Showing 1–50 of 99 results for author: Ding, F

Calibration of systematic distortions in quantum emitter localization microscopy for deterministic nanophotonic fabrication

Authors: Chenxi Ma, Maximilian Heller, Timon Handrup, Yiteng Zhang, Tobias M. Krieger, Thomas Oberleitner, Zenghui Jiang, Xian Zheng, Eddy P. Rugeramigabo, Folke Dencker, Armando Rastelli, Fei Ding, Michael Zopf

Abstract: Quantum photonic technologies greatly benefit from quantum light emitters with high brightness, indistinguishability, and reliable polarization characteristics. Achieving optimal performance relies on the accurate localization of emitters and their deterministic integration into tailored photonic structures with nanometer-scale accuracy. Although marker-based photoluminescence imaging techniques c… ▽ More Quantum photonic technologies greatly benefit from quantum light emitters with high brightness, indistinguishability, and reliable polarization characteristics. Achieving optimal performance relies on the accurate localization of emitters and their deterministic integration into tailored photonic structures with nanometer-scale accuracy. Although marker-based photoluminescence imaging techniques can achieve statistical fitting uncertainties below 10 nm, the ultimate integration yield is often limited by uncorrected systematic distortions in custom cryo-optical setups that compromise metrological accuracy. Here, we present an in situ calibration protocol that uses lithographically defined gold nanodisk arrays as references to calibrate optical distortions with a Zernike vector-field model. On held-out validation patterns beyond the calibration dataset, this correction reduces the residual systematic bias to 5.3 nm with a 2D scatter of 24.6 nm across the analyzed field of view. Furthermore, we demonstrate that applying this correction to the deterministic fabrication of circular mesa structures around semiconductor quantum dots reduces the variance in emission polarization by 49%, indicating improved registration accuracy. This calibration strategy offers a practical route to high-yield deterministic integration of quantum emitters into scalable quantum photonic circuits. △ Less

Submitted 6 July, 2026; originally announced July 2026.

Trillion-atom molecular dynamics simulations with ab initio accuracy

Authors: Pengfei Suo, Wudi Cao, Xingxing Wu, Wenjie Zhang, Zheyong Fan, Shuanghan Xian, Rui Wang, Cheng Qian, Chao Liang, Qinghong Yuan, Xiaoshuang Chen, Pengfei Guan, Jingde Bu, Hongzhen Tian, Yanjing Su, Feng Ding, Lin-Wang Wang

Abstract: Material properties are fundamentally dictated by multiscale phenomena, which often reach mesoscale in size. The μm mesoscale is also the size which can be observed directly under an optical microscope, bridging the atomistic microscopic description with the continuous model macroscopic world. In this work, we report an unprecedented molecular dynamics (MD) simulation comprising 1.62 trillion atom… ▽ More Material properties are fundamentally dictated by multiscale phenomena, which often reach mesoscale in size. The μm mesoscale is also the size which can be observed directly under an optical microscope, bridging the atomistic microscopic description with the continuous model macroscopic world. In this work, we report an unprecedented molecular dynamics (MD) simulation comprising 1.62 trillion atoms. Utilizing the neuroevolution potential (NEP) framework, we attained ab initio accuracy on China's New-generation Intelligent Supercomputer. Our implementation achieves a time-to-solution (s/step/atom) 100 times faster than previous state-of-the-art machine learning force field simulations, and 1,000 times faster than the Gordon Bell Prize-winning application from six years ago. Furthermore, we demonstrate an 86.9% weak scaling efficiency from a single GPGPU to 45,000 GPGPUs. These results redefine atomistic simulation boundaries, enabling direct mesoscopic modeling with quantum-level precision. △ Less

Submitted 27 April, 2026; originally announced April 2026.

Phase transformation kinetics in MoS2 governed by S-S repulsive interactions and defect-interface compatibility

Authors: Pai Li, Ziao Tian, ZengFeng Di, Feng Ding

Abstract: The metastable T' phase in monolayer MoS2 exhibits remarkable persistence despite a strong thermodynamic driving force toward the stable H phase. Using machine learning-accelerated molecular dynamics and first-principles calculations, we reveal that this kinetic arrest originates from repulsive S-S interactions, which impose high energy barriers during both nucleation and grain boundary propagatio… ▽ More The metastable T' phase in monolayer MoS2 exhibits remarkable persistence despite a strong thermodynamic driving force toward the stable H phase. Using machine learning-accelerated molecular dynamics and first-principles calculations, we reveal that this kinetic arrest originates from repulsive S-S interactions, which impose high energy barriers during both nucleation and grain boundary propagation. While sulfur vacancies can alleviate these barriers in certain interfaces, they fail to accelerate transformation at the most stable interface, ZZ-Mo|-, due to their thermodynamic instability there. Instead, vacancies migrate into the T' phase, leaving the advancing front defect-free. Direct simulations of nanostructures confirm that H-phase nucleation initiates at corners or edges, and all observed growth fronts adopt the ZZ-Mo|- configuration, consistent with its low interfacial energy but slow kinetics. Our work establishes that phase transformation in 2D materials is governed not by global defect concentration, but by the local compatibility between defects and moving interfaces, offering a new paradigm for controlling structural transitions through interface-specific design. △ Less

Submitted 27 April, 2026; originally announced April 2026.

Facet-dependent Chemical Kinetics Governed Growth of Twisted Graphene Layers with Pre-designed Angles

Authors: Chaowu Xue, Mengzhao Sun, Zixuan Zhou, Zhuoran Yao, Li-Qun Shen, Xiao Kong, Honglong Zhao, Feng Ding, Marc Willinger, Zhongkai Liu, Zhu-Jun Wang

Abstract: Twisted graphene layers (TGLs) provide a powerful platform for investigating multiple quantum phenomena, yet their scalable deployment is hindered by the lack of reliable synthesis with precise angle. Here, benefited from a deeper understanding of the interplay between grain index and graphene growth kinetics, we report a scalable strategy to grow TGLs with pre-designed twist angles on platinum (P… ▽ More Twisted graphene layers (TGLs) provide a powerful platform for investigating multiple quantum phenomena, yet their scalable deployment is hindered by the lack of reliable synthesis with precise angle. Here, benefited from a deeper understanding of the interplay between grain index and graphene growth kinetics, we report a scalable strategy to grow TGLs with pre-designed twist angles on platinum (Pt) via chemical vapor deposition (CVD), Through a combination of complementary in situ methods, we identified the activity sequence of different Pt grains and attributed it to the area ratio of exposed (110) facets during graphene-induced surface reconstruction. Moreover, we revealed that CVD-grown graphene orientation is determined by the grain-orientation-dependent surface morphology. By leveraging the so-established correlations between grain index with both graphene growth priority and its orientation, we achieve controlled folding and tearing of graphene overlayer using a pair of adjacent grains with dramatically different catalytical activity and kink-free atomic steps. We reveal that overlayer-induced step bunching and terrace reconfiguration critically govern the domain morphology and folding direction. Building on this mechanistic insight, we demonstrate a substrate-engineering framework where specific platinum grains are rationally selected to yield TGLs with pre-designed twist angles, including magic angle with flat band dispersion. This work not only highlights fundamental kinetics of Pt catalyzed graphene CVD growth, but also offers a generalizable methodology for manipulating foldable two-dimensional materials via dynamic substrate reconstruction, exampled by programmable growth of high-quality TGLs on open surfaces. △ Less

Submitted 16 April, 2026; originally announced April 2026.

Comments: 43 pages, 6 figures. Main text only, including Methods and References

Turning Insulators into Accelerators: Deciphering the Interfacial Conductivity Boost in ZrO2-Li2ZrCl6 Composites through Machine Learning Molecular Dynamics Simulations

Authors: Boyuan Xu, Chen Qian, Liyi Bai, Chenlu Wang, Feng Ding, Qisheng Wu

Abstract: Halide solid-state electrolytes have emerged as promising candidates for all-solid-state lithium batteries due to their high oxidative stability and deformability, yet their moderate ionic conductivity remains a bottleneck. While incorporating ionically insulating ZrO2 nanoparticles (Nat. Commun. 2023, 14, 2459) has been experimentally shown to enhance the ionic conductivity of Li2ZrCl6, the atomi… ▽ More Halide solid-state electrolytes have emerged as promising candidates for all-solid-state lithium batteries due to their high oxidative stability and deformability, yet their moderate ionic conductivity remains a bottleneck. While incorporating ionically insulating ZrO2 nanoparticles (Nat. Commun. 2023, 14, 2459) has been experimentally shown to enhance the ionic conductivity of Li2ZrCl6, the atomistic origin governing this interfacial phenomenon remains unclear. Here, we bridge the spatiotemporal gap in modeling complex heterostructures by developing an accurate machine-learned force fields based on neuroevolution potential, enabling large-scale molecular dynamics simulations of ZrO2/Li2ZrCl6 heterostructures. By systematically investigating four representative low-lattice-mismatch ZrO2/Li2ZrCl6 interfaces, we identify spontaneous interfacial amorphization driven by space-charge effects upon surface cleavage, trapping Li+ and leading to under-coordinated Li+ polyhedrons with pronounced geometric distortion. These distorted amorphous interfacial regions exhibit markedly enhanced Li+ hopping activity, significantly outperforming the bulk lattice, provided that local mobile Li+ inventory is not depleted by surface charge redistribution. This work establishes a computational framework for training validated machine-learned force fields for interfaces and provides mechanistic understandings of the interfacial conductivity boost in the insulator-conductor composites, guiding the rational design of electrolytes toward next-generation solid-state batteries. △ Less

Submitted 30 January, 2026; originally announced January 2026.

Observation of robust macroscale structural superlubricity

Authors: Minhao Han, Deli Peng, Dinglin Yang, Jin Wang, Yi Zheng, Guofeng Hu, Meng Qi, Yifan Shao, Jiaying Li, Feng Ding, Zhiping Xu, Michael Urbakh, Quanshui Zheng

Abstract: Structural superlubricity (SSL) promises nearly frictionless and wearless sliding, but has until now been considered a special and extreme interfacial phenomenon limited to micro- and nanoscale contacts. Here, we demonstrate robust macroscale SSL within a single sub-millimeter graphite contact. Previously reported near-zero friction coefficients, where friction is nearly independent of normal load… ▽ More Structural superlubricity (SSL) promises nearly frictionless and wearless sliding, but has until now been considered a special and extreme interfacial phenomenon limited to micro- and nanoscale contacts. Here, we demonstrate robust macroscale SSL within a single sub-millimeter graphite contact. Previously reported near-zero friction coefficients, where friction is nearly independent of normal load, have only been observed at microscale contacts under low loads. Our system expands both contact size and load into the macroscopic regime, exhibiting friction coefficients that fluctuate around zero and reach values as low as $10^{-6}$ across a broad load range from 1 mN to 0.5 N. Negative friction coefficients are also observed. Similar behavior is observed at graphite/MoS$_2$ interfaces, indicating that macroscale SSL is a generalizable phenomenon across flat layered materials. These findings overturn long-standing scaling limitations and establish macroscale SSL as a paradigm-shifting platform for next-generation mechanical and electromechanical systems. △ Less

Submitted 31 December, 2025; originally announced January 2026.

