ps, both free and trapped QPs continue to grow in a self-sustained manner--hallmarks of the long-anticipated QP-vortex interactions in nonequilibrium superconductivity. We estimate a substantial increase of $\sim$34\% in QP density at vortex densities of $\sim$ 100 magnetic flux quanta per $\mathrm{μm^{2}}$. Our findings establish a powerful spectroscopic tool for uncovering QP multiplication and reveal vortex-assisted QP relaxation as a critical materials bottleneck whose mitigation will be essential for resolving QP poisoning and enhancing coherence in superconducting qubits. △ Less

Submitted 5 November, 2025; originally announced November 2025.

Bidirectional ultrafast control of charge density waves via phase competition

Authors: Honglie Ning, Kyoung Hun Oh, Yifan Su, Zhengyan Darius Shi, Dong Wu, Qiaomei Liu, B. Q. Lv, Alfred Zong, Gyeongbo Kang, Hyeongi Choi, Hyun-Woo J. Kim, Seunghyeok Ha, Jaehwon Kim, Suchismita Sarker, Jacob P. C. Ruff, B. J. Kim, N. L. Wang, Todadri Senthil, Hoyoung Jang, Nuh Gedik

Abstract: The intricate competition between coexisting charge density waves (CDWs) can lead to rich phenomena, offering unique opportunities for phase manipulation through electromagnetic stimuli. Leveraging time-resolved X-ray diffraction, we demonstrate ultrafast control of a CDW in EuTe$_4$ upon optical excitation. At low excitation intensities, the amplitude of one of the coexisting CDW orders increases… ▽ More The intricate competition between coexisting charge density waves (CDWs) can lead to rich phenomena, offering unique opportunities for phase manipulation through electromagnetic stimuli. Leveraging time-resolved X-ray diffraction, we demonstrate ultrafast control of a CDW in EuTe$_4$ upon optical excitation. At low excitation intensities, the amplitude of one of the coexisting CDW orders increases at the expense of the competing CDW, whereas at high intensities, it exhibits a nonmonotonic temporal evolution characterized by both enhancement and reduction. This transient bidirectional controllability, tunable by adjusting photo-excitation intensity, arises from the interplay between optical quenching and phase-competition-induced enhancement. Our findings, supported by phenomenological time-dependent Ginzburg-Landau theory simulations, not only clarify the relationship between the two CDWs in EuTe$_4$, but also highlight the versatility of optical control over order parameters enabled by phase competition. △ Less

Submitted 30 September, 2025; originally announced October 2025.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. Lett. 135, 246504 (2025)

Dynamics of a jointly commensurate moiré charge density wave

Authors: Kyoung Hun Oh, Yifan Su, Honglie Ning, B. Q. Lv, Alfred Zong, Dong Wu, Qiaomei Liu, Gyeongbo Kang, Hyeongi Choi, Hyun-Woo J. Kim, Seunghyeok Ha, Jaehwon Kim, Suchismita Sarker, Jacob P. C. Ruff, Xiaozhe Shen, Duan Luo, Stephen Weathersby, Patrick Kramer, Xinxin Cheng, Dongsung Choi, Doron Azoury, Masataka Mogi, B. J. Kim, N. L. Wang, Hoyoung Jang , et al. (1 additional authors not shown)

Abstract: The advent of two-dimensional moiré systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moiré superstructure formed by two coexisting charge density waves (CDWs) with slightly mismatched wavevectors has been realized. These incommensurate CDWs can collectively exhibit commensurability, resulting in the jointly commensur… ▽ More The advent of two-dimensional moiré systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moiré superstructure formed by two coexisting charge density waves (CDWs) with slightly mismatched wavevectors has been realized. These incommensurate CDWs can collectively exhibit commensurability, resulting in the jointly commensurate CDW (JC-CDW) and establishing a new paradigm for controlling moiré potential and periodicity. Achieving such functionality, however, hinges on a key open question: how do the amplitude, phase coherence, and periodicity of this order respond to external perturbations? Here, we address this question using a suite of time- and momentum-resolved diffraction and spectroscopic techniques to probe light-induced CDW dynamics in EuTe$_4$. Our time-resolved diffraction measurements distinguish the instantaneous quenching of the JC-CDW amplitude, as verified by time-resolved photoemission spectroscopy, from the much slower evolution of phase fluctuations. Furthermore, while the JC-CDW wavevector remains locked along the CDW direction upon photoexcitation, indicating a preserved moiré periodicity, the correlation length of JC-CDW shows an exclusive reduction perpendicular to its wavevector, unveiling the formation of previously unexplored shear-type defects. Together, this multimodal methodology reconstructs the spatiotemporal evolution of the JC-CDW upon excitation. These findings not only highlight the remarkable robustness of JC-CDWs out of equilibrium, but also provide insight into optical manipulation and engineering of moiré quantum materials through defect control. △ Less

Submitted 2 July, 2026; v1 submitted 19 September, 2025; originally announced September 2025.

Comments: 24 pages, 4 figures

Intertwined Orders in a Quantum-Entangled Metal

Authors: Junyoung Kwon, Jaehwon Kim, Gwansuk Oh, Seyoung Jin, Kwangrae Kim, Hoon Kim, Seunghyeok Ha, Hyun-Woo J. Kim, GiBaik Sim, Bjorn Wehinger, Gaston Garbarino, Nour Maraytta, Michael Merz, Matthieu Le Tacon, Christoph J. Sahle, Alessandro Longo, Jungho Kim, Ara Go, Gil Young Cho, Beom Hyun Kim, B. J. Kim

Abstract: Entanglement underpins quantum information processing and computing, yet its experimental quantification in complex, many-body condensed matter systems remains a considerable challenge. Here, we reveal a highly entangled electronic phase proximate to a quantum metal-insulator transition, identified by resonant inelastic x-ray scattering interferometry. This approach reveals that entanglement acros… ▽ More Entanglement underpins quantum information processing and computing, yet its experimental quantification in complex, many-body condensed matter systems remains a considerable challenge. Here, we reveal a highly entangled electronic phase proximate to a quantum metal-insulator transition, identified by resonant inelastic x-ray scattering interferometry. This approach reveals that entanglement across atomic sites generates characteristic interference patterns, which our model accurately reproduces, enabling extraction of a full entanglement spectrum and resolution of the underlying quantum states. Our analysis of the pyrochlore iridate Nd2Ir2O7 demonstrates that the system undergoes pronounced quantum fluctuations in its spin, orbital and charge degrees of freedom, even in the presence of a long-range 'all-in-all-out' antiferromagnetic order. Importantly, the observed entanglement signatures facilitate the coexistence of multiple exotic symmetry-breaking orders. Complementary investigations using Raman spectroscopy corroborate the presence of these hidden orders and their emergent excitations. In particular, we observe a two-magnon-bound state below the lowest single-magnon excitation energy, which, together with split phonon modes, provides strong evidence for cubic symmetry-breaking orders of magnetic origin juxtaposed with the all-in-all-out order. Our work thus establishes a direct link between quantum entanglement and emergent unconventional orders, opening new avenues for investigating quantum materials. △ Less

Submitted 6 July, 2025; originally announced July 2025.

