Higher-order elastic constants of La1.84Sr0.16CuO4 high-temperature superconductor

Finite deformation theory is used to obtain the strain energy density of a tetragonal 2–1–4-type single crystal of the high-temperature superconductor La_2−xSr_xCuO₄. The complete set of second and third-order elastic constants of the high-temperature superconductor La_2−xSr_xCuO₄ (x = 0.16) is calculated by taking into account the interactions between nine nearest-neighbour atoms in the lattice and using Mie–Grüneisen interatomic potential. For the sake of comparison we have also computed the values of these constants for x = 0.13–0.20. The values of third-order elastic constants of La_2−xSr_xCuO₄ (x = 0.13–0.20) are negative and their absolute magnitudes are one order higher than those of the second-order elastic constants.     

Effect of Cu doping in MgB2 superconductor at various processing temperatures

The effects of Cu doping in MgB₂ superconductor has been studied at different processing temperatures. The polycrystalline samples of Mg_1−xCu_xB₂ with x = 0.05 were synthesized through the in-situ solid sate reaction method in argon atmosphere at different temperature range between 800–900 °C. The samples were characterized through X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and low temperature R–T measurement techniques for the phase verification, microstructure and superconducting transition temperature, respectively. The XRD patterns of Mg_1−xCu_xB₂ (x = 0.05) do not exhibit any impurity traces of MgB₄ or MgB₆ and they show the sharp transition in the samples prepared at 850 °C. The onset transition temperature of the prepared samples is around 39 K, which is almost the same as that for the pure MgB₂. This indicates that Cu doping in MgB₂ does not affect the transition temperature. The SEM micrograph of Mg_0.95Cu_0.05B₂ has shown that the sample is dense with grain size smaller than 1 μm.     

Study of superconducting state parameters of alkali–alkali binary alloys by a pseudopotential

A detailed study of the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ^*, transition temperature T_C, isotope effect exponent and effective interaction strength N_OV of ten alkali–alkali binary alloys i.e. Li_1−xNa_x, Li_1−xK_x, Li_1−xRb_x, Li_1−xCs_x, Na_1−xK_x, Na_1−xRb_x, Na_1−xCs_x, K_1−xRb_x, K_1−xCs_x and Rb_1−xCs_x are made within the framework of the model potential formalism and employing the pseudo-alloy-atom (PAA) model for the first time. We use the Ashcroft’s empty core (EMC) model potential for evaluating the superconducting properties of alkali alloys. Five different forms of local field correction functions viz. Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used to incorporate the exchange and correlation effects. A considerable influence of various exchange and correlation functions on λ and μ^* is found from the present study. Reasonable agreement with the theoretical values of the SSP of pure components is found (corresponding to the concentration x = 0 or 1). It is also concluded that nature of the SSP strongly depends on the value of the atomic volume Ω₀ of alkali–alkali binary alloys.     

Erbium in silicon–germanium quantum wells

Strained Si_1−xGe_x/Si quantum wells have been doped with erbium by implantation. A comparison is made with strained Si_1−xGe_x/Si quantum wells and relaxed Si_1−xGe_x, with x between 10% and 25%, doped with erbium during MBE growth. The erbium concentration was between 1×10¹⁸ and 5×10¹⁸ cm⁻³ throughout the active regions. Transmission electron microscopy, X-ray diffraction, and photoluminescence studies indicate that good regrowth can been achieved after full amorphisation by implantation of the strained quantum wells. The erbium luminescence is more intense in the Si_1−xGe_x/Si layers, but erbium-implanted samples containing Si_1−xGe_x exhibit defect luminescence in the region of 0.9–1.0 eV. These defects are also present when Si_1−xGe_x/Si quantum wells are implanted with an amorphising dose of silicon, and then regrown. They are attributed to small germanium-rich platelets, rather than to erbium-related defects. Electroluminescence is presented from a forward biased erbium-implanted Si_0.87Ge_0.13/Si structure at a drive current density of only 1.8 mA/cm².     

Luminescence from Si-Si1−xGex/Si1−yCy-Si structures

Near band edge photoluminescence has been obtained from Si_1−yC_y quantum well (QW) and neighboring Si_1−xGe_x/Si_1−yC_y double QW (DQW) structures. Enhanced no-phonon recombination is observed from the DQW structures and it is attributed to a breaking of the k-selection rule in the presence of the heterointerface. The luminescence persists for measurement temperatures up to 30–50 K and the intensity exhibits a quenching behavior with an activation energy equal to 8–20 meV. In electroluminescence only recombination in the Si_1−xGe_x layer has been observed from neighboring Si_1−xGe_x and Si_1−yC_y DQW structures.     

