Publications

How metals bind: The deformable-jellium model with correlated electrons

Atoms cohere to form solids largely due to exchange and correlation. The volume is set by a balance between the expansive electronic kinetic energy and the compressive exchange-correlation energy. These effects are simply illustrated by the jellium model, in which the valence electrons neutralize a positive background charge that is rigidly uniform. But the formation of free atoms under extreme expansion is found only in the deformable-jellium model. Deformable jellium is condensed matter in miniature, displaying not only bulk cohesion with a realistic equation of state and surface effects, but also phonons and plasmons and their soft mode instabilities. By drawing an analogy with the motion of shoppers in a mall, we also discuss an intuitive picture of exchange and correlation (the tendency of electrons not to bump into other electrons or into themselves).

Computational study of mechanical properties of poly-phenylene terephthalamide

CCDC 2174206: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Erratum: Surfaces of real metals by the variational self-consistent method

tion, which describes the situation with the crystal in its ground state and no core hole, is also the finalstate Hamiltonian for emission.Initial-and final-state pictures refer to the ways in which transition ma- trix elements are calculated.In the initial-state picture both states entering the matrix element are ei-

Composite photoanodes for photoelectrochemical solar water splitting

Photoelectrochemical (PEC) water splitting is an attractive approach to capturing and storing the earth's abundant solar energy influx. The challenging four-electron water-oxidation half-cell reaction has hindered this technology, giving rise to slow water oxidation kinetics at the photoanode surfaces relative to competitive loss processes. In this perspective, we review recent efforts to improve PEC efficiencies by modification of semiconductor photoanode surfaces with water-oxidation catalysts that can operate at low overpotentials. This approach allows separation of the tasks of photon absorption, charge separation, and surface catalysis, allowing each to be optimized independently. In particular, composite photoanodes marrying nanocrystalline and molecular/non-crystalline components provide flexibility in adjusting the properties of each component, but raise new challenges in interfacial chemistries.

High yield production of ultrathin fibroid semiconducting nanowire of Ta$_2$Pd$_3$Se$_8$

Immediately after the demonstration of the high-quality electronic properties in various two dimensional (2D) van der Waals (vdW) crystals fabricated with mechanical exfoliation, many methods have been reported to explore and control large scale fabrications. Comparing with recent advancements in fabricating 2D atomic layered crystals, large scale production of one dimensional (1D) nanowires with thickness approaching molecular or atomic level still remains stagnant. Here, we demonstrate the high yield production of a 1D vdW material, semiconducting Ta2Pd3Se8 nanowires, by means of liquid-phase exfoliation. The thinnest nanowire we have readily achieved is around 1 nm, corresponding to a bundle of one or two molecular ribbons. Transmission electron microscopy and transport measurements reveal the as-fabricated Ta2Pd3Se8 nanowires exhibit unexpected high crystallinity and chemical stability. Our low frequency Raman spectroscopy reveals clear evidence of the existing of weak inter-ribbon bindings. The fabricated nanowire transistors exhibit high switching performance and promising applications for photodetectors.

Assessing r2SCAN meta-GGA functional for structural parameters, cohesive energy, mechanical modulus, and thermophysical properties of 3<i>d</i>, 4<i>d</i>, and 5<i>d</i> transition metals

The recent development of accurate and efficient semilocal density functionals on the third rung of Jacob's ladder of density functional theory, such as the revised regularized strongly constrained and appropriately normed (r2SCAN) density functional, could enable rapid and highly reliable prediction of the elasticity and temperature dependence of thermophysical parameters of refractory elements and their intermetallic compounds using the quasi-harmonic approximation (QHA). Here, we present a comparative evaluation of equilibrium cell volumes, cohesive energy, mechanical moduli, and thermophysical properties (Debye temperature and thermal expansion coefficient) for 22 transition metals using semilocal density functionals, including the local density approximation (LDA), Perdew-Burke-Ernzerhof (PBE) and PBEsol generalized gradient approximations (GGAs), and the r2SCAN meta-GGA. PBEsol and r2SCAN deliver the same level of accuracies for structural, mechanical, and thermophysical properties. PBE and r2SCAN perform better than LDA and PBEsol for calculating cohesive energies of transition metals. Among the tested density functionals, r2SCAN provides an overall well-balanced performance for reliably computing cell volumes, cohesive energies, mechanical properties, and thermophysical properties of various 3d, 4d, and 5d transition metals using QHA. Therefore, we recommend that r2SCAN could be employed as a workhorse method to evaluate thermophysical properties of transition metal compounds and alloys in high throughput workflows.

