Search results for "Vacancy defect"
showing 10 items of 178 documents
Cationic vacancies and anomalous spectral-weight transfer in Ti1−xTaxO2thin films studied via polarization-dependent near-edge x-ray absorption fine …
2013
We report the electronic structures of Ta-doped anatase TiO 2 thin films grown by pulsed laser deposition (PLD) with varying magnetization using a combination of first-principles calculations and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The roles of Ta doping and Ti vacancies are clarified, and the observed room-temperature ferromagnetism is attributed to the localized magnetic moments at Ti vacancy sites ferromagnetically ordered by electron charge carriers. O K -edge spectra exhibit significant polarization dependence which is discussed and supported by first-principles calculations in relation to both the crystal symmetry and the formation of defects. In particula…
Pressure-induced band anticrossing in two adamantine ordered-vacancy compounds: CdGa2S4 and HgGa2S4
2021
Abstract This paper reports a joint experimental and theoretical study of the electronic band structure of two ordered-vacancy compounds with defect-chalcopyrite structure: CdGa2S4 and HgGa2S4. High-pressure optical-absorption experiments (up to around 17 GPa) combined with first-principles electronic band-structure calculations provide compelling evidence of strong nonlinear pressure dependence of the bandgap in both compounds. The nonlinear pressure dependence is well accounted for by the band anticrossing model that was previously established mostly for selenides with defect chalcopyrite structure. Therefore, our results on two sulfides with defect chalcopyrite structure under compressio…
Detection of magnetic thin film impurity phases using nitrogen vacancy centers in diamond crystal
2019
We demonstrate the possibility to detect magnetic impurity phases in thin films using magnetic field imaging technique based on a layer of nitrogen-vacancy centers in the diamond lattice. We demonstrate results of magnetic field distributions created by impurity phases as well as mechanical defects on the thin film surface.
Melting Process of the Sn-45 wt.% In and Sn-77 wt.% In Alloys as Viewed by Positron Annihilation
2006
The temperature dependences of the positron annihilation parameters F and R were determined for Sn–45 wt.% In and Sn–77 wt.% In alloys. The obtained results enabled to verify the vacancy and the quasicrystalline models of the melting process. It was found that for metals transforming into the liquid state from the structures different than the close packed structure, the melting transition are hardly explainable in terms of the vacancy model of melting. The strong trapping of positrons in metallic liquids is an indication of the presence of microvoids (vacancy clusters) or microcrystals containing defects more immense than vacancies. The share of these positron trapping centers in the volum…
Electronic structure and thermodynamic stability ofLaMnO3andLa1−xSrxMnO3(001) surfaces:Ab initiocalculations
2008
We present the results of ab initio hybrid density-functional calculations of the atomic and the electronic structures of ${\text{LaMnO}}_{3}$ (LMO) and ${\text{La}}_{1\ensuremath{-}{x}_{b}}{\text{Sr}}_{{x}_{b}}{\text{MnO}}_{3}$ (001) surfaces. The total energies obtained from these calculations were used to analyze thermodynamic stability of the surfaces. We predict Sr and O vacancy segregation to the surface to occur with similar energies ($\ensuremath{\sim}0.5\text{ }\text{eV}$ per defect). In pure LMO only ${\text{MnO}}_{2}$ termination is thermodynamically favorable under typical operational conditions of a cathode in solid oxide fuel cells, whereas Sr doping makes La(Sr)O termination …
First-principles calculations of the atomic and electronic structure ofFcenters in the bulk and on the (001) surface ofSrTiO3
2006
The atomic and electronic structure, formation energy, and the energy barriers for migration have been calculated for the neutral O vacancy point defect F center in cubic SrTiO3 employing various implementations of density functional theory DFT. Both bulk and TiO2-terminated 001 surface F centers have been considered. Supercells of different shapes containing up to 320 atoms have been employed. The limit of an isolated single oxygen vacancy in the bulk corresponds to a 270-atom supercell, in contrast to commonly used supercells containing 40– 80 atoms. Calculations carried out with the hybrid B3PW functional show that the F center level approaches the conduction band bottom to within 0.5 eV…
Calculations of the Electronic and Atomic Structure and Diffusion of Point Defects in KNbO3 Perovskite Crystals and Relevant KTN Solid Solutions
2002
AbstractIn this paper we review our recent achievements in large scale computer simulations of point defects in advanced perovskite crystals. We have calculated the defect migration energies in the KNbO3 cubic phase using quantum chemical method of the Intermediate Neglect of Differential Overlap (INDO) and classical shell model (SM). The migration energies for the O vacancy obtained by means of these two quite different methods are reasonably close (0.68 eV and 0.79 eV, respectively) and also agree with the only experimental estimate available (ca. 1 eV). Atomic relaxations calculated by these two methods also agree quite well. We used INDO method for a large-scale modeling of the atomic a…
Atomic structure relaxation in nanocrystalline NiO studied by EXAFS spectroscopy: Role of nickel vacancies
2012
Nanocrystalline NiO samples have been studied using the Ni K-edge extended x-ray absorption fine structure (EXAFS) spectroscopy and recently developed modeling technique, combining classical molecular dynamics with ab initio multiple-scattering EXAFS calculations (MD-EXAFS). Conventional analysis of the EXAFS signals from the first two coordination shells of nickel revealed that (i) the second shell average distance R(Ni–Ni2) expands in nanocrystalline NiO compared to microcrystalline NiO, in agreement with overall unit cell volume expansion observed by x-ray diffraction; (ii) on the contrary, the first shell average distance R(Ni–O1) in nanocrystalline NiO shrinks compared to microcrystall…
Eddy-Current Imaging with Nitrogen-Vacancy Centers in Diamond
2018
We demonstrate microwave-free eddy-current imaging using nitrogen-vacancy centers in diamond. By detecting the eddy-current induced magnetic field of conductive samples, we can distinguish between different materials and shapes and identify structural defects. Our technique allows for the discrimination of different materials according to their conductivity. The sensitivity of the measurements is calculated as 8$\times 10 ^{5}$\,S/m\,$\sqrt[]{\textrm{Hz}}$ at 3.5\,MHz, for a cylindrical sample with radius $r_0$\,=\,1\,mm and height $h$\,=\,0.1\,mm (volume $\sim$\,0.3\,mm$^3$), at a distance of 0.5\,mm. In comparison with existing technologies, the diamond-based device exhibits a superior ba…
Optically Enhanced Electric Field Sensing Using Nitrogen-Vacancy Ensembles
2021
Nitrogen-vacancy (NV) centers in diamond have shown promise as inherently localized electric-field sensors, capable of detecting individual charges with nanometer resolution. Working with NV ensembles, we demonstrate that a detailed understanding of the internal electric field environment enables enhanced sensitivity in the detection of external electric fields. We follow this logic along two complementary paths. First, using excitation tuned near the NV's zero-phonon line, we perform optically detected magnetic resonance (ODMR) spectroscopy at cryogenic temperatures in order to precisely measure the NV center's excited-state susceptibility to electric fields. In doing so, we demonstrate th…