Properties of Mg(Fe<Subscript>1 − <Emphasis Type=Italic>x</Emphasis></Subscript>Ga<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>)<Subscript>2</Subscript>O<Subscript>4 + δ</Subscript> solid solutions in stable and metastable states

Using pyrohydrolytic synthesis, we have obtained a continuous series of Mg(Fe_{1 − x} Ga _x )₂O_{4 + δ} solid solutions, as checked by X-ray diffraction. The magnetization and coercivity of the samples have been determined from field dependences in applied magnetic fields of ±5 T, and their conductivity has been assessed using current-voltage measurements at 300 K in electric fields from −200 to +200 V. The effective band gap of the solid solutions has been evaluated from their absorption spectra obtained by diffuse reflectance measurements. The optimal composition for spintronic applications is Mg(Fe_0.8Ga_0.2)₂O_{4 + δ}.     

Potassium ion conducting K<Subscript>1 − 2<Emphasis Type=Italic>x</Emphasis></Subscript>Cd<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>FeO<Subscript>2</Subscript> solid electrolytes

New potassium ion conducting solid electrolytes based on potassium monoferrite have been prepared through partial substitution of the divalent cation Cd²⁺ on the potassium site, and their properties have been investigated. The introduction of cadmium cations sharply increases the electrical conductivity of KFeO₂ over the entire temperature range studied. In addition to the maximum in conductivity at the boundary of the K_{1 − 2x} Cd _x FeO₂ solid solution, there is a maximum at a higher cadmium content (x = 0.30–0.35). The possible origins of this maximum are discussed.     

Effects of Out-of-Plane Disorder in CaFe<Subscript>1−<Emphasis Type=Italic>x</Emphasis></Subscript>Co<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>AsF System

We perform oxygen doping experiments in the CaFeAsF and CaFe_0.88Co_0.12AsF compounds. In the parent compound CaFeAsF, partial replacement of F by O leads to an increase of resistivity and a weakening of spin-density wave transition, indicating that the out-of-plane disorder strongly affects the electronic and magnetic properties of the system. In the CaFe_0.88Co_0.12AsF_1−x O _x system, the doping of oxygen leads to the suppression of superconductivity, reflecting the importance of Ca–F layer disorder in this system.     

Phase structure and vibrational spectra of rare-earth-oxide ceramics of Dy<Subscript>2(1−<Emphasis Type=Italic>x</Emphasis>)</Subscript>Tm<Subscript>2<Emphasis Type=Italic>x</Emphasis></Subscript>O<Subscript>3</Subscript>

Rare-earth-sesquioxide ceramics have been found to possess potential applications in solid-state lasers due to their excellent physical and chemical properties as well as low cost. In this paper, composite powders with the composition of Dy_2(1−x)Tm_2x O₃ were prepared by ball milling method and corresponding ceramics were obtained using the pressureless sintering technique. Phase structure and vibrational spectra were investigated using X-ray diffraction, Raman spectrometer, and FT-IR spectrometer. It is shown that the mixture of Dy₂O₃ and Tm₂O₃ converts to an ordered solid solution of body-centered cubic structure after heat treatment at 1,100 °C for 4 h. It is also found that the cell constants of ceramics decrease linearly with the increase of Tm₂O₃ content. Raman spectra analysis demonstrates that bond length plays a major role in determining the frequencies of Raman bands at high-frequency range and that peak positions exhibit a blue shift with the increase of Tm₂O₃ content due to decreasing cell constant. Similar phenomenon is also observed in infrared spectra, which shows linearly increasing infrared band frequency with decreasing cell volume. The ball milling method used for preparing composite powders and vibrational spectra analysis in this work provide some important references for the study of laser ceramics containing Dy₂O₃ and Tm₂O₃.     

Doping Dependence of Phase Coherence Between Superconducting Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript><Emphasis Type="Bold">O</Emphasis><Subscript>8+<Emphasis Type="Italic">δ</Emphasis></Subscript> Grains

In the present work, we report the new findings on the doping level dependence of the phase coherence between superconducting Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) grains. The experimental results from the strongly underdoped and overdoped regimes deviated from the expectation based on the doping level dependence of the superfluid density at T = 0K. These findings appear to be governed by interplay between competing orders inside the superconducting dome of cuprate superconductors. Two quantum critical points are likely to exist at the underdoped and overdoped regimes beneath the superconducting dome.     

Misfit dislocations and stress in In<Subscript>1 − <Emphasis Type=Italic>x</Emphasis></Subscript>Ga<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>As/GaAs heterostructures

We have estimated the elastic properties of In_{1 − x} Ga _x As/GaAs heterostructures and the characteristics of misfit dislocations in such heterostructures: misfit dislocation spacing, Burgers vector length in various interfaces, surface density of dangling bonds, film/substrate interface energy, critical film thickness below which pseudomorphic growth is possible without misfit dislocations, elastic strain energy of the film-substrate system, average elastic strain of a thin-film island as a function of its radius, thermal stresses induced by the thermal-expansion and lattice mismatches between the layers in contact, and crack length in the film.     

