Showing 1–50 of 111 results for author: Kong, J

Resource-Efficient Bio-Molecular Docking on a NISQ-era Digital Quantum Computer

Authors: Tianqi Chen, Adrian M. Mak, Jianguo Li, Jian Feng Kong, Chandra Verma, Sebastian Maurer-Stroh

Abstract: Molecular docking is a vital computational task in drug discovery, wherein the objective is to efficiently identify optimal binding poses between a ligand and a target receptor protein. Due to the combinatorial explosion of possible binding configurations, docking of large and flexible molecules remains a computationally intensive problem, especially at scale. Early studies have revealed that the… ▽ More Molecular docking is a vital computational task in drug discovery, wherein the objective is to efficiently identify optimal binding poses between a ligand and a target receptor protein. Due to the combinatorial explosion of possible binding configurations, docking of large and flexible molecules remains a computationally intensive problem, especially at scale. Early studies have revealed that the molecular docking can be re-cast as a maximum vertex-weighted clique problem (MVWCP) problem on a compatibility graph to be solved classically. In this work, we proposed a hybrid quantum-classical approach for molecular docking leveraging the MVWCP formalism with a variational full-basis encoding (FBE) strategy, which enables efficient encoding of classical binary variables with Bloch sphere vectors. We further prove that a global minimizer of the FBE objective can always be chosen to be a pure product state, thereby providing a rigorous justification for its optimization using a unitary variational circuit. The molecular docking problem is first mapped to a cost Hamiltonian that is minimized within a variational framework, optimized via a randomized imaginary time evolution (ITE)-inspired warm start, and gradient-based techniques. Finally, we also executed the circuit on an IBM quantum computer, underlying the feasibility and of quantum-assisted optimization for structure-based drug design and point towards the broader utility of advanced encoding techniques in quantum optimization for computational biology. △ Less

Submitted 20 August, 2026; originally announced August 2026.

Comments: 22 pages, 8 figures; Comments are welcome

Grassmann tensor networks

Authors: Jian-Gang Kong, Jia-Ji Zhu, Z. Y. Xie

Abstract: Developing non-perturbative methods to reveal exotic properties of strongly correlated fermionic systems remains one of the most essential tasks of theoretical physics. Tensor network methods with Grassmann algebra offer powerful numerical tools for fermionic many-body systems in the coherent-state path-integral representation. Despite their vast potential for both condensed-matter and particle-ph… ▽ More Developing non-perturbative methods to reveal exotic properties of strongly correlated fermionic systems remains one of the most essential tasks of theoretical physics. Tensor network methods with Grassmann algebra offer powerful numerical tools for fermionic many-body systems in the coherent-state path-integral representation. Despite their vast potential for both condensed-matter and particle-physics communities, Grassmann tensor network methods are somewhat underexploited in practical simulations. In this work, we provide a detailed, self-contained introduction to Grassmann tensor network methods, from the basics of the Grassmann tensor operations to the Grassmannization of typical tensor network algorithms. Furthermore, the resulting Grassmann tensor network methods are validated in several interesting models in both particle physics and condensed matter physics. △ Less

Submitted 11 June, 2026; v1 submitted 12 May, 2026; originally announced May 2026.

Comments: 75 pages, 10 figures

Journal ref: Annals of Physics 491 (2026) 170511

Grassmann corner transfer-matrix renormalization group approach to one-dimensional fermionic models

Authors: Jian-Gang Kong, Zhi-Yuan Xie

Abstract: The strongly correlated fermions play a vital role in modern physics. For a given fermionic Hamiltonian system, the most widely used approach to explore the underlying physics is to study the wave function that incorporates Fermi-Dirac statistics, which can be obtained variationally by energy minimization or by imaginary-time evolution. In this work, we develop an accurate tensor network method fo… ▽ More The strongly correlated fermions play a vital role in modern physics. For a given fermionic Hamiltonian system, the most widely used approach to explore the underlying physics is to study the wave function that incorporates Fermi-Dirac statistics, which can be obtained variationally by energy minimization or by imaginary-time evolution. In this work, we develop an accurate tensor network method for one-dimensional interacting fermionic models based on the coherent-state path-integral representation of the fermionic partition function. Employing the coherent-state representation, the partition function is effectively represented as a (1+1)-dimensional anisotropic Grassmann-valued tensor network, and the Grassmann version of the corner transfer-matrix renormalization group algorithm is developed to contract the tensor network and evaluate physical quantities. We validate our method in the one-dimensional fermionic Hubbard model with a magnetic field, where the essential features of the phase diagram in the $(μ, B)$ plane are quantitatively captured. Our work offers a promising approach to interacting fermionic models within the framework of tensor networks. △ Less

Submitted 11 June, 2026; v1 submitted 7 April, 2026; originally announced April 2026.

Comments: 15 pages, 10 figures

Journal ref: Chin. Phys. B 35, 067101 (2026)

Phase diagram of the single-flavor Gross--Neveu--Wilson model from the Grassmann corner transfer matrix renormalization group

Authors: Jian-Gang Kong, Shinichiro Akiyama, Tao Shi, Z. Y. Xie

Abstract: We investigate the phase structure of the single-flavor Gross--Neveu model with Wilson fermions using the Grassmann corner transfer matrix renormalization group (CTMRG). The path integral is formulated as a two-dimensional Grassmann tensor network and approximately contracted by the Grassmann CTMRG algorithm. We investigate the phase diagram by varying the fermion mass and the four-fermion couplin… ▽ More We investigate the phase structure of the single-flavor Gross--Neveu model with Wilson fermions using the Grassmann corner transfer matrix renormalization group (CTMRG). The path integral is formulated as a two-dimensional Grassmann tensor network and approximately contracted by the Grassmann CTMRG algorithm. We investigate the phase diagram by varying the fermion mass and the four-fermion coupling, using the pseudoscalar condensate as an order parameter for the $\mathbb{Z}_{2}$ parity symmetry breaking phase. The universality classes of the phase boundaries are identified through the central charge $c$ obtained via scaling analysis of the entanglement entropy. Furthermore, we extract the quantity related to the entanglement spectrum from the converged CTMRG environments, allowing us to distinguish the topological insulator phase and the trivial phase. The resulting phase structure suggests that the Aoki phase is separated from the other phases by critical lines characterized by $c=1/2$, while the critical lines with $c=1$ separate the topological insulating and trivial phases. Our numerical results also indicate that the Aoki phase does not persist in the strong-coupling regime for the single-flavor theory. △ Less

Submitted 10 June, 2026; v1 submitted 25 February, 2026; originally announced February 2026.

Comments: 26 pages, 20 figures

Report number: UTHEP-816, UTCCS-P-174

Journal ref: Phys. Rev. D 113, 114504(2026)

Beyond Seamless: Unexpected Defective Merging in Single-Orientation Graphene

Authors: Zhien Wang, Jiangtao Wang, Diego Exposito, Andrey Krayev, Shih-Ming He, Xudong Zheng, Zachariah Hennighausen, Ivan Brihuega, Se-Young Jeong, Jing Kong

Abstract: Single-orientation stitching of graphene has emerged as the predominant method for growth of large-area, high-quality graphene films. Particularly noteworthy is graphene grown on single-crystalline Cu(111)/sapphire substrates, which exhibits exceptionally planar oriented stitching due to the atomically smooth substrate, facilitating the formation of continuous, high-quality graphene monolayer. The… ▽ More Single-orientation stitching of graphene has emerged as the predominant method for growth of large-area, high-quality graphene films. Particularly noteworthy is graphene grown on single-crystalline Cu(111)/sapphire substrates, which exhibits exceptionally planar oriented stitching due to the atomically smooth substrate, facilitating the formation of continuous, high-quality graphene monolayer. These single-orientation stitches have conventionally been regarded as seamless with negligible defect concentrations. In this report, we present experimental observations regarding graphene grown on single-crystalline Cu(111)/sapphire substrates. Among the graphene flakes with single-orientation, our findings reveal two major merging behaviors: one producing the expected seamless stitching, and another unexpectedly generating structural defects that create nanoscale pathways permitting water permeation. Notably, we identify a unique merging structure--overlapped junction, in which the edge of one graphene flake overlaps and lies atop the edge of another flake, rather than forming a continuous atomic stitch. This discovery challenges the conventional anticipation of single-orientation stitched graphene films as seamless single crystalline film, while offers unique perspective for graphene applications in molecular sieving, selective filtration membranes, and protective coatings. △ Less

Submitted 26 September, 2025; originally announced September 2025.

Comments: 17 pages 4 figures

Revealing Atomic-Scale Switching Pathways in van der Waals Ferroelectrics

Authors: Xinyan Li, Kenna Ashen, Chuqiao Shi, Nannan Mao, Saagar Kolachina, Kaiwen Yang, Tianyi Zhang, Sajid Husain, Ramamoorthy Ramesh, Jing Kong, Xiaofeng Qian, Yimo Han

Abstract: Two-dimensional van der Waals (vdW) materials hold the potential for ultra-scaled ferroelectric (FE) devices due to their silicon compatibility and robust polarization down to atomic scale. However, the inherently weak vdW interactions enable facile sliding between layers, introducing complexities beyond those encountered in conventional ferroelectric materials and presenting significant challenge… ▽ More Two-dimensional van der Waals (vdW) materials hold the potential for ultra-scaled ferroelectric (FE) devices due to their silicon compatibility and robust polarization down to atomic scale. However, the inherently weak vdW interactions enable facile sliding between layers, introducing complexities beyond those encountered in conventional ferroelectric materials and presenting significant challenges in uncovering intricate switching pathways. Here, we combine atomic-resolution imaging under in-situ electrical biasing conditions with first-principles calculations to unravel the atomic-scale switching mechanisms in SnSe, a vdW group-IV monochalcogenide. Our results uncover the coexistence of a consecutive 90 degrees switching pathway and a direct 180 degrees switching pathway from antiferroelectric (AFE) to FE order in this vdW system. Atomic-scale investigations and strain analysis reveal that the switching processes simultaneously induce interlayer sliding and compressive strain, while the lattice remains coherent despite the presence of multidomain structures. These findings elucidate vdW ferroelectric switching dynamics at atomic scale and lay the foundation for the rational design of 2D ferroelectric nanodevices. △ Less

Submitted 28 July, 2025; originally announced July 2025.