Meta-generalized gradient approximations: What they can and cannot do for you

Molecular and solid-state tests of density functional approximations: LSD, GGAs, and meta-GGAs

Results from numerical tests of nine approximate exchange–correlation energy functionals are reported for various systems—atoms, molecules, surfaces, and bulk solids. The functional forms can be divided into three categories: (1) the local spin density (LSD) approximation, (2) generalized gradient approximations (GGAs), and (3) meta-GGAs. In addition to the spin densities and their first gradients, the input to a meta-GGA includes other semilocal information such as Laplacians of the spin densities or orbital kinetic energy densities. We present a way to visualize meta-GGA nonlocality which generalizes that for GGA nonlocality, and which stresses the different meta-GGA descriptions of iso-orbital and orbital overlap regions of space. While some of the tested approximations were constructed semiempirically with many parameters fitted to chemical data, others were constructed to incorporate key properties of the exact exchange–correlation energy. The latter functionals perform well for both small and extended systems, with the best performance achieved by a meta-GGA which recovers the correct gradient expansion. While the semiempirical functionals can achieve high accuracy for atoms and molecules, they are typically less accurate for surfaces and solids. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 889–909, 1999

Uniform Density Limit of Exchange-Correlation Energy Functionals

We present theoretical and practical reasons why a density functional for the exchange-correlation energy should be essentially exact in the uniform density limit. In this limit, the exchange energy is known exactly, and the correlation energy is known to within less than 1 millihartree in the range of valence-electron or lower densities. Some density functionals perform well in this limit, while others do not. Functionals with many parameters fitted to chemical data tend to fail in this limit, and also for real solids. The spin resolution of the correlation energy of the spin-unpolarized uniform electron gas seems simple but unlike that of the widely-used ansatz of Stoll et al., and its low-density limit brings a surprise: a positive parallel-spin contribution in the spin-unpolarized case.

Layered Semiconductor Cr_0.32Ga_0.68Te_2.33 with Concurrent Broken Inversion Symmetry and Ferromagnetism: A Bulk Ferrovalley Material Candidate

The valleytronic state found in group-VI transition-metal dichalcogenides such as MoS₂ has attracted immense interest since its valley degree of freedom could be used as an information carrier. However, valleytronic applications require spontaneous valley polarization. Such an electronic state is predicted to be accessible in a new ferroic family of materials, i.e., ferrovalley materials, which features the coexistence of spontaneous spin and valley polarization. Although many atomic monolayer materials with hexagonal lattices have been predicted to be ferrovalley materials, no bulk ferrovalley material candidates have been reported or proposed. Here, we show that a new non-centrosymmetric van der Waals (vdW) semiconductor Cr_0.32Ga_0.68Te_2.33, with intrinsic ferromagnetism, is a possible candidate for bulk ferrovalley material. This material exhibits several remarkable characteristics: (i) it forms a natural heterostructure between vdW gaps, a quasi-two-dimensional (2D) semiconducting Te layer with a honeycomb lattice stacked on the 2D ferromagnetic slab comprised of the (Cr, Ga)-Te layers, and (ii) the 2D Te honeycomb lattice yields a valley-like electronic structure near the Fermi level, which, in combination with inversion symmetry breaking, ferromagnetism, and strong spin-orbit coupling contributed by heavy Te element, creates a possible bulk spin-valley locked electronic state with valley polarization as suggested by our DFT calculations. Further, this material can also be easily exfoliated to 2D atomically thin layers. Therefore, this material offers a unique platform to explore the physics of valleytronic states with spontaneous spin and valley polarization in both bulk and 2D atomic crystals.

Composition-induced type I and direct bandgap transition metal dichalcogenides alloy vertical heterojunctions

While members of the 2D semiconducting transition metal dichalcogenide (TMD) family MX₂ (M = {Mo, W}, X = {S, Se}) are promising for device applications, stacked layer (vertical) heterojunctions exhibit features that make them inappropriate for light-emitting applications. Such vertical heterojunctions exhibit type II, rather than the preferred type I band alignment. Using density functional theory calculations, we identify the pseudo-binary and quaternary alloy composition range for which band alignment is type I. While broad regions of composition space lead to type I band alignment, most light-emitting devices require direct bandgaps. We demonstrate that by taking advantage of alloying and/or twisting between layers, a wide range of type I, direct bandgap stacked layer (vertical) heterojunctions are achievable. These results and the underlying method developed here provide new opportunities for TMD vertical heterojunction device optimization and opens the door to new classes of TMD vertical heterojunction device applications.

