Optical transmittance and microgravimetric studies of the oxidation of 〈100〉 single crystal films of copper

Thin single crystal copper films have been grown and oxidized on (100) faces of cleaved sodium chloride discs suspended from a vacuum ultramicrobalance. Optical transmittance measurements between 400–800 nm and electron microscopic investigations were also used to characterize the oxidation process. Polycrystalline copper films grown at room temperature are substantially the same as those grown previously on glass substrates. Single crystalline growth at 325 ° C on rock salt produces a characteristic transmittance curve due to the “island” nature of the films. These curves compare favorably with other previously published results. Single crystal copper films oxidized to CuO_0.67 at temperatures of 117–159°C in 100 Torr of oxygen for films less than 500 A thick. For films 378 to 1000 Å thick, compositions of CuO_0.52 to CuO_0.62 were obtained between 123–176°C. The oxidation to less than CuO_0.67 is attributed to the existence of islands in these films which are thicker than the average film thickness, and require higher temperatures or thinner films to permit oxidation to CuO_0.67 before the nucleation of CuO sets in.     

Origin of large spin Hall magnetoresistance in Fe/CuOx bilayers

We have compared the spin Hall magnetoresistance (SMR) in Fe/Pt and Fe/CuO_x (with natural oxidation) bilayers with varying the thickness of Fe layer. A larger SMR in Fe/CuO_x bilayers has been found when the thickness of Fe layer is 3 nm. Moreover, the SMR of the two bilayers decrease with increasing the thickness of Fe from 3 nm to 10 nm, but that of Fe/CuO_x drops more sharply due to shunting current effect. Through harmonic measurements, the emergent spin current is proved to be generated in the Fe/CuO_x bilayers. The mixed phase of CuO_x has been confirmed including CuO, Cu₂O and Cu, which performs strong spin-orbit coupling and produce large spin current. On the other hand, the interface-generated spin current should be ruled out. All the results have been compared with those in Fe/Al₂O₃ bilayers with negligible spin current.     

X-Ray Spectral Studies of the Interface Interaction in CuO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/MWCNTs Nanocomposite

Results of a comprehensive study of the interface interaction of a nanostructured CuO_x and multiwalled carbon nanotubes (MWCNTs) in CuO_x/MWCNT nanocomposite by X-ray absorption spectroscopy (XANES, NEXAFS) and X-ray photoelectron spectroscopy (XPS) methods using a synchrotron radiation are presented. It is established that a nanostructured CuO_x in CuO_x/MWCNT nanocomposite is predominantly formed by CuO and has the form of flakelike particles 200–500 nm in size uniformly dispersed over an array of nanotubes. A chemical interaction of CuO_x and nanotubes with formation of covalent carbon–oxygen bonds, which does not lead to a significant destruction of the outer layers of carbon nanotubes, is observed at the interfaces of the nanocomposite.     

On the interpretation of the phase diagram of La<Subscript>2−x</Subscript>M<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

The difference between the phase diagrams of La_2?x(Ba,Sr)_xCuO₄ and Nd_2?xCe_xCuO₄ is discussed. It is proposed that the discrepancy of x-values corresponding to the transition from antiferromagnetic dielectric state to conducting one (respectively, x≈0.06 in La_2?x(Ba,Sr)_xCuO₄ and x≈0.125 in Nd_2?xCe_xCuO₄) results from non-homogeneous doping of La_2?x(Ba,Sr)_xCuO₄ over the range 0.06<x<0.125, when localized holes are added to each second CuO₂ layer. Therefore the actual phase diagram of the “holedoped” superconductor La_2?x(Ba,Sr)_xCuO₄ coincides with one for the “electron-doped” superconductor Nd_2?xCe_xCuO₄. It is shown that all features observed in La_2?x(Ba,Sr)_xCuO₄ around x=0.125 are clarified on this basis.     