Zero-shot Autonomous Microscopy for Scalable and Intelligent Characterization of 2D Materials

Authors: Jingyun Yang, Ruoyan Avery Yin, Chi Jiang, Yuepeng Hu, Xiaokai Zhu, Xingjian Hu, Sutharsika Kumar, Xiao Wang, Xiaohua Zhai, Keran Rong, Yunyue Zhu, Tianyi Zhang, Zongyou Yin, Jing Kong, Neil Zhenqiang Gong, Zhichu Ren, Haozhe Wang

Abstract: Characterization of atomic-scale materials traditionally requires human experts with months to years of specialized training. Even for trained human operators, accurate and reliable characterization remains challenging when examining newly discovered materials such as two-dimensional (2D) structures. This bottleneck drives demand for fully autonomous experimentation systems capable of comprehendin… ▽ More Characterization of atomic-scale materials traditionally requires human experts with months to years of specialized training. Even for trained human operators, accurate and reliable characterization remains challenging when examining newly discovered materials such as two-dimensional (2D) structures. This bottleneck drives demand for fully autonomous experimentation systems capable of comprehending research objectives without requiring large training datasets. In this work, we present ATOMIC (Autonomous Technology for Optical Microscopy & Intelligent Characterization), an end-to-end framework that integrates foundation models to enable fully autonomous, zero-shot characterization of 2D materials. Our system integrates the vision foundation model (i.e., Segment Anything Model), large language models (i.e., ChatGPT), unsupervised clustering, and topological analysis to automate microscope control, sample scanning, image segmentation, and intelligent analysis through prompt engineering, eliminating the need for additional training. When analyzing typical MoS2 samples, our approach achieves 99.7% segmentation accuracy for single layer identification, which is equivalent to that of human experts. In addition, the integrated model is able to detect grain boundary slits that are challenging to identify with human eyes. Furthermore, the system retains robust accuracy despite variable conditions including defocus, color temperature fluctuations, and exposure variations. It is applicable to a broad spectrum of common 2D materials-including graphene, MoS2, WSe2, SnSe-regardless of whether they were fabricated via chemical vapor deposition or mechanical exfoliation. This work represents the implementation of foundation models to achieve autonomous analysis, establishing a scalable and data-efficient characterization paradigm that fundamentally transforms the approach to nanoscale materials research. △ Less

Submitted 14 April, 2025; originally announced April 2025.

Comments: 13 pages, 4 figures

Three-dimensional imaging of individual carbon atoms

Authors: Na Yeon Kim, Hanfeng Zhong, Jianhua Zhang, Colum M. O'Leary, Yuxuan Liao, Ji Zou, Haozhi Sha, Minh Pham, Weiyi Li, Yakun Yuan, Ji-Hoon Park, Dennis Kim, Huaidong Jiang, Jing Kong, Miaofang Chi, Jianwei Miao

Abstract: Carbon is fundamental to science and technology due to its diverse bonding configurations, structural versatility, and essential role in defining the mechanical, chemical, electronic, and quantum properties of materials. However, direct three-dimensional (3D) imaging of individual carbon atoms remains a long-standing challenge. Here, we use twisted bilayer graphene (TBG) as a model system and demo… ▽ More Carbon is fundamental to science and technology due to its diverse bonding configurations, structural versatility, and essential role in defining the mechanical, chemical, electronic, and quantum properties of materials. However, direct three-dimensional (3D) imaging of individual carbon atoms remains a long-standing challenge. Here, we use twisted bilayer graphene (TBG) as a model system and demonstrate ptychographic atomic electron tomography (pAET) for determining the 3D atomic coordinates of individual carbon atoms with a precision of 0.11 angstrom. The resulting 3D atomic model uncovers chiral lattice distortions driven by van der Waals interactions that exhibit meron-like and skyrmion-like structural textures. These findings provide direct insight into the interplay between 3D chiral lattice deformation and electronic properties in moire-engineered carbon systems. Beyond TBG, pAET offers a versatile approach for 3D atomic-scale imaging of carbon-based and other light-element materials that are central to advances in physics, chemistry, materials science, and nanotechnology. △ Less

Submitted 3 January, 2026; v1 submitted 10 April, 2025; originally announced April 2025.

Synthesis-related nanoscale defects in Mo-based Janus monolayers revealed by cross-correlated AFM and TERS imaging

Authors: Tianyi Zhang, Andrey Krayev, Tilo H. Yang, Nannan Mao, Lauren Hoang, Zhien Wang, Hongwei Liu, Yu-Ren Peng, Yunyue Zhu, Eleonora Isotta, Maria E. Kira, Ariete Righi, Marcos A. Pimenta, Yu-Lun Chueh, Eric Pop, Andrew J. Mannix, Jing Kong

Abstract: Two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) are promising candidates for various applications in non-linear optics, energy harvesting, and catalysis. These materials are usually synthesized via chemical conversion of pristine TMDs. Nanometer-scale characterization of the obtained Janus materials' morphology and local composition is crucial for both the synthesis optimization… ▽ More Two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) are promising candidates for various applications in non-linear optics, energy harvesting, and catalysis. These materials are usually synthesized via chemical conversion of pristine TMDs. Nanometer-scale characterization of the obtained Janus materials' morphology and local composition is crucial for both the synthesis optimization and the future device applications. In this work, we present a cross-correlated atomic force microscopy (AFM) and tip-enhanced Raman spectroscopy (TERS) study of Janus $\mathrm{Mo}_{\mathrm{Se}}^{\mathrm{S}}$ and Janus $\mathrm{Mo}_{\mathrm{S}}^{\mathrm{Se}}$ monolayers synthesized by the hydrogen plasma-assisted chemical conversion of $\mathrm{MoSe}_2$ and $\mathrm{MoS}_2$, respectively. We demonstrate how the choice of the growth substrate and the starting TMD affects the morphology of the resulting Janus material. Furthermore, by employing TERS imaging, we demonstrate the presence of nanoscale islands (~20 nm across) of $\mathrm{MoSe}_2$-$\mathrm{Mo}_{\mathrm{Se}}^{\mathrm{S}}$ ($\mathrm{MoS}_2$-$\mathrm{Mo}_{\mathrm{S}}^{\mathrm{Se}}$) vertical heterostructures originating from the bilayer nanoislands in the precursor monolayer crystals. The understanding of the origins of nanoscale defects in Janus TMDs revealed in our study can help with further optimization of the Janus conversion process towards uniform and wrinkle-/crack-free Janus materials. Moreover, our work shows that cross-correlated AFM and TERS imaging is a powerful and accessible method for studying nanoscale composition and defects in Janus TMD monolayers. △ Less

Submitted 28 March, 2025; originally announced March 2025.

Non-perturbative cathodoluminescence microscopy of beam-sensitive materials

Authors: Malcolm Bogroff, Gabriel Cowley, Ariel Nicastro, David Levy, Yueh-Chun Wu, Nannan Mao, Tilo H. Yang, Tianyi Zhang, Jing Kong, Rama Vasudevan, Kyle P. Kelley, Benjamin J. Lawrie

Abstract: Cathodoluminescence microscopy is now a well-established and powerful tool for probing the photonic properties of nanoscale materials, but in many cases, nanophotonic materials are easily damaged by the electron-beam doses necessary to achieve reasonable cathodoluminescence signal-to-noise ratios. Two-dimensional materials have proven particularly susceptible to beam-induced modifications, yieldin… ▽ More Cathodoluminescence microscopy is now a well-established and powerful tool for probing the photonic properties of nanoscale materials, but in many cases, nanophotonic materials are easily damaged by the electron-beam doses necessary to achieve reasonable cathodoluminescence signal-to-noise ratios. Two-dimensional materials have proven particularly susceptible to beam-induced modifications, yielding both obstacles to high spatial-resolution measurement and opportunities for beam-induced patterning of quantum photonic systems. Here pan-sharpening techniques are applied to cathodoluminescence microscopy in order to address these challenges and experimentally demonstrate the promise of pan-sharpening for minimally-perturbative high-spatial-resolution spectrum imaging of beam-sensitive materials. △ Less

Submitted 15 December, 2024; originally announced December 2024.

Classical Shadows with Improved Median-of-Means Estimation

Authors: Winston Fu, Dax Enshan Koh, Siong Thye Goh, Jian Feng Kong

Abstract: The classical shadows protocol, introduced by Huang et al. [Nat. Phys. 16, 1050 (2020)], makes use of the median-of-means (MoM) estimator to efficiently estimate the expectation values of $M$ observables with failure probability $δ$ using only $\mathcal{O}(\log(M/δ))$ measurements. In their analysis, Huang et al. used loose constants in their asymptotic performance bounds for simplicity. However,… ▽ More The classical shadows protocol, introduced by Huang et al. [Nat. Phys. 16, 1050 (2020)], makes use of the median-of-means (MoM) estimator to efficiently estimate the expectation values of $M$ observables with failure probability $δ$ using only $\mathcal{O}(\log(M/δ))$ measurements. In their analysis, Huang et al. used loose constants in their asymptotic performance bounds for simplicity. However, the specific values of these constants can significantly affect the number of shots used in practical implementations. To address this, we studied a modified MoM estimator proposed by Minsker [PMLR 195, 5925 (2023)] that uses optimal constants and involves a U-statistic over the data set. For efficient estimation, we implemented two types of incomplete U-statistics estimators, the first based on random sampling and the second based on cyclically permuted sampling. We compared the performance of the original and modified estimators when used with the classical shadows protocol with single-qubit Clifford unitaries (Pauli measurements) for an Ising spin chain, and global Clifford unitaries (Clifford measurements) for the Greenberger-Horne-Zeilinger (GHZ) state. While the original estimator outperformed the modified estimators for Pauli measurements, the modified estimators showed improved performance over the original estimator for Clifford measurements. Our findings highlight the importance of tailoring estimators to specific measurement settings to optimize the performance of the classical shadows protocol in practical applications. △ Less

Submitted 4 December, 2024; originally announced December 2024.

Comments: 15 pages, 13 figures

Journal ref: Quantum Science and Technology 10, 035043 (2025)

Protected chaos in a topological lattice

Authors: Haydar Sahin, Hakan Akgün, Zhuo Bin Siu, S. M. Rafi-Ul-Islam, Jian Feng Kong, Mansoor B. A. Jalil, Ching Hua Lee

Abstract: The erratic nature of chaotic behavior is thought to erode the stability of periodic behavior, including topological oscillations. However, we discover that in the presence of chaos, non-trivial topology not only endures but also provides robust protection to chaotic dynamics within a topological lattice hosting non-linear oscillators. Despite the difficulty in defining topological invariants in n… ▽ More The erratic nature of chaotic behavior is thought to erode the stability of periodic behavior, including topological oscillations. However, we discover that in the presence of chaos, non-trivial topology not only endures but also provides robust protection to chaotic dynamics within a topological lattice hosting non-linear oscillators. Despite the difficulty in defining topological invariants in non-linear settings, non-trivial topological robustness still persists in the parametric state of chaotic boundary oscillations. We demonstrate this interplay between chaos and topology by incorporating chaotic Chua's circuits into a topological Su-Schrieffer-Heeger (SSH) circuit. By extrapolating from the linear limit to deep into the non-linear regime, we find that distinctive correlations in the bulk and edge scroll dynamics effectively capture the topological origin of the protected chaos. Our findings suggest that topologically protected chaos can be robustly achieved across a broad spectrum of periodically-driven systems, thereby offering new avenues for the design of resilient and adaptable non-linear networks. △ Less

Submitted 2 July, 2025; v1 submitted 11 November, 2024; originally announced November 2024.

