Vacuum ultraviolet photoluminescence of NaMgF3:Sm and NaMgF3:Sm,Ce: energy levels of the lanthanides in NaMgF3:Ln compounds.

The luminescence properties of NaMgF3:Sm and NaMgF3:Ce,Sm were studied in the vacuum ultraviolet spectral region. Excitation bands corresponding to the charge transfer processes F- â†’ Sm3+, O2- â†’ Sm3+, and O2- â†’ Ce3+, and the energy transfer processes Ce3+ â†’ Sm3+and O2- â†’ Sm3+, were observed. The energies of the Sm3+charge transfer transitions and the crystal field split Ce3+4f05d1transitions were used to construct a complete host referred binding energy diagram for the series of lanthanide-doped NaMgF3:Ln compounds. We demonstrate that the optical and luminescence properties predicted by the binding energy diagram are in good agreement with those predicted by the binding energy diagram constructed via the alternative impurity-informed method, and all available experimental data regarding the NaMgF3:Ln compounds. We demonstrate that NaMgF3:Ln compounds are model systems for the study of charge trapping phenomena and divalent lanthanide luminescence. Ultimately, we validate that the impurity-informed method can be used to establish the energy levels of lanthanides in fluoride systems.

Modelling the radioluminescence of Sm2+ and Sm3+ in the dosimeter material NaMgF3:Sm.

Photoluminescence (PL) and radioluminescence (RL) measurements were made on NaMgF3:Sm before, during and after exposure to high doses of ionising radiation. Magnetic measurements prior to irradiation showed that approximately 10% of the total Sm concentration was in the divalent state. The RL from Sm3+ was found to increase while the Sm2+ RL decreased with increasing x-ray dose before reaching steady-state values for high doses. This behaviour is opposite to that previously reported for Sm3+ and Sm2+ PL. We show that this apparent discrepancy can be accounted for by a RL model where there is a hole trap, an electron trap, and direct x-ray induced carrier recombination at Sm2+ and Sm3+. Furthermore, a good fit to the dose-dependence of all of the Sm RL emissions can be obtained by assuming that the relevant electron and hole traps are close to Sm3+. Our model accounts for F3-centre production during irradiation that affects some of the Sm3+ RL emissions via reabsorption of the RL by the F3-centres. Thus, the rate of F3-centre production can be conveniently monitored by the RL intensity ratio, I RL(567 nm)/I RL(650 nm). Additionally, the Sm2+ RL emissions may be expressed as [1.94 × I RL(721 nm)] - I RL(695 nm) to determine the real-time dose rate, independent of dose history.

Optical Modulation of the Diffraction Efficiency in an Indoline Azobenzene/Amorphous Polycarbonate Film.

We have made a diffraction grating in an indoline azobenzene/amorphous polycarbonate film by two-beam interference at 532 nm that periodically photodegrades the indoline azobenzene dye. Subsequent illumination of the film with 532-nm light into the trans-isomer band leads to trans-cis isomerization in the indoline azobenzene dye and results in a decrease in the trans-isomer band absorption coefficient. This causes the diffraction efficiency to decrease when probed at 655 nm. The diffraction efficiency returns to its original value when the 532-nm light is blocked by thermal relaxation from the indoline azobenzene cis-isomer to the trans-isomer. Thus, we have been able to optically modulate the diffraction efficiency in a thin film diffraction grating.

Synthesis and Compositional Analysis of Permalloy Powder Prepared by Arc-Discharge.

We report the synthesis, compositional, structural and magnetic properties of permalloy powders prepared using an arc-discharge method under different atmospheres. Ion beam analysis results showed that powder prepared in air had a higher concentration of oxygen than those prepared under nitrogen or argon atmospheres. X-ray diffraction measurements showed that powders prepared in air contained magnetite (Fe3O4) and other phases, while powders prepared under nitrogen or argon predominately contained permalloy. The permalloy powders contained a broad range of particle sizes, and nanoparticles as small as 10 nm were evident from transmission electron microscopy data. The saturation magnetizations were significantly lower for the powders prepared in air than those prepared under nitrogen or argon. This can be attributed to oxidation, where the saturation magnetization is predominately from Fe3O4 for powders made in air. The coercive fields were also significantly larger for powders prepared in air, which is consistent with the powders containing different phases when compared with the permalloy powders. Our results show that permalloy powders can be made in nitrogen and argon, allowing for the production of low oxygen content permalloy powders for device applications. Our results also suggest that the use of an iron anode could result in Fe3O4 powders.

Optically switchable diffraction grating in a photochromic/polymer thin film.

An optically switchable diffraction grating has been made in a thin film containing a photochromic dye and amorphous polycarbonate. We show that a film containing the dye 5-chloro-1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'-(3H)naphth[2,1-b](1,4)oxazine] can be optically processed so that a diffraction grating can be completely switched on by ultraviolet light and turned off by thermal relaxation. The ability to switch or modulate the diffraction efficiency has a number of practical applications that include optical switches and add/drop multiplexers.

Dielectric versus magnetic pairing mechanisms in high-temperature cuprate superconductors investigated using Raman scattering.

