Showing 1–49 of 49 results for author: Wong, C H

Evaluating Synthetic Images as Effective Substitutes for Experimental Data in Surface Roughness Classification

Authors: Binwei Chen, Huachao Leng, Chi Yeung Mang, Tsz Wai Cheung, Yanhua Chen, Wai Keung Anthony Loh, Chi Ho Wong, Chak Yin Tang

Abstract: Hard coatings play a critical role in industry, with ceramic materials offering outstanding hardness and thermal stability for applications that demand superior mechanical performance. However, deploying artificial intelligence (AI) for surface roughness classification is often constrained by the need for large labeled datasets and costly high-resolution imaging equipment. In this study, we explor… ▽ More Hard coatings play a critical role in industry, with ceramic materials offering outstanding hardness and thermal stability for applications that demand superior mechanical performance. However, deploying artificial intelligence (AI) for surface roughness classification is often constrained by the need for large labeled datasets and costly high-resolution imaging equipment. In this study, we explore the use of synthetic images, generated with Stable Diffusion XL, as an efficient alternative or supplement to experimentally acquired data for classifying ceramic surface roughness. We show that augmenting authentic datasets with generative images yields test accuracies comparable to those obtained using exclusively experimental images, demonstrating that synthetic images effectively reproduce the structural features necessary for classification. We further assess method robustness by systematically varying key training hyperparameters (epoch count, batch size, and learning rate), and identify configurations that preserve performance while reducing data requirements. Our results indicate that generative AI can substantially improve data efficiency and reliability in materials-image classification workflows, offering a practical route to lower experimental cost, accelerate model development, and expand AI applicability in materials engineering. △ Less

Submitted 26 March, 2026; originally announced March 2026.

Exotic Pressure-Driven Band Gap Widening in Carbon Chain-Filled KFI Zeolite and Its Pathway to High-Pressure Semiconducting Electronics and High-Temperature Superconductivity

Authors: C. T. Wat, K. C. Lam, W. Y. Chan, C. P. Chau, S. P. Ng, W. K. Loh, L. Y. F. Lam, X. Hu, C. H. Wong

Abstract: Semiconducting devices face persistent challenges in operating at high pressure, as the band theory predicts that materials transition to a more metallic state under compression. However, our findings with carbon chains in KFI substrates reveal a conditional deviation from this norm. We not only witness the transition from polyyne (semiconductor) to cumulene (metal) at medium pressure, but we also… ▽ More Semiconducting devices face persistent challenges in operating at high pressure, as the band theory predicts that materials transition to a more metallic state under compression. However, our findings with carbon chains in KFI substrates reveal a conditional deviation from this norm. We not only witness the transition from polyyne (semiconductor) to cumulene (metal) at medium pressure, but we also observe an unexpected re-entrance of the polyyne at high pressures, where the band gap in the polyyne increases with pressure. In addition, the synthesis of long cumulene chains has posed a longstanding challenge in the quest for high-temperature organic superconductivity. We have identified critical conditions for synthesizing extended cumulene chains within zeolite frameworks, highlighting the interplay between unconventional charge density waves and significant torsions. The KFI zeolite facilitates the formation of carbon chains exceeding 5,000 atoms, in stark contrast to around 100 other zeolites that are limited to ~10 atoms. The cumulene@KFI system demonstrates a superconducting transition temperature reaching ~62 K, surpassing the highest reported values for bulk iron-based superconductors. This interplay between carbon structures and superconductivity not only advances our understanding of charge density waves but also heralds a new era in the study of novel applications △ Less

Submitted 6 March, 2026; originally announced March 2026.

Room-temperature superconductivity in ultra-thin carbon nanotube zeolite composites: a conventional or unconventional superconductor?

Authors: Chi Ho Wong, Rolf Lortz

Abstract: The recent report of signs of room-temperature superconductivity in ultrathin single-walled carbon nanotubes (CNT) of types (2,1) and (3,0) holds significant promise for energy applications due to their ability to conduct current without dissipation. However, the McMillan Tc formula fails to calculate their superconducting transition temperatures (Tc) accurately, which raises an important question… ▽ More The recent report of signs of room-temperature superconductivity in ultrathin single-walled carbon nanotubes (CNT) of types (2,1) and (3,0) holds significant promise for energy applications due to their ability to conduct current without dissipation. However, the McMillan Tc formula fails to calculate their superconducting transition temperatures (Tc) accurately, which raises an important question: what is the pairing mechanism driving their room-temperature superconductivity? To explore this further, we first investigate whether the strong curvature of ultrathin CNT leads to exotic phenomena in unconventional superconductors. If no evidence of these exotic characteristics is found and the McMillan formalism indicates that it is not a BCS-type superconductor, could we be observing a new class of unconventional superconductivity that functions independently of phonons and typical exotic features? In this paper, we demonstrate that factors such as the chiral angle of CNT, boron dopants and lattice regularity can be used to tune the theoretical Tc to experimental values. Our finding suggests that combining CNT with a harder substrate could be vital for further enhancing Tc while minimizing lattice distortion under doping. We propose a reconsideration of the common belief regarding whether the McMillan and BCS Tc formulas are adequate for classifying materials as BCS or non-BCS superconductors. △ Less

Submitted 25 September, 2025; originally announced September 2025.

High-precision baryon number cumulants from lattice QCD in a finite box: cumulant ratios, Lee-Yang zeros and critical endpoint predictions

Authors: Alexander Adam, Szabolcs Borsányi, Zoltan Fodor, Jana N. Guenther, Piyush Kumar, Paolo Parotto, Attila Pásztor, Chik Him Wong

Abstract: We have performed high-statistics lattice simulations using 4HEX improved staggered fermions on $16^3 \times 8$ lattices. We calculated the Taylor expansion coefficients of the pressure with respect to the baryochemical potential to the tenth order at zero, and fourth order at purely imaginary chemical potentials. We used this data to construct rational function approximations of the free energy.… ▽ More We have performed high-statistics lattice simulations using 4HEX improved staggered fermions on $16^3 \times 8$ lattices. We calculated the Taylor expansion coefficients of the pressure with respect to the baryochemical potential to the tenth order at zero, and fourth order at purely imaginary chemical potentials. We used this data to construct rational function approximations of the free energy. We use a rational ansatz that explicitly satisfies the charge conjugation symmetry and the Roberge-Weiss periodicity, which are exact properties of the QCD free energy. We use this ansatz to estimate the position of Lee-Yang zeros in the complex chemical potential plane. The temperature dependence of the imaginary part of the Lee-Yang zeros is then fitted with ansätze motivated by the universal behavior of the free energy near a 3D Ising critical point. In principle, this allows one to estimate the temperature of the critical endpoint. We consider several sources of systematic errors. On this single lattice spacing we find that with $84\%$ probability, the chiral critical endpoint is either below $103$~MeV temperature or it does not exist. We also identify some caveats of the method, which do not disappear even with the extremely high statistics of this present study. We discuss to what extent these can be eliminated by future high statistics lattice analyses. △ Less

Submitted 17 July, 2025; originally announced July 2025.

Comments: 14 pages, 10 figures

Tuning Charge Density Wave in the Transition from Magnetically Frustrated Conductor to Ferrimagnetic Insulator in Carbon Nanowire within Boron Nitride Nanotube

Authors: Chi Ho Wong, Zong Liang Guo, King Cheong Lam, Chun Pong Chau, Wing Yu Chan, Chak-yin Tang, Yuen Hong Tsang, Leung Yuk Frank Lam, Xijun Hu

Abstract: The emergence of exotic charge density wave (CDW) alongside ferrimagnetism materials opens exciting new possibilities for quantum switching, particularly in field-tuning CDW electronics. However, these two phenomena often compete and rely heavily on strong electronic correlations. While carbon nanowire arrays have been experimentally shown to exhibit ferromagnetism above 400 K, our research shows… ▽ More The emergence of exotic charge density wave (CDW) alongside ferrimagnetism materials opens exciting new possibilities for quantum switching, particularly in field-tuning CDW electronics. However, these two phenomena often compete and rely heavily on strong electronic correlations. While carbon nanowire arrays have been experimentally shown to exhibit ferromagnetism above 400 K, our research shows that encapsulating a linear carbon chain (LCC) within zigzag boron nitride nanotubes (BNT) induces a short-range CDW state under a competing effect of ferrimagnetism and magnetic frustrations. However, for this exotic feature to occur, the LCC needs to break the symmetry along the circular plane of the BNT. Then we utilize a Monte Carlo model to identify the optimal length of LCC@BNT to tackle its size effect, while also comparing the stability of chains provided by carbon nanotubes. The shorter LCC@BNT displays a more prominent long-range CDW pattern with a tunneling barrier of 2.3 eV on the Fermi surface, transitioning into an unconventional insulator. Meanwhile, magnetic frustrations disappear, and ferrimagnetism remains stable up to 280 K. Our discovery of ferrimagnetic CDW carbyne insulators, which function without conventional periodic lattice distortion, spin-orbit coupling, or complex d and f hybridization represents a groundbreaking shift in thinking, which demonstrates that such exotic properties are not exclusive to transition metal elements. We anticipate that spin fluctuations in LCC@BNT could enable fine-tuning of the CDW pattern, and applying an electric excitation of 2.3 eV triggers an abrupt insulator-to-conductor transition for quantum switching applications. △ Less

Submitted 10 April, 2025; originally announced April 2025.

