Conversion of ultraviolet and infrared to visible light in Er3+ and Dy3+ co-doped CsGd2F7

The CsGd₂F₇ crystal co-doped with 1 at% Er³⁺ and 0.2 at% Dy³⁺ ions was studied using the excitation of both synchrotron radiation and 355 nm laser. All the emission bands were assigned by using the Dieke diagram together with the conclusions from relaxation kinetics studies. The onset of the 4f–5d transition bands and the centers of charge transfer bands were also calculated for the lanthanide series CsLn₂F₇ and the results were used to assign the bands and peaks in the excitation spectra. The energy transfer and relaxation processes in CsGd₂F₇ were thoroughly analyzed. Quantum cutting downconversion of one vacuum ultraviolet to two visible photons was realized under 154 nm excitation. Upconverted green emission was observed under 980 nm excitation.     

Photo-luminescence properties of Cu and Ag doped Li2B4O7 single crystals at low temperatures

UV excited photo luminescence from Li₂B₄O₇:Cu and Li₂B₄O₇:Cu, Ag single crystals has been investigated in the temperature range from 77 K to 300 K. An excitation band having a doublet structure at 240 nm and 262 nm was observed for the emission at 370 nm that corresponds to ¹A_1g→¹E_g and ¹A_1g→¹T_2g crystal field components of the 3d¹⁰→3d⁹4s¹ transition of Cu⁺. The relative intensity of these components and their temperature dependence provide a measure of the off-center displacement of the Cu⁺ ground state in the crystal lattice site. The co-doped Ag plays a role of a sensitizer when doped with Cu and increases the overall emission as the emission between Ag states lies in the excitation region of Cu states. The 370 nm emission in both the crystals slightly decreases with temperature; however a sudden increase in the intensity around 264 K was observed.     

Dielectric relaxations in undoped,Ce-doped and Ce,Zr-codoped Lu3Al5O12 single crystals

Dielectric spectroscopy was performed on single crystals of pure, Ce-doped or Ce,Zr-codoped Lu₃Al₅O₁₂, before and after UV- or X-irradaiation, at various frequencies within the range 100 Hz–1 MHz as the temperature was scanned from 110 to 353 K. All samples previously subjected to ionising radiations gave spectra showing loss peaks with Arrhenius characteristics of permanent dipoles relaxation. We attribute the dipoles to defect-stabilised pairs of anion–anion vacancies (oxygen ions and oxygen vacancies) that have captured holes and photo-electrons separately, thus forming O^?- and F⁺-like centers. The dielectric relaxation peaks disappeared in undoped or doped samples annealed at 573 K, suggesting that charge carrier traps are relatively deep. UV–visible absorption spectra have also been measured, which tend to support our proposed interpretation. Further evidence for deep traps has come from thermally stimulated luminescence experiments.     

High intensity photoluminescence of microcrystalline CsPbBr3 films: Evidence for enhanced stimulated emission at room temperature

Photoluminescence of microcrystalline CsPbBr₃ films grown from the amorphous phase shows stimulated emission not only at cryogenic temperature but also at room temperature, in great contrast to the case for bulk CsPbBr₃ single crystals, where no stimulated emission occurs even at 4.2 K. This is the first demonstration of room temperature stimulated emission from metal halide compounds.The stimulated emission is so strong that single-path-light-amplification stimulated emission across the film thickness is observed at relatively low threshold excitation intensities of ∼50 kW cm⁻² at 77 K and ∼100 kW cm⁻² at 295 K suggesting a large optical gain. The physical origin of the stimulated emission is assigned as due to free exciton-free exciton inelastic collision. The large-gain mechanism is attributable to giant oscillator strength effect characteristic of excitonic superradiance recently reported in this issue.     

Strongly enhanced flux pining of GdBCO coated conductors with BaSnO3 nanorods by pulsed laser deposition

We report the effects of BSO addition on the crystallinity, texture, and the field dependency of critical current density (J_c) of GdBCO coated conductors (CCs) prepared by pulsed laser deposition (PLD). Undoped and BSO-doped GdBCO films showed only c-axis oriented growth, and the incorporated BSO nanorods exhibited epitaxial relationship with the GdBCO matrix. In comparison with undoped film, BSO-doped GdBCO film exhibited greatly enhanced J_c and higher pinning force densities in the entire field region of 0–5 T (H//c) at 77 and 65 K. The BSO-doped GdBCO film showed the maximum pinning force densities (F_p) of 6.5 GN/m³ (77 K, H//c) and 32.5 GN/m³ (65 K, H//c), ～2.8 times higher than those of the undoped sample. Cross-sectional TEM analyses exhibited nano-structured BSO nanorods roughly aligned along the c-axis of the GdBCO film, which are believed effective flux pinning centers responsible for strongly improved critical current densities in magnetic fields.     

