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Chang, Kai

Publications and source records attributed to Chang, Kai.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Reducing Operator Complexity of Galerkin Coarse-grid Operators with Machine Learning

Here, we propose a data-driven and machine-learning-based approach to compute non-Galerkin coarse-grid operators in multigrid (MG) methods, addressing the well-known issue of increasing operator complexity. Guided by the MG theory on spectrally equivalent coarse-grid operators, we have developed novel machine learning algorithms that utilize neural networks combined with smooth test vectors from multigrid eigenvalue problems. The proposed method demonstrates promise in reducing the complexity of coarse-grid operators while maintaining overall MG convergence for solving parametric partial differential equation problems. Numerical experiments on anisotropic rotated Laplacian and linear elasticity problems are provided to showcase the performance and comparison with existing methods for computing non-Galerkin coarse-grid operators.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2024

Defect‐Free Nanowelding of Bilayer SnSe Nanoplates

Nanowelding is a bottom‐up technique to create custom‐designed nanostructures and devices beyond the precision of lithographic methods. Here, a new technique is reported based on anisotropic lubricity at the van der Waals interface between monolayer and bilayer SnSe nanoplates and a graphene substrate to achieve precise control of the crystal orientation and the interface during the welding process. As‐grown SnSe monolayer and bilayer nanoplates are commensurate with graphene's armchair direction but lack commensuration along graphene's zigzag direction, resulting in a reduced friction along that direction and a rail‐like, 1D movement that permits joining nanoplates with high precision. This way, molecular beam epitaxially grown SnSe nanoplates of lateral sizes 30–100 nm are manipulated by the tip of a scanning tunneling microscope at room temperature. In situ annealing is applied afterward to weld contacting nanoplates without atomic defects at the interface. This technique can be generalized to any van der Waals interfaces with anisotropic lubricity and is highly promising for the construction of complex quantum devices, such as field effect transistors, quantum interference devices, lateral tunneling junctions, and solid‐state qubits.

2D ferroelectrics↗
DOE OSTI2023

Elucidating the mechanism underlying the augmented capacity of MoO 2 as an anode material in Li-ion batteries

Transition-metal oxide anode materials have been observed to possess an intriguing surplus of capacity beyond the expected values based on conversion reaction. However, the mechanisms behind this phenomenon have remained contentious and elusive. Here, this study focuses on synthesized nanosized molybdenum dioxide and its electrochemical performance as an anode material for Li-ion batteries. Our findings reveal a substantial increase in capacity upon cycling, achieving approximately 1688 mA h g -1 , nearly double the theoretical capacity, after 700 cycles at a 1C rate. To elucidate the mechanisms underlying this augmented capacity, a comprehensive analysis employing in situ and ex situ X-ray diffraction, X-ray absorption spectroscopy, scanning electron microscopy, and transmission electron microscopy was conducted at various stages of the Li-ion cell cycling. Our results indicate that no conversion reaction occurs during the initial discharge phase, with Li 2 O and Mo remaining undetected. Instead, Li 0.98 MoO 2 is generated upon lithiation. Further materials characterization employing electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy on the cycled electrode suggests the potential formation of a metallic Li-rich layer at the interface of the Li-ion intercalated phase subsequent to the formation of Li 0.98 MoO 2 , contributing to the surplus Li storage. Moreover, electrochemical impedance spectroscopy coupled with ex situ SEM and TEM analyses reveals that alterations in particle size and morphology, along with changes in the solid electrolyte interphase (SEI) resistance, are instrumental in the capacity variation observed upon cycling.

25 ENERGY STORAGE↗
DOE OSTI2021

Colloquium : Physical properties of group-IV monochalcogenide monolayers

The state-of-the-art knowledge of ferroelectric and ferroelastic group-IV monochalcogenide monolayers is surveyed. These semiconductors feature remarkable structural and mechanical properties, such as a switchable in-plane spontaneous polarization, soft elastic constants, structural degeneracies, and thermally driven two-dimensional structural transformations. Additionally, these 2D materials display selective valley excitations, valley Hall effects, and persistent spin helix behavior. After a description of their Raman spectra, a discussion of optical properties arising from their lack of centrosymmetry (such as an unusually strong second-harmonic intensity, large bulk photovoltaic effects, photostriction, and tunable exciton binding energies) is provided as well. Here, the physical properties observed in these materials originate from (correlate with) their intrinsic and switchable electric polarization, and the physical behavior hereby reviewed could be of use in nonvolatile memory, valleytronic, spintronic, and optoelectronic devices: these 2D multiferroics enrich and diversify the 2D material toolbox.