Comments: 24 figures, 43 pages

Journal ref: Adv. Sci. 2025, e03216

Enhanced Radiation Hardness of InAs/GaAs Quantum Dot Lasers for Space Communication

Authors: Manyang Li, Jianan Duan, Zhiyong Jin, Shujie Pan, Wenkang Zhan, Jinpeng Chen, Jinling Yu, Xiaotian Cheng, Zhibo Ni, Chaoyuan Jin, Tien Khee Ng, Jinxia Kong, Xiaochuan Xu, Yong Yao, Bo Xu, Siming Chen, Zhanguo Wang, Chao Zhao

Abstract: Semiconductor lasers have great potential for space laser communication. However, excessive radiation in space can cause laser failure. In principle, quantum dot (QD) lasers are more radiation-resistant than traditional semiconductor lasers because of their superior carrier confinement and smaller active regions. However, the multifaceted nature of radiation effects on QDs resulted in ongoing cont… ▽ More Semiconductor lasers have great potential for space laser communication. However, excessive radiation in space can cause laser failure. In principle, quantum dot (QD) lasers are more radiation-resistant than traditional semiconductor lasers because of their superior carrier confinement and smaller active regions. However, the multifaceted nature of radiation effects on QDs resulted in ongoing controversies. Comprehensive testing under simulated space conditions is also necessary to validate their performance. In this work, we conducted radiation tests on various In(Ga)As/GaAs QD and quantum well (QW) materials and devices. Our results revealed that InAs/GaAs QDs with filling factors greater than 50% exhibit greater radiation hardness than those below 50%. Furthermore, most InAs/GaAs QDs showed superior radiation resistance compared to InGaAs/GaAs QW when exposed to low proton fluences of 1E11 and 1E12 cm-2, resulting from radiation-induced defects. The linewidth enhancement factor (LEF) of well-designed QD lasers remains remarkably stable and close to zero, even under proton irradiation at a maximum fluence of 7E13 cm-2, owing to their inherent insensitivity to irradiation-induced defects. These QD lasers demonstrate an exceptional average relative intensity noise (RIN) level of -162 dB/Hz, with only a 1 dB/Hz increase in RIN observed at the highest fluence, indicating outstanding stability. Furthermore, the lasers exhibit remarkable robustness against optical feedback, sustaining stable performance even under a feedback strength as high as -3.1 dB. These results highlight the significant potential of QD lasers for space laser communication applications, where high reliability and resilience to radiation and environmental perturbations are critical. △ Less

Submitted 26 December, 2024; v1 submitted 30 July, 2024; originally announced July 2024.

Comments: 5 figures

Anomalous properties of spark plasma sintered boron nitride solids

Authors: Abhijit Biswas, Peter Serles, Gustavo A. Alvarez, Jesse Schimpf, Michel Hache, Jonathan Kong, Pedro Guerra Demingos, Bo Yuan, Tymofii S. Pieshkov, Chenxi Li, Anand B. Puthirath, Bin Gao, Tia Gray, Xiang Zhang, Jishnu Murukeshan, Robert Vajtai, Pengcheng Dai, Chandra Veer Singh, Jane Howe, Yu Zou, Lane W. Martin, James Patrick Clancy, Zhiting Tian, Tobin Filleter, Pulickel M. Ajayan

Abstract: Hexagonal boron nitride (h-BN) is a brittle ceramic with a layered structure, however, recent experiments have suggested that inter-layer structural engineering could be key to new structural and functional properties. Here we report the scalable bulk synthesis of high-density crystalline h-BN solids, by using high-temperature spark plasma sintering (SPS) of h-BN powders, which show high values of… ▽ More Hexagonal boron nitride (h-BN) is a brittle ceramic with a layered structure, however, recent experiments have suggested that inter-layer structural engineering could be key to new structural and functional properties. Here we report the scalable bulk synthesis of high-density crystalline h-BN solids, by using high-temperature spark plasma sintering (SPS) of h-BN powders, which show high values of mechanical strength, ductility, dielectric constant, thermal conductivity, and exceptional neutron radiation shielding capability. Through exhaustive characterizations we reveal that SPS induces non-basal plane crystallinity, twisting of layers, and facilitates inter-grain fusion with a high degree of in-plane alignment across macroscale dimensions, resulting in near-theoretical density and improved properties. Our findings highlight the importance of material design, via new approaches such as layer twisting and interlayer interconnections, to create novel ceramics with properties that could go beyond their intrinsic limits. △ Less

Submitted 20 May, 2025; v1 submitted 9 May, 2024; originally announced May 2024.

Comments: Authors revised version, 46 pages, 4 Main figures, 16 SI figures

Journal ref: Materials Today 86, 42-51 (2025)

Remote-contact catalysis for target-diameter semiconducting carbon nanotube array

Authors: Jiangtao Wang, Xudong Zheng, Gregory Pitner, Xiang Ji, Tianyi Zhang, Aijia Yao, Jiadi Zhu, Tomás Palacios, Lain-Jong Li, Han Wang, Jing Kong

Abstract: Electrostatic catalysis has been an exciting development in chemical synthesis (beyond enzymes catalysis) in recent years, boosting reaction rates and selectively producing certain reaction products. Most of the studies to date have been focused on using external electric field (EEF) to rearrange the charge distribution in small molecule reactions such as Diels-Alder addition, carbene reaction, et… ▽ More Electrostatic catalysis has been an exciting development in chemical synthesis (beyond enzymes catalysis) in recent years, boosting reaction rates and selectively producing certain reaction products. Most of the studies to date have been focused on using external electric field (EEF) to rearrange the charge distribution in small molecule reactions such as Diels-Alder addition, carbene reaction, etc. However, in order for these EEFs to be effective, a field on the order of 1 V/nm (10 MV/cm) is required, and the direction of the EEF has to be aligned with the reaction axis. Such a large and oriented EEF will be challenging for large-scale implementation, or materials growth with multiple reaction axis or steps. Here, we demonstrate that the energy band at the tip of an individual single-walled carbon nanotube (SWCNT) can be spontaneously shifted in a high-permittivity growth environment, with its other end in contact with a low-work function electrode (e.g., hafnium carbide or titanium carbide). By adjusting the Fermi level at a point where there is a substantial disparity in the density of states (DOS) between semiconducting (s-) and metallic (m-) SWCNTs, we achieve effective electrostatic catalysis for s-SWCNT growth assisted by a weak EEF perturbation (200V/cm). This approach enables the production of high-purity (99.92%) s-SWCNT horizontal arrays with narrow diameter distribution (0.95+-0.04 nm), targeting the requirement of advanced SWCNT-based electronics for future computing. These findings highlight the potential of electrostatic catalysis in precise materials growth, especially for s-SWCNTs, and pave the way for the development of advanced SWCNT-based electronics. △ Less

Submitted 3 April, 2024; originally announced April 2024.

Comments: 4 figures, 23 pages

Expanding the Pressure Frontier in Grüneisen Parameter Measurement: Study of Sodium Chloride

Authors: Jun Kong, Kaiyuan Shi, Xingbang Dong, Xiao Dong, Xin Zhang, Jiaqing Zhang, Lei Su, Guoqiang Yang

Abstract: The Grüneisen parameter (γ) is crucial for determining many thermal properties, including the anharmonic effect, thermostatistics, and equation of state (EOS) of materials. However, the isentropic adiabatic compression conditions required to measure the Grüneisen parameter under high pressure are difficult to achieve. Thus, direct experimental Grüneisen parameter data in a wide range of pressures… ▽ More The Grüneisen parameter (γ) is crucial for determining many thermal properties, including the anharmonic effect, thermostatistics, and equation of state (EOS) of materials. However, the isentropic adiabatic compression conditions required to measure the Grüneisen parameter under high pressure are difficult to achieve. Thus, direct experimental Grüneisen parameter data in a wide range of pressures is sparse. In this work, we developed a new device that can apply pressure (up to tens of GPa) with an extremely short time about 0.5 ms, confidently achieving isentropic adiabatic compression. Then, we applied our new technique to sodium chloride and measured its Grüneisen parameter, which conforms to previous theoretical predictions. According to our obtained sodium chloride Grüneisen parameters, the calculated Hugoniot curve of the NaCl B1 phase appears up to 20 GPa and 960 K, which compares very well with the shock compression experiment data by Fritz et al. and other calculation works. Our results suggest that this new method can reliably measure the Grüneisen parameter of even more materials, which is significant for researching the equation of state in substances. △ Less

Submitted 21 December, 2023; originally announced December 2023.

Electrical Tuning of Neutral and Charged Excitons with 1-nm Gate

Authors: Jawaher Almutlaq, Jiangtao Wang, Linsen Li, Chao Li, Tong Dang, Vladimir Bulović, Jing Kong, Dirk Englund

Abstract: Electrical control of individual spins and photons in solids is key for quantum technologies, but scaling down to small, static systems remains challenging. Here, we demonstrate nanoscale electrical tuning of neutral and charged excitons in monolayer WSe2 using 1-nm carbon nanotube gates. Electrostatic simulations reveal a confinement radius below 15 nm, reaching the exciton Bohr radius limit for… ▽ More Electrical control of individual spins and photons in solids is key for quantum technologies, but scaling down to small, static systems remains challenging. Here, we demonstrate nanoscale electrical tuning of neutral and charged excitons in monolayer WSe2 using 1-nm carbon nanotube gates. Electrostatic simulations reveal a confinement radius below 15 nm, reaching the exciton Bohr radius limit for few-layer dielectric spacing. In situ photoluminescence spectroscopy shows gate-controlled conversion between neutral excitons, negatively charged trions, and biexcitons at 4 K. Important for quantum information processing applications, our measurements indicate gating of a local 2D electron gas in the WSe2 layer, coupled to photons via trion transitions with binding energies exceeding 20 meV. The ability to deterministically tune and address quantum emitters using nanoscale gates provides a pathway towards large-scale quantum optoelectronic circuits and spin-photon interfaces for quantum networking. △ Less

Submitted 30 October, 2023; originally announced October 2023.

Comments: 21 pages, 11 figures

Precise Fermi-level engineering in a topological Weyl semimetal via fast ion implantation

Authors: Manasi Mandal, Abhijatmedhi Chotrattanapituk, Kevin Woller, Haowei Xu, Nannan Mao, Ryotaro Okabe, Artittaya Boonkird, Thanh Nguyen, Nathan C. Drucker, Takashi Momiki, Ju Li, Jing Kong, Mingda Li

Abstract: The precise controllability of the Fermi level is a critical aspect of quantum materials. For topological Weyl semimetals, there is a pressing need to fine-tune the Fermi level to the Weyl nodes and unlock exotic electronic and optoelectronic effects associated with the divergent Berry curvature. However, in contrast to 2D materials, where the Fermi level can be controlled through various techniqu… ▽ More The precise controllability of the Fermi level is a critical aspect of quantum materials. For topological Weyl semimetals, there is a pressing need to fine-tune the Fermi level to the Weyl nodes and unlock exotic electronic and optoelectronic effects associated with the divergent Berry curvature. However, in contrast to 2D materials, where the Fermi level can be controlled through various techniques, the situation for bulk crystals beyond laborious chemical doping poses significant challenges. Here, we report the meV-level ultra-fine-tuning of the Fermi level of bulk topological Weyl semimetal TaP using accelerator-based high-energy hydrogen implantation and theory-driven planning. By calculating the desired carrier density and controlling the accelerator profiles, the Fermi level can be fine-tuned from 5 meV to only $\sim$0.5 meV (DFT calculations) away from the Weyl nodes. The Weyl nodes are preserved, while the carrier mobility is largely retained. Our work demonstrates the viability of this generic approach to tune the Fermi level in semimetal systems and could serve to achieve property fine-tuning for other bulk quantum materials with ultrahigh precision. △ Less

Submitted 11 October, 2023; originally announced October 2023.

