Nonzero Gaps Research Articles

Machine Learning Accelerated Discovery of Functional MXenes with Giant Piezoelectric Coefficients.

Efficient and rapid screening of target materials in a vast material space remains a significant challenge in the field of materials science. In this study, first-principles calculations and machine learning algorithms are performed to search for high out-of-plane piezoelectric stress coefficient materials in the MXene functional database among the 1757 groups of noncentrosymmetric MXenes with nonzero band gaps, which meet the criteria for piezoelectric properties. For the monatomic MXene testing set, the random forest regression (RFR), gradient boosting regression (GBR), support vector regression (SVR), and multilayer perceptron regression (MLPR) exhibit R2 values of 0.80, 0.80, 0.89, and 0.87, respectively. Expanding our analysis to the entire MXene data set, the best active learning cycle finds more than 140 and 22 MXenes with out-of-plane piezoelectric stress coefficients (e31) exceeding 3 × 10-10 and 5 × 10-10 C/m, respectively. Moreover, thermodynamic stabilities were confirmed in 22 MXenes with giant piezoelectric stress coefficients and 9 MXenes with both large in-plane (d11 > 15 pm/V) and out-of-plane (d31 > 2 pm/V) piezoelectric strain coefficients. These findings highlight the remarkable capabilities of machine learning and its optimization algorithms in accelerating the discovery of novel piezoelectric materials, and MXene materials emerge as highly promising candidates for piezoelectric materials.

Graphyne and graphdiyne nanoribbons: from their structures and properties to potential applications.

Graphyne (GY) and graphdiyne (GDY) have properties including unique sp- and sp2-hybrid carbon atomic structures, natural non-zero band gaps, and highly conjugated π electrons. GY and GDY have good application prospects in many fields, including catalysis, solar cells, sensors, and modulators. Under the influence of the boundary effect and quantum size effect, quasi-one-dimensional graphyne nanoribbons (GYNRs) and graphdiyne nanoribbons (GDYNRs) show novel physical properties. The various structures available give GYNRs and GDYNRs greater band structure and electronic properties, and their excellent physical and chemical properties differ from those of two-dimensional GY and GDY. However, the development of GYNRs and GDYNRs still faces problems, including issues with accurate synthesis, advanced structural characterization, the structure-performance correlation of materials, and potential applications. In this review, the structures and physical properties of quasi-one-dimensional GYNRs and GDYNRs are reviewed, their advantages and disadvantages are summarized, and their potential applications are highlighted. This review provides a meaningful basis and research foundation for the design and development of high-performance materials and devices based on GYNRs and GDYNRs in the field of energy.

Explainable machine learning for predicting the band gaps of ABX3 perovskites

In this study, we trained and compared explainable machine learning algorithms for predicting the band gaps of perovskite materials that have the formula ABX3 containing both zero and non-zero band gaps. Six supervised learning models: 5 ensemble learning methods and 1 neural network (CompoundNet) were employed to study the non-linear relationship that exists between the band gap and the characteristics of its constituent elements such as electronegativity, covalent radius, first ionization energy, and row in the periodic table. The machine learning (ML) models were trained on datasets obtained from density functional theory (DFT) calculations. The results show that CatBoost and XGBoost models yielded the least predictive errors and the highest coefficient of determination of R2 ≥ 88% than other approaches in the testing phase. Furthermore, the Shapley Additive Explanation (SHAP) was used for explaining the model based on the elemental composition of each perovskite compound from the physics standpoint, and a novel holistic feature ranking of the explained models was proposed. One key insight gained from the SHAP analysis is that the Pauling electronegativity of the B site cation in the cubic perovskites which characteristically plays an important role in the electronic properties of this class of materials is the feature that contributes most to the prediction of the band gaps. These results reveal the potential of ML to predict materials properties quickly and accurately with datasets useful in the engineering of efficient solar cell devices.

A Hyper-Heuristic for the Preemptive Single Machine Scheduling Problem to Minimize the Total Weighted Tardiness

Abstract A problem of minimizing the total weighted tardiness in the preemptive single machine scheduling for discrete manufacturing is considered. A hyper-heuristic is presented, which is composed of 24 various heuristics, to find an approximately optimal schedule whenever finding the exact solution is practically intractable. The three heuristics are based on the well-known rules, whereas the 21 heuristics are introduced first. Therefore, the hyper-heuristic selects the best heuristic schedule among 24 schedule versions, whose total weighted tardiness is minimal. Each of the 24 heuristics can solely produce a schedule which is the best one for a given scheduling problem. Despite the percentage of zero gap instances decreases as the greater number of jobs is scheduled, the average and maximal gaps decrease as well. In particular, the percentage is not less than 80 % when up to 10 jobs are scheduled. The average gap calculated over nonzero gaps does not exceed 4 % in the case of scheduling 7 jobs. When manufacturing consists of hundreds of jobs, the hyper-heuristic is made an online scheduling algorithm by applying it only to a starting part of the manufacturing process.