Comments: 11 pages, 4 figures

Journal ref: Physical Review Letters 136, 076201 (2026)

Non-Euclidean interfaces decode the continuous landscape of graphene-induced surface reconstructions

Authors: Li-Qun Shen, Hao-Jin Wang, Mengzhao Sun, Yang Xiang, Xin-Ning Tian, Yue Chai, Yue Yang, Feng Ding, Xiao Kong, Marc-Georg Willinger, Zhu-Jun Wang

Abstract: Interfacial reconstruction between two-dimensional (2D) materials and metal substrates fundamentally governs heterostructure properties, yet conventional flat substrates fail to capture the continuous crystallographic landscape. Here, we overcome this topological limitation using non-Euclidean interfaces-curved 2D graphene-copper surfaces as a model system-to traverse the infinite spectrum of latt… ▽ More Interfacial reconstruction between two-dimensional (2D) materials and metal substrates fundamentally governs heterostructure properties, yet conventional flat substrates fail to capture the continuous crystallographic landscape. Here, we overcome this topological limitation using non-Euclidean interfaces-curved 2D graphene-copper surfaces as a model system-to traverse the infinite spectrum of lattice orientations. By integrating multimodal microscopy with a deep-learning-enhanced dimensional upscaling framework, we translate 2D scanning electron microscopy (SEM) contrast into quantitative three-dimensional (3D) morphologies with accurate facet identification. Coupling these observations with machine-learning-assisted density functional theory, we demonstrate that reconstruction is governed by a unified thermodynamic mechanism where high-index facets correspond to specific local minima in the surface energy landscape. This work resolves the long-standing complexity of graphene-copper faceting and establishes non-Euclidean surface topologies as a generalizable paradigm for decoding and controlling interfacial reconstruction in diverse metal-2D material systems. △ Less

Submitted 30 December, 2025; originally announced December 2025.

From cluster to nanocrystal: the continuous evolution and critical size of copper clusters revealed by machine learning

Authors: Hongsheng Liu, Luneng Zhao, Yaning Li, Yuan Chang, Shi Qiu, Xiao Wang, Junfeng Gao, Feng Ding

Abstract: The evolution of cluster structure with size and the critical size for the transition from cluster to nanocrystal have long been fundamental problems in nanoscience. Due to limitations of experimental technology and computational methods, the exploration of the continuous evolution of clusters towards nanocrystal is still a big challenge. Here, we proposed a machine learning force field (MLFF) tha… ▽ More The evolution of cluster structure with size and the critical size for the transition from cluster to nanocrystal have long been fundamental problems in nanoscience. Due to limitations of experimental technology and computational methods, the exploration of the continuous evolution of clusters towards nanocrystal is still a big challenge. Here, we proposed a machine learning force field (MLFF) that can generalize well to various copper systems ranging from small clusters to large clusters and bulk. The continuous evolution of copper clusters CuN towards nanocrystal was revealed by investigating clusters in a wide size range (7 <= N <= 17885) based on MLFF simulated annealing. For small CuN (N < 40), electron counting rule plays a major role in stability. For large CuN (N > 80), geometric magic number rule plays a dominant role and the evolution of clusters is based on the formation of more and more icosahedral shells. For medium size CuN (40 <= N <= 80), both rules contribute. The critical size from cluster to nanocrystal was calculated to be around 8000 atoms (about 6 nm in diameter). Our work terminates the long-term challenge in nanoscience, and lay the methodological foundation for subsequent research on other cluster systems. △ Less

Submitted 24 December, 2025; originally announced December 2025.

Artificial Intelligence-Enabled Holistic Design of Catalysts Tailored for Semiconducting Carbon Nanotube Growth

Authors: Liu Qian, Yue Li, Ying Xie, Jian Zhang, Pai Li, Yue Yu, Zhe Liu, Feng Ding, Jin Zhang

Abstract: Catalyst design is crucial for materials synthesis, especially for complex reaction networks. Strategies like collaborative catalytic systems and multifunctional catalysts are effective but face challenges at the nanoscale. Carbon nanotube synthesis contains complicated nanoscale catalytic reactions, thus achieving high-density, high-quality semiconducting CNTs demands innovative catalyst design.… ▽ More Catalyst design is crucial for materials synthesis, especially for complex reaction networks. Strategies like collaborative catalytic systems and multifunctional catalysts are effective but face challenges at the nanoscale. Carbon nanotube synthesis contains complicated nanoscale catalytic reactions, thus achieving high-density, high-quality semiconducting CNTs demands innovative catalyst design. In this work, we present a holistic framework integrating machine learning into traditional catalyst design for semiconducting CNT synthesis. It combines knowledge-based insights with data-driven techniques. Three key components, including open-access electronic structure databases for precise physicochemical descriptors, pre-trained natural language processing-based embedding model for higher-level abstractions, and physical - driven predictive models based on experiment data, are utilized. Through this framework, a new method for selective semiconducting CNT synthesis via catalyst - mediated electron injection, tuned by light during growth, is proposed. 54 candidate catalysts are screened, and three with high potential are identified. High-throughput experiments validate the predictions, with semiconducting selectivity exceeding 91% and the FeTiO3 catalyst reaching 98.6%. This approach not only addresses semiconducting CNT synthesis but also offers a generalizable methodology for global catalyst design and nanomaterials synthesis, advancing materials science in precise control. △ Less

Submitted 17 December, 2025; originally announced December 2025.

Comments: 16 pages and 4 figures in main text

Atomically-precise synthesis and simultaneous integration of 2D transition metal dichalcogenides enabled by nano-confinement

Authors: Ce Bian, Yifan Zhao, Roger Guzman, Hongtao Liu, Hao Hu, Qi Qi, Ke Zhu, Hao Wang, Kang Wu, Hui Guo, Wanzhen He, Zhaoqing Wang, Peng Peng, Zhiping Xu, Wu Zhou, Feng Ding, Haitao Yang, Hong-Jun Gao

Abstract: Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs), and hBN, exhibit intriguing properties that are sensitive to their atomic-scale structures and can be further enriched through van der Waals (vdW) integration. However, the precise synthesis and clean integration of 2D materials remain challenging. Here, using graphene or hBN as a vdW capping layer, we creat… ▽ More Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs), and hBN, exhibit intriguing properties that are sensitive to their atomic-scale structures and can be further enriched through van der Waals (vdW) integration. However, the precise synthesis and clean integration of 2D materials remain challenging. Here, using graphene or hBN as a vdW capping layer, we create a nano-confined environment that directs the growth kinetics of 2D TMDs (e.g., NbSe2 and MoS2), enabling precise formation of TMD monolayers with tailored morphologies, from isolated monolayer domains to large-scale continuous films and intrinsically-patterned rings. Moreover, Janus S-Mo-Se monolayers are synthesized with atomic precision via vdW-protected bottom-plane chalcogen substitution. Importantly, our approach simultaneously produces ultraclean vdW interfaces. This in situ encapsulation reliably preserves air-sensitive materials, as evidenced by the enhanced superconductivity of nano-confined NbSe2 monolayers. Altogether, our study establishes a versatile platform for the controlled synthesis and integration of 2D TMDs for advanced applications. △ Less

Submitted 17 December, 2025; originally announced December 2025.

Comments: Accepted in principle by Nature Materials

Spectral shadows of a single GaAs quantum dot

Authors: Kai Hühn, Lena Klar, Fei Ding, Arne Ludwig, Andreas D. Wieck, Jens Hübner, Michael Oestreich

Abstract: Semiconductor quantum dots are a promising platform for generating single and entangled photons.Still, their use is limited even in the most advanced structures by changes in the charge state of the quantum dot and its environment. Here, we present detailed time-resolved resonance fluorescence measurements on a single charge-tunable GaAs quantum dot, shedding new light on the spectral shadows invo… ▽ More Semiconductor quantum dots are a promising platform for generating single and entangled photons.Still, their use is limited even in the most advanced structures by changes in the charge state of the quantum dot and its environment. Here, we present detailed time-resolved resonance fluorescence measurements on a single charge-tunable GaAs quantum dot, shedding new light on the spectral shadows invoked by the complex impurity environment. Detuning-dependent measurements reveal the existence of multiple Stark-shifted resonances, which are associated with rare spectral jumps smaller than the homogeneous linewidth and, therefore, typically concealed in the measurement noise. We observe similar environmentally induced Stark shifts for both the neutral exciton and negatively charged trion transitions, while the positively and doubly negatively charged trions exhibit significant differences. Our investigation quantifies the underlying impurity charge dynamics over a range from well below milliseconds to seconds, revealing that the hole occupation of the positively charged trion transition is constrained by rapid hole loss and slow hole recapture dynamics. Utilizing a second non-resonant laser, we increase the hole occupancy by over an order of magnitude and identify both a prolonged hole residence time and an enhanced hole tunneling rate into the quantum dot. These findings are supported by complementary spin noise spectroscopy measurements, which offer a significantly higher bandwidth compared to the time-resolved resonance fluorescence measurements. △ Less

Submitted 22 April, 2026; v1 submitted 27 July, 2025; originally announced July 2025.

Step-directed Epitaxy of Uni-directional Hexagonal Boron Nitride on Vicinal Ge(110)

Authors: Ju-Hyun Jung, Chao Zhao, Seong-Jun Yang, Jun-Ho Park, Woo-Ju Lee, Su-Beom Song, Jonghwan Kim, Chan-Cuk Hwang, Seung-Hwa Baek, Feng Ding, Cheol-Joo Kim

Abstract: Insulating hexagonal boron nitride (hBN) films with precisely controlled thickness are ideal dielectric components to modulate various interfaces in electronic devices. To achieve this, high-quality hBN with controlled atomic configurations must be able to form pristine interfaces with various materials in devices. However, previously reported large-scale hBN films with uniform thickness either ar… ▽ More Insulating hexagonal boron nitride (hBN) films with precisely controlled thickness are ideal dielectric components to modulate various interfaces in electronic devices. To achieve this, high-quality hBN with controlled atomic configurations must be able to form pristine interfaces with various materials in devices. However, previously reported large-scale hBN films with uniform thickness either are polycrystalline or are not suitable for atomically clean assembly via mechanical exfoliation, limiting their applications in device technology. Here, we report the large-scale growth of monolayer single crystalline hBN films on Ge(110) substrates by using chemical vapor deposition (CVD). Vicinal Ge(110) substrates are used for the step-directed epitaxial growth of hBN, where Ge atomic steps act as the hBN nucleation sites, guiding the uni-directional alignments of multiple hBN domains. Density functional theory (DFT) calculations reveal that the optimum hydrogen passivations on both hBN edges and Ge surfaces enable the epitaxial coupling between hBN and the Ge step edges and the single crystallinity of the final hBN films. Using epitaxially grown monolayer hBN films, we fabricate a few hBN films with controlled stacking orders and pristine interfaces through a layer-by-layer assembly process. These films function as high-quality dielectrics to enhance carrier transport in graphene and MoS₂ channels. △ Less

Submitted 25 July, 2025; originally announced July 2025.