Signatures of pressure-enhanced helimagnetic order in van der Waals multiferroic NiI$_2$

Authors: Connor A. Occhialini, Luiz G. P. Martins, Qian Song, Jesse S. Smith, Jesse Kapeghian, Danila Amoroso, Joshua J. Sanchez, Paolo Barone, Bertrand Dupé, Matthieu j. Verstraete, Jing Kong, Antia S. Botana, Riccardo Comin

Abstract: The van der Waals (vdW) type-II multiferroic NiI$_2$ has emerged as a candidate for exploring non-collinear magnetism and magnetoelectric effects in the 2D limit. Frustrated intralayer exchange interactions on a triangular lattice result in a helimagnetic ground state, with spin-induced improper ferroelectricity stabilized by the interlayer interactions. Here we investigate the magnetic and struct… ▽ More The van der Waals (vdW) type-II multiferroic NiI$_2$ has emerged as a candidate for exploring non-collinear magnetism and magnetoelectric effects in the 2D limit. Frustrated intralayer exchange interactions on a triangular lattice result in a helimagnetic ground state, with spin-induced improper ferroelectricity stabilized by the interlayer interactions. Here we investigate the magnetic and structural phase transitions in bulk NiI$_2$, using high-pressure Raman spectroscopy, optical linear dichroism, and x-ray diffraction. We obtain evidence for a significant pressure enhancement of the antiferromagnetic and helimagnetic transition temperatures, at rates of $\sim15.3/14.4$ K/GPa, respectively. These enhancements are attributed to a cooperative effect of pressure-enhanced interlayer and third-nearest-neighbor intralayer exchange. These results reveal a general path for obtaining high-temperature type-II multiferroicity via high pressures in vdW materials. △ Less

Submitted 20 June, 2023; originally announced June 2023.

Effects of Pressure on the Electronic and Magnetic Properties of Bulk NiI$_{2}$

Authors: Jesse Kapeghian, Danila Amoroso, Connor A. Occhialini, Luiz G. P. Martins, Qian Song, Jesse S. Smith, Joshua J. Sanchez, Jing Kong, Riccardo Comin, Paolo Barone, Bertrand Dupé, Matthieu J. Verstraete, Antia S. Botana

Abstract: Transition metal dihalides have recently garnered interest in the context of two-dimensional van der Waals magnets as their underlying geometrically frustrated triangular lattice leads to interesting competing exchange interactions. In particular, NiI$_{2}$ is a magnetic semiconductor that has been long known for its exotic helimagnetism in the bulk. Recent experiments have shown that the helimagn… ▽ More Transition metal dihalides have recently garnered interest in the context of two-dimensional van der Waals magnets as their underlying geometrically frustrated triangular lattice leads to interesting competing exchange interactions. In particular, NiI$_{2}$ is a magnetic semiconductor that has been long known for its exotic helimagnetism in the bulk. Recent experiments have shown that the helimagnetic state survives down to the monolayer limit with a layer-dependent magnetic transition temperature that suggests a relevant role of the interlayer coupling. Here, we explore the effects of hydrostatic pressure as a means to enhance this interlayer exchange and ultimately tune the electronic and magnetic response of NiI$_{2}$. We study first the evolution of the structural parameters as a function of external pressure using first-principles calculations combined with x-ray diffraction measurements. We then examine the evolution of the electronic structure and magnetic exchange interactions via first-principles calculations and Monte Carlo simulations. We find that the leading interlayer coupling is an antiferromagnetic second-nearest neighbor interaction that increases monotonically with pressure. The ratio between isotropic third- and first-nearest neighbor intralayer exchanges, which controls the magnetic frustration and determines the magnetic propagation vector $\mathbf{q}$ of the helimagnetic ground state, is also enhanced by pressure. As a consequence, our Monte Carlo simulations show a monotonic increase in the magnetic transition temperature, indicating that pressure is an effective means to tune the magnetic response of NiI$_{2}$. △ Less

Submitted 7 June, 2023; originally announced June 2023.

Quantifying the magnetic interactions governing chiral spin textures using deep neural networks

Authors: Jian Feng Kong, Yuhua Ren, M. S. Nicholas Tey, Pin Ho, Khoong Hong Khoo, Xiaoye Chen, Anjan Soumyanarayanan

Abstract: The interplay of magnetic interactions in chiral multilayer films gives rise to nanoscale topological spin textures, which form attractive elements for next-generation computing. Quantifying these interactions requires several specialized, time-consuming, and resource-intensive experimental techniques. Imaging of ambient domain configurations presents a promising avenue for high-throughput extract… ▽ More The interplay of magnetic interactions in chiral multilayer films gives rise to nanoscale topological spin textures, which form attractive elements for next-generation computing. Quantifying these interactions requires several specialized, time-consuming, and resource-intensive experimental techniques. Imaging of ambient domain configurations presents a promising avenue for high-throughput extraction of the parent magnetic interactions. Here we present a machine learning-based approach to determine the key interactions -- symmetric exchange, chiral exchange, and anisotropy -- governing chiral domain phenomenology in multilayers. Our convolutional neural network model, trained and validated on over 10,000 domain images, achieved $R^2 > 0.85$ in predicting the parameters and independently learned physical interdependencies between them. When applied to microscopy data acquired across samples, our model-predicted parameter trends are consistent with independent experimental measurements. These results establish ML-driven techniques as valuable, high-throughput complements to conventional determination of magnetic interactions, and serve to accelerate materials and device development for nanoscale electronics. △ Less

Submitted 4 May, 2023; originally announced May 2023.

Comments: 7 pages, 6 figures

Journal ref: ACS Applied Materials and Interfaces (2024) 16, 1025-32

Giant room-temperature nonlinearities from a monolayer Janus topological semiconductor

Authors: Jiaojian Shi, Haowei Xu, Christian Heide, Changan HuangFu, Chenyi Xia, Felipe de Quesada, Hongzhi Shen, Tianyi Zhang, Leo Yu, Amalya Johnson, Fang Liu, Enzheng Shi, Liying Jiao, Tony Heinz, Shambhu Ghimire, Ju Li, Jing Kong, Yunfan Guo, Aaron M. Lindenberg

Abstract: Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and topological properties, such as Berry curvature and the quantum metric tensor, have stimulated great interest. Here, we report giant room-temperature nonlinearities in an emergent non-centrosymmetric two-di… ▽ More Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and topological properties, such as Berry curvature and the quantum metric tensor, have stimulated great interest. Here, we report giant room-temperature nonlinearities in an emergent non-centrosymmetric two-dimensional topological material, the Janus transition metal dichalcogenides in the 1T' phase, which are synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently reveal orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities of 1T' MoSSe (e.g., > 50 times higher than 2H MoS$_2$ for 18th order harmonic generation; > 20 times higher than 2H MoS$_2$ for terahertz emission). It is elucidated that such colossal nonlinear optical responses come from topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and preludes the applications of topological materials in optoelectronics down to the monolayer limit. This two-dimensional form of topological materials also constitute a unique platform for examining origin of the anomalous high-harmonic generation, with potential applications as building blocks for scalable attosecond sources. △ Less

Submitted 3 April, 2023; originally announced April 2023.

Comments: 22 pages, 4 figures

Van der Waals device integration beyond the limits of van der Waals forces via adhesive matrix transfer

Authors: Peter F. Satterthwaite, Weikun Zhu, Patricia Jastrzebska-Perfect, Melbourne Tang, Hongze Gao, Hikari Kitadai, Ang-Yu Lu, Qishuo Tan, Shin-Yi Tang, Yu-Lun Chueh, Chia-Nung Kuo, Chin Shan Lue, Jing Kong, Xi Ling, Farnaz Niroui

Abstract: Pristine van der Waals (vdW) interfaces between two-dimensional (2D) and other materials are core to emerging optical and electronic devices. Their direct fabrication is, however, challenged as the vdW forces are weak and cannot be tuned to accommodate integration of arbitrary layers without solvents, sacrificial-layers or high-temperatures, steps that can introduce damage. To address these limita… ▽ More Pristine van der Waals (vdW) interfaces between two-dimensional (2D) and other materials are core to emerging optical and electronic devices. Their direct fabrication is, however, challenged as the vdW forces are weak and cannot be tuned to accommodate integration of arbitrary layers without solvents, sacrificial-layers or high-temperatures, steps that can introduce damage. To address these limitations, we introduce a single-step 2D material-to-device integration approach in which forces promoting transfer are decoupled from the vdW forces at the interface of interest. We use this adhesive matrix transfer to demonstrate conventionally-forbidden direct integration of diverse 2D materials (MoS2, WSe2, PtS2, GaS) with dielectrics (SiO2, Al2O3), and scalable, aligned heterostructure formation, both foundational to device development. We then demonstrate a single-step integration of monolayer-MoS2 into arrays of transistors. With no exposure to polymers or solvents, clean interfaces and pristine surfaces are preserved, which can be further engineered to demonstrate both n- and p-type behavior. Beyond serving as a platform to probe the intrinsic properties of sensitive nanomaterials without the influence of processing steps, our technique allows efficient formation of unconventional device form-factors, with an example of flexible transistors demonstrated. △ Less

Submitted 12 February, 2023; originally announced February 2023.

Synergistic Photon Management and Strain-Induced Band Gap Engineering of Two-Dimensional MoS2 Using Semimetal Composite Nanostructures

Authors: Xiaoxue Gao, Sidan Fu, Tao Fang, Xiaobai Yu, Haozhe Wang, Qingqing Ji, Jing Kong, Xiaoxin Wang, Jifeng Liu

Abstract: 2D MoS2 attracts increasing attention for its application in flexible electronics and photonic devices. For 2D material optoelectronic devices, light absorption of the molecularly thin 2D absorber would be one of the key limiting factors in device efficiency, and conventional photon management techniques are not necessarily compatible with them. In this paper, we show two semimetal composite nanos… ▽ More 2D MoS2 attracts increasing attention for its application in flexible electronics and photonic devices. For 2D material optoelectronic devices, light absorption of the molecularly thin 2D absorber would be one of the key limiting factors in device efficiency, and conventional photon management techniques are not necessarily compatible with them. In this paper, we show two semimetal composite nanostructures for synergistic photon management and strain-induced band gap engineering of 2D MoS2: (1) pseudo-periodic Sn nanodots, (2) conductive SnOx (x<1) core-shell nanoneedle structures. Without sophisticated nanolithography, both nanostructures are self-assembled from physical vapor deposition. 2D MoS2 achieves up to >15x enhancement in absorption at λ=650-950 nm under Sn nanodots, and 20-30x at λ=700-900 nm under SnOx (x<1) nanoneedles, both spanning from visible to near infrared regime. Enhanced absorption in MoS2 results from strong near field enhancement and reduced MoS2 band gap due to the tensile strain induced by the Sn nanostructures, as confirmed by Raman and photoluminescence spectroscopy. Especially, we demonstrate that up to 3.5% biaxial tensile strain is introduced to 2D MoS2 using conductive nanoneedle-structured SnOx (x<1), which reduces the band gap by ~0.35 eV to further enhance light absorption at longer wavelengths. To the best of our knowledge, this is the first demonstration of a synergistic triple-functional photon management, stressor, and conductive electrode layer on 2D MoS2. Such synergistic photon management and band gap engineering approach for extended spectral response can be further applied to other 2D materials for future 2D photonic devices. △ Less

Submitted 3 January, 2023; originally announced January 2023.