Structural and electronic properties of single- and double-walled BeN4 nanotubes: First-principles calculations

The structural and electronic properties of single- and double-walled BeN4 nanotubes are studied using first-principles calculations. Two sets of single-walled BeN4 nanotubes, namely, (n,0) and (0,n) are formed by rolling up the BeN4 sheets along two different sides into a cylinder. The cohesive energies and molecular dynamics simulations confirm the thermal stability of BeN4 nanotubes. The (n,0) BeN4 nanotubes exhibit semiconducting properties with non-zero band gaps. Their band gaps are dependent on the tube size. The band gap of (0,n) BeN4 nanotubes is not considerable and these tubes behave as semimetals. The double-walled BeN4 nanotubes with interlayers larger than 3 Å are energetically favorable. These nanotubes show semiconducting and semimetallic properties depending on their chirality. The results propose BeN4 nanotubes with various electronic properties to use in future nanoelectronic devices.

Generalized quantum circuit differentiation rules

Variational quantum algorithms that are used for quantum machine learning rely on the ability to automatically differentiate parametrized quantum circuits with respect to underlying parameters. Here, we propose the rules for differentiating quantum circuits (unitaries) with arbitrary generators. Unlike the standard parameter shift rule valid for unitaries generated by operators with spectra limited to at most two unique eigenvalues (represented by involutory and idempotent operators), our approach also works for generators with a generic non-degenerate spectrum. Based on a spectral decomposition, we derive a simple recipe that allows explicit derivative evaluation. The derivative corresponds to the weighted sum of measured expectations for circuits with shifted parameters. The number of function evaluations is equal to the number of unique positive non-zero spectral gaps (eigenvalue differences) for the generator. We apply the approach to relevant examples of two-qubit gates, among others showing that the fSim gate can be differentiated using four measurements. Additionally, we present generalized differentiation rules for the case of Pauli string generators, based on distinct shifts (here named as the triangulation approach), and analyse the variance for derivative measurements in different scenarios. Our work offers a toolbox for the efficient hardware-oriented differentiation needed for circuit optimization and operator-based derivative representation.

Opening of Band Gap of Graphene with High Electronic Mobility by Codoping BN Pairs

Two-dimensional(2D) materials with a high density and low power consumption have become the most popular candidates for next-generation semiconductor electronic devices. As a prototype 2D material, graphene has attracted much attention owing to its stability and ultrahigh mobility. However, zero band gap of graphene leads to very low on-off ratios and thus limits its applications in electronic devices, such as transistors. Although some new 2D materials and doped graphene have nonzero band gaps, the electronic mobility is sacrificed. In this study, to open the band gap of graphene with high electronic mobility, the structure and property of BN-doped graphene were evaluated using first-principles calculations. The formation energies indicate that the six-membered BN rings doped graphene has the most favorable configuration. The band structures show that the band gaps can be opened by such type of doping. Also, the Dirac-cone-like band dispersion of graphene is mostly inhibited, ensuring high electronic mobility. Therefore, codoping BN into graphene might provide 2D materials with nonzero band gaps and high electronic mobility.

Thermally driven Fermi glass states in warm dense matter: Effects on terahertz and direct-current conductivities

There is a growing interest in the electrical conductivity of warm dense matter from terahertz-frequency alternating current to direct current. Herein, using first-principles molecular dynamics simulations, we show that ionic thermal motion in warm dense matter drives thermal fluctuations in the electronic valence band that produce localized states in Lifshitz tails on the top and bottom of the bands. We predict Fermi glass states when these localized states extend and fill the gap between valence and conduction bands. This significantly affects the ultralow-frequency and direct current conductivity because of the very small but nonzero energy gaps between these localized states. An order parameter is proposed to describe the degree of glassiness of an electron energy band using the local density-of-state distribution. To take into account thermal hopping, we introduce electron energy-level broadening as a thermal correction term in the Kubo–Greenwood equation. The calculated terahertz conductivities of warm dense helium and argon show the differences between the Fermi glass and normal metal states.

