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Isolated energy level in the band gap of Yb2Si2O7 identified by electron energy-loss spectroscopy

Physical Review B 2016年第20期93卷 201107(R)-201107(R)页

作者： Takafumi Ogawa Shunsuke Kobayashi Masashi Wada Craig A. J. Fisher Akihide Kuwabara Takeharu Kato Masato Yoshiya Satoshi Kitaoka Hiroki Moriwake Nanostructures Research Laboratory Japan Fine Ceramics Center Nagoya 456-8587 Japan Materials Research and Development Laboratory Japan Fine Ceramics Center Nagoya 456-8587 Japan Department of Adaptive Machine Systems Osaka University Osaka 565-0871 Japan

We report the detection of an isolated energy level in the band gap of crystalline Yb2Si2O7 in the low-energy-loss region of its electron energy-loss (EEL) spectrum, obtained using a monochromated scanning transmission electron microscope. The experimental results are corroborated by first-principles calculations of the theoretical EEL spectrum. The calculations reveal that unoccupied Yb 4f orbitals constitute an isolated energy level about 1 eV below the conduction band minimum (CBM), resulting in a terrace about 1 eV wide at the band edge of the EEL spectrum. In the case of Yb2O3, no band edge terrace is present because the unoccupied f level lies just below the CBM. We also examined optical absorption properties of Yb2Si2O7 using UV-vis diffuse reflectance spectroscopy, which shows that the isolated energy level could not be detected in the band edge of the obtained absorbance spectrum. These findings demonstrate the utility of low-loss EEL spectroscopy with high energy resolution for probing semilocalized electronic features.

关键词： Band gap First-principles calculations Local density of states Crystalline systems Oxides DFT+U density functional theory development Electron energy loss spectroscopy Electronic structure Methods in electromagnetism Scanning electron microscopy

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Defects and oxidation resilience in InSe

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Physical Review B 2017年第5期96卷 054112-054112页

作者： K. J. Xiao A. Carvalho A. H. Castro Neto Centre for Advanced 2D Materials and Graphene Research Centre National University of Singapore Singapore 117542 Singapore Boston University 590 Commonwealth Avenue Boston Massachusetts 02215 USA

We use density functional theory to study intrinsic defects and oxygen related defects in indium selenide. We find that InSe is prone to oxidation, but however not reacting with oxygen as strongly as phosphorene. The dominant intrinsic defects in In-rich material are the In interstitial, a shallow donor, and the Se vacancy, which introduces deep traps. The latter can be passivated by oxygen, which is isoelectronic with Se. The dominant intrinsic defects in Se-rich material have comparatively higher formation energies.

关键词： Crystal defects Dopants Interstitials Point defects Semiconductors Dichalcogenides Doped semiconductors Honeycomb lattice Multiferroics Band structure methods density functional theory density functional theory development Electronic structure First-principles calculations

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van der Waals corrected density functionals for cylindrical surfaces: Ammonia and nitrogen dioxide adsorbed on a single-walled carbon nanotube

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Physical Review B 2021年第19期103卷 195410-195410页

作者： Shah Tanvir ur Rahman Chowdhury Hong Tang John P. Perdew Department of Physics Temple University Philadelphia Pennsylvania 19122 USA Department of Chemistry Temple University Philadelphia Pennsylvania 19122 USA

We extend the damped Zaremba-Kohn model (dZK) for long-range dispersion interaction between a molecule and a planar surface [J. Tao, H. Tang, A. Patra, P. Bhattarai, and J. P. Perdew, Phys. Rev. B 97, 165403 (2018)] to molecules adsorbed on a curved cylindrical surface, and employ this extended model as an additive correction to the semilocal density functionals PBE (Perdew-Burke-Ernzerhof) and SCAN (strongly constrained and appropriately normed). The resulting PBE+vdW (van der Waals)-dZK and SCAN+vdW-dZK are applied to two systems, NH3 and NO2 molecules adsorbed on a single-wall carbon nanotube (CNT), for calculations of binding energies and equilibrium distances. For comparison, the results from vdW nonlocal functionals, such as SCAN+rVV10 and PBE+rVV10, are also presented. The binding energies from PBE+rVV10 (Vydrov and Van Voorhis), SCAN+rVV10, PBE+vdW-dZK, and SCAN+vdW-dZK are about 70–115 meV for the system of CNT+NH3 and 300–500 meV for the system of CNT+NO2. The results from PBE+vdW-dZK and SCAN+vdW-dZK are closer to each other than those from PBE+rVV10 and SCAN+rVV10 are. The relatively closer results from PBE+vdW-dZK and SCAN+vdW-dZK indicate the consistency of our developed vdW−dZK model for cylindrical surfaces. All methods, including PBE, SCAN, PBE+rVV10, SCAN+rVV10, PBE+vdW-dZK, and SCAN+vdW-dZK, give approximately the same binding energy differences between two adsorption configurations (types I and II) for the two systems. This implies that the two adsorption sites have approximately the same adsorption stability. The exponent of the vdW interaction power law from our vdW-dZK model for the two systems is about 0 at short distance, largely due to the damping factor, and tends slowly to −4 to −4.5 at distances D about 20–50 Å. At even larger distances, the vdW power-law exponent approaches −5. This feature is very similar to the one calculated with random-phase approximation and renormalization group approaches, supporting the applicability of our meth