Comments: 38 pages,16 figures

Journal ref: Small Structures 5 (2024) 2400297

Ultrastable, low-error dynamic polarization encoding of deterministically generated single photons

Authors: Joscha Hanel, Zenghui Jiang, Jipeng Wang, Frederik Benthin, Tom Fandrich, Eddy Patrick Rugeramigabo, Raphael Joos, Michael Jetter, Simone Luca Portalupi, Jingzhong Yang, Michael Zopf, Peter Michler, Fei Ding

Abstract: The ability to inscribe information on single photons at high speeds is a crucial requirement for quantum applications such as quantum communication and measurement-based photonic quantum computation. Nowadays, most experimental implementations employ phase modulators in single-pass, Mach-Zehnder interferometer or Michelson interferometer configurations to encode information on photonic qubits. Ho… ▽ More The ability to inscribe information on single photons at high speeds is a crucial requirement for quantum applications such as quantum communication and measurement-based photonic quantum computation. Nowadays, most experimental implementations employ phase modulators in single-pass, Mach-Zehnder interferometer or Michelson interferometer configurations to encode information on photonic qubits. However, these approaches are intrinsically sensitive to environmental influences, limiting the achievable quantum error rates in practice. We report on the first demonstration of a polarization encoder for single-photon qubits based on a free-space Sagnac interferometer, showcasing inherent phase stability and overcoming previous error rate limitations. Telecom-wavelength single photons emitted by a quantum dot are modulated by the encoder under a repetition rate of 152 MHz. A quantum bit error rate of 0.69(2)% is achieved, marking the lowest error rate reported to date for high-speed information encoding on single photons. This work represents a key advance towards robust, scalable, and low-error quantum information processing with single photon sources. △ Less

Submitted 22 July, 2025; originally announced July 2025.

Comments: 13 pages, 4 figures

Preferred Synthesis of Armchair Transition Metal Dichalcogenide Nanotubes

Authors: Abid, Luneng Zhao, Ju Huang, Yongjia Zheng, Yuta Sato, Tianyu Wang, Dmitry Levshov, Lingfeng Wang, Qingyun Lin, Zhen Han, Chunxia Yang, Bill Herve Nduwarugira, Yicheng Ma, Yige Zheng, Hang Wang, Salman Ullah, Afzal Khan, Qi Zhang, Wenbin Li, Junfeng Gao, Bingfeng Ju, Feng Ding, Yan Li, Wouter Herrebout, Kazu Suenaga , et al. (3 additional authors not shown)

Abstract: In this work, we present the synthesis of transition-metal dichalcogenide (TMDC) nanotubes with a preferred chiral angle. SnS2, MoS2, and WS2 are formed with high yield and structural purity inside the channels of boron nitride nanotubes. Atomic-resolution imaging, nano-area electron diffraction, and Circular Dichroism spectroscopy reveal that these synthesized TMDC nanotubes prefer to have an arm… ▽ More In this work, we present the synthesis of transition-metal dichalcogenide (TMDC) nanotubes with a preferred chiral angle. SnS2, MoS2, and WS2 are formed with high yield and structural purity inside the channels of boron nitride nanotubes. Atomic-resolution imaging, nano-area electron diffraction, and Circular Dichroism spectroscopy reveal that these synthesized TMDC nanotubes prefer to have an armchair configuration, with a probability up to 84%. Density functional theory reveals a negligible difference in the formation energy between armchair and zigzag nanotubes, suggesting that the chirality preference does not originate from the differences in structural stability. However, a detailed TEM investigation revealed that these TMDC nanotubes formed via a transition state of nanoribbons, and these nanoribbons are energetically more stable in a zigzag configuration. Subsequent machine learning potential molecular dynamics simulations verify that zigzag nanoribbons do roll up to form an armchair SnS2 nanotubes. Finally, this "zigzag nanoribbon to armchair nanotube" transition process is directly observed in real time by in-situ transmission electron microscopy. This work demonstrates the first, but likely general, experimental strategy for synthesizing chirality-preferred TMDC nanotubes. △ Less

Submitted 12 March, 2026; v1 submitted 19 June, 2025; originally announced June 2025.

Time-bin encoded quantum key distribution over 120 km with a telecom quantum dot source

Authors: Jipeng Wang, Joscha Hanel, Zenghui Jiang, Raphael Joos, Michael Jetter, Eddy Patrick Rugeramigabo, Simone Luca Portalupi, Peter Michler, Xiao-Yu Cao, Hua-Lei Yin, Shan Lei, Jingzhong Yang, Michael Zopf, Fei Ding

Abstract: Quantum key distribution (QKD) with deterministic single photon sources has been demonstrated over intercity fiber and free-space channels. The previous implementations relied mainly on polarization encoding schemes, which are susceptible to birefringence, polarization-mode dispersion and polarization-dependent loss in practical fiber networks. In contrast, time-bin encoding offers inherent robust… ▽ More Quantum key distribution (QKD) with deterministic single photon sources has been demonstrated over intercity fiber and free-space channels. The previous implementations relied mainly on polarization encoding schemes, which are susceptible to birefringence, polarization-mode dispersion and polarization-dependent loss in practical fiber networks. In contrast, time-bin encoding offers inherent robustness and has been widely adopted in mature QKD systems using weak coherent laser pulses. However, its feasibility in conjunction with a deterministic single-photon source has not yet been experimentally demonstrated. In this work, we construct a time-bin encoded QKD system employing a high-brightness quantum dot (QD) single-photon source operating at telecom wavelength. Our proof-of-concept experiment successfully demonstrates the possibility of secure key distribution over fiber link of 120 km, while maintaining extraordinary long-term stability over 6 hours of continuous operation. This work provides the first experimental validation of integrating a quantum dot single-photon source with time-bin encoding in a telecom-band QKD system. In addition, it demonstrates the highest secure key rate among the time-bin QKDs based on single-photon sources. This development signifies a substantial advancement in the establishment of a robust and scalable QKD network based on solid-state single-photon technology △ Less

Submitted 26 February, 2026; v1 submitted 18 June, 2025; originally announced June 2025.

Comments: 15 pages, 4 figures

Journal ref: Light Sci Appl 15, 126 (2026)

How oxygen influences the catalytic activity of iron during carbon nanotube nucleation

Authors: Ben McLean, Alister J. Page, Feng Ding

Abstract: The catalytic activity of metal nanoparticles toward nucleation of single-walled carbon nanotubes (SWCNTs) is fundamental to achieving structure-controlled growth using catalytic chemical vapor deposition (CVD). Despite the success of oxidized catalysts in SWCNT growth, there is a lack of understanding regarding how oxygen influences the catalysts and the nucleation process. Quantum chemical molec… ▽ More The catalytic activity of metal nanoparticles toward nucleation of single-walled carbon nanotubes (SWCNTs) is fundamental to achieving structure-controlled growth using catalytic chemical vapor deposition (CVD). Despite the success of oxidized catalysts in SWCNT growth, there is a lack of understanding regarding how oxygen influences the catalysts and the nucleation process. Quantum chemical molecular dynamics (MD) simulations employing density functional tight binding (DFTB) demonstrate that the kinetics of carbon nucleation on an iron nanoparticle catalyst can be tuned via oxygen loading. Increasing the oxygen content in the catalyst leads to activation of surface-bound carbon species and enhanced carbon chain growth due to respective weakening and strengthening of the C-C and Fe-C bonding. This is due to oxygen modulating the electronic structure of the iron catalyst, with the Fermi level of the catalyst increasing proportionally with oxygen content until the iron:oxygen stoichiometry reaches parity. The increase in Fe 3d states near the Fermi level also promotes the donation of electron density into unoccupied C 2p states, activating C-C bonds which in turn facilitates carbon chain growth and slows carbon ring condensation. △ Less

Submitted 13 June, 2025; originally announced June 2025.

Comments: 19 pages, 3 Figures

Advances in modeling complex materials: The rise of neuroevolution potentials

Authors: Penghua Ying, Cheng Qian, Rui Zhao, Yanzhou Wang, Feng Ding, Shunda Chen, Zheyong Fan

Abstract: Interatomic potentials are essential for driving molecular dynamics (MD) simulations, directly impacting the reliability of predictions regarding the physical and chemical properties of materials. In recent years, machine-learned potentials (MLPs), trained against first-principles calculations, have become a new paradigm in materials modeling as they provide a desirable balance between accuracy an… ▽ More Interatomic potentials are essential for driving molecular dynamics (MD) simulations, directly impacting the reliability of predictions regarding the physical and chemical properties of materials. In recent years, machine-learned potentials (MLPs), trained against first-principles calculations, have become a new paradigm in materials modeling as they provide a desirable balance between accuracy and computational cost. The neuroevolution potential (NEP) approach, implemented in the open-source GPUMD software, has emerged as a promising machine-learned potential, exhibiting impressive accuracy and exceptional computational efficiency. This review provides a comprehensive discussion on the methodological and practical aspects of the NEP approach, along with a detailed comparison with other representative state-of-the-art MLP approaches in terms of training accuracy, property prediction, and computational efficiency. We also demonstrate the application of the NEP approach to perform accurate and efficient MD simulations, addressing complex challenges that traditional force fields typically can not tackle. Key examples include structural properties of liquid and amorphous materials, chemical order in complex alloy systems, phase transitions, surface reconstruction, material growth, primary radiation damage, fracture in two-dimensional materials, nanoscale tribology, and mechanical behavior of compositionally complex alloys under various mechanical loadings. This review concludes with a summary and perspectives on future extensions to further advance this rapidly evolving field. △ Less

Submitted 19 January, 2025; originally announced January 2025.

Comments: 43 pages, 28 figures

Journal ref: Chem. Phys. Rev. 6, 011310 (2025)

Theoretical Studies on Sodium Storage Mechanism in Hard Carbon Anodes of Sodium-Ion Batteries: Molecular Simulations Based on Machine Learning Force Fields

Authors: Zhaoming Wang, Guanghui Shi, Guanghui Wang, Man Wang, Xiao Wang, Feng Ding

Abstract: Sodium-ion batteries (SIBs) have garnered significant attention in recent years as a promising alternative to lithium-ion batteries (LIBs) due to their low cost, abundant sodium resources, and excellent cycling performance. Hard carbon materials, characterized by their high specific capacity, outstanding cycling stability, and low cost, have emerged as potential candidates for SIB anodes. However,… ▽ More Sodium-ion batteries (SIBs) have garnered significant attention in recent years as a promising alternative to lithium-ion batteries (LIBs) due to their low cost, abundant sodium resources, and excellent cycling performance. Hard carbon materials, characterized by their high specific capacity, outstanding cycling stability, and low cost, have emerged as potential candidates for SIB anodes. However, the sodium storage mechanism in hard carbon anodes remains highly complex, especially in disordered structures, and is yet to be fully understood. To address this, we employed relative machine learning force fields (MLFFs) in conjunction with multiscale simulation techniques to systematically investigate the sodium storage behavior in hard carbon. By integrating simulations, this study provides a detailed exploration of sodium adsorption, intercalation, and filling mechanisms. High-precision, large-scale simulations reveal the dynamic behavior and distribution patterns of sodium ions in hard carbon. The findings not only deepen our understanding of sodium storage mechanisms in hard carbon anodes, but also offer a theoretical foundation for optimizing future SIB designs, while introducing novel simulation methodologies and technical frameworks to enhance battery performance. △ Less

Submitted 3 December, 2024; v1 submitted 29 November, 2024; originally announced December 2024.