Unveiling the Dirac feature in the Metallated Carbyne as a Potential Platform for Exploring Widely Separated Majorana Fermions

Authors: Chi Ho Wong

Abstract: The realization of next-generation quantum computing devices is hindered by the formidable challenge of detecting and manipulating Majorana Fermion in nanomaterials. In this study, we explore a new approach of detecting Majorana Fermion in a metallated carbyne nanowire array. Through comprehensive optimizations, we successfully achieved a local magnetic moment exceeding 3μB, with the average magne… ▽ More The realization of next-generation quantum computing devices is hindered by the formidable challenge of detecting and manipulating Majorana Fermion in nanomaterials. In this study, we explore a new approach of detecting Majorana Fermion in a metallated carbyne nanowire array. Through comprehensive optimizations, we successfully achieved a local magnetic moment exceeding 3μB, with the average magnetic moment of the entire metallated carbyne surpassing 1μB. Surprisingly, in the absence of spin-orbit coupling, the ferromagnetic Ru metallated carbyne, when coupled with a superconducting Ru substrate, is already able to demonstrate the symmetric opening of a Dirac gap at the gamma point. We discovered that the kink structure of the metallated carbyne plays a crucial role in modulating its topological properties. Moreover, we identified the origin of magnetic hybridization which is intricately linked to the distinctive features found in one-dimensional carbon structures. Our findings not only uncover the unconventional ferromagnetism observed in metallated carbyne but also present an exciting opportunity to realize carbon-based materials capable of hosting Majorana Zero Modes (MZM). This discovery has the potential to further stabilize MZM by decoupling the orbital perturbation from the MZM itself. △ Less

Submitted 19 March, 2024; originally announced March 2024.

Exploring the Unconventional Electron Distribution Patterns in Iron-based Superconductors

Authors: Chi Ho Wong, Rolf Lortz

Abstract: For more than a decade, the unusual distribution of electrons observed in ARPES (angle-resolved photoemission spectroscopy) data within the energy range of ~30meV to ~300meV below the Fermi level, known as the ARPES range, has remained a puzzle in the field of iron-based superconductivity. However, in this study, we have made a noteworthy observation: although the electron-phonon coupling alone is… ▽ More For more than a decade, the unusual distribution of electrons observed in ARPES (angle-resolved photoemission spectroscopy) data within the energy range of ~30meV to ~300meV below the Fermi level, known as the ARPES range, has remained a puzzle in the field of iron-based superconductivity. However, in this study, we have made a noteworthy observation: although the electron-phonon coupling alone is insufficient to account for the observed ARPES pattern, our analysis reveals that when the instantaneous electron-phonon coupling occurring in selective phonon channel is enhanced by the coexistence of antiferromagnetic spin density wave and charge density wave phenomena, the amplified interaction becomes comparable to the ARPES range. This finding suggests that the instantaneous interplay between these intricate phenomena should play a crucial role in generating the observed energy range in ARPES. Our work may provide a valuable clue towards achieving a deeper understanding of the complex relationship between electronic correlations, lattice structure, and superconductivity in iron-based materials for uncovering the origin of the unconventional ARPES pattern. △ Less

Submitted 4 January, 2024; originally announced January 2024.

Synergistic interplays between the selective electron-phonon coupling, antiferromagnetic fluctuations and charge density wave in the YBa2Cu3Ox cuprate superconductor

Authors: Chi Ho Wong, Rolf Lortz

Abstract: This research aims to investigate the synergistic effect between charge density wave, selective electron-phonon coupling under antiferromagnetic fluctuations, as well as the unusual electron distribution observed in ARPES data in YBa2Cu3Ox superconductors (YBCO). By considering their synergistic impact, our model can calculate the superconducting transition temperature Tc of YBa2Cu3Ox as a functio… ▽ More This research aims to investigate the synergistic effect between charge density wave, selective electron-phonon coupling under antiferromagnetic fluctuations, as well as the unusual electron distribution observed in ARPES data in YBa2Cu3Ox superconductors (YBCO). By considering their synergistic impact, our model can calculate the superconducting transition temperature Tc of YBa2Cu3Ox as a function of pressure for x = 6.5 and 7 at a reasonable level. Moreover, we have identified a specific antiferromagnetic phonon that contributes significantly to the high Tc observed in YBCO. This finding highlights the significance of these effects in achieving high Tc values. Our study not only identifies an imbalanced charge-density wave effect for triggering selective electron-phonon coupling but also explains why the charge density wave usually occurs around the magnetic copper atoms. Furthermore, our research reveals limitations in the conventional mean-field ab-initio approximation used for antiferromagnetic fluctuations in YBCO. It shows that the dynamic behavior of electrons in YBCO might not be accurately captured by this approximation, as non-uniform magnetic fields under antiferromagnetic fluctuations induce an additional electric potential on electrons across the boundary between non-magnetic to magnetic sites instantaneously. This instantaneous electric potential, in turn, suggest that the influence of the antiferromagnetic phonon-based pairing mechanism might not have been optimized in theory △ Less

Submitted 4 January, 2024; originally announced January 2024.

Decoding 122-Type Iron-Based Superconductors: A Comprehensive Simulation of Phase Diagrams and Transition Temperatures

Authors: Chi Ho Wong, Rolf Lortz

Abstract: Iron-based superconductors, a cornerstone of low-temperature physics, have been the subject of numerous theoretical models aimed at deciphering their complex behavior. In this study, we present a comprehensive approach that amalgamates several existing models and incorporates experimental data to simulate the superconducting phase diagrams of the principal 122-type iron-based compounds. Our model… ▽ More Iron-based superconductors, a cornerstone of low-temperature physics, have been the subject of numerous theoretical models aimed at deciphering their complex behavior. In this study, we present a comprehensive approach that amalgamates several existing models and incorporates experimental data to simulate the superconducting phase diagrams of the principal 122-type iron-based compounds. Our model considers a multitude of factors including the momentum dependence of the superconducting gap, spin-orbital coupling, antiferromagnetism, spin density wave, induced XY potential on the tetrahedral structure, and electron-phonon coupling. We have refined the electron-phonon scattering matrix using experimental angle-resolved photoemission spectroscopy (ARPES) data, ensuring that all electrons pertinent to iron-based superconductivity are accounted for. This innovative approach allows us to calculate theoretical critical temperature Tc values for Ba1-xKxFe2As2, CaFe2As2 and SrFe2As2 as functions of pressure. These calculated values exhibit remarkable agreement with experimental findings. Furthermore, our model predicts that MgFe2As2 remains non-superconducting irrespective of the applied pressure. Given that 122-type superconductivity at low pressure or low doping concentration has been experimentally validated, our combined model serves as a powerful predictive tool for generating superconducting phase diagrams at high pressure. This study underscores that the high transition temperatures and the precise doping and pressure dependence of iron-based superconductors are intrinsically linked to an intertwined mechanism involving a strong interplay between structural, magnetic and electronic degrees of freedom. △ Less

Submitted 22 September, 2023; originally announced September 2023.

Microwave heating as a universal method to transform confined molecules into armchair graphene nanoribbons

Authors: Haoyuan Zhang, Yingzhi Chen, Kunpeng Tang, Ziheng Lin, Xuan Li, Hongwei Zhang, Yifan Zhang, Takeshi Saito, Chi Ho Wong, Chi Wah Leung, Chee Leung Mak, Yuan Hu, Weili Cui, Kecheng Cao, Lei Shi

Abstract: Armchair graphene nanoribbons (AGNRs) with sub-nanometer width are potential materials for fabrication of novel nanodevices thanks to their moderate direct band gaps. AGNRs are usually synthesized by polymerizing precursor molecules on substrate surface. However, it is time-consuming and not suitable for large-scale production. AGNRs can also be grown by transforming precursor molecules inside sin… ▽ More Armchair graphene nanoribbons (AGNRs) with sub-nanometer width are potential materials for fabrication of novel nanodevices thanks to their moderate direct band gaps. AGNRs are usually synthesized by polymerizing precursor molecules on substrate surface. However, it is time-consuming and not suitable for large-scale production. AGNRs can also be grown by transforming precursor molecules inside single-walled carbon nanotubes via furnace annealing, but the obtained AGNRs are normally twisted. In this work, microwave heating is applied for transforming precursor molecules into AGNRs. The fast heating process allows synthesizing the AGNRs in seconds. Several different molecules were successfully transformed into AGNRs, suggesting that it is a universal method. More importantly, as demonstrated by Raman spectroscopy, aberration-corrected high-resolution transmission electron microscopy and theoretical calculations, less twisted AGNRs are synthesized by the microwave heating than the furnace annealing. Our results reveal a route for rapid production of AGNRs in large scale, which would benefit future applications in novel AGNRs-based semiconductor devices. △ Less

Submitted 24 January, 2023; originally announced January 2023.