2D ferroelectrics↗
DOE OSTI2020

Microscopic Manipulation of Ferroelectric Domains in SnSe Monolayers at Room Temperature

Two-dimensional (2D) van der Waals ferroelectrics provide an unprecedented architectural freedom for the creation of artificial multiferroics and nonvolatile electronic devices based on vertical and coplanar heterojunctions of 2D ferroic materials. Nevertheless, controlled microscopic manipulation of ferroelectric domains is still rare in monolayer-thick 2D ferroelectrics with in-plane polarization. Here we report the discovery of robust ferroelectricity with a critical temperature close to 400 K in SnSe monolayer plates grown on graphene and the demonstration of controlled room-temperature ferroelectric domain manipulation by applying appropriate bias voltage pulses to the tip of a scanning tunneling microscope (STM). This study shows that STM is a powerful tool for detecting and manipulating the microscopic domain structures in 2D ferroelectric monolayers, which are difficult for conventional approaches such as piezoresponse force microscopy, thus facilitating the hunt for other 2D ferroelectric monolayers with in-plane polarization with important technological applications.

2D ferroelectric↗
DOE OSTI2020

Spin-Triplet Excitonic Insulator: The Case of Semihydrogenated Graphene

While various excitonic insulators have been studied in the literature, due to the perceived too-small spin splitting, spin-triplet excitonic insulator is rare. In two-dimensional systems such as a semihydrogenated graphene (known as graphone), however, it is possible, as revealed by first-principles calculations coupled with Bethe-Salpeter equation. Here, the critical temperature, given by an effective Hamiltonian, is 11.5 K. While detecting excitonic insulators is still a daunting challenge, the condensation of triplet excitons will result in spin superfluidity, which can be directly measured by a transport experiment. Nonlocal dielectric screening also leads to an unexpected phenomenon, namely, an indirect-to-direct transition crossover between single-particle band and exciton dispersion in the semihydrogenated graphene, which offers yet another test by experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Standing Waves Induced by Valley-Mismatched Domains in Ferroelectric SnTe Monolayers

Two-dimensional (2D) quasiparticle standing waves originate from the interference of coherent quantum states and are usually created by the scattering off edges, atomic steps, or adatoms that induce large potential barriers. We report standing waves close to the valence band maximum (E V ), confined by electrically neutral domain walls of newly discovered ferroelectric SnTe monolayers, as revealed by spatially resolved scanning tunneling spectroscopy. Ab initio calculations show that this novel confinement arises from the polarization lifted hole valley degeneracy and a ~90° rotation of the Brillouin zones that render holes’ momentum mismatched across neighboring domains. These results show a potential for polarization-tuned valleytronics in 2D ferroelectrics.

2-dimensional systems↗
DOE OSTI2019

From an atomic layer to the bulk: Low-temperature atomistic structure and ferroelectric and electronic properties of SnTe films

SnTe hosts ferroelectricity that competes with its weak nontrivial band topology: in the high-symmetry rocksalt structure—in which its intrinsic electric dipole is quenched—this material develops metallic surface bands, but in its rhombic ground-state configuration—which hosts a nonzero spontaneous electric dipole—the crystalline symmetry is lowered, and the presence of surface electronic bands is not guaranteed. In this work, the type of ferroelectric coupling and the atomistic and electronic structure of SnTe films ranging from 2 to 40 atomic layers (ALs) are examined on freestanding samples, to which atomic layers were gradually added. Four-AL SnTe films are antiferroelectrically coupled, while thicker freestanding SnTe films are ferroelectrically coupled. The electronic band gap reduces its magnitude in going from 2 to 40 ALs, but it does not close due to the rhombic nature of the structure. Lastly, these results bridge the structure of SnTe films from the monolayer to the bulk.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure

2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in–plane polarization. Remarkably, they exhibit transition temperatures (T c ) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the Tc enhancement is unveiled, which relies on the formation of γ–SnTe, a van der Waals orthorhombic phase with antipolar inter–layer coupling in few–AL thick SnTe films. In this phase, 4n – 2 AL (n = 1, 2, 3…) thick films are found to possess finite in–plane polarization (space group Pmn2 1 ), while 4n AL thick films have zero total polarization (space group Pnma). Above 8 AL, the γ–SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS–type monochalcogenide monolayers. Here, the observed high transition temperature, together with the strong spin–orbit coupling and van der Waals structure, underlines the potential of atomically thin γ–SnTe films for the development of novel spontaneous polarization–based devices.