关键词： Carbon-based materials Nanotubes Surfaces density functional theory density functional theory development

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Flat plane based double-counting free and parameter free many-body DFT+U

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Physical Review B 2024年第20期110卷 205150-205150页

作者： Andrew C. Burgess David D. O'Regan School of Physics Trinity College Dublin The University of Dublin D02 PN40 Ireland

The recently introduced BLOR corrective exchange-correlation functional [A. C. Burgess et al., Phys. Rev. B 107, L121115 (2023)] is, by construction, the unique simplified rotationally invariant DFT+U functional that enforces the flat plane condition separately on each effective orbital of a localized subspace. Detached from the Hubbard model, functionals of this type are both double-counting correction free and, when optimized in situ using appropriate error quantifiers, effectively parameter free. They are as computationally undemanding as conventional DFT+U functionals. In this work, the extension of the BLOR functional to address many-body errors (mBLOR) is derived. The mBLOR functional is built to enforce the flat plane condition on the entire subspace, rather than on each orbital individually. It depends solely on the total subspace occupancy and spin magnetization, bringing consistency with how Hubbard U and Hund J values are typically calculated, and very low complexity. In this way, interorbital errors are corrected on the same footing as the single-particle ones. Focusing on exact test cases with strong interorbital interactions, the BLOR and mBLOR functionals were benchmarked against contemporary DFT+U functionals using the total-energy extensivity condition on stretched homonuclear p-block dimers that represent various self-interaction and static-correlation error regimes, namely, singlet N2 and F2, non-spin polarized O2, and doublet Ne2+. The BLOR functional outperformed all other DFT+U functionals tested, which often act to increase total-energy errors, yet it still yielded large errors in some systems. mBLOR instead yielded low-energy errors across all four strongly correlated dimers, while being constructed using only semilocal approximation ingredients. As mBLOR would not otherwise introduce a band-gap correction in the manner that is a desirable feature of DFT+U, we developed a cost-free technique to reintroduce it automatically by moving the func

关键词： Electronic structure Strongly correlated systems DFT+U density functional theory density functional theory development

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Configurational forces in electronic structure calculations using Kohn-Sham density functional theory

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Physical Review B 2018年第16期97卷 165132-165132页

作者： Phani Motamarri Vikram Gavini Department of Mechanical Engineering University of Michigan Ann Arbor Michigan 48109 USA Department of Materials Science and Engineering University of Michigan Ann Arbor Michigan 48109 USA

We derive the expressions for configurational forces in Kohn-Sham density functional theory, which correspond to the generalized variational force computed as the derivative of the Kohn-Sham energy functional with respect to the position of a material point x. These configurational forces that result from the inner variations of the Kohn-Sham energy functional provide a unified framework to compute atomic forces as well as stress tensor for geometry optimization. Importantly, owing to the variational nature of the formulation, these configurational forces inherently account for the Pulay corrections. The formulation presented in this work treats both pseudopotential and all-electron calculations in a single framework, and employs a local variational real-space formulation of Kohn-Sham density functional theory (DFT) expressed in terms of the nonorthogonal wave functions that is amenable to reduced-order scaling techniques. We demonstrate the accuracy and performance of the proposed configurational force approach on benchmark all-electron and pseudopotential calculations conducted using higher-order finite-element discretization. To this end, we examine the rates of convergence of the finite-element discretization in the computed forces and stresses for various materials systems, and, further, verify the accuracy from finite differencing the energy. Wherever applicable, we also compare the forces and stresses with those obtained from Kohn-Sham DFT calculations employing plane-wave basis (pseudopotential calculations) and Gaussian basis (all-electron calculations). Finally, we verify the accuracy of the forces on large materials systems involving a metallic aluminum nanocluster containing 666 atoms and an alkane chain containing 902 atoms, where the Kohn-Sham electronic ground state is computed using a reduced-order scaling subspace projection technique [P. Motamarri and V. Gavini, Phys. Rev. B 90, 115127 (2014)].

关键词： density functional theory density functional theory development Electronic structure Finite-element method

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Time-dependent spin-density-functional-theory description of He+-He collisions

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Physical Review A 2017年第3期96卷 032708-032708页

作者： Matthew Baxter Tom Kirchner Eberhard Engel Center for Scientific Computing J. W. Goethe-Universität D-60438 Frankfurt/Main Germany

Theoretical total cross-section results for all ionization and capture processes in the He+-He collision system are presented in the approximate impact energy range of 10–1000 keV/amu. Calculations were performed within the framework of time-dependent spin-density functional theory. The Krieger-Li-Iafrate approximation was used to determine an accurate exchange-correlation potential in the exchange-only limit. The results of two models, one where electron translation factors in the orbitals used to calculate the potential are ignored and another where partial electron translation factors are included, are compared with available experimental data as well as a selection of previous theoretical calculations.

