Showing 1–50 of 95 results for author: Kang, M

Oxygen Vacancies at Dislocation Core Modulate Plasticity in Strontium Titanate

Authors: Min-Chul Kang, Chunxu Yan, Alexander Frisch, Xufei Fang, Liming Xiong, Lin Zhou

Abstract: Dislocation core chemistry in oxides critically influences mechanical behavior and functionality; yet the evolution of core chemistry during the dislocation motion in them has not been directly observed. Here, using SrTiO3 as a model material, we combine aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy with atomic-level molecular dynamics (MD) si… ▽ More Dislocation core chemistry in oxides critically influences mechanical behavior and functionality; yet the evolution of core chemistry during the dislocation motion in them has not been directly observed. Here, using SrTiO3 as a model material, we combine aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy with atomic-level molecular dynamics (MD) simulations to correlate the <110>{1-10} dislocation core structure, oxygen vacancy density, charge state, and mobility with each other. We find that the mechanically induced dislocation loops exhibit dissociated cores, whose oxygen vacancy density depends on the gliding distance: short loops are Ti-reduced and oxygen-deficient at the edge dislocation core, whereas longer loops remain close to stoichiometry in both the edge and screw components. MD simulations reveal that kink-assisted edge dislocation glide in SrTiO3 leaves oxygen-deficient trails behind, modulating the oxygen content inside the edge core. These results demonstrate that oxygen-vacancy evolution at the dislocation core intrinsically couples with plasticity in ionic crystals, suggesting a mechanism for oxygen vacancy-dependent dislocation mobility in plastically deformed oxides. △ Less

Submitted 1 May, 2026; originally announced May 2026.

Journal ref: Acta Materialia, 2026, 313: 122283

Giant Hole-doping in 2H-WSe2 via Ta Substitution

Authors: Minhee Kang, Woojin Choi, Choongyu Hwang, Jinwoong Hwang

Abstract: The family of transition metal dichalcogenides (TMDCs) has been regarded as promising candidates for future electronics, valleytronics, spintronics, and optoelectronics. While most of TMDCs are intrinsic n-type semiconductors due to electron donation from chalcogen vacancies, realizing intrinsic p-type TMDCs and achieving precise control over their electronic properties remain challenging. In this… ▽ More The family of transition metal dichalcogenides (TMDCs) has been regarded as promising candidates for future electronics, valleytronics, spintronics, and optoelectronics. While most of TMDCs are intrinsic n-type semiconductors due to electron donation from chalcogen vacancies, realizing intrinsic p-type TMDCs and achieving precise control over their electronic properties remain challenging. In this work, we introduce a powerful approach to obtain intrinsic hole doping by substituting Ta atom into 2H-WSe2. A combining study of molecular beam epitaxy growth and in-situ angle-resolved photoemission spectroscopy characterization clearly reveals that Ta substitution induces a significant hole doping and provides a possible way of a semiconductor-to-metal transition in 2H-WSe2. △ Less

Submitted 13 January, 2026; originally announced January 2026.

Journal ref: Applied Science and Convergence Technology, 34 (6); 194-197 (2025)

Optical Signatures and Quantum Geometry in Proximity-Induced Topological Superconductors

Authors: Myungjun Kang, Yogeshwar Prasad, Nikhil Danny Babu, Rasoul Ghadimi, Jae Hoon Kim, Sangmo Cheon

Abstract: Topological-insulator-superconductor (TI-SC) heterostructures provide a promising platform for proximity-induced topological superconductivity, but diagnosing superconductivity at a buried interface remains challenging for conventional surface-sensitive probes. Here, we develop a quantitative theory of the longitudinal optical response of a TI-SC heterostructure and show that the complex sheet con… ▽ More Topological-insulator-superconductor (TI-SC) heterostructures provide a promising platform for proximity-induced topological superconductivity, but diagnosing superconductivity at a buried interface remains challenging for conventional surface-sensitive probes. Here, we develop a quantitative theory of the longitudinal optical response of a TI-SC heterostructure and show that the complex sheet conductance provides an interface-selective route to isolating and diagnosing the buried proximitized interface state. Starting from a minimal model, we derive a low-energy description of the heterointerface in which the induced gap emerges directly from the TI-SC coupling. Combined with a slab-based thickness-extrapolation procedure, this framework yields a practical protocol for separating the buried interfacial sheet conductance from bulk and exposed-surface optical contributions. The extracted interface response exhibits a robust, thickness-independent coherence peak at an energy set by the proximity-induced gap, clearly distinguishable from both the pair-breaking scale of the parent superconductor and the Dirac cone on the exposed TI surface. At low energies, the heterointerface is described by an effective time-reversal-invariant topological-superconducting theory, while the associated low-frequency optical spectral weight admits a quantum-geometric interpretation through the optical sum rule. Our results establish terahertz/infrared spectroscopy of thickness-extracted sheet conductance as a noninvasive route to identifying and quantifying proximity-induced superconductivity at buried TI-SC interfaces. △ Less

Submitted 13 August, 2026; v1 submitted 8 January, 2026; originally announced January 2026.

Higher-Order Topological Systems and Their Sub-Symmetry-Protected Topology

Authors: Myungjun Kang, Wonjun Sung, Sonu Verma, Sangmo Cheon

Abstract: Symmetry and topology are essential principles in topological physics. Recently, the idea of sub-symmetry-protected topology -- where some of the original symmetries are broken while a remaining subset, called sub-symmetries, continues to protect specific boundary states -- has been developed. Here, we extend sub-symmetry-protected topology to higher-order topological systems from second-order top… ▽ More Symmetry and topology are essential principles in topological physics. Recently, the idea of sub-symmetry-protected topology -- where some of the original symmetries are broken while a remaining subset, called sub-symmetries, continues to protect specific boundary states -- has been developed. Here, we extend sub-symmetry-protected topology to higher-order topological systems from second-order topological insulators to semimetals. By introducing a sub-symmetry-protecting perturbation that acts on a single sublattice and selectively preserves specific topological boundary states, we track the evolution of these states and their topological features using numerical and analytical methods, and we show that state-resolved quadrupole moments diagnose which corner or hinge modes remain topological. As a representative example of a second-order topological insulator, we begin with the Benalcazar-Bernevig-Hughes model. We demonstrate that, under a sub-symmetry-protecting perturbation, sub-symmetry-protected corner states remain pinned at zero energy and maintain quantized state-resolved quadrupole moments. In contrast, corner states on sub-symmetry-broken boundaries shift away from zero energy and lose their quantized character. We further extend this framework to a three-dimensional second-order topological semimetal, constructed by stacking second-order topological insulator layers, and analyze how second-order Fermi arc states -- hinge-localized modes that link the projections of bulk Dirac points, in contrast to conventional surface Fermi arcs -- evolve under a sub-symmetry-protecting perturbation. While one second-order Fermi arc becomes dispersive and loses its quadrupolar character under a sub-symmetry-breaking perturbation, the remaining second-order Fermi arcs retain chiral symmetry and preserve quantized quadrupolar characters. △ Less

Submitted 19 December, 2025; originally announced December 2025.

Role of Phase Fluctuation in Dynamic Competition Between Charge Order and Superconductivity in Cuprates

Authors: Mingu Kang, Pavel E. Dolgirev, Chao C. Zhang, Hoyoung Jang, Byungjune Lee, Minseok Kim, Sang-Youn Park, Ronny Sutarto, Eugene Demler, Jae-Hoon Park, John Y. T. Wei, Riccardo Comin

Abstract: Phase fluctuations are a key factor distinguishing nonthermal (ultrafast) and thermal phase transitions. Charge order in cuprates is characterized by short-range coherence while competing with superconductivity, and as such, it provides a representative case to study the role of phase fluctuation in coupled order parameter dynamics. In this work, we investigated the intertwined evolution of charge… ▽ More Phase fluctuations are a key factor distinguishing nonthermal (ultrafast) and thermal phase transitions. Charge order in cuprates is characterized by short-range coherence while competing with superconductivity, and as such, it provides a representative case to study the role of phase fluctuation in coupled order parameter dynamics. In this work, we investigated the intertwined evolution of charge order and superconductivity in cuprate/manganite heterostructures using time-resolved resonant X-ray scattering. The resulting dynamics are analyzed within a space- and time-dependent nonperturbative model capturing both amplitude and phase dynamics. At low fluence, photo-induced suppression of superconductivity results in a nonthermal enhancement of charge order, underscoring the dynamic competition between charge order and superconductivity. With increasing fluence, the slowing down of melting and recovery dynamics is observed, indicating a critical role of phase fluctuations. At high fluence, both charge order and superconductivity remain suppressed for an extended time window due to decoupling between amplitude and phase dynamics and the delayed recovery of phase coherence. Our work underscores the importance of phase fluctuation for understanding the dynamic competition between order parameters in cuprates. △ Less

Submitted 30 October, 2025; originally announced October 2025.

Comments: 19 pages, 7 figures

Ultrathin bismuth-yttrium iron garnet films with tunable magnetic anisotropy

Authors: Hanchen Wang, William Legrand, Davit Petrosyan, Min-Gu Kang, Emir Karadža, Hiroki Matsumoto, Richard Schlitz, Michaela Lammel, Myriam H. Aguirre, Pietro Gambardella

Abstract: We report on the epitaxial growth of nm-thick films of bismuth-substituted yttrium iron garnet (BiYIG) by high-temperature off-axis radio-frequency magnetron sputtering. We demonstrate accurate control of the magnetic properties by tuning of the sputtering parameters and epitaxial strain on various (111)-oriented garnet substrates. BiYIG films with up to -0.80\% lattice mismatch with the substrate… ▽ More We report on the epitaxial growth of nm-thick films of bismuth-substituted yttrium iron garnet (BiYIG) by high-temperature off-axis radio-frequency magnetron sputtering. We demonstrate accurate control of the magnetic properties by tuning of the sputtering parameters and epitaxial strain on various (111)-oriented garnet substrates. BiYIG films with up to -0.80\% lattice mismatch with the substrate remain fully strained up to 60~nm-thick, maintaining a high crystalline quality. Transmission electron microscopy and energy-dispersive X-ray spectroscopy confirm coherent epitaxial growth, the absence of defects, and limited interdiffusion at the BiYIG/substrate interface. Varying the tensile or compressive strain between -0.80\% and +0.56\% in BiYIG allows for accurate compensation of the total magnetic anisotropy through magneto-elastic coupling. The effective magnetic anisotropy of sputtered BiYIG films can be further tuned via the off-axis deposition angle and the oxygen flow during growth, which determine the cation stoichiometry. Under optimized growth conditions, a ferromagnetic resonance (FMR) linewidth of 1~mT at 10~GHz is reliably obtained even for thicknesses as low as 10~nm. We also report small FMR linewidths in ultrathin (2-5~nm) BiYIG films grown on diamagnetic substrate yttrium scandium gallium garnet. These findings highlight the promise of low-damping, strain-engineered nm-thick BiYIG films for implementing advanced functionalities in spin-orbitronic and magnonic devices. Specifically, the magnetic-anisotropy compensation and low damping enable large cone-angle magnetization dynamics immune to magnon-magnon nonlinear scattering. △ Less

Submitted 13 January, 2026; v1 submitted 8 October, 2025; originally announced October 2025.

