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2022 ◽  
Author(s):  
Yogendra Limbu ◽  
Gopi Chandra Kaphle ◽  
Alok Lal Karn ◽  
Niraj Kumar Shah ◽  
Durga Paudyal

From first principles electronic structure calculations, we unravel the evolution of structural, electronic, and magnetic properties of pristine, defected, and strained titanium nitride MXene with different functional groups (-F, -O, -H, and -OH). The formation and cohesive energies reveal their chemical stability. The MAX phase and defect free functionalized MXenes are metallic except for oxygen terminated (Ti 2 NO 2 ) one which is 100% spin polarized half-metallic ferromagnet. The spin-orbit coupling significantly influences the bare MXene (Ti 2 N) to exhibit Dirac topology and band inversion near the high symmetry directions and Fermi level. The strain effect sways the Fermi level thereby shifting it toward lower energy state under compression and toward higher energy state under tensile strain in Ti 2 NH 2 . The Ti 2 NO 2 exhibits exotic electronic structure and magnetic states not only in pristine but also in strained and defected structures. Its half-metallic nature changes to semi-metallic under 1% compression and it is completely destroyed under 2% compression. In single vacancy defect, its band structure remarkably transforms from half-metallic to semi-conducting with large band gap in 12.5% Ti, weakly semi-conducting in 5.5% Ti, and semi-metallic in 12.5% O. The 25% N defect changes it’s half-metallic characteristic to metallic. Further, the 12.5% Co substitution preserves it’s half-metallic character, whereas Mn substitution allows it to convert half-metallic characteristic into weak semi-metallic characteristic preserving ferromagnetism. However, Cr substitution converts half-metallic ferromagnetic state to half-metallic anti-ferromagnetic state. The understanding made here on collective structural stability, and electronic band structure, and magnetic phenomena in novel 2D Ti 2 N derived MXenes open up their possibility in designing them for synthesis and thereby taking to applications.


2022 ◽  
Author(s):  
Jorge-Alberto Peralta-Ángeles ◽  
Jorge-Alejandro Reyes-Esq

Abstract An analytical and numerical study of hybrid photonic-plasmonic crystals is presented. The proposed theoretical model describes a system composed of a dielectric photonic crystal on a metallic thin film. To show the validity and usefulness of the model, four particular structures are analyzed, a one-dimensional crystal and three lattices of two-dimensional crystals. The model can calculate the photonic band structure of photonic-plasmonic crystals as a function of structural characteristics, showing two partial bandgaps for a square lattice, and complete bandgaps for triangular lattices. Furthermore, using a particular high-symmetry path, a full bandgap emerges in rectangular lattices, even with a small index of refraction contrast. Using the analytical model, a dataset is generated to train an artificial neural network to predict the center and width of the bandgap, that is, the forward design. In addition, an artificial neural network is trained to tune the optical response, that is, to perform the inverse design. The analytical results are consistent with the physics of the system studied and are supported by numerical simulations. Moreover, the prediction accuracy of the artificial neural networks is better than 95%. Overall, this paper reports a useful tool for tuning the optical properties of hybrid photonic-plasmonic crystals with potential applications in waveguides, nanocavities, mirrors, etc.