2d ferroelectrics↗
DOE OSTI2018

Apparatus and methods for memory using in-plane polarization

A memory device includes a semiconductor layer with an in-plane polarization component switchable between a first direction and a second direction. A writing electrode is employed to apply a writing voltage to the semiconductor layer to change the in-plane polarization component between the first direction and the second direction. A reading electrode is employed to apply a reading voltage to the semiconductor layer to measure a tunneling current substantially perpendicular to the polarization direction of the in-plane polarization component. The directions of the reading voltage and the writing voltage are substantially perpendicular to each other. Therefore, the reading process is non-destructive. Thin films (e.g., one unit cell thick) of ferroelectric material can be used in the memory device to increase the miniaturization of the device.

42 ENGINEERING↗
DOE OSTI2017

Evidence for a topological excitonic insulator in InAs/GaSb bilayers

Electron–hole pairing can occur in a dilute semimetal, transforming the system into an excitonic insulator state in which a gap spontaneously appears at the Fermi surface, analogous to a Bardeen–Cooper–Schrieffer (BCS) superconductor. Here, we report optical spectroscopic and electronic transport evidence for the formation of an excitonic insulator gap in an inverted InAs/GaSb quantum-well system at low temperatures and low electron–hole densities. Terahertz transmission spectra exhibit two absorption lines that are quantitatively consistent with predictions from the pair-breaking excitation dispersion calculated based on the BCS gap equation. Low-temperature electronic transport measurements reveal a gap of ~2 meV (or ~25 K) with a critical temperature of ~10 K in the bulk, together with quantized edge conductance, suggesting the occurrence of a topological excitonic insulator phase.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Discovery of robust in-plane ferroelectricity in atomic-thick SnTe

Stable ferroelectricity with high transition temperature in nanostructures is needed for miniaturizing ferroelectric devices. Here, we report the discovery of the stable in-plane spontaneous polarization in atomic-thick tin telluride (SnTe), down to a 1–unit cell (UC) limit. The ferroelectric transition temperature T c of 1-UC SnTe film is greatly enhanced from the bulk value of 98 kelvin and reaches as high as 270 kelvin. Moreover, 2- to 4-UC SnTe films show robust ferroelectricity at room temperature. The interplay between semiconducting properties and ferroelectricity in this two-dimensional material may enable a wide range of applications in nonvolatile high-density memories, nanosensors, and electronics.

36 MATERIALS SCIENCE↗
NASA NTRS2006

A Cassegrain Offset-Fed Dual-Band Reflectarray

An X/Ka dual-band microstrip reflectarray with circular polarization (CP) has been constructed using thin membranes and Cassegrain offset-fed configuration. It is believed that this is the first Cassegrain reflectarray ever been developed. This antenna has a 75-cm-diameter aperture and uses metallic subreflector and angular-rotated annular ring elements. It achieved a measured -3dB-gain bandwidth of 700 MHz at X-band and 1.5 GHz at Ka-band, as well as a CP bandwidth (3dB axial ratio) of more than 700 MHz at X-band and more than 2 GHz at Ka-band. The measured peak efficiencies are 49.8% at X-band and 48.2% at Ka-band.

offset fed↗
NASA NTRS1999

Ka-Band Beam Steering Reflectarray Study

A one-meter diameter, 32 GHz active microstrip reflectarray capable of electronic beam steering has been studied. The 15_500 element array consists of rnicrostrip patch antennas, microstrip phase shifters, and dc bias lines. Specifically, the study focused on the solid-state devices and dc bias layouts for scanning the beam linearly 45 from boresight using 2-bit phase shifters. Three configurations are recommended for the reflectarray with integrated 2-bit phase shifters where the total substrate and cladding mass varies from 8 kg to 14 kg. Additionally, dc power requirements for p-i-n diodes in 1-bit, 2-bit and 3-bit phase shifting circuits are approximately 19 W, 19 W, and 39 W, respectively. The dc power requirement for phase shifters using passive MESFETs is much less than 1 W.

McSpadden, James O.↗
NASA NTRS1994

Rule based design of conceptual models for formative evaluation

A Human-Computer Interface (HCI) Prototyping Environment with embedded evaluation capability has been investigated. This environment will be valuable in developing and refining HCI standards and evaluating program/project interface development, especially Space Station Freedom on-board displays for payload operations. This environment, which allows for rapid prototyping and evaluation of graphical interfaces, includes the following four components: (1) a HCI development tool, (2) a low fidelity simulator development tool, (3) a dynamic, interactive interface between the HCI and the simulator, and (4) an embedded evaluator that evaluates the adequacy of a HCI based on a user's performance. The embedded evaluation tool collects data while the user is interacting with the system and evaluates the adequacy of an interface based on a user's performance. This paper describes the design of conceptual models for the embedded evaluation system using a rule-based approach.

Moore, Loretta A.↗