Simple Views of Metallic Clusters

Test of a nonempirical density functional: Short-range part of the van der Waals interaction in rare-gas dimers

It is known that the nonempirical generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) provides a much more realistic description of the short-range part of the van der Waals (vdW) interaction than does the local spin density (LSD) approximation. In the present work, the ability of the higher-level nonempirical meta-GGA of Tao, Perdew, Staroverov, and Scuseria (TPSS) to describe vdW interaction is tested self-consistently in ten rare-gas dimers with Z⩽36. The one-parameter hybrid version (TPSSh) of the TPSS exchange-correlation functional is also included in this test. Calculations show that both TPSS and TPSSh functionals correctly yield vdW bonds in these dimers and significantly improve the prediction of bond lengths, binding energies, and harmonic vibrational frequencies over LSD. The rather close agreement of TPSS with PBE for these dimers confirms a principle of the TPSS construction: preservation of the PBE large-gradient behavior. More importantly, it suggests that TPSS can serve as a platform on which to construct a still-higher level of nonempirical functionals. Compared with the PBE GGA, TPSS, and TPSSh yield a slightly weaker binding. As for normally bonded molecules, TPSSh yields the most accurate vibrational frequencies. The typically too-long bond lengths and too-small binding energies of TPSS meta-GGA suggest the need for some long-range vdW interaction correction even in this class of systems. The effect of basis-set superposition error on the calculated properties of these vdW systems is investigated. We also show that the relatively strong anharmonic effects in the rare-gas dimers are described remarkably well by the Morse potential.

Orbital shifts: An easy way to calculate the exact Kohn-Sham exchange potential

Refined description of liquid and supercooled silicon from <i>ab initio</i> simulations

Despite silicon being of great technological importance, an understanding of its behavior across the phase diagram is still lacking, especially near liquid-solid coexistence. The difficulty in describing silicon near coexistence from first principles lies in discriminating between the metallic and covalent bonds present in the material. Using the strongly constrained and appropriately normed (SCAN) density functional, which can describe a wide variety of bonds with quantitative accuracy, we report a thorough investigation of liquid silicon in the vicinity of liquid-solid coexistence using ab initio molecular dynamics simulations. We observe a structural transition in the supercooled regime that is rooted in a change in the electronic structure of the material. This transition is found to occur at a higher temperature than previous predictions. We also discuss implications of the observed change in interatomic interactions for empirical models of transitions between two distinct liquids.

Symmetry-breaking polymorphous descriptions for correlated materials without interelectronic <i>U</i>

Correlated materials with open-shell $d$ and $f$ ions having degenerate band-edge states show a rich variety of interesting properties ranging from metal-insulator transition to unconventional superconductivity. The textbook view for the electronic structure of these materials is that mean-field approaches are inappropriate, as the interelectronic interaction U is required to open a band gap between the occupied and unoccupied degenerate states while retaining symmetry. We show that the latter scenario often defining what Mott insulators are is in fact not needed for the $3d$ binary oxides MnO, FeO, CoO, and NiO. The mean-field-like band theory can indeed lift such degeneracies in the binaries when nontrivial unit-cell representations (polymorphous networks) are allowed to break symmetries, in conjunction with a recently developed nonempirical exchange and correlation density functional without an on-site interelectronic interaction U. We explain how density-functional theory in the polymorphous representation achieves band-gap opening in correlated materials through a separate mechanism from the Mott-Hubbard approach. We show the method predicts magnetic moments and gaps for the four binary monoxides in both the antiferromagnetic and paramagnetic phases, offering an effective alternative to symmetry-conserving approaches for studying a range of functionalities in open $d$- and $f$-shell complex materials.

Correction to “Accurate and Numerically Efficient r²SCAN Meta-Generalized Gradient Approximation”

RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to "Accurate and Numerically Efficient r2SCAN Meta-Generalized Gradient Approximation"James W. Furness*James W. Furness*[email protected]More by James W. Furnesshttp://orcid.org/0000-0003-3146-0977, Aaron D. KaplanAaron D. KaplanMore by Aaron D. Kaplanhttp://orcid.org/0000-0003-3439-4856, Jinliang NingJinliang NingMore by Jinliang Ninghttp://orcid.org/0000-0002-3691-5291, John P. PerdewJohn P. PerdewMore by John P. Perdew, and Jianwei Sun*Jianwei Sun*[email protected]More by Jianwei Sunhttp://orcid.org/0000-0002-2361-6823Cite this: J. Phys. Chem. Lett. 2020, 11, 21, 9248Publication Date (Web):October 19, 2020Publication History Published online19 October 2020Published inissue 5 November 2020https://doi.org/10.1021/acs.jpclett.0c03077Copyright © 2020 American Chemical SocietyRIGHTS & PERMISSIONSACS AuthorChoiceArticle Views863Altmetric-Citations3LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (617 KB) Get e-Alerts Get e-Alerts