Local dynamic deformation of the superconducting CuO2 plane in the Nd2 − x Ce x CuO4 + δ compound

The local structure of the electron-doped high-temperature superconductor Nd_{2 ? x} Ce _x CuO_{4 + δ} has been investigated by EXAFS spectroscopy using synchrotron radiation in the temperature range 5–300 K. The radii, coordination numbers, and Debye-Waller factors of the nearest coordination spheres of the copper, neodymium, and cerium local environment have been obtained. The Einstein temperatures, characterizing the strength of Cu-O, Nd-O, and Ce-O interatomic bonds have been determined. The dynamic local deformation of the superconducting CuO₂ plane in the form of vibrations of some part of oxygen ions in a double-well potential due to the difference in the electronic filling of neighboring CuO₄ complexes has been established.     

New class of T′-structure cuprate superconductors

High-temperature superconductivity has been discovered in La_2−xBa_xCuO₄ [J.G. Bednorz, K.A. Müller, Z. Phys. B 64 (1986) 189. [1]], a compound that derives from the undoped La₂CuO₄ crystallizing in the perovskite T-structure. In this structure oxygen octahedra surround the copper ions. It is common knowledge that charge carriers induced by doping in such an undoped antiferromagnetic Mott-insulator lead to high-temperature superconductivity [V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794; C.M. Varma, S. Schmitt-Rink, E. Abrahams, Solid State Commun. 62 (1987) 681; E. Dagotto, Rev. Mod. Phys. 66 (1994) 763. [2], [3] and [4]]. The undoped material La₂CuO₄ is also the basis of the electron-doped cuprate superconductors [Y. Tokura, H. Takagi, S. Uchida, Nature (London) 337 (1989) 345. [5]] of the form La_2−xCe_xCuO_4+y [M. Naito, M. Hepp, Jpn. J. Appl. Phys. 39 (2000) L485; A. Sawa, M. Kawasaki, H. Takagi, Y. Tokura, Phys. Rev. B 66 (2002) 014531. [6] and [7]] which, however, crystallize in the so-called T′-structure, i.e. without apical oxygen above or below the copper ions of the CuO₂-plane. It is well known that for La_2−xCe_xCuO_4+y the undoped T′-structure parent compound cannot be prepared due to the structural phase transition back into the T-structure occurring around x∼0.05. Here, we report that if La is substituted by RE=Y, Lu, Sm, Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La, nominally undoped La_2−xRE_xCuO₄ can be synthesized by molecular beam epitaxy in the T′-structure. The second important result is that all these new T′-compounds are superconductors with fairly high critical temperatures up to 21 K. For this new class of cuprates La_2−xRE_xCuO₄, which forms the T′-parent compounds of the La-based electron doped cuprates, we have not been able to obtain the Mott-insulating ground state for small x before the structural phase transition into the T-structure takes place.     

High temperature superconductivity in (M1−xM′x)2CuO4−δ and related compounds (M = Y,La, Eu,Sm; M′ = Ba,Sr)

Compounds of the form (M_1−xM′_x)₂CuO_4−δ and related compounds where M and M′ are Y, various rare earths from La to Lu, and the alkaline earths Sr and Ba, have been investigated in connection with high temperature superconductivity. High temperature superconductivity is confirmed for the system (La_1−xBa_x)₂CuO_4−δ, (La_1−xSr_x)₂CuO_4−δ and (Y_1−xBa_x)₂CuO_4−δ with superconducting transition temperature T_c onsets of 30 K, 38 K and 90 K, respectively. We have found that the related systems (Eu_1−xBa_x)₂CuO_4−δ and (Sm_1−xBa_x)₂CuO_4−δ also exhibit high temperature superconductivity with T_c onsets of 95 K and 65 K, respectively. The highest T_c onset observed in this investigation was 97 K for a sample with the nominal composition of the spinel structure Y_0.33Ba_0.67Cu₂O_4−δ. Measurements of the specific heat C as a function of temperature T on a La_0.8Sr_0.2CuO_4−δ sample reveal a break in slope in the C/T vs T curve at the T_c midpoint, but no clearly discernable jump in C at T_c. A linear term ≈ λ′T in C was observed at low temperature in the superconducting state.     