Journal ref: Phys. Rev. Materials 10, 034404 (2026)

Effect of Magnetic Anisotropy on Magnetoelastic Waves in Ni/LiNbO3 Hybrid Device

Authors: Minwoo Yu, Moojune Song, Minseok Kang, Mujin You, Yunyoung Hwang, Albert Min Gyu Park, Byong-Guk Park, Kab-Jin Kim, Junho Suh

Abstract: We study the effects of magnetic anisotropy and crystalline axes in surface acoustic waves (SAWs) driven magnetic resonances of Ni/LiNbO3 hybrid devices. SAW absorption from the interaction with magnons in Ni displays a strong anisotropic dependence on the direction of the applied in-plane magnetic field. Magnetic anisotropy is further investigated by magneto-optical Kerr effect measurements to sh… ▽ More We study the effects of magnetic anisotropy and crystalline axes in surface acoustic waves (SAWs) driven magnetic resonances of Ni/LiNbO3 hybrid devices. SAW absorption from the interaction with magnons in Ni displays a strong anisotropic dependence on the direction of the applied in-plane magnetic field. Magnetic anisotropy is further investigated by magneto-optical Kerr effect measurements to show both uniaxial and biaxial anisotropy components in Ni films on LiNbO3. By introducing a dipolar interaction term in addition to the anisotropies, we successfully explain the anisotropic SAW absorption in our devices. These findings show the importance of substrate-induced anisotropy and long-range dipolar effects in SAW-magnons hybrid devices and indicate future directions for optimizing these spin-acoustic devices through comprehensive anisotropy engineering. △ Less

Submitted 3 September, 2025; originally announced September 2025.

Comments: 20 pages, 6 figures, 1 table

Aligning Reasoning LLMs for Materials Discovery with Physics-aware Rejection Sampling

Authors: Lee Hyun, Sohee Yoon, Jinwoo Park, Sue In Chae, Seongeon Park, Jooyeon Ahn, Yebin Jung, Youjung Chung, Hogeun Chang, Sujin Park, Myeonginn Kang, Jina Kim, Ho-Gyeong Kim, Myeonghun Jeong

Abstract: AI-driven materials discovery that couples automated experimentation with algorithmic decision-making requires process aware recipe to property predictors that are accurate, calibrated, and physically admissible. We approach this as a reasoning problem with large reasoning models (LRMs). To instill reasoning capability into language models, we curate reasoning traces from a teacher model to train… ▽ More AI-driven materials discovery that couples automated experimentation with algorithmic decision-making requires process aware recipe to property predictors that are accurate, calibrated, and physically admissible. We approach this as a reasoning problem with large reasoning models (LRMs). To instill reasoning capability into language models, we curate reasoning traces from a teacher model to train a student model. However, most training pipelines select reasoning traces using binary correctness or learned preference signals that poorly reflect physical admissibility. We introduce Physics-aware Rejection Sampling (PaRS), a training-time trace selection scheme that favors traces consistent with fundamental physics and numerically close to targets, with lightweight halting to control compute. We instantiate our framework with a large student model fine-tuned on traces synthesized by a larger teacher model, and evaluate under matched token budgets against various rejection sampling baselines. Our method improves accuracy and calibration, reduces physics-violation rates, and lowers sampling cost relative to baselines. These results indicate that modest, domain-aware constraints combined with trace-level selection provide a practical path toward reliable, efficient LRMs for process-aware property prediction and closed-loop materials design. △ Less

Submitted 2 October, 2025; v1 submitted 31 August, 2025; originally announced September 2025.

Comments: 16 pages, 6 figures

Observation of gapless collective charge fluctuations in an Anderson insulating state

Authors: Jong Mok Ok, Beom Jun Park, Junik Hwang, Seonghoon Park, Myeongjun Kang, Jun Sung Kim, Ki-Seok Kim, Seung-Ho Baek

Abstract: Understanding the nature of collective charge dynamics in the Coulomb gap phase is essential for revealing the existence of many-body localization. However, the corresponding many-particle excitation spectra remain poorly understood. Here, we present a comprehensive investigation of $^{27}$Al and $^{63}$Cu nuclear magnetic/quadrupole resonance (NMR/NQR), along with specific heat ($C_p$) measuremen… ▽ More Understanding the nature of collective charge dynamics in the Coulomb gap phase is essential for revealing the existence of many-body localization. However, the corresponding many-particle excitation spectra remain poorly understood. Here, we present a comprehensive investigation of $^{27}$Al and $^{63}$Cu nuclear magnetic/quadrupole resonance (NMR/NQR), along with specific heat ($C_p$) measurements, in the $p$-type semiconductor CuAlO$_2$. Our study unveils distinct changes in charge dynamics at two crossover temperature scales which separate three regimes associated with Anderson localization of charge carriers: thermally activated transport ($T>150$ K) $\rightarrow$ Mott variable-range hopping (VRH) $\rightarrow$ Efros-Shklovskii (ES) VRH with Coulomb gap formation ($T<50$ K). In the ES VRH regime, we observe a striking divergence in the zero-field $^{63}$Cu spin-lattice relaxation rate, $(T_1T)^{-1}$, which is strongly suppressed by an applied magnetic field, indicative of quantum critical charge fluctuations. This is further supported by a distinct magnetic field-dependence of $C_p/T$ deep within the Coulomb gap phase. Taken together, these results provide compelling evidence for the emergence of strong, gapless collective charge fluctuations within the Anderson insulating phase where single-particle excitations are gapped. △ Less

Submitted 10 August, 2025; originally announced August 2025.

Comments: 16 pages, 5 figures

Journal ref: Scientific Reports 16, 2881 (2026)

Non-Gaussian Phase Transition and Cascade of Instabilities in the Dissipative Quantum Rabi Model

Authors: Mingyu Kang, Yikang Zhang, Kenneth R. Brown, Thomas Barthel

Abstract: The open quantum Rabi model describes a two-level system coupled to a harmonic oscillator. A Gaussian phase transition for the nonequilibrium steady states has been predicted when the bosonic mode is soft and subject to damping. We show that oscillator dephasing is a relevant perturbation, which leads to a non-Gaussian phase transition and an intriguing cascade of instabilities for $k$-th order bo… ▽ More The open quantum Rabi model describes a two-level system coupled to a harmonic oscillator. A Gaussian phase transition for the nonequilibrium steady states has been predicted when the bosonic mode is soft and subject to damping. We show that oscillator dephasing is a relevant perturbation, which leads to a non-Gaussian phase transition and an intriguing cascade of instabilities for $k$-th order bosonic operators, as well as a jump in the steady-state qubit polarization. For the soft-mode limit, the equations of motion form a closed hierarchy and spectral properties can be efficiently studied. To this purpose, we establish a fruitful connection to non-Hermitian Hamiltonians. The results for the phase diagram, stability boundaries, and relevant observables are based on mean-field analysis, exact diagonalization, perturbation theory, and Keldysh field theory. △ Less

Submitted 12 June, 2026; v1 submitted 9 July, 2025; originally announced July 2025.

Comments: 5 pages main text, 13 pages appendix, 8 figures; additional appendices on interpretation in terms of Wigner function dynamics, Fock distributions, and convergence of data in the number of retained oscillator levels; minor improvements; published version

Journal ref: Phys. Rev. A 113, L061703 (2026)

Residual Connection-Enhanced ConvLSTM for Lithium Dendrite Growth Prediction

Authors: Hosung Lee, Byeongoh Hwang, Dasan Kim, Myungjoo Kang

Abstract: The growth of lithium dendrites significantly impacts the performance and safety of rechargeable batteries, leading to short circuits and capacity degradation. This study proposes a Residual Connection-Enhanced ConvLSTM model to predict dendrite growth patterns with improved accuracy and computational efficiency. By integrating residual connections into ConvLSTM, the model mitigates the vanishing… ▽ More The growth of lithium dendrites significantly impacts the performance and safety of rechargeable batteries, leading to short circuits and capacity degradation. This study proposes a Residual Connection-Enhanced ConvLSTM model to predict dendrite growth patterns with improved accuracy and computational efficiency. By integrating residual connections into ConvLSTM, the model mitigates the vanishing gradient problem, enhances feature retention across layers, and effectively captures both localized dendrite growth dynamics and macroscopic battery behavior. The dataset was generated using a phase-field model, simulating dendrite evolution under varying conditions. Experimental results show that the proposed model achieves up to 7% higher accuracy and significantly reduces mean squared error (MSE) compared to conventional ConvLSTM across different voltage conditions (0.1V, 0.3V, 0.5V). This highlights the effectiveness of residual connections in deep spatiotemporal networks for electrochemical system modeling. The proposed approach offers a robust tool for battery diagnostics, potentially aiding in real-time monitoring and optimization of lithium battery performance. Future research can extend this framework to other battery chemistries and integrate it with real-world experimental data for further validation △ Less

Submitted 21 June, 2025; originally announced June 2025.

Comments: 14pages, 6figures, accepted to Journal of The Electrochemical Society

Time-domain decoding of unconventional charge order mechanisms in nonmagnetic and magnetic kagome metals

Authors: Seongyong Lee, Byungjune Lee, Hoyoung Jang, Xueliang Wu, Jimin Kim, Gyeongbo Kang, Choongjae Won, Hyeongi Choi, Sang-Youn Park, Kyle M. Shen, Federico Cilento, Aifeng Wang, Jae-Hoon Park, Mingu Kang

Abstract: In kagome lattice materials, quantum interplay between charge, spin, orbital, and lattice degrees of freedom gives rise to a remarkably rich set of emergent phenomena, ranging from unconventional charge order and superconductivity to topological magnetism. While the exact nature of these exotic orders is often challenging to comprehend in static experiments, time-resolved techniques can offer crit… ▽ More In kagome lattice materials, quantum interplay between charge, spin, orbital, and lattice degrees of freedom gives rise to a remarkably rich set of emergent phenomena, ranging from unconventional charge order and superconductivity to topological magnetism. While the exact nature of these exotic orders is often challenging to comprehend in static experiments, time-resolved techniques can offer critical insights by disentangling coupled degrees of freedom on the time-axis. In this work, we demonstrate that the nature of charge orders in two representative kagome metals - nonmagnetic ScV6Sn6 and magnetic FeGe - which has been highly controversial in static studies, can be directly deciphered in the time-domain through their fundamentally distinct order parameter dynamics measured via time-resolved X-ray scattering at an X-ray free electron laser. In nonmagnetic ScV6Sn6, the dynamics are characterized by ultrafast melting and coherent amplitudon oscillations, typical of a phonon-coupled charge order. In stark contrast, magnetic FeGe exhibits resilient metastable charge order dynamics, hitherto unobserved in any other charge-ordered system - this unique time-domain behavior directly signifies an unconventional magnetism-interlocked charge order state realized in this kagome magnet. Our results not only provide a model case where unconventional nature of electronic order, hidden in equilibrium, is directly unraveled in the time-domain, but also pave the way for future out-of-equilibrium engineering of novel quantum orders in kagome lattice platforms. △ Less

Submitted 17 June, 2025; originally announced June 2025.