Impedance responses and size-dependent resonances in topolectrical circuits via the method of images

Authors: Haydar Sahin, Zhuo Bin Siu, S. M. Rafi-Ul-Islam, Jian Feng Kong, Mansoor B. A. Jalil, Ching Hua Lee

Abstract: Resonances in an electric circuit occur when capacitive and inductive components are present together. Such resonances appear in admittance measurements depending on the circuit's parameters and the driving AC frequency. In this study, we analyze the impedance characteristics of nontrivial topolectrical circuits such as one- and two-dimensional Su-Schrieffer-Heeger circuits and reveal that size-de… ▽ More Resonances in an electric circuit occur when capacitive and inductive components are present together. Such resonances appear in admittance measurements depending on the circuit's parameters and the driving AC frequency. In this study, we analyze the impedance characteristics of nontrivial topolectrical circuits such as one- and two-dimensional Su-Schrieffer-Heeger circuits and reveal that size-dependent anomalous impedance resonances inevitably arise in finite $LC$ circuits. Through the \textit{method of images}, we study how resonance modes in a multi-dimensional circuit array can be nontrivially modified by the reflection and interference of current from the structure and boundaries of the lattice. We derive analytic expressions for the impedance across two corner nodes of various lattice networks with homogeneous and heterogeneous circuit elements. We also derive the irregular dependency of the impedance resonance on the lattice size, and provide integral and dimensionally-reduced expressions for the impedance in three dimensions and above. △ Less

Submitted 18 August, 2023; v1 submitted 12 December, 2022; originally announced December 2022.

Comments: 24 pages, 10 figures

Journal ref: PhysRevB.107.245114 (2023)

Self-supervised Representations and Node Embedding Graph Neural Networks for Accurate and Multi-scale Analysis of Materials

Authors: Jian-Gang Kong, Ke-Lin Zhao, Jian Li, Qing-Xu Li, Yu Liu, Rui Zhang, Jia-Ji Zhu, Kai Chang

Abstract: Supervised machine learning algorithms, such as graph neural networks (GNN), have successfully predicted material properties. However, the superior performance of GNN usually relies on end-to-end learning on large material datasets, which may lose the physical insight of multi-scale information about materials. And the process of labeling data consumes many resources and inevitably introduces erro… ▽ More Supervised machine learning algorithms, such as graph neural networks (GNN), have successfully predicted material properties. However, the superior performance of GNN usually relies on end-to-end learning on large material datasets, which may lose the physical insight of multi-scale information about materials. And the process of labeling data consumes many resources and inevitably introduces errors, which constrains the accuracy of prediction. We propose to train the GNN model by self-supervised learning on the node and edge information of the crystal graph. Compared with the popular manually constructed material descriptors, the self-supervised atomic representation can reach better prediction performance on material properties. Furthermore, it may provide physical insights by tuning the range information. Applying the self-supervised atomic representation on the magnetic moment datasets, we show how they can extract rules and information from the magnetic materials. To incorporate rich physical information into the GNN model, we develop the node embedding graph neural networks (NEGNN) framework and show significant improvements in the prediction performance. The self-supervised material representation and the NEGNN framework may investigate in-depth information from materials and can be applied to small datasets with increased prediction accuracy. △ Less

Submitted 5 June, 2024; v1 submitted 19 October, 2022; originally announced November 2022.

Comments: 19 pages, 12 figures, 8 tables

Journal ref: Machine Learning: Science and Technology 5, 035018 (2024)

Dynamical quantum phase transitions in a spinor Bose-Einstein condensate and criticality enhanced quantum sensing

Authors: Lu Zhou, Jia Kong, Zhihao Lan, Weiping Zhang

Abstract: Quantum phase transitions universally exist in the ground and excited states of quantum many-body systems, and they have a close relationship with the nonequilibrium dynamical phase transitions, which however are challenging to identify. In the system of spin-1 Bose-Einstein condensates, though dynamical phase transitions with correspondence to equilibrium phase transitions in the ground state and… ▽ More Quantum phase transitions universally exist in the ground and excited states of quantum many-body systems, and they have a close relationship with the nonequilibrium dynamical phase transitions, which however are challenging to identify. In the system of spin-1 Bose-Einstein condensates, though dynamical phase transitions with correspondence to equilibrium phase transitions in the ground state and uppermost excited state have been probed, those taken place in intermediate excited states remain untouched in experiments thus far. Here we unravel that both the ground and excited-state quantum phase transitions in spinor condensates can be diagnosed with dynamical phase transitions. A connection between equilibrium phase transitions and nonequilibrium behaviors of the system is disclosed in terms of the quantum Fisher information. We also demonstrate that near the critical points parameter estimation beyond standard quantum limit can be implemented. This work not only advances the exploration of excited-state quantum phase transitions via a scheme that can immediately be applied to a broad class of few-mode quantum systems, but also provides new perspective on the relationship between quantum criticality and quantum enhanced sensing. △ Less

Submitted 22 March, 2025; v1 submitted 23 September, 2022; originally announced September 2022.

Comments: close to published version

Journal ref: Phys. Rev. Research 5, 013087 (2023)

Self-supervised graph neural networks for accurate prediction of Néel temperature

Authors: Jian-Gang Kong, Qing-Xu Li, Jian Li, Yu Liu, Jia-Ji Zhu

Abstract: Antiferromagnetic materials are exciting quantum materials with rich physics and great potential for applications. It is highly demanded of the accurate and efficient theoretical method for determining the critical transition temperatures, Néel temperatures, of antiferromagnetic materials. The powerful graph neural networks (GNN) that succeed in predicting material properties lose their advantage… ▽ More Antiferromagnetic materials are exciting quantum materials with rich physics and great potential for applications. It is highly demanded of the accurate and efficient theoretical method for determining the critical transition temperatures, Néel temperatures, of antiferromagnetic materials. The powerful graph neural networks (GNN) that succeed in predicting material properties lose their advantage in predicting magnetic properties due to the small dataset of magnetic materials, while conventional machine learning models heavily depend on the quality of material descriptors. We propose a new strategy to extract high-level material representations by utilizing self-supervised training of GNN on large-scale unlabeled datasets. According to the dimensional reduction analysis, we find that the learned knowledge about elements and magnetism transfers to the generated atomic vector representations. Compared with popular manually constructed descriptors and crystal graph convolutional neural networks, self-supervised material representations can help us obtain a more accurate and efficient model for Néel temperatures, and the trained model can successfully predict high Néel temperature antiferromagnetic materials. Our self-supervised GNN may serve as a universal pre-training framework for various material properties. △ Less

Submitted 27 May, 2022; originally announced June 2022.

Comments: 7 pages, 6 figures, 2 tables

Journal ref: Chinese Physics Letters 39, 067503 (2022)

Giant enhancement of third-harmonic generation in graphene-metal heterostructures

Authors: Irati Alonso Calafell, Lee A. Rozema, David Alcaraz Iranzo, Alessandro Trenti, Joel D. Cox, Avinash Kumar, Hlib Bieliaiev, Sebastian Nanot, Cheng Peng, Dmitri K. Efetov, Jin Yong Hong, Jing Kong, Dirk R. Englund, F. Javier García de Abajo, Frank H. L. Koppens, Philp Walther

Abstract: Nonlinear nanophotonics leverages engineered nanostructures to funnel light into small volumes and intensify nonlinear optical processes with spectral and spatial control. Due to its intrinsically large and electrically tunable nonlinear optical response, graphene is an especially promising nanomaterial for nonlinear optoelectronic applications. Here we report on exceptionally strong optical nonli… ▽ More Nonlinear nanophotonics leverages engineered nanostructures to funnel light into small volumes and intensify nonlinear optical processes with spectral and spatial control. Due to its intrinsically large and electrically tunable nonlinear optical response, graphene is an especially promising nanomaterial for nonlinear optoelectronic applications. Here we report on exceptionally strong optical nonlinearities in graphene-insulator-metal heterostructures, demonstrating an enhancement by three orders of magnitude in the third-harmonic signal compared to bare graphene. Furthermore, by increasing the graphene Fermi energy through an external gate voltage, we find that graphene plasmons mediate the optical nonlinearity and modify the third-harmonic signal. Our findings show that graphene-insulator-metal is a promising heterostructure for optically-controlled and electrically-tunable nano-optoelectronic components. △ Less

Submitted 25 May, 2022; originally announced May 2022.

Journal ref: Nature Nanotechnology 16, 318-324, (2021)

Multiplication of freestanding semiconductor membranes from a single wafer by advanced remote epitaxy

Authors: Hyunseok Kim, Yunpeng Liu, Kuangye Lu, Celesta S. Chang, Kuan Qiao, Ki Seok Kim, Bo-In Park, Junseok Jeong, Menglin Zhu, Jun Min Suh, Yongmin Baek, You Jin Ji, Sungsu Kang, Sangho Lee, Ne Myo Han, Chansoo Kim, Chanyeol Choi, Xinyuan Zhang, Haozhe Wang, Lingping Kong, Jungwon Park, Kyusang Lee, Geun Young Yeom, Sungkyu Kim, Jinwoo Hwang , et al. (4 additional authors not shown)

Abstract: Freestanding single-crystalline membranes are an important building block for functional electronics. Especially, compounds semiconductor membranes such as III-N and III-V offer great opportunities for optoelectronics, high-power electronics, and high-speed computing. Despite huge efforts to produce such membranes by detaching epitaxial layers from donor wafers, however, it is still challenging to… ▽ More Freestanding single-crystalline membranes are an important building block for functional electronics. Especially, compounds semiconductor membranes such as III-N and III-V offer great opportunities for optoelectronics, high-power electronics, and high-speed computing. Despite huge efforts to produce such membranes by detaching epitaxial layers from donor wafers, however, it is still challenging to harvest epitaxial layers using practical processes. Here, we demonstrate a method to grow and harvest multiple epitaxial membranes with extremely high throughput at the wafer scale. For this, 2D materials are directly formed on III-N and III-V substrates in epitaxy systems, which enables an advanced remote epitaxy scheme comprised of multiple alternating layers of 2D materials and epitaxial layers that can be formed by a single epitaxy run. Each epilayer in the multi-stack structure is then harvested by layer-by-layer peeling, producing multiple freestanding membranes with unprecedented throughput from a single wafer. Because 2D materials allow peeling at the interface without damaging the epilayer or the substrate, wafers can be reused for subsequent membrane production. Therefore, this work represents a meaningful step toward high-throughput and low-cost production of single-crystal membranes that can be heterointegrated. △ Less

Submitted 7 April, 2022; originally announced April 2022.

