Thermal stability and UV–Vis-NIR spectroscopy of a new erbium-doped fluorotellurite glass

A new transparent bulk glass from the system 76TeO₂?·?10ZnO?·?9.0PbO?·?1.0PbF₂?·?3.0Na₂O doped with Er³⁺ (TZPPN doped with Er³⁺) has been prepared using the conventional melt-quenching method. Results of differential thermal analysis (DTA) measurements indicate good thermal stability of this glass. The refractive indices at different wavelengths, the optical energy gap, the Sellmeier gap energy and the dispersion energy have been estimated. The Judd–Ofelt parameters, Ω _t (t?=?2,?4,?6) of Er³⁺ were evaluated from optical absorption spectra. Electric dipole, magnetic dipole type transition probabilities, spectroscopic quality factors, branching ratio and radiative lifetimes of several excited states of Er³⁺ have been predicted using intensity Judd–Ofelt parameters. The spectroscopic properties indicate that TZPPN glass doped with Er³⁺ is a promising candidate for laser applications and may be suitable for upconversion fibre optical devices.     

Band structures of symmetrical graphene superlattice with cells of three regions

We study the electronic band structures of massless Dirac fermions in symmetrical graphene superlattice with cells of three regions. opening gaps and additional Dirac points. Finally, we inspect the potential effect on minibands, the anisotropy of group velocity and the energy bands contours near Dirac points. We also discuss the evolution of gap edges and cutoff region near the vertical Dirac points.     

Oxidation of bismuth nanodroplets deposit on GaAs substrate

Bismuth nanodroplets on GaAs substrate were obtained by metalorganic vapor phase epitaxy (MOVPE). New products have been synthesized when Bi nanodroplets are heated under oxygen atmosphere. The oxidation process of Bi nanodroplets consists of a heating from the room temperature to different oxidation temperatures (350, 500, 600 ^°C) with a temperature rate of 14 ^°C/min. The annealing duration was fixed to 30 min. The presence of oxygen in the products was confirmed by energy dispersive X-ray (EDX) measurements using a scanning electron microscope (SEM). SEM images show that Bi microcomposites density decrease and their size increases with increasing annealing temperature. After X-ray diffraction analysis of the products no obvious peaks could be observed. The reflectance spectra of the products were studied in spectral domains ranged from 200 nm to 1100 nm. By fitting the reflectivity signal, we extracted the thickness of the products and their refractive index variation versus wavelength. The results show that the thickness of the samples increases with increasing annealing temperature. The photoluminescence (PL) spectra under excitation at 325 nm shows a broad emission centered at around 1.92 eV.     

Magnetotransport properties in \text{ Ni }_{81}\text{ Fe }_{19}\text{/Zr } multi-layers

In this work, we present a study of the magneto transport properties in magnetic multilayered structure $\text{ Ni }_{81}\text{ Fe }_{19}\text{/Zr }$ Ni 81 Fe 19 /Zr . The magnetic $(\text{ Ni }_{81}\text{ Fe }_{19})$ ( Ni 81 Fe 19 ) and non magnetic (Zr) layer thickness $(\mathbf{t}_\mathbf{NiFe}, \mathbf{t}_\mathbf{zr})$ ( t NiFe , t zr ) effects on the magneto resistance (MR) are discussed theoretically in the framework of the Johnson–Camley semi classical approach based on the Boltzmann transport equation. A comparison between calculated and measured MR is obtained. The observed MR ratio oscillates for Zr layer thickness with an average period of 7Å. A generally weak $\text{ MR }(\text{ t }_{\mathrm{NiFe}})$ MR ( t NiFe ) ratio for fixed $\mathbf{t}_\mathbf{zr}$ t zr is obtained and it shows a maxima peak of the MR with a value of 1.8 % located at $\mathbf{t}_\mathbf{NiFe}= 80$ t NiFe = 80 Å.     

Structural and lithium intercalation studies of Mn(0.5−x)CaxTi2(PO4)3 phases (0≤x≤0.50)

Materials from the Mn_(0.5−x)Ca_xTi₂(PO₄)₃ (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, ³¹P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R space group. Mn²⁺/Ca²⁺ ions occupy only the M1 sites in the Ti₂(PO₄)₃ framework. The divalent cations are ordered in one of two M1 sites, except for the Mn_0.50Ti₂(PO₄)₃ phase, where a small departure from the ideal order is observed by XRD and ³¹P MAS NMR. The electrochemical behaviour of Mn_0.50Ti₂(PO₄)₃ and Mn_(0.5−x)Ca_xTi₂(PO₄)₃ phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti₂(PO₄)₃ framework. In the 0≤y≤2 range, the OCV curves of Li//Li_yMn_0.50Ti₂(PO₄)₃ cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li_(1+y)Ti₂(PO₄)₃ and Li//Li_yMn_0.50Ti₂(PO₄)₃ shows that the intercalation occurs first in the unoccupied M1 site of Mn_0.50Ti₂(PO₄)₃ at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn_0.50Ti₂(PO₄)₃ on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn_0.50Ti₂(PO₄)₃, the stability of Mn²⁺ either in an octahedral or tetrahedral environment facilitates cationic migration.     

Radiation Effects on Optical Properties of Ethylene Vinyl Acetate Copolymer Films

The optical properties of ethylene vinyl acetate (EVA) film have been studied. The effects of gamma irradiations on the optical spectrum of EVA films have been investigated using spectrophotometric measurements of reflectance and transmittance in the wavelength range 200–1100 nm. The absorption spectra were recorded in the UV–vis region for the unirradiated and irradiated films (from 0 to 50 kGy). Optical constants such as refractive index (n), extinction coefficient (K), and complex dielectric constant have been determined, as well as the optical dispersion parameters and high frequency dielectric constants. A large dependence of the fundamental optical constants on the irradiation dose was noticed. On irradiation, a higher refractive index was obtained as compared with that for unirradiated film. The dispersion parameters, such as E ₀ (single‐oscillator energy), E _d (dispersive energy), and M _?1 and M _?3 (moments), are discussed in terms of the single‐oscillator Wemple–DiDomenico model.     

Raman study of LiTaO3-related non-stoichiometric solid solutions isolated inside the ternary systems Li2O–Ta2O5–(M′O)2 with M′=Mn,Co

We have characterized by Raman spectroscopy the disorder and the local modifications of cation environment in the lithium tantalate structure resulting from the incorporation of bivalent cations Co²⁺ and Mn²⁺ as potential substitutes for Li and/or Ta ions. Frequency and damping of the E(TO1), E(TO6) and E(TO8) phonon modes of ceramic powders are studied along seven lines in the ternary phase diagrams Li₂O–Ta₂O₅–(M′O)₂ with M′=Mn and Co, and compared to those of the pure stoichiometric LiTaO₃. Raman spectroscopy is found to be very sensitive to the substitution ions and defects generated in the lattice vibration. Dopants occupy primarily the Li site in the region of lithium oxide excess. The site of Ta becomes progressively implicated in the substitution process when the concentration of dopant increases. In the Li-poor region of the ternary-phase diagram, corresponding to under-stoichiometric compositions, we retain the charge compensation mechanism involving both Li and Ta site according to: 3Li⁺+Ta⁵⁺→4M′²⁺ with M′²⁺=Mn²⁺ or Co²⁺.