Surface ◽  
2021 ◽  
Vol 13(28) ◽  
pp. 3-14
Author(s):  
A.G. Grebenyuk ◽  

Oxides of tetravalent elements are well known to have a lot of crystalline modifications. For example, most of silica polymorphs are characterized by tetrahedral coordination environment of silicon atoms. On the contrary, crystals of stishovite and of some silicate minerals have their silicon atoms in octahedral coordination spheres. It has been found experimentally that the phase transitions between silica polymorphs accompanied by a rearrangement of silica-oxygen tetrahedrons into octahedra require an energy income (preference energy) of 54 kJ/mol. When increasing the atomic mass of the oxide forming element, the former decreases extremely and for tin dioxide is equal to -59 kJ/mol. These values can be reproduced in a theoretical way, within the frameworks of modern quantum chemical methods and periodic models. High disperse silica nanoparticles (as well as those for other oxides) have only the nearest order of atomic stationing, so that theoretical approaches developed for crystals cannot be applied to small particles. These particles can be transformed into stishovite form under hydrothermal conditions. Such a process can be simulated within cluster approximation by use of molecular models. This work is devoted to quantum chemical simulation of formation of the fragments with hexa-coordinated atoms of silicon and of its analogs in the structure of oxane dendrimers. A row of high symmetry models was examined containing two, three, five, and seventeen atoms of silicon and of germanium, titanium and tin, terminated with hydroxyl groups. These structures can be rearranged into another ones including oxide forming atoms with elevated (equal to 5 or 6) coordination number, so mimicking the rutile-like structure. Such models let it possible to fulfill the procedure of transformation without rupturing siloxane bonds, so remaining within a deformation approach. Another advantage is the exclusion of the basis set superposition error due to use of molecular models of the same total formula for all the coordination states. All calculations were carried out by Hartree-Fock and density functional theory methods with the all-electron (3-21G*) and valent (SBKJC) basis sets by means of the GAMESS program. Models of various size have been examined, in particular, disiloxane (HO)3Si-O-Si(OH)3 witch can be transformed into a self-coordinated form where one of silicon atoms becomes a five-coordinated; trisiloxane (HO)3Si-O-Si(OH)2-O-Si(OH)3 can be rearranged into symmetric isomer with one hexa-coordinated silicon atom. Pentasiloxane with neo-structure of [(HO)3Si-O]4Si forms three coordination structures, the most stable of them mimicking the stishovite crystal; it contains one 6-coordinated and two 5-coordinated silicon atoms. Siloxane containing 17 silicon atoms has a super-neo-structure of {[(HO)3Si-O]3Si-O}4Si; it includes seven six-coordinated and four five-coordinated silicon atoms. Relative models for silicon analogs have been also examined. When analyzing a dependence of the energy differences between open and coordinated oxane structures on the number of atoms of the oxide forming element in the cluster, one can jump to the conclusion that the specific value of this characteristic monotonously decreases with the increase in the number of atoms of the molecular model, so becoming close to the experimental data.


2021 ◽  
Author(s):  
Sterling G. Baird ◽  
Eric R. Homer ◽  
David T. Fullwood ◽  
Oliver K. Johnson

We apply a newly developed Voronoi fundamental zone (VFZ) framework to gain insights about grain boundary (GB) structure-property relationships in the five degree-of-freedom (5DOF) space of cubic GBs. We analyze the shape and size of a 5DOF fundamental zone (FZ), molecular statics energy uncertainty, property similarity of GBs that are crystallographically \close" (i.e. correlations), and energy pathways through 5DOF space. Considered together, these insights are important for managing tradeoffs between accuracy, complexity, and design considerations for electron backscatter diffraction/serial sectioning, high-energy diffraction microscopy, molecular statics, and density-functional theory. In terms of the shape and size of a 5DOF FZ, we discover that a FZ is smaller than expected at only ∼65° in the largest principal component. Thus, a 10° difference between two GBs, which may have previously been considered small, is actually quite large. We represent a GB by five transformed Cartesian coordinates equipped with a Euclidean distance metric. Using this representation, we find that the FZ has a low aspect-ratio shape (i.e. width, length, height, etc. are similar) which is important for 5DOF numerical differentiation. Semivariogram and numerical optimization methods reveal that grain boundary energy (GBE) in Ni and Fe are globally correlated within ∼6° to 8° in the grain boundary octonion (GBO) sense (multiply by 2 to convert to misorientation angle). For local correlation lengths of high-symmetry GBs of interest, we notice significant variation relative to global correlation lengths and an inverse relationship with the Brandon criterion. We suggest that property data with no more than ± ∼3 % error and point sets with GBs that are no more than ∼3−4° apart should be used and then paired with high-fidelity interpolation strategies. Finally, in terms of dynamic material behavior, geodesic paths through 5DOF space for Ni suggest that, under appropriate conditions, a certain low-energy Σ7 GB may transform into the frequently observed Σ3 coherent-twin GB which may be interesting to verify by experiment or simulation.


2021 ◽  
Author(s):  
Kazuki Morita ◽  
Daniel Davies ◽  
Keith Butler ◽  
Aron Walsh

While traditional crystallographic representations of structure play an important role in materials science, they are unsuitable for efficient machine learning. A range of effective numerical descriptors have been developed for molecular and crystal structures. We are interested in a special case, where distortions emerge relative to an ideal high-symmetry parent structure. We demonstrate that irreducible representations form an efficient basis for the featurisation of polyhedral deformations with respect to such an aristotype. Applied to dataset of 552 octahedra in ABO3 perovskite-type materials, we use unsupervised machine learning with irreducible representation descriptors to identify four distinct classes of behaviour, associated with predominately corner, edge, face, and mixed connectivity between neighbouring octahedral units. Through this analysis, we identify SrCrO3 as a material with tuneable multiferroic behaviour. We further show, through supervised machine learning, that thermally activated structural distortions of CsPbI3 are well described by this approach.