Anomalous fractal scaling in two-dimensional electric networks

Authors: Xiao Zhang, Boxue Zhang, Haydar Sahin, Zhuo Bin Siu, S. M. Rafi-Ul-Islam, Jian Feng Kong, Mansoor B. A. Jalil, Ronny Thomale, Ching Hua Lee

Abstract: Much of the qualitative nature of physical systems can be predicted from the way it scales with system size. Contrary to the continuum expectation, we observe a profound deviation from logarithmic scaling in the impedance of a two-dimensional $LC$ circuit network. We find this anomalous impedance contribution to sensitively depend on the number of nodes $N$ in a curious erratic manner, and experim… ▽ More Much of the qualitative nature of physical systems can be predicted from the way it scales with system size. Contrary to the continuum expectation, we observe a profound deviation from logarithmic scaling in the impedance of a two-dimensional $LC$ circuit network. We find this anomalous impedance contribution to sensitively depend on the number of nodes $N$ in a curious erratic manner, and experimentally demonstrate its robustness against perturbations from the contact and parasitic impedance of individual components. This impedance anomaly is traced back to a generalized resonance condition reminiscent of the Harper's equation for electronic lattice transport in a magnetic field, even though our circuit network does not involve magnetic translation symmetry. It exhibits an emergent fractal parametric structure of anomalous impedance peaks for different $N$ that cannot be reconciled with continuum theory and does not correspond to regular waveguide resonant behavior. This anomalous fractal scaling extends to the transport properties of generic systems described by a network Laplacian whenever a resonance frequency scale is simultaneously present. △ Less

Submitted 2 July, 2023; v1 submitted 11 April, 2022; originally announced April 2022.

Comments: 15 pages, 9 figures, 2 tables

Journal ref: Communications Physics 6(1):151 2023

Thermal evolution of skyrmion formation mechanism in chiral multilayer films

Authors: Xiaoye Chen, Edwin Chue, Jian Feng Kong, Hui Ru Tan, Hang Khume Tan, Anjan Soumyanarayanan

Abstract: Magnetic skyrmions form in chiral multilayers from the shrinking or fission of elongated stripe textures. Here we report an experimental and theoretical study of the temperature dependence of this stripe-to-skyrmion transition in Co/Pt-based multilayers. Field-reversal magnetometry and Lorentz microscopy experiments over 100 - 350 K establish the increased efficacy of stripe-to-skyrmion fission at… ▽ More Magnetic skyrmions form in chiral multilayers from the shrinking or fission of elongated stripe textures. Here we report an experimental and theoretical study of the temperature dependence of this stripe-to-skyrmion transition in Co/Pt-based multilayers. Field-reversal magnetometry and Lorentz microscopy experiments over 100 - 350 K establish the increased efficacy of stripe-to-skyrmion fission at higher temperatures - driven primarily by the thermal evolution of key magnetic interactions - thereby enhancing skyrmion density. Atomistic calculations elucidate that the energy barrier to fission governs the thermodynamics of the skyrmion formation. Our results establish a mechanistic picture of the stripe-to-skyrmion transition and advance the use of thermal knobs for efficient skyrmion generation. △ Less

Submitted 23 March, 2022; originally announced March 2022.

Comments: 11 pages, 11 figures

Journal ref: Phys. Rev. Applied 17, 044039 (2022)

Unveiling the emergent traits of chiral spin textures in magnetic multilayers

Authors: Xiaoye Chen, Ming Lin, Jian Feng Kong, Hui Ru Tan, Anthony K. C. Tan, Soong-Geun Je, Hang Khume Tan, Khoong Hong Khoo, Mi-Young Im, Anjan Soumyanarayanan

Abstract: Magnetic skyrmions are topologically wound nanoscale textures of spins whose ambient stability and electrical manipulation in multilayer films have led to an explosion of research activities. While past efforts focused predominantly on isolated skyrmions, recently ensembles of chiral spin textures, consisting of skyrmions and magnetic stripes, were shown to possess rich interactions with potential… ▽ More Magnetic skyrmions are topologically wound nanoscale textures of spins whose ambient stability and electrical manipulation in multilayer films have led to an explosion of research activities. While past efforts focused predominantly on isolated skyrmions, recently ensembles of chiral spin textures, consisting of skyrmions and magnetic stripes, were shown to possess rich interactions with potential for device applications. However, several fundamental aspects of chiral spin texture phenomenology remain to be elucidated, including their domain wall structure, thermodynamic stability, and morphological transitions. Here we unveil the evolution of these textural characteristics on a tunable multilayer platform - wherein chiral interactions governing spin texture energetics can be widely varied - using a combination of full-field electron and soft X-ray microscopies with numerical simulations. With increasing chiral interactions, we demonstrate the emergence of Néel helicity, followed by a marked reduction in domain compressibility, and finally a transformation in the skyrmion formation mechanism. Together with an analytical model, these experiments establish a comprehensive microscopic framework for investigating and tailoring chiral spin texture character in multilayer films. △ Less

Submitted 23 March, 2022; originally announced March 2022.

Journal ref: Advanced Science 9, 2103978 (2022)

Panoramic mapping of phonon transport from ultrafast electron diffraction and machine learning

Authors: Zhantao Chen, Xiaozhe Shen, Nina Andrejevic, Tongtong Liu, Duan Luo, Thanh Nguyen, Nathan C. Drucker, Michael E. Kozina, Qichen Song, Chengyun Hua, Gang Chen, Xijie Wang, Jing Kong, Mingda Li

Abstract: One central challenge in understanding phonon thermal transport is a lack of experimental tools to investigate mode-based transport information. Although recent advances in computation lead to mode-based information, it is hindered by unknown defects in bulk region and at interfaces. Here we present a framework that can reveal microscopic phonon transport information in heterostructures, integrati… ▽ More One central challenge in understanding phonon thermal transport is a lack of experimental tools to investigate mode-based transport information. Although recent advances in computation lead to mode-based information, it is hindered by unknown defects in bulk region and at interfaces. Here we present a framework that can reveal microscopic phonon transport information in heterostructures, integrating state-of-the-art ultrafast electron diffraction (UED) with advanced scientific machine learning. Taking advantage of the dual temporal and reciprocal-space resolution in UED, we are able to reliably recover the frequency-dependent interfacial transmittance with possible extension to frequency-dependent relaxation times of the heterostructure. This enables a direct reconstruction of real-space, real-time, frequency-resolved phonon dynamics across an interface. Our work provides a new pathway to experimentally probe phonon transport mechanisms with unprecedented details. △ Less

Submitted 12 February, 2022; originally announced February 2022.

Spectroscopic Signatures of Interlayer Coupling in Janus MoSSe/MoS2 Heterostructures

Authors: Kunyan Zhang, Yunfan Guo, Daniel T. Larson, Ziyan Zhu, Shiang Fang, Efthimios Kaxiras, Jing Kong, Shengxi Huang

Abstract: The interlayer coupling in van der Waals heterostructures governs a variety of optical and electronic properties. The intrinsic dipole moment of Janus transition metal dichalcogenides (TMDs) offers a simple and versatile approach to tune the interlayer interactions. In this work, we demonstrate how the van der Waals interlayer coupling and charge transfer of Janus MoSSe/MoS2 heterobilayers can be… ▽ More The interlayer coupling in van der Waals heterostructures governs a variety of optical and electronic properties. The intrinsic dipole moment of Janus transition metal dichalcogenides (TMDs) offers a simple and versatile approach to tune the interlayer interactions. In this work, we demonstrate how the van der Waals interlayer coupling and charge transfer of Janus MoSSe/MoS2 heterobilayers can be tuned by the twist angle and interface composition. Specifically, the Janus heterostructures with a sulfur/sulfur (S/S) interface display stronger interlayer coupling than the heterostructures with a selenium/sulfur (Se/S) interface as shown by the low-frequency Raman modes. The differences in interlayer interactions are explained by the interlayer distance computed by density-functional theory (DFT). More intriguingly, the built-in electric field contributed by the charge density redistribution and interlayer coupling also play important roles in the interfacial charge transfer. Namely, the S/S and Se/S interfaces exhibit different levels of PL quenching of MoS2 A exciton, suggesting the enhanced and reduced charge transfer at the S/S and Se/S interface, respectively. Our work demonstrates how the asymmetry of Janus TMDs can be used to tailor the interfacial interactions in van der Waals heterostructures. △ Less

Submitted 24 August, 2021; originally announced August 2021.

Tuning color centers at a twisted interface

Authors: Cong Su, Fang Zhang, Salman Kahn, Brian Shevitski, Jingwei Jiang, Chunhui Dai, Alex Ungar, Ji-Hoon Park, Kenji Watanabe, Takashi Taniguchi, Jing Kong, Zikang Tang, Wenqing Zhang, Feng Wang, Michael Crommie, Steven G. Louie, Shaul Aloni, Alex Zettl

Abstract: Color center is a promising platform for quantum technologies, but their application is hindered by the typically random defect distribution and complex mesoscopic environment. Employing cathodoluminescence, we demonstrate that an ultraviolet-emitting single photon emitter can be readily activated and controlled on-demand at the twisted interface of two hexagonal boron nitride flakes. The brightne… ▽ More Color center is a promising platform for quantum technologies, but their application is hindered by the typically random defect distribution and complex mesoscopic environment. Employing cathodoluminescence, we demonstrate that an ultraviolet-emitting single photon emitter can be readily activated and controlled on-demand at the twisted interface of two hexagonal boron nitride flakes. The brightness of the color center can be enhanced by two orders of magnitude by altering the twist angle. Additionally, a brightness modulation of nearly 100% of this color center is achieved by an external voltage. Our ab-initio GW calculations suggest that the emission is correlated to nitrogen vacancies and that a twist-induced moiré potential facilitates electron-hole recombination. This mechanism is further exploited to draw nanoscale color center patterns using electron beams. △ Less

Submitted 10 August, 2021; originally announced August 2021.

Enhanced Raman Scattering on Functionalized Graphene Substrates

Authors: Václav Valeš, Petr Kovaříček, Michaela Fridrichová, Xiang Ji, Xi Ling, Jing Kong, Mildred S. Dresselhaus, Martin Kalbáč

Abstract: The graphene-enhanced Raman scattering of Rhodamine 6G molecules on pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates was studied. The uniformity of the Raman signal enhancement was studied by making large Raman maps. The relative enhancement of the Raman signal is demonstrated to be dependent on the functional groups, which was rationalized by the different doping levels… ▽ More The graphene-enhanced Raman scattering of Rhodamine 6G molecules on pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates was studied. The uniformity of the Raman signal enhancement was studied by making large Raman maps. The relative enhancement of the Raman signal is demonstrated to be dependent on the functional groups, which was rationalized by the different doping levels of pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates. The impact of the Fermi energy of graphene and the phonon energy of the molecules was considered together for the first time in order to explain the enhancement. Such approach enables to understand the enhancement without assuming anything about the uniformity of the molecules on the graphene surface. The agreement between the theory and our measured data was further demonstrated by varying excitation energy. △ Less

Submitted 26 July, 2021; originally announced July 2021.

Journal ref: 2D Materials, 4, 025087 (2017)

Strain-induced large injection current in twisted bilayer graphene

Authors: Arpit Arora, Jian Feng Kong, Justin C. W. Song

Abstract: The electronic wavefunctions in moiré materials are highly sensitive to the details of the local atomic configuration enabling Bloch band geometry and topology to be controlled by stacking and strain. Here we predict that large injection currents (under circular polarized irradiation) can develop in strained twisted bilayer graphene (TBG) heterostructures with broken sublattice symmetry; such bulk… ▽ More The electronic wavefunctions in moiré materials are highly sensitive to the details of the local atomic configuration enabling Bloch band geometry and topology to be controlled by stacking and strain. Here we predict that large injection currents (under circular polarized irradiation) can develop in strained twisted bilayer graphene (TBG) heterostructures with broken sublattice symmetry; such bulk photovoltaic currents flow even in the absence of a p-n junction and can be controlled by the helicity of incident light. As we argue, large injection current rates proceed from strong and highly peaked interband Berry curvature dipole distributions (arising from the texturing of Bloch wavefunctions in strained TBG heterostructures). Strikingly, we find that TBG injection current displays pronounced responses in the THz regime and can be tuned by chemical potential. These render injection currents a useful photocurrent probe of symmetry breaking in TBG heterostructures and make TBG a promising material for THz technology. △ Less

Submitted 15 July, 2021; originally announced July 2021.