Comments: 7 pages, 7 figures

Simulation of environmental impacts on the synthesis of carbyne with more than 6000 atoms for emerging continuously tunable energy barriers in CNT-based transistors

Authors: Chi Ho Wong, Yan Ming Yeung, Xin Zhao, Wing Cheung Law, Chak-yin Tang, Chee Leung Mak, Chi Wah Leung, Lei Shi, Rolf Lortz

Abstract: Transistors made up of carbon nanotubes CNT have demonstrated excellent current-voltage characteristics which outperform some high-grade silicon-based transistors. A continuously tunable energy barrier across semiconductor interfaces is desired to make the CNT-based transistors more robust. Despite the direct band gap of carbyne inside a CNT can be widely tuned by strain, the size of carbyne canno… ▽ More Transistors made up of carbon nanotubes CNT have demonstrated excellent current-voltage characteristics which outperform some high-grade silicon-based transistors. A continuously tunable energy barrier across semiconductor interfaces is desired to make the CNT-based transistors more robust. Despite the direct band gap of carbyne inside a CNT can be widely tuned by strain, the size of carbyne cannot be controlled easily. The production of a monoatomic chain with more than 6000 carbon atoms is an enormous technological challenge. To predict the optimal chain length of a carbyne in different molecular environments, we have developed a Monte Carlo model in which a finite-length carbyne with a size of 4000-15000 atoms is encapsulated by a CNT at finite temperatures. Our simulation shows that the stability of the carbyne@nanotube is strongly influenced by the nature and porosity of the CNT, the external pressure, the temperature and the chain length. We have observed an initiation of chain-breaking process in a compressed carbyne@nanotube. Our work provides much needed input for optimising the carbyne length to produce carbon chains much longer than 6000 atoms at ~300K. Design rules are proposed for synthesizing ~1% strained carbyne@(6,5)CNT as a component in CNT-based transistors to tune the energy barriers continuously. △ Less

Submitted 19 January, 2023; originally announced January 2023.

Comments: arXiv admin note: substantial text overlap with arXiv:2207.14558

Routines to synthesize carbyne of more than 6000 atoms

Authors: Chi Ho Wong

Abstract: The superior electronic, optical and magnetic properties of carbyne have been called for optoelectronic and magnetoelectronic applications. However, manufacturing a monoatomic chain of more than 6000 carbon atoms presents a huge technical challenge. In order to predict the optimal chain length in different environments, we develop a Monte Carlo model in which a finite-length carbyne in the size of… ▽ More The superior electronic, optical and magnetic properties of carbyne have been called for optoelectronic and magnetoelectronic applications. However, manufacturing a monoatomic chain of more than 6000 carbon atoms presents a huge technical challenge. In order to predict the optimal chain length in different environments, we develop a Monte Carlo model in which a finite-length carbyne in the size of 4000-15000 atoms is encapsulated by a carbon nanotube at finite temperatures. Our Monte Carlo simulation shows that the stability of the carbyne-nanotube is influenced by the charity and porosity of carbon nanotube, external pressure, temperature and the chain length. When the geometric structure of carbon nanotube and environmental parameters are provided, our Monte Carlo algorithm can predict the maximum length of the internal carbyne. Our work presents a path to manufacture a carbon chain much longer than 6000 atoms at room temperature. △ Less

Submitted 17 October, 2022; v1 submitted 29 July, 2022; originally announced July 2022.

Comments: Monte Carlo method

The GoogLeNet-assisted phase transition detectors

Authors: C. H. Wong, Raymond P. H. Wu, X. Lei, A. F. Zatsepin

Abstract: In the presence of the phase fluctuations in superconducting nanowires array, the electrical resistance of the superconducting nanowires is always non-zero unless the system undergoes Berezinskii-Kosterlitz-Thouless (BKT) transition where the superconducting vortices and anti-vortices form pairs. The two-dimensional XY model can mimic the superconducting transition temperature Tc and the BKT trans… ▽ More In the presence of the phase fluctuations in superconducting nanowires array, the electrical resistance of the superconducting nanowires is always non-zero unless the system undergoes Berezinskii-Kosterlitz-Thouless (BKT) transition where the superconducting vortices and anti-vortices form pairs. The two-dimensional XY model can mimic the superconducting transition temperature Tc and the BKT transition at a lower critical temperature TBKT by observing the heat capacity anomalies upon cooling. If the Josephson coupling across the nanowires is strong, the heat capacity anomalies almost overlap with each other so that it is difficult to distinguish between the Tc and the TBKT. To solve this issue, we apply an artificial-intelligence technique to split the nearly overlapped heat capacity anomalies. After the GoogLeNet-assisted phase transition detector is built, the GoogLeNet model can learn from the features of the phase transitions and then interpret the Tc and TBKT in the unseen system precisely. Our work opens a path for the GoogLeNet model to enter the world of magnetism and superconductivity. △ Less

Submitted 26 July, 2021; originally announced July 2021.

The effectiveness of data augmentation in porous substrate, nanowire, fiber and tip images at the level of deep learning intelligence

Authors: C. H. Wong, S. M. Ng, C. W. Leung, A. F. Zatsepin

Abstract: To prepare for identifying the composition of nanowire-fiber mixtures in Scanning Electron Microscope (SEM) images, we optimize the performance of image classification between nanowires, fibers and tips due to their geometric similarities. The SEM images are analyzed by deep learning techniques where the validation accuracies of 11 convolutional neural network (CNN) models are compared. By increas… ▽ More To prepare for identifying the composition of nanowire-fiber mixtures in Scanning Electron Microscope (SEM) images, we optimize the performance of image classification between nanowires, fibers and tips due to their geometric similarities. The SEM images are analyzed by deep learning techniques where the validation accuracies of 11 convolutional neural network (CNN) models are compared. By increasing the diversity of data such as reflection, translation and scale factor approaches, the highest validation accuracy of recognizing nanowires, fibers and tips is 97.1%. We proceed to classify the level of porosity in anodized aluminum oxide for the self-assisted nanowire growth where the validation accuracy is optimized at 93%. Our software allow scientists to count the percentage of fibers in any nanowire-fiber composite and design the porous substrate for embedding different sizes of nanowires automatically, which assists the software development in Nanoscience Foundries & Fine Analysis (NFFA) Europe Projects. △ Less

Submitted 11 March, 2021; originally announced March 2021.

Schottky-diode design for the world's leading telecommunication

Authors: Chi Ho Wong, Frank Leung Yuk Lam, Xijun Hu, Anatoly Fedorovich Zatsepin

Abstract: The Schottky diode, BN/GaN layered composite contacting to bulk aluminum, is theoretically plausible to harvest wireless energy above X-band. According to our first principle calculation, the insertion of GaN layers dramatically influences the optical properties of the layered composite. The relative dielectric constant of BN/GaN layered composite as a function of layer-to-layer separation is inve… ▽ More The Schottky diode, BN/GaN layered composite contacting to bulk aluminum, is theoretically plausible to harvest wireless energy above X-band. According to our first principle calculation, the insertion of GaN layers dramatically influences the optical properties of the layered composite. The relative dielectric constant of BN/GaN layered composite as a function of layer-to-layer separation is investigated where the optimized dielectric constant is 3.1. Furthermore, we design another Schottky diode via nanostructuring. Our first principle calculation suggests that the relative dielectric constant of boron nitride monolayer can be minimized to 1.5 only if it is deposited on aluminum monolayer. It is rare to find a semiconductor with the dielectric constant close to 1 which may push the cut-off frequency of Al/BN-based rectenna to the high-band 5G network. △ Less

Submitted 28 January, 2021; originally announced January 2021.

Spin and charge distributions in Graphene/Nickel (111) substrate under Rashba spin-orbital coupling

Authors: C. H. Wong, A. F. Zatsepin

Abstract: To understand the coupling factor between Rashba spin-orbital interaction and ferromagnetic proximity effect, we design a Monte Carlo algorithm to simulate the spin and charge distributions for the room-temperature Rashba material, Graphene/Nickel(111) substrate, at finite temperature. We observe that the rate of exchange fluctuation is a key player to produce giant Rashba spin-orbit splitting in… ▽ More To understand the coupling factor between Rashba spin-orbital interaction and ferromagnetic proximity effect, we design a Monte Carlo algorithm to simulate the spin and charge distributions for the room-temperature Rashba material, Graphene/Nickel(111) substrate, at finite temperature. We observe that the rate of exchange fluctuation is a key player to produce giant Rashba spin-orbit splitting in graphene. More importantly, we monitor the Rashba spin-splitting phenomenon where the spin-polarized electrons may be escaped from two opposite edges upon heating. However, the escaped electrons show Gaussian-like distribution in interior area that is important for spintronic engineers to optimize the efficiency of spin-state detection. In addition, we investigate if our Monte Carlo model can explain why room-temperature Rashba effect is observed in Graphene/Nickel(111) substrate experimentally. All results are presented in physical units. △ Less

Submitted 28 July, 2020; originally announced July 2020.

Enormous enhancement of p-orbital magnetism and band gap in the lightly doped carbyne

Authors: C. H. Wong, R. Lortz, A. F. Zatsepin

Abstract: This paper presents a path to tailor adapted magnetic and optical properties in carbyne. Although p-orbital magnetism is generally much weaker than d-orbital magnetism, we demonstrate that the charge fluctuation of the free radical electrons triggered by a time-varying electric dipole moment leads to enormous p-orbital magnetism. By introducing 25% arsenic and 12.5% fluorine into the monoatomic ca… ▽ More This paper presents a path to tailor adapted magnetic and optical properties in carbyne. Although p-orbital magnetism is generally much weaker than d-orbital magnetism, we demonstrate that the charge fluctuation of the free radical electrons triggered by a time-varying electric dipole moment leads to enormous p-orbital magnetism. By introducing 25% arsenic and 12.5% fluorine into the monoatomic carbon chain, the magnetic moment of the arsenic atom reaches 2.9 Bohr Magneton, which is ~1.3 times stronger than magnetic moment of bulk Fe. This magnetically optimized carbyne composite carries an exchange-correlation energy of 22meV (~270K). On the other hand, we convert the carbyne (in beta-form) from metallic to a semiconducting state by using anionic dopants. After doping 12.5% nitrogen and 12.5% oxygen into the beta-carbyne, the semiconducting gap of this composite is optimized at 1.6eV, which is 1.4 times larger than the band gap of bulk silicon. △ Less

Submitted 23 October, 2019; v1 submitted 1 October, 2019; originally announced October 2019.