Comments: 23 pages, 7 figures

Intermediates of Forming Transition Metal Dichalcogenide Heterostructures Revealed by Machine Learning Simulations

Authors: Luneng Zhao, Hongsheng Liu, Yuan Chang, Xiaoran Shi, Jijun Zhao, Feng Ding, Junfeng Gao

Abstract: Two-dimensional (2D) transition metal dichalcogenide (TMD) van der Waals heterostructures (vdWHs) hold promise for high-performance electronics, but their large-scale synthesis remains limited by size constraints and alloying contaminations. Recently, a two-step vapor deposition method was reported for growing wafer-size TMD vdWHs with minimal impurities. In this study, we develop a machine learni… ▽ More Two-dimensional (2D) transition metal dichalcogenide (TMD) van der Waals heterostructures (vdWHs) hold promise for high-performance electronics, but their large-scale synthesis remains limited by size constraints and alloying contaminations. Recently, a two-step vapor deposition method was reported for growing wafer-size TMD vdWHs with minimal impurities. In this study, we develop a machine learning potential (MLP) that accurately captures the atomic-scale dynamic growth process of bilayer MoS$_2$/WS$_2$ vdWHs under feasible growth conditions. Our simulations uncover a crucial metastable SMMS (M = Mo or W) intermediate structure that facilitates metal atom swap and alloying. Eliminating the alloying contamination requires preventing the embedding of bare metal atoms. The results also show that the SMMS structure exhibits favourable electronic properties and emerges as a low Schottky barrier contact electrode for MoS$_2$ field-effect transistors (FETs). △ Less

Submitted 24 February, 2026; v1 submitted 8 May, 2024; originally announced May 2024.

Ultra-Long Homochiral Graphene Nanoribbons Grown Within h-BN Stacks for High-Performance Electronics

Authors: Bosai Lyu, Jiajun Chen, Sen Wang, Shuo Lou, Peiyue Shen, Jingxu Xie, Lu Qiu, Izaac Mitchell, Can Li, Cheng Hu, Xianliang Zhou, Kenji Watanabe, Takashi Taniguchi, Xiaoqun Wang, Jinfeng Jia, Qi Liang, Guorui Chen, Tingxin Li, Shiyong Wang, Wengen Ouyang, Oded Hod, Feng Ding, Michael Urbakh, Zhiwen Shi

Abstract: Van der Waals encapsulation of two-dimensional materials within hexagonal boron nitride (h-BN) stacks has proven to be a promising way to create ultrahigh-performance electronic devices. However, contemporary approaches for achieving van der Waals encapsulation, which involve artificial layer stacking using mechanical transfer techniques, are difficult to control, prone to contamination, and unsca… ▽ More Van der Waals encapsulation of two-dimensional materials within hexagonal boron nitride (h-BN) stacks has proven to be a promising way to create ultrahigh-performance electronic devices. However, contemporary approaches for achieving van der Waals encapsulation, which involve artificial layer stacking using mechanical transfer techniques, are difficult to control, prone to contamination, and unscalable. Here, we report on the transfer-free direct growth of high-quality graphene nanoribbons (GNRs) within h-BN stacks. The as-grown embedded GNRs exhibit highly desirable features being ultralong (up to 0.25 mm), ultranarrow ( < 5 nm), and homochiral with zigzag edges. Our atomistic simulations reveal that the mechanism underlying the embedded growth involves ultralow GNR friction when sliding between AA'-stacked h-BN layers. Using the grown structures, we demonstrate the transfer-free fabrication of embedded GNR field-effect devices that exhibit excellent performance at room temperature with mobilities of up to 4,600 $cm^{2} V^{-1} s^{-1}$ and on-off ratios of up to $10^{6}$. This paves the way to the bottom-up fabrication of high-performance electronic devices based on embedded layered materials. △ Less

Submitted 18 March, 2024; originally announced March 2024.

Profiles of static liquid-gas interfaces in axisymmetrical containers under different accelerations

Authors: Shangtong Chen, Yong Gao, Wen Li, Fenglin Ding, Jintao Liu, Yong Li

Abstract: Second order perturbation solutions of profiles of bubbles suspended in liquid and liquid gas interfaces when liquid all sinks in the bottom under different accelerations are derived. Six procedures are developed based on these solutions, and they are divided into two types. One takes coordinates of endpoints of profiles as inputs, and the other takes liquid volume or gas volume as inputs. Numeric… ▽ More Second order perturbation solutions of profiles of bubbles suspended in liquid and liquid gas interfaces when liquid all sinks in the bottom under different accelerations are derived. Six procedures are developed based on these solutions, and they are divided into two types. One takes coordinates of endpoints of profiles as inputs, and the other takes liquid volume or gas volume as inputs. Numerical simulation are performed with the Volume of Fluid method and numerical results are in good agreement with predictions of these procedures. Besides, the bigger the acceleration, the more flatter the bubble will be until all liquid sinks to the bottom. Effects of accelerations on bubbles shape must be considered. When liquid all sinks to the bottom, predictions of liquid volume with the same liquid meniscus height as inputs differs a lot under different accelerations. The most significant change of liquid volume is when Bond is much smaller than 1. Effects of accelerations and liquid contact angle on liquid gas interfaces must be considered during evaluating liquid residue, and these findings will be great helpful for liquid residue measurement and fine management in space. △ Less

Submitted 30 January, 2024; originally announced January 2024.

Comments: 33 pages. 15 figures

MSC Class: 76Txx

Probing charge redistribution at the interface of self-assembled cyclo-P5 pentamers on Ag(111)

Authors: Outhmane Chahib, Yulin Yin, Jung-Ching Liu, Chao Li, Thilo Glatzel, Feng Ding, Qinghong Yuan, Ernst Meyer, Rémy Pawlak

Abstract: Phosphorus pentamer (cyclo-P5-) ions are unstable in nature but can be synthesized at the Ag(111) surface. Unlike monolayer black phosphorous, little is known about their electronic properties when in contact with metal electrodes, although this is crucial for future applications. Here we characterize the atomic structure of cyclo-P5 assembled on Ag(111) using atomic force microscopy with function… ▽ More Phosphorus pentamer (cyclo-P5-) ions are unstable in nature but can be synthesized at the Ag(111) surface. Unlike monolayer black phosphorous, little is known about their electronic properties when in contact with metal electrodes, although this is crucial for future applications. Here we characterize the atomic structure of cyclo-P5 assembled on Ag(111) using atomic force microscopy with functionalized tips and density functional theory. Combining force and tunneling spectroscopy, we find that a strong charge transfer induces an inward dipole moment at the cyclo-P5/Ag interface as well as the formation of an interface state. We probe the image potential states by field-effect resonant tunneling and quantify the increase of the local change of work function of 0.46 eV at the cyclo-P5 assembly. Our results suggest that the high-quality of the cyclo-P5/Ag interface might serve as a prototypical system for electric contacts in phosphorus-based semiconductor devices. △ Less

Submitted 23 January, 2024; originally announced January 2024.

Comments: 25 pages, 4 figures

Circular photonic crystal grating design for charge-tunable quantum light sources in the telecom C-band

Authors: Chenxi Ma, Jingzhong Yang, Pengji Li, Eddy P. Rugeramigabo, Michael Zopf, Fei Ding

Abstract: Efficient generation of entangled photon pairs at telecom wavelengths is a key ingredient for long-range quantum networks. While embedding semiconductor quantum dots into hybrid circular Bragg gratings has proven effective, it conflicts with $p$-$i$-$n$ diode heterostructures which offer superior coherence. We propose and analyze hybrid circular photonic crystal gratings, incorporating air holes t… ▽ More Efficient generation of entangled photon pairs at telecom wavelengths is a key ingredient for long-range quantum networks. While embedding semiconductor quantum dots into hybrid circular Bragg gratings has proven effective, it conflicts with $p$-$i$-$n$ diode heterostructures which offer superior coherence. We propose and analyze hybrid circular photonic crystal gratings, incorporating air holes to facilitate charge carrier transport without compromising optical properties. Through numerical simulations, a broad cavity mode with a Purcell factor of 23 enhancing both exciton and biexciton transitions, and exceptional collection efficiency of 92.4% into an objective with numerical aperture of 0.7 are achieved. Furthermore, our design demonstrates direct coupling efficiency over 90% into a single-mode fiber over the entire telecom C-band. The hybrid circular photonic crystal grating thereby emerges as a promising solution for the efficient generation of highly coherent, polarization-entangled photon pairs. △ Less

Submitted 12 January, 2024; v1 submitted 2 January, 2024; originally announced January 2024.

Comments: 12 pages, 5 figures

Journal ref: Opt. Express 32, 14789-14800 (2024)

Corrosion-resistant aluminum alloy design through machine learning combined with high-throughput calculations

Authors: Yucheng Ji, Xiaoqian Fu, Feng Ding, Yongtao Xu, Yang He, Min Ao, Fulai Xiao, Dihao Chen, Poulumi Dey, Kui Xiao, Jingli Ren, Xiaogang Li, Chaofang Dong

Abstract: Efficiently designing lightweight alloys with combined high corrosion resistance and mechanical properties remains an enduring topic in materials engineering. To this end, machine learning (ML) coupled ab-initio calculations is proposed within this study. Due to the inadequate accuracy of conventional stress-strain ML models caused by corrosion factors, a novel reinforcement self-learning ML algor… ▽ More Efficiently designing lightweight alloys with combined high corrosion resistance and mechanical properties remains an enduring topic in materials engineering. To this end, machine learning (ML) coupled ab-initio calculations is proposed within this study. Due to the inadequate accuracy of conventional stress-strain ML models caused by corrosion factors, a novel reinforcement self-learning ML algorithm (accuracy R2 >0.92) is developed. Then, a strategy that integrates ML models, calculated energetics and mechanical moduli is implemented to optimize the Al alloys. Next, this Computation Designed Corrosion-Resistant Al alloy is fabricated that verified the simulation. The performance (elongation reaches ~30%) is attributed to the H-captured Al-Sc-Cu phases (-1.44 eV H-1) and Cu-modified η/η' precipitation inside the grain boundaries (GBs). The developed Al-Mg-Zn-Cu interatomic potential (energy accuracy 6.50 meV atom-1) proves the cracking resistance of the GB region enhanced by Cu-modification. Conceptually, our strategy is of practical importance for designing new alloys exhibiting corrosion resistance and mechanical properties. △ Less

Submitted 26 December, 2023; originally announced December 2023.