Correlating Superconducting Qubit Performance Losses to Sidewall Near-Field Scattering via Terahertz Nanophotonics

Authors: Richard H. J. Kim, Samuel J. Haeuser, Joong-Mok Park, Randall K. Chan, Jin-Su Oh, Thomas Koschny, Lin Zhou, Matthew J. Kramer, Akshay A. Murthy, Mustafa Bal, Francesco Crisa, Sabrina Garattoni, Shaojiang Zhu, Andrei Lunin, David Olaya, Peter Hopkins, Alex Romanenko, Anna Grassellino, Jigang Wang

Abstract: Elucidating dielectric losses, structural heterogeneity, and interface imperfections is critical for improving coherence in superconducting qubits. However, most diagnostics rely on destructive electron microscopy or low-throughput millikelvin quantum measurements. Here, we demonstrate noninvasive terahertz (THz) nano-imaging/-spectroscopy of encapsulated niobium transmon qubits, revealing sidewal… ▽ More Elucidating dielectric losses, structural heterogeneity, and interface imperfections is critical for improving coherence in superconducting qubits. However, most diagnostics rely on destructive electron microscopy or low-throughput millikelvin quantum measurements. Here, we demonstrate noninvasive terahertz (THz) nano-imaging/-spectroscopy of encapsulated niobium transmon qubits, revealing sidewall near-field scattering that correlates with qubit coherence. We further employ a THz hyperspectral line scan to probe dielectric responses and field participation at Al junction interfaces. These findings highlight the promise of THz near-field methods as a high-throughput proxy characterization tool for guiding material selection and optimizing processing protocols to improve qubit and quantum circuit performance. △ Less

Submitted 5 June, 2025; originally announced June 2025.

Journal ref: Appl. Phys. Lett., 127, 114002 (2025)

New Insights into Refractive Indices and Birefringence of Undoped and MgO-Doped Lithium Niobate Crystals at High Temperatures

Authors: Nina Hong, Jiarong R. Cui, Hyun Jung Kim, Ross G. Shaffer, Nguyen Q. Vinh

Abstract: The lithium niobate single crystal is a well-known optical material that has been employed in a wide range of photonic applications. To realize further applications of the crystal, the birefringence properties need to be determined over a large range of temperatures. We report refractive indices and birefringence properties of undoped and MgO-doped lithium niobate crystals with high accuracy using… ▽ More The lithium niobate single crystal is a well-known optical material that has been employed in a wide range of photonic applications. To realize further applications of the crystal, the birefringence properties need to be determined over a large range of temperatures. We report refractive indices and birefringence properties of undoped and MgO-doped lithium niobate crystals with high accuracy using spectroscopic ellipsometry in the spectral range from 450 to 1700 nm and a temperature range from ambient temperature to 1000 °C. The birefringence results indicate a transition temperature, where the crystal transforms from an anisotropic to isotropic property, and the advance of MgO doping in the crystal, which is related to the optical damage threshold of the materials. In addition, the lattice dynamics of the crystals have been analyzed by revisiting the Raman spectroscopy. The results establish the foundation of optical properties of lithium niobate crystals, providing pathways for their photonic applications. △ Less

Submitted 11 April, 2025; originally announced April 2025.

Comments: 17 pages, 6 figures and Supplementary Material

Journal ref: Optical Materials 144, 114365 (2023)

High-Efficiency Multilevel Phase Lenses with Nanostructures on Polyimide Membranes

Authors: Leslie Howe, Tharindu D. Rajapaksha, Kalani H. Ellepola, Vinh X. Ho, Zachary Aycock, Minh L. P. Nguyen, John P. Leckey, Dave G. Macdonnell, Hyun Jung Kim, Nguyen Q. Vinh

Abstract: The emergence of planar meta-lenses on flexible materials has profoundly impacted the long-standing perception of diffractive optics. Despite their advantages, these lenses still face challenges in design and fabrication to obtain high focusing efficiency and resolving power. A nanofabrication technique is demonstrated based on photolithography and polyimide casting for realizing membrane-based mu… ▽ More The emergence of planar meta-lenses on flexible materials has profoundly impacted the long-standing perception of diffractive optics. Despite their advantages, these lenses still face challenges in design and fabrication to obtain high focusing efficiency and resolving power. A nanofabrication technique is demonstrated based on photolithography and polyimide casting for realizing membrane-based multilevel phase-type Fresnel zone plates (FZPs) with high focusing efficiency. By employing advantageous techniques, these lenses with nanostructures are directly patterned into thin polyimide membranes. The computational and experimental results have indicated that the focusing efficiency of these nanostructures at the primary focus increases significantly with increasing the number of phase levels. Specifically, 16-level phase lenses on a polyimide membrane can achieve a focusing efficiency of more than 91.6% of the input signal (9.5 times better than that of a conventional amplitude-type FZP) and focus light into a diffraction-limited spot together with very weak side-lobes. Furthermore, these lenses exhibit considerably reduced unwanted diffraction orders and produce extremely low background signals. The potential impact of these lenses extends across various applications and techniques including microscopy, imaging, micro-diffraction, remote sensing, and space flight instruments which require lightweight and flexible configurations. △ Less

Submitted 25 February, 2025; originally announced February 2025.

Comments: 27 pages, 7 figures with supporting information

Journal ref: Advanced Optical Materials 12, 2400847, 2024

Effect of Grafting Density on the Two-dimensional Assembly of Nanoparticles

Authors: Binay P. Nayak, James Ethan Batey, Hyeong Jin Kim, Wenjie Wang, Wei Bu, Honghu Zhang, Surya K. Mallapragada, David Vaknin

Abstract: Employing grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR), we demonstrate that films composed of polyethylene glycol (PEG)-grafted silver nanoparticles (AgNP) and gold nanoparticles (AuNP), as well as their binary mixtures, form highly stable hexagonal structures at the vapor-liquid interface. These nanoparticles exhibit remarkable stability under varying envir… ▽ More Employing grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR), we demonstrate that films composed of polyethylene glycol (PEG)-grafted silver nanoparticles (AgNP) and gold nanoparticles (AuNP), as well as their binary mixtures, form highly stable hexagonal structures at the vapor-liquid interface. These nanoparticles exhibit remarkable stability under varying environmental conditions, including changes in pH, mixing concentration, and PEG chain length. Short-chain PEG grafting produces dense, well-ordered films, while longer chains produce more complex, less dense quasi-bilayer structures. AuNPs exhibit higher grafting densities than AgNPs, leading to more ordered in-plane arrangements. In binary mixtures, AuNPs dominate the population at the surface, while AgNPs integrate into the system, expanding the lattice without forming a distinct binary superstructure. These results offer valuable insights into the structural behavior of PEG-grafted nanoparticles and provide a foundation for optimizing binary nanoparticle assemblies for advanced nanotechnology applications. △ Less

Submitted 7 October, 2024; originally announced October 2024.

Comments: 17 pages, 8 figures, 2 tables

Unravelling and circumventing failure mechanisms in chalcogenide optical phase change materials

Authors: Cosmin Constantin Popescu, Kiumars Aryana, Brian Mills, Tae Woo Lee, Louis Martin-Monier, Luigi Ranno, Jia Xu Brian Sia, Khoi Phuong Dao, Hyung-Bin Bae, Vladimir Liberman, Steven Vitale, Myungkoo Kang, Kathleen A. Richardson, Carlos A. Ríos Ocampo, Dennis Calahan, Yifei Zhang, William M. Humphreys, Hyun Jung Kim, Tian Gu, Juejun Hu

Abstract: Chalcogenide optical phase change materials (PCMs) have garnered significant interest for their growing applications in programmable photonics, optical analog computing, active metasurfaces, and beyond. Limited endurance or cycling lifetime is however increasingly becoming a bottleneck toward their practical deployment for these applications. To address this issue, we performed a systematic study… ▽ More Chalcogenide optical phase change materials (PCMs) have garnered significant interest for their growing applications in programmable photonics, optical analog computing, active metasurfaces, and beyond. Limited endurance or cycling lifetime is however increasingly becoming a bottleneck toward their practical deployment for these applications. To address this issue, we performed a systematic study elucidating the cycling failure mechanisms of Ge$_2$Sb$_2$Se$_4$Te (GSST), a common optical PCM tailored for infrared photonic applications, in an electrothermal switching configuration commensurate with their applications in on-chip photonic devices. We further propose a set of design rules building on insights into the failure mechanisms, and successfully implemented them to boost the endurance of the GSST device to over 67,000 cycles. △ Less

Submitted 18 September, 2024; originally announced September 2024.