The influence of antiferromagnetism, soft out-of plane phonons and heavy electrons on the superconducting pairing mechanism of Ba1-xKxFe2As2

Authors: Chi Ho Wong, Rolf Lortz

Abstract: Based on ab-initio calculated parameters, we apply a theoretical model on the iron-based BaFe2As2 superconductor that takes into account dramatic enhancements of the electron-phonon coupling of soft transverse phonons in the FeAs layers and antiferromagnetism. Our model is able to reproduce the Tc values of BaFe2As2 found under pressure in experiments. To calculate the Tc of the K-doped Ba1-xKxFe2… ▽ More Based on ab-initio calculated parameters, we apply a theoretical model on the iron-based BaFe2As2 superconductor that takes into account dramatic enhancements of the electron-phonon coupling of soft transverse phonons in the FeAs layers and antiferromagnetism. Our model is able to reproduce the Tc values of BaFe2As2 found under pressure in experiments. To calculate the Tc of the K-doped Ba1-xKxFe2As2 system as a function of the K content, we additionally consider the experimentally observed effective mass enhancements and Kondo temperatures in the strongly over-doped region (0.8 < x < 1), which decouple the antiferromagnetism and electron-phonon scattering. The highest theoretical Tc at the optimal doping concentration is reproduced after optimization of antiferromagnetic fluctuations and electron-phonon coupling. Our model is also able to reproduce the dip-like structure in Tc in the region where a re-entrant tetragonal phase of C4 symmetry is found (0.24 < x < 0.28) and indicates the weakening effect of local exchange correlation energy as responsible for Tc reduction. Our model thus demonstrates that the high transition temperatures and the exact doping and pressure dependence of this iron-based superconductor can be explained within an extended electron-phonon coupling model in which the structural, magnetic and electronic degrees of freedom are strongly intertwined. △ Less

Submitted 13 June, 2019; originally announced June 2019.

The interactions between antiferromagnetism, tetrahedral sites and electron-phonon coupling in FeSe1-xTex and FeSe/SrTiO3

Authors: C. H. Wong, R. Lortz

Abstract: We show that the superconducting transition temperature Tc of FeSe1-xTex can be computed to reasonable values in a modified McMillan approach in which the electron-phonon coupling is amplified by the antiferromagnetism and the out-of-plane phonons triggered by the tetrahedral lattice sites. This interplay is not only effective at ambient pressure, but also under hydrostatic compression. According… ▽ More We show that the superconducting transition temperature Tc of FeSe1-xTex can be computed to reasonable values in a modified McMillan approach in which the electron-phonon coupling is amplified by the antiferromagnetism and the out-of-plane phonons triggered by the tetrahedral lattice sites. This interplay is not only effective at ambient pressure, but also under hydrostatic compression. According to our model, the theoretical Tc of the compressed FeSe0.5Te0.5 agrees with experiment results. More importantly, by taking into account the interfacial effect between an FeSe monolayer and its SrTiO3 substrate as an additional gain factor, our calculated Tc value is up to 91 K high, and provides evidence that the strong Tc enhancement recently observed in such monolayers with Tc reaching 100 K may be due to an enhanced-electron phonon coupling. △ Less

Submitted 28 June, 2019; v1 submitted 31 May, 2019; originally announced May 2019.

The antiferromagnetic and phonon-mediated model of the NaFeAs, LiFeAs and FeSe superconductors

Authors: Chi Ho Wong, Rolf Lortz

Abstract: Recently it has been suggested that the role of electron-phonon coupling in the mechanism of iron-based superconductors may have been underestimated and that the antiferromagnetism and the induced xy potential may even have a dramatic amplification effect on electron-phonon coupling. To substantiate the recently announced xy potential in the literature, we create a two-channel model to separately… ▽ More Recently it has been suggested that the role of electron-phonon coupling in the mechanism of iron-based superconductors may have been underestimated and that the antiferromagnetism and the induced xy potential may even have a dramatic amplification effect on electron-phonon coupling. To substantiate the recently announced xy potential in the literature, we create a two-channel model to separately superimpose the dynamics of the electron in the upper and lower tetrahedral plane. The results of our two-channel model support the literature data. While the scientists are still searching for a universal DFT functional that can describe the pairing mechanism of all iron-based superconductors, we are designing an empirical combination of DFT functional to calculate the electron-phonon coupling and antiferromagetism of LiFeAs, NaFeAs and FeSe. We use ARPES data to revise the electron-phonon scattering matrix in superconducting state to ensure that all electrons involved in iron-based superconductivity are included in the ab-inito calculation. We present an ab-initio theoretical approach that takes into account this amplifying effect of antiferromagnetism and the correction of the electron-phonon scattering matrix together with the abnormal soft out-of-plane lattice vibration of the layered structure, which allows us to calculate theoretical Tc values of LiFeAs, NaFeAs and FeSe as a function of pressure that correspond reasonably well to the experimental values. △ Less

Submitted 7 March, 2021; v1 submitted 18 February, 2019; originally announced February 2019.

Electron-electron interactions of the multi-Cooper-pairs in the 1D limit and their role in the formation of global phase coherence in quasi-one-dimensional superconducting nanowire arrays

Authors: C. H. Wong, E. A. Buntov, A. F. Zatsepin, R. Lortz

Abstract: Nanostructuring of superconducting materials to form dense arrays of thin parallel nanowires with significantly large transverse Josephson coupling has proven to be an effective way to increase the upper critical field of superconducting elements by as much as two orders of magnitude as compared to the corresponding bulk materials and, in addition, may cause considerable enhancements in their crit… ▽ More Nanostructuring of superconducting materials to form dense arrays of thin parallel nanowires with significantly large transverse Josephson coupling has proven to be an effective way to increase the upper critical field of superconducting elements by as much as two orders of magnitude as compared to the corresponding bulk materials and, in addition, may cause considerable enhancements in their critical temperatures. Such materials have been realized in the linear pores of mesoporous substrates or exist intrinsically in the form of various quasi-1D crystalline materials. The transverse coupling between the superconducting nanowires is determined by the size-dependent coherence length E0. In order to obtain E0 over the Langer-Ambegaokar- McCumber-Halperin (LAMH) theory, extensive experimental fitting parameters have been required over the last 40 years. We propose a novel Monte Carlo algorithm for determining E0 of the multi-Cooper pair system in the 1D limit. The concepts of uncertainty principle, Pauli-limit, spin flip mechanism, electrostatic interaction, thermal perturbation and co-rotating of electrons are considered in the model. We use Pb nanowires as an example to monitor the size effect of E0 as a result of the modified electron-electron interaction without the need for experimental fitting parameters. We investigate how the coherence length determines the transverse coupling of nanowires in dense arrays. This determines whether or not a global phase-coherent state with zero resistance can be formed in such arrays. Our Monte Carlo results are in very good agreement with experimental data from various types of superconducting nanowire arrays △ Less

Submitted 2 July, 2018; originally announced July 2018.

Revisiting the semi-flexible entangled chains of polymer in the carbyne model

Authors: C. H. Wong, L. Xue, E. A. Buntov, A. F. Zatsepin

Abstract: The Monte Carlo carbyne model is modified to investigate the glass transition of the semi-flexible entangled polymer chains. The stochastic bombardment between monomers are monitored by Metropolis algorithm with help of the consideration of hard potential while the mobility of monomers is governed by its mass, scattering rate and temperature. Our model is capable to show that the glass transition… ▽ More The Monte Carlo carbyne model is modified to investigate the glass transition of the semi-flexible entangled polymer chains. The stochastic bombardment between monomers are monitored by Metropolis algorithm with help of the consideration of hard potential while the mobility of monomers is governed by its mass, scattering rate and temperature. Our model is capable to show that the glass transition temperature reduces with decreasing film thickness and the formation of critical voids in the thinner polymer contributing to the glass transition that is much easier than the bulk polymer. △ Less

Submitted 2 July, 2018; originally announced July 2018.

A tunable quantum dissipator for active resonator reset in circuit QED

Authors: Clement H. Wong, Chris Wilen, Robert McDermott, Maxim G. Vavilov

Abstract: We propose a method for fast, deterministic resonator reset based on tunable dissipative modes. The dissipator is based on a Josephson junction with relatively low quality factor. When the dissipator is tuned into resonance with a high quality microwave resonator, resonator photons are absorbed by the dissipator at a rate orders of magnitude faster than the resonator relaxation rate. We determine… ▽ More We propose a method for fast, deterministic resonator reset based on tunable dissipative modes. The dissipator is based on a Josephson junction with relatively low quality factor. When the dissipator is tuned into resonance with a high quality microwave resonator, resonator photons are absorbed by the dissipator at a rate orders of magnitude faster than the resonator relaxation rate. We determine the optimal parameters for realization of the tunable dissipator, and examine application of the dissipator to removing spurious photon population in the qubit readout resonator in circuit quantum electrodynamics. We show that even in the nonlinear large photon occupation regime, this enhanced resonator decay rate can be attained by appropriate modulation of the dissipator frequency. △ Less

Submitted 24 November, 2018; v1 submitted 5 June, 2018; originally announced June 2018.

