Search results for "defect"
showing 10 items of 879 documents
Quenched molecular dynamics studies on the extraction energy of aluminum atoms
2007
The extraction energy of an aluminum atom is calculated at 0 K as a function of coordination number and defect depth for three surface orientations [(100), (110) and (111)]. For each orientation, atoms are selected and extracted one by one. A linear relationship is obtained between the extraction energy of surface atoms and their coordination numbers (with slight variations due to the geometrical configuration of the atoms). However, the study of the influence of the defect depth on the extraction energy highlights the role played by intrinsic stress on the extraction energy. Copyright © 2008 John Wiley & Sons, Ltd.
Luminescence from nearly isolated surface defects in silica nanoparticles
2015
A structured emission/excitation pattern, proper of isolated defects, arises in a vacuum from silica nanoparticles. The luminescence, centered around 3.0-3.5 eV, is characterised by a vibronic progression due to the phonon coupling with two localised modes of frequency ∼1370 cm(-1) and ∼360 cm(-1), and decays in about 300 ns at 10 K. On increasing the temperature, the intensity and the lifetime decrease due to the activation of a non-radiative rate from the excited state. Concurrently, the temperature dependence of the lineshape evidences the low coupling with non-localised modes of the matrix (Huang-Rhys factor S ~ 0.2) and the poor influence of the inhomogeneous broadening. These findin…
Semiflexible Polymers in Spherical Confinement: Bipolar Orientational Order Versus Tennis Ball States
2017
Densely packed semiflexible polymers with contour length L confined in spheres with radius R of the same order as L cannot exhibit uniform nematic order. Depending on the chain stiffness (which we vary over a wide range), highly distorted structures form with topological defects on the sphere surface. These structures are completely different from previously observed ones of very long chains winding around the inner surface of spheres and from nematic droplets. At high densities, a thin shell of polymers close to the sphere surface exhibits a tennis ball texture due to the confinement-induced gradual bending of polymer bonds. In contrast, when the contour length of the chains is significant…
Roughening of the Cu(110) surface
1993
The structure of the Cu(110) surface is studied at high temperatures using a combination of lattice-gas Monte Carlo and molecular dynamics methods with identical many-atom interactions derived from the effective medium theory. The anisotropic six-vertex model is used in the interpretation of the lattice-gas results. We find a clear roughening transition around T_R=1000K and T_R/T_M=0.81. Molecular dynamics reveals the clustering of surface defects as the atomistic mechanism of the transition and allows us to estimate characteristic time scales. For the system of size 50x50, the time scale of the local roughening at 1150 K of an initially smooth surface is of the order of 100 ps.
Computer Modeling of Defects and Surfaces in Advanced Perovskite Ferroelectrics
2000
The (110) surface relaxations are calculated for SrTiO3 and BaTiO3 perovskites. The positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized. Strong surface rumpling and surface-induced dipole moments perpendicular to the surface are predicted for both the O-terminated and Ti-terminated surfaces. Calculated optical properties of basic point defects – F-type centres and hole polarons – in KNbO3 are used for the interpretation of available experimental data.
Luminescence of the surface nonbridging oxygen hole center in silica: Spectral and decay properties
2008
We investigated the red luminescence in a porous film of silica nanoparticles, originating from surface nonbridging oxygen hole centers. The excitation spectrum was measured from 1.8 to 8.0 eV by a tunable laser system and a synchrotron radiation source; this spectrum evidences a peak at 2.0 eV, nearly overlapping with the emission, and an ultraviolet broadband with peaks at 4.8 and 6.0 eV. The emission is characterized by a spectrum with two subbands split by 0.07 eV, its decay occurs with lifetime longer than 30 microsec and undergoes a thermal quenching by a factor aboout 2 with increasing temperature from 10 to 290 K. The optical characteristics of surface and bulk centers are discussed…
Vacancy Defects in Ga2O3: First-Principles Calculations of Electronic Structure
2021
This research was funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP08856540) as well as by the Latvian research council via the Latvian National Research Program under the topic ?High-Energy Physics and Accelerator Technologies?, Agreement No: VPP-IZM-CERN-2020/1-0002 for A.I. Popov. In addition, J. Purans is grateful to the ERAF project 1.1.1.1/20/A/057 while A. Platonenko was supported by Latvian Research Council No. LZP-2018/1-0214. The authors thank A. Lushchik and M. Lushchik for many useful discussions. The research was (partly) performed in the Institute of Solid State Physics, University of Latvia ISSP UL. ISSP UL as…
Longitudinal spin-relaxation in nitrogen-vacancy centers in electron irradiated diamond
2015
We present systematic measurements of longitudinal relaxation rates ($1/T_1$) of spin polarization in the ground state of the nitrogen-vacancy (NV$^-$) color center in synthetic diamond as a function of NV$^-$ concentration and magnetic field $B$. NV$^-$ centers were created by irradiating a Type 1b single-crystal diamond along the [100] axis with 200 keV electrons from a transmission electron microscope with varying doses to achieve spots of different NV$^-$ center concentrations. Values of ($1/T_1$) were measured for each spot as a function of $B$.
Intrinsic Point Defects in Silica for Fiber Optics Applications
2021
Due to its unique properties, amorphous silicon dioxide (a-SiO2) or silica is a key material in many technological fields, such as high-power laser systems, telecommunications, and fiber optics. In recent years, major efforts have been made in the development of highly transparent glasses, able to resist ionizing and non-ionizing radiation. However the widespread application of many silica-based technologies, particularly silica optical fibers, is still limited by the radiation-induced formation of point defects, which decrease their durability and transmission efficiency. Although this aspect has been widely investigated, the optical properties of certain defects and the correlation betwee…
Phase separation of symmetric polymer mixtures in a common good solvent in the semidilute concentration regime
1994
Monte Carlo simulations of lattice models of binary (AB) symmetric polymer mixtures (chain lengthsN A=N B=N) in a common good solvent are carried out and the phase diagrams and critical properties of the unmixing transitions are estimated and interpreted in terms of recent theories. Polymers are modeled by self-avoiding walks of lengthN=16, 32 and 64 on the simple cubic lattice. Data for vacancy concentrations of φV=0.6, 0.8 and 0.85 are analyzed. It is shown that forN=16, φV=0.85 no phase separation occurs, down to the lowest temperature, while forN=32, φV=0.85 still phase separation occurs but no longer is complete. Our results are compatible with a scaling theory based on a “renormalizat…