Robust electrothermal switching of optical phase change materials through computer-aided adaptive pulse optimization

Authors: Parth Garud, Kiumars Aryana, Cosmin Constantin Popescu, Steven Vitale, Rashi Sharma, Kathleen Richardson, Tian Gu, Juejun Hu, Hyun Jung Kim

Abstract: Electrically tunable optical devices present diverse functionalities for manipulating electromagnetic waves by leveraging elements capable of reversibly switching between different optical states. This adaptability in adjusting their responses to electromagnetic waves after fabrication is crucial for developing more efficient and compact optical systems for a broad range of applications including… ▽ More Electrically tunable optical devices present diverse functionalities for manipulating electromagnetic waves by leveraging elements capable of reversibly switching between different optical states. This adaptability in adjusting their responses to electromagnetic waves after fabrication is crucial for developing more efficient and compact optical systems for a broad range of applications including sensing, imaging, telecommunications, and data storage. Chalcogenide-based phase change materials (PCMs) have shown great promise due to their stable, non-volatile phase transition between amorphous and crystalline states. Nonetheless, optimizing the switching parameters of PCM devices and maintaining their stable operation over thousands of cycles with minimal variation can be challenging. In this paper, we report on the critical role of PCM pattern as well as electrical pulse form in achieving reliable and stable switching, extending the operational lifetime of the device beyond 13,000 switching events. To achieve this, we have developed a computer-aided algorithm that monitors optical changes in the device and adjusts the applied voltage in accordance with the phase transformation process, thereby significantly enhancing the lifetime of these reconfigurable devices. Our findings reveal that patterned PCM structures show significantly higher endurance compared to blanket PCM thin films. △ Less

Submitted 22 April, 2024; originally announced April 2024.

Holographic dual effective field theory in the Luttinger-Ward functional approach: Application to an SYK model

Authors: Yoon-Seok Choun, Hyeon Jung Kim, Ki-Seok Kim

Abstract: We construct an emergent holographic dual description in the Luttinger-Ward functional approach, where the renormalization group (RG) flows of collective bi-local fields appear manifestly in the bulk effective action with an emergent extra dimension. This holographic dual effective field theory reproduces $1/N$ quantum corrections in a self-consistent manner when we take the UV limit in the bulk e… ▽ More We construct an emergent holographic dual description in the Luttinger-Ward functional approach, where the renormalization group (RG) flows of collective bi-local fields appear manifestly in the bulk effective action with an emergent extra dimension. This holographic dual effective field theory reproduces $1/N$ quantum corrections in a self-consistent manner when we take the UV limit in the bulk effective action. Going into the IR regime in the extra dimension, we observe that a partial class of the field theoretic $1/N$, $1/N^{2}$, ... quantum corrections are resummed in the all-loop order and reorganized to form a holographic dual effective field theory in a large $N$ fashion living on the one-higher dimensional spacetime. In this study, we apply this theoretical framework into an Sachdev-Ye-Kitaev (SYK) model. Taking the large $N$ limit in the holographic dual effective field theory, we obtain nonlinearly coupled second-order bulk differential equations of motion for the three bi-local order-parameter fields of fermion self-energy, Green's function, and polarization function. Here, both UV and IR boundary conditions are derived self-consistently from the boundary effective action. We solve these highly intertwined nonlinear differential equations based on the so called matching method. Our ansatz for the bi-local order-parameter fields coincide with the conformally invariant solution of the field theoretic large $N$ limit in the UV limit, but their overall coefficients $RG-flow$ along the extra dimensional space, respectively, reflecting effects of higher-order quantum corrections. As a result, we find an insulating behavior, where the self-energy diverges at IR. ... △ Less

Submitted 9 September, 2024; v1 submitted 19 February, 2024; originally announced February 2024.

Comments: Regularizations have been clarified and a general scope has been discussed

Observation of the Magnonic Dicke Superradiant Phase Transition

Authors: Dasom Kim, Sohail Dasgupta, Xiaoxuan Ma, Joong-Mok Park, Hao-Tian Wei, Liang Luo, Jacques Doumani, Xinwei Li, Wanting Yang, Di Cheng, Richard H. J. Kim, Henry O. Everitt, Shojiro Kimura, Hiroyuki Nojiri, Jigang Wang, Shixun Cao, Motoaki Bamba, Kaden R. A. Hazzard, Junichiro Kono

Abstract: Two-level atoms coupled with single-mode cavity photons are predicted to exhibit a quantum phase transition when the coupling strength exceeds a critical value, entering a phase in which atomic polarization and photonic field are finite even at zero temperature and without external driving. However, this phenomenon, the superradiant phase transition (SRPT), is forbidden by a no-go theorem due to t… ▽ More Two-level atoms coupled with single-mode cavity photons are predicted to exhibit a quantum phase transition when the coupling strength exceeds a critical value, entering a phase in which atomic polarization and photonic field are finite even at zero temperature and without external driving. However, this phenomenon, the superradiant phase transition (SRPT), is forbidden by a no-go theorem due to the existence of the diamagnetic term in the Hamiltonian. Here, we present spectroscopic evidence for a magnonic SRPT in ErFeO$_3$, where the role of the photonic mode (two-level atoms) in the photonic SRPT is played by an Fe$^{3+}$ magnon mode (Er$^{3+}$ spins). The absence of the diamagnetic term in the Fe$^{3+}$-Er$^{3+}$ exchange coupling ensures that the no-go theorem does not apply. Terahertz and gigahertz magnetospectroscopy experiments revealed the signatures of the SRPT -- a kink and a softening, respectively, of two spin-magnon hybridized modes at the critical point. △ Less

Submitted 3 January, 2024; originally announced January 2024.

Journal ref: Science Advances 11, adt1691 (2025)

Origin of chirality in transition-metal dichalcogenides

Authors: Kwangrae Kim, Hyun-Woo J. Kim, Seunghyeok Ha, Hoon Kim, Jin-Kwang Kim, Jaehwon Kim, Hyunsung Kim, Junyoung Kwon, Jihoon Seol, Saegyeol Jung, Changyoung Kim, Ahmet Alatas, Ayman Said, Michael Merz, Matthieu Le Tacon, Jin Mo Bok, Ki-Seok Kim, B. J. Kim

Abstract: Chirality is a ubiquitous phenomenon in which a symmetry between left- and right-handed objects is broken, examples in nature ranging from subatomic particles and molecules to living organisms. In particle physics, the weak force is responsible for the symmetry breaking and parity violation in beta decay, but in condensed matter systems interactions that lead to chirality remain poorly understood.… ▽ More Chirality is a ubiquitous phenomenon in which a symmetry between left- and right-handed objects is broken, examples in nature ranging from subatomic particles and molecules to living organisms. In particle physics, the weak force is responsible for the symmetry breaking and parity violation in beta decay, but in condensed matter systems interactions that lead to chirality remain poorly understood. Here, we unravel the mechanism of chiral charge density wave formation in the transition-metal dichalcogenide 1T-TiSe2. Using representation analysis, we show that charge density modulations and ionic displacements, which transform as a continuous scalar field and a vector field on a discrete lattice, respectively, follow different irreducible representations of the space group, despite the fact that they propagate with the same wave-vectors and are strongly coupled to each other. This charge-lattice symmetry frustration is resolved by further breaking of all symmetries not common to both sectors through induced lattice distortions, thus leading to chirality. Our theory is verified using Raman spectroscopy and inelastic x-ray scattering, which reveal that all but translation symmetries are broken at a level not resolved by state-of-the-art diffraction techniques. △ Less

Submitted 19 December, 2023; originally announced December 2023.