Synthesis and characterization of LiMn1-x Fe x PO4/carbon nanotubes composites as cathodes for Li-ion batteries

Carbon nanotubes (CNT) coated with LiMn_1-x Fe _x PO₄ (0.2?≤?x?≤?0.8), as possible cathode materials, was synthesized by using a sol–gel process (Polyol method), after annealing under flowing nitrogen. X-ray diffraction (XRD) patterns of the composites confirmed the formation of the olivine structured LiMn_1-x Fe _x PO₄ phase and no secondary phases were detected. The morphological investigation revealed the formation of agglomerates with particles size ranging between 300 and 700 nm. XRD investigation of composites shows difference of the morphology by doping CNT and carbon black in the composites. Transmission electron microscopy shows the growth of nano-sized particles on CNT (20–70 nm) and the agglomeration of primary particles to form secondary particles. The X-ray photoelectron spectroscopy showed that the Fe and Mn ions are in divalent states in the LiMn_1-x Fe _x PO₄ composites. The cyclic voltamograms showed the oxidation peaks of iron and manganese ions at 3.53–3.63 and 4.05–4.33 V, respectively, while the reduction peaks were found at 3.21–3.42 V (iron reduction) and 3.85–3.93 V (manganese reduction) depending on the iron content in the composition. The LiMn_0.6Fe_0.4PO₄/CNT composite (x?=?0.4) (with 20 %?wt CNT) delivered a specific capacity of 120 mAhg^?1 (at a discharge rate of C/20 and RT).     

Reactance of the n-Au/p-La0.67Ca0.33MnO3 film contact

The responses of the resistance and reactance of Au/(20 nm)La_0.67Ca_0.33MnO₃ film heterostructures to temperature variation and magnetic field (f = 100 kHz) are investigated. At T = 300 K, the capacitance per unit area of the interface between a gold contact and the Au/(20 nm)La_0.67Ca_0.33MnO₃ epitaxial film is found to be about 1 μF/cm². The maximum value of the negative magnetoreactance (≈60% at μ₀ H = 0.4 T) of the heterostructures is almost twice as high as the extremal value of the active magnetoresistance at T ≈ 235 K. The effective depth of magnetic field penetration into the manganite film on the side of the gold contact deposited on its surface is about 3 nm at room temperature.     

Theory for the Gaussian Component of the 63Cu Nuclear Spin-Echo Decay Rate 1/T 2G in La2−x Sr x CuO4

We report calculations of the Gaussian component of the ⁶³Cu nuclear spin-echo decay rate 1/T _2G employing the theory for spin susceptibility as derived within the t-J model starting from carrier-free La₂CuO₄ and right up to optimally doped superconducting layered copper oxide La_2?x Sr _x CuO₄. The theory reproduces the temperature and doping behavior of the experimental data for 1/T _2G. A quantitative agreement with experimental 1/T _2G data in doped La_2?x Sr _x CuO₄ compounds is obtained with account of both the spin–spin and “fermion”–“fermion” correlations.     

Intercalation and staging behavior in super-oxygenated La2CuO4 + δ

A high temperature electrochemical oxidation process has been used to produce large single crystals of La₂CuO_{4 + δ} suitable for neutron scattering experiments. Below room temperature the oxygen-rich phases have structural superlattice scattering peaks which indicate new periodicities ranging from 2 to 6.6 layers perpendicular to the copper oxide planes. A model structure originally proposed for La₂NiO_{4 + δ} can account for the superlattice peaks as a result of anti-phase domain boundaries between different tilt directions of the CuO₆ octahedra. Within this model, the changes in CuO₆ tilt directions are induced by segregated layers of interstitial oxygen which order in a manner similar to intercalants in graphite. This structural model thus clarifies previous work and establishes La₂CuO_{4 + δ} as a unique lamellar superconducting system with annealed disorder.     

The direct-indirect transition in In1−xGaxP

Data (300°K) are presented on laser quality liquid phase epitaxial (LPE) In_1?xGa_xP, grown on lattice-matched {100} GaAs_1?yP_y substrates, confirming a theoretical account presented by Altarelli that the direct-indirect transition occurs at x_c ≈ 0.74. Photoluminescence data show that crystals ranging in composition from x = 0.52 to x = 0.70, all of which operate as lasers at 77°K, exhibit the same decrease in luminescence intensity from 77 to 300°K, indicating that x_c > 0.70 at 300°K. The steep I–V characteristics (at 77 and 300°K) of Zn-diffused diodes prepared on crystals of composition x = 0.70 in contrast to the significantly more resistive behavior of x = 0.74 crystals is consistent also with the assignment x_c > 0.70. The Λ band edge (300°K), determined by the photoluminescence data points, intersects the X band edge (300°K) at x_c ≈ 0.74. A discussion, in terms of the problems inherent in In_1?xGa_xP crystals growth and quality, is presented to reconcile the difference in the crossover value (x_c ≈ 0.74) reported here on laser crystals and the lower value (x_c ≈ 0.63) of some other recent reports.     