Comments: 4 figures

Orbital Pumping in Ferrimagnetic Insulators

Authors: Hanchen Wang, Min-Gu Kang, Davit Petrosyan, Shilei Ding, Richard Schlitz, Lauren J. Riddiford, William Legrand, Pietro Gambardella

Abstract: We report the detection of pure orbital currents generated by both coherent and thermal magnons in the magnetic insulator Bi-doped yttrium iron garnet (BiYIG). The pumping of orbital and spin currents is jointly investigated in nano-devices made of naturally oxidized Cu, pure Cu, Pt, and Cr. The absence of charge conduction in BiYIG and the negligible spin-to-charge conversion of oxidized Cu allow… ▽ More We report the detection of pure orbital currents generated by both coherent and thermal magnons in the magnetic insulator Bi-doped yttrium iron garnet (BiYIG). The pumping of orbital and spin currents is jointly investigated in nano-devices made of naturally oxidized Cu, pure Cu, Pt, and Cr. The absence of charge conduction in BiYIG and the negligible spin-to-charge conversion of oxidized Cu allows us to disambiguate the orbital current contribution. Comparative measurements on YIG and BiYIG show that the origin of the orbital pumping in BiYIG/oxidized Cu is the dynamics of the orbital magnetization in the magnetic insulator. In Cr, the pumping signal is dominated by the negative spin Hall effect rather than the positive orbital Hall effect, indicating that orbital currents represent a minority of the total angular momentum current pumped from the magnetic insulator. Our results also evidence that improving the interfacial transparency significantly enhances pumping efficiencies not only for spin, but also for orbital currents. △ Less

Submitted 13 June, 2025; originally announced June 2025.

Journal ref: Physical Review Letters 134, 126701 (2025)

Phonon fluctuation diagnostics: Origin of charge order in AV$_3$Sb$_5$ kagome metals

Authors: Stefan Enzner, Jan Berges, Arne Schobert, Dongjin Oh, Mingu Kang, Riccardo Comin, Ronny Thomale, Tim O. Wehling, Domenico Di Sante, Giorgio Sangiovanni

Abstract: The microsopic origin of the charge-density wave (CDW) in AV$_3$Sb$_5$ (A = K, Rb, Cs) kagome metals remains a longstanding question, often revolving around electron-phonon coupling and purely electronic mechanisms involving Van Hove scenarios, nesting, and sublattice interference. To reveal the processes driving the CDW transition, we combine ab-initio calculations analysis of the phonon self-ene… ▽ More The microsopic origin of the charge-density wave (CDW) in AV$_3$Sb$_5$ (A = K, Rb, Cs) kagome metals remains a longstanding question, often revolving around electron-phonon coupling and purely electronic mechanisms involving Van Hove scenarios, nesting, and sublattice interference. To reveal the processes driving the CDW transition, we combine ab-initio calculations analysis of the phonon self-energy and angle-resolved photoemission spectroscopy (ARPES). Our momentum-resolved study, supported by ARPES data, reveals that lattice instabilities in the V-135 family of kagome metals appear to also be driven by electronic states far from high-symmetry points, where these states exhibit the strongest coupling with the phonon modes responsible for the CDW distortion. Footing on an interpretation scheme based on phonon fluctuation diagnostics, our work challenges and revises theories that so far have exclusively attributed CDW formation to nesting effects close to the Fermi level. △ Less

Submitted 22 April, 2025; v1 submitted 10 April, 2025; originally announced April 2025.

Comments: 13 pages, 11 figures

Rapid analysis of point-contact Andreev reflection spectra via machine learning with adaptive data augmentation

Authors: Dongik Lee, Valentin Stanev, Xiaohang Zhang, Mijeong Kang, Ichiro Takeuchi, Seunghun Lee

Abstract: Delineating the superconducting order parameters is a pivotal task in investigating superconductivity for probing pairing mechanisms, as well as their symmetry and topology. Point-contact Andreev reflection (PCAR) measurement is a simple yet powerful tool for identifying the order parameters. The PCAR spectra exhibit significant variations depending on the type of the order parameter in a supercon… ▽ More Delineating the superconducting order parameters is a pivotal task in investigating superconductivity for probing pairing mechanisms, as well as their symmetry and topology. Point-contact Andreev reflection (PCAR) measurement is a simple yet powerful tool for identifying the order parameters. The PCAR spectra exhibit significant variations depending on the type of the order parameter in a superconductor, including its magnitude ($\mathitΔ$), as well as temperature, interfacial quality, Fermi velocity mismatch, and other factors. The information on the order parameter can be obtained by finding the combination of these parameters, generating a theoretical spectrum that fits a measured experimental spectrum. However, due to the complexity of the spectra and the high dimensionality of parameters, extracting the fitting parameters is often time-consuming and labor-intensive. In this study, we employ a convolutional neural network (CNN) algorithm to create models for rapid and automated analysis of PCAR spectra of various superconductors with different pairing symmetries (conventional $s$-wave, chiral $p_x+ip_y$-wave, and $d_{x^2-y^2}$-wave). The training datasets are generated based on the Blonder-Tinkham-Klapwijk (BTK) theory and further modified and augmented by selectively incorporating noise and peaks according to the bias voltages. This approach not only replicates the experimental spectra but also brings the model's attention to important features within the spectra. The optimized models provide fitting parameters for experimentally measured spectra in less than 100 ms per spectrum. Our approaches and findings pave the way for rapid and automated spectral analysis which will help accelerate research on superconductors with complex order parameters. △ Less

Submitted 13 March, 2025; originally announced March 2025.

Comments: 18 pages, 3 figures

Measurements of the quantum geometric tensor in solids

Authors: Mingu Kang, Sunje Kim, Yuting Qian, Paul M. Neves, Linda Ye, Junseo Jung, Denny Puntel, Federico Mazzola, Shiang Fang, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Jun Fuji, Ivana Vobornik, Jae-Hoon Park, Joseph G. Checkelsky, Bohm-Jung Yang, Riccardo Comin

Abstract: Understanding the geometric properties of quantum states and their implications in fundamental physical phenomena is at the core of modern physics. The Quantum Geometric Tensor (QGT) is a central physical object in this regard, encoding complete information about the geometry of the quantum state. The imaginary part of the QGT is the well-known Berry curvature, which plays a fundamental role in th… ▽ More Understanding the geometric properties of quantum states and their implications in fundamental physical phenomena is at the core of modern physics. The Quantum Geometric Tensor (QGT) is a central physical object in this regard, encoding complete information about the geometry of the quantum state. The imaginary part of the QGT is the well-known Berry curvature, which plays a fundamental role in the topological magnetoelectric and optoelectronic phenomena. The real part of the QGT is the quantum metric, whose importance has come to prominence very recently, giving rise to a new set of quantum geometric phenomena, such as anomalous Landau levels, flat band superfluidity, excitonic Lamb shifts, and nonlinear Hall effect. Despite the central importance of the QGT, its experimental measurements have been restricted only to artificial two-level systems. In this work, we develop a framework to measure the QGT (both quantum metric and Berry curvature) in crystalline solids using polarization-, spin-, and angle-resolved photoemission spectroscopy. Using this framework, we demonstrate the effective reconstruction of the QGT in solids in the archetype kagome metal CoSn, which hosts topological flat bands. The key idea is to introduce another geometrical tensor, the quasi-QGT, whose components, the band Drude weight and orbital angular momentum, are experimentally accessible and can be used for extracting the QGT. Establishing such a momentum- and energy-resolved spectroscopic probe of the QGT is poised to significantly advance our understanding of quantum geometric responses in a wide range of crystalline systems. △ Less

Submitted 23 December, 2024; originally announced December 2024.

Journal ref: Nat. Phys. (2024)

Colossal optical anisotropy in wide-bandgap semiconductor CuAlO2

Authors: Baekjune Kang, Junhee Shin, Myeongjun Kang, Uksam Choi, Uihyeon Seo, Kunook Chung, Jong Mok Ok, Hosub Jin, Changhee Sohn

Abstract: Colossal optical anisotropy in the entire visible spectrum is crucial for advanced photonic applications, enabling precise light manipulation without optical loss across a broad spectral range. Here, we demonstrate that CuAlO2 exhibits colossal optical anisotropy and transparency across the visible spectrum, enabled by its unique three-dimensional O-Cu-O dumbbell structure and two-dimensionally co… ▽ More Colossal optical anisotropy in the entire visible spectrum is crucial for advanced photonic applications, enabling precise light manipulation without optical loss across a broad spectral range. Here, we demonstrate that CuAlO2 exhibits colossal optical anisotropy and transparency across the visible spectrum, enabled by its unique three-dimensional O-Cu-O dumbbell structure and two-dimensionally confined excitons. Using mm-sized single crystals, we independently measured ab-plane and c-axis optical properties, revealing maximum birefringence (= 3.67) and linear dichroism (= 5.21), the highest reported to date. CuAlO2 retains birefringence over 0.5 throughout the entire visible range and possesses a wide direct bandgap of 3.71 eV, surpassing the birefringence of commercial anisotropic crystals transparent in the visible spectrum. From the two-dimensional screened hydrogen model and first-principles calculations, we demonstrate that the colossal anisotropy arises from a unique excitonic Cu d-p transition confined to the atomic-thick layer. This colossal optical anisotropy and transparency across the entire visible spectrum makes CuAlO2 a promising candidate for future photonic technologies. △ Less

Submitted 17 December, 2024; originally announced December 2024.

Skyrmion Emergence via Domain Wall Anchoring through Vertical Bloch Line

Authors: Suyeong Jeong, Dae-Han Jung, Hee-Sung Han, Ganghwi Kim, Myeonghwan Kang, Mi-Young Im, Younggun Park, Ki-Suk Lee

Abstract: Skyrmions, topologically stable magnetic solitons characterized by whirling magnetization in nanoscale magnetic elements, show promise information carriers in spintronics and spin-based quantum computing due to their unique properties: small size, stability, and controllability. In this study, we introduce a novel method of skyrmion generation through domain wall deformation dynamics. Our analytic… ▽ More Skyrmions, topologically stable magnetic solitons characterized by whirling magnetization in nanoscale magnetic elements, show promise information carriers in spintronics and spin-based quantum computing due to their unique properties: small size, stability, and controllability. In this study, we introduce a novel method of skyrmion generation through domain wall deformation dynamics. Our analytical and micromagnetic simulations demonstrate that domain wall motion exceeding the Walker threshold induces topological deformation of magnetic domain walls exhibiting Dzyaloshinskii-Moriya interaction. This deformation process catalyzes the emergence of skyrmions from magnetic domain wall structure distortion, specifically through the Anchoring of domain walls due to the vertical Bloch line. We elucidate the underlying mechanism of skyrmion generation, correlating it with topological transitions accompanied by burst energy dissipation through spin-wave radiation. Notably, we present robust skyrmion generation conditions through a comprehensive classification of domain wall distortion, including vertical Bloch line generation and annihilation in magnetic domain wall dynamics within a DMI system. These findings provide noble insights into topological behaviors of spin structures and offer a potential pathway for efficient, controlled skyrmion creation in the next-generation spintronic devices. △ Less

Submitted 6 November, 2024; originally announced November 2024.