Journal ref: Phys. Rev. B 104, L241404 (2021)

Detecting photoelectrons from spontaneously formed excitons

Authors: Keisuke Fukutani, Roland Stania, Chang Il Kwon, Jun Sung Kim, Ki Jeong Kong, Jaeyoung Kim, Han Woong Yeom

Abstract: Excitons, quasiparticles of electrons and holes bound by Coulombic attraction, are created transiently by light and play an important role in optoelectronics, photovoltaics and photosynthesis. While they are also predicted to form spontaneously in a small gap semiconductor or a semimetal, leading to a Bose-Einstein condensate at low temperature, their material realization has been elusive without… ▽ More Excitons, quasiparticles of electrons and holes bound by Coulombic attraction, are created transiently by light and play an important role in optoelectronics, photovoltaics and photosynthesis. While they are also predicted to form spontaneously in a small gap semiconductor or a semimetal, leading to a Bose-Einstein condensate at low temperature, their material realization has been elusive without any direct evidence. Here we detect the direct photoemission signal from spontaneously formed excitons in a debated excitonic insulator candidate Ta2NiSe5. Our symmetry-selective angle-resolved photoemission spectroscopy reveals a characteristic excitonic feature above the transition temperature, which provides detailed properties of excitons such as anisotropic Bohr radius. The present result evidences so called preformed excitons and guarantees the excitonic insulator nature of Ta2NiSe5 at low temperature. Direct photoemission can be an important tool to characterize steady-state excitons. △ Less

Submitted 24 May, 2021; originally announced May 2021.

Revealing the Brønsted-Evans-Polanyi Relation in Halide-Activated Fast MoS2 Growth Towards Millimeter-Sized 2D Crystals

Authors: Qingqing Ji, Cong Su, Nannan Mao, Xuezeng Tian, Juan-Carlos Idrobo, Jianwei Miao, William A. Tisdale, Alex Zettl, Ju Li, Jing Kong

Abstract: Achieving large-size two-dimensional (2D) crystals is key to fully exploiting their remarkable functionalities and application potentials. Chemical vapor deposition (CVD) growth of 2D semiconductors such as monolayer MoS2 has been reported to be activated by halide salts, yet clear identification of the underlying mechanism remains elusive. Here we provide unambiguous experimental evidence showing… ▽ More Achieving large-size two-dimensional (2D) crystals is key to fully exploiting their remarkable functionalities and application potentials. Chemical vapor deposition (CVD) growth of 2D semiconductors such as monolayer MoS2 has been reported to be activated by halide salts, yet clear identification of the underlying mechanism remains elusive. Here we provide unambiguous experimental evidence showing that the MoS2 growth dynamics are halogen-dependent through the Brønsted-Evans-Polanyi relation, based on which we build a growth model by considering MoS2 edge passivation by halogens, and theoretically reproduces the trend of our experimental observations. These mechanistic understandings enable us to further optimize the fast growth of MoS2 and reach record-large domain sizes that should facilitate practical applications. △ Less

Submitted 11 March, 2021; originally announced March 2021.

Comments: 18 pages, 5 figures

Journal ref: Science Advances 7, 44 (2021)

Colossal switchable photocurrents in topological Janus transition metal dichalcogenides

Authors: Haowei Xu, Hua Wang, Jian Zhou, Yunfan Guo, Jing Kong, Ju Li

Abstract: Nonlinear optical properties, such as bulk photovoltaic effects, possess great potential in energy harvesting, photodetection, rectification, etc. To enable efficient light-current conversion, materials with strong photo-responsivity are highly desirable. In this work, we predict that monolayer Janus transition metal dichalcogenides (JTMDs) in the 1T' phase possess colossal nonlinear photoconducti… ▽ More Nonlinear optical properties, such as bulk photovoltaic effects, possess great potential in energy harvesting, photodetection, rectification, etc. To enable efficient light-current conversion, materials with strong photo-responsivity are highly desirable. In this work, we predict that monolayer Janus transition metal dichalcogenides (JTMDs) in the 1T' phase possess colossal nonlinear photoconductivity owing to their topological band mixing, strong inversion symmetry breaking, and small electronic bandgap. 1T' JTMDs have inverted bandgaps on the order of 10 meV and are exceptionally responsive to light in the terahertz (THz) range. By first-principles calculations, we reveal that 1T' JTMDs possess shift current (SC) conductivity as large as $2300 ~\rm nm \cdot μA / V^2$, equivalent to a photo-responsivity of $2800 ~\rm mA/W$. The circular current (CC) conductivity of 1T' JTMDs is as large as $10^4~ \rm nm \cdot μA / V^2$. These remarkable photo-responsivities indicate that the 1T' JTMDs can serve as efficient photodetectors in the THz range. We also find that external stimuli such as the in-plane strain and out-of-plane electric field can induce topological phase transitions in 1T' JTMDs and that the SC can abruptly flip their directions. The abrupt change of the nonlinear photocurrent can be used to characterize the topological transition and has potential applications in 2D optomechanics and nonlinear optoelectronics. △ Less

Submitted 26 February, 2021; originally announced March 2021.

Journal ref: npj Computational Materials 7, 31 (2021)

Designing Artificial Two-Dimensional Landscapes via Room-Temperature Atomic-Layer Substitution

Authors: Yunfan Guo, Yuxuan Lin, Kaichen Xie, Biao Yuan, Jiadi Zhu, Pin-Chun Shen, Ang-Yu Lu, Cong Su, Enzheng Shi, Kunyan Zhang, Zhengyang Cai, Jihoon Park, Qingqing Ji, Jiangtao Wang, Xiaochuan Dai, Xuezeng Tian, Shengxi Huang, Letian Dou, Ju Li, Yi Yu, Juan-Carlos Idrobo, Ting Cao, Tomás Palacios, Jing Kong

Abstract: Manipulating materials with atomic-scale precision is essential for the development of next-generation material design toolbox. Tremendous efforts have been made to advance the compositional, structural, and spatial accuracy of material deposition and patterning. The family of 2D materials provides an ideal platform to realize atomic-level material architectures. The wide and rich physics of these… ▽ More Manipulating materials with atomic-scale precision is essential for the development of next-generation material design toolbox. Tremendous efforts have been made to advance the compositional, structural, and spatial accuracy of material deposition and patterning. The family of 2D materials provides an ideal platform to realize atomic-level material architectures. The wide and rich physics of these materials have led to fabrication of heterostructures, superlattices, and twisted structures with breakthrough discoveries and applications. Here, we report a novel atomic-scale material design tool that selectively breaks and forms chemical bonds of 2D materials at room temperature, called atomic-layer substitution (ALS), through which we can substitute the top layer chalcogen atoms within the 3-atom-thick transition-metal dichalcogenides using arbitrary patterns. Flipping the layer via transfer allows us to perform the same procedure on the other side, yielding programmable in-plane multi-heterostructures with different out-of-plane crystal symmetry and electric polarization. First-principle calculations elucidate how the ALS process is overall exothermic in energy and only has a small reaction barrier, facilitating the reaction to occur at room temperature. Optical characterizations confirm the fidelity of this design approach, while TEM shows the direct evidence of Janus structure and suggests the atomic transition at the interface of designed heterostructure. Finally, transport and Kelvin probe measurements on MoXY (X,Y=S,Se; X and Y corresponding to the bottom and top layers) lateral multi-heterostructures reveal the surface potential and dipole orientation of each region, and the barrier height between them. Our approach for designing artificial 2D landscape down to a single layer of atoms can lead to unique electronic, photonic and mechanical properties previously not found in nature. △ Less

Submitted 15 November, 2020; originally announced November 2020.

Direct prediction of phonon density of states with Euclidean neural networks

Authors: Zhantao Chen, Nina Andrejevic, Tess Smidt, Zhiwei Ding, Yen-Ting Chi, Quynh T. Nguyen, Ahmet Alatas, Jing Kong, Mingda Li

Abstract: Machine learning has demonstrated great power in materials design, discovery, and property prediction. However, despite the success of machine learning in predicting discrete properties, challenges remain for continuous property prediction. The challenge is aggravated in crystalline solids due to crystallographic symmetry considerations and data scarcity. Here we demonstrate the direct prediction… ▽ More Machine learning has demonstrated great power in materials design, discovery, and property prediction. However, despite the success of machine learning in predicting discrete properties, challenges remain for continuous property prediction. The challenge is aggravated in crystalline solids due to crystallographic symmetry considerations and data scarcity. Here we demonstrate the direct prediction of phonon density of states using only atomic species and positions as input. We apply Euclidean neural networks, which by construction are equivariant to 3D rotations, translations, and inversion and thereby capture full crystal symmetry, and achieve high-quality prediction using a small training set of $\sim 10^{3}$ examples with over 64 atom types. Our predictive model reproduces key features of experimental data and even generalizes to materials with unseen elements,and is naturally suited to efficiently predict alloy systems without additional computational cost. We demonstrate the potential of our network by predicting a broad number of high phononic specific heat capacity materials. Our work indicates an efficient approach to explore materials' phonon structure, and can further enable rapid screening for high-performance thermal storage materials and phonon-mediated superconductors. △ Less

Submitted 2 February, 2021; v1 submitted 10 September, 2020; originally announced September 2020.

Comments: 21 pages total, 5 main figures + 16 supplementary figures. To appear in Advanced Science (2021)

Journal ref: Advanced Science 202004214 (2021)

Room Temperature Terahertz Electroabsorption Modulation by Excitons in Monolayer Transition Metal Dichalcogenides

Authors: Jiaojian Shi, Edoardo Baldini, Simone Latini, Shunsuke A. Sato, Yaqing Zhang, Brandt C. Pein, Pin-Chun Shen, Jing Kong, Angel Rubio, Nuh Gedik, Keith A. Nelson

Abstract: The interaction between off-resonant laser pulses and excitons in monolayer transition metal dichalcogenides is attracting increasing interest as a route for the valley-selective coherent control of the exciton properties. Here, we extend the classification of the known off-resonant phenomena by unveiling the impact of a strong THz field on the excitonic resonances of monolayer MoS$_2$. We observe… ▽ More The interaction between off-resonant laser pulses and excitons in monolayer transition metal dichalcogenides is attracting increasing interest as a route for the valley-selective coherent control of the exciton properties. Here, we extend the classification of the known off-resonant phenomena by unveiling the impact of a strong THz field on the excitonic resonances of monolayer MoS$_2$. We observe that the THz pump pulse causes a selective modification of the coherence lifetime of the excitons, while keeping their oscillator strength and peak energy unchanged. We rationalize these results theoretically by invoking a hitherto unobserved manifestation of the Franz-Keldysh effect on an exciton resonance. As the modulation depth of the optical absorption reaches values as large as 0.05 dB/nm at room temperature, our findings open the way to the use of semiconducting transition metal dichalcogenides as compact and efficient platforms for high-speed electroabsorption devices. △ Less

Submitted 8 June, 2020; originally announced June 2020.