Fabrication of cuprous and cupric oxide thin films by heat treatment

Cuprous oxide (Cu₂O) and cupric oxide (CuO) thin films were prepared by thermal oxidation of copper films coated on indium tin oxide (ITO) glass and non-alkaline glass substrates. The formation of Cu₂O and CuO was controlled by varying oxidation conditions such as, oxygen partial pressure, heat treatment temperature, and oxidation time. The microstructure, crystal direction, and optical properties of copper oxide films were measured with X-ray diffraction, atomic force microscopy, and optical spectroscopy. The results indicated that the phase-pure Cu₂O and CuO films were produced in the oxidation process. Optical transmittance and reflectance spectra of Cu₂O and CuO clearly exhibited distinct characteristics related to their phases. The electrical properties indicated that these films formed ohmic contacts with Cu and ITO electrode materials. Multilayers of Cu₂O/CuO were fabricated by choosing the oxidation sequence. The experimental results in this paper suggest that the thermal oxidation method can be employed to fabricate device quality Cu₂O and CuO films that are up to 200–300 nm thick.     

Effects of post-thermal annealing temperature on the optical and structural properties of gold particles on silicon suboxide films

In this work, silicon suboxide (SiO_x) thin films were deposited using a RF magnetron sputtering system. A thin layer of gold (Au) with a thickness of about 10 nm was sputtered onto the surface of the deposited SiO_x films prior to the thermal annealing process at 400 °C, 600 °C, 800 °C and 1000 °C. The optical and structural properties of the samples were studied using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and optical transmission and reflection spectroscopy. SEM analyses demonstrated that the samples annealed at different temperatures produced different Au particle sizes and shapes. SiO_x nanowires were found in the sample annealed at 1000 °C. Au particles induce the crystallinity of SiO_x thin films in the post-thermal annealing process at different temperatures. These annealed samples produced silicon nanocrystallites with sizes of less than 4 nm, and the Au nanocrystallite sizes were in the range of 7-23 nm. With increased annealing temperature, the bond angle of the Si-O bond increased and the optical energy gap of the thin films decreased. The appearance of broad surface plasmon resonance absorption peaks in the region of 590-740 nm was observed due to the inclusion of Au particles in the samples. The results show that the position and intensity of the surface plasmon resonance peaks can be greatly influenced by the size, shape and distribution of Au particles.     

Low-voltage cathodoluminescence property of Li-doped Gd2−xYxO3:Eu

Cathodoluminescent (CL) spectra of Li-doped Gd_2−xY_xO₃:Eu³⁺ solid-solution (0.0?x?0.8) were investigated at low voltages (300 V-1 kV). The CL intensity is maximum for the composition of x=0.2 and gradually reduces with increasing the amount of substituted Y content. In particular, small (∼100 nm) particles of Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ are obtained by firing the citrate precursors at only 650°C for 18 h. Relative red-emission intensity at 300 V of this phosphor is close to 180% in comparison with that of commercial red phosphor Y₂O₃:Eu³⁺. An increase of firing temperature to 900°C results in 400-600 nm sized spherical particles. At low voltages (300-800 V), the CL emission of 100 nm sized particles is much stronger than that of 400-600 nm sized ones. In contrast, the larger particles exhibit the higher CL emission intensity at high voltages (1-10 kV). Taking into consideration small spherical morphology and effective CL emission, Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ appears to be an efficient phosphor material for low voltage field emission display.     

