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Photostimulated luminescence phosphors are promising candidates for next‐generation optical data storage devices. Herein, optically‐reversible luminescence modulation is demonstrated using UV wavelengths in the fluoroperovskite RbCdF3:Mn, where the modulation is mediated by photostimulated luminescence processes. UV‐C stimulation enhances the luminescence from Mn2+ centers and simultaneously fills electron traps. This charging process occurs via electron transfer from Mn2+ ions to fluorine vacancies, yielding Mn3+ ions and F‐centers, and is mediated by conduction band transport. UV‐A stimulation restores the material to the initial state. This discharging process occurs via electron transfer from F‐centers to Mn3+ ions and is similarly mediated by conduction band transport. Moreover, the discharging process manifests Mn2+ photostimulated luminescence. The primary trap state has activation energies in the range 1.46 to 1.73 eV and has room temperature lifetimes exceeding 40 000 years. A kinetic model is presented and evaluated that accurately describes the charge transport and luminescence properties of the material. Thus, a material is presented via which ultra‐long term, multi‐level luminescent data storage can be realized, and a model via which precise control over the luminescence modulation and photostimulated luminescence intensities can be achieved. [ABSTRACT FROM AUTHOR]

We report the results from a comprehensive study of the effect that annealing FeSe0.35Te0.65 single crystals in different atmospheres has on the surface chemistry, the superconducting transition temperature, Tc, and the critical current density, Jc. A review of the literature and our data shows that annealing in oxygen, nitrogen, air or vacuum atmosphere leads to a universal superconducting phase diagram that is independent of the variance in the as-grown properties. We show that atmospheres with an oxygen partial pressure of p > 10−3 hPa during annealing are necessary to improve the superconducting properties in this compound. This is demonstrated by Raman and magnetisation measurements on air, nitrogen, or low vacuum annealed FeSe0.35Te0.65 samples, which show that the improvement in the superconducting properties is strongly correlated to the formation of a thin iron oxide surface layer. However, annealing under high vacuum does not lead to the formation of an iron oxide layer and no improvement in the superconducting properties is observed. Our findings show that the previously reported improvements in the superconducting properties after annealing under vacuum or in nitrogen are thus likely to have been caused by residual oxygen. Furthermore, we propose a diffusion model for the formation of the iron oxide layer as the driving force for the reduction of interstitial excess iron that suppresses superconductivity in this compound. Overall, our results show that the presence of an iron reactant, such as oxygen, during annealing is necessary to improve the superconducting properties of FeSe1−xTex. [ABSTRACT FROM AUTHOR]

We report the results from a comprehensive study of the effect that annealing FeSe0.35Te0.65 single crystals in different atmospheres has on the surface chemistry, the superconducting transition temperature, Tc, and the critical current density, Jc. A review of the literature and our data shows that annealing in oxygen, nitrogen, air or vacuum atmosphere leads to a universal superconducting phase diagram that is independent of the variance in the as-grown properties. We show that atmospheres with an oxygen partial pressure of p > 10−3 hPa during annealing are necessary to improve the superconducting properties in this compound. This is demonstrated by Raman and magnetisation measurements on air, nitrogen, or low vacuum annealed FeSe0.35Te0.65 samples, which show that the improvement in the superconducting properties is strongly correlated to the formation of a thin iron oxide surface layer. However, annealing under high vacuum does not lead to the formation of an iron oxide layer and no improvement in the superconducting properties is observed. Our findings show that the previously reported improvements in the superconducting properties after annealing under vacuum or in nitrogen are thus likely to have been caused by residual oxygen. Furthermore, we propose a diffusion model for the formation of the iron oxide layer as the driving force for the reduction of interstitial excess iron that suppresses superconductivity in this compound. Overall, our results show that the presence of an iron reactant, such as oxygen, during annealing is necessary to improve the superconducting properties of FeSe1−xTex. [ABSTRACT FROM AUTHOR]

Nuclear relaxation is an important thermodynamic probe of electronic excitations, in particular in conducting and superconducting systems. Here, an empirical phenomenology based on all available literature data for planar Cu in hole-doped cuprates is developed. It is found that most of the seemingly different relaxation rates among the systems are due to a temperature-independent anisotropy that affects mostly measured 1/T1∥, the rate with an external magnetic field along the crystal c-axis, while 1/T1⊥ is largely independent on doping and material above the critical temperature of superconductivity (Tc). This includes very strongly overdoped systems that show Fermi liquid behavior and obey the Korringa law. Below Tc, the relaxation rates are similar, as well, if plotted against the reduced temperature T/Tc. Thus, planar Cu nuclear relaxation is governed by a simple, dominant mechanism that couples the nuclei with varying anisotropy to a rather ubiquitous bath of electronic excitations that appear Fermi liquid-like irrespective of doping and family. In particular, there is no significant enhancement of the relaxation due to electronic spin fluctuations, different from earlier conclusions. Only the La2−xSrxCuO4 family appears to be an outlier as additional relaxation is present; however, the anisotropy remains temperature independent. Also systems with very low doping levels, for which there is a lack of data, may behave differently. [ABSTRACT FROM AUTHOR]

