Search results for "Functional theory"
showing 10 items of 1012 documents
First principles simulations of F centers in cubic SrTiO 3
2005
Atomic and electronic structure of regular and O-deficient SrTiO3 have been studied. Several types of first principles atomistic simulations: Hartree-Fock method, Density Functional Theory, and hybrid HF-DFT functionals, have been applied to periodic models that consider supercells of different sizes (ranging between 40 and 240 atoms). We confirm the ionic character of the Sr-O bonds and the high covalency of the Ti-O2 substructure. For the stoichiometric cubic crystal; the lattice constant and bulk modulus correctly reproduce the experimental data whereas the band gap is only properly obtained by the B3PW functional. The relaxed geometry around the F center shows a large expansion of the t…
Calculations of radiation-induced point defects, polarons and excitons in ferroelectric perovskites
2002
Abstract We review results of our recent large-scale computer simulations of radiation-induced point defects, excitons and polarons in ABO3 perovskite crystals, focusing mostly on KNbO3 and KTaO3 as representative examples. We have calculated the atomic and electronic structure of defects, their optical absorption, defect-induced electron density redistribution, and activation energies for defect migration. The majority of our results were obtained using the quantum-chemical method of the intermediate neglect of differential overlap (INDO) based on the Hartree–Fock formalism, as well as the shell model (SM). The main findings are compared with those obtained by means of ab initio density fu…
Colossal Density-Driven Resistance Response in the Negative Charge Transfer Insulator MnS2
2021
A reversible density driven insulator to metal to insulator transition in high-spin MnS_{2} is experimentally observed, leading with a colossal electrical resistance drop of 10^{8} Ω by 12 GPa. Density functional theory simulations reveal the metallization to be unexpectedly driven by previously unoccupied S_{2}^{2-} σ_{3p}^{*} antibonding states crossing the Fermi level. This is a unique variant of the charge transfer insulator to metal transition for negative charge transfer insulators having anions with an unsaturated valence. By 36 GPa the emergence of the low-spin insulating arsenopyrite (P2_{1}/c) is confirmed, and the bulk metallicity is broken with the system returning to an insula…
Ab initio simulation of yttrium oxide nanocluster formation on fcc Fe lattice
2010
Using results of density functional theory (DFT) calculations the first attempt towards the understanding of Y2O3 particles formation in oxide dispersed strengthened (ODS) ferritic–martensitic steels was performed. The present work includes modeling of single defects (O impurity atom, Fe vacancy and Y substitute atom), interaction between substituted Y atoms, Y–Fe vacancy pairs and oxygen impurity atoms in the iron matrix. The calculations have showed the repulsive interaction between the two Y substitute atoms at any separation distances that might mean that the oxygen atoms or O atoms with vacancies are required to form binding between atoms in the yttrium oxide nanoclusters.
Competition of Dzyaloshinskii-Moriya and Higher-Order Exchange Interactions in Rh/Fe Atomic Bilayers on Ir(111)
2018
Using spin-polarized scanning tunneling microscopy and density functional theory we demonstrate the occurrence of a novel type of noncollinear spin structure in $\mathrm{Rh}/\mathrm{Fe}$ atomic bilayers on Ir(111). We find that higher-order exchange interactions depend sensitively on the stacking sequence. For fcc-$\mathrm{Rh}/\mathrm{Fe}/\mathrm{Ir}(111)$, frustrated exchange interactions are dominant and lead to the formation of a spin spiral ground state with a period of about 1.5 nm. For hcp-$\mathrm{Rh}/\mathrm{Fe}/\mathrm{Ir}(111)$, higher-order exchange interactions favor an up-up-down-down ($\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$) s…
Influence of CO in the structural and electrical properties of Pt nanocontacts: comparison with H2 molecule addition.
2009
Density-functional theory is used to determine the atomic structure and the zero-bias conductance of Pt nanocontacts upon CO addition. Three stable atomic configurations of the nanocontacts, in conjunction with a statistical analysis of their electrical response, are enough to explain the main features of the experimental conductance histograms. Remarkably, the role of nonlocal effects when approaching the exchange-correlation potential within density-functional theory turns out to be crucial for CO molecular-based systems. Finally, a comparison with H(2) molecule addition on Pt nanocontacts is presented.
Dipole surface plasmon in K+N clusters
1992
Abstract The technique of sum rules has been used to investigate the dipole surface plasmon for K + N clusters within a Density Functional Theory and the spherical jellium model. The role played by non-local effects is discussed comparing the results obtained from different functionals. Band-structure and core-polarization effects have been phenomenologically included in the calculation by means of an electron effective mass and a dielectric constant. Comparison with recent experimental data is presented.
Calculations of the atomic and electronic structure for SrTiO3 perovskite thin films
2001
The results of calculations of SrTiO3 (100) surface relaxation and rumpling with two different terminations (SrO and TiO2) are presented and discussed. We have used the ab initio Hartree–Fock (HF) method with electron correlation corrections and the density functional theory (DFT) with different exchange–correlation functionals, including hybrid exchange techniques. All methods agree well on surface energies and on atomic displacements, as well as on the considerable increase of covalency effects near the surface. More detailed experiments on surface rumpling and relaxation are necessary for further testing of theoretical predictions.
First-principles phonon calculations of Fe4+impurity in SrTiO3
2012
The results of hybrid density functional theory calculations on phonons in Sr(Fe(x)Ti(1-x))O(3) solid solution within the formalism of a linear combination of atomic orbitals are presented. The phonon density of states (DOS) calculated for 6.25% Fe(4+) impurities is reported and defect-induced phonon modes are identified. Based on our calculations and group-theoretical analysis, we suggest for the first time an interpretation of experimentally observed Raman- and IR-active modes.
A density functional theory evaluation of hydrophobic solvation: Ne, Ar and Kr in a 50-water cluster – Implications for the hydrophobic effect
2012
The physical explanation for the hydrophobic effect has been the subject of disagreement. Physical organic chemists tend to use a explanation related to pressure, while many biochemists prefer an explanation that involves decreased entropy of the aqueous solvent. We present DFT calculations at the B3LYP/6-31G(d,p) and X3LYP/6-31G(d,p) levels on the solvation of three noble gases (Ne, Ar, and Kr) in clusters of 50 waters. Vibrational analyses show no substantial decreases in the vibrational entropies of the waters in any of the three clusters. The observed positive free energies of transfer from the gas phase or from nonpolar solvents to water appear to be due to the work needed to make a su…