A Reduction Approach to the Multiple-Unicast Conjecture in Network Coding

The multiple-unicast conjecture in network coding states that for multiple unicast sessions in an undirected network, network coding has no advantage over routing in improving the throughput or saving bandwidth. In this paper, we propose a reduction method to study the multiple-unicast conjecture, and prove the conjecture for a new class of networks that are characterized by relations between cut-sets and source-receiver paths. This class subsumes all the known types of networks with non-zero max-flow min-cut gaps but zero coding advantage. Combining this result with a computer-aided search, we derive as a corollary that network coding is unnecessary in networks with up to six coding nodes. We also prove the multiple-unicast conjecture for almost all unit-link-length networks with up to three sessions and seven nodes.

Theoretical investigation on electronic properties and carrier mobilities of BN-substituted graphyne nanoribbons

This paper presents the self-consistent field crystal orbital studies of the BN-substituted graphyne nanoribbons (NRs) based on density functional theory. Two kinds of the substitution are considered: one is that the six-membered rings in the graphyne NRs are replaced by the corresponding BN hexagonal rings; another is the substitution of acetylenic linkages by BN units. The calculations show that all the optimized one-dimensional BN-substituted graphyne NRs still keep plane structures. These NRs are semiconductors with non-zero band gaps and their band gaps are wider than those of corresponding graphyne NRs but narrower than those of BNNRs. The substitution of hexagonal rings leads to wider band gap than the substitution of acetylenic linkages. The relationship of band gaps with the ribbon widths is different from different NR patterns. The stabilities of these BN-substituted graphyne NRs monotonically decrease as their widths increase and the molecular dynamics simulations show that these NRs would be stable when T<600K. Natural population analysis shows that BN bonds have stronger ionicity in the hexagonal rings than in the BN linkages. The dependence of the charge carrier mobilities on the ribbon widths is also quite different from different types of the BN-substituted graphyne NRs. It is found that both the substituting types and edge structures have important influence on the transport property of the BN-substituted NRs. Thus, BN substituting indeed plays an important role in tuning the electronic structure of graphyne.

Metal decorated graphdiyne: A first principle study

In this work, we performed first principles calculations based on self-consistent charge density functional tight-binding to investigate mechanical and electronic properties of the optimized graphdiyne sheet, nanoribbons and transition metal decorated with Ag, Fe and Co atoms in the different sites. Then we calculated electronic properties of the graphdiyne sheet under uniaxial elastic strain in two different crystal directions. It is shown that the carbon bond length values for the graphdiyne structure are agreeable with experimental and other theoretical values. Our results show that the charge transferred to the graphdiyne from Ag and Fe atoms but to the Co decorated charge transferred to the Co atom from the graphdiyne. From the results, it can be seen that the graphdiyne is a direct semiconductor with doubly degenerated for both the valence band maximum and the conduction band minimum in the 6C hole site. Also, by Metal decorated of the graphdiyne, the structures become also direct semiconductor for the Ag decorated and indirect for Co and Fe decorated for the 6C- hole, top and bridge sites while the structure have metallic behavior for the 18C- hole site. Our results from the band structure of the GDY nanoribbons with the size of N = 1, 2, 3 and 4, indicate that all of them are nonzero band gaps at the Γ point. It can be seen that the energy gap decreases with increasing of nanoribbons size. Finally, we found that the gap energy increase with applying strain and also the structure remain direct semiconductor.

Solution-Processed Two-Dimensional Ultrathin InSe Nanosheets

The rise of two-dimensional (2D) graphene-cognated crystals with nonzero band gaps like transition metal dichalcogenides has led to a rapidly increasing interest in their dimensionality-dependent anisotropic properties, which bear high potential for ultrathin electronics. 2D crystals of the III–VI metal chalcogenide InSe represent a new kind of material class predestined for the use in optoelectronic applications as highly responsive photodetectors and field-effect transistors. We present a solution-processable method for 2D ultrathin InSe nanosheets (≤5 nm with ligands, lateral sizes up to ∼800 μm) with a detailed characterization of the sheet formation by a lamellar ligand templated growth. Optical and electrical transport properties, as well as in depth analysis of the crystal structure and stoichiometry of the colloidal nanosheets by electron and atomic force microscopy, X-ray photoelectron spectroscopy, and scattering methods complete this comprehensive study on a wet-chemical alternative to produce ...