Conductivity and dielectric relaxation phenomena in (NH4)2SO4 single crystal

AC impedance measurements have been carried out on (NH₄)₂SO₄ single crystals for the temperatures from 300 to 473 K and frequency range between 100 Hz and 4 MHz. The results reveal two distinct relaxation processes in the sample crystal. One is the dipolar relaxation with a peak at frequency slightly higher than 4 × 10⁶ Hz. The other is the charge carrier relaxation at lower frequencies. The frequency dependence of conductivity is described by the relation σ(ω) = Bωⁿ, and n = 1.32 is obtained at temperatures below 413 K. This value drops to 0.2 and then decreases slightly with increasing temperature. The dipolar response of the (NH₄)₂SO₄ single crystal under an ac field is attributed to the reorientation of dipoles. The contribution of charge carriers is increasing substantially with increasing temperature at temperatures above 413 K. The temperature variation of conductivity relaxation peaks follows the Arrhenius relation. The obtained activation energy for migration of the mobile ions for (NH₄)₂SO₄ single crystal was 1.24 eV in the temperature range between 433 and 468 K in this intrinsic region. It is proposed that the NH₄⁺ in the sample crystal has the contribution to the electrical conduction.     

Field-assisted sustainable O− ion emission from fluorine-substituted 12CaO·7Al2O3 with improved thermal stability

We examined the electric field-assisted thermionic emission of atomic oxygen radical anion (O^?) in a vacuum from fluorine-substituted derivatives of 12CaO·7Al₂O₃ (C12A7) with a composition of (12 ? x)CaO·7Al₂O₃·xCaF₂ (0 ≤ x ≤ 0.8). Unsubstituted C12A7 easily decomposed into 5CaO?3Al₂O₃ (C5A3) and 3CaO?Al₂O₃ (C3A) above 830 °C during the emission experiment in a vacuum. The decomposition temperature range became narrower as the amount of F^? ion substitution increased, e.g. the sample with x = 0.4 kept a single phase after the emission experiment at 900 °C. The emitted anionic species from the x = 0.4 sample were dominated by O^? ions (～ 92%) together with a small amount of O₂^? ions (～ 4%) and F^? ions (～ 4%). The absence of an O₂ gas supply to the opposite side of the emission surface led to a nearly steady co-emission of O^? ions and electrons with a ratio of < 1/1. The O₂ gas supply markedly enhanced the O^? ion emission, and suppressed the electron emission. A sustainable and high-purity O^? ion emission with a current density of 11 nA cm^? 2 was achieved at 830 °C with the supply of 40 Pa O₂ gas. The similarity in these emission features to the unsubstituted C12A7, together with the improved thermal stability demonstrates that the F^? ion-substituted C12A7 is a promising material for higher intensity O^? ion emission at higher temperatures.     

Effect of structural disorder on magnetic frustrations in the Fe80Nb6B14 amorphous alloy

The paper presents measurements of magnetic frustrations for the Fe₈₀Nb₆B₁₄ amorphous alloy preliminary annealed for 1 h at temperatures ranging from 300 to 770 K in comparison with other parameters characterizing structural relaxation. It was shown that annealing out of free volume and internal stresses cause a reduction in number of frustrated states observed in the range 400>T>50 K. In the range 2<T<50 K additional frustrations due to iron clusters or nanograins were detected.     

Proton diffusion in the superprotonic phase of [(NH4)1 − xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation

Proton diffusion in [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 < x < 1) has been studied by means of ¹H spin-lattice relaxation times, T₁. The relaxation times were measured at 200.13 MHz in the range of 296–490 K and at 19.65 MHz in the range of 300–470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 ≤ x ≤ 0.54). Rb₃H(SO₄)₂ does not obey this trend. The results of NMR well explain the macroscopic proton conductivity.     