Comments: 10 pages, 3 figures, 1 table

Journal ref: Nature Physics 20, 1919-1926 (2024)

Discovery of an Unconventional Quantum Echo by Interference of Higgs Coherence

Authors: C. Huang, M. Mootz, L. Luo, D. Cheng, J. M. Park, R. H. J. Kim, Y. Qiang, V. L. Quito, Yongxin Yao, P. P. Orth, I. E. Perakis, J. Wang

Abstract: Nonlinearities in quantum systems are fundamentally characterized by the interplay of phase coherences, their interference, and state transition amplitudes. Yet the question of how quantum coherence and interference manifest in transient, massive Higgs excitations, prevalent within both the quantum vacuum and superconductors, remains elusive. One hallmark example is photon echo, enabled by the gen… ▽ More Nonlinearities in quantum systems are fundamentally characterized by the interplay of phase coherences, their interference, and state transition amplitudes. Yet the question of how quantum coherence and interference manifest in transient, massive Higgs excitations, prevalent within both the quantum vacuum and superconductors, remains elusive. One hallmark example is photon echo, enabled by the generation, preservation, and retrieval of phase coherences amid multiple excitations. Here we reveal an unconventional quantum echo arising from the Higgs coherence in superconductors, and identify distinctive signatures attributed to Higgs anharmonicity. A terahertz pulse-pair modulation of the superconducting gap generates a "time grating" of coherent Higgs population, which scatters echo signals distinct from conventional spin- and photon-echoes in atoms and semiconductors. These manifestations appear as Higgs echo spectral peaks occurring at frequencies forbidden by equilibrium particle-hole symmetry, an asymmetric delay in the echo formation from the dynamics of the "reactive" superconducting state, and negative time signals arising from Higgs-quasiparticle anharmonic coupling. The Higgs interference and anharmonicity control the decoherence of driven superconductivity and may enable applications in quantum memory and entanglement. △ Less

Submitted 17 December, 2023; originally announced December 2023.

Journal ref: Science Advances, 11, eads8740 (2025)

Assembling PNIPAM-Capped Gold Nanoparticles in Aqueous Solutions

Authors: Binay P. Nayak, Hyeong Jin Kim, Srikanth Nayak, Wenjie Wang, Wei Bu, Surya K. Mallapragada, David Vaknin

Abstract: Employing small angle X-ray scattering (SAXS), we explore the conditions under which the assembly of gold nanoparticles (AuNPs) grafted with the thermo-sensitive polymer Poly(N-isopropylacrylamide) (PNIPAM) emerges. We find that short-range order assembly emerges by combining the addition of electrolytes or poly-electrolytes with raising the temperature of the suspensions above the lower-critical… ▽ More Employing small angle X-ray scattering (SAXS), we explore the conditions under which the assembly of gold nanoparticles (AuNPs) grafted with the thermo-sensitive polymer Poly(N-isopropylacrylamide) (PNIPAM) emerges. We find that short-range order assembly emerges by combining the addition of electrolytes or poly-electrolytes with raising the temperature of the suspensions above the lower-critical solution temperature (LCST) of PNIPAM. Our results show that the longer the PNIPAM chain is, the better organization in the assembled clusters. Interestingly, without added electrolytes, there is no evidence of AuNP assembly as a function of temperature, although untethered PNIPAM is known to undergo a coil-to-globule transition above its LCST. This study demonstrates another approach to assembling potential thermo-sensitive nanostructures for devices by leveraging the unique properties of PNIPAM. △ Less

Submitted 7 December, 2023; originally announced December 2023.

Comments: Published at ACS Macro Letters, DOI - https://doi.org/10.1021/acsmacrolett.3c00617

Journal ref: ACS Macro Lett. 2023, 12, XXX, 1659 to 1664

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)

Quantum spin nematic phase in a square-lattice iridate

Authors: Hoon Kim, Jin-Kwang Kim, Jimin Kim, Hyun-Woo J. Kim, Seunghyeok Ha, Kwangrae Kim, Wonjun Lee, Jonghwan Kim, Gil Young Cho, Hyeokjun Heo, Joonho Jang, J. Strempfer, G. Fabbris, Y. Choi, D. Haskel, Jungho Kim, J. -W. Kim, B. J. Kim

Abstract: Spin nematic (SN) is a magnetic analog of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid. Particularly intriguing is a valence-bond SN, in which spins are quantum entangled to form a multi-polar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here, we establish a SN phase in the sq… ▽ More Spin nematic (SN) is a magnetic analog of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid. Particularly intriguing is a valence-bond SN, in which spins are quantum entangled to form a multi-polar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here, we establish a SN phase in the square-lattice iridate Sr$_2$IrO$_4$, which approximately realizes a pseudospin one-half Heisenberg antiferromagnet (AF) in the strong spin-orbit coupling limit. Upon cooling, the transition into the SN phase at T$_C$ $\approx$ 263 K is marked by a divergence in the static spin quadrupole susceptibility extracted from our Raman spectra, and concomitant emergence of a collective mode associated with the spontaneous breaking of rotational symmetries. The quadrupolar order persists in the antiferromagnetic (AF) phase below T$_N$ $\approx$ 230 K, and becomes directly observable through its interference with the AF order in resonant x-ray diffraction, which allows us to uniquely determine its spatial structure. Further, we find using resonant inelastic x-ray scattering a complete breakdown of coherent magnon excitations at short-wavelength scales, suggesting a resonating-valence-bond-like quantum entanglement in the AF state. Taken together, our results reveal a quantum order underlying the Néel AF that is widely believed to be intimately connected to the mechanism of high temperature superconductivity (HTSC). △ Less

Submitted 14 December, 2023; v1 submitted 2 October, 2023; originally announced October 2023.

Comments: Published in https://www.nature.com/articles/s41586-023-06829-4

Highly-Sensitive Resonance-Enhanced Organic Photodetectors for Shortwave Infrared Sensing

Authors: Hoang Mai Luong, Chokchai Kaiyasuan, Ahra Yi, Sangmin Chae, Brian Minki Kim, Patchareepond Panoy, Hyo Jung Kim, Vinich Promarak, Yasuo Miyata, Hidenori Nakayama, Thuc-Quyen Nguyen

Abstract: Shortwave infrared (SWIR) has various applications, including night vision, remote sensing, and medical imaging. SWIR organic photodetectors (OPDs) offer advantages such as flexibility, cost-effectiveness, and tunable properties, however, lower sensitivity and limited spectral coverage compared to inorganic counterparts are major drawbacks. Here, we propose a simple yet effective and widely applic… ▽ More Shortwave infrared (SWIR) has various applications, including night vision, remote sensing, and medical imaging. SWIR organic photodetectors (OPDs) offer advantages such as flexibility, cost-effectiveness, and tunable properties, however, lower sensitivity and limited spectral coverage compared to inorganic counterparts are major drawbacks. Here, we propose a simple yet effective and widely applicable strategy to extend the wavelength detection range of OPD to a longer wavelength, using resonant optical microcavity. We demonstrate a proof-of-concept in PTB7-Th:COTIC-4F blend system, achieving external quantum efficiency (EQE) > 50 % over a broad spectrum 450 - 1100 nm with a peak specific detectivity (D*) of 1.1E13 Jones at 1100 nm, while cut-off bandwidth, speed, and linearity are preserved. By employing a novel small-molecule acceptor IR6, a record high EQE = 35 % and D* = 4.1E12 Jones are obtained at 1150 nm. This research emphasizes the importance of optical design in optoelectronic devices, presenting a considerably simpler method to expand the photodetection range compared to a traditional approach that involves developing absorbers with narrow optical gaps. △ Less

Submitted 13 September, 2023; originally announced September 2023.