Experimental verification of the inverse Anomalous spin Hall effect with perpendicular magnetic anisotropy Materials

Authors: J. E. Abrão, A. R. Rodrigues, H. F. Ding, S. Bedanta, A. Azevedo

Abstract: In this work, the spin pumping technique was employed to investigate the anomalous inverse spin Hall effect in BIG/NiO/Fe samples where BIG[(Bi,Tm)3(Fe,Ga)5O12] exhibits perpendicular magnetic anisotropy. Our results reveal an intriguing phenomenon: when the magnetizations of both ferromagnetic layers align perpendicularly, a distinct spin-to-charge current conversion mechanism occurs. This conver… ▽ More In this work, the spin pumping technique was employed to investigate the anomalous inverse spin Hall effect in BIG/NiO/Fe samples where BIG[(Bi,Tm)3(Fe,Ga)5O12] exhibits perpendicular magnetic anisotropy. Our results reveal an intriguing phenomenon: when the magnetizations of both ferromagnetic layers align perpendicularly, a distinct spin-to-charge current conversion mechanism occurs. This conversion is intricately linked to the magnetization of the converting layer, spin polarization, and the spin current orientation. △ Less

Submitted 11 December, 2023; originally announced December 2023.

Comments: 11 pages, 4 figures

Converse Flexoelectricity of Low-Dimensional Bismuth Selenite (Bi2Se3) Revealed by Piezoresponse Force Microscopy (PFM)

Authors: Qiong Liu, S. S. Nanthakumar, Bin Li, Teresa Cheng, Florian Bittner, Chenxi Ma, Fei Ding, Lei Zheng, Bernhard Roth, Xiaoying Zhuang

Abstract: Many kinds of two-dimensional (2D) van der Waals (vdW) have been demonstrated to exhibit electromechanical coupling effects, which makes them promising candidates for next-generation devices, such as piezotronics and nanogenerators. Recently, flexoelectricity was found to account for the out-of-plane electromechanical coupling in many 2D transition metal dichalcogenides (TMDs) who only exhibit in-… ▽ More Many kinds of two-dimensional (2D) van der Waals (vdW) have been demonstrated to exhibit electromechanical coupling effects, which makes them promising candidates for next-generation devices, such as piezotronics and nanogenerators. Recently, flexoelectricity was found to account for the out-of-plane electromechanical coupling in many 2D transition metal dichalcogenides (TMDs) who only exhibit in-plane piezoelectricity. However, low dimensional vdW three-dimensional (3D) topological insulators (TIs) have been overlooked regarding their electromechanical properties. In this study, for the first time, we experimentally investigate the electromechanical coupling of low dimensional 3D TIs with a centrosymmetric crystal structure, where a binary compound, bismuth selenite (Bi2Se3), is taken as an example. The results of piezoresponse force microscope (PFM) tests on the Bi2Se3 nanoflakes show that the material exhibits both out-of-plane and in-plane electromechanical responses. The Bi2Se3 nanoflake with a thickness of 37 nm possesses an effective out-of-plane piezoelectric coefficient of ~0.65 pm V-1. With careful analyses, the electromechanical responses are verified to arise from the converse flexoelectricity. The measured effective out-of-plane piezoelectric coefficient is mainly contributed by flexoelectric coefficient, μ_39, which is estimated to be approximately 0.13 nC m-1. However, it is rather difficult to obtain the in-plane component of the flexoelectric tensor from the in-plane PFM measurements since the direction of the in-plane stress is always not normal to the AFM cantilever axis. The results provide useful guidance for understanding the flexoelectric effect of low dimensional vdW materials with centrosymmetric crystal structures. Moreover, the work can pave to way to explore the electromechanical devices based on the flexoelectricity of vdW TIs. △ Less

Submitted 10 November, 2023; originally announced November 2023.

Comments: 6 figures

Anomalous tensile strength and thermal expansion, and low thermal conductivity in wide band gap boron monoxide monolayer

Authors: Bohayra Mortazavi, Fazel Shojaei, Fei Ding, Xiaoying Zhuang

Abstract: Most recently the formation of boron monoxide (BO) in the two-dimensional (2D) form has been confirmed experimentally (J. Am. Chem. Soc. 2023, 145, 14660). Motivated by the aforementioned finding, herein we theoretically explore the key physical properties of the single-layer and suspended BO. Density functional theory (DFT) results reveal that BO monolayer yields a large indirect band gap of 3.78… ▽ More Most recently the formation of boron monoxide (BO) in the two-dimensional (2D) form has been confirmed experimentally (J. Am. Chem. Soc. 2023, 145, 14660). Motivated by the aforementioned finding, herein we theoretically explore the key physical properties of the single-layer and suspended BO. Density functional theory (DFT) results reveal that BO monolayer yields a large indirect band gap of 3.78 (2.18) eV on the basis of HSE06(PBE) functional. Ab-initio molecular dynamics results reveal the remarkable thermal stability of the BO monolayer at 1000 K. The thermal and mechanical properties at room temperature are furthermore investigated using a machine learning interatomic potential (MLIP). The developed MLIP-based model close to the ground state could very precisely reproduce the DFT predictions for the mechanical properties of the BO monolayer. The elastic modulus, tensile strength and lattice thermal conductivity of the BO monolayer at room temperature are predicted to be 107 GPa, 25 GPa and 5.6 W/mK, respectively. At the room temperature the BO monolayer is noticeably predicted to yield an ultrahigh negative thermal expansion coefficient, by almost 17 folds larger than that of the single-layer graphene. The presented results reveal the large indirect electronic band gap, decent thermal and dynamical stability, anomalously low elastic modulus to tensile strength ratio, ultrahigh negative thermal expansion coefficients and low lattice thermal conductivity of the BO monolayer. △ Less

Submitted 30 October, 2023; originally announced October 2023.

Journal ref: FlatChem 2023

Intralayer Negative Poisson's Ratio in Two-Dimensional Black Arsenic by Strain Engineering

Authors: Jingjing Zhang, Weihan Zhang, Leining Zhang, Guoshuai Du, Yunfei Yu, Qinglin Xia, Xu Wu, Yeliang Wang, Wei Ji, Jingsi Qiao, Feng Ding, Yabin Chen

Abstract: Negative Poisson's ratio as the anomalous characteristic generally exists in artificial architectures, such as re-entrant and honeycomb structures. The structures with negative Poisson's ratio have attracted intensive attention due to their unique auxetic effect and many promising applications in shear resistant and energy absorption fields. However, experimental observation of negative Poisson's… ▽ More Negative Poisson's ratio as the anomalous characteristic generally exists in artificial architectures, such as re-entrant and honeycomb structures. The structures with negative Poisson's ratio have attracted intensive attention due to their unique auxetic effect and many promising applications in shear resistant and energy absorption fields. However, experimental observation of negative Poisson's ratio in natural materials barely happened, although various two-dimensional layered materials are predicted in theory. Herein, we report the anisotropic Raman response and the intrinsic intralayer negative Poisson's ratio of two-dimensional natural black arsenic (b-As) via strain engineering strategy. The results were evident by the detailed Raman spectrum of b-As under uniaxial strain together with density functional theory calculations. It is found that b-As was softer along the armchair than zigzag direction. The anisotropic mechanical features and van der Waals interactions play essential roles in strain-dependent Raman shifts and negative Poisson's ratio in the natural b-As along zigzag direction. This work may shed a light on the mechanical properties and potential applications of two-dimensional puckered materials. △ Less

Submitted 7 September, 2023; originally announced September 2023.

Comments: 23 pages, 4 figures

Magic momenta and three dimensional Landau levels from a three dimensional graphite moiré superlattice

Authors: Xin Lu, Bo Xie, Yue Yang, Xiao Kong, Jun Li, Feng Ding, Zhu-Jun Wang, Jianpeng Liu

Abstract: Twisted bilayer graphene (TBG) and other quasi-two-dimensional moiré superlattices have attracted significant attention due to the emergence of various correlated and topological states associated with the flat bands in these systems. In this work, we theoretically explore the physical properties of a new type of \textit{three dimensional graphite moiré superlattice}, the bulk alternating twisted… ▽ More Twisted bilayer graphene (TBG) and other quasi-two-dimensional moiré superlattices have attracted significant attention due to the emergence of various correlated and topological states associated with the flat bands in these systems. In this work, we theoretically explore the physical properties of a new type of \textit{three dimensional graphite moiré superlattice}, the bulk alternating twisted graphite (ATG) system with homogeneous twist angle, which is grown by in situ chemical vapor decomposition method. Compared to TBG, the bulk ATG system is bestowed with an additional wavevector degrees of freedom due to the extra dimensionality. As a result, we find that when the twist angle of bulk ATG is smaller than twice of the magic angle of TBG, there always exist ``magic momenta" at which the in-plane Fermi velocities of the moiré bands vanish. Moreover, topologically distinct flat bands of TBG at different magic angles can even co-exist at different out-of-plane wavevectors in a single bulk ATG system. Most saliently, when the twist angle is relatively large, exactly dispersionless three dimensional zeroth Landau level would emerge in the bulk ATG, which may give rise to robust three dimensional quantum Hall effects over a large range of twist angles. △ Less

Submitted 2 September, 2023; originally announced September 2023.

Comments: 6 pages + 3 figures in main text, 17 pages + 13 figures in Supplementary Information

Journal ref: Phys. Rev. Lett. 132, 056601 (2024)

High-rate intercity quantum key distribution with a semiconductor single-photon source

Authors: Jingzhong Yang, Zenghui Jiang, Frederik Benthin, Joscha Hanel, Tom Fandrich, Raphael Joos, Stephanie Bauer, Sascha Kolatschek, Ali Hreibi, Eddy Patrick Rugeramigabo, Michael Jetter, Simone Luca Portalupi, Michael Zopf, Peter Michler, Stefan Kück, Fei Ding

Abstract: Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of sin… ▽ More Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 * 10^{-5} with an average quantum bit error ratio of ~ 0.65 % are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications. △ Less

Submitted 2 July, 2024; v1 submitted 30 August, 2023; originally announced August 2023.

Comments: 10 pages, 4 figures

Journal ref: Light Sci Appl 13, 150 (2024)

A solid-state source of single and entangled photons at diamond SiV$^-$-center transitions operating at 80K

Authors: Xin Cao, Jingzhong Yang, Tom Fandrich, Yiteng Zhang, Eddy P. Rugeramigabo, Benedikt Brechtken, Rolf J. Haug, Michael Zopf, Fei Ding

Abstract: Large-scale quantum networks require the implementation of long-lived quantum memories as stationary nodes interacting with qubits of light. Epitaxially grown quantum dots hold great potential for the on-demand generation of single and entangled photons with high purity and indistinguishability. Coupling these emitters to memories with long coherence times enables the development of hybrid nanopho… ▽ More Large-scale quantum networks require the implementation of long-lived quantum memories as stationary nodes interacting with qubits of light. Epitaxially grown quantum dots hold great potential for the on-demand generation of single and entangled photons with high purity and indistinguishability. Coupling these emitters to memories with long coherence times enables the development of hybrid nanophotonic devices incorporating the advantages of both systems. Here we report the first GaAs/AlGaAs quantum dots grown by droplet etching and nanohole infilling method, emitting single photons with a narrow wavelength distribution (736.2 $\pm$ 1.7 nm) close to the zero-phonon line of Silicon-vacancy centers. Polarization entangled photons are generated via the biexciton-exciton cascade with a fidelity of (0.73 $\pm$ 0.09). High single photon purity is maintained from 4 K (g$^($$^2$$^)$(0) = 0.07 $\pm$ 0.02) up to 80 K (g$^($$^2$$^)$(0) = 0.11 $\pm$ 0.01), therefore making this hybrid system technologically attractive for real-world quantum photonic applications. △ Less

Submitted 27 April, 2023; originally announced April 2023.