Effects on Jc of pinning center morphology for multiple-in-line-damage in coated conductor and bulk,melt-textured HTS

The properties of discontinuous aligned pinning centers (PCs) created by high-energy heavy-ions are compared for bulk melt-textured and coated conductor HTS. Properties of PCs, which increase J_c (pinning potential and entanglement), and negative properties which decrease J_c (e.g., decreased T_c and percolation paths) are evaluated. Mechanisms are proposed to explain the very large increases in J_c resulting from multiple-in-line-damage (MILD) compared to continuous columnar pinning centers (CCPC). In particular, a mechanism which results in fluxoid entanglement, even for parallel (unsplayed) PCs, is discussed. The same mechanism is found to also account for restoration of much of the pinning potential expected to be lost due to the gaps in MILD PCs. It also accounts for the fact that at high fluence, J_c increases as fluence is increased, instead of decreasing as expected. The very low self-field in coated conductor permits separation of the negative and positive effects of PCs. It is found that parameters developed to quantify the negative effects in bulk melt-textured YBCO, by 63 GeV U²³⁸ ions, successfully describe damage to 2.1 μm thick coated conductor by 1 GeV Ru⁴⁴ ions. Coated conductor at 77 K and self-field is generally known to have J_c about 100 times that of melt-textured YBCO. However, at 77 K and applied field of 1 T, when both forms of HTS are processed with comparable numbers of near-optimum MILD PCs, the difference in J_c is reduced to a factor of 1.3–2. Whereas J_c for melt-textured YBCO increased sharply, by a factor of up to 16.8 for high-fluence MILD PCs, J_c in coated conductor increased by a smaller factor of 2.5–3.0. Nevertheless, 2.1 μm thick coated conductor, with near-optimum MILD PCs, exhibits J_c = 543 kA/cm² at 77 K and applied field of 1.0 T, and I_c = 114 A/cm-width of conductor. This is the highest value we find in the literature. The phenomenology developed indicates that for optimum MILD PCs in coated conductor, J_c ～ 700 ± 70 kA/cm² should be achievable at 77 K, 1.0 T.     

The effect of chemical pressure in rutheno-cuprates

We have studied the effect of negative chemical pressure in the RuGd_1.5(Ce_0.5?xPr_x)Sr₂Cu₂O_10?δ with Pr content of 0.0 ? x ? 0.2. This is also investigated using the bond length results obtained from the Rietveld refinement analysis. The c parameter and cell volume increase with x for 0.0 ? x ? 0.15. The width of the resistivity transition also increases with Pr concentration, indicating higher inhomogeneity and oxygen deficiency. The difference in the ionic valences of Pr^3+,4+ and Ce⁴⁺ causing different hole doping, the difference in the ionic radii, and oxygen stoichiometry affect the superconducting transition. The magnetoresistance shows a cusp around 135 K which lies between the antiferromagnetic and ferromagnetic transition temperatures, which is probably due to the presence of a spin glass region. There exist two magnetic transition temperatures for 0.0 ? x ? 0.2 which respectively change from T_M = 155 K to 144 K and from T_irr = 115 K to 70 K. The magnetization versus applied magnetic field isotherms at 77 K and 300 K show that the remanent magnetization and coercivity are lower for samples with higher Pr content.     

Low-temperature magnetic ordering in the perovskites Pr1−xAxCoO3 (A=Ca,Sr)

The magnetic and electrical properties of polycrystalline Pr_1?xA_xCoO₃ cobaltites with A=Ca, Sr and 0≤x≤0.5 were studied in the temperature range 4 K≤T≤1000 K and field up to 7 T. The X-ray analyses show the presence of only one phase having monoclinic or orthorhombic symmetry. The magnetic measurements indicate that the Ca-doped samples have at low temperatures, similar properties to the frustrated magnetic materials. PrCoO₃ is a paramagnetic insulator in the range from 4 to 1000 K. The Sr-doped cobaltites exhibit two phase transitions: a paramagnetic–ferromagnetic (or magnetic phase separated state) phase transition at about 240 K and a second one at about 100 K. The magnetic measurements suggest the presence of magnetic clusters and a change in the nature of magnetic coupling between Co ions at low temperatures. A semiconducting type behavior and high negative magnetoresistance was found for the Ca-doped samples, while the Sr-doped ones were metallic and with negligible magnetoresistance. The results are analyzed in the frame of a phase separation scenario in the presence of the spin-state transitions of Co ions.     