The effects of noise vocoding on gap detection thresholds

Gap detection threshold (GDT), the shortest silent interval a person can perceive, is a commonly used measure of temporal processing resolution. The purposes of this study were: (1) to examine the effects of noise vocoding, which has been used to simulate what signals sound like through a cochlear implant, on GDTs in normal-hearing subjects, and (2) to further the understanding of neural mechanisms underlying gap detection using the Auditory Late Response (ALR). Thirteen normal listeners participated. In behavioral tests, the GDTs were determined for the original and vocoded stimuli. In ALR recordings, the subjects were presented with auditory stimuli with and without containing gaps and stimuli with and without being vocoded. Results showed that GDTs were significantly elevated for vocoded stimuli with spectral resolutions of 4 and 20 channels compared to those for the original stimuli. A gap effect was observed in the post-gap ALR. Current densities for N1 peaks evoked by stimuli with zero- vs. non-zero ms gaps, pre- vs. post-gap markers, and original vs. vocoded stimuli were obtained using the standardized low-resolution brain electromagnetic tomography (sLORETA) method. Paired comparisons of pre- and post-gap current density values were made. Results showed a statistical difference between the N1s evoked by pre- vs. post-gap markers, with the activation in the middle frontal gyrus and precentral gyrus. The results suggest that: (1) noise vocoding does affect temporal processing resolution assessed with GDTs, (2) gap detection may involve the recruitment of cognitive neural resources, and (3) the ALR has a potential value of objectively estimating temporal processing resolution.

Crystal orbital study on the double walls made of nanotubes encapsulated inside zigzag carbon nanotubes

The structure stabilities and electronic properties are investigated by using ab initio self-consistent-field crystal orbital method based on density functional theory for the one-dimensional (1D) double-wall nanotubes made of n-gon SiO2 nanotubes encapsulated inside zigzag carbon nanotubes. It is found that formation of the combined systems is energetically favorable when the distance between the two constituents is around the Van der Waals scope. The obtained band structures show that all the combined systems are semiconductors with nonzero energy gaps. The frontier energy bands (the highest occupied band and the lowest unoccupied band) of double-wall nanotubes are mainly derived from the corresponding carbon nanotubes. The mobilities of charge carriers are calculated to be within the range of 102–104cm2V−1s−1 for the hybrid double-wall nanotubes. Young’s moduli are also calculated for the combined systems. For the comparison, geometrical and electronic properties of n-gon SiO2 nanotubes are also calculated and discussed.

SU‐E‐T‐357: Error Detection with a 3D Patient Specific QA System

Purpose: To determine if our in‐house patient specific QA process, which utilizes the COMPASS (v2.1, IBA‐Dosimetry) system, can detect delivery errors introduced in clinical IMRT plans. Methods: The COMPASS system consists of a 2D chamber array mounted in the gantry, an independent gantry angle sensor and associated software. Patient specific QA at our clinic involves the creation of two QA structures, one consisting of all clinical PTVs, the other generated from the 20% isodose line. The gamma agreement index (GAI) within the two QA structures is used to compare 3D COMPASS reconstructed dose with planned dose distributions. Ten clinical DMLC‐IMRT plans (5 prostate, 5 head &amp; neck) were included in this study. Plans were modified by systematically shifting leaf pair positions so that that all nonzero leaf gaps were increased by 2.0 mm and 1.0 mm for the two sites, respectively. Verification plans were created for both the original and modified plans in order to quantify the dosimetric effect of the introduced errors via in‐phantom ion chamber measurements. Finally, plans were delivered to COMPASS and 3D dose distributions were reconstructed on both the phantom and patient CT datasets. Reconstructed doses on the patient CT were analyzed as per our QA procedure. Results: On average, measured ion chamber values for the modified plans deviated from the original planning doses by 3.1 ± 1.2% (Range: 1.0%–5.3%). For each of the six modified plans in which this metric exceeded 3%, GAI values for at least one of the QA structures were below 95%, which would be flagged by our QA process. Only one of the other modified and none of the original plans had GAI values &lt;95%. Conclusions: Gamma analysis of patient specific QA with COMPASS can effectively detect dosimetrically significant delivery errors introduced by systematically increasing leaf gaps in clinical IMRT plans. CancerCare Manitoba has a research agreement with IBA‐Dosimetry.