Chemical pressure effect on the superconductor MgCNi3

In order to investigate the positive and negative pressure effects on superconducting properties for MgCNi₃, chemical pressure was applied by means of Zn-doping to Mg site (Mg_1−xZn_xCNi₃) and by substituting Mg with Cd (CdCNi₃). The lattice constant decreases (increases) with increasing Zn (Cd) content. In the magnetic measurements, superconducting transition temperature (T_c) is decreasing with increasing Zn content and disappears at x > 0.3. While for CdCNi₃, T_c also decreases down to 3.4 K. The result seems not to be fully understandable in the case of CdCNi₃ since T_c seems to rise owing to the increase of density of states at Fermi energy caused by lattice expansion.     

Structural and compositional analysis of InBixAsySb(1−x−y) films grown on GaAs(0 0 1) substrates by liquid phase epitaxy

The growth of epitaxial InBi_xAs_ySb_(1−x−y) layers on highly lattice mis-matched semi-insulating GaAs substrates has been successfully achieved via the traditional liquid phase epitaxy. Orientation and single crystalline nature of the film have been confirmed by X-ray diffraction. Scanning electron micrograph shows abrupt interface at micrometer resolution. Surface composition of Bi(x) and As(y) in the InBi_xAs_ySb_(1−x−y) film was measured using energy dispersive X-ray analysis and found to be 2.5 and 10.5 at.%, respectively, and was further confirmed with X-ray photoelectron spectroscopy. Variation of the composition with depth of the film was studied by removing the layers with low current (20 μA) Ar⁺ etching. It was observed that with successive Ar⁺ etching, In/Sb ratio remained the same, while the As/Sb and Bi/Sb ratios changed slightly with etching time. However after about 5 min etching the As/Sb and Bi/Sb ratios reached constant values. The room temperature band gap of InBi_0.025As_0.105Sb_0.870 was found to be in the range of 0.113–0.120 eV. The measured values of mobility and carrier density at room temperature are 3.1×10⁴ cm² V⁻¹ s⁻¹ and 8.07×10¹⁶ cm⁻³, respectively.     

Nax/2Bi1−x/2MoxV1−xO4 and Bi1−x/3MoxV1−xO4 New Scheelite-related phases

New Scheelite-related solid solutions of the compositions Na_x/2Bi_1−x/2Mo_xV_1−xO₄ (0≤x≤1) and Bi_1−x/3 Mo_xV_1−xO₄(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO₄ type ferroelastic BiVO₄. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na_x/2Bi_1−x/2Mo_xV_1−xO₄, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi_1−x/3 Mo_xV_1−xO₄ where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.     

Far-infrared and millimeter wave spectroscopy of superionic copper conductors

Far-infrared and millimeter wave spectra of copper ion conducting crystal RbCu₄Cl_3+xI_2−x, which has the same structure as the room temperature silver ion conductor RbAg₄I₅, were investigated. Broad absorption peaks observed around 40, 80, and 110–200 cm⁻¹ at room temperature show doublet structures at low temperature; this may be attributed to the difference of local structure by chlorine and iodine ion. The 110–200 cm⁻¹ bands seem to be symmetric breathing modes of CuX₄ (X = Cl or I) tetrahedron and the frequency shift coincides with the square root of the mass ratio of conduction ions. The 80 cm⁻¹ band seems to be Rb-X vibration in RbX₆ octahedron. The 40 cm⁻¹ band seems to be the attempt mode which is an outward motion of the mobile ion in halogen cage. The increase of the absorption intensity at the low energy side with temperature corresponds to an increase of the DC conductivity. Plasmon fitting in energy loss function spectra was attempted.     

Magnetic properties of Sm–Co–B-based nanocomposite magnets

The magnetic properties of nanocomposite melt-spun magnets with composition Sm_16−xCo_68+xB₁₆ (x=0–10, 2 at% interval) and Sm₈Co_92−yB_y (y=10–18, 2 at% interval) have been studied systematically. Several ribbons were fabricated with a wheel speed of 50 m/s, followed by annealing in the temperature range of 700–800°C for 2.5–40 min. XRD results and magnetization versus temperature curves showed that almost all of the samples were composed of the tetragonal Sm₂Co₁₄B and rhombohedral SmCo₁₂B₆ phases which are not magnetically hard at room temperature. However, a relatively high coercivity in the range of 3.5–5.5 kOe has been obtained in these samples. The highest coercivity of 5.5 kOe and a very promising β value of −0.28%/°C were obtained in Sm₈Co₇₄B₁₈ ribbons annealed at 750°C for 5 min. The high coercivities are attributed to the small grain size of the 2 : 14 : 1 phase, in which the large surface areas enhance its effective anisotropy, and make it uniaxial type.     