Transport phenomena in metal nanocomposites

Ferromagnetic nanocomposites are the special case of metal composites; they are of practical interest for spintronics. Temperature dependences of resistivity ρ(T) and thermoelectric power α(T) of ferromagnetic nanocomposites with the composition Co _x (Al₂O₃)_{100 ? x} (36.6 ≤ x ≤ 52.5 at %) are investigated near the percolation threshold (x _p ≈ 43.3 at %) in a temperature range of 77–300 K. Sizes of Co nanoparticles are no larger than 25 nm. Specific features are observed in the dependences α(T) in the form of a kink at T _b ≈ 170 K. The analysis of the structural and electrical schematics as well as energy diagrams of percolation channels of electrons shows that only the diffusion thermoelectric power appears in Co nanoparticles, at which α(T) is the linear function. No mechanisms of the thermoelectric power caused by nanosizes of Co particles or by electron tunneling between them are found. The kink of the α(T) linear dependence is explained by the existence of the oxide shell of Co nanoparticles. It is assumed that the temperature dependences of energy barriers of oxide shells of metal nanoparticles (including ferromagnetic ones) in oxygen-containing dielectric matrices determine the features of α(T) and ρ(T) dependences of such nanocomposites.     

Spinel oxides growing on Fe-Cr alloy particles during plasma spraying

Oxidation reactions during plasma spraying of metallic powders give rise to oxide crusts on powder particle surfaces. The first oxidation stage occurs in flight of molten particles. It is usually followed by the second stage after hitting a substrate. To investigate the oxidation products immediately after the first stage, abrupt stopping of in-flight oxidation is possible by trapping and quenching the flying particles in liquid nitrogen. In oxide crusts on plasma sprayed and liquid nitrogen quenched particles of a Fe-12%Cr alloy, two spinel oxides were indicated by Mössbauer spectroscopy and X-ray diffraction. Both are solid solutions of the type Fe₃O₄ - Cr₃O₄ (i.e., Fe_3?x Cr _x O₄, 0 ≤x ≤ 3). One of the oxides, tetragonally distorted spinel, is characterized by the mean value ofx ≈ 2.3. It is only stable at very high temperatures. The other spinel oxide is cubic withx slightly lower than 2, i.e. almost stoichiometric chromite FeCr₂O₄. From thermodynamic considerations it follows that in the Fe₃O₄ - Cr₃O₄ system there is no miscibility gap at high temperatures. The simultaneous existence of both oxides is probably due to non-equilibrium conditions during liquid nitrogen quenching of trapped particles.     

Analytical TEM studies of Nd-Fe-B containing V,Co and Dy

Microstructural changes resulting from the addition of V, Co and Dy to Nd-Fe-B to yield the composition Nd_14.4Dy_1.6Fe₆₇Co₅V₄B₈ have been studied using transmission electron microscopy (TEM). Two types of precipitates ranging in size from ≈ 0.1 μm to several micrometers in diameter were observed embedded in the intergranular Nd-rich region and within the hard magnetic matrix grains. These two types of precipitates were round and lamellar, respectively. They were both borides (V boride and V-Fe boride) as shown by electron energy loss spectroscopy (EELS). TEM diffraction patterns revealed that the lamellar phase could be indexed as an orthorhombic structure with a = 0.291 nm, b = 0.278 nm and c = 0.7837 nm, which is close to VB and has not been reported before in V containing Nd-Fe-B material. The round particles have a tetragonal structure with a = 0.53 nm and b = 0.30 nm, which is similar to that of the V_2-x Fe_xB₂ phase reported previously. Co was found to form Nd₃Co precipitates in the Nd-rich boundary regions. The Co dissolved in the matrix phase is responsible for the increase of Curie temperature ( ≈ 354°C).     

Anomalous Hall effect in highly Mn-Doped silicon films

The transport and magnetic properties of Mn _x Si_{1 ? x} films with a high (x ≈ 0.35) content of Mn produced by laser deposition at growth temperatures of 300–350°C have been studied in a temperature range of 5–300 K in magnetic fields of up to 2.5 T. The films exhibit a hole-type metallic conductivity and a relatively weak change of magnetization in a temperature range of 50–200 K. An anomalous Hall effect with an essentially hysteretic behavior from 50 K up to ≈230 K has been discovered. The properties of the films are explained by the two-phase model, in which ferromagnetic clusters containing interstitial Mn ions with a localized magnetic moment are embedded in the matrix of a weak band MnSi_{2 ? x} (x ≈ 0.3) type ferromagnet with delocalized spin density.