Spontaneous and stimulated emission in InAsSb-based LED heterostructures

Electroluminescence of LED heterostructures with active layer made of InAsSb films grown on InAs substrates was studied in the temperature range T = 4.2–300 K. At low temperatures (T = 4.2–100 K), stimulated emission was observed with an optical cavity formed normal to the growth plane. The emission became spontaneous at higher temperatures due to the resonant “switch-on” of the CHHS Auger recombination process in which the energy of a recombining electron-hole pair is transferred to a hole transitioning to the spin-orbit-splitted band. The spontaneous character of emission continued up to room temperature because of the influence of other Auger processes. The results obtained suggest that InAsSb-based LED heterostructures are promising for the fabrication of vertically-emitting mid-infrared lasers.     

Mid-wavelength type II InAs/GaSb superlattice infrared focal plane arrays

We have demonstrated 384 × 288 pixels mid-wavelength infrared focal plane arrays (FPA) using type II InAs/GaSb superlattice (T2SL) photodetectors with pitch of 25 μm. Two p-i-n T2SL samples were grown by molecular beam epitaxy with both GaAs-like and InSb-like interface. The diode chips were realized by pixel isolation with both dry etching and wet etching method, and passivation with SiN_x layer. The device one with 50% cutoff wavelength of 4.1 μm shows NETD ∼ 18 mK from 77 K to 100 K. The NETD of the other device with 50% cutoff wavelength at 5.6 μm is 10 mK at 77 K. Finally, the T2SL FPA shows high quality imaging capability at the temperature ranging from 80 K to 100 K which demonstrates the devices’ good temperature performance.     

Long RE123 coated conductors with high critical current over 500 A/cm by IBAD/PLD technique

We have developed long RE₁Ba₂Cu₃O_7?X (RE123) coated conductors with large current capacity by the ion beam assisted deposition (IBAD) and the pulsed laser deposition using hot wall heating (HW-PLD) technique. As a result, we could fabricate an 8 m-long Gd₁Ba₂Cu₃O_7?X (Gd123) coated wire with the minimum and maximum critical current (I_c) of 951 A/cm-w and 1003 A/cm-w at 77 K, 0 T, respectively, measured in 0.7 m-long sections by the standard 4-probe technique. Furthermore, we succeeded in preparation of over 600 m-long Gd123 coated wire with the uniform I_c distribution over 600 A/cm-w. It had average, maximum and minimum I_c of 665, 698, 609 A/cm-w, respectively. The n-values of the sample showed the maximum I_c and minimum I_c were 40 and 36, respectively. As a result, we set the new world record of I_c × L value as 374535 A m (= 609 A × 615 m). The in-field performance of this long wire was quite high as well; the minimum I_c exceeded 50 A/cm-w at 77 K, 3 T.     

Near-infrared broadband luminescence and energy transfer in Bi–Tm–Er co-doped lanthanum aluminosilicate glasses

The Bi–Tm–Er co-doped SiO₂–Al₂O₃–La₂O₃ (SAL) glasses, which exhibited a broadband near-infrared (NIR) emission, were investigated by the optical absorption and photoluminescence spectra. A super broadband NIR emission extending from 0.95 to 1.6 μm with a full-width at half-maximum (FWHM) of 430 nm which covered the whole O, E, S, C and L bands, was observed in Bi–Tm–Er co-doped samples under 808 nm excitation, as a result of the overlap of the Bi-related emission band (centered at 1270 nm) and the emission from Tm^{3+ 3}H₄→³F₄ transition (1450 nm) as well as Er^{3+ 4}I_13/2→⁴I_15/2 transition (1545 nm). In addition, a super broadband emission with amplitude relatively flat from 0.95 to 2.1 μm has been observed. The possible energy transfer between Bi-related centers, Tm³⁺ ions and Er³⁺ ions was proposed.     

Muon spin relaxation in Li0.6TiO2 anode material

In Li_0.6TiO₂ the longitudinal muon spin relaxation function has been measured for temperatures between 10 and 600 K. The μSR spectra were analyzed with a Markov process for multiple collisions. The time scale found for the Li⁺ diffusion is of the order of the microsecond or shorter. Above T = 100 K the magnetic field distribution at the muon is decreasing with increasing temperature.     