Comments: 22 pages, 5 figures

Superconductivity in the parent infinite-layer nickelate NdNiO$_2$

Authors: C. T. Parzyck, Y. Wu, L. Bhatt, M. Kang, Z. Arthur, T. M. Pedersen, R. Sutarto, S. Fan, J. Pelliciari, V. Bisogni, G. Herranz, A. B. Georgescu, D. G. Hawthorn, L. F. Kourkoutis, D. A. Muller, D. G. Schlom, K. M. Shen

Abstract: We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO$_2$. A combination of oxide molecular-beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples… ▽ More We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO$_2$. A combination of oxide molecular-beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples with a variety of techniques, including electrical transport, scanning transmission electron microscopy, x-ray absorption spectroscopy, and resonant inelastic x-ray scattering, to investigate the possible origins of superconductivity. We propose that superconductivity could be intrinsic to the undoped infinite-layer nickelates but suppressed by disorder due to its nodal order parameter, a finding which would necessitate a reconsideration of the nickelate phase diagram. Another possible hypothesis is that the parent materials can be hole doped from randomly dispersed apical oxygen atoms, which would suggest an alternative pathway for achieving superconductivity. △ Less

Submitted 2 October, 2024; originally announced October 2024.

Comments: Main: 10 pages, 6 figures. Supplementary: 9 pages, 10 figures

Journal ref: Phys. Rev. X 15, 021048 (2025)

Competing Ordinary and Hanle Magnetoresistance in Pt and Ti Thin Films

Authors: Sebastian Sailler, Giacomo Sala, Denise Reustlen, Richard Schlitz, Min-Gu Kang, Pietro Gambardella, Sebastian T. B. Goennenwein, Michaela Lammel

Abstract: One of the key elements in spintronics research is the spin Hall effect, allowing to generate spin currents from charge currents. A large spin Hall effect is observed in materials with strong spin orbit coupling, e.g., Pt. Recent research suggests the existence of an orbital Hall effect, the orbital analogue to the spin Hall effect, which also arises in weakly spin orbit coupled materials like Ti,… ▽ More One of the key elements in spintronics research is the spin Hall effect, allowing to generate spin currents from charge currents. A large spin Hall effect is observed in materials with strong spin orbit coupling, e.g., Pt. Recent research suggests the existence of an orbital Hall effect, the orbital analogue to the spin Hall effect, which also arises in weakly spin orbit coupled materials like Ti, Mn or Cr. In Pt both effects are predicted to coexist. In any of these materials, a magnetic field perpendicular to the spin or orbital accumulation leads to additional Hanle dephasing and thereby the Hanle magnetoresistance (MR). To reveal the MR behavior of a material with both spin and orbital Hall effect, we thus study the MR of Pt thin films over a wide range of thicknesses. Careful evaluation shows that the MR of our textured samples is dominated by the ordinary MR rather than by the Hanle effect. We analyze the intrinsic properties of Pt films deposited by different groups and show that next to the resistivity also the structural properties of the film influence which MR dominates. We further show that this correlation can be found in both spin Hall active materials like Pt and orbital Hall active materials, like Ti. For both materials, the crystalline samples shows a MR attributed to the ordinary MR, whereas we find a large Hanle MR for the samples without apparent structural order. We then provide a set of rules to distinguish between the ordinary and the Hanle MR. We conclude that in all materials with a spin or orbital Hall effect the Hanle MR and the ordinary MR coexist and the purity and crystallinity of the thin film determine the dominating effect. △ Less

Submitted 26 September, 2024; originally announced September 2024.

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.

Anisotropic Spin Stripe Domains in Bilayer La$_3$Ni$_2$O$_7$

Authors: N. K Gupta, R. Gong, Y. Wu, M. Kang, C. T. Parzyck, B. Z. Gregory, N. Costa, R. Sutarto, S. Sarker, A. Singer, D. G. Schlom, K. M. Shen, D. G. Hawthorn

Abstract: The discovery of superconductivity in La$_3$Ni$_2$O$_7$ under pressure has motivated the investigation of a parent spin density wave (SDW) state, which could provide the underlying pairing interaction. Here, we employ resonant soft x-ray scattering and polarimetry on thin films of bilayer La$_3$Ni$_2$O$_7$ to determine that the magnetic structure of the SDW forms unidirectional diagonal spin strip… ▽ More The discovery of superconductivity in La$_3$Ni$_2$O$_7$ under pressure has motivated the investigation of a parent spin density wave (SDW) state, which could provide the underlying pairing interaction. Here, we employ resonant soft x-ray scattering and polarimetry on thin films of bilayer La$_3$Ni$_2$O$_7$ to determine that the magnetic structure of the SDW forms unidirectional diagonal spin stripes with moments lying within the NiO$_2$ plane and perpendicular to $\mathbf{Q}_{SDW}$, but without evidence of the strong charge disproportionation typically associated with other nickelates. These stripes form anisotropic domains with shorter correlation lengths perpendicular versus parallel to $\mathbf{Q}_{SDW}$, revealing nanoscale rotational and translational symmetry breaking analogous to the cuprate and Fe-based superconductors, with possible Bloch-like antiferromagnetic domain walls separating orthogonal domains. △ Less

Submitted 2 September, 2025; v1 submitted 4 September, 2024; originally announced September 2024.

Comments: 18 pages, 5 figures; replaced with published version. Supplementary information available at https://doi.org/10.1038/s41467-025-61653-w

Journal ref: Nature Communications 16, 6560 (2025)

Band-selective simulation of photoelectron intensity and converging Berry phase in trilayer graphene

Authors: Hayoon Im, Sue Hyeon Hwang, Minhee Kang, Kyoo Kim, Haeyong Kang, Choongyu Hwang

Abstract: Berry phase is one of the key elements to understand quantum-mechanical phenomena such as the Aharonov-Bohm effect and the unconventional Hall effect in graphene. The Berry phase in monolayer and bilayer graphene has been manifested by the anisotropic distribution of photoelectron intensity along a closed loop in the momentum space as well as its rotation by a characteristic angle upon rotating li… ▽ More Berry phase is one of the key elements to understand quantum-mechanical phenomena such as the Aharonov-Bohm effect and the unconventional Hall effect in graphene. The Berry phase in monolayer and bilayer graphene has been manifested by the anisotropic distribution of photoelectron intensity along a closed loop in the momentum space as well as its rotation by a characteristic angle upon rotating light polarization. Here we report the band-selective simulation of photoelectron intensity of trilayer graphene to understand its Berry phase within the tight-binding formalism. ABC- and ABA-stacked trilayer graphene show characteristic rotational angles of photoelectron intensity distribution, as predicted from their well-known Berry phases. Surprisingly, however, in ABA-stacked trilayer graphene, the rotational angle changes upon approaching toward the band touching point between the conduction and valence bands, which suggest that Berry phase changes as a function of binding energy. The binding energy-dependent Berry phase is attributed to the enhanced hybridization of the two electron bands of ABA-stacked trilayer graphene that converge at the band touching point, resulting in the converging Berry phase. These findings will provide an efficient way of tuning Berry phase and hence exotic phenomena stemming from the Berry phase. △ Less

Submitted 14 August, 2024; originally announced August 2024.

Journal ref: Appl. Sci. Converg. Technol. 33, 91 (2024)

Giant Uniaxial Magnetocrystalline Anisotropy in SmCrGe$_3$

Authors: Mingyu Xu, Yongbin Lee, Xianglin Ke, Min-Chul Kang, Matt Boswell, Sergey. L. Bud'ko, Lin Zhou, Liqin Ke, Mingda Li, Paul. C. Canfield, Weiwei Xie

Abstract: Magnetic anisotropy is a crucial characteristic for enhancing spintronic device performance. The synthesis of SmCrGe$_3$ single crystals through a high-temperature solution method has led to the determination of uniaxial magnetocrystalline anisotropy. Phase verification was achieved using scanning transmission electron microscopy (STEM), powder, and single-crystal X-ray diffraction techniques. Ele… ▽ More Magnetic anisotropy is a crucial characteristic for enhancing spintronic device performance. The synthesis of SmCrGe$_3$ single crystals through a high-temperature solution method has led to the determination of uniaxial magnetocrystalline anisotropy. Phase verification was achieved using scanning transmission electron microscopy (STEM), powder, and single-crystal X-ray diffraction techniques. Electrical transport and specific heat measurements indicate a Curie temperature ($T_C$) of approximately 160 K, while magnetization measurements were utilized to determine the anisotropy fields and constants. Curie-Weiss fitting applied to magnetization data suggests the contribution of both Sm and Cr in the paramagnetic phase. Additionally, density functional theory (DFT) calculations explored the electronic structures and magnetic properties of SmCrGe$_3$, revealing a significant easy-axis single-ion Sm magnetocrystalline anisotropy of 16 meV/f.u.. Based on the magnetization measurements, easy-axis magnetocrystalline anisotropy at 20 K is 13 meV/f.u.. △ Less

Submitted 7 August, 2024; originally announced August 2024.

Comments: 27 pages, 5+5 figures

Orbital Torque in Rare-Earth Transition-Metal Ferrimagnets

Authors: Shilei Ding, Min-Gu Kang, William Legrand, Pietro Gambardella

Abstract: Orbital currents have recently emerged as a promising tool to achieve electrical control of the magnetization in thin-film ferromagnets. Efficient orbital-to-spin conversion is required in order to torque the magnetization. Here we show that the injection of an orbital current in a ferrimagnetic GdyCo100-y alloy generates strong orbital torques whose sign and magnitude can be tuned by changing the… ▽ More Orbital currents have recently emerged as a promising tool to achieve electrical control of the magnetization in thin-film ferromagnets. Efficient orbital-to-spin conversion is required in order to torque the magnetization. Here we show that the injection of an orbital current in a ferrimagnetic GdyCo100-y alloy generates strong orbital torques whose sign and magnitude can be tuned by changing the Gd content and temperature. The effective spin-orbital Hall angle reaches up to -0.25 in a GdyCo100-y/CuOx bilayer compared to +0.03 in Co/CuOx and +0.13 in GdyCo100-y/Pt. This behavior is attributed to the local orbital-to-spin conversion taking place at the Gd sites, which is about five times stronger and of the opposite sign relative to Co. Furthermore, we observe a manyfold increase in the net orbital torque at low temperature, which we attribute to the improved conversion efficiency following the magnetic ordering of the Gd and Co sublattices. △ Less

Submitted 28 June, 2024; originally announced June 2024.

