Using density functional theory, we show that the long-believed transition-metal tetraborides (TB(4)) of tungsten and molybdenum are in fact triborides (TB(3)). This finding is supported by ...thermodynamic, mechanical, and phonon instabilities of TB(4), and it challenges the previously proposed origin of superhardness of these compounds and the predictability of the generally used hardness model. Theoretical calculations for the newly identified stable TB(3) structure correctly reproduce their structural and mechanical properties, as well as the experimental x-ray diffraction pattern. However, the relatively low shear moduli and strengths suggest that TB(3) cannot be intrinsically stronger than c-BN. The origin of the lattice instability of TB(3) under large shear strain that occurs at the atomic level during plastic deformation can be attributed to valence charge depletion between boron and metal atoms, which enables easy sliding of boron layers between the metal ones.
The stability of the stacked two-dimensional (2D) transition metal carbides and their interlayered friction in different configurations are comparatively studied by means of density functional theory ...(DFT). At equilibrium, a larger interlayer distance corresponds to a smaller binding energy, suggesting an easier sliding between them. The oxygen-functionalized M 2 CO 2 possesses much lower sliding resistance than the bare ones due to the strong metallic interactions between the stacked M 2 C layers. Compared to the parallel stacking order of M 2 CO 2 -I, the mirror stacked M 2 CO 2 -II possesses better lubricant properties. At strained states, normal compression substantially enhances the sliding barrier owing to more charges transferring from the M to O atom. Furthermore, the in-plane biaxial strain may effectively hinder the interlayer sliding, while the uniaxial strain fundamentally modifies the preferred sliding pathway due to anisotropic expansion of surface electronic state. These results highlight that the functionalized MXenes with strain-controllable frictional properties are promising lubricating materials because of their low sliding energy barrier and excellent mechanical properties.
Dislocation is one of the most critical and fundamental crystal defects that dominate the mechanical behavior of crystalline solids, however, a quantitative determination of its character and ...property in experiments is quite challenging and limited so far. In this paper, a fully automated Peierls–Nabarro (P–N) analyzer named PNADIS is presented; a complete set of the character and property of dislocation can be automatically derived, including the dislocation core structure, Peierls energy and stress, pressure field around dislocation core, solute/dislocation interaction energy, as well as the energy barrier and increase in critical-resolved shear stress at 0 K for solid solution strengthening. Furthermore, both one-dimensional (1D) and two-dimensional (2D) P–N models are implemented to meet the demand to analyze the character and property of dislocation for not only simple FCC and HCP structures but also complex crystals. The implementation of this code has been critically validated by a lot of evaluations and tests including 1D P–N model for complex crystals, 2D P–N model for FCC and HCP metals, pressure field around dislocation core, and solid solution strengthening for alloys. We expect that the automated feature of this code would provide a high-efficiency solution for determining the character and property of dislocation.
Program title: PNADIS
Program Files doi:http://dx.doi.org/10.17632/whk6wdy3nn.1
Licensing provisions: GNU General Public License 3
Programming language: MATLAB
Nature of problem: To determine automatically the character and property of dislocation, including dislocation core structure, Peierls stress, pressure field around dislocation core and solid solution strengthening, for not only FCC and HCP structures but also complex crystals.
Solution method: The generalized stacking fault energy is firstly fitted by Fourier expansion, and meanwhile an appropriate trial function of disregistry vector is chosen. Afterwards, a least square minimization of the difference between elastic resistance and restoring force for one-dimensional Peierls–Nabarro model, or a global minimization of the total dislocation energy via particle swarm optimization or genetic algorithm for two-dimensional Peierls–Nabarro model, will be performed to determine the dislocation core structure of complex crystals, or FCC and HCP structures. Finally, the Peierls stress, pressure field around dislocation core and solid solute strengthening are derived from the calculated dislocation core structure.