Emulating Expert Insight: A Robust Strategy for Optimal Experimental Design

Authors: Matthew R. Carbone, Hyeong Jin Kim, Chandima Fernando, Shinjae Yoo, Daniel Olds, Howie Joress, Brian DeCost, Bruce Ravel, Yugang Zhang, Phillip M. Maffettone

Abstract: The challenge of optimal design of experiments (DOE) pervades materials science, physics, chemistry, and biology. Bayesian optimization has been used to address this challenge in vast sample spaces, although it requires framing experimental campaigns through the lens of maximizing some observable. This framing is insufficient for epistemic research goals that seek to comprehensively analyze a samp… ▽ More The challenge of optimal design of experiments (DOE) pervades materials science, physics, chemistry, and biology. Bayesian optimization has been used to address this challenge in vast sample spaces, although it requires framing experimental campaigns through the lens of maximizing some observable. This framing is insufficient for epistemic research goals that seek to comprehensively analyze a sample space, without an explicit scalar objective (e.g., the characterization of a wafer or sample library). In this work, we propose a flexible formulation of scientific value that recasts a dataset of input conditions and higher-dimensional observable data into a continuous, scalar metric. Intuitively, the scientific value function measures where observables change significantly, emulating the perspective of experts driving an experiment, and can be used in collaborative analysis tools or as an objective for optimization techniques. We demonstrate this technique by exploring simulated phase boundaries from different observables, autonomously driving a variable temperature measurement of a ferroelectric material, and providing feedback from a nanoparticle synthesis campaign. The method is seamlessly compatible with existing optimization tools, can be extended to multi-modal and multi-fidelity experiments, and can integrate existing models of an experimental system. Because of its flexibility, it can be deployed in a range of experimental settings for autonomous or accelerated experiments. △ Less

Submitted 25 July, 2023; originally announced July 2023.

An Open-Source Multi-functional Testing Platform for Optical Phase Change Materials

Authors: Cosmin-Constantin Popescu, Khoi Phuong Dao, Luigi Ranno, Brian Mills, Louis Martin, Yifei Zhang, David Bono. Brian Neltner, Tian Gu, Juejun Hu, Kiumars Aryana, William M. Humphreys, Hyun Jung Kim, Steven Vitale, Paul Miller, Christopher Roberts, Sarah Geiger, Dennis Callahan, Michael Moebius, Myungkoo Kang, Kathleen Richardson, Carlos A. Ríos Ocampo

Abstract: Owing to their unique tunable optical properties, chalcogenide phase change materials are increasingly being investigated for optics and photonics applications. However, in situ characterization of their phase transition characteristics is a capability that remains inaccessible to many researchers. In this article, we introduce a multi-functional silicon microheater platform capable of in situ mea… ▽ More Owing to their unique tunable optical properties, chalcogenide phase change materials are increasingly being investigated for optics and photonics applications. However, in situ characterization of their phase transition characteristics is a capability that remains inaccessible to many researchers. In this article, we introduce a multi-functional silicon microheater platform capable of in situ measurement of structural, kinetic, optical, and thermal properties of these materials. The platform can be fabricated leveraging industry-standard silicon foundry manufacturing processes. We fully open-sourced this platform, including complete hardware design and associated software codes. △ Less

Submitted 12 July, 2023; originally announced July 2023.

Comments: 13 pages main text, 5 figures +1 (supplementary)

Two-dimensional assembly of gold nanoparticles grafted with charged-end-group polymers

Authors: Hyeong Jin Kim, Binay P. Nayak, Honghu Zhang, Benjamin M. Ocko, Alex Travesset, David Vaknin, Surya K. Mallapragada, Wenjie Wang

Abstract: Hypothesis: Introducing charged terminal groups to polymers that graft nanoparticles enables Coulombic control over their assembly by tuning the pH and salinity of aqueous suspensions. Experiments: Gold nanoparticles (AuNPs) are grafted with poly(ethylene glycol) (PEG) terminated with CH3 (charge-neutral), COOH (negatively charged), or NH2 (positively charged) groups. The nanoparticles are charact… ▽ More Hypothesis: Introducing charged terminal groups to polymers that graft nanoparticles enables Coulombic control over their assembly by tuning the pH and salinity of aqueous suspensions. Experiments: Gold nanoparticles (AuNPs) are grafted with poly(ethylene glycol) (PEG) terminated with CH3 (charge-neutral), COOH (negatively charged), or NH2 (positively charged) groups. The nanoparticles are characterized using dynamic light scattering, zeta-potential, and thermal gravimetric analysis. Liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) techniques are employed to determine the density profile and in-plane structure of the AuNP assembly across and on the aqueous surface. Findings: The assembly of PEG-AuNPs at the liquid/vapor interface can be tuned by adjusting pH or salinity, particularly for COOH terminals. However, the effect is less pronounced for NH2 terminals. These distinct assembly behaviors are attributed to the overall charge of PEG-AuNPs and the conformation of PEG. The COOH-PEG corona is the most compact, resulting in smaller superlattice constants. The net charge per particle depends not only on the PEG terminal groups but also on the cation sequestration of PEG and the intrinsic negative charge of the AuNP surface. NH2-PEG, due to its closeness to overall charge neutrality and the presence of hydrogen bonding, enables the assembly of NH2-PEG-AuNPs more readily. △ Less

Submitted 7 December, 2023; v1 submitted 25 May, 2023; originally announced May 2023.

Comments: Published in the Journal of Colloid and Interface Science, DOI - 10.1016/j.jcis.2023.07.095

Journal ref: J. Colloid Interface Sci. Volume 650, Part B (2023); Pages 1941-1948

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)

Microscopic origin of local electric polarization in NiPS$_3$

Authors: Hyeon Jung Kim, Ki-Seok Kim

Abstract: Recently, Zhang-Rice triplet to singlet excitations have been measured experimentally and verified numerically in a van der Waals antiferromagnet NiPS\textsubscript{3}, which reveals a collective local change of an electronic structure. In particular, such numerical simulations predicted that these electronic excitations occur simultaneously with local electric polarizations. In this study, we unc… ▽ More Recently, Zhang-Rice triplet to singlet excitations have been measured experimentally and verified numerically in a van der Waals antiferromagnet NiPS\textsubscript{3}, which reveals a collective local change of an electronic structure. In particular, such numerical simulations predicted that these electronic excitations occur simultaneously with local electric polarizations. In this study, we uncover the microscopic origin of this local electric polarization in the Zhang-Rice triplet to singlet excitation. Our lattice-model calculation predicts that the electric polarization can be controlled by applied magnetic fields, where the atomic spin-orbit coupling plays an important role. We speculate emergence of real space Berry curvature to describe the electric polarization in this strongly correlated system. △ Less

Submitted 3 July, 2023; v1 submitted 16 November, 2022; originally announced November 2022.