Evidence of surface $p$-wave superconductivity and higher-order topology in MoTe$_2$

Authors: Sangyun Lee, Myungjun Kang, Duk Y. Kim, Jihyun Kim, Suyeon Cho, Sangmo Cheon, Tuson Park

Abstract: Exploration of nontrivial superconductivity and electronic band topology is at the core of condensed matter physics and applications to quantum information. The transition-metal dichalcogenide (TMDC) MoTe$_2$ has been proposed as an ideal candidate to explore the interplay between topology and superconductivity, but their studies remain limited regarding the required high-pressure environments. He… ▽ More Exploration of nontrivial superconductivity and electronic band topology is at the core of condensed matter physics and applications to quantum information. The transition-metal dichalcogenide (TMDC) MoTe$_2$ has been proposed as an ideal candidate to explore the interplay between topology and superconductivity, but their studies remain limited regarding the required high-pressure environments. Here, we observe proximity-induced surface $p$-wave superconductivity, and investigate the higher-order topological nature of MoTe$_2$ in its 1T$'$ phase, which emerges from the T$_d$ phase through a high-pressure-induced topological phase transition. Using surface-sensitive soft-point-contact Andreev reflection spectroscopy, we confirm the emergence of surface $s+p$-wave superconductivity via the BTK model as well as a zero-bias conductance peak. Such surface $p$-wave superconductivity emerges via the proximity effect between an $s$-wave superconducting band and a second-order topological band, which is protected by the time-reversal and inversion symmetries. The temperature dependence of the surface $p$-wave superconducting gap shows a correlation with that of the bulk $s$-wave gap, as well as its suppression by an external magnetic field or a reduction in pressure, implying its proximity-induced origin. Moreover, we suggest that the topological hinge states, derived from second-order topological bands, evolve into zero-energy Majorana corner states in this proximity-effect-induced third-order topological superconducting phase. These results demonstrate the potential realization of topological superconductivity in MoTe$_2$, thus opening a pathway for studying various topological natures of TMDC materials. △ Less

Submitted 14 May, 2025; v1 submitted 11 June, 2024; originally announced June 2024.

Sub-symmetry Protected Topology in Topological Insulators and Superconductors

Authors: Myungjun Kang, Mingyu Lee, Sangmo Cheon

Abstract: Exploration of topology protected by a certain symmetry is central in condensed matter physics. A recent idea of sub-symmetry-protected (SSP) topology--remains of a broken symmetry can still protect specific topological boundary states--has been developed and demonstrated in an optical system [Nat. Phys. 19, 992-998 (2023)]. Here, we extend this idea further by applying sub-symmetry-protecting per… ▽ More Exploration of topology protected by a certain symmetry is central in condensed matter physics. A recent idea of sub-symmetry-protected (SSP) topology--remains of a broken symmetry can still protect specific topological boundary states--has been developed and demonstrated in an optical system [Nat. Phys. 19, 992-998 (2023)]. Here, we extend this idea further by applying sub-symmetry-protecting perturbation (SSPP) to one-dimensional topological insulating and superconducting systems using the Su-Schrieffer-Hegger (SSH) and Kitaev models. Using the tight-binding and low-energy effective theory, we show that the SSP boundary states retain topological properties while the SSPP results in the asymmetry of boundary states. For the SSH model, an SSP zero-energy edge state localized on one edge possesses quantized polarization. In contrast, the other edge state is perturbed to have non-zero energy, and its polarization is not quantized. For topological superconductors, zero-energy SSP Majorana boundary states for spinful Kitaev models emerge on only one edge, contrary to the conventional belief that Majorana fermions emerge at opposite edges. Our findings can be used as a platform to expand our understanding of topological materials as they broaden our understanding of the symmetry in a topological system and a method to engineer Majorana fermions. △ Less

Submitted 3 June, 2024; originally announced June 2024.

Hardware-efficient ansatz without barren plateaus in any depth

Authors: Chae-Yeun Park, Minhyeok Kang, Joonsuk Huh

Abstract: Variational quantum circuits have recently gained much interest due to their relevance in real-world applications, such as combinatorial optimizations, quantum simulations, and modeling a probability distribution. Despite their huge potential, the practical usefulness of those circuits beyond tens of qubits is largely questioned. One of the major problems is the so-called barren plateaus phenomeno… ▽ More Variational quantum circuits have recently gained much interest due to their relevance in real-world applications, such as combinatorial optimizations, quantum simulations, and modeling a probability distribution. Despite their huge potential, the practical usefulness of those circuits beyond tens of qubits is largely questioned. One of the major problems is the so-called barren plateaus phenomenon. Quantum circuits with a random structure often have a flat cost-function landscape and thus cannot be trained efficiently. In this paper, we propose two novel parameter conditions in which the hardware-efficient ansatz (HEA) is free from barren plateaus for arbitrary circuit depths. In the first condition, the HEA approximates to a time-evolution operator generated by a local Hamiltonian. Utilizing a recent result by [Park and Killoran, Quantum 8, 1239 (2024)], we prove a constant lower bound of gradient magnitudes in any depth both for local and global observables. On the other hand, the HEA is within the many-body localized (MBL) phase in the second parameter condition. We argue that the HEA in this phase has a large gradient component for a local observable using a phenomenological model for the MBL system. By initializing the parameters of the HEA using these conditions, we show that our findings offer better overall performance in solving many-body Hamiltonians. Our results indicate that barren plateaus are not an issue when initial parameters are smartly chosen, and other factors, such as local minima or the expressivity of the circuit, are more crucial. △ Less

Submitted 7 March, 2024; originally announced March 2024.

Comments: 5+15 pages, 3+3 figures

Nodal fermions in a strongly spin-orbit coupled frustrated pyrochlore superconductor

Authors: Dongjin Oh, Junha Kang, Yuting Qian, Shiang Fang, Mingu Kang, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Joseph G. Checkelsky, Liang Fu, Tomasz Klimczuk, Michal J. Winiarski, Bohm-Jung Yang, Riccardo Comin

Abstract: The pyrochlore lattice, a three-dimensional network of corner-sharing tetrahedra, is a promising material playground for correlated topological phases arising from the interplay between spin-orbit coupling (SOC) and electron-electron interactions. Due to its geometrically frustrated lattice structure, exotic correlated states on the pyrochlore lattice have been extensively studied using various sp… ▽ More The pyrochlore lattice, a three-dimensional network of corner-sharing tetrahedra, is a promising material playground for correlated topological phases arising from the interplay between spin-orbit coupling (SOC) and electron-electron interactions. Due to its geometrically frustrated lattice structure, exotic correlated states on the pyrochlore lattice have been extensively studied using various spin Hamiltonians in the localized limit. On the other hand, the topological properties of the electronic structure in the pyrochlore lattice have rarely been explored, due to the scarcity of pyrochlore materials in the itinerant paramagnetic limit. Here, we explore the topological electronic band structure of pyrochlore superconductor RbBi$_{2}$ using angle-resolved photoemission spectroscopy. Thanks to the strong SOC of the Bi pyrochlore network, we experimentally confirm the existence of three-dimensional (3D) massless Dirac fermions enforced by nonsymmorphic symmetry, as well as a 3D quadratic band crossing protected by cubic crystalline symmetry. Furthermore, we identify an additional 3D linear Dirac dispersion associated with band inversion protected by threefold rotation symmetry. These observations reveal the rich non-trivial band topology of itinerant pyrochlore lattice systems in the strong SOC regime. Through manipulation of electron correlations and SOC of the frustrated pyrochlore lattices, this material platform is a natural host for exotic phases of matter, including the fractionalized quantum spin Hall effect in the topological Mott insulator phase, as well as axion electrodynamics in the axion insulator phase. △ Less

Submitted 6 February, 2024; originally announced February 2024.

Comments: 18 pages, 4 figures

Topological Domain-Wall States Hosting Quantized Polarization and Majorana Zero Modes Without Bulk Boundary Correspondence

Authors: Sang-Hoon Han, Myungjun Kang, Moon Jip Park, Sangmo Cheon

Abstract: Bulk-boundary correspondence is a concept for topological insulators and superconductors that determines the existence of topological boundary states within the tenfold classification table. Contrary to this belief, we demonstrate that topological domain-wall states can emerge in all forbidden 1D classes in the classification table using representative generalized Su-Schrieffer-Heeger and Kitaev m… ▽ More Bulk-boundary correspondence is a concept for topological insulators and superconductors that determines the existence of topological boundary states within the tenfold classification table. Contrary to this belief, we demonstrate that topological domain-wall states can emerge in all forbidden 1D classes in the classification table using representative generalized Su-Schrieffer-Heeger and Kitaev models, which manifests as quantized electric dipole moments and Majorana zero modes, respectively. We first show that a zero-energy domain-wall state can possess a quantized polarization, even if the polarization of individual domains is not inherently quantized. A quantized Berry phase difference between the domains confirms the non-trivial nature of the domain-wall states, implying a general-bulk-boundary principle, further confirmed by the tight-binding, topological field, and low-energy effective theories. Our methodology is then extended to a superconducting system, resulting in Majorana zero modes on the domain wall of a generalized Kitaev model. Finally, we suggest potential systems where our results may be realized, spanning from condensed matter to optical. △ Less

Submitted 15 November, 2023; originally announced November 2023.

Transport collapse in dynamically evolving networks

Authors: Geoffroy Berthelot, Liubov Tupikina, Min-Yeong Kang, Jérôme Dedecker, Denis S. Grebenkov

Abstract: Transport in complex networks can describe a variety of natural and human-engineered processes including biological, societal and technological ones. However, how the properties of the source and drain nodes can affect transport subject to random failures, attacks or maintenance optimization in the network remain unknown. In this paper, the effects of both the distance between the source and drain… ▽ More Transport in complex networks can describe a variety of natural and human-engineered processes including biological, societal and technological ones. However, how the properties of the source and drain nodes can affect transport subject to random failures, attacks or maintenance optimization in the network remain unknown. In this paper, the effects of both the distance between the source and drain nodes and of the degree of the source node on the time of transport collapse are studied in scale-free and lattice-based transport networks. These effects are numerically evaluated for two strategies, which employ either transport-based or random link removal. Scale-free networks with small distances are found to result in larger times of collapse. In lattice-based networks, both the dimension and boundary conditions are shown to have a major effect on the time of collapse. We also show that adding a direct link between the source and the drain increases the robustness of scale-free networks when subject to random link removals. Interestingly, the distribution of the times of collapse is then similar to the one of lattice-based networks. △ Less

Submitted 18 October, 2023; originally announced October 2023.

