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Under ultraviolet (UV) excitation with photon energy larger than 4.5 eV, a charge-state conversion from negatively to neutrally charged nitrogen-vacancy (NV− to NV0) centers in diamond samples is realized. The UV-induced charge-state conversion is found to strongly depend on the N concentration in the sample and the irradiation fluence of the electron beam. For the samples with high N concentrations, low-fluence (2.5 × 1017 cm−2) 10-MeV electron beam irradiation usually leads to UV-induced charge-state conversion efficiency higher than that of the samples irradiated with high fluences (≥2.5 × 1018 cm−2). For the samples with a few ppm N, however, the charge-state conversion efficiency is relatively low in the cases of irradiation fluences in this work. Meanwhile, UV-induced NV0 luminescence exhibits temperature dependence different from that of visible-light excited NV0 or NV− centers; that is, the photoluminescence intensity does not reach saturation at temperatures lower than 135 K but decreases with the decrease in temperature. Based on the photoluminescence excitation spectra of NV0 centers in the UV region, the UV-induced charge-state conversion is suggested to involve free-hole generation, diffusion, and the capture by ground-state NV− centers. [ABSTRACT FROM AUTHOR]

By solving the continuity equation of excitons under steady excitation, a theoretical model for the excitonic luminescence of semiconductors was developed taking into account the exciton diffusion and surface recombination. The theoretical model was used to analyze the photoluminescence (PL) spectra of ZnO obtained from the bulk single-crystal samples with and without surface passivation, showing that the nonradiative recombination on the surface is an important channel of losing excitons, thus substantially reducing the PL quantum efficiency of excitons at room temperature. In addition, the surface recombination was found to have impacts on the fine structure of excitonic luminescence at low temperature. Using the theoretical model, the diffusion length of excitons at room temperature was estimated and found to be different from sample to sample, strongly depending on the sample processing. The theoretical model was demonstrated to be capable of accurately fitting the temperature-dependent PL intensity of passivated samples and showed that the exciton diffusion has significant impacts on the dynamics of excitonic luminescence at high temperature. [ABSTRACT FROM AUTHOR]

Osmium (Os) is a hexagonal-close-packed metal with a non-spherical Fermi surface that seriously deviates from the assumption in the Mayadas-Shatzkes electrical-resistivity model (MS model) for the size effects of polycrystalline films of metals due to electron scattering by grain boundaries. In this work, we studied the resistivity of the Os films with different thicknesses as a function of temperature in the range of 20 to 296 K. The electron scattering by the surface was found to be unimportant in the contributions to the size effects of resistivity of Os films with a sufficient thickness. Based on the first-principles calculations, an analytical equation was suggested for correction to the MS model and used for fitting the temperature-dependent resistivity of the Os films. The results show that correction to the MS model is necessary and the residual resistivity caused by the defects and impurities cannot be neglected. In addition, the inhomogeneity of resistivity in the direction perpendicular to the film surface was discussed under an assumption of parallel circuits. [ABSTRACT FROM AUTHOR]

So far available experimental data on the magnetic ground state and transport behaviors of CaCrO3 are conflicting, suggesting that the electronic structure may be fragile against intrinsic or external perturbations. In this work we perform full-scale first principles calculations on the magnetic ground state and electronic structure of CaCrO3. It is revealed that the electronic structure and spin order are sensitive to the on-site Coulomb interaction and lattice distortion. The magnetic ground state is of the marginal stability, close to the boundary between various antiferromagnetically ordered phases. A crossing over the magnetic boundaries and associated metal-insulator transitions can be allowed by even weak lattice distortion (compressive strain) on the ab plane. The numerical results suggest that the on-site Coulomb interaction and lattice distortion represent two major ingredients of physics to determine the magnetic ground state, thus reasonably interpreting the scattered experimental data on the spin order and transport behaviors for CaCrO3. [ABSTRACT FROM AUTHOR]