Robust 3.7 V-Na$_{2/3}$[Cu$_{1/3}$Mn$_{2/3}$]O$_2$ Cathode for Na-ion Batteries

Authors: Xiaohui Rong, Xingguo Qi, Quan Zhou, Libin Kang, Dongdong Xiao, Ruijuan Xiao, Feixiang Ding, Yang Yang, Yuan Liu, Yun Su, Shiguang Zhang, Lunhua He, Yaxiang Lu, Liquan Chen, Yong-Sheng Hu

Abstract: Na-ion batteries (NIBs), which are recognized as a next-generation alternative technology for energy storage, still suffer from commercialization constraints due to the lack of low-cost, high-performance cathode materials. Since our first discovery of Cu$^{3+}$/Cu$^{2+}$ electrochemistry in 2014, numerous Cu-substituted/doped materials have been designed for NIBs. However for almost ten years, the… ▽ More Na-ion batteries (NIBs), which are recognized as a next-generation alternative technology for energy storage, still suffer from commercialization constraints due to the lack of low-cost, high-performance cathode materials. Since our first discovery of Cu$^{3+}$/Cu$^{2+}$ electrochemistry in 2014, numerous Cu-substituted/doped materials have been designed for NIBs. However for almost ten years, the potential of Cu$^{3+}$/Cu$^{2+}$ electrochemistry has been grossly underappreciated and normally regarded as a semielectrochemically active redox. Here, we re-synthesized P2-Na$_{2/3}$[Cu$_{1/3}$Mn$_{2/3}$]O$_2$ and reinterpreted it as a high-voltage, cost-efficient, air-stable, long-life, and high-rate cathode material for NIBs, which demonstrates a high operating voltage of 3.7 V and a completely active Cu$^{3+}$/Cu$^{2+}$ redox reaction. The 2.3 Ah cylindrical cells exhibit excellent cycling (93.1% capacity after 2000 cycles), high rate (97.2% capacity at 10C rate), good low-temperature performance (86.6% capacity at -30$^\circ$C), and high safety, based on which, a 56 V-11.5 Ah battery pack for E-bikes is successfully constructed, exhibiting stable cycling (96.5% capacity at the 800th cycle) and a long driving distance (36 km, tester weight 65 kg). This work offers a commercially feasible cathode material for low-cost, high-voltage NIBs, paving the way for advanced NIBs in power and stationary energy storage applications. △ Less

Submitted 27 March, 2023; originally announced March 2023.

Comments: 15 pages, 3 figures, 1 table

Strain control of exciton and trion spin-valley dynamics in monolayer transition metal dichalcogenides

Authors: Zhao An, Pedro Soubelet, Yaroslav Zhumagulov, Michael Zopf, Alex Delhomme, Chenjiang Qian, Paulo E. Faria Junior, Jaroslav Fabian, Xin Cao, Jingzhong Yang, Andreas V. Stier, Fei Ding, Jonathan J. Finley

Abstract: The electron-hole exchange interaction is a fundamental mechanism that drives valley depolarization via intervalley exciton hopping in semiconductor multi-valley systems. Here, we report polarization-resolved photoluminescence spectroscopy of neutral excitons and negatively charged trions in monolayer MoSe$_2$ and WSe$_2$ under biaxial strain. We observe a marked enhancement(reduction) on the WSe… ▽ More The electron-hole exchange interaction is a fundamental mechanism that drives valley depolarization via intervalley exciton hopping in semiconductor multi-valley systems. Here, we report polarization-resolved photoluminescence spectroscopy of neutral excitons and negatively charged trions in monolayer MoSe$_2$ and WSe$_2$ under biaxial strain. We observe a marked enhancement(reduction) on the WSe$_2$ triplet trion valley polarization with compressive(tensile) strain while the trion in MoSe$_2$ is unaffected. The origin of this effect is shown to be a strain dependent tuning of the electron-hole exchange interaction. A combined analysis of the strain dependent polarization degree using ab initio calculations and rate equations shows that strain affects intervalley scattering beyond what is expected from strain dependent bandgap modulations. The results evidence how strain can be used to tune valley physics in energetically degenerate multi-valley systems. △ Less

Submitted 27 March, 2023; originally announced March 2023.

Successive magnetic orderings in the Ising spin chain magnet DyNi$_5$Ge$_3$

Authors: H. Ge, L. Zhang, N. Zhao, J. Yang, L. Wang, L. Zhou, Y. Fu, T. T. Li, Z. M. Song, F. Ding, J. B. Xu, Y. F. Zhang, S. M. Wang, J. W. Mei, X. Tong, P. Miao, H. He, Q. Zhanghang, L. S. Wu, J. M. Sheng

Abstract: In this report, we investigated a new rare earth based one-dimensional Ising spin chain magnet~\DNG~by means of magnetization, specific heat and powder neutron diffraction measurements. Due to the crystalline electrical field splitting, the magnetic Dy ions share an Ising like ground doublet state. Owning to the local point symmetry, these Ising moments form into two canted magnetic sublattices, w… ▽ More In this report, we investigated a new rare earth based one-dimensional Ising spin chain magnet~\DNG~by means of magnetization, specific heat and powder neutron diffraction measurements. Due to the crystalline electrical field splitting, the magnetic Dy ions share an Ising like ground doublet state. Owning to the local point symmetry, these Ising moments form into two canted magnetic sublattices, which were further confirmed by the angle-dependent magnetization measurement. In zero fields, two successive antiferromagnetic phase transitions were found at temperatures $T_{\mathrm{N1}}=6~\rm K$ and $T_{\mathrm{N2}}=5~\rm K$, respectively. Only part of the moments are statically ordered in this intermediate state between $T_{\mathrm{N1}}$ and $T_{\mathrm{N2}}$. Powder neutron diffraction experiments at different temperatures were performed as well. An incommensurate magnetic propagation vector of $\mathbf{k_{\rm m}}=(0.5,0.4,0.5)$ was identified. The refined spin configurations through the irreducible representation analysis confirmed that these Ising spins are canted in the crystal $ab$~plane. △ Less

Submitted 15 March, 2023; originally announced March 2023.

Comments: 9 pages, 7 figures

Journal ref: Physical Review MATERIALS 6, 085001 (2022)

Dynamics of growing carbon nanotube interfaces probed by machine learning-enabled molecular simulations

Authors: Daniel Hedman, Ben McLean, Christophe Bichara, Shigeo Maruyama, J. Andreas Larsson, Feng Ding

Abstract: Carbon nanotubes (CNTs) are currently considered a successor to silicon in future nanoelectronic devices. To realize this, controlled growth of defect-free nanotubes is required. Until now, the understanding of atomic-scale CNT growth mechanisms provided by molecular dynamics simulations has been hampered by their short timescales. Here, we develop an efficient and accurate machine learning force… ▽ More Carbon nanotubes (CNTs) are currently considered a successor to silicon in future nanoelectronic devices. To realize this, controlled growth of defect-free nanotubes is required. Until now, the understanding of atomic-scale CNT growth mechanisms provided by molecular dynamics simulations has been hampered by their short timescales. Here, we develop an efficient and accurate machine learning force field, DeepCNT-22, to simulate the complete growth of defect-free single-walled CNTs (SWCNTs) on iron catalysts at near-microsecond timescales. We provide atomic-level insight into the nucleation and growth processes of SWCNTs, including the evolution of the tube-catalyst interface and the mechanisms underlying defect formation and healing. Our simulations highlight the maximization of SWCNT-edge configurational entropy during growth and how defect-free CNTs can grow ultralong if carbon supply and temperature are carefully controlled. △ Less

Submitted 15 March, 2023; v1 submitted 19 February, 2023; originally announced February 2023.

Comments: Supporting Videos can be found on YouTube at the following links S1: https://youtu.be/K90Ca6uDNEQ S2: https://youtu.be/x8Z5Go5iW58 S3: https://youtu.be/e1Yx14PQjkg S4: https://youtu.be/JFKhklSHgA4

Atomically Sharp, Closed Bilayer Phosphorene Edges by Self-Passivation

Authors: Sol Lee, Yangjin Lee, Li Ping Ding, Kihyun Lee, Feng Ding, Kwanpyo Kim

Abstract: Two-dimensional (2D) crystals' edge structures not only influence their overall properties but also dictate their formation due to edge-mediated synthesis and etching processes. Edges must be carefully examined because they often display complex, unexpected features at the atomic scale, such as reconstruction, functionalization, and uncontrolled contamination. Here, we examine atomic-scale edge st… ▽ More Two-dimensional (2D) crystals' edge structures not only influence their overall properties but also dictate their formation due to edge-mediated synthesis and etching processes. Edges must be carefully examined because they often display complex, unexpected features at the atomic scale, such as reconstruction, functionalization, and uncontrolled contamination. Here, we examine atomic-scale edge structures and uncover reconstruction behavior in bilayer phosphorene. We use in situ transmission electron microscopy (TEM) of phosphorene/graphene specimens at elevated temperatures to minimize surface contamination and reduce e-beam damage, allowing us to observe intrinsic edge configurations. Bilayer zigzag (ZZ) edge was found the most stable edge configuration under e-beam irradiation. Through first-principles calculations and TEM image analysis under various tilting and defocus conditions, we find that bilayer ZZ edges undergo edge reconstruction and so acquire closed, self-passivated edge configurations. The extremely low formation energy of the closed bilayer ZZ edge and its high stability against e-beam irradiation are confirmed by first-principles calculations. Moreover, we fabricate bilayer phosphorene nanoribbons with atomically-sharp closed ZZ edges. The identified bilayer ZZ edges will aid in the fundamental understanding of the synthesis, degradation, reconstruction, and applications of phosphorene and related structures. △ Less

Submitted 2 August, 2022; originally announced August 2022.