We suggest, and demonstrate, a systematic approach to the study of cuprate superconductors, namely, progressive change of ion size in order to systematically alter the interaction strength and other key parameters. R(Ba,Sr)2Cu3Oy (R={La,…,Lu,Y}) is such a system where potentially obscuring structural changes are minimal. We thereby systematically alter both dielectric and magnetic properties. Dielectric fluctuation is characterized by ionic polarizability while magnetic fluctuation is characterized by exchange interactions measurable by Raman scattering. The range of transition temperatures is 70-107 K, and we find that these correlate only with the dielectric properties, a behavior which persists with external pressure. The ultimate significance may remain to be proven, but it highlights the role of dielectric screening in the cuprates and adds support to a previously proposed novel pairing mechanism involving exchange of quantized waves of electronic polarization.

63Cu nuclear magnetic resonance study of Pr(1.85)Ce(0.15)Cu(1-x)Ni(x)O(4): Ni-induced spin density oscillation and modification of the low energy spin fluctuations.

We report the results from a (63)Cu nuclear magnetic resonance (NMR) study of the electron-doped high temperature superconducting cuprate (HTSC) Pr(1.85)Ce(0.15)Cu(1-x)Ni(x)O(4). We find that Ni induces a magnetic broadening of the (63)Cu NMR spectra that can be interpreted in terms of an induced spin density oscillation about the Ni site, similar to that reported from (63)Cu NMR measurements on the hole-doped HTSCs when Zn is partially substituted for Cu. There is also an additional temperature-dependent contribution to the (63)Cu spin-lattice relaxation rate that can be interpreted in terms of an Ni-induced modification of the low energy spin fluctuations. Furthermore, the spin fluctuations are intrinsically spatially inhomogeneous and additional inhomogeneities are induced by Ni.

Fabrication of surface magnetic nanoclusters using low energy ion implantation and electron beam annealing.

Magnetic nanoclusters have novel applications as magnetic sensors, spintronic and biomedical devices, as well as applications in more traditional materials such as high-density magnetic storage media and high performance permanent magnets. We describe a new synthesis protocol which combines the advantages of ion implantation and electron beam annealing (EBA) to produce surface iron nanoclusters. We compare the structure, composition and magnetic properties of iron nanoclusters fabricated by low dose 15 keV Fe implantation into SiO(2) followed by 1000 °C EBA or furnace annealing. Atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) images together with superconducting quantum interference device (SQUID) magnetometry measurements show that only EBA leads to the rapid formation of surface crystalline Fe spherical nanoclusters, showing magnetic moments per Fe atom comparable to that of bulk bcc Fe and superparamagnetic properties. We propose a fabrication mechanism which includes e-beam enhanced desorption of SiO(2). This method has potential for fabricating nanoscale magnetic sensors integrated in microelectronic devices.

Evidence for two electronic components in high-temperature superconductivity from NMR.

A new analysis of (63)Cu and (17)O NMR shift data on La(1.85)Sr(0.15)CuO(4) is reported that supports earlier work arguing for a two-component description of La(1.85)Sr(0.15)CuO(4), but conflicts with the widely held view that the cuprates are a one-component system. The data are analyzed in terms of two components A and B with susceptibilities χ(AA), χ(AB)(= χ(BA)) and χ(BB). We find that above T(c), χ(AB) and χ(BB) are independent of temperature and obtain for the first time the temperature dependence of all three susceptibilities above T(c) as well as the complete temperature dependence of χ(AA)+χ(AB) and χ(AB)+χ(BB) below T(c). The form of the results agrees with that recently proposed by Barzykin and Pines.

Isotope effect in the superfluid density of high-temperature superconducting cuprates: stripes, pseudogap, and impurities.

Underdoped cuprates exhibit a normal-state pseudogap, and their spins and doped carriers tend to spatially separate into 1D or 2D stripes. Some view these as central to superconductivity and others as peripheral and merely competing. Using La(2-x)Sr(x)Cu(1-y)Zn(y)O4 we show that an oxygen isotope effect in Tc and in the superfluid density can be used to distinguish between the roles of stripes and pseudogap and also to detect the presence of impurity scattering. We conclude that stripes and pseudogap are distinct, and both compete and coexist with superconductivity.

Nuclear magnetic resonance study of the electron-doped high-temperature superconducting cuprates.

Nuclear magnetic resonance (NMR) measurements have been made on two of the electron-doped high-temperature superconducting cuprates (HTSCs), Pr(2-x)Ce(x)CuO(4) and Sr(0.9)La(0.1)CuO(2) that represent the two known electron-doped structures. The results are compared with the more-studied hole-doped HTSCs. We show that the electron and hole-doped HTSCs probe a similar antiferromagnetic spin fluctuation spectrum in the normal state, which provides support for theories of superconductivity where the pairing is mediated by antiferromagnetic spin fluctuations and the superconducting order parameter has a [Formula: see text] symmetry. Contrary to results from underdoped and hole-doped HTSCs, there is no evidence for a normal-state pseudogap in the NMR data from measurements on the electron-doped HTSCs. Therefore, the electron-doped HTSCs can be better compared with overdoped and hole-doped HTSCs where the normal-state pseudogap is absent. The antiferromagnetic spin fluctuation spectrum as probed by the Cu spin-lattice relaxation rate, is independent of the doped electrons per Cu. A similar effect is observed in the overdoped and hole-doped HTSC, Y(1-x)Ca(x)Ba(2)Cu(3)O(7-delta) for a hole concentration range of approximately 0.063. The anomalous Cu NMR linewidth anisotropy observed in the electron-doped HTSCs suggests a small and static spin variation for temperatures up to room temperature.