We report a computational theoretical investigation of electron spin-flip scattering induced by the electron-phonon interaction in the transition-metal ferromagnets bcc Fe, fcc Co, and fcc Ni. The ...Elliott-Yafet electron-phonon spin-flip scattering is computed from first principles, employing a generalized spin-flip Eliashberg function as well as ab initio computed phonon dispersions. Aiming at investigating the amount of electron-phonon mediated demagnetization in femtosecond laser-excited ferromagnets, the formalism is extended to treat laser-created thermalized as well as nonequilibrium, nonthermal hot electron distributions. Using the developed formalism we compute the phonon-induced spin lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated demagnetization rate is evaluated for laser-created thermalized and nonequilibrium electron distributions. Nonthermal distributions are found to lead to a stronger demagnetization rate than hot, thermalized distributions, yet their demagnetizing effect is not enough to explain the experimentally observed demagnetization occurring in the subpicosecond regime.
Two-dimensional (2D) materials have attracted considerable interest due to their remarkable properties and potential applications for nanoelectronics, electrodes, energy storage devices, among ...others. However, many well-studied 2D materials lack appreciable conductivity and tunable mechanical strength, limiting their applications in flexible devices. Newly developed MXenes open up the opportunity to design novel flexible conductive electronic materials. Here, using density functional theory (DFT), we investigate systematically the effects of several functional groups on the stabilization, mechanical properties, and electronic structures of a representative MXene. It is found that oxygen possesses the largest adsorption energy as compared to other functional groups, indicating its good thermodynamic stabilization. In comparison with bare and other functionalized titanium carbides, the oxygen functionalized one exhibits the most superior ideal strength; however, the premature softening of the long-wave phonon modes might limit the intrinsic strength for Ti sub(3) C sub(2) O sub(2). Furthermore, the introduction of functional groups can induce a strong anisotropy under tensile loading. By analyzing the deformation paths and the electronic instability under various loadings, we demonstrate that the unique strengthening by oxygen functional groups is attributed to a significant charge transfer from inner bonds to outer surface ones after functionalization. Our results shed a novel view into exploring a variety of MXenes for their potential applications in flexible electronic and energy storage devices.
In this work, we present the program MAELAS to calculate magnetocrystalline anisotropy energy, anisotropic magnetostrictive coefficients and magnetoelastic constants in an automated way by Density ...Functional Theory calculations. The program is based on the length optimization of the unit cell proposed by Wu and Freeman to calculate the magnetostrictive coefficients for cubic crystals. In addition to cubic crystals, this method is also implemented and generalized for other types of crystals that may be of interest in the study of magnetostrictive materials. As a benchmark, some tests are shown for well-known magnetic materials.
Program Title: MAELAS
CPC Library link to program files: https://doi.org/10.17632/gxcdg3z7t6.1
Developer’s repository link:https://github.com/pnieves2019/MAELAS
Code Ocean capsule: https://codeocean.com/capsule/0361425
Licensing provisions: BSD 3-clause
Programming language: Python3
Nature of problem: To calculate anisotropic magnetostrictive coefficients and magnetoelastic constants in an automated way based on Density Functional Theory methods.
Solution method: In the first stage, the unit cell is relaxed through a spin-polarized calculation without spin-orbit coupling. Next, after a crystal symmetry analysis, a set of deformed lattice and spin configurations are generated using the pymatgen library 1. The energy of these states is calculated by the first-principles code VASP 3, including the spin-orbit coupling. The anisotropic magnetostrictive coefficients are derived from the fitting of these energies to a quadratic polynomial 2. Finally, if the elastic tensor is provided 4, then the magnetoelastic constants are also calculated.
Additional comments including restrictions and unusual features: This version supports the following crystal systems: Cubic (point groups 432, 4̄3m, m3̄m), Hexagonal (6mm, 622, 6̄2m, 6∕mmm), Trigonal (32, 3m, 3̄m), Tetragonal (4mm, 422, 4̄2m, 4∕mmm) and Orthorhombic (222, 2mm, mmm).
References:
1 S. P. Ong, W. D. Richards, A. Jain, G. Hautier, M. Kocher, S. Cholia, D. Gunter, V. L. Chevrier, K. A. Persson, and G. Ceder, Comput. Mater. Sci. 68, 314 (2013).
