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Chi Ho Wong

Publications and source records attributed to Chi Ho Wong.

At least 19 recordsLinked to original sources

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

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.

cs.CV

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

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.

cond-mat.supr-con

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

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.

cond-mat.mes-hall

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

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.

cond-mat.mes-hall

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

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

cond-mat.supr-con

Exploring the Unconventional Electron Distribution Patterns in Iron-based Superconductors

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.

cond-mat.supr-con

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

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.

cond-mat.supr-con

Source-Aware Embedding Training on Heterogeneous Information Networks

Heterogeneous information networks (HINs) have been extensively applied to real-world tasks, such as recommendation systems, social networks, and citation networks. While existing HIN representation learning methods can effectively learn the semantic and structural features in the network, little awareness was given to the distribution discrepancy of subgraphs within a single HIN. However, we find that ignoring such distribution discrepancy among subgraphs from multiple sources would hinder the effectiveness of graph embedding learning algorithms. This motivates us to propose SUMSHINE (Scalable Unsupervised Multi-Source Heterogeneous Information Network Embedding) -- a scalable unsupervised framework to align the embedding distributions among multiple sources of an HIN. Experimental results on real-world datasets in a variety of downstream tasks validate the performance of our method over the state-of-the-art heterogeneous information network embedding algorithms.

cs.AI

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

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.

cond-mat.mtrl-sci

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

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.

cond-mat.mtrl-sci

Routines to synthesize carbyne of more than 6000 atoms

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.

cond-mat.mtrl-sci

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

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.

cond-mat.supr-con

Schottky-diode design for the world's leading telecommunication

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.

cond-mat.mtrl-sci

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

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.

cond-mat.supr-con

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

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).

cond-mat.supr-con

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

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.

cond-mat.supr-con

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

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.

cond-mat.supr-con

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

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.

cond-mat.supr-con