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The terahertz response of a slot diode with a two-dimensional electron channel is calculated on the basis of the first principles of electromagnetism. It is shown that all characteristic electromagnetic lengths (scattering, absorption, and extinction lengths), as well as the impedance of the diode, exhibit resonances at plasmon excitation frequencies in the channel. The fundamental resonance behaves similarly to the current resonance in an RLC circuit. It has been concluded that, even at room temperature, a slot diode with a two-dimensional electron channel provides a resonant circuit at terahertz frequencies that couples effectively to external electromagnetic radiation with a loaded Q-factor exceeding unity. The diode resistance may be measured from contactless measurements of the characteristic electromagnetic lengths of the diode. [ABSTRACT FROM AUTHOR]

The formation of ZrO2 nanocrystals during heat treatment of amorphous ZrO(OH)2 is studied. The results indicate that the high-temperature phases of ZrO2 (T-ZrO2 and trace amounts of G-ZrO2) can be produced and stabilized at 350-375°C in nanoparticles with a size of coherently scattering domains on the order of 20 nm. Heating to above 500-600°C leads to the formation and growth of the stable phase M-ZrO2. The nature of the starting zirconium salt is shown to have a significant effect on the thermal decomposition of zirconium hydroxides. [ABSTRACT FROM AUTHOR]

The crystal/local structure and morphology of hydrated molybdenum oxides particles obtained by acidic precipitation of molybdates solutions were studied using X-ray diffraction, X-ray absorption spectroscopy, scanning electron microscopy, Raman and IR spectroscopy. The precipitation of ammonium heptamolybdate solutions (pH ​= ​1.00) at room temperature result in the formation of poorly crystallized hydrated molybdenum oxides. At the same time, acidification of the sodium molybdate solutions does not lead to the appearance of precipitates for several days. Heating to 90 ​°C and aging the reaction suspension prepared from both ammonium heptamolybdate and sodium molybdate results in the formation of prismatic h-MoO 3 particles with the hexagonal crystal structure. It was shown that distorted octahedra MoO 6 are the basic structural units in the formed hydrated molybdenum oxides. Along with the presence of Mo O and Mo–O bonds, the presence of water molecules, hydroxyl ions, and N H 4 + (or Na+) ions was detected in the structure of hydrated MoO 3. The type of Mo salt and acid significantly affects the morphology of h-MoO 3 particles. Large h-MoO 3 particles are formed by aggregation of small ones and Ostwald ripening due to the "dissolution - precipitation" process. SEM micrographs of hydrated molybdenum oxides particles prepared from ammonium heptamolybdate tetrahydrate and sodium molybdate dihydrate under different precipitation conditions. [Display omitted] • The formation of hydrated MoO 3 particles prepared in aqueous solutions was studied. • A combination of XRD, XANES/EXAFS, SEM, Raman and IR spectroscopy were used. • The local structure of MoO 3 involves distorted MoO 6 octahedra with Mo O and Mo–O bonds. • The type of Mo salt and acid significantly affects the morphology of h-MoO 3 particles. • Large h-MoO 3 particles are formed by aggregation of small ones and Ostwald ripening. [ABSTRACT FROM AUTHOR]

The effect of the lanthanide cation type and calcination temperature on the structure and phase transitions in Ln 2 (MoO 4) 3 molybdates was studied using synchrotron X-ray diffraction, infrared and Raman spectroscopy, photoluminescence, simultaneous thermal analysis and inductively coupled plasma atomic emission spectroscopy. It was found that the crystallization of the precursors at 500∘ C/3 h results in the formation of nanocrystalline powders with the monoclinic structure (sp. gr. I 112/ b (15)) for Ln 2 (MoO 4) 3 (Ln = La–Ho), and a mixture of orthorhombic (sp. gr. Pnca (60)) and tetragonal (sp. gr. I 4 1 / a (88)) phases for Yb 2 (MoO 4) 3. A further increase in temperature to 1000∘ C leads to a significant rearrangement of the crystal structure, which is largely determined by the lanthanide type. The diagram of phase relations in the Ln 2 (MoO 4) 3 molybdates as a function of the Ln 3+ cation radius and the calcination temperature has been refined. According to the results of Raman and IR spectroscopy, MoO 4 tetrahedra are the main structural units in Ln molybdates. The increased hygroscopicity of heavy lanthanide molybdates (Ln = Ho, Yb) is associated with the presence of empty cavities in the orthorhombic structure. The photoluminescence spectra have shown that EuO n polyhedra in Eu 2 (MoO 4) 3 are distorted, and Eu3+ ions occupy several nonequivalent crystallographically different sites with a low symmetry point group. Image 1 • Phase transitions in Ln molybdates (Ln = La, Pr, Nd, Eu, Gd, Dy, Ho, Yb) were studied. • A combination of s-XRD, IR, Raman spectroscopy, PL, and STA methods were used. • The ordering of both cationic and anionic sublattices in the Ln 2 (MoO 4) 3 was observed. • A diagram of Ln 2 (MoO 4) 3 crystal structure vs. Ln 3+ cation radius was presented. [ABSTRACT FROM AUTHOR]