Comments: updated references

Journal ref: Quantum Sci. Technol. 4 025001 (2019)

Antiferromagnetically assisted electron-phonon coupling and spin-lattice interaction in Fe-based superconductors

Authors: Chi Ho Wong, Rolf Lortz

Abstract: We present a theoretical ab-initio approach that allows us to explicitly calculate the superconducting transition temperatures (Tc) of the iron-based superconductors of LaFeAsO1-xFx, SmFeAsO1-xFx, NdFeAsO1-xFx, Ba1-xKxFe2As2, FeSe and LiFeAs that fit perfectly with the experiments. We consider recent evidence that electron-phonon coupling may have been underestimated previously, and a prediction t… ▽ More We present a theoretical ab-initio approach that allows us to explicitly calculate the superconducting transition temperatures (Tc) of the iron-based superconductors of LaFeAsO1-xFx, SmFeAsO1-xFx, NdFeAsO1-xFx, Ba1-xKxFe2As2, FeSe and LiFeAs that fit perfectly with the experiments. We consider recent evidence that electron-phonon coupling may have been underestimated previously, and a prediction that antiferromagnetism can greatly enhance electron-phonon coupling through localized iron d orbitals. We then include the contribution of these localized orbitals in a McMillan formalism. In addition, we take into account the spin-lattice interaction between the spin-polarized electrons at the Fermi surface and the iron orbitals in combination with a modified exchange Hamiltonian involving a ferrimagnetic coupling between Fe and As. With this approach we can accurately calculate the Tc of FeSe (11 family), LiFeAs (111 family), LaFeAsO0.9F0.1 (1111 family) and BaFe2As2 (122 family) as a function of pressure. In addition, we also obtain the correct doping dependence of Tc of LaFeAsO0.9F0.1 (1111 family) and BaFe2As2 (122 family). △ Less

Submitted 18 February, 2019; v1 submitted 24 May, 2018; originally announced May 2018.

Comments: The magnetic moment of arsenic atom is arisen from the poor accuracy of our simulation. Therefore I prefer to withdraw this article. Thank you

High-pressure effects on isotropic superconductivity in the iron-free layered pnictide superconductor BaPd2As2

Authors: M. Abdel-Hafiez, Y. Zhao, Z. Huang, C. -w. Cho, C. H. Wong, A. Hassen, M. Ohkuma, Y. -W. Fang, B. -J. Pan, Z. -A. Ren, A. Sadakov, A. Usoltsev, V. Pudalov, M. Mito, R. Lortz, C. Krellner, W. Yang

Abstract: While the layered 122 iron arsenide superconductors are highly anisotropic, unconventional, and exhibit several forms of electronic orders that coexist or compete with superconductivity in different regions of their phase diagrams, we find in the absence of iron in the structure that the superconducting characteristics of the end member BaPd2As2 are surprisingly conventional. Here we report on com… ▽ More While the layered 122 iron arsenide superconductors are highly anisotropic, unconventional, and exhibit several forms of electronic orders that coexist or compete with superconductivity in different regions of their phase diagrams, we find in the absence of iron in the structure that the superconducting characteristics of the end member BaPd2As2 are surprisingly conventional. Here we report on complementary measurements of specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and synchrotron high pressure x-ray diffraction measurements supplemented with theoretical calculations for BaPd2As2. Its superconducting properties are completely isotropic as demonstrated by the critical fields, which do not depend on the direction of the applied field. Under the application of high pressure, Tc is linearly suppressed, which is the typical behavior of classical phonon-mediated superconductors with some additional effect of a pressure-induced decrease in the electronic density of states and the electron-phonon coupling parameters. Structural changes in the layered BaPd2As2 have been studied by means of angle-dispersive diffraction in a diamond-anvil cell. At 12 GPa and 24.2 GPa we observed pressure induced lattice distortions manifesting as the discontinuity and, hence discontinuity in the Birch-Murnaghan equation of state. The bulk modulus is B0=40(6) GPa below 12 GPa and B0=142(3) GPa below 27.2 GPa. △ Less

Submitted 20 May, 2018; originally announced May 2018.

Journal ref: Phys. Rev. B 97, 134508 (2018)

Thermal disorder in finite-length carbon nanowire

Authors: C. H. Wong, E. A. Buntov, A. F. Zatsepin, M. B. Guseva

Abstract: Chemisorption is one of the active research areas in carbon materials. The occurrence of the monoatomic carbon chain can be made by surrounding the double walled carbon nanotube and meanwhile worldwide efforts have been made to create the extraction technique for unlashing the carbon chains from the enclosure. Here we report an extensive study of the kink structure in the free standing carbon nano… ▽ More Chemisorption is one of the active research areas in carbon materials. The occurrence of the monoatomic carbon chain can be made by surrounding the double walled carbon nanotube and meanwhile worldwide efforts have been made to create the extraction technique for unlashing the carbon chains from the enclosure. Here we report an extensive study of the kink structure in the free standing carbon nanowires. Our Monte Carlo simulation considers the multi-monoatomic carbon chains laterally interacted by the Van der Waal force. Despite the linearity of the carbon nanowires is independent of chain length at low temperatures, the same situation does not hold at high temperatures. Disordered kink structure is observed in the short carbon chains especially above Peierls transition temperature. For instance, the average kink angle of 50-atoms carbon nanowire is as large as 35 degree at 800K. We have provided an important inspection that any physical property of the finite-length carbon chain predicted by ab-initio calculation should reconsider the atomic rearrangement due to the thermal instability. Apart from this, the kink structure in the nanowires likely increases the probability of attaching negatively charged atoms which is an encouragement to find the next generation materials for chemisorption. △ Less

Submitted 9 February, 2018; originally announced February 2018.

Superconductivity in ultra-thin carbon nanotubes and carbyne-nanotube composites: an ab-initio approach

Authors: C. H. Wong, E. A. Buntov, R. E. Kasimova, A. F. Zatsepin

Abstract: The superconductivity of the 4-angstrom single-walled carbon nanotubes (SWCNTs) was discovered more than a decade ago, and marked the breakthrough of finding superconductivity in pure elemental undoped carbon compounds. The van Hove singularities in the electronic density of states at the Fermi level in combination with a large Debye temperature of the SWCNTs are expected to cause an impressively… ▽ More The superconductivity of the 4-angstrom single-walled carbon nanotubes (SWCNTs) was discovered more than a decade ago, and marked the breakthrough of finding superconductivity in pure elemental undoped carbon compounds. The van Hove singularities in the electronic density of states at the Fermi level in combination with a large Debye temperature of the SWCNTs are expected to cause an impressively large superconducting gap. We have developed an innovative computational algorithm specially tailored for the investigation of superconductivity in ultrathin SWCNTs. We predict the superconducting transition temperature of various thin carbon nanotubes resulting from electron-phonon coupling by an ab-initio method, taking into account the effect of radial pressure, symmetry, chirality (N,M) and bond lengths. By optimizing the geometry of the carbon nanotubes, a maximum Tc of 60K is found. We also use our method to calculate the Tc of a linear carbon chain embedded in the center of (5,0) SWCNTs. The strong curvature in the (5,0) carbon nanotubes in the presence of the inner carbon chain provides an alternative path to increase the Tc of this carbon composite by a factor of 2.2 with respect to the empty (5,0) SWCNTs. △ Less

Submitted 15 July, 2017; originally announced July 2017.

Comments: 15 pages, 8 figures

The role of the coherence length for the establishment of global phase coherence in three-dimensional arrays of ultra-thin quasi-one-dimensional superconducting Pb and NbN nanowires

Authors: Chi Ho Wong, Frank L. Y. Lam, Junying Shen, Minquan He, Xijun Hu, Rolf Lortz

Abstract: We have fabricated 5 nm ultra-thin NbN nanowires that form a dense and regular array in the linear channels of mesoporous SBA-15 silica substrates. Bulk NbN is a well-known classical superconductor with Tc of 16 K. We show that, by being incorporated into this nanostructure, the composite material exhibits typical quasi-one-dimensional characteristics. We compare the superconducting properties wit… ▽ More We have fabricated 5 nm ultra-thin NbN nanowires that form a dense and regular array in the linear channels of mesoporous SBA-15 silica substrates. Bulk NbN is a well-known classical superconductor with Tc of 16 K. We show that, by being incorporated into this nanostructure, the composite material exhibits typical quasi-one-dimensional characteristics. We compare the superconducting properties with those of superconducting Pb nanowires of same dimensionality in identical configuration within the linear SBA-15 pores. While Pb nanowire arrays show a pronounced crossover from 1D superconductivity at high temperatures to a 3D bulk superconducting state in the low temperature regime with true zero resistance triggered by transversal Josephson interaction, this transition appears to be completely absent in the NbN nanowire array. The small coherence length in NbN, which strongly suppresses the Josephson coupling is discussed as the origin of this difference. △ Less

Submitted 15 May, 2017; originally announced May 2017.