2021 ◽  
Vol 104 (6) ◽  
Author(s):  
Joseph R. Rain ◽  
PeiYu Cai ◽  
Alexander Baekey ◽  
Matthew A. Reinhard ◽  
Roman I. Vasquez ◽  
...  

Author(s):  
Jonathan D Denlinger ◽  
J Kang ◽  
Lenart Dudy ◽  
J W Allen ◽  
Kyoo Kim ◽  
...  

Abstract Previous high-resolution angle-resolved photoemission (ARPES) studies of URu2Si2 have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level (E F) at low photon energies near the zone center, with an emphasis on electronic reconstruction due to Brillouin zone folding. A substantial challenge to a proper description is that these states interact with other hole-band states that are generally absent from bulk-sensitive soft x-ray ARPES measurements. Here we provide a more global k-space context for the presence of such states and their relation to the bulk Fermi surface topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure using photon energies intermediate between the low-energy regime and the high-energy soft x-ray regime. Small-spot spatial dependence, f-resonant photoemission, Si 2p core-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of the E F-crossing bands and their relation to specific U- or Si-terminations of the cleaved surface. The bulk Fermi surface topology is critically compared to density functional theory and to dynamical mean field theory calculations. In addition to clarifying some aspects of the previously measured high symmetry Γ, Z and X points, incommensurate 0.6a* nested Fermi-edge states located along Z-N-Z are found to be distinctly different from the density functional theory Fermi surface prediction. The temperature evolution of these states above THO, combined with a more detailed theoretical investigation of this region, suggests a key role of the N-point in the hidden order transition.


2021 ◽  
Vol 38 (11) ◽  
pp. 117101
Author(s):  
Di Wang ◽  
Jihai Yu ◽  
Feng Tang ◽  
Yuan Li ◽  
Xiangang Wan

Magnetic exchange interactions (MEIs) define networks of coupled magnetic moments and lead to a surprisingly rich variety of their magnetic properties. Typically MEIs can be estimated by fitting experimental results. Unfortunately, how many MEIs need to be included in the fitting process for a material is unclear a priori, which limits the results obtained by these conventional methods. Based on linear spin-wave theory but without performing matrix diagonalization, we show that for a general quadratic spin Hamiltonian, there is a simple relation between the Fourier transform of MEIs and the sum of square of magnon energies (SSME). We further show that according to the real-space distance range within which MEIs are considered relevant, one can obtain the corresponding relationships between SSME in momentum space. By directly utilizing these characteristics and the experimental magnon energies at only a few high-symmetry k points in the Brillouin zone, one can obtain strong constraints about the range of exchange path beyond which MEIs can be safely neglected. Our methodology is also generally applicable for other Hamiltonian with quadratic Fermi or Boson operators.


2021 ◽  
Vol 7 (1) ◽  
Author(s):  
Longsheng Feng ◽  
Michael J. Mills ◽  
Yunzhi Wang

AbstractEven though the fundamental rules governing dislocation activities have been well established in the past century, we report a phenomenon, dislocation transformation, governed by the generalized-stacking-fault energy surface mismatch (GSF mismatch for short) between two co-existing phases. By carrying out ab-initio-informed microscopic phase-field simulations, we demonstrate that the GSF mismatch between a high symmetry matrix phase and a low symmetry precipitate phase can transform an array of identical full dislocations in the matrix into an array of two different types of full dislocations when they shear through the precipitates. The precipitates serve as a passive Shockley partial source, creating new Shockley partial dislocations that are neither the ones from the dissociation of the full dislocation. This phenomenon enriches our fundamental understanding of partial dislocation nucleation and dislocation-precipitate interactions, offering additional opportunities to tailor work-hardening and twinning processes in alloys strengthened by low-symmetry precipitate phases.


2021 ◽  
Vol 57 (11) ◽  
pp. 1149
Author(s):  
I. Geru ◽  
N. Gorinchoy ◽  
I. Balan

The pseudo Jahn–Teller (PJT) origin of the proton transfer barrier in the Zundel cation at different O–O distances and in an H5O2+(H2O)4 cluster is revealed by means of  ab initio calculations of their electronic structures and the adiabatic potential energy curves. The vibronic constants in this approach were estimated by fitting the ab initio calculated adiabatic potential to its analytical expression. It is shown also that the high-symmetry nuclear configurations ofproton-centered water clusters of the type H+(H2O)n (n = 6, 4, 3) are unstable with respect to the low-symmetry nuclear distortions leading to forming the dihydronium cation H5O2+ and the appropriate number of water molecules: H2n + 1On+ →  (n – 2)H2O + H5O2+. The reason for this instability and the subsequent decay is the PJT coupling between the ground and excited electronic states.


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