关键词： Atomic & molecular collisions Charge-transfer collisions Electronic excitation & ionization Atomic orbital density functional theory density functional theory development Nonperturbative methods Time-dependent DFT

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Time-dependent current-density-functional theory taking into consideration the effect of energy dissipation

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Physical Review A 2019年第6期100卷 062503-062503页

作者： Katsuhiko Higuchi Yasuhiro Fujie Hisashi Shimizu Masahiko Higuchi Graduate School of Advanced Sciences of Matter Hiroshima University Higashi-Hiroshima 739-8527 Japan Graduate School of Science and Technology Shinshu University Matsumoto 390-8621 Japan Department of Physics Faculty of Science Shinshu University Matsumoto 390-8621 Japan

As a microscopic theory to express the electromagnetic response of the system to the external fields, we present the time-dependent current-density-functional theory (td-CDFT), taking into account the effect of the energy dissipation from the system. In order to express such an effect, the non-Hermitian term which corresponds to the lifetime of the higher-energy component of the many-electron state is added to the Hamiltonian of the system. Specifically, the occupation number of the Bloch states, the energies of which are higher than the Fermi energy, is chosen as a basic variable in addition to the electron density and paramagnetic current density. In order to confirm its validity, the present td-CDFT is applied to the two-level system. We successfully revisit the electric susceptibility of the Drude-Lorentz model via the present theory.

关键词： Optical & microwave phenomena density functional theory development

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Using forces to accelerate first-principles anharmonic vibrational calculations

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Physical Review Materials 2017年第2期1卷 023801-023801页

作者： Joseph C. A. Prentice R. J. Needs TCM Group Cavendish Laboratory University of Cambridge J. J. Thomson Avenue Cambridge CB3 0HE United Kingdom

High-level vibrational calculations have been used to investigate anharmonicity in a wide variety of materials using density functional theory methods. We have developed an efficient approach for describing strongly anharmonic systems using a vibrational self-consistent field method. By far the most computationally expensive part of the calculations is the mapping of an accurate Born-Oppenheimer (BO) energy surface within the region of interest. Here we present an improved method which reduces the computational cost of the mapping. In this approach we use data from a set of energy calculations for different vibrational distortions of the materials and the corresponding forces on the atoms. Results using both energies and forces are presented for the test cases of the hydrogen molecule, solid hydrogen under high pressure including mapping of two-dimensional subspaces of the BO surface, and the bcc phases of the metals Li and Zr. The use of force data speeds up the anharmonic calculations by up to 40%.

关键词： Anharmonic lattice dynamics Electronic structure of atoms & molecules First-principles calculations Phonons density functional theory density functional theory development

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Driving-laser ellipticity dependence of high-order harmonic generation in graphene

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Physical Review A 2018年第6期97卷 063412-063412页

作者： Candong Liu Yinghui Zheng Zhinan Zeng Ruxin Li State Key Laboratory of High Field Laser Physics Shanghai Institute of Optics and Fine Mechanics Chinese Academy of Sciences Shanghai 201800 China

High-order harmonic generation in graphene driven by the elliptically polarized midinfrared driving pulse is theoretically investigated by solving the two-band semiconductor Bloch equation in the Houston basis under the tight-binding approximation. It is found that the maximal harmonic yield occurs at a specific driver ellipticity whose value is determined by relaxation processes, so that the proper choice of the dephasing time is key to quantitatively reproducing the recent experiment. We show that the ellipticity dependence mainly originates from the interband harmonic emission, and exhibits an almost stable behavior against the crystal orientation. Furthermore, we also demonstrate that both the large magnitude and the circular vortex structure of the transition dipole near the graphene Dirac point are responsible for forming the unique characteristics. These results demonstrate that high-harmonic generation by the elliptically polarized driving field carries significant information on the dephasing time and the electronic structure signature in the graphene.

关键词： High-order harmonic generation Quantum fluids & solids density functional theory development Dipole approximation

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Local density Approximation for Excited States

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Physical Review X 2024年第4期14卷 041045-041045页

作者： Tim Gould Stefano Pittalis Queensland Micro- and Nanotechnology Centre Griffith University Nathan Qld 4111 Australia

The ground state of an homogeneous electron gas is a paradigmatic state that has been used to model and predict the electronic structure of matter at equilibrium for nearly a century. For half a century, it has been successfully used to predict ground states of quantum systems via the local density approximation (LDA) of density functional theory (DFT), and systematic improvements in the form of generalized gradient approximations and evolution thereon. Here, we introduce the LDA for excited states by considering a particular class of nonthermal ensemble states of the homogeneous electron gas. These states find sound foundation and application in ensemble DFT—a generalization of DFT that can deal with ground and excited states on equal footing. The ensemble LDA is shown to successfully predict difficult low-lying excitations in atoms and molecules for which approximations based on local spin density approximation and time-dependent LDA fail.

关键词： Chemical Physics & Physical Chemistry Electronic structure density functional theory density functional theory development

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