InAs/GaSb strained layer superlattice detectors with nBn design

We have investigated the electrical and optical properties of an nBn based Type-II InAs/GaSb strained layer superlattice detector as a function of absorber region background carrier concentration. Temperature-dependent dark current, responsivity and detectivity were measured. At T = 77 K and V_b = 0.1 V, with two orders of magnitude change in doping concentration, the dark current density increased from ～0.3 mA/cm² to ～0.3 A/cm². We attribute this to a depletion region that exists at the AlGaSb barrier and the SLS absorber interface. The device with non-intentionally doped absorption region demonstrated the lowest dark current density (0.3 mA/cm² at 0.1 V) with a specific detectivity D¹ at zero bias equal to 1.2 × 10¹¹ Jones at 77 K. The D¹ value decreased to 6 × 10¹⁰ cm Hz^1/2/W at 150 K. This temperature dependence is significantly different from conventional PIN diodes, in which the D¹ decreases by over two orders of magnitude from 77 K to 150 K, making nBn devices a promising alternative for higher operating temperatures.     

High performance batch production of LREBa2Cu3Oy using novel thin film Nd-123 seed

A batch production for fabrication of LREBa₂Cu₃O_y (LRE: Sm, Gd, NEG) “LRE-123” pellets are developed in air and Ar-1% O₂ using a novel thin film Nd-123 seeds grown on MgO crystals. The SEM and XRD results conformed that the quality and orientation of the seed crystals are excellent. On the other hand, new seeds can withstand temperatures >1100 °C, as a result, the cold seeding process was applied even to grow Sm-123 material in Air. The trapped field observed in the best 45 mm single-grain puck of Gd-123 was in the range of 1.35 T and 0.35 T at 77.3 K and 87.3 K, respectively. The average trapped field at 77.3 K in the 24 mm diameter NEG-123 samples batch lies between 0.9 and 1 T. The maximum trapped field of 1.2 T was recorded at the sample surface. Further, the maximum trapped field of 0.23 T at 77 K was recorded in a sample with 16 mm diameter of Sm-123 with 3 mol% BaO₂ addition. As a result we made more then 130 single grain pucks within a couple of months. Taking advantage of the single grain batch processed material, we constructed self-made chilled levitation disk, which was used on the open day of railway technical research Institute. More then 150 children stood on the levitation disk and revel the experience of levitation. The present results prove that a high-performance good-quality class of LREBa₂Cu₃O_y material can be made by using a novel thin film Nd-123 seeds.     

Effects of ultrasound and temperature on copper electro reduction in Deep Eutectic Solvents (DES)

This paper concerns a preliminary study for a new copper recovery process from ionic solvent. The aim of this work is to study the reduction of copper in Deep Eutectic Solvent (choline chloride–ethylene glycol) and to compare the influence of temperature and the ultrasound effects on kinetic parameters. Solutions were prepared by dissolution of chloride copper salt CuCl₂ (to obtain Copper in oxidation degree II) or CuCl (to obtain Copper in oxidation degree I) and by leaching metallic copper directly in DES. The spectrophotometry UV–visible analysis of the leached solution showed that the copper soluble form obtained is at oxidation degree I (Copper I). Both cyclic voltammetry and linear voltammetry were performed in the three solutions at three temperatures (25, 50 and 80 °C) and under ultrasonic conditions (F = 20 kHz, PT = 5.8 W) to calculate the mass transfer diffusion coefficient kD and the standard rate coefficient k°. These parameters are used to determine that copper reduction is carried out via a mixed kinetic-diffusion control process. Temperature and ultrasound have the same effect on mass transfer for reduction of Cu^II/Cu^I. On the other hand, temperature is more beneficial than ultrasound for mass transfer of Cu^I/Cu. Standard rate constant improvement due to temperature increase is of the same order as that obtained with ultrasound. But, by combining higher temperature and ultrasound (F = 20 kHz, PT = 5.6 W at 50 °C), reduction limiting current is increased by a factor of 10 compared to initial conditions (T = 25 °C, silent), because ultrasonic stirring is more efficient in lower viscosity fluid. These values can be considered as key-parameters in the design of copper recovery in global processes using ultrasound.