Abstract: The layers’ structure, interlayer boundaries and magnetic properties of Co/CoO films sputtered on single-crystal MgO(100) and Al2O3(110) substrates have been investigated. The relationship between the structure and magnetic properties is analyzed. The NGR investigations of 57Co atomic probes on the Co film surface and Co/CoO interface have been carried out. [Copyright &y& Elsevier]

Amorphous-to-crystalline and fluorite-to-pyrochlore phase transitions in complex oxides Ln 2 M 2 O 7 (Ln = Gd, Tb, Dy; M = Ti, Zr) prepared by sol-gel and coprecipitation methods were studied using x-ray absorption spectroscopy in combination with x-ray diffraction and Raman spectroscopy. The Ln titanates crystallize directly from the amorphous state to the pyrochlore structure, while the Ln zirconates form the intermediate fluorite phase first. We have found that both XANES and EXAFS data reflect the considerable difference between the crystallization processes of Ln titanates and zirconates. In particular the evolution of the K –Ti pre-edge features is a good indicator of the phase transition from amorphous to crystalline state. The splitting of the first Ln -O shell in the FT modulus of L 3 - Ln edge EXAFS spectra in Ln titanates can be regarded as a fingerprint of the ordered pyrochlore structure formation which is accompanied also by specific changes in the main features of the XANES spectra. For the Ln zirconates these features are less pronounced. • EXAFS spectra in Ln titanates provide detection of the ordered pyrochlore structure. • K–Ti XANES spectra reflect an evolution of the Ln titanates electronic structure. • Splitting of the first Ln-O coordination shell is less pronounced for Ln zirconates. [ABSTRACT FROM AUTHOR]

The structure of Nb filaments of different geometry artificially doped with NbTi alloy in Nb/Cu-Sn composites after various regimes of deformation beginning from the very first stages of fabrication of thin multifilamentary wires has been studied by transmission electron microscopy, optical microscopy and X-ray analysis. The structure of Nb filaments in composites of different designs is compared, and the development of dislocation and grain structure and texture of niobium in monolithic and assembled filaments under deformation with intermediate annealings is analyzed. • Assembled Nb filaments have better conformity with bronze in Nb/Cu-Sn composites. • Grain structure develops faster in assembled Nb filaments than in monolithic. • Application of assembled filaments ensures grain refinement in Nb. • Microhardness and dislocation density are higher in assembled Nb filaments. [ABSTRACT FROM AUTHOR]

The influence of the cation radii ratio (γ = 1.37–1.61) and temperature of the isothermal annealing on the specific features of fluorite – pyrochlore phase transition in Ln 2 O 3 : ZrO 2 = 1:2 (Ln = La, Nd, Tb, Yb, and Y) systems was investigated by means of Neutron (NPD) and X-ray (XRD) powder diffractions as well as Raman spectroscopy. It was shown that with a decrease in the radius of the Ln cation in the La → Nd → Tb → Y → Yb series, the following sequence of phase transitions is observed: the pyrochlore structure → the structure of defect fluorite → the rhombohedral δ-phase. Significant advantages of NPD over XRD are demonstrated for determining the structural features of the anion sublattice. A detailed description of the parameters of the vibrational modes in Raman spectra for samples with a pyrochlore type of ordering is given, depending on the refined structural parameters. Image 1 • Phase transitions in Ln zirconates (Ln = La, Nd, Tb, Yb and Y) were investigated. • A combination of XRD, NPD and Raman spectroscopy methods were used. • The advantage of neutron diffraction for crystal structure analysis is demonstrated. • The ordering of both anionic and cationic sublattices in the Ln 2 Zr 2 O 7 was studied. • An analysis of the Raman modes using refined structural parameters is carried out. [ABSTRACT FROM AUTHOR]