Entanglement of condensed magnons via momentum-space fragmentation

Authors: Clement H. Wong, Ari Mizel

Abstract: A scheme is presented for engineering momentum-space entanglement of fragmented magnon condensates. We consider easy plane frustrated antiferromagnets in which the magnon dispersion has degenerate minima that represent umbrella chiral spin textures. With an applied magnetic field, we tune the Hamiltonian near a quantum critical point that is is signaled by a singularity in the entanglement entropy… ▽ More A scheme is presented for engineering momentum-space entanglement of fragmented magnon condensates. We consider easy plane frustrated antiferromagnets in which the magnon dispersion has degenerate minima that represent umbrella chiral spin textures. With an applied magnetic field, we tune the Hamiltonian near a quantum critical point that is is signaled by a singularity in the entanglement entropy. The ground state develops momentum-space entanglement of the chiral spin textures. The size of the entangled superposition is accessible experimentally through the magnetic structure factor. Our model is motivated by equilibrium magnon condensates in frustrated antiferromagnets such as CsCuCl3, and it can also be simulated in spin-orbit coupled Mott insulators in atomic optical lattices and circuit quantum electrodynamics. △ Less

Submitted 17 March, 2017; originally announced March 2017.

Journal ref: Phys. Rev. B 96, 054412 (2017)

Effect of symmetry on the electronic DOS, charge fluctuations and electron-phonon coupling in carbon chains

Authors: C. H. Wong, J. Y. Dai, M. B. Guseva, V. N. Rychkov, E. A. Buntov, A. F. Zatsepin

Abstract: A theoretical model is provided to address the parameters influencing the electronic properties of kink-structured carbon chain at 0K. It is studied by the principle of DFT and solving the numerical 1D time-independent Schrödinger equation of electron and phonon simultaneously. Two different lengths of branches A and B, are occupied alternatively to generate the asymmetric carbon chain. The ratio… ▽ More A theoretical model is provided to address the parameters influencing the electronic properties of kink-structured carbon chain at 0K. It is studied by the principle of DFT and solving the numerical 1D time-independent Schrödinger equation of electron and phonon simultaneously. Two different lengths of branches A and B, are occupied alternatively to generate the asymmetric carbon chain. The ratio of the asymmetric branch length,RAB=A/B, plays an important role in the electronic density of states DOS around Fermi level Ef . The highest DOS(Ef) occurs if the RAB equals to 2 and while the Fermi level coincides with the Von-Hove singularity at RAB=3. The location of the singularity point relative to the Ef is controllable via branch length interestingly. By comparison with the symmetric case, tuning the branch length asymmetrically shows a stronger impact to shift the Ef to the singularity point. The numerical solution of the 1D time independent Schrodinger equation of phonon indicates that the kink reinforces the charge fluctuations but the fluctuations are minimized when the RAB goes up. Based on the simulation results, the electron phonon coupling of the carbon nanowire decreases with chain length. In comparison to the symmetric structure, the electron phonon coupling of the asymmetric carbon chain is higher. The maximum electron phonon coupling of the asymmetric carbon chain takes place at RAB=2 . However, the weakening of the electron phonon coupling is observed owing to ultrahigh concentration of kinks. The reduction of the electron phonon coupling of the carbon chain is occurred under pressure. △ Less

Submitted 17 November, 2016; originally announced November 2016.

Simulation of chemical bond distributions and phase transformation in carbon chains

Authors: C. H. Wong, E. A. Buntov, V. N. Rychkov, M. B. Guseva, A. F. Zatsepin

Abstract: In the present work we develop a Monte Carlo algorithm of the carbon chains ordered into 2D hexagonal array. The chemical bond of the chained carbon is computed from 1K to 1300K. Our model confirms that the beta phase is more energetic preferable at low temperatures but the system prefers the alpha phase at high temperatures. Based on the thermal effect on the bond distributions and 3D atomic vibr… ▽ More In the present work we develop a Monte Carlo algorithm of the carbon chains ordered into 2D hexagonal array. The chemical bond of the chained carbon is computed from 1K to 1300K. Our model confirms that the beta phase is more energetic preferable at low temperatures but the system prefers the alpha phase at high temperatures. Based on the thermal effect on the bond distributions and 3D atomic vibrations in the carbon chains, the bond softening temperature is observed at 500K. The bond softening temperature is higher in the presence of interstitial doping but it does not change with the length of nanowire. The elastic modulus of the carbon chains is 1.7TPa at 5K and the thermal expansion is +7 x 10-5 K-1 at 300K via monitoring the collective atomic vibrations and bond distributions. Thermal fluctuation in terms of heat capacity as a function of temperatures is computed in order to study the phase transition across melting point. The heat capacity anomaly initiates around 3800K. △ Less

Submitted 15 November, 2016; originally announced November 2016.

Dramatic increase of the onset critical temperature and critical field of elemental Sn in the form of thin nanowires

Authors: Ying Zhang, Chi Ho Wong, Junying Shen, Sin Ting Sze, Yan Dong, Hui Xu, Zifeng Yan, Yingying Li, Xijun Hu, Rolf Lortz

Abstract: Sn is a well-known classical superconductor on the border between type I and type II with critical temperature of 3.722K and critical field of 0.031T. We show by means of specific heat and electric magneto-transport data that its critical parameters can be dramatically increased if it is brought in the form of loosely bound bundles of thin nanowires. The specific heat displays a pronounced double… ▽ More Sn is a well-known classical superconductor on the border between type I and type II with critical temperature of 3.722K and critical field of 0.031T. We show by means of specific heat and electric magneto-transport data that its critical parameters can be dramatically increased if it is brought in the form of loosely bound bundles of thin nanowires. The specific heat displays a pronounced double phase transition at 3.7K and 5.5K, which we attribute to the inner 'bulk' contribution of the nanowires and to the surface contribution, respectively. The latter is visible only because of the large volume fraction of the surface layer in relation to their bulk inner volume. The upper transition coincides with the onset of the resistive transition, while zero resistance is gradually approached below the lower transition. The large coherence length of 230nm at 0K likely actuates a Josephson coupling between adjacent neighboring nanowires and thus suppresses the effect of 1D phase fluctuations along the nanowires, and stabilizes 3D phase coherence throughout the entire network with zero resistance. A magnetic field of more than 3T is required to restore the normal state, which means that the critical field is enhanced by about two orders of magnitude with respect to Sn in its bulk form. △ Less

Submitted 20 January, 2016; originally announced January 2016.

Edge effect and significant increase of the superconducting transition onset temperature of 2D superconductors in flat and curved geometries

Authors: Chi Ho Wong, Rolf Lortz

Abstract: In this paper, we present a simple method to model the curvature activated phonon softening in a 2D superconducting layer. The superconducting transition temperature Tc in the case of a 2D rectangular sheet, a hollow cylinder and a hollow sphere of one coherence length thickness is calculated by the quantum mechanical electron-phonon scattering matrix, and a series of collective lattice vibrations… ▽ More In this paper, we present a simple method to model the curvature activated phonon softening in a 2D superconducting layer. The superconducting transition temperature Tc in the case of a 2D rectangular sheet, a hollow cylinder and a hollow sphere of one coherence length thickness is calculated by the quantum mechanical electron-phonon scattering matrix, and a series of collective lattice vibrations in the surface state. We will show that being extremely thin in a flat rectangular shape is not enough to significantly enhance the Tc through phonon softening. However, if a curvature is added, Tc can be strongly enhanced. The increase in Tc with respect to the bulk is greatest in a hollow sphere, intermediate in a hollow cylinder and weakest for the rectangular sheet, when systems of identical length scale are considered. In addition, we find that the edge effect of such a 2D sheet has a strong broadening effect on Tc in addition to the effect of order parameter phase fluctuations. △ Less

Submitted 18 January, 2016; originally announced January 2016.

Comments: Physica C, in press

Quantum efficiency of a microwave photon detector based on a double quantum dot

Authors: Clement H. Wong, Maxim G. Vavilov

Abstract: Motivated by recent interest in implementing circuit quantum electrodynamics with semiconducting quantum dots, we consider a double quantum dot (DQD) capacitively coupled to a superconducting resonator that is driven by the microwave field of a superconducting transmission line. We analyze the DQD current response using input-output theory and show that the resonator-coupled DQD is a sensitive mic… ▽ More Motivated by recent interest in implementing circuit quantum electrodynamics with semiconducting quantum dots, we consider a double quantum dot (DQD) capacitively coupled to a superconducting resonator that is driven by the microwave field of a superconducting transmission line. We analyze the DQD current response using input-output theory and show that the resonator-coupled DQD is a sensitive microwave single photon detector. Using currently available experimental parameters of DQD-resonator coupling and dissipation, including the effects of $1/f$ charge noise and phonon noise, we determine the parameter regime for which incident photons are completely absorbed and near unit $\gtrsim$ 98\% efficiency can be achieved. We show that this regime can be reached by using very high quality resonators with quality factor $Q\simeq 10^5$. △ Less

Submitted 15 June, 2016; v1 submitted 21 December, 2015; originally announced December 2015.

Journal ref: Physical Review A 95, 012325 (2017)

High fidelity ac gate operations of the quantum dot hybrid qubit

Authors: Clement H. Wong

Abstract: Semiconductor quantum dots in silicon are promising qubits because of long spin coherence times and their potential for scalability. However, such qubits with complete electrical control and fidelities above the threshold for quantum error correction have not yet been achieved. We show theoretically that the threshold fidelity can be achieved with ac gate operation of the quantum dot hybrid qubit.… ▽ More Semiconductor quantum dots in silicon are promising qubits because of long spin coherence times and their potential for scalability. However, such qubits with complete electrical control and fidelities above the threshold for quantum error correction have not yet been achieved. We show theoretically that the threshold fidelity can be achieved with ac gate operation of the quantum dot hybrid qubit. Formed by three electrons in a double dot, this qubit is electrically controlled, does not require magnetic fields, and runs at GHz gate speeds. We analyze the decoherence caused by 1/f charge noise in this qubit, find the parameter regime for tunnel couplings and detuning that minimize the charge noise dependence in the qubit frequency, and determine the optimal working points for ac gate operations that drive the detuning and tunnel coupling. △ Less

Submitted 22 March, 2016; v1 submitted 15 July, 2015; originally announced July 2015.