The narrow band gap semiconductors Bi2Te3 and Sb2Te3 are well known for their room temperature thermoelectric performance. Recently, they were shown to serve as model systems of three‐dimensional topological insulators with a bulk band gap and very robust, spin‐resolved surface states due to a special band inversion in their periodic bulk. Evidently, it is of interest to investigate the special surface states with a local probe like nuclear magnetic resonance (NMR). However, especially the bulk NMR of these materials shows peculiar and rather unexpected phenomena, e. g., a magnetic field induced deformation of the local charge symmetry and excessive line broadening due to a special internuclear coupling. Here we report a comprehensive account of the room temperature NMR properties of the spin‐1/2 nucleus 125Te of single crystalline Bi2Te3 and Sb2Te3. This includes a very unusual spin‐lattice relaxation that seems not to be reflected in the NMR shift, as well as an exchange enhanced NMR line broadening. [ABSTRACT FROM AUTHOR]

The local distribution of electrons and holes in the CuO2 plane of the high-temperature superconducting cuprates is determined with nuclear magnetic resonance through the quadrupole splittings of 17O and 63Cu. Based on new data for single crystals of electron-doped Pr2-xCexCuO4 (x = 0, 0.05, 0.10, 0.15) as well as Nd2-xCexCuO4 (x = 0, 0.13) the changes in hole contents nd of Cu 3dx2-y2 and np of O 2pσ orbitals are determined and these account for the stoichiometrically doped charges, similar to hole-doped La2-xSrxCuO4. It emerges that while nd + 2np = 1 in all parent materials as expected, nd and np vary substantially between different groups of materials. Doping holes increases predominantly np, but also nd. To the contrary, doping electrons predominantly decreases nd and only slightly np. However, np for the electron-doped systems is higher than that in hole-doped La1.85Sr0.15CuO4. Cuprates with the highest maximum Tc's appear to have a comparatively low nd and high np. The rather high oxygen hole content of the Pr2CuO4 and Nd2CuO4 with the low nd seems to make them ideal candidates for hole doping to obtain a potentially high Tc. [ABSTRACT FROM AUTHOR]

Topological insulators constitute a new class of materials with an energy gap in the bulk and peculiar metallic states on the surface. We report on new features resulting from the bulk electronic structure, based on a comprehensive nuclear magnetic resonance (NMR) study of 77Se on Bi2Se3 and Cu0.15Bi2Se3 single crystals. First, we find two resonance lines and show that they originate from the two inequivalent Se lattice sites. Second, we observe unusual field-independent linewidths and attribute them to an unexpectedly strong internuclear coupling mediated by bulk electrons. In order to support this interpretation, we present a model calculation of the indirect internuclear coupling and show that the Bloembergen-Rowland coupling is much stronger than the Ruderman-Kittel-Kasuya-Yosida coupling. Our results call for a revision of earlier NMR studies and add information concerning the bulk electronic properties. [ABSTRACT FROM AUTHOR]

Electrospinning has been used to fabricate ferromagnetic Ni 0. 47Fe 0.53 nanofiber mats that were composed of individual, orientated Ni 0.47 Fe 0.53 nanofibers. The key steps were processing a polyvinylpyrrolidone nanofiber template containing ferric nitrate and nickel acetate metal precursors in Ar at 300°C and then 95% Ar: 5% H 2 at 600°C. The Ni 0.47 Fe 0.53 fibers were nanostructured and contained Ni 0.47 Fe 0.53 nanocrystals with average diameters of ~14 nm. The Ni 0.47 Fe 0.53 ferromagnetic mats had a high saturation magnetic moment per formula unit that was comparable to those reported in other studies of nanostructured Ni 1- x Fe x . There is a small spin-disordered fraction that is typically seen in nanoscale ferromagnets and is likely to be caused by the surface of the nanofibers. There was an additional magnetic contribution that could possibly stem from a small Fe 1- z Ni z O phase fraction surrounding the fibers. The coercivity was found to be enhanced when compared with the bulk material., (Copyright © 2020 Bhugra, Williams, Chong and Nann.)