Sb-doped CuAlO<Subscript>2</Subscript>: widening of band gap and nonlinear <Emphasis Type=Italic>J</Emphasis>–<Emphasis Type=Italic>E</Emphasis> characteristics

The effect of antimony doping on the optical and electrical properties of copper aluminum oxide synthesized by solid state reaction technique was investigated. Formation of single phase CuAlO₂ was confirmed by x-ray diffraction studies. Variation of strain, particle size, and preferred orientation of the crystallites with Sb doping concentrations were analyzed by x-ray diffraction. Using the x-ray photoelectron spectroscopy trivalent state of Sb ions was confirmed. A blue shift in the energy band gap with Sb doping was observed, and it was supported by ab initio calculation based on density functional theory. A nonlinear current–voltage characteristic was observed for the Sb-doped CuAlO₂ samples whereas the same was linear for the undoped samples. The nonlinearity of the current–voltage characteristics changed significantly at higher temperature, and this behavior was explained.     

Scaling of the Temperature Dependent Resistivity and Hall Effect in Ba(Fe<Subscript>1−<Emphasis Type=Italic>x</Emphasis></Subscript>Co<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>)As<Subscript>2</Subscript>

We show that the temperature-dependent resistivity ρ(T), Hall number n _H(T) and the cotangent of the Hall angle cot θ _H(T) of Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2) can be scaled using a recently proposed model-independent scaling method (Luo et al. in Phys. Rev. B 77:014529, 2008). The zero field normal-state resistivity above T _c can be reproduced by the expresion r(T) = r₀ +cTexp(- frac2varDelta T )rho(T) = rho_{0} +cTexp(- frac{2varDelta }{T} ) and scaled using the energy scale Δ, c and the residual resistivity ρ ₀ as scaling parameters. The scaling parameters have been calculated and the compositional variation of 2Δ and ρ ₀ has been determined. The 2Δ(x) dependence show almost linear decreasing in underdoped regime, minimum corresponding to the T _c maximum and increasing in overdoped regime. The latter is different from that reported for cuprates. The existence of a universal metallic ρ(T) curve which, however, is restricted for the underdoped compounds to temperatures above a structural and antiferromagnetic transition is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2).     

Thermoelectric power factor of Ti<Subscript>1 − <Emphasis Type=Italic>x</Emphasis></Subscript>V<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>NiSn alloys

Ti_{1 − x} V _x NiSn (x = 0–0.10) substitutional solid solutions have been prepared by doping the intermetallic semiconductor n-TiNiSn (half-Heusler phase) with vanadium, a donor impurity, and their resistivity and thermopower have been measured at temperatures from 80 to 380 K. The results demonstrate that, when doping of TiNiSn causes no type inversion, the thermoelectric power factor of the solid solution markedly exceeds that of the undoped ternary compound.     

Critical Current Densities and Vortex Dynamics in Ba(Fe<Subscript>1−<Emphasis Type=Italic>x</Emphasis></Subscript>Co<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Superconducting properties are evaluated for high-quality single crystals of Ba(Fe_1−x Co _x )₂As₂ in a wide range of doping levels. The critical current density, J _c , in an optimally-doped crystal (T _c ∼24 K) shows a fishtail effect with its value over 10⁵ A/cm² even at 5 T below 10 K. Magneto-optical imaging has clarified rather homogeneous supercurrent flow in the crystal, in spite of a large amount of impurities. In the heavy-ion irradiated sample, the presence of columnar defects are confirmed and J _c has been enhanced by a factor of five at low temperatures, reaching 6×10⁶ A/cm² at 2 K under zero field. Flux creep rate in the heavy-ion irradiated sample has been reduced in accordance with the enhancement of J _c .     

A Study of the Thermovoltaic Effect in Heterostructures Based on Sm<Subscript>1–<Emphasis Type=Italic>x</Emphasis></Subscript>Gd<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>S Solid Solutions

The influence of doping of samarium monosulfide with gadolinium has been investigated. Experiments have been carried out in the temperature range of 300–450 K. A decrease in the effect value with increasing gadolinium content in the Sm1–xGdxS heterostructure with x ranged from 0 to 0.13 has been found and explained. Formulas for calculating the output signal in dependence of the doping value have been derived.     

Lead adsorption study on combustion derived <Emphasis Type=Italic>γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> surface

New combustion synthetic route for the synthesis of nanosized γ-Fe₂O₃ by microwave-assisted route is reported. X-ray density, tap density and powder density of prepared γ-Fe₂O₃ are calculated. Adsorption study of Pb²⁺ on combustion derived nanosized γ-Fe₂O₃ is studied by dynamic method. The γ-Fe₂O₃ structure and lead adsorbed γ-Fe₂O₃ (Pb-γ-Fe₂O₃) are studied by X-ray diffraction (XRD). Additional lead peaks in Pb-γ-Fe₂O₃ sample pattern confirm the lead adsorption. Morphology of as prepared γ-Fe₂O₃ and Pb-γ-Fe₂O₃ is studied by scanning electron micrograph (SEM) technique. Varied morphology for Pb-γ-Fe₂O₃ compared to its γ-Fe₂O₃ is observed. Variation of bonding in Pb-γ-Fe₂O₃ sample due to lead adsorption is viewed by infrared spectroscopic (IR) technique. Energy dispersive X-ray microanalysis (EDX) is scanned for the lead adsorbed γ-Fe₂O₃ to know the presence of lead on γ-Fe₂O₃ surface. The eluent lead solution is characterized by atomic absorption spectroscopy (AAS) and solution conductivity (SC). Reduction in the concentration and increase in conductance of eluent lead solution is observed. The potential use of solid adsorbents for the adsorption of heavy metal pollutants is envisaged in the present work.     