2 R. Wu, A. J. Freeman, Journal of Applied Physics 79, 6209–6212 (1996).
3 G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169.
4 S. Zhang and R. Zhang, Comput. Phys. Commun. 220, 403 (2017).
•Software to calculate anisotropic magnetostrictive coefficients.•It also calculates anisotropic magnetoelastic constants.•Evaluation of magnetocrystalline anisotropy energy.•Calculations in an automated way by Density Functional Theory calculations.•It supports the main crystal symmetries in the research field of magnetostriction.
Uranium monocarbide, a potential fuel material for the generation IV reactors, is investigated within density functional theory. Its electronic, magnetic, elastic, and phonon properties are analyzed ...and discussed in terms of spin-orbit interaction and localized versus itinerant behavior of the 5f electrons. The localization of the 5f states is tuned by varying the local Coulomb repulsion interaction parameter. We demonstrate that the theoretical electronic structure, elastic constants, phonon dispersions, and their densities of states can reproduce accurately the results of x-ray photoemission and bremsstrahlung isochromat measurements as well as inelastic neutron scattering experiments only when the 5f states experience the spin-orbit interaction and simultaneously remain partially localized. The partial localization of the 5f electrons could be represented by a moderate value of the on-site Coulomb interaction parameter of about 2 eV. The results of the present studies indicate that both strong electron correlations and spin-orbit effects are crucial for realistic theoretical description of the ground-state properties of uranium carbide.
Spin-lattice model for cubic crystals Nieves, P.; Tranchida, J.; Arapan, S. ...
Physical review. B,
03/2021, Letnik:
103, Številka:
9
Journal Article
Recenzirano
Odprti dostop
We present a methodology based on the Néel model to build a classical spin-lattice Hamiltonian for cubic crystals capable of describing magnetic properties induced by the spin-orbit coupling like ...magnetocrystalline anisotropy and anisotropic magnetostriction, as well as exchange magnetostriction. Taking advantage of the analytical solutions of the Néel model, we derive theoretical expressions for the parametrization of the exchange integrals and Néel dipole and quadrupole terms that link them to the magnetic properties of the material. This approach allows us to build accurate spin-lattice models with the desired magnetoelastic properties. We also explore a possible way to model the volume dependence of magnetic moment based on the Landau energy. This feature allows us to consider the effects of hydrostatic pressure on the saturation magnetization. We apply this method to develop a spin-lattice model for BCC Fe and FCC Ni, and we show that it accurately reproduces the experimental elastic tensor, magnetocrystalline anisotropy under pressure, anisotropic magnetostrictive coefficients, volume magnetostriction, and saturation magnetization under pressure at zero temperature. This work could constitute a step towards large-scale modeling of magnetoelastic phenomena.
Thorium is a chemical element that is beginning to attract attention because of its potential use as a nuclear fuel. It is not easy to carry out experiments because of its radioactive nature, and ...therefore theoretical works are highly appreciated. Thorium contains only a small number of the 5f states, and it is generally accepted that these states are itinerant, that they form a chemical bond, and that their nature does not need to be corrected with the Hubbard model. On the other hand, there is a well-known problem with the description of the 6p1/2 states when the spin-orbit coupling is added as the perturbation to a scalar-relativistic calculation. Electronic, elastic, phonon, and thermodynamic properties are analyzed in terms of the importance of the spin-orbit coupling acting on the 6d and 5f states. The importance of the spin-orbit coupling acting on the semicore 6p states is discussed. The same properties are analyzed for thorium monocarbide, and a difference caused by adding a carbon atom into the structure is discussed. Detailed analysis of the thermal conductivity (both phonon and electronic contributions) is also included. We have given extra attention to the thermal conductivity of ThC to explain why the optical phonon modes account only for approximately 6% of the phonon thermal conductivity.
4f spin to phonon coupling is crucial for ultrafast spin-wave generation and magnetization dynamics in rare-earth metals.
Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ...ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin–lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.
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