Journal ref: New J. Phys. 25, 083029 (2023)

Cryogenic Magneto-Terahertz Scanning Near-field Optical Microscope (cm-SNOM)

Authors: Richard H. J. Kim, Joong-Mok Park, Samuel J. Haeuser, Liang Luo, Jigang Wang

Abstract: We have developed a versatile near-field microscopy platform that can operate at high magnetic fields and below liquid-helium temperatures. We use this platform to demonstrate an extreme terahertz (THz) nanoscope operation and to obtain the first cryogenic magneto-THz time-domain nano-spectroscopy/imaging at temperatures as low as 1.8 K and magnetic fields of up to 5 T simultaneously. Our cryogeni… ▽ More We have developed a versatile near-field microscopy platform that can operate at high magnetic fields and below liquid-helium temperatures. We use this platform to demonstrate an extreme terahertz (THz) nanoscope operation and to obtain the first cryogenic magneto-THz time-domain nano-spectroscopy/imaging at temperatures as low as 1.8 K and magnetic fields of up to 5 T simultaneously. Our cryogenic magneto-THz scanning near-field optical microscopy, or cm-SNOM, instrument comprises three main equipment: i) a 5 T split pair magnetic cryostat with a custom made insert for mounting SNOM inside; ii) an atomic force microscope (AFM) unit that accepts ultrafast THz excitation and iii) a MHz repetition rate, femtosecond laser amplifier for high-field THz pulse generation and sensitive detection. We apply the cm-SNOM to obtain proof of principle measurements of superconducting and topological materials. The new capabilities demonstrated break grounds for studying quantum materials that requires extreme environment of cryogenic operation and applied magnetic fields simultaneously in nanometer space, femtosecond time, and terahertz energy scales. △ Less

Submitted 13 October, 2022; originally announced October 2022.

Journal ref: Review of Scientific Instruments, 94, 043702 (2023)

Single crystal growth of iridates without platinum impurities

Authors: Jimin Kim, Hoon Kim, Hyun-Woo J. Kim, Sunwook Park, Jin-Kwang Kim, Junyoung Kwon, Jungho Kim, Hyeong Woo Seo, Jun Sung Kim, B. J. Kim

Abstract: Iridates have attracted much interest in the last decade for their novel magnetism emerging in the limit of strong spin-orbit coupling and possible unconventional superconductivity. A standard for growing iridate single crystals has been the flux method using platinum crucibles. Here, we show that this widely used method compromises the sample quality by inclusion of platinum impurities. We find t… ▽ More Iridates have attracted much interest in the last decade for their novel magnetism emerging in the limit of strong spin-orbit coupling and possible unconventional superconductivity. A standard for growing iridate single crystals has been the flux method using platinum crucibles. Here, we show that this widely used method compromises the sample quality by inclusion of platinum impurities. We find that Sr2IrO4 single crystals grown in iridium crucibles show remarkable differences from those grown in platinum crucibles in their sample characterizations using Raman spectroscopy, resistivity, magnetization, optical third harmonic generation, resonant X-ray diffraction, and resonant inelastic X-ray scattering measurements. In particular, we show that several peaks of sizable intensities disappear in the Raman spectra of samples free of platinum impurities, and a significantly larger activation energy is extracted from the resistivity data compared to previously reported values. Furthermore, we find no evidence of the previously reported glide symmetry breaking structural distortions and confirm the I41/acd space group of the lattice symmetry. Although the platinum impurities are not apparent in the magnetic properties and thus went unnoticed in the stoichiometric insulating phase for a long time, their effects can be much more detrimental to transport properties in chemically doped compounds. Therefore, our result suggests using growth methods that avoid platinum impurities for an investigation of intrinsic physical properties of iridates, and possible superconducting phases. △ Less

Submitted 27 September, 2022; originally announced September 2022.

Comments: to be published in Physical Review Materials

Visualizing heterogeneous dipole fields by terahertz light coupling in individual nano-junctions used in transmon qubits

Authors: R. H. J. Kim, J. M. Park, S. Haeuser, C. Huang, D. Cheng, T. Koschny, J. Oh, C. Kopas, H. Cansizoglu, K. Yadavalli, J. Mutus, L. Zhou, L. Luo, M. Kramer, J. Wang

Abstract: The fundamental challenge underlying superconducting quantum computing is to characterize heterogeneity and disorder in the underlying quantum circuits. These nonuniform distributions often lead to local electric field concentration, charge scattering, dissipation and ultimately decoherence. It is particularly challenging to probe deep sub-wavelength electric field distribution under electromagnet… ▽ More The fundamental challenge underlying superconducting quantum computing is to characterize heterogeneity and disorder in the underlying quantum circuits. These nonuniform distributions often lead to local electric field concentration, charge scattering, dissipation and ultimately decoherence. It is particularly challenging to probe deep sub-wavelength electric field distribution under electromagnetic wave coupling at individual nano-junctions and correlate them with structural imperfections from interface and boundary, ubiquitous in Josephson junctions (JJ) used in transmon qubits. A major obstacle lies in the fact that conventional microscopy tools are incapable of measuring simultaneous at nanometer and terahertz, "nano-THz" scales, which often associate with frequency-dependent charge scattering in nano-junctions. Here we directly visualize interface nano-dipole near-field distribution of individual Al/AlO$_{x}$/Al junctions used in transmon qubits. Our THz nanoscope images show a remarkable asymmetry across the junction in electromagnetic wave-junction coupling response that manifests as "hot" vs "cold" cusp spatial electrical field structures and correlates with defected boundaries from the multi-angle deposition processes in JJ fabrication inside qubit devices. The asymmetric nano-dipole electric field contrast also correlates with distinguishing, "overshoot" frequency dependence that characterizes the charge scattering and dissipation at nanoscale, hidden in responses from topographic, structural imaging and spatially-averaged techniques. The real space mapping of junction dipole fields and THz charge scattering can be extended to guide qubit nano-fabrication for ultimately optimizing qubit coherence times. △ Less

Submitted 13 July, 2022; originally announced July 2022.

Journal ref: Commun Phys 6, 147 (2023)

Preparation of large Cu3Sn single crystal by Czochralski method

Authors: Minsik Kong, Sang-Eon Park, Hye Jung Kim, Sehwan Song, Dong-Choon Ryu, Baekjune Kang, Changhee Sohn, Hyun Jung Kim, Youngwook Kim, Sangmoon Yoon, Ara Go, Hyoungjeen Jeen, Sungkyun Park, Se-Young Jeong, Chang-Jong Kang, Jong Mok Ok

Abstract: Cu3Sn was recently predicted to host topological Dirac fermions, but related research is still in its infancy. The growth of large and high-quality Cu3Sn single crystals is, therefore, highly desired to investigate the possible topological properties. In this work, we report the single crystal growth of Cu3Sn by Czochralski (CZ) method. Crystal structure, chemical composition, and transport proper… ▽ More Cu3Sn was recently predicted to host topological Dirac fermions, but related research is still in its infancy. The growth of large and high-quality Cu3Sn single crystals is, therefore, highly desired to investigate the possible topological properties. In this work, we report the single crystal growth of Cu3Sn by Czochralski (CZ) method. Crystal structure, chemical composition, and transport properties of Cu3Sn single crystals were analyzed to verify the crystal quality. Notably, compared to the mm-sized crystals from a molten Sn-flux, the cm-sized crystals obtained by the CZ method are free from contamination from flux materials, paving the way for the follow-up works. △ Less

Submitted 14 June, 2022; originally announced June 2022.

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

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

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

Submitted 8 March, 2022; originally announced March 2022.

