Giant shift upon strain on the fluorescence spectrum of VNNB color centers in h-BN

We study the effect of strain on the physical properties of the nitrogen antisite-vacancy pair in hexagonal boron nitride (h-BN), a color center that may be employed as a quantum bit in a two-dimensional material. With group theory and ab initio analysis we show that strong electron–phonon coupling plays a key role in the optical activation of this color center. We find a giant shift on the zero-phonon-line (ZPL) emission of the nitrogen antisite-vacancy pair defect upon applying strain that is typical of h-BN samples. Our results provide a plausible explanation for the experimental observation of quantum emitters with similar optical properties but widely scattered ZPL wavelengths and the experimentally observed dependence of the ZPL on the strain.

Analysis of Defect Modes in One-Dimensional Photonic Crystal with a Dielectric-Superconducting Pair Defect

One-dimensional dielectric photonic crystals (PCs) with complex substitutional defect layers, consisting of superconducting (SC) and dielectric sublayers are theoretically studied. The influence of the incidence angle on the photonic gap spectra is theoretically analyzed. The pronounced difference in the transmittivity spectra of the PCs with right-handed (RH) and left-handed (LH) positions of the superconducting defect sublayer with respect to the dielectric defect sublayer is demonstrated. It is observed that, for the case of RH SC defect sublayer, the position of the defect mode and the transmittivity at the defect mode frequency strongly depend on the thickness of the superconducting sublayer as well as on the temperature. But the defect modes of the PCs with LH SC defect sublayer are nearly invariant upon the change of the thickness of the superconducting sublayer and the temperature.

Simulating Radiation-Induced Defect Formation in Pyrochlores

ABSTRACTThe accuracy and robustness of new Buckingham potentials for the pyrochlores Gd2Ti2O7 and Gd2Zr2O7 is demonstrated by calculating and comparing values for a selection of point defects with those calculated using a selection of other published potentials and our own ab inito values. Frenkel pair defect formation energies are substantially lowered in the presence of a small amount of local cation disorder. The activation energy for oxygen vacancy migration between adjacent O48f sites is calculated for Ti and Zr pyrochlores with the energy found to be lower for the non-defective Ti than for the Zr pyrochlore by ∼0.1 eV. The effect of local cation disorder on the VO48f → VO48f migration energy is minimal for Gd2Ti2O7, while the migration energy is lowered typically by ∼43 % for Gd2Zr2O7. As the healing mechanisms of these pyrochlores are likely to rely upon the availability of oxygen vacancies, the healing of a defective Zr pyrochlore is predicted to be faster than for the equivalent Ti pyrochlore.

An ab initio study of the effect of charge localization on oxygen defect formation and migration energies in magnesium oxide

Plane-wave density functional theory was used to study the properties of oxygen vacancies and interstitials, with different charge states, in MgO. The calculated properties were the relaxed configurations, the Frenkel defect formation energies and the energies of the migration barriers, and all properties were found to be strongly dependent on the defect charge state. The lowest energy configuration of the O 2− interstitial was found to be the cube centre; however, the O − and O 0 interstitials formed dumb-bell configurations. The Frenkel defect energies were also strongly dependent on the defect charge, with the neutral pair energy calculated to be 3 eV lower than the doubly charged Frenkel pair defect energy. The migration barriers of the oxygen vacancies were found to increase as the net charge of the oxygen vacancies decreased, which suggests that vacancies with trapped electrons are much less mobile than the F 2+ vacancies modelled by classical potentials. The migration of the oxygen interstitials showed particularly interesting behaviour. The O 0 interstitial was found to have a higher migration barrier than the O 2− interstitial but a very low barrier (0.06 eV) was found for the O − interstitial. The results have significant implications for the reliability of classical radiation damage simulations.