Journal ref: J. Royal Soc. Interface 20, 20220906 (2023)

Theoretical investigation of delafossite-Cu2ZnSnO4 as a promising photovoltaic absorber

Authors: Seoung-Hun Kang, Myeongjun Kang, Sang Woon Hwang, Sinchul Yeom, Mina Yoon, Jong Mok Ok, Sangmoon Yoon

Abstract: In the quest for efficient and cost-effective photovoltaic absorber materials beyond silicon, considerable attention has been directed toward exploring alternatives. One such material, zincblende-derived Cu2ZnSnS4 (CZTS), has shown promise due to its ideal band-gap size and high absorption coefficient. However, challenges such as structural defects and secondary phase formation have hindered its d… ▽ More In the quest for efficient and cost-effective photovoltaic absorber materials beyond silicon, considerable attention has been directed toward exploring alternatives. One such material, zincblende-derived Cu2ZnSnS4 (CZTS), has shown promise due to its ideal band-gap size and high absorption coefficient. However, challenges such as structural defects and secondary phase formation have hindered its development. In this study, we examine the potential of another compound Cu2ZnSnO4 (CZTO) with a similar composition to CZTS as a promising alternative. Employing ab initio density function theory (DFT) calculations in combination with an evolutionary structure prediction algorithm, we identify that the crystalline phase of the delafossite structure is the most stable among the 900 (meta)stable CZTO. Its thermodynamic stability at room temperature is also confirmed by the molecular dynamics study. Excitingly, this new phase of CZTO displays a direct band gap where the dipole-allowed transition occurs, making it a strong candidate for efficient light absorption. Furthermore, the estimation of spectroscopic limited maximum efficiency (SLME) directly demonstrates the high potential of delafossite-CZTO as a photovoltaic absorber. Our numerical results suggest that delafossite-CZTO holds another promise for future photovoltaic applications. △ Less

Submitted 17 October, 2023; originally announced October 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)

Photoinduced dynamics of flat bands in the kagome metal CoSn

Authors: Denny Puntel, Wibke Bronsch, Manuel Tuniz, Mingu Kang, Paul M. Neves, Shiang Fang, Efthimios Kaxiras, Joseph G. Checkelsky, Riccardo Comin, Fulvio Parmigiani, Federico Cilento

Abstract: CoSn is a prototypical kagome compound showing lattice-born flat bands with suppressed bandwidth over large parts of the Brillouin zone. Here, by means of time- and angle-resolved photoelectron spectroscopy, we provide direct evidence of the response to photoexcitation of the flat bands, that underlie information about localization in real space. In particular, we detect a sudden shift and broaden… ▽ More CoSn is a prototypical kagome compound showing lattice-born flat bands with suppressed bandwidth over large parts of the Brillouin zone. Here, by means of time- and angle-resolved photoelectron spectroscopy, we provide direct evidence of the response to photoexcitation of the flat bands, that underlie information about localization in real space. In particular, we detect a sudden shift and broadening of the flat bands, while after one picosecond only a broadening survives. We ascribe both these effects to an ultrafast disruption of electron localization, which renormalizes the effective electron-electron interaction and affects the flat band dispersion. Since both variations are in the order of a few meV, our measurements suggest that the flat bands are resilient to near-infrared photoexcitation. △ Less

Submitted 16 May, 2023; originally announced May 2023.

Nature of charge density wave in kagome metal ScV6Sn6

Authors: Seongyong Lee, Choongjae Won, Jimin Kim, Jonggyu Yoo, Sudong Park, Jonathan Denlinger, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Riccardo Comin, Mingu Kang, Jae-Hoon Park

Abstract: Kagome lattice materials offer a fertile ground to discover novel quantum phases of matter, ranging from unconventional superconductivity and quantum spin liquids to charge orders of various profiles. However, understanding the genuine origin of the quantum phases in kagome materials is often challenging, owing to the intertwined atomic, electronic, and structural degrees of freedom. Here, we comb… ▽ More Kagome lattice materials offer a fertile ground to discover novel quantum phases of matter, ranging from unconventional superconductivity and quantum spin liquids to charge orders of various profiles. However, understanding the genuine origin of the quantum phases in kagome materials is often challenging, owing to the intertwined atomic, electronic, and structural degrees of freedom. Here, we combine angle-resolved photoemission spectroscopy, phonon mode calculation, and chemical doping to elucidate the driving mechanism of the root3*root3 charge order in a newly discovered kagome metal ScV6Sn6. In contrast to the case of the archetype kagome system AV3Sb5 (A= K, Rb, Cs), the van Hove singularities in ScV6Sn6 remain intact across the charge order transition, indicating a marginal role of the electronic instability from the V kagome lattice. Instead, we identified a three-dimensional band with dominant planar Sn character opening a large charge order gap of 260 meV and strongly reconstructing the Fermi surface. Our complementary phonon dispersion calculations further emphasize the role of the structural components other than the V kagome lattice by revealing the unstable planar Sn and Sc phonon modes associated to the root3*root3 phase. Finally, in the constructed phase diagram of Sc(V1-xCrx)6Sn6, the charge order remains robust in a wide doping range x = 0 ~ 0.10 against the Fermi level shift up to ~ 120 meV, further making the electronic scenarios such as Fermi surface or saddle point nesting unlikely. Our multimodal investigations demonstrate that the physics of ScV6Sn6 is fundamentally different from the canonical kagome metal AV3Sb5, uncovering a new mechanism to induce symmetry-breaking phase transition in kagome lattice materials. △ Less

Submitted 24 April, 2023; originally announced April 2023.

Collective nature of orbital excitations in layered cuprates in the absence of apical oxygens

Authors: Leonardo Martinelli, Krzysztof Wohlfeld, Jonathan Pelliciari, Riccardo Arpaia, Nicholas B. Brookes, Daniele Di Castro, Mirian G. Fernandez, Mingu Kang, Yoshiharu Krockenberger, Kurt Kummer, Daniel E. McNally, Eugenio Paris, Thorsten Schmitt, Hideki Yamamoto, Andrew Walters, Ke-Jin Zhou, Lucio Braicovich, Riccardo Comin, Marco Moretti Sala, Thomas P. Devereaux, Maria Daghofer, Giacomo Ghiringhelli

Abstract: We have investigated the 3d orbital excitations in CaCuO2 (CCO), Nd2CuO4 (NCO), and La2CuO4 (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the dxy orbital clearly disperse, pointing to a collective character of this excitation (orbiton) in compounds without apica… ▽ More We have investigated the 3d orbital excitations in CaCuO2 (CCO), Nd2CuO4 (NCO), and La2CuO4 (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the dxy orbital clearly disperse, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. We show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy. △ Less

Submitted 9 February, 2024; v1 submitted 4 April, 2023; originally announced April 2023.

Comments: 24 pages, 10 figures

Journal ref: Phys. Rev. Lett. 132, 066004 (2024)

2D MXene Electrochemical Transistors

Authors: Jyoti Shakya, Min-A Kang, Jian Li, Armin VahidMohammadi, Weiqian Tian, Erica Zeglio, Mahiar Max Hamedi

Abstract: In the past two decades another transistor based on conducting polymers, called the organic electrochemical transistor (ECT) was shown and largely studied. The main difference between organic ECTs and FETs is the mode and extent of channel doping: while in FETs the channel only has surface doping through dipoles, the mixed ionic-electronic conductivity of the channel material in Organic ECTs enabl… ▽ More In the past two decades another transistor based on conducting polymers, called the organic electrochemical transistor (ECT) was shown and largely studied. The main difference between organic ECTs and FETs is the mode and extent of channel doping: while in FETs the channel only has surface doping through dipoles, the mixed ionic-electronic conductivity of the channel material in Organic ECTs enables bulk electrochemical doping. As a result, the organic ECT maximizes conductance modulation at the expense of speed. Until now ECTs have been based on conducting polymers, but here we show that MXenes, a class of 2D materials beyond graphene, have mixed ionic-electronic properties that enable the realization of electrochemical transistors (ECTs). We show that the formulas for organic ECTs can be applied to these 2D ECTs and used to extract parameters like mobility. These MXene ECTs have high transconductance values but low on-off ratios. We further show that conductance switching data measured using ECT, in combination with other in-situ ex-situ electrochemical measurements, is a powerful tool for correlating the change in conductance to that of redox state: to our knowledge, this is the first report of this important correlation for MXene films. Many future possibilities exist for MXenes ECTs, and we think other 2D materials with bandgaps can also form ECTs with single or heterostructured 2D materials. 2D ECTs can draw great inspiration and theoretical tools from the field of organic ECTs and have the potential to considerably extend the capabilities of transistors beyond that of conducting polymer ECTs, with added properties such as extreme heat resistance, tolerance for solvents, and higher conductivity for both electrons and ions than conducting polymers. △ Less

Submitted 10 January, 2024; v1 submitted 19 March, 2023; originally announced March 2023.

Comments: 14 pages, 4 figures

MSC Class: 82D37

Small Fermi pockets intertwined with charge stripes and pair density wave order in a kagome superconductor

Authors: Hong Li, Dongjin Oh, Mingu Kang, He Zhao, Brenden R Ortiz, Yuzki Oey, Shiang Fang, Zheng Ren, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Joseph G. Checkelsky, Ziqiang Wang, Stephen D. Wilson, Riccardo Comin, Ilija Zeljkovic

Abstract: The kagome superconductor family AV3Sb5 (A=Cs, K, Rb) emerged as an exciting platform to study exotic Fermi surface instabilities. Here we use spectroscopic-imaging scanning tunneling microscopy (SI-STM) and angle-resolved photoemission spectroscopy (ARPES) to reveal how the surprising cascade of higher and lower-dimensional density waves in CsV3Sb5 is intimately tied to a set of small reconstruct… ▽ More The kagome superconductor family AV3Sb5 (A=Cs, K, Rb) emerged as an exciting platform to study exotic Fermi surface instabilities. Here we use spectroscopic-imaging scanning tunneling microscopy (SI-STM) and angle-resolved photoemission spectroscopy (ARPES) to reveal how the surprising cascade of higher and lower-dimensional density waves in CsV3Sb5 is intimately tied to a set of small reconstructed Fermi pockets. ARPES measurements visualize the formation of these pockets generated by a 3D charge density wave transition. The pockets are connected by dispersive q* wave vectors observed in Fourier transforms of STM differential conductance maps. As the additional 1D charge order emerges at a lower temperature, q* wave vectors become substantially renormalized, signaling further reconstruction of the Fermi pockets. Remarkably, in the superconducting state, the superconducting gap modulations give rise to an in-plane Cooper pair-density-wave at the same q* wave vectors. Our work demonstrates the intrinsic origin of the charge-stripes and the pair-density-wave in CsV3Sb5 and their relationship to the Fermi pockets. These experiments uncover a unique scenario of how Fermi pockets generated by a parent charge density wave state can provide a favorable platform for the emergence of additional density waves. △ Less

Submitted 13 March, 2023; originally announced March 2023.