Journal ref: Phys. Rev. B 93, 035409 (2016)

Spin-heat relaxation and thermo-spin diffusion in atomic Bose and Fermi gases

Authors: Clement H. Wong, H. T. C. Stoof, R. A. Duine

Abstract: We study spin-dependent heat transport in quantum gases, focusing on transport phenomena related to pure spin currents and spin-dependent temperatures. Using the Boltzmann equation, we compute the coupled spin and heat transport coefficients as a function of temperature and interaction strength for energy dependent $s$-wave scattering. We address the issue of whether spin-dependent temperatures ca… ▽ More We study spin-dependent heat transport in quantum gases, focusing on transport phenomena related to pure spin currents and spin-dependent temperatures. Using the Boltzmann equation, we compute the coupled spin and heat transport coefficients as a function of temperature and interaction strength for energy dependent $s$-wave scattering. We address the issue of whether spin-dependent temperatures can be sustained on a time and length scale relevant for experiments by computing the spin-heat relaxation time and diffusion length. We find that the time scale for spin-heat relaxation time diverges at low temperatures for both bosons and fermions, indicating that the concept of spin-heat accumulation is well defined for degenerate gases. For bosons, we find power-law behavior on approach to Bose condensation above the critical temperature, as expected from the theory of dynamical critical phenomena. △ Less

Submitted 5 February, 2015; v1 submitted 20 October, 2014; originally announced October 2014.

Journal ref: Phys. Rev. A 91, 043602 (2015)

High Fidelity Singlet-Triplet ${S}$-${T_-}$ Qubits in Inhomogeneous Magnetic Fields

Authors: Clement H. Wong, M. A. Eriksson, S. N. Coppersmith, Mark Friesen

Abstract: We propose an optimal set of quantum gates for a singlet-triplet qubit in a double quantum dot with two electrons utilizing the $S$-$T_-$ subspace. Qubit rotations are driven by the applied magnetic field and an orthogonal field gradient provided by a micromagnet. We optimize the fidelity of this qubit as a function of magnetic fields, taking advantage of "sweet spots" where the rotation frequenci… ▽ More We propose an optimal set of quantum gates for a singlet-triplet qubit in a double quantum dot with two electrons utilizing the $S$-$T_-$ subspace. Qubit rotations are driven by the applied magnetic field and an orthogonal field gradient provided by a micromagnet. We optimize the fidelity of this qubit as a function of magnetic fields, taking advantage of "sweet spots" where the rotation frequencies are independent of the energy level detuning, providing protection against charge noise. We simulate gate operations and qubit rotations in the presence of quasistatic noise from charge and nuclear spins as well as leakage to nonqubit states, and predict that in silicon quantum dots gate fidelities greater than $99\%$ can be achieved for two nearly-orthogonal rotation axes. △ Less

Submitted 16 July, 2015; v1 submitted 8 October, 2014; originally announced October 2014.

Journal ref: Phys. Rev. B 92 045403 (2015)

Large gap, a pseudogap and proximity effect in the Bi2Te3/Fe1+yTe interfacial superconductor

Authors: M. Q. He, Q. L. He, J. Y. Shen, H. C. Liu, Y. Zheng, C. H. Wong, Q. H. Chen, J. N. Wang, K. T. Law, I. K. Sou, A. P. Petrovic, R. Lortz

Abstract: We report directional point-contact spectroscopy data on the novel Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically sharp interface. Our data show a very large superconducting twin-gap structure with an energy scale exceeding that of bulk FeSe or FeSe1-xTex by a factor of 4. While the larger g… ▽ More We report directional point-contact spectroscopy data on the novel Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically sharp interface. Our data show a very large superconducting twin-gap structure with an energy scale exceeding that of bulk FeSe or FeSe1-xTex by a factor of 4. While the larger gap is isotropic and attributed to a thin FeTe layer in proximity of the interface, the smaller gap has a pronounced anisotropy and is associated with proximity-induced superconductivity in the topological insulator Bi2Te3. Zero resistance is lost above 8 K, but superconducting fluctuations are visible up to at least 12 K and the large gap is replaced by a pseudogap that persists up to 40 K. The spectra show a pronounced zero-bias conductance peak in the superconducting state, which may be a signature of an unconventional pairing mechanism. △ Less

Submitted 29 October, 2014; v1 submitted 22 July, 2014; originally announced July 2014.

Quasiparticle Berry curvature and Chern numbers in spin-orbit coupled bosonic Mott insulators

Authors: Clement H. Wong, R. A. Duine

Abstract: We study the ground-state topology and quasiparticle properties in bosonic Mott insulators with two- dimensional spin-orbit couplings in cold atomic optical lattices. We show that the many-body Chern and spin-Chern number can be expressed as an integral of the quasihole Berry curvatures over the Brillouin zone. Using a strong-coupling perturbation theory, for an experimentally feasible spin-orbit… ▽ More We study the ground-state topology and quasiparticle properties in bosonic Mott insulators with two- dimensional spin-orbit couplings in cold atomic optical lattices. We show that the many-body Chern and spin-Chern number can be expressed as an integral of the quasihole Berry curvatures over the Brillouin zone. Using a strong-coupling perturbation theory, for an experimentally feasible spin-orbit coupling, we compute the Berry curvature and the spin Chern number and find that these quantities can be generated purely by interactions. We also compute the quasiparticle dispersions, spectral weights, and the quasimomentum space distribution of particle and spin density, which can be accessed in cold-atom experiments and used to deduce the Berry curvature and Chern numbers. △ Less

Submitted 25 November, 2013; v1 submitted 12 July, 2013; originally announced July 2013.

Journal ref: Phys. Rev. A 88, 053631 (2013)

Topological transport in a spin-orbit coupled bosonic Mott insulator

Authors: C. H. Wong, R. A. Duine

Abstract: We investigate topological transport in a spin-orbit coupled bosonic Mott insulator. We show that interactions can lead to anomalous quasi-particle dynamics even when the spin-orbit coupling is abelian. To illustrate the latter, we consider the spin-orbit coupling realized in the experiment of Lin \textit{et al}. [Nature (London) \textbf{471}, 83 (2011)]. For this spin-orbit coupling, we compute t… ▽ More We investigate topological transport in a spin-orbit coupled bosonic Mott insulator. We show that interactions can lead to anomalous quasi-particle dynamics even when the spin-orbit coupling is abelian. To illustrate the latter, we consider the spin-orbit coupling realized in the experiment of Lin \textit{et al}. [Nature (London) \textbf{471}, 83 (2011)]. For this spin-orbit coupling, we compute the quasiparticle dispersions and spectral weights, the interaction-induced momentum space Berry curvature, and the momentum space distribution of spin density, and propose experimental signatures. Furthermore, we find that in our approximation for the single-particle propagator, the ground state can in principle support an integer Hall conductivity if the sum of the Chern numbers of the hole bands is nonzero. △ Less

Submitted 11 December, 2012; v1 submitted 29 November, 2012; originally announced November 2012.

Journal ref: Phys. Rev. Lett. 110, 115301 (2013)

1D to 3D Dimensional Crossover in the Superconducting Transition of the Quasi-One-Dimensional Carbide Superconductor Sc3CoC4

Authors: Mingquan He, Chi Ho Wong, Dian Shi, Pok Lam Tse, Ernst-Wilhelm Scheidt, Georg Eickerling, Wolfgang Scherer, Ping Sheng, Rolf Lortz

Abstract: The transition metal carbide superconductor Sc3CoC4 may represent a new benchmark system of quasi-1D superconducting behavior. We investigate the superconducting transition of a high-quality single crystalline sample by electrical transport experiments. Our data show that the superconductor goes through a complex dimensional crossover below the onset Tc of 4.5 K. First, a quasi-1D fluctuating supe… ▽ More The transition metal carbide superconductor Sc3CoC4 may represent a new benchmark system of quasi-1D superconducting behavior. We investigate the superconducting transition of a high-quality single crystalline sample by electrical transport experiments. Our data show that the superconductor goes through a complex dimensional crossover below the onset Tc of 4.5 K. First, a quasi-1D fluctuating superconducting state with finite resistance forms in the CoC4 ribbons which are embedded in a Sc matrix in this material. At lower temperature, the transversal Josephson or proximity coupling of neighboring ribbons establishes a 3D bulk superconducting state. This dimensional crossover is very similar to Tl2Mo6Se6, which for a long time has been regarded as the most appropriate model system of a quasi-1D superconductor. Sc3CoC4 appears to be even more in the 1D limit than Tl2Mo6Se6. △ Less

Submitted 29 October, 2014; v1 submitted 23 November, 2012; originally announced November 2012.

