Advanced Materials Research Vol. 501

Abstract: The effects of nano particles Fe3O4 addition on the superconducting and transport properties of YBa2Cu3O7-δ (YBCO) were studied. YBa2Cu3O7-δ superconductor powders were prepared by using high purity oxide powders via solid state reaction method. Nano Fe3O4 with 0.01 – 0.05 wt.% with average size 28 nm was added into YBCO. The transition temperatures (Tc) of the samples were measured by using four point probe method. The critical current (Ic) of the samples has been determined by using the 1 μV/cm criterion from 30 – 77 K. Sample with 0.02 wt.% nano Fe3O4 showed the highest Tc at 87 K. It is interesting to note the same sample also exhibited the highest Jc at 77 K up to 1683 mA/cm2. Nano Fe3O4 has acted as effective flux pinning centers in YBCO. A small amount of nano particles Fe3O4 addition has successfully improved the superconducting and transport properties of YBCO. The excessive addition of nano Fe3O4 (> 0.02 wt.%) suppressed the Tc and Jc. Overall, Jc decreases with increasing temperature (30 – 77 K) as a consequence of thermally activated flux creep. Magnetic impurities normally suppress superconductivity. However, by adding magnetic nano particles, current carrying capacity of superconductors YBCO was enhanced significantly.

Abstract: The effect of LCMO addition in YBCO system to investigate changes in structure and transport properties is reported. Samples with addition of (La_0.67Ca_0.33MnO₃)x (LCMO) (x=0.0-0.6) in YBa₂Cu₃O_7-δ (YBCO) superconducting system were prepared via solid state reaction. The effect of magneto resistive components on the superconducting properties of YBCO was investigated through X-Ray diffraction (XRD), resistance-temperature (R-T) measurement, and Scanning Electron Microscope (SEM). XRD analysis showed the addition of LCMO (0.2 wt% - 0.6 wt%) formed four new phases. The crystal system still orthorhombic with space group Pmmm after addition 0.2 wt% and 0.4 wt% of LCMO but the crystal system was changed to tetragonal and space group P4/mmm when addition with 0.6 wt% of LCMO. The temperature dependent of electrical resistance showed the samples were metallic above the transition temperature even though after addition. The T_c(onset) superconducting transition temperature decreased from 84 K to 66 K while the T_{c (offset)} decreased from 72°C to 54°C. This suppression was due to the magnetic behaviour of the LCMO. The average grain size decreased when addition with LCMO were increased.

Abstract: We report the room temperature thermal conductivity κ and thermal diffusivity α of polycrystalline La0.7Ca0.3-xSrxMnO3 for x = 0 to 0.1. The samples were prepared by heating at 1220 and 1320oC. The insulator-metal transition temperature, TIM and thermal diffusivity increased with Sr content. Phonon was the dominant contributor to thermal conductivity and the electronic contribution was less than 1%. Enhancement of electrical conductivity σ and thermal diffusivity for x ≥ 0.08 was observed in both series of samples. The grain size of the samples (28 to 46 µm) does not show any affect on the thermal and electrical properties.

Abstract: The polycrystalline Yttrium Iron Garnet (YIG) powder with the chemical formula Y3Fe5O12 has been synthesized by using High Energy Ball Milling technique. The effect of various preparation parameters on the crystallinity, morphology and complex permeability of YIG, which includes milling time and annealing temperature were studied respectively by using XRD, SEM and Impedance Material Analyzer. The frequency dependence of complex permeability namely real permeability, µ’ and magnetic loss, µ’’ were measured at room temperature for samples sintered from 600°C to 1400°C, in the frequency range 10 MHz to 1 GHz. The results showed that milling time plays a role in determining the crystallinity of the milled powder where higher milling time results in better crystallinity due to high reactivity of the particles. From complex permeability measurement, it was observed that the initial permeability and magnetic loss increased with increasing grain size. The permeability values increased with annealing temperature and the absolute values of permeability decreased after attaining the natural resonance frequency of the material.