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

Detection of Weyl Fermions and the Metal to Weyl-Semimetal phase transition in WTe$_2$ via broadband High Resolution NMR

Authors: Wassilios Papawassiliou, José P. Carvalho, Hae Jin Kim, Chang-Yeon Kim, Seung Jo Yoo, Jin Bae Lee, Saeed Alhassan, Savvas Orfanidis, Vassilios Psycharis, Marina Karagianni, Michael Fardis, Nikolaos Panopoulos, Georgios Papavassiliou, Andrew J. Pell

Abstract: Weyl Fermions (WFs) in the type-II Weyl Semimetal (WSM) WTe$_2$ are difficult to resolve experimentally because the Weyl bands disperse in an extremely narrow region of the (E-k) space. Here, by using DFT-assisted high-resolution $^{125}$Te solid-state NMR (ssNMR) in the temperature range $50$K - $700$K, we succeeded in detecting low energy WF excitations and monitor their evolution with temperatu… ▽ More Weyl Fermions (WFs) in the type-II Weyl Semimetal (WSM) WTe$_2$ are difficult to resolve experimentally because the Weyl bands disperse in an extremely narrow region of the (E-k) space. Here, by using DFT-assisted high-resolution $^{125}$Te solid-state NMR (ssNMR) in the temperature range $50$K - $700$K, we succeeded in detecting low energy WF excitations and monitor their evolution with temperature. Remarkably, WFs appear to emerge at T$\sim 120$K; at lower temperatures WTe$_2$ behaves as a metal. This intriguing metal-to-WSM phase transition is shown to be induced by the rapid raise of the Fermi level with temperature, crossing solely the electron and hole pockets in the low-T metallic phase, while crossing the Weyl bands near the nodal points - a prerequisite for the emergence of WFs - only for T$>120$K. △ Less

Submitted 6 December, 2021; v1 submitted 4 October, 2021; originally announced October 2021.

Comments: 23 pages (5 main article, 18 supplementary material), 16 figures (4 main article, 12 supplementary material), 1 table (supplementary material)

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

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

PCM-net: A refractive index database of chalcogenide phase change materials for tunable nanophotonic device modelling

Authors: Hyun Jung Kim, Jung-woo Sohn, Nina Hong, Calum Williams, William Humphreys

Abstract: Recently, chalcogenide glass based phase change materials (PCMs) have shown utility as a tuning material for a range of nanophotonic devices. Owing to their low loss, ultrafast switching speeds and wide waveband operation, PCMs are integrated in an increasing number of next generation tunable components, including integrated photonic switches, metasurface optics and tunable spectral filters. Nonet… ▽ More Recently, chalcogenide glass based phase change materials (PCMs) have shown utility as a tuning material for a range of nanophotonic devices. Owing to their low loss, ultrafast switching speeds and wide waveband operation, PCMs are integrated in an increasing number of next generation tunable components, including integrated photonic switches, metasurface optics and tunable spectral filters. Nonetheless, modelling of PCM-based devices is challenging, both in terms of accurate representation of experimentally-derived material properties in different phase states, and standardization of results across the research community. In this work, we introduce PCMnet, an online database of the complex refractive indices of a variety of chalcogenide glass PCMs (such as GeSbTe), as an accessible and indexed repository for data sharing across the PCM community. Refractive indices (n) and extinction coefficients (k) between amorphous and crystalline states are directly extracted from experimentally-derived data in numerous academic research articles, and collated into the material resource database. Due to the inaccuracies associated with our data collection methods, this data is supplemented with additional computationally-generated data, obtained through WVASE, a commercial ellipsometry analysis software package. To demonstrate the utility of PCMnet, we provide a NASA application-driven device optimization example using the optical properties of PCMs collected with our database. We anticipate the database providing great use to the PCM community and coordinated research efforts enabled by PCMnet will promote the shared repository for the selection of appropriate PCMs for tunable nanophotonic device design for a range of applications. △ Less

Submitted 28 December, 2020; originally announced December 2020.

Comments: 19 pages, 8 figures, 1 table

Light Quantum Control of Persisting Higgs Modes in Iron-Based Superconductors

Authors: C. Vaswani, J. H. Kang, M. Mootz, L. Luo, X. Yang, C. Sundahl, D. Cheng, C. Huang, R. H. J. Kim, Z. Liu, Y. G. Collantes, E. E. Hellstrom, I. E. Perakis, C. B. Eom, J. Wang

Abstract: The Higgs mechanism, i.e., spontaneous symmetry breaking of the quantum vacuum, is a cross-disciplinary principle, universal for understanding dark energy, antimatter and quantum materials, from superconductivity to magnetism. Yet, Higgs modes in one-band superconductors (SCs) are currently under debate due to their competition with charge-density fluctuations. A distinct Higgs mode, controllable… ▽ More The Higgs mechanism, i.e., spontaneous symmetry breaking of the quantum vacuum, is a cross-disciplinary principle, universal for understanding dark energy, antimatter and quantum materials, from superconductivity to magnetism. Yet, Higgs modes in one-band superconductors (SCs) are currently under debate due to their competition with charge-density fluctuations. A distinct Higgs mode, controllable by terahertz (THz) laser pulses, can arise in multi-band, unconventional SCs via strong {\em interband} Coulomb interaction, but is yet to be accessed. Here we both discover and demonstrate quantum control of such collective mode in iron-based high-temperature superconductors. Using two-pulse, phase coherent THz spectroscopy, we observe a tunable and coherent 2$Δ_{\mathrm{SC}}$ amplitude oscillation of the complex order parameter in such SC with coupled lower and upper bands. The nonlinear dependence of the amplitude mode oscillations on the THz driving fields is distinct from any one-band and conventional SC results: we observe a large nonlinear change of resonance strength, yet with a persisting mode frequency. We argue that this result provides compelling evidence for a transient coupling between the electron and hole amplitude modes via strong interband coherent interaction. To support this scenario, we perform quantum kinetic modeling of a hybrid Higgs mechanism without invoking extra disorder or phonons. In addition to distinguishing between collective modes and charge fluctuations, the light quantum control of multiband SCs can be extended to probe and manipulate many-body entanglement and hidden symmetries in different quantum materials. △ Less

Submitted 29 November, 2020; v1 submitted 25 November, 2020; originally announced November 2020.

Journal ref: Nat Commun 12, 258 (2021)

Spin and mass currents near a moving magnetic obstacle in a two-component Bose-Einstein condensate

Authors: Jong Heum Jung, Hyung Jin Kim, Y. Shin

Abstract: We study the spatial distributions of the spin and mass currents generated by a moving Gaussian magnetic obstacle in a symmetric, two-component Bose-Einstein condensate in two dimensions. We analytically describe the current distributions for a slow obstacle and show that the spin and the mass currents exhibit characteristic spatial structures resembling those of electromagnetic fields around dipo… ▽ More We study the spatial distributions of the spin and mass currents generated by a moving Gaussian magnetic obstacle in a symmetric, two-component Bose-Einstein condensate in two dimensions. We analytically describe the current distributions for a slow obstacle and show that the spin and the mass currents exhibit characteristic spatial structures resembling those of electromagnetic fields around dipole moments. When the obstacle's velocity increases, we numerically observe that the flow pattern maintains its overall structure while the spin polarization induced by the obstacle is enhanced with an increased spin current. We investigate the critical velocity of the magnetic obstacle based on the local criterion of Landau energetic instability and find that it decreases almost linearly as the magnitude of the obstacle's potential increases, which can be directly tested in current experiments. △ Less

Submitted 30 October, 2020; originally announced October 2020.

Comments: 8 pages, 5 figures

Journal ref: J. Korean Phys. Soc. 78, 19 (2021)

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)

Tunable mid-wave infrared Fabry-Perot bandpass filters using phase-change GeSbTe

Authors: Calum Williams, Nina Hong, Matthew Julian, Stephen Borg, Hyun Jung Kim

Abstract: We demonstrate spectrally-tunable Fabry-Perot bandpass filters operating across the MWIR by utilizing the phase-change material GeSbTe (GST) as a tunable cavity medium between two (Ge:Si) distributed Bragg reflectors. The induced refractive index modulation of GST increases the cavity's optical path length, red-shifting the passband. Our filters have spectral-tunability of ~300 nm, transmission ef… ▽ More We demonstrate spectrally-tunable Fabry-Perot bandpass filters operating across the MWIR by utilizing the phase-change material GeSbTe (GST) as a tunable cavity medium between two (Ge:Si) distributed Bragg reflectors. The induced refractive index modulation of GST increases the cavity's optical path length, red-shifting the passband. Our filters have spectral-tunability of ~300 nm, transmission efficiencies of 60-75% and narrowband FWHMs of 50-65 nm (Q-factor ~70-90). We further show multispectral thermal imaging and gas sensing. By matching the filter's initial passband to a CO$_{2}$ vibrational-absorption mode (~4.25 $μ$m), tunable atmospheric CO$_{2}$ sensing and dynamic plume visualization of added CO$_{2}$ is realized. △ Less

Submitted 14 February, 2020; originally announced February 2020.