Journal ref: Phys. Rev. X 13, 031030 (2023)

Effect of Annealing Temperature on Minimum Domain Size of Ferroelectric Hafnia

Authors: Seokjung Yun, Hoon Kim, Myungsoo Seo, Min-Ho Kang, Taeho Kim, Seongwoo Cho, Min Hyuk Park, Sanghun Jeon, Yang-Kyu Choi, Seungbum Hong

Abstract: Here, we optimized the annealing temperature of HZO/TiN thin film heterostructure via multiscale analysis of remnant polarization, crystallographic phase, minimum ferroelectric domain size, and average grain size. We found that the remnant polarization was closely related to the relative amount of the orthorhombic phase whereas the minimum domain size was to the relative amount of the monoclinic p… ▽ More Here, we optimized the annealing temperature of HZO/TiN thin film heterostructure via multiscale analysis of remnant polarization, crystallographic phase, minimum ferroelectric domain size, and average grain size. We found that the remnant polarization was closely related to the relative amount of the orthorhombic phase whereas the minimum domain size was to the relative amount of the monoclinic phase. The minimum domain size was obtained at the annealing temperature of 500$^\cird$C while the optimum remnant polarization and capacitance at the annealing temperature of 600$^\circ$C. We conclude that the minimum domain size is more important than the sheer magnitude of remnant polarization considering the retention and fatigue of switchable polarization in nanoscale ferroelectric devices. Our results are expected to contribute to the development of ultra-low-power logic transistors and next-generation non-volatile memory devices. △ Less

Submitted 12 January, 2023; originally announced January 2023.

Comments: 28 pages, 11 figures, 1 table

Growth of delafossite CuAlO2 single crystals in a reactive crucible

Authors: Du hyung Kim, Minsik Kong, Myeongjun Kang, Minjae Kim, Seohee Kim, Youngwook Kim, Sangmoon Yoon, Jong Mok Ok

Abstract: Delafossite oxide CuAlO2 has engaged great attention as a promising p-type conducting oxide. In this work, high-quality CuAlO2 single crystals with a size of several millimeters (mm) are successfully achieved with a reactive crucible melting method. The crystals are characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, transport measurement, and magn… ▽ More Delafossite oxide CuAlO2 has engaged great attention as a promising p-type conducting oxide. In this work, high-quality CuAlO2 single crystals with a size of several millimeters (mm) are successfully achieved with a reactive crucible melting method. The crystals are characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, transport measurement, and magnetic susceptibility measurement. The grown single crystals are free of contamination from a copper oxide flux. This work provides a new approach to growing high-quality delafossite oxide CuAlO2 with a few mm size. △ Less

Submitted 22 May, 2022; originally announced May 2022.

Discovery of an electronic crystal in a cuprate Mott insulator

Authors: Mingu Kang, Charles Zhang, Enrico Schierle, Stephen McCoy, Jiarui Li, Ronny Sutarto, Feizhou He, Andreas Suter, Thomas Prokscha, Zaher Salman, Eugen Weschke, Shane Cybart, John Y. T. Wei, Riccardo Comin

Abstract: Copper oxide high temperature superconductors universally exhibit multiple forms of electronically ordered phases that break the native translational symmetry of the CuO2 planes. The interplay between these orders and the superconducting ground state, as well as how they arise through doping a Mott insulator, is essential to decode the mechanisms of high-temperature superconductivity. Over the yea… ▽ More Copper oxide high temperature superconductors universally exhibit multiple forms of electronically ordered phases that break the native translational symmetry of the CuO2 planes. The interplay between these orders and the superconducting ground state, as well as how they arise through doping a Mott insulator, is essential to decode the mechanisms of high-temperature superconductivity. Over the years, various forms of electronic liquid crystal phases including charge/spin stripes and incommensurate charge-density-waves (CDWs) were found to emerge out of a correlated metallic ground state in underdoped cuprates. Early theoretical studies also predicted the emergence of a Coulomb-frustrated 'charge crystal' phase in the very lightly-doped, insulating limit of the CuO2 planes. Here, we use resonant X-ray scattering, electron transport, and muon spin rotation measurements to fully resolve the electronic and magnetic ground state and search for signatures of charge order in very lightly hole-doped cuprates from the RBa2Cu3O7-d family (RBCO; R: Y or rare earth). X-ray scattering data from RBCO films reveal a breaking of translational symmetry more pervasive than was previously known, extending down to the Mott limit. The ordering vector of this charge crystal state is linearly connected to the charge-density-waves of underdoped RBCO, suggesting that the former phase is a precursor to the latter as hole doping is increased. Most importantly, the coexistence of charge and spin order in RBCO suggests that this electronic symmetry-breaking state is common to the CuO2 planes in the very lightly-doped regime. These findings bridge the gap between the Mott insulating state and the underdoped metallic state and underscore the prominent role of Coulomb-frustrated electronic phase separation among all cuprates. △ Less

Submitted 16 March, 2022; originally announced March 2022.

Charge order landscape and competition with superconductivity in kagome metals

Authors: Mingu Kang, Shiang Fang, Jonggyu Yoo, Brenden R. Ortiz, Yuzki Oey, Jonghyeok Choi, Sae Hee Ryu, Jimin Kim, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Efthimios Kaxiras, Joseph G. Checkelsky, Stephen D. Wilson, Jae-Hoon Park, Riccardo Comin

Abstract: In kagome metals AV3Sb5 (A = K, Rb, Cs), three-dimensional charge order (3D-CO) is the primary instability that sets the stage for other collective orders to emerge, including unidirectional stripe order, orbital flux order, electronic nematicity, and superconductivity. Here, we use high-resolution angle-resolved photoemission spectroscopy to determine the microscopic structure of three-dimensiona… ▽ More In kagome metals AV3Sb5 (A = K, Rb, Cs), three-dimensional charge order (3D-CO) is the primary instability that sets the stage for other collective orders to emerge, including unidirectional stripe order, orbital flux order, electronic nematicity, and superconductivity. Here, we use high-resolution angle-resolved photoemission spectroscopy to determine the microscopic structure of three-dimensional charge order (3D-CO) in AV3Sb5 and its interplay with superconductivity. Our approach is based on identifying an unusual splitting of kagome bands induced by 3D-CO, which provides a sensitive way to refine the spatial charge patterns in neighboring kagome planes. We found a marked dependence of the 3D-CO structure on composition and doping. The observed difference between CsV3Sb5 and the other compounds potentially underpins the double-dome superconductivity in CsV3(Sb,Sn)5 and the suppression of Tc in KV3Sb5 and RbV3Sb5. Our results provide fresh insights into the rich phase diagram of AV3Sb5. △ Less

Submitted 22 December, 2022; v1 submitted 3 February, 2022; originally announced February 2022.

Comments: 20 pages, 5 figures

Journal ref: Nature Materials (2022)

Low-threshold exciton transport and control in atomically thin semiconductors

Authors: Hyeongwoo Lee, Yeonjeong Koo, Jinseong Choi, Shailabh Kumar, Hyoung-Taek Lee, Gangseon Ji, Soo Ho Choi, Mingu Kang, Ki Kang Kim, Hyeong-Ryeol Park, Hyuck Choo, Kyoung-Duck Park

Abstract: Understanding and controlling the nanoscale transport of excitonic quasiparticles in atomically thin 2D semiconductors is crucial to produce highly efficient nano-excitonic devices. Here, we present a nano-gap device to selectively confine excitons or trions of 2D transition metal dichalcogenides at the nanoscale, facilitated by the drift-dominant exciton funnelling into the strain-induced local s… ▽ More Understanding and controlling the nanoscale transport of excitonic quasiparticles in atomically thin 2D semiconductors is crucial to produce highly efficient nano-excitonic devices. Here, we present a nano-gap device to selectively confine excitons or trions of 2D transition metal dichalcogenides at the nanoscale, facilitated by the drift-dominant exciton funnelling into the strain-induced local spot. We investigate the spatio-spectral characteristics of the funnelled excitons in a WSe2 monolayer (ML) and converted trions in a MoS2 ML using hyperspectral tip-enhanced photoluminescence (TEPL) imaging with <15 nm spatial resolution. In addition, we dynamically control the exciton funnelling and trion conversion rate by the GPa scale tip pressure engineering. Through a drift-diffusion model, we confirm an exciton funnelling efficiency of ~25 % with a significantly low strain threshold (~0.1 %) which sufficiently exceeds the efficiency of ~3 % in previous studies. This work provides a new strategy to facilitate efficient exciton transport and trion conversion of 2D semiconductor devices. △ Less

Submitted 15 September, 2021; originally announced September 2021.

Comments: 20 pages, 5 figures

A flat band-induced correlated kagome metal

Authors: Linda Ye, Shiang Fang, Min Gu Kang, Josef Kaufmann, Yonghun Lee, Jonathan Denlinger, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Efthimios Kaxiras, David C. Bell, Oleg Janson, Riccardo Comin, Joseph G. Checkelsky

Abstract: The notion of an electronic flat band refers to a collectively degenerate set of quantum mechanical eigenstates in periodic solids. The vanishing kinetic energy of flat bands relative to the electron-electron interaction is expected to result in a variety of many-body quantum phases of matter. Despite intense theoretical interest, systematic design and experimental realization of such flat band-dr… ▽ More The notion of an electronic flat band refers to a collectively degenerate set of quantum mechanical eigenstates in periodic solids. The vanishing kinetic energy of flat bands relative to the electron-electron interaction is expected to result in a variety of many-body quantum phases of matter. Despite intense theoretical interest, systematic design and experimental realization of such flat band-driven correlated states in natural crystals have remained a challenge. Here we report the realization of a partially filled flat band in a new single crystalline kagome metal Ni$_3$In. This flat band is found to arise from the Ni $3d$-orbital wave functions localized at triangular motifs within the kagome lattice plane, where an underlying destructive interference among hopping paths flattens the dispersion. We observe unusual metallic and thermodynamic responses suggestive of the presence of local fluctuating magnetic moments originating from the flat band states, which together with non-Fermi liquid behavior indicate proximity to quantum criticality. These results demonstrate a lattice and orbital engineering approach to designing flat band-based many-body phenomena that may be applied to integrate correlation with topology and as a novel means to construct quantum criticality. △ Less

Submitted 20 June, 2021; originally announced June 2021.

Comments: 18 pages, 4 figures

Journal ref: Nature Physics, 20, 610-614 (2024)

Long-Range Orbital Magnetoelectric Torque in Ferromagnets

Authors: Dongwook Go, Daegeun Jo, Kyoung-Whan Kim, Soogil Lee, Min-Gu Kang, Byong-Guk Park, Stefan Blügel, Hyun-Woo Lee, Yuriy Mokrousov

Abstract: While it is often assumed that the orbital response is suppressed and short-ranged due to strong crystal field potential and orbital quenching, we show that the orbital magnetoelectric response can be remarkably long-ranged in ferromagnets. In a bilayer consisting of a nonmagnet and a ferromagnet, spin injection from the interface results in spin accumulation and torque in the ferromagnet, which r… ▽ More While it is often assumed that the orbital response is suppressed and short-ranged due to strong crystal field potential and orbital quenching, we show that the orbital magnetoelectric response can be remarkably long-ranged in ferromagnets. In a bilayer consisting of a nonmagnet and a ferromagnet, spin injection from the interface results in spin accumulation and torque in the ferromagnet, which rapidly oscillate and decay by spin dephasing. In contrast, we find that even when an external electric field is applied only on the nonmagnet, we find substantially long-ranged orbital magnetoelectric response in the FM, which can go far beyond the spin dephasing length. This unusual feature is attributed to nearly degenerate orbital characters imposed by the crystal symmetry, which form hotspots for the intrinsic orbital response. Because only the states near the hotspots contribute dominantly, the induced orbital angular momentum does not exhibit destructive interference among states with different momentum as in the case of the spin dephasing. This gives rise to a distinct type of orbital torque on the magnetization, increasing with the thickness of the ferromagnet. Such behavior may serve as critical long-sought evidence of orbital transport to be directly tested in experiments. Our findings open the possibility of using long-range orbital magnetoelectric effect in orbitronic device applications. △ Less

Submitted 16 May, 2022; v1 submitted 15 June, 2021; originally announced June 2021.