Spin-Seebeck effect in a strongly interacting Fermi gas

Authors: C. H. Wong, H. T. C. Stoof, R. A. Duine

Abstract: We study the spin-Seebeck effect in a strongly interacting, two-component Fermi gas and propose an experiment to measure this effect by relatively displacing spin up and spin down atomic clouds in a trap using spin-dependent temperature gradients. We compute the spin-Seebeck coefficient and related spin-heat transport coefficients as functions of temperature and interaction strength. We find that… ▽ More We study the spin-Seebeck effect in a strongly interacting, two-component Fermi gas and propose an experiment to measure this effect by relatively displacing spin up and spin down atomic clouds in a trap using spin-dependent temperature gradients. We compute the spin-Seebeck coefficient and related spin-heat transport coefficients as functions of temperature and interaction strength. We find that when the inter-spin scattering length becomes larger than the Fermi wavelength, the spin-Seebeck coefficient changes sign as a function of temperature, and hence so does the direction of the spin-separation. We compute this zero-crossing temperature as a function of interaction strength and in particular in the unitary limit for the inter-spin scattering. △ Less

Submitted 16 March, 2012; originally announced March 2012.

Journal ref: Phys. Rev. A 85, 063613 (2012)

Spin Caloritronics in Noncondensed Bose Gases

Authors: C. H. Wong, H. J. van Driel, R. Kittinaradorn, H. T. C. Stoof, R. A. Duine

Abstract: We consider coupled spin and heat transport in a two-component, atomic Bose gas in the noncon- densed state. We find that the transport coefficients show a temperature dependence reflecting the bosonic enhancement of scattering, and discuss experimental signatures of the spin-heat coupling in spin accumulation and total dissipation. Inside the critical region of Bose-Einstein condensation, we find… ▽ More We consider coupled spin and heat transport in a two-component, atomic Bose gas in the noncon- densed state. We find that the transport coefficients show a temperature dependence reflecting the bosonic enhancement of scattering, and discuss experimental signatures of the spin-heat coupling in spin accumulation and total dissipation. Inside the critical region of Bose-Einstein condensation, we find anomalous behavior of the transport coefficients, and in particular, an enhancement for the spin caloritronics figure of merit that determines the thermodynamic efficiency of spin-heat conversion. △ Less

Submitted 18 October, 2011; originally announced October 2011.

Journal ref: Phys. Rev. Lett. 108, 075301 (2012)

Spin-transfer mechanism for magnon-drag thermopower

Authors: M. E. Lucassen, C. H. Wong, R. A. Duine, Y. Tserkovnyak

Abstract: We point out a relation between the dissipative spin-transfer-torque parameter $β$ and the contribution of magnon drag to the thermoelectric power in conducting ferromagnets. Using this result we estimate $β$ in iron at low temperatures, where magnon drag is believed to be the dominant contribution to the thermopower. Our results may be used to determine $β$ from magnon-drag-thermopower experiment… ▽ More We point out a relation between the dissipative spin-transfer-torque parameter $β$ and the contribution of magnon drag to the thermoelectric power in conducting ferromagnets. Using this result we estimate $β$ in iron at low temperatures, where magnon drag is believed to be the dominant contribution to the thermopower. Our results may be used to determine $β$ from magnon-drag-thermopower experiments, or, conversely, to infer the strength of magnon drag via experiments on spin transfer. △ Less

Submitted 12 September, 2011; originally announced September 2011.

Report number: ITP-UU-11/34

Quantum kinetic equation in phase-space textured multiband systems

Authors: Clement H. Wong, Yaroslav Tserkovnyak

Abstract: Starting from the density-matrix equation of motion, we derive a semiclassical kinetic equation for a general two-band electronic Hamiltonian, systematically including quantum-mechanical corrections up to second order in space-time gradients. We find, in addition to band-projected corrections to the single-particle equation of motion due to phase-space Berry curvature, interband terms that we attr… ▽ More Starting from the density-matrix equation of motion, we derive a semiclassical kinetic equation for a general two-band electronic Hamiltonian, systematically including quantum-mechanical corrections up to second order in space-time gradients. We find, in addition to band-projected corrections to the single-particle equation of motion due to phase-space Berry curvature, interband terms that we attribute to the nonorthorgonality of the projected Hilbert spaces. As examples, we apply our kinetic equation to electronic systems in the presence of spatially inhomogeneous and dynamical spin textures stemming from electromagnetic gauge potentials. Specifically, we consider the electromagnetic response of massive two-dimensional Dirac fermions and three-dimensional Weyl fermions, and reproduce the anomalous currents known as the parity and the Adler-Bell-Jackiw anomaly in particle physics. △ Less

Submitted 25 November, 2013; v1 submitted 5 February, 2011; originally announced February 2011.

Journal ref: Phys. Rev. B 84, 115209 (2011)

Dissipative dynamics of magnetic solitons in metals

Authors: Clement H. Wong, Yaroslav Tserkovnyak

Abstract: Soliton dynamics in spin-textured metals generate electrical currents, which produce backaction through spin torques. We modify the Landau-Lifshitz-Gilbert equation and the corresponding solitonic equations of motion to include such higher-order texture effects. We also find a quasistatic equation for the induced electrochemical potential, which needs to be solved for self-consistently, in the i… ▽ More Soliton dynamics in spin-textured metals generate electrical currents, which produce backaction through spin torques. We modify the Landau-Lifshitz-Gilbert equation and the corresponding solitonic equations of motion to include such higher-order texture effects. We also find a quasistatic equation for the induced electrochemical potential, which needs to be solved for self-consistently, in the incompressible limit. As an example, we consider the orbital motion of a vortex in a point-contact spin valve, and discuss modifications of orbit radius, frequency, and dissipation power. △ Less

Submitted 14 February, 2010; v1 submitted 30 October, 2009; originally announced October 2009.

Comments: 1 figure

Journal ref: Phys. Rev. B 81, 060404(R) (2010)

Hydrodynamic theory of coupled current and magnetization dynamics in spin-textured ferromagnets

Authors: Clement H. Wong, Yaroslav Tserkovnyak

Abstract: We develop the hydrodynamical theory of collinear spin currents coupled to magnetization dynamics in metallic ferromagnets. The collective spin density couples to the spin current through a U(1) Berry-phase gauge field determined by the local texture and dynamics of the magnetization. We determine phenomenologically the dissipative corrections to the equation of motion for the electronic current… ▽ More We develop the hydrodynamical theory of collinear spin currents coupled to magnetization dynamics in metallic ferromagnets. The collective spin density couples to the spin current through a U(1) Berry-phase gauge field determined by the local texture and dynamics of the magnetization. We determine phenomenologically the dissipative corrections to the equation of motion for the electronic current, which consist of a dissipative spin-motive force generated by magnetization dynamics and a magnetic texture-dependent resistivity tensor. The reciprocal dissipative, adiabatic spin torque on the magnetic texture follows from the Onsager principle. We investigate the effects of thermal fluctuations and find that electronic dynamics contribute to a nonlocal Gilbert damping tensor in the Landau-Lifshitz-Gilbert equation for the magnetization. Several simple examples, including magnetic vortices, helices, and spirals, are analyzed in detail to demonstrate general principles. △ Less

Submitted 16 November, 2009; v1 submitted 27 May, 2009; originally announced May 2009.

Comments: 14 pages, 4 figures

Journal ref: Phys. Rev. B 80, 184411 (2009)

Theory of spin magnetohydrodynamics

Authors: Yaroslav Tserkovnyak, Clement H. Wong

Abstract: We develop a phenomenological hydrodynamic theory of coherent magnetic precession coupled to electric currents. Exchange interaction between electron spin and collective magnetic texture produces two reciprocal effects: spin-transfer torque on the magnetic order parameter and the Berry-phase gauge field experienced by the itinerant electrons. The dissipative processes are governed by three coeff… ▽ More We develop a phenomenological hydrodynamic theory of coherent magnetic precession coupled to electric currents. Exchange interaction between electron spin and collective magnetic texture produces two reciprocal effects: spin-transfer torque on the magnetic order parameter and the Berry-phase gauge field experienced by the itinerant electrons. The dissipative processes are governed by three coefficients: the ohmic resistance, Gilbert damping of the magnetization, and the "beta coefficient" describing viscous coupling between magnetic dynamics and electric current, which stems from spin mistracking of the magnetic order. We develop general magnetohydrodynamic equations and discuss the net dissipation produced by the coupled dynamics. The latter in particular allows us to determine a lower bound on the magnetic-texture resistivity. △ Less

Submitted 6 January, 2009; v1 submitted 28 June, 2008; originally announced June 2008.

Comments: 6 pages, 1 figure

Journal ref: Phys. Rev. B 79 (2009) 014402

Dynamics of Localized Waves

Authors: Z. Q. Zhang, A. A. Chabanov, S. K. Cheung, C. H. Wong, A. Z. Genack

Abstract: We have measured pulsed microwave transmission through quasi-1D samples with lengths up to three localization lengths. For times approaching four times the diffusion time τ_D, transmission is diffusive in accord with the self-consistent theory of localization for the renormalized diffusion coefficient in space and frequency, D(z,Ω). For longer times, the transmission decay rate first agrees with… ▽ More We have measured pulsed microwave transmission through quasi-1D samples with lengths up to three localization lengths. For times approaching four times the diffusion time τ_D, transmission is diffusive in accord with the self-consistent theory of localization for the renormalized diffusion coefficient in space and frequency, D(z,Ω). For longer times, the transmission decay rate first agrees with and later falls increasingly below the self-consistent theory. Beyond the Heisenberg time, the decay rate approaches the predictions of a dynamic single parameter scaling model which reflects the decay of long-lived localized modes and converges to the results of 1D simulations. △ Less

Submitted 15 March, 2008; v1 submitted 16 October, 2007; originally announced October 2007.

Comments: 5 RevTex pages, 3 figures; submitted to PRL