We studied powder samples of europium titanates Eu 1− x 2+Eu x 3+TiO 3+ x /2 prepared by sol-gel and coprecipitation methods with subsequent thermal treatment of precursors at different conditions (reducing or oxidizing atmosphere, temperature, exposure time). We have revealed a radical rearrangement in the local, electronic and crystal structure of europium titanates depending on synthesis conditions. The combination of synchrotron X-ray diffraction (s-XRD), X-ray absorption (XANES and EXAFS), infrared and Raman spectroscopies supplemented by photoluminescence and simultaneous thermal analysis was used in our study. It is shown that the combined XANES and thermogravimetric analysis gives clear evidence of a change in the oxidation state of europium cations from Eu2+ to Eu3+. The comparison of s-XRD, XANES, and EXAFS data allowed us to explain the local structure rearrangement during the phase transition from cubic perovskite Eu2+TiO 3 (space group P m 3 ¯ m) to pyrochlore phase Eu 2 3+Ti 2 O 7 (space group F d 3 ¯ m) through intermediate layered perovskite Eu 2 3+Ti 2 O 7 with monoclinic structure (space group P 2 1) under annealing in oxidizing atmosphere. The Raman spectroscopy study pointed to significant changes in the local structure of the anionic sublattice upon the observed phase transitions. The analysis of luminescence spectra has shown that Eu3+ cations occupy positions with low local site symmetry in all synthesized compounds regardless of the type of the crystal structure. Image 1 • Local, electronic and crystal structure rearrangement in Eu titanates was studied. • s-XRD, XAFS, IR, Raman spectroscopy, TGA and PL methods were used. • XANES and TGA are effective tools for determining the Eu2+/Eu3+ cations ratio. • Eu3+ cations occupy low local site symmetry positions in synthesized compounds. [ABSTRACT FROM AUTHOR]

We report on the resonant, voltage tunable emission of terahertz radiation (0.4–1.0 THz) from a gated two-dimensional electron gas in a 60 nm InGaAs high electron mobility transistor. The emission is interpreted as resulting from a current driven plasma instability leading to oscillations in the transistor channel (Dyakonov–Shur instability). © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This study is inspired by the potential of application-designed aluminium-copper conductors. This work combines recently discovered advantages of hybrids with one constituent having a helical architecture with the benefits provided by severe plastic deformation (SPD) methods. The hybrids are made by embedding copper helixes in melted aluminium and subjecting cast hybrid ingots to different SPD techniques. The electrical conductivity, microstructure features and strength of the produced samples are discussed in relation to effect of SPD and annealing on both constituents and an interface zone formed during the hybrids' production. It was shown that between all processing techniques the reciprocal extrusion (RE) has the great potential for production of lightweight conductors with high conductivity and enhanced strength. A new model describing the effective electrical conductivity of hybrid samples, consisting of an aluminium matrix with an embedded copper helix and intermetallic containing interface, is developed and justified by experimental data. The model is shown to be instrumental for analysis of the effect of the helix's parameters and interface width on effective conductivity of the hybrid samples and could be used for optimal design of hybrid conductors. Unlabelled Image • The new design for Al Cu conductor with copper used as helical wire fully embedded in the aluminium matrix is suggested. • Samples with helical reinforcement exhibit increased load-bearing capability and higher strain hardening during deformation. • The SPD processing is important for strengthening of the hybrid due to grain refinement and dissolution of intermetallic. • New analytical model predicting the electrical conductivity of hybrids and accounting for intermetallic layer is developed. • The model depends on two parameters determined by copper volume fraction and the geometry of the helical constituent. [ABSTRACT FROM AUTHOR]