Control over the Size Effect in the Spectroscopic Properties of Zn<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>Cd<Subscript>1 – <Emphasis Type=Italic>x</Emphasis></Subscript>S Colloidal Quantum Dots

Approaches are developed for controlling the size effect in the absorption and photoluminescence properties of colloidal CdS and Zn_0.5Cd_0.5S quantum dots (QDs) in a gelatin matrix and thioglycolic acid. We analyze spectroscopic manifestations of dark Ostwald ripening and size-selective photoetching and demonstrate that the illumination of molten colloidal CdS and Zn_0.5Cd_0.5S QD solutions at a wavelength lying at the optical absorption edge in the presence of hydrogen peroxide as a catalyst for photoetching leads to a blue shift of the optical absorption and photoluminescence spectra of the QDs. Analysis of TEM images of QD samples leads us to conclude that photoetching reduces the average QD size as a consequence of the photodissolution of the CdS and Zn_0.5Cd_0.5S compounds. We demonstrate that, by contrast, heat treatment of the synthesized CdS and Zn_0.5Cd_0.5S QD sols at a temperature between 75 and 90°C leads to an increase in nanocrystal size through recrystallization, which shows up as a redshift of the optical absorption and photoluminescence spectra of the corresponding samples.     

Structural,optical and electrical properties of ZnTe<Subscript>1−<Emphasis Type=Italic>x</Emphasis></Subscript>Se<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript> thin films

ZnTe_1−x Se _x films were deposited on glass substrates kept at 200 °C by the electron beam evaporation technique. These films exhibited cubic structure and the lattice parameter increased with increase of Tellurium concentration in the films which confirmed the solid solution formation. The grain size is found to increase with Te content. The dislocation density and lattice strain show a decreasing trend with increasing of Te content. Band gap values of 2.73 eV, 2.63 eV, 2.52 eV and 2.41 eV have been calculated for the films of composition ‘x’ = 0.2, 0.4, 0.6 and 0.8, respectively, which confirmed the formation of solid solution between ZnSe and ZnTe. Refractive index of the films increased from 2.535 to 2.826 as the concentration of Te increased. All the films showed high resistivity values. Laser Raman spectral studies of ZnTe_1−x Se _x revealed LO phonon frequencies whose values are located in between the LO phonon frequencies of ZnSe and ZnTe.     

Electron diffraction study of TlGa<Subscript>1 − <Emphasis Type=Italic>x</Emphasis></Subscript>Ge<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>X<Subscript>2</Subscript> (X = S,Se, Te) nanofilms

The formation of thin TlGa_{1 − x} Ge _x X₂ (X = S, Se, Te) films has been studied by high-energy electron diffraction. The results demonstrate that the dopant (Ge) concentration influences the structure of the films (30 nm in thickness). The epilayer-substrate orientation relationships are identified.     

Probing the Superconducting Gap from Tunneling Conductance on NdFeAsO<Subscript>0.7</Subscript> with <Emphasis Type=Italic>T</Emphasis><Subscript>C</Subscript>=51 K

We report the tunneling spectroscopy of an iron-based oxypnictide NdFeAsO_0.7 with T _C=51 K, measured by a mechanical point contact technique. Mainly two kinds of tunneling spectra have been observed reproducibly. One is tunneling conductance displaying sharp superconducting gap peaks at 6.0±1.0 mV, in which hump (or kink) structures are also observed at 20–30 mV. Another is that showing dominantly the larger superconducting gap Δ _L with sharp conductance peaks at 14±1.0 meV, in which the trace of a smaller gap (Δ _S=5–7 meV) is simultaneously observed. Our results give direct evidence for the existence of multiple gaps in the quasiparticle excitation spectrum of this multiband system, although the origin of the hump at 20–30 mV is still unclear.     

Synthesis and laser-excited IR luminescence of Y<Subscript>1 − <Emphasis Type=Italic>x</Emphasis></Subscript>Yb<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>PO<Subscript>4</Subscript> solid solutions

We have determined the composition range of solid solutions in the binary system YPO₄-YbPO₄ and identified the key trends in the composition dependence of their IR Stokes luminescence intensity and decay time in the range 0.96–1.1 μm. The results were used to develop a new narrow-band IR phosphor, YPO₄:Yb³⁺. Under 0.940-μm laser excitation, this phosphor offers an increased IR Stokes luminescence intensity in the range 0.96–1.1 μm (about twice as high as that of the well-known commercially available phosphors L-54 and FSD-546-2), with an afterglow time of 1700–1800 μs.