Comments: 22 pages, 5 figures

Journal ref: ACS Nano, 2022

Surface quantum dots with pure, coherent, and blinking-free single photon emission

Authors: Xin Cao, Jingzhong Yang, Pengji Li, Tom Fandrich, Eddy P. Rugeramigabo, Vlastimil Křápek, Chenxi Ma, Frederik Benthin, Robert Keil, Benedikt Brechtken, Rolf J. Haug, Michael Oestreich, Yiteng Zhang, Constantin Schmidt, Zhao An, Michael Zopf, Fei Ding

Abstract: The surface of semiconductor nanostructures has a major impact on their electronic and optical properties. Disorder and defects in the surface layer typically cause degradation of charge carrier transport and radiative recombination dynamics. However, surface vicinity is inevitable for many scalable nano-optical applications. Epitaxially grown quantum dots are the best candidate for high-performan… ▽ More The surface of semiconductor nanostructures has a major impact on their electronic and optical properties. Disorder and defects in the surface layer typically cause degradation of charge carrier transport and radiative recombination dynamics. However, surface vicinity is inevitable for many scalable nano-optical applications. Epitaxially grown quantum dots are the best candidate for high-performance single photon emission and show great potential for quantum technologies. Yet, these emitters only reveal their excellent properties if they are deeply embedded in a semiconductor host. Until today, quantum dots close to surfaces yield weak, broad, and unstable emissions. Here, we show the complete restoration of optical properties from quantum dots grown directly on a semiconductor surface. The vanishing luminescence from the as-grown sample turns into bright, ultra-stable, coherent and blinking-free single photon emission after sulphur passivation. Under quasi-resonant excitation, single photons are generated with 98.8% purity, 77% indistinguishability, linewidths down to 4 $μ$eV and 99.69% persistency across 11 orders of magnitude in time. The emission is stable even after two years and when being subjected to nanomanufacturing processes. Some long-standing stumbling blocks for surface-dominated quantum dots are thereby removed, unveiling new possibilities for hybrid nano-devices and applications in quantum communication or sensing. △ Less

Submitted 12 May, 2023; v1 submitted 27 July, 2022; originally announced July 2022.

Comments: New experiments show that the results have to be re-interpreted, most likely leading to a new manuscript

Catalytic growth of ultralong graphene nanoribbons on insulating substrates

Authors: Bosai Lyu, Jiajun Chen, Shuo Lou, Can Li, Lu Qiu, Wengen Ouyang, Jingxu Xie, Izaac Mitchell, Tongyao Wu, Aolin Deng, Cheng Hu, Xianliang Zhou, Peiyue Shen, Saiqun Ma, Zhenghan Wu, Kenji Watanabe, Takashi Taniguchi, Xiaoqun Wang, Qi Liang, Jinfeng Jia, Michael Urbakh, Oded Hod, Feng Ding, Shiyong Wang, Zhiwen Shi

Abstract: Graphene nanoribbons (GNRs) with widths of a few nanometres are promising candidates for future nano-electronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometre-long GNRs on insulating substrates, which is essential for the fabrication of nano-electronic devices, remains an immense challenge.… ▽ More Graphene nanoribbons (GNRs) with widths of a few nanometres are promising candidates for future nano-electronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometre-long GNRs on insulating substrates, which is essential for the fabrication of nano-electronic devices, remains an immense challenge. Here, we report the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalysed chemical vapor deposition (CVD). Ultra-narrow GNRs with lengths of up to 10 μm are synthesized. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming one-dimensional (1D) moiré superlattices. Scanning tunnelling microscopy reveals an average width of 2 nm and a typical bandgap of ~1 eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes (CNTs) during the nucleation stage, and van der Waals sliding of the GNRs on the h-BN substrate throughout the growth stage. Our study provides a scalable, single-step method for growing micrometre-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moiré superlattices. △ Less

Submitted 27 May, 2022; originally announced May 2022.

Interplay of itinerant electrons and Ising moments in a hybrid honeycomb quantum magnet TmNi$_3$Al$_9$

Authors: H. Ge, C. J. Huang, Q. Zhang, N. Zhao, L. Wang, J. Yang, Y. Fu, L. Zhang, Z. M. Song, T. T. Li, F. Ding, J. B. Xu, Y. F. Zhang, X. Tong, S. M. Wang, J. W. Mei, A. Podlesnyak, L. S. Wu, Gang Chen, J. M. Sheng

Abstract: The interplay between itinerant electrons and local magnetic moments in quantum materials brings about rich and fascinating phenomena and stimulates various developments in the theoretical framework. In this work, thermodynamic, electric transport, and neutron diffraction measurements were performed on a newly synthesized honeycomb lattice magnet TmNi$_3$Al$_9$. Based on the experimental data, a m… ▽ More The interplay between itinerant electrons and local magnetic moments in quantum materials brings about rich and fascinating phenomena and stimulates various developments in the theoretical framework. In this work, thermodynamic, electric transport, and neutron diffraction measurements were performed on a newly synthesized honeycomb lattice magnet TmNi$_3$Al$_9$. Based on the experimental data, a magnetic field temperature phase diagram was constructed, exhibiting three essentially different magnetic regions. Below ${T_{\rm N}=2.97 \pm 0.02}\ \rm K$ Tm$^{3+}$ moments order antiferromagnetically in zero field. We found that the Tm$^{3+}$ ions form a pseudo-doublet ground state with the Ising-like moments lying normal to the two-dimensional honeycomb layers. Application of a magnetic field along the easy axis gradually suppresses the antiferromagnetic order in favor of an induced ferromagnetic state above the critical field ${B_c=0.92 \pm 0.05}\ \rm T$. In the vicinity of $B_c$, a strong enhancement of the quantum spin fluctuations was observed. The quantum Ising nature of the local moments and the coupling to itinerant electrons are discussed. △ Less

Submitted 2 September, 2022; v1 submitted 18 January, 2022; originally announced January 2022.

Comments: 9 pages, 9 figures

Journal ref: Physical Review B 106, 054434 (2022)

Bottom-up Growth of Graphene Nanospears and Nanoribbons

Authors: Haibin Sun, Fengning Liu, Leining Zhang, Ben McLean, Hao An, Ming Huang, Marc-Georg Willinger, Rodney Ruoff, Zhujun Wang, Feng Ding

Abstract: Graphene nanoribbons (GNRs) are considered one of the most promising materials for next generation electronics, however a reliable and controllable synthesis method is still lacking. Here, we report the CVD growth of GNRs on a copper surface and the corresponding mechanisms of growth. One-dimensional GNR growth is enabled by a vapor-liquid-solid (VLS) graphene growth guided by on-surface propagati… ▽ More Graphene nanoribbons (GNRs) are considered one of the most promising materials for next generation electronics, however a reliable and controllable synthesis method is still lacking. Here, we report the CVD growth of GNRs on a copper surface and the corresponding mechanisms of growth. One-dimensional GNR growth is enabled by a vapor-liquid-solid (VLS) graphene growth guided by on-surface propagation of a liquid catalyst particle. Controlling the suppression of vapor-solid-solid (VSS) graphene growth along the width direction of the GNR by tuning the flow of H2 during growth gives rise to a spear head-shaped graphene that we term graphene nanospears (GNSs). The real-time visual and spatially resolved observations confirm the VSS growth of graphene can be fully suppressed and lead to GNR formation on Cu surface. These findings reveal key insight into the growth mechanism of graphene and open a door for achieving a facile and scalable method of synthesizing free standing GNRs. △ Less

Submitted 3 January, 2022; originally announced January 2022.

Muon spin rotation and relaxation study on topological noncentrosymmetric superconductor PbTaSe$_2$

Authors: Z. H. Zhu, C. Tan, J. Zhang, P. K. Biswas, A. D. Hillier, M. X. Wang, Y. X. Yang, C. S. Chen, Z. F. Ding, S. Y. Li, L. Shu

Abstract: Topological superconductivity is an exotic phenomenon due to the symmetry-protected topological surface state, in which a quantum system has an energy gap in the bulk but supports gapless excitations confined to its boundary. Symmetries including central and time-reversal (TRS), along with their relations with topology, are crucial for topological superconductivity. We report muon spin relaxation/… ▽ More Topological superconductivity is an exotic phenomenon due to the symmetry-protected topological surface state, in which a quantum system has an energy gap in the bulk but supports gapless excitations confined to its boundary. Symmetries including central and time-reversal (TRS), along with their relations with topology, are crucial for topological superconductivity. We report muon spin relaxation/rotation ($μ$SR) experiments on a topological noncentrosymmetric superconductor PbTaSe$_2$ to study its TRS and gap symmetry. Zero-field $μ$SR experiments indicate the absence of internal magnetic field in the superconducting state, consistent with previous $μ$SR results. Furthermore, transverse-field $μ$SR measurements reveals that the superconducting gap of PbTaSe$_2$ is an isotropic three-dimensional fully-gapped single-band. The fully-gapped results can help understand the pairing mechanism and further classify the topological superconductivity in this system. △ Less

Submitted 11 December, 2021; originally announced December 2021.

Photoneutralization of charges in GaAs quantum dot based entangled photon emitters

Authors: Jingzhong Yang, Tom Fandrich, Frederik Benthin, Robert Keil, Nand Lal Sharma, Weijie Nie, Caspar Hopfmann, Oliver G. Schmidt, Michael Zopf, Fei Ding

Abstract: Semiconductor-based emitters of pairwise photonic entanglement are a promising constituent of photonic quantum technologies. They are known for the ability to generate discrete photonic states on-demand with low multiphoton emission, near-unity entanglement fidelity, and high single photon indistinguishability. However, quantum dots typically suffer from luminescence blinking, lowering the efficie… ▽ More Semiconductor-based emitters of pairwise photonic entanglement are a promising constituent of photonic quantum technologies. They are known for the ability to generate discrete photonic states on-demand with low multiphoton emission, near-unity entanglement fidelity, and high single photon indistinguishability. However, quantum dots typically suffer from luminescence blinking, lowering the efficiency of the source and hampering their scalable application in quantum networks. In this paper, we investigate and adjust the intermittence of the neutral exciton emission in a GaAs/AlGaAs quantum dot under two-photon resonant excitation of the neutral biexciton. We investigate the spectral and quantum optical response of the quantum dot emission to an additional wavelength tunable gate laser, revealing blinking caused by the intrinsic Coulomb blockade due to charge capture processes. Our finding demonstrates that the emission quenching can be actively suppressed by controlling the balance of free electrons and holes in the vicinity of the quantum dot and thereby significantly increasing the quantum efficiency by 30%. △ Less

Submitted 14 February, 2024; v1 submitted 5 October, 2021; originally announced October 2021.

Comments: 7 pages, 4 figures

Journal ref: Physical Review B 105, 115301 (2022)

Statistical limits for quantum networks with semiconductor entangled photon sources

Authors: Jingzhong Yang, Michael Zopf, Pengji Li, Nand Lal Sharma, Weijie Nie, Frederik Benthin, Tom Fandrich, Eddy Patrick Rugeramigabo, Caspar Hopfmann, Robert Keil, Oliver G. Schmidt, Fei Ding

Abstract: Semiconductor quantum dots are promising building blocks for quantum communication applications. Although deterministic, efficient, and coherent emission of entangled photons has been realized, implementing a practical quantum repeater remains outstanding. Here we explore the statistical limits for entanglement swapping with sources of polarization-entangled photons from the commonly used biexcito… ▽ More Semiconductor quantum dots are promising building blocks for quantum communication applications. Although deterministic, efficient, and coherent emission of entangled photons has been realized, implementing a practical quantum repeater remains outstanding. Here we explore the statistical limits for entanglement swapping with sources of polarization-entangled photons from the commonly used biexciton-exciton cascade. We stress the necessity of tuning the exciton fine structure, and explain why the often observed time evolution of photonic entanglement in quantum dots is not applicable for large quantum networks. We identify the critical, statistically distributed device parameters for entanglement swapping based on two sources. A numerical model for benchmarking the consequences of device fabrication, dynamic tuning techniques, and statistical effects is developed, in order to bring the realization of semiconductor-based quantum networks one step closer to reality. △ Less

Submitted 10 June, 2022; v1 submitted 14 September, 2021; originally announced September 2021.