Efficient conversion of orbital Hall current to spin current for spin-orbit torque switching

Authors: Soogil Lee, Min-Gu Kang, Dongwook Go, Dohyoung Kim, Jun-Ho Kang, Taekhyeon Lee, Geun-Hee Lee, Nyun Jong Lee, Sanghoon Kim, Kab-Jin Kim, Kyung-Jin Lee, Byong-Guk Park

Abstract: Spin Hall effect, an electric generation of spin current, allows for efficient control of magnetization. Recent theory revealed that orbital Hall effect creates orbital current, which can be much larger than spin Hall-induced spin current. However, orbital current cannot directly exert a torque on a ferromagnet, requiring a conversion process from orbital current to spin current. Here, we report t… ▽ More Spin Hall effect, an electric generation of spin current, allows for efficient control of magnetization. Recent theory revealed that orbital Hall effect creates orbital current, which can be much larger than spin Hall-induced spin current. However, orbital current cannot directly exert a torque on a ferromagnet, requiring a conversion process from orbital current to spin current. Here, we report two effective methods of the conversion through spin-orbit coupling engineering, which allows us to unambiguously demonstrate orbital-current-induced spin torque, or orbital Hall torque. We find that orbital Hall torque is greatly enhanced by introducing either a rare-earth ferromagnet Gd or a Pt interfacial layer with strong spin-orbit coupling in Cr/ferromagnet structures, indicating that the orbital current generated in Cr is efficiently converted into spin current in the Gd or Pt layer. Furthermore, we show that the orbital Hall torque can facilitate the reduction of switching current of perpendicular magnetization in spin-orbit-torque-based spintronic devices. △ Less

Submitted 3 May, 2022; v1 submitted 4 June, 2021; originally announced June 2021.

Comments: 19 pages, 3 figures

Journal ref: Communications Physics 4, 234 (2021)

Ultra-compact nonvolatile phase shifter based on electrically reprogrammable transparent phase change materials

Authors: Carlos Ríos, Qingyang Du, Yifei Zhang, Cosmin-Constantin Popescu, Mikhail Y. Shalaginov, Paul Miller, Christopher Roberts, Myungkoo Kang, Kathleen A. Richardson, Tian Gu, Steven A. Vitale, Juejun Hu

Abstract: Energy-efficient programmable photonic integrated circuits (PICs) are the cornerstone of on-chip classical and quantum optical technologies. Optical phase shifters constitute the fundamental building blocks which enable these programmable PICs. Thus far, carrier modulation and thermo-optical effect are the chosen phenomena for ultrafast and low-loss phase shifters, respectively; however, the state… ▽ More Energy-efficient programmable photonic integrated circuits (PICs) are the cornerstone of on-chip classical and quantum optical technologies. Optical phase shifters constitute the fundamental building blocks which enable these programmable PICs. Thus far, carrier modulation and thermo-optical effect are the chosen phenomena for ultrafast and low-loss phase shifters, respectively; however, the state and information they carry are lost once the power is turned off-they are volatile. The volatility not only compromises energy efficiency due to their demand for constant power supply, but also precludes them from emerging applications such as in-memory computing. To circumvent this limitation, we introduce a novel phase shifting mechanism that exploits the nonvolatile refractive index modulation upon structural phase transition of Sb$_{2}$Se$_{3}$, a bi-stable transparent phase change material. A zero-static power and electrically-driven phase shifter was realized on a foundry-processed silicon-on-insulator platform, featuring record phase modulation up to 0.09 $π$/$μ$m and a low insertion loss of 0.3 dB/$π$, which can be further improved upon streamlined design. We also pioneered a one-step partial amorphization scheme to enhance the speed and energy efficiency of PCM devices. A diverse cohort of programmable photonic devices were demonstrated based on the ultra-compact PCM phase shifter. △ Less

Submitted 21 March, 2022; v1 submitted 12 May, 2021; originally announced May 2021.

Comments: 15 pages with 6 figures and 1 table

Twofold van Hove singularity and origin of charge order in topological kagome superconductor CsV3Sb5

Authors: Mingu Kang, Shiang Fang, Jeong-Kyu Kim, Brenden R. Ortiz, Sae Hee Ryu, Jimin Kim, Jonggyu Yoo, Giorgio Sangiovanni, Domenico Di Sante, Byeong-Gyu Park, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Efthimios Kaxiras, Stephen D. Wilson, Jae-Hoon Park, Riccardo Comin

Abstract: The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently discovered family of topological kagome metals with a rich phenomenology of strongly correlated electronic phases including charge order and superconductivity. Understanding how the singularities inherent to the kagome electronic structure are linked to the observed many-body phases is a topic of great interest and relevance. H… ▽ More The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently discovered family of topological kagome metals with a rich phenomenology of strongly correlated electronic phases including charge order and superconductivity. Understanding how the singularities inherent to the kagome electronic structure are linked to the observed many-body phases is a topic of great interest and relevance. Here, we combine angle-resolved photoemission spectroscopy and density functional theory to reveal multiple kagome-derived van Hove singularities (vHs) coexisting near the Fermi level of CsV3Sb5 and analyze their contribution to electronic symmetry breaking. Intriguingly, the vHs in CsV3Sb5 have two distinct flavors - p-type and m-type - which originate from their pure and mixed sublattice characters, respectively. This twofold vHs is unique property of the kagome lattice, and its flavor critically determines the pairing symmetry and ground states emerging in AV3Sb5 series. We establish that, among the multiple vHs in CsV3Sb5, the m-type vHs of the dxz/dyz kagome band and the p-type vHs of the dxy/dx2-y2 kagome band cross the Fermi level to set the stage for electronic symmetry breaking. The former band exhibits pronounced Fermi surface nesting, while the latter contributes via higher-order vHs. Our work reveals the essential role of kagome-derived vHs for the collective phenomena realized in the AV3Sb5 family, paving the way to a deeper understanding of strongly correlated topological kagome systems. △ Less

Submitted 5 November, 2021; v1 submitted 4 May, 2021; originally announced May 2021.

Comments: Submitted to Nature Physics on May 28th, 2021. A revised version will appear in Nature Physics

High-speed ionic synaptic memory based on two-dimensional titanium carbide MXene

Authors: Armantas Melianas, Min-A Kang, Armin VahidMohammadi, Weiqian Tian, Yury Gogotsi, Alberto Salleo, Mahiar Max Hamedi

Abstract: Synaptic devices with linear high-speed switching can accelerate learning in artificial neural networks (ANNs) embodied in hardware. Conventional resistive memories however suffer from high write noise and asymmetric conductance tuning, preventing parallel programming of ANN arrays as needed to surpass conventional computing efficiency. Electrochemical random-access memories (ECRAMs), where resist… ▽ More Synaptic devices with linear high-speed switching can accelerate learning in artificial neural networks (ANNs) embodied in hardware. Conventional resistive memories however suffer from high write noise and asymmetric conductance tuning, preventing parallel programming of ANN arrays as needed to surpass conventional computing efficiency. Electrochemical random-access memories (ECRAMs), where resistive switching occurs by ion insertion into a redox-active channel address these challenges due to their linear switching and low noise. ECRAMs using two-dimensional (2D) materials and metal oxides suffer from slow ion kinetics, whereas organic ECRAMs enable high-speed operation but face significant challenges towards on-chip integration due to poor temperature stability of polymers. Here, we demonstrate ECRAMs using 2D titanium carbide (Ti3C2Tx) MXene that combines the high speed of organics and the integration compatibility of inorganic materials in a single high-performance device. Our ECRAMs combine the speed, linearity, write noise, switching energy and endurance metrics essential for parallel acceleration of ANNs, and importantly, they are stable after heat treatment needed for back-end-of-line integration with Si electronics. The high speed and performance of these ECRAMs introduces MXenes, a large family of 2D carbides and nitrides with more than 30 compositions synthesized to date, as very promising candidates for devices operating at the nexus of electrochemistry and electronics. △ Less

Submitted 23 April, 2021; v1 submitted 12 April, 2021; originally announced April 2021.

Comments: Fig. 4 now shows film stability up to 400C (replacing earlier 300C data). Section headings were also added to the main text

Ferromagnetic helical nodal line and Kane-Mele spin-orbit coupling in kagome metal Fe3Sn2

Authors: Shiang Fang, Linda Ye, Madhav Prasad Ghimire, Min Gu Kang, Junwei Liu, Liang Fu, Manuel Richter, Jeroen van den Brink, Efthimios Kaxiras, Riccardo Comin, Joseph G. Checkelsky

Abstract: The two-dimensional kagome lattice hosts Dirac fermions at its Brillouin zone corners K and K', analogous to the honeycomb lattice. In the density functional theory electronic structure of ferromagnetic kagome metal Fe$_3$Sn$_2$, without spin-orbit coupling we identify two energetically split helical nodal lines winding along $z$ in the vicinity of K and K' resulting from the trigonal stacking of… ▽ More The two-dimensional kagome lattice hosts Dirac fermions at its Brillouin zone corners K and K', analogous to the honeycomb lattice. In the density functional theory electronic structure of ferromagnetic kagome metal Fe$_3$Sn$_2$, without spin-orbit coupling we identify two energetically split helical nodal lines winding along $z$ in the vicinity of K and K' resulting from the trigonal stacking of the kagome layers. We find that hopping across A-A stacking introduces a layer splitting in energy while that across A-B stacking controls the momentum space amplitude of the helical nodal lines. The effect of spin-orbit coupling is found to resemble that of a Kane-Mele term, where the nodal lines can either be fully gapped to quasi-two-dimensional massive Dirac fermions, or remain gapless at discrete Weyl points depending on the ferromagnetic moment orientation. Aside from numerically establishing Fe$_3$Sn$_2$ as a model Dirac kagome metal, our results provide insights into materials design of topological phases from the lattice point of view, where paradigmatic low dimensional lattice models often find realizations in crystalline materials with three-dimensional stacking. △ Less

Submitted 15 March, 2021; originally announced March 2021.

Comments: 46 pages, 17 figures

Journal ref: Phys. Rev. B 105, 035107 (2022)