Structure and electronic properties of the double-wall nanotubes constructed from SiO2 nanotubes encapsulated inside zigzag carbon nanotubes

This paper presents ab initio self-consistent field crystal orbital calculations on the structures, stabilities, elastic and electronic properties of the double-wall nanotubes made of SiO2 nanotubes encapsulated inside zigzag carbon nanotubes based on density functional theory. It is found that formation of the combined systems is energetically favorable when the nearest distance between the two constituents is in the area of the van der Waals effect. The obtained band structures show that all the combined systems are semiconductors with nonzero energy gaps. Based on the deformation potential theory and effective mass approximation, the mobilities of charge carriers are calculated to be in the range of 102–104 cm 2 V −1 s −1, the same order of magnitude as those of the corresponding zigzag carbon nanotubes. The Young’s moduli are also calculated for the combined systems.

On zero duality gap in surrogate constraint optimization: The case of rational-valued functions of constraints

This paper is concerned with the constrained optimization problem. A detailed discussion of surrogate constraints with zero duality gaps is presented. Readily available surrogate multipliers are considered that close the duality gaps where constraints are rational-valued. Through illustrative examples, the sources of duality gaps are examined in detail. While in the published literature, in many situations conclusions have been made about the existence of non-zero duality gaps, we show that taking advantage of full problem information can close the duality gaps. Overlooking such information can produce shortcomings in the research in which a non-zero duality gap is observed. We propose theorems to address the shortcomings and report results regarding implementation issues.

A new seam-tracking algorithm through characteristic-point detection for a portable welding robot

The welding task in double-hulled structures in shipyards and in steel-frame structures is hazardous and difficult due to the toxic gas and limited workspace. Therefore, many efforts have been undertaken for automation. The main challenge for automation is the development of a simple and robust seam-tracking algorithm that can be applied to a portable welding robot that operates under irregular and diverse task conditions in the workspace. We developed a seam-tracking algorithm for weaving weld path planning using a laser displacement sensor. The goal of the proposed algorithm is to detect the seam of single-butt welding with manually tack-welded non-zero gaps. The focus is on keeping the algorithm simple and affordable so that it can be applied to portable robots that operate in hazardous fields. The algorithm consists of four steps: scanning, filtering, generation of the reference points, and path planning. In the scanning process, the depth data of a cross-section of the seam profile is obtained. Next, a Gaussian filter is used to remove noise from the raw data. A differential characteristic-point detection algorithm is applied to the filtered data to detect the reference points that represent the shape and location of the gap to be welded. Finally, path planning for single-V butt multi-pass welding is done based on the detected reference points. A portable four-axis welding robot is built using the developed algorithm. The algorithm is validated through welding experiments regarding a single-V butt welding task with a manually tack-welded non-zero gap.

Cooper pairsʼ magnetic moment in MCFL color superconductivity

We investigate the effect of the alignment of the magnetic moments of Cooper pairs of charged quarks that form at high density in three-flavor quark matter. The high-density phase of this matter in the presence of a magnetic field is known to be the Magnetic Color–Flavor-Locked (MCFL) phase of color superconductivity. We derive the Fierz identities of the theory and show how the explicit breaking of the rotational symmetry by the uniform magnetic field opens new channels of interactions and allows the formation of a new diquark condensate. The new order parameter is a spin-1 condensate proportional to the component in the field direction of the average magnetic moment of the pairs of charged quarks. The magnitude of the spin-1 condensate becomes comparable to the larger of the two scalar gaps in the region of large fields. The existence of the spin-1 condensate is unavoidable, as in the presence of a magnetic field there is no solution of the gap equations with nonzero scalar gaps and zero magnetic moment condensate. This is consistent with the fact that the extra condensate does not break any symmetry that has not already been broken by the known MCFL gaps. The spin-1 condensate enhances the condensation energy of pairs formed by charged quarks and the magnetization of the system. We discuss the possible consequences of the new order parameter on the issue of the chromomagnetic instability that appears in color superconductivity at moderate density.

Dispersion interactions between semiconducting wires

The dispersion energy between extended molecular chains (or equivalently infinite wires) with non-zero band gaps is generally assumed to be expressible as a pair-wise sum of atom-atom terms which decay as $R^{-6}$. Using a model system of two parallel wires with a variable band gap, we show that this is not the case. The dispersion interaction scales as $z^{-5}$ for large interwire separations $z$, as expected for an insulator, but as the band gap decreases the interaction is greatly enhanced; while at shorter (but non-overlapping) separations it approaches a power-law scaling given by $z^{-2}$, \emph{i.e.} the dispersion interaction expected between \emph{metallic} wires. We demonstrate that these effects can be understood from the increasing length scale of the plasmon modes (charge fluctuations), and their increasing contribution to the molecular dipole polarizability and the dispersion interaction, as the band gaps are reduced. This result calls into question methods which invoke locality assumptions in deriving dispersion interactions between extended small-gap systems.