Comments: 9 pages, 4 figures

Journal ref: Physical Review B 105, 235305

Room-temperature on-chip orbital angular momentum single-photon sources

Authors: Cuo Wu, Shailesh Kumar, Yinhui Kan, Danylo Komisar, Zhiming Wang, Sergey I. Bozhevolnyi, Fei Ding

Abstract: On-chip photon sources carrying orbital angular momentum (OAM) are in demand for high-capacity optical information processing in both classical and quantum regimes. However, currently-exploited integrated OAM sources have been primarily limited to the classical regime. Herein, we demonstrate a room-temperature on-chip integrated OAM source that emits well-collimated single photons, with a single-p… ▽ More On-chip photon sources carrying orbital angular momentum (OAM) are in demand for high-capacity optical information processing in both classical and quantum regimes. However, currently-exploited integrated OAM sources have been primarily limited to the classical regime. Herein, we demonstrate a room-temperature on-chip integrated OAM source that emits well-collimated single photons, with a single-photon purity of g(2)(0) = 0.22, carrying entangled spin and orbital angular momentum states and forming two spatially separated entangled radiation channels with different polarization properties. The OAM-encoded single photons are generated by efficiently outcoupling diverging surface plasmon polaritons excited with a deterministically positioned quantum emitter via Archimedean spiral gratings. Our OAM single-photon sources bridge the gap between conventional OAM manipulation and nonclassical light sources, enabling high-dimensional and large-scale photonic quantum systems for information processing. △ Less

Submitted 2 April, 2021; originally announced April 2021.

Angstrom-wide conductive channels in black phosphorus by Cu intercalation

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

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

Submitted 21 January, 2021; originally announced January 2021.

Crystallographic Reconstruction Driven Modified Mechanical Properties in Anisotropic Rhenium Disulfides

Authors: Jung Hwa Kim, Xinyue Dai, Feng Ding, Zonghoon Lee

Abstract: Atomic-scale investigation on mechanical behaviors is highly necessary to fully understand the fracture mechanics especially of brittle materials, which are determined by atomic-scale phenomena (e.g., lattice trapping). Here, exfoliated anisotropic rhenium disulfide (ReS2) flakes are used to investigate atomic-scale crack propagation depending on the propagation directions. While the conventional… ▽ More Atomic-scale investigation on mechanical behaviors is highly necessary to fully understand the fracture mechanics especially of brittle materials, which are determined by atomic-scale phenomena (e.g., lattice trapping). Here, exfoliated anisotropic rhenium disulfide (ReS2) flakes are used to investigate atomic-scale crack propagation depending on the propagation directions. While the conventional strain-stress curves exhibit a strong anisotropy depending on the cleavage direction of ReS2, but our experimental results show a reduced cleavage anisotropy due to the lattice reconstruction in [100] cracking with high resistance to fracture. In other words, [010] and [110] cracks with low barriers to cleavage exhibit the ultimate sharpness of the crack tip without plastic deformation, whereas [100] cracks drive lattice rotation on one side of the crack, leading to a non-flat grain boundary formation. Finally, crystallographic reconstruction associated with the high lattice randomness of two-dimensional materials drives to a modified cleavage tendency, further indicating the importance of atomic-scale studies for a complete understanding of the mechanics. △ Less

Submitted 2 December, 2020; originally announced December 2020.

Comments: 15 pages, 4 figures

Deterministic preparation of spin qubits in droplet etched GaAs quantum dots using quasi-resonant excitation

Authors: Caspar Hopfmann, Nand Lal Sharma, Weijie Nie, Robert Keil, Fei Ding, Oliver G. Schmidt

Abstract: We present a first comprehensive study on deterministic spin preparation employing excited state resonances of droplet etched GaAs quantum dots. This achievement facilitates future investigations of spin qubit based quantum memories using the GaAs quantum dot material platform. By observation of excitation spectra for a range of fundamental excitonic transitions the properties of different quantum… ▽ More We present a first comprehensive study on deterministic spin preparation employing excited state resonances of droplet etched GaAs quantum dots. This achievement facilitates future investigations of spin qubit based quantum memories using the GaAs quantum dot material platform. By observation of excitation spectra for a range of fundamental excitonic transitions the properties of different quantum dot energy levels, i.e. shells, are revealed. The innovative use of polarization resolved excitation and detection in quasi-resonant excitation spectroscopy facilitates determination of $85$ $\%$ maximum spin preparation fidelity - irrespective of the relative orientations of lab and quantum dot polarization eigenbases. Additionally, the characteristic non-radiative decay time is investigated as a function of ground state, excitation resonance and excitation power level, yielding decay times as low as $29$ ps for s-p-shell exited state transitions. Finally, by time resolved correlation spectroscopy it is demonstrated that the employed excitation scheme has a significant impact on the electronic environment of quantum dot transitions thereby influencing its charge and coherence. △ Less

Submitted 30 November, 2020; originally announced November 2020.

Comments: 13 pages, 11 figures

Journal ref: Phys. Rev. B 104, 075301 (2021)

Maximally entangled and GHz-clocked on-demand photon pair source

Authors: Caspar Hopfmann, Weijie Nie, Nand Lal Sharma, Carmen Weigelt, Fei Ding, Oliver G. Schmidt

Abstract: We present a 1 GHz-clocked, maximally entangled and on-demand photon pair source based on droplet etched GaAs quantum dots using two-photon excitation. By employing these GaP microlensenhanced devices in conjunction with their substantial brightness, raw entanglement fidelities of up to $0.95 \pm 0.01$ and post-selected photon indistinguishabilities of up to $0.93 \pm 0.01$, the suitability for qu… ▽ More We present a 1 GHz-clocked, maximally entangled and on-demand photon pair source based on droplet etched GaAs quantum dots using two-photon excitation. By employing these GaP microlensenhanced devices in conjunction with their substantial brightness, raw entanglement fidelities of up to $0.95 \pm 0.01$ and post-selected photon indistinguishabilities of up to $0.93 \pm 0.01$, the suitability for quantum repeater based long range quantum entanglement distribution schemes is shown. Comprehensive investigations of a complete set of polarization selective two-photon correlations as well as time resolved Hong-Ou-Mandel interferences facilitate innovative methods that determine quantities such as photon extraction and excitation efficiencies as well as pure dephasing directly - opposed to commonly employed indirect techniques. △ Less

Submitted 22 October, 2020; originally announced October 2020.

Comments: 9 pages, 10 figures

Journal ref: Phys. Rev. B 103, 075413 (2021)

Distinct Kondo Screening Behaviors in Heavy Fermion Filled Skutterudites with 4f1 and 4f2 Configurations

Authors: X. Lou, H. C. Xu, T. L. Yu, Y. H. Song, C. H. P. Wen, W. Z. Wei, A. Leithe-Jasper, Z. F. Ding, L. Shu, S. Kirchner, R. Peng, D. L. Feng

Abstract: Filled-skutterudite heavy fermion (HF) compounds host rich ground states depending on the f electron configurations. CeOs4Sb12 (COS) with Ce 4f1, and PrOs4Sb12 (POS) with Pr 4f2 configurations show distinct properties of Kondo insulating and HF superconductivity, respectivity. We unveiled the underlying microscopic origin by angle-resolved photoemission spectroscopy studies. Their eV-scale band st… ▽ More Filled-skutterudite heavy fermion (HF) compounds host rich ground states depending on the f electron configurations. CeOs4Sb12 (COS) with Ce 4f1, and PrOs4Sb12 (POS) with Pr 4f2 configurations show distinct properties of Kondo insulating and HF superconductivity, respectivity. We unveiled the underlying microscopic origin by angle-resolved photoemission spectroscopy studies. Their eV-scale band structure matches well, representing the common characters of conduction electrons in ROs4Sb12 systems (R = rare earth). However, f electrons interact differently with conduction electrons in them. Strong hybridization between conduction electrons and f electrons is observed in COS with band dependent hybridization gaps, and the development of Kondo insulating state is directly revealed. Although the ground state of POS is a singlet, finite but incoherent hybridization exists due to Kondo scattering with the thermally excited triplet crystalline electric field (CEF) state. Our results help to understand the intriguing properties in COS and POS, and provide a clean demonstration of the microscopic differences in HF systems with 4f1 and 4f2 configurations. △ Less

Submitted 9 June, 2020; originally announced June 2020.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 126, 136402 (2021)

Unusual slow magnetic fluctuations and critical slowing down in Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$

Authors: C. Tan, Z. F. Ding, J. Zhang, Z. H. Zhu, O. O. Bernal, P. -C. Ho, A. D. Hillier, A. Koda, H. Luetkens, G. D. Morris, D. E. MacLaughlin, L. Shu

Abstract: Hidden magnetic order of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$, $x = 0.05$ and 0.1, has been studied using muon spin relaxation spectroscopy. In zero applied field and weak longitudinal fields ($μ_0H_L \lesssim 2$~mT), muon spin relaxation data can be well described by exponentially-damped static Lorentzian Kubo-Toyabe functions, indicating that static and dynamic local fields coexist at each muon site. For… ▽ More Hidden magnetic order of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$, $x = 0.05$ and 0.1, has been studied using muon spin relaxation spectroscopy. In zero applied field and weak longitudinal fields ($μ_0H_L \lesssim 2$~mT), muon spin relaxation data can be well described by exponentially-damped static Lorentzian Kubo-Toyabe functions, indicating that static and dynamic local fields coexist at each muon site. For $μ_0H_L \gtrsim 2$~mT, the static rate is completely decoupled, and the exponential decay is due to dynamic spin fluctuations. In both zero field and $μ_0H_L = 1$--2~mT, the temperature dependencies of the exponential muon spin relaxation rate exhibit maxima at 215~K for $x = 0.05$ and 175~K for $x = 0.1$, suggesting critical slowing down of electronic spin fluctuations. The field dependencies of the dynamic spin fluctuation rates can be well described by the Redfield relation. The correlation time of this electronic spin fluctuation is in the range of~2--5~ns for Sr$_2$Ir$_{0.9}$Rh$_{0.1}$O$_4$, and shorter than 2~ns for Sr$_2$Ir$_{0.95}$Rh$_{0.05}$O$_4$. The rms fluctuating field is on the order of 1 mT, which is consistent with the polarized neutron diffraction cross-section. △ Less

Submitted 26 March, 2020; v1 submitted 17 October, 2019; originally announced October 2019.

Journal ref: Phys. Rev. B 101, 195108 (2020)