Time-resolved photoluminescence study of the red emission in nanoporous SiGe alloys

Porous Si_1−xGe_x (PSiGe) layers with efficient room temperature visible photoluminescence (PL) were elaborated by anodical etching from p-type doped epitaxial layers with Ge contents from 5 to 30%. The luminescence is characterised by a broad PL band centred at 1.8 eV. Time resolved photoluminescence decay is studied in porous silicon germanium as a function of germanium content, temperature, emission energies and surface passivation. The PL decay line shape is well described by a stretched exponential in all cases. The effective lifetime at low temperature in as prepared porous Si_1−xGe_x is 400 μs, i.e. an order of magnitude less than in porous silicon. After the formation of a 20 Å thick oxide surface layer we observe a decrease of the effective lifetime to 20 μs at T=4 K.     

Temporary phase segregation processes during the oxidation of (Fe0.7Mn0.3)0.99O in N2---H2O atmosphere

A two-step thermochemical cycle with the ternary metal oxide system (Fe_{1 − x}Mn_x)₃O₄/(Fe_{1 − x}Mn_x)_{1 − y}O is applied to convert solar energy to chemical energy. Experimental investigations on the water splitting reaction of (Fe_{1 − x}Mn_x)_{1 − y}O revealed temporary formation of a manganese rich rock salt phase and an iron rich spinel phase due to phase segregation processes.     

Positive linear magnetoresistance in Fex–C1−x composites

A large positive magnetoresistance (MR) has been found in micro-sized Fe_x–C_1−x composites. At a magnetic field of 5 T, the Fe_0.2–C_0.8 composite has the largest MR, 53.8% and 190% at room temperature and at 5 K, respectively. The magnetic field dependence of the MR can be described approximately as MR∝Bⁿ, and the value of exponent n is determined by the Fe weight concentration and temperature, ranging from 1/4 to 6/4. It appears that Fe_x–C_1−x has a linear field dependence of the positive MR at different temperatures. The possible mechanism for the positive MR is discussed.     

On the appearance of ferromagnetism in Y(Co1−xAlx)2 alloys

The appearance of ferromagnetism in Y(Co_1−xAl_x)₂ is discussed in terms of a d band model. The approximate d bands for YCo₂ and Y(Co_1−x)₂ are calculated and the decrease in the electronic energy due to magnetization of the spin of estimated. The energy decrease is the largest in YCo₂, and it gradually decreases as the Al content increases, if the lattice constant is fixed, while this energy decrease increases if the lattice constant increases with increasing Al content. These results of calculations give a good account of the appearance of ferromagnetism in Y(Co_1−xAl_x)₂ around x = 0.15. The ferromagnetism in Sc(CO_1−xAl_x)₂ is also discussed, leading to the appearance of ferromagnetism between x = 0.15 and 0.30.     

Spin fluctuations of Y(Mn1−xAlx)2 and Y1−xScxMn2

The thermal expansion and low temperature and low temperature specific heat were measured for Y_1−xScxMn2. The results are discussed in terms of spin fluctuations and compared with those of Y(Mn_1−xAl_x)₂, which show al local moment character. It is revealed that Y_1−xSc_xMn₂ is a typical nearly antiferromagnet in which giant spin fluctuations are thermally excited.     

First-principles study of electronic structure and optical properties of barium strontium titanates (BaxSr1−xTiO3)

Using first-principles calculations based on density-functional theory in its local-density approximation, we investigate the energy band structures and total density of states (TDOS) of barium strontium titanates (BSTs). Direct band gaps of 1.89 and 1.87 eV at the Γ point in the Brillouin zone are calculated for Ba_xSr_1−xTiO₃ (x = 2/3 and 1/3), respectively. The optical properties of the above perovskites in the core-level spectra are investigated by the first-principles under scissor approximation. The real and imaginary parts of the complex dielectric function and derive optical constants, viz., the refractive index, extinction coefficient, reflectivity, and the electron energy-loss spectrum are calculated.     

Magneto-optical and magnetic properties of some solid solutions of C1b-Heusler compounds

The large Kerr effect in PtMnSb has been ascribed to details of its unusual band structure. Substitution of atoms will modify the band structure and consequently alter the Kerr effect. In the present investigation we have determined the magneto-optical properties at room temperature in the energy range 0.5–5 eV of several solid solutions of C1_b−Heusler compounds, comprising the solid solution series Pt_1−xNi_xMnSb, Pt_1−xRh_xMnSb, Ni_1−xCr_xMnSb, Ni_1−xFe_xMnSb and PtMnSb_0.9Sn_0.1. The magnetic and crystallographic properties were determined for the solid solution series Pt_1−xNi_xMnSb, Pt_1−xRh_xMnSb and Ni_1−xCr_xMnSb. Neither of the substitutions leads to a larger Kerr effect than that present in the pure compound. Moreover, it is found that the shape of the spectra does not change significantly. For one case, Ni substituted in PtMnSb, band structure calculations were made for hypothetically ordered compounds. The results agree with the experimental findings that near 1.7 eV magneto-optically active transitions persist on substituting Ni in PtMnSb.     

Itinerant metamagnetic transition in Y(Co1−xFex)2

High field magnetization measurements have been performed to examine the existence of itinerant metamagnetism in exchange-enhanced systems related to YCo₂ together with Fe_1−x Co_xSi. In the Y(Co_−xFe_x)₂ system, the meta magnetism inherent in YCo₂ has been observed in 0.04 x0.07. The transition is not as sharp as in the Y(Co_1−xAl_x)₂ system. Other exchange-enhanced paramagnets Y(Co_−xCu_x)₂ and Y_1−xLa_xCo_x2 and weakly itinerant ferromagnet Fe_1−xCo_xSi exhibit no metamagnetic transition up to 430 kOe.     

Structure, oxygen stoichiometry and electrical conductivity in the system Sr-Ce-Co-O

Mixed oxides in the system S-Ce-Co-O were prepared by solid state reaction and by freeze-drying of precursor compounds followed by thermal treatment. Two types of perovskite oxides exist in the system: Solid solutions of the type Sr_{1 − y}Ce_yCoO_{3 − x} and mixed oxides of the type (1 − y)SrCeO₃ − ySrCoO_{3 − x}. Microstructures and phase compositions were determined by electron microscopy and X-ray diffraction. SrCoO_{3 − x} forms a solid solution of ceria on the A-site in the strontium cobaltite lattice up to 0.15 mol Ce. This solid solution corresponds to the high-temperature structure of pure SrCoO_{3 − x} and is characterized by high oxygen exchange and electrical conductivity. The oxygen deficiency x was measured by solid electrolyte coulometry. The oxygen deficiency of solid solutions Sr_{1 − y}Ce_yCoO_{3 − x} increases with temperature and decreases with pO₂ in the ambient atmosphere and with increasing Ce dopant concentration. The pO₂-T-x diagram of the solid solution was determined. The T, pO₂ and dopant concentration dependencies of electrical conductivity were measured by a four-point d.c. technique. By Ce doping strontium cobaltite becomes a stabilized high-conductive material (maximum conductivity: 500 S cm⁻¹ at 400 °C, E_a = 0.025 eV, p-type). Above this temperature the T-coefficient of the conductivity changes from positive (semiconducting) to negative values.     

An investigation of MBE growth of (Zn, Mg)Se using RHEED oscillations

We have used reflection high-energy electron diffraction (RHEED) oscillations to measure the growth rates of Zn_1−x.Mg_xSe and ZnSe as functions of substrate temperature, II/VI flux ratio, and electron-beam filament current. Although the ZnSe and Zn_1−xMg_xSe growth rates vary with these growth parameters, the growth rate difference attributable to the addition of magnesium does not depend on them, if growth is done with a selenium excess. Thus, unlike the other common components of II/VI materials (Zn, Cd, S, Se and Te), the sticking coefficient of magnesium is independent of substrate temperature and II/VI flux ratio. Electron-beam-induced desorption of magnesium is also negligible. For growth with a selenium excess, the composition of Zn_1−xMg_xSe films determined by RHEED oscillations agrees well with X-ray fluorescence measurements. However, if growth is done without an excess of selenium, then a simple composition calculation using the Zn_1−xMg_xSe and ZnSe growth rates indicates a magnesium content which is lower than the true magnesium content. This occurs because of a decrease in the sticking coefficient of zinc induced by the presence of magnesium.