Comments: 13 pages, 5 figures

Journal ref: Optics Express 28 (7), 10583-10594 (2020)

All-optical continuous tuning of phase-change plasmonic metasurfaces for multispectral thermal imaging

Authors: Matthew N. Julian, Calum Williams, Stephen Borg, Scott Bartram, Hyun Jung Kim

Abstract: Actively tunable, narrowband spectral filtering across arbitrary optical wavebands is highly desirable in a plethora of applications, from chemical sensing, hyperspectral imaging to infrared astronomy. Yet, the ability to actively reconfigure the optical properties of a solid-state narrowband filter remains elusive. Existing solutions require either moving parts, have slow response times or provid… ▽ More Actively tunable, narrowband spectral filtering across arbitrary optical wavebands is highly desirable in a plethora of applications, from chemical sensing, hyperspectral imaging to infrared astronomy. Yet, the ability to actively reconfigure the optical properties of a solid-state narrowband filter remains elusive. Existing solutions require either moving parts, have slow response times or provide limited spectral coverage. Here, we demonstrate a continuously tunable, spectrally-agnostic, all-solid-state, narrowband phase-change metasurface filter based on a GeSbTe (GST)-embedded plasmonic nanohole array. The passband of the presented tunable filter is ~74 nm with ~70% transmittance and operates across 3 - 5 $μ$m; the thermal imaging waveband. Continuous, reconfigurable tuning is achieved by exploiting intermediate GST phases via optical switching with a single nanosecond laser pulse and material stability is verified through multiple switching cycles. We further demonstrate multispectral thermal imaging in the mid-wave infrared using our phase-change metasurfaces. Our results pave the way for highly functional, reduced power, compact hyperspectral imaging systems and optical filters. △ Less

Submitted 17 December, 2019; originally announced December 2019.

Comments: Main text: 19 pages, 5 Figures, Supplementary: 10 pages, 9 Figures

Journal ref: Optica 7 (7), 746-754 (2020)

Light-Driven Raman Coherence as a Non-Thermal Route to Ultrafast Topology Switching

Authors: C. Vaswani, L. -L. Wang, D. H. Mudiyanselage, Q. Li, P. M. Lozano, G. Gu, D. Cheng, B. Song, L. Luo, R. H. J. Kim, C. Huang, Z. Liu, M. Mootz, I. E. Perakis, Y. Yao, K. M. Ho, J. Wang

Abstract: A grand challenge underlies the entire field of topology-enabled quantum logic and information science: how to establish topological control principles driven by quantum coherence and understand the time-dependence of such periodic driving? Here we demonstrate a THz pulse-induced phase transition in Dirac materials that is periodically driven by vibrational coherence due to excitation of the lowes… ▽ More A grand challenge underlies the entire field of topology-enabled quantum logic and information science: how to establish topological control principles driven by quantum coherence and understand the time-dependence of such periodic driving? Here we demonstrate a THz pulse-induced phase transition in Dirac materials that is periodically driven by vibrational coherence due to excitation of the lowest Raman-active mode. Above a critical field threshold, there emerges a long-lived metastable phase with unique Raman coherent phonon-assisted switching dynamics, absent for optical pumping. The switching also manifest itself by non-thermal spectral shape, relaxation slowing down near the Lifshitz transition where the critical Dirac point (DP) occurs, and diminishing signals at the same temperature that the Berry curvature induced Anomalous Hall Effect varnishes. These results, together with first-principles modeling, identify a mode-selective Raman coupling that drives the system from strong to weak topological insulators, STI to WTI, with a Dirac semimetal phase established at a critical atomic displacement controlled by the phonon pumping. Harnessing of vibrational coherence can be extended to steer symmetry-breaking transitions, i.e., Dirac to Weyl ones, with implications on THz topological quantum gate and error correction applications. △ Less

Submitted 4 December, 2019; originally announced December 2019.

Journal ref: Phys. Rev. X 10, 021013 (2020)

Discovery of Terahertz Second Harmonic Generation from Lightwave Acceleration of Symmetry--Breaking Nonlinear Supercurrents

Authors: C. Vaswani, C. Sundahl, M. Mootz, D. H. Mudiyanselage, J. H. Kang, X. Yang, D. Cheng, C. Huang, R. H. J. Kim, Z. Liu, L. Luo, I. E. Perakis, C. B. Eom, J. Wang

Abstract: We report terahertz (THz) second harmonic generation (SHG) in superconductors (SC) with inversion symmetric equilibrium states that forbid even-order nonlinearities. Such SHG signal is observed in single-pulse emission by periodic driving with a multi-cycle THz electric field tuned below the SC energy gap and vanishes above the SC critical temperature. We explain the microscopic physics by a dynam… ▽ More We report terahertz (THz) second harmonic generation (SHG) in superconductors (SC) with inversion symmetric equilibrium states that forbid even-order nonlinearities. Such SHG signal is observed in single-pulse emission by periodic driving with a multi-cycle THz electric field tuned below the SC energy gap and vanishes above the SC critical temperature. We explain the microscopic physics by a dynamical symmetry breaking principle at sub-THz-cycle by using quantum kinetic modeling of the interplay between strong THz-lightwave nonlinearity and pulse propagation. The resulting non-zero integrated pulse area inside the SC drives lightwave nonlinear supercurrents due to sub--cycle Cooper pair acceleration, in contrast to d.c.-biased superconductors, which can be controlled by the bandstructure and the THz pump field. △ Less

Submitted 3 December, 2019; originally announced December 2019.

Journal ref: Phys. Rev. Lett. 124, 207003 (2020)

Direct observation of Dirac states in Bi2Te3 nanoplatelets by 125Te NMR

Authors: Wassilios Papawassiliou, Aleksander Jaworski, Andrew J. Pell, Jae Hyuck Jang, Yeonho Kim, Sang-Chul Lee, Hae Jin Kim, Yasser Alwahedi, Saeed Alhassan, Ahmed Subrati, Michael Fardis, Marina Karagianni, Nikolaos Panopoulos, Janez Dolinsek, Georgios Papavassiliou

Abstract: Detection of the metallic Dirac electronic states on the surface of Topological Insulators (TIs) is a tribune for a small number of experimental techniques the most prominent of which is Angle Resolved Photoemission Spectroscopy. However, there is no experimental method showing at atomic scale resolution how the Dirac electrons extend inside TI systems. This is a critical issue in the study of imp… ▽ More Detection of the metallic Dirac electronic states on the surface of Topological Insulators (TIs) is a tribune for a small number of experimental techniques the most prominent of which is Angle Resolved Photoemission Spectroscopy. However, there is no experimental method showing at atomic scale resolution how the Dirac electrons extend inside TI systems. This is a critical issue in the study of important surface quantum properties, especially topological quasiparticle excitations. Herein, by applying advanced DFT-assisted solid-state 125Te Nuclear Magnetic Resonance on Bi2Te3 nanoplatelets, we succeeded in uncovering the hitherto invisible NMR signals with magnetic shielding influenced by the Dirac electrons, and subsequently showed how Dirac electrons spread and interact with the bulk interior of the nanoplatelets. △ Less

Submitted 25 September, 2019; originally announced September 2019.

Comments: Includes main manuscript (9 pages, 4 figures) and supplementary material (17 pages, 11 figures)