Comments: 40 pages, 11 figures

Understanding disorder in 2D materials: the case of carbon doping of Silicene

Authors: Ricardo Pablo-Pedro, Miguel Angel Magana-Fuentes, Marcelo Videa, Jing Kong, Mingda Li, Jose L. Mendoza-Cortes, Troy Van Voorhis

Abstract: We investigate the effect of lattice disorder and local correlation effects in finite and periodic silicene structures caused by carbon doping using first-principles calculations. For both finite and periodic silicene structures, the electronic properties carbon-doped monolayers are dramatically changed by controlling the doping sites in the structures, which is related to the amount of disorder i… ▽ More We investigate the effect of lattice disorder and local correlation effects in finite and periodic silicene structures caused by carbon doping using first-principles calculations. For both finite and periodic silicene structures, the electronic properties carbon-doped monolayers are dramatically changed by controlling the doping sites in the structures, which is related to the amount of disorder introduced in the lattice and electron-electron correlation effects. By changing the position of the carbon dopants, we found that a Mott-Anderson transition is achieved. Moreover, the band gap is determined by the level of lattice disorder and electronic correlation effects. Finally, these structures are ferromagnetic even under disorder which has potential applications in Si-based nanoelectronics, such as field-effect transistors (FETs). △ Less

Submitted 15 August, 2020; v1 submitted 1 December, 2019; originally announced December 2019.

Comments: 10 pages, 6 figures

Coexistence of Van Hove Singularities and Pseudomagnetic Fields in Modulated Graphene Bilayer

Authors: Jana Vejpravova, Barbara Pacakova, Mildred S. Dresselhaus, Jing Kong, Martin Kalbac

Abstract: The stacking and bending of graphene are trivial but extremely powerful agents of control over graphene's manifold physics. By changing the twist angle, one can drive the system over a plethora of exotic states via strong electron correlation, thanks to the moiré superlattice potentials, while the periodic or triaxial strains induce discretization of the band structure into Landau levels without t… ▽ More The stacking and bending of graphene are trivial but extremely powerful agents of control over graphene's manifold physics. By changing the twist angle, one can drive the system over a plethora of exotic states via strong electron correlation, thanks to the moiré superlattice potentials, while the periodic or triaxial strains induce discretization of the band structure into Landau levels without the need for an external magnetic field. We fabricated a hybrid system comprising both the stacking and bending tuning knobs. We have grown the graphene monolayers by chemical vapor deposition, using $^{12}$C and $^{13}$C precursors, which enabled us to individually address the layers through Raman spectroscopy mapping. We achieved the long-range spatial modulation by sculpturing the top layer ($^{13}$C) over uniform magnetic nanoparticles (NPs) deposited on the bottom layer ($^{12}$C). An atomic force microscopy study revealed that the top layer tends to relax into pyramidal corrugations with C$_3$ axial symmetry at the position of the NPs, which have been widely reported as a source of large pseudomagnetic fields (PMFs) in graphene monolayers. The modulated graphene bilayer (MGBL) also contains a few micrometer large domains, with the twist angle ~ 10$^{\circ}$, which were identified via extreme enhancement of the Raman intensity of the G-mode due to formation of Van Hove singularities (VHSs). We thereby conclude that the twist induced VHSs coexist with the PMFs generated in the strained pyramidal objects without mutual disturbance. The graphene bilayer modulated with magnetic NPs is a non-trivial hybrid system that accommodates features of twist induced VHSs and PMFs in environs of giant classical spins. △ Less

Submitted 3 November, 2019; originally announced November 2019.

Comments: 6 figures, 13 pages

Evidence for a pressure-induced phase transition of few-layer graphene to 2D diamond

Authors: Luiz G. Pimenta Martins, Diego L. Silva, Jesse S. Smith, Ang-Yu Lu, Cong Su, Marek Hempel, Connor Occhialini, Xiang Ji, Ricardo Pablo, Rafael S. Alencar, Alan C. R. Souza, Alan B. de Oliveira, Ronaldo J. C. Batista, Tomás Palacios, Matheus J. S. Matos, Mário S. C. Mazzoni, Riccardo Comin, Jing Kong, Luiz G. Cançado

Abstract: We unveil the diamondization mechanism of few-layer graphene compressed in the presence of water, providing robust evidence for the pressure-induced formation of 2D diamond. High-pressure Raman spectroscopy provides evidence of a phase transition occurring in the range of 4-7 GPa for 5-layer graphene and graphite. The pressure-induced phase is partially transparent and indents the silicon substrat… ▽ More We unveil the diamondization mechanism of few-layer graphene compressed in the presence of water, providing robust evidence for the pressure-induced formation of 2D diamond. High-pressure Raman spectroscopy provides evidence of a phase transition occurring in the range of 4-7 GPa for 5-layer graphene and graphite. The pressure-induced phase is partially transparent and indents the silicon substrate. Our combined theoretical and experimental results indicate a gradual top-bottom diamondization mechanism, consistent with the formation of diamondene, a 2D ferromagnetic semiconductor. High-pressure x-ray diffraction on graphene indicates the formation of hexagonal diamond, consistent with the bulk limit of eclipsed-conformed diamondene. △ Less

Submitted 16 October, 2019; v1 submitted 3 October, 2019; originally announced October 2019.

Comments: 30 pages, 17 figures

Deep-Learning-Enabled Fast Optical Identification and Characterization of Two-Dimensional Materials

Authors: Bingnan Han, Yuxuan Lin, Yafang Yang, Nannan Mao, Wenyue Li, Haozhe Wang, Kenji Yasuda, Xirui Wang, Valla Fatemi, Lin Zhou, Joel I-Jan Wang, Qiong Ma, Yuan Cao, Daniel Rodan-Legrain, Ya-Qing Bie, Efrén Navarro-Moratalla, Dahlia Klein, David MacNeill, Sanfeng Wu, Hikari Kitadai, Xi Ling, Pablo Jarillo-Herrero, Jing Kong, Jihao Yin, Tomás Palacios

Abstract: Advanced microscopy and/or spectroscopy tools play indispensable role in nanoscience and nanotechnology research, as it provides rich information about the growth mechanism, chemical compositions, crystallography, and other important physical and chemical properties. However, the interpretation of imaging data heavily relies on the "intuition" of experienced researchers. As a result, many of the d… ▽ More Advanced microscopy and/or spectroscopy tools play indispensable role in nanoscience and nanotechnology research, as it provides rich information about the growth mechanism, chemical compositions, crystallography, and other important physical and chemical properties. However, the interpretation of imaging data heavily relies on the "intuition" of experienced researchers. As a result, many of the deep graphical features obtained through these tools are often unused because of difficulties in processing the data and finding the correlations. Such challenges can be well addressed by deep learning. In this work, we use the optical characterization of two-dimensional (2D) materials as a case study, and demonstrate a neural-network-based algorithm for the material and thickness identification of exfoliated 2D materials with high prediction accuracy and real-time processing capability. Further analysis shows that the trained network can extract deep graphical features such as contrast, color, edges, shapes, segment sizes and their distributions, based on which we develop an ensemble approach topredict the most relevant physical properties of 2D materials. Finally, a transfer learning technique is applied to adapt the pretrained network to other applications such as identifying layer numbers of a new 2D material, or materials produced by a different synthetic approach. Our artificial-intelligence-based material characterization approach is a powerful tool that would speed up the preparation, initial characterization of 2D materials and other nanomaterials and potentially accelerate new material discoveries. △ Less

Submitted 27 January, 2020; v1 submitted 26 June, 2019; originally announced June 2019.

Molecular monolayer stabilizer for multilayer 2D materials

Authors: Cong Su, Zongyou Yin, Qing-Bo Yan, Zegao Wang, Hongtao Lin, Lei Sun, Wenshuo Xu, Tetsuya Yamada, Xiang Ji, Nobuyuki Zettsu, Katsuya Teshima, Jamie H. Warner, Mircea Dincă, Juejun Hu, Mingdong Dong, Gang Su, Jing Kong, Ju Li

Abstract: 2D van der Waals materials have rich and unique functional properties, but many are susceptible to corrosion under ambient conditions. Here we show that linear alkylamines are highly effective in protecting the optoelectronic properties of these materials such as black phosphorous (BP) and transitional metal dichalcogenides. As a representative example, n-hexylamine can be applied in the form of t… ▽ More 2D van der Waals materials have rich and unique functional properties, but many are susceptible to corrosion under ambient conditions. Here we show that linear alkylamines are highly effective in protecting the optoelectronic properties of these materials such as black phosphorous (BP) and transitional metal dichalcogenides. As a representative example, n-hexylamine can be applied in the form of thin molecular monolayers on BP flakes with less-than-2nm thickness and can prolong BP's lifetime from a few hours to several weeks and even months in ambient environments. Characterizations combined with our theoretical analysis show that the thin monolayers selectively sift out water molecules, forming a drying layer to achieve the passivation of the protected 2D materials. The monolayer coating is also stable in air, hydrogen annealing, and organic solvents, but can be removed by certain organic acids. △ Less

Submitted 2 June, 2019; originally announced June 2019.

Giant intrinsic photoresponse in pristine graphene

Authors: Qiong Ma, Chun Hung Lui, Justin C. W. Song, Yuxuan Lin, Jian Feng Kong, Yuan Cao, Thao H. Dinh, Nityan L. Nair, Wenjing Fang, Kenji Watanabe, Takashi Taniguchi, Su-Yang Xu, Jing Kong, Tomás Palacios, Nuh Gedik, Nathaniel M. Gabor, Pablo Jarillo-Herrero

Abstract: When the Fermi level matches the Dirac point in graphene, the reduced charge screening can dramatically enhance electron-electron (e-e) scattering to produce a strongly interacting Dirac liquid. While the dominance of e-e scattering already leads to novel behaviors, such as electron hydrodynamic flow, further exotic phenomena have been predicted to arise specifically from the unique kinematics of… ▽ More When the Fermi level matches the Dirac point in graphene, the reduced charge screening can dramatically enhance electron-electron (e-e) scattering to produce a strongly interacting Dirac liquid. While the dominance of e-e scattering already leads to novel behaviors, such as electron hydrodynamic flow, further exotic phenomena have been predicted to arise specifically from the unique kinematics of e-e scattering in massless Dirac systems. Here, we use optoelectronic probes, which are highly sensitive to the kinematics of electron scattering, to uncover a giant intrinsic photocurrent response in pristine graphene. This photocurrent emerges exclusively at the charge neutrality point and vanishes abruptly at non-zero charge densities. Moreover, it is observed at places with broken reflection symmetry, and it is selectively enhanced at free graphene edges with sharp bends. Our findings reveal that the photocurrent relaxation is strongly suppressed by a drastic change of fast photocarrier kinematics in graphene when its Fermi level matches the Dirac point. The emergence of robust photocurrents in neutral Dirac materials promises new energy-harvesting functionalities and highlights intriguing electron dynamics in the optoelectronic response of Dirac fluids. △ Less

Submitted 17 December, 2018; originally announced December 2018.

Comments: Originally submitted version

Journal ref: Nature Nanotechnology (2018)