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The influence of the growth temperature T[sub s] on the structure, optical absorption, and electrical conductivity of BiSrCaCuO films has been studied. It was observed that nonmonotonic changes in the parameters of the films with T[sub s] are caused by (111)→(001) phase transitions at T[sub s] ≈ 550 °C for the 2212 phase and T[sub s] ≈ 600 °C for the 2223 phase. These phase transitions stimulate the formation of metallic conductivity and are caused by a change in the system of preferential ordering planes of the atoms. [ABSTRACT FROM AUTHOR]

Calculates the equilibrium states of a hexagonal planar collinear antiferromagnet in a planar magnetic field. Observation of first-order phase transitions; Range and equilibrium parameters of the thermodynamically stable domain structure; Analysis of the magnetization curves.

Focuses on the influence of zero-point and thermal spin fluctuations on the electronic structure and magnetic properties of weak itinerant magnets. Possibility of considerable complex reconstruction of electron state density in fluctuating exchange and charge fields; Hamiltonian expression of the electron system.

Focuses on the hopping conductivity of single crystals of metal-oxide compounds with excess oxygen. Transition from variable-range hopping to simple activation conductivity at a temperature lower than 20 degrees Kelvin; Changes in the behavior of magnetoresistance and current-voltage characteristics.

Using only two principles: (i) high-temperature superconductivity requires hypercharged oxygen, and (ii) the superconducting condensates are located in those parts of the crystal structures where they are unaffected by magnetic pair breaking, we are able to explain why certain rare-earth ions R are compatible with superconductivity and others are not, in the compounds RBa2Cu3O7, RBa2Cu4O8, RBa2Cu2NbO8, R2 − zCe zCuO4, and R2 − zCe zSr2Cu2NbO10. Various defects are proposed as having central roles in the superconductivity or the suppression of superconductivity in these compounds. Many experiments for testing this physical picture are suggested. [ABSTRACT FROM AUTHOR]

The Monte Carlo method has been used to investigate the magnetic properties of doped copper-oxide high-temperature superconductors and phase separation in LaCuO. The calculations were based on a model in accordance with which a hole induced by the dopant or excess oxygen changes the antiferromagnetic interaction between neighboring copper spins to a ferromagnetic interaction in a group of Cu−Cu bonds forming a nest of defect bonds. For copper oxides with impurities in which the nests are distributed in accordance with a random law and frozen the concentration of impurities at which an antiferromagnet-spin-glass phase transition occurs has been calculated. Good agreement with the experimental value is found. A mechanism that explains the phenomenon, of phase separation in LaCuO is found. It is shown that the phase separation is due to magnetic effects. Mobile nests formed with excess oxygen atoms exhibit a clear tendency to clustering forming regions with high oxygen concentration. The clustering is energetically advantageous, since it leads to a decrease of the frustration energy. [ABSTRACT FROM AUTHOR]

The monotone susceptibility component has been examined at 4.2-420 K for CdMg ordering single crystals for 0.18 < x < 0.33. The temperature and concentration curves for the susceptibility components show features related to structural and electronic topological transitions. The susceptibility anomalies at the electron-transition points are correlated with the boundaries to the existence of the allotropic forms. [ABSTRACT FROM AUTHOR]

Presents a study that examined ferromagnet uranium-nickel. Information