Abstract: Polycrystalline Bi1-xSmxFeO3 multiferroic samples in the range 0 ≤ x ≤ 0.1 were prepared via solid state reaction method. The effect of samarium ion (Sm3+) doping at the bismuth site in multiferroic BiFeO3 structure and its relation with magnetic and dielectric properties were investigated. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Vibrating Sample Magnetometer (VSM) and Impedance Analyzer were used to characterize the structure and the properties of the composition. The XRD results showed that all the compounds are rhombohedral distorted perovskite structure (R3C). The average grain size identified by SEM were 2.61 µm and decreased to 1.18 µm by introducing the element of Sm doping. At room temperature, all samples showed a spontaneous magnetization which was enhanced by doping with Sm3+ ion. The dielectric constant and dielectric loss decreased inversely proportional to the frequency from 102 to 106 Hz.

Abstract: We present the calculated band edge energies altered by strain in a nanostructure system of a pyramidal InAs quantum dot buried in a GaAs substrate. Our zinc-blende supercell system of dimension 11.9 nm × 11.9 nm × 8.5 nm and 55119 atoms contains a pyramidal In770As886 quantum dot of 1656 atoms with height of 3.03 nm and square base of length 6.06 nm. The strain energy of this nanostructure system is minimized by employing the Keating formulation of interatomic potential and Monte Carlo relaxation method via the Metropolis algorithm. This relaxation is run for 20 million Monte Carlo steps at simulation temperature of 4.2 K. The calculated strain is then used to determine the conduction and valence band edge energies of the nanostructure. We find that along the [001] growth direction in the InAs quantum dot region, strain increases the conduction band edge energy by 0.6 eV and in the valence band strain results in relatively sharp wells at the dot base for heavy holes and at the dot tip for light holes. Thus, our calculation predicts that strain leads to increased band gap and spatial splitting of holes in this nanostructure system.

Abstract: The electronic band structure, density of state and elastic properties of lead-free perovskite oxide SnTiO3 (ST) were investigated by employing first principles calculation using the Density Functional Theory (DFT) within local density approximation (LDA). The energy band gap was calculated from the separation between the Ti 3d (conduction band) and the maximum of O 2p (valence band). This gives an indirect band gap of 2.36 eV. The elastic constants and their pressure dependence were calculated up to 30 GPa and the independent elastic constants (C11, C12, and C44), bulk modules, B were obtained and analyzed. The results showed that SnTiO3 have a mechanical stability in cubic phase (Pm3m).

Abstract: We have calculated the properties of electron states in an InAs/GaAs quantum dot system based on the effective mass approximation of a one-band Hamiltonian model. This semiconductor nanostructure system consisted of an InAs quantum dot embedded in a GaAs substrate. In this paper, the Schrödinger equation of an ideal cubic quantum dot with infinite barrier was solved using a finite difference approach. The sparse matrix of N3 x N3 for the Hamiltonian was diagonalized to calculate the lowest states of electrons in this nanostructure system. The calculation was performed for different dot size and the obtained energy levels are comparable to those calculated analytically. The finite difference method was relatively faster and applicable to quantum dots of any geometry or potential profile. This was proven by applying the developed computational procedure to quantum dots of cubic, spherical and pyramidal geometries for the InAs/GaAs nanostructure system.

Abstract: The dynamical properties of a new perovskite GeTiO3 materials have been investigated by using first principle calculation based on Density Functional Theory (DFT) within the gradient generalized approximation (GGA). All calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) computer code. The calculations include the structural parameter, Born effective charges, and phonon dispersion. The calculated Born effective charges and phonon dispersion have been analyzed and the possibility of ferroelectric feature in GeTiO3 compounds has been discussed. From the analysis, the calculated Born effective charge ZGe and ZTi showed large anomaly compared to the nominal charge which contributed to the large atomic displacement. The calculated phonon dispersion showed the most unstable mode was at G point and the unstable modes were dominated by Ge branch. The dynamical properties and ferroelectric properties in GeTiO3 are discussed and compared with the ferroelectric feature in PbTiO3.

Abstract: A computational study using the density fuctional through linear augmented plane wave (LAPW) and gradient generalized approximation (GGA) methods on the electronic properties of cadmium telluride (CdTe) in two modes namely with relativistic effect and non-relativistic effect is presented. Two electronic properties were obtained and compared between the computation with and without the relativistic effects. Firstly, plots of density of states were produced which were for the total CdTe. The total DOS showed that the conduction band was dominated by the states of Te atom, whereas the valence band is dominated by the states of Cd atom. Secondly, the total band structure plot obtained showed that the direct energy band gap, Eg calculated value with relativistic effect was about 1.0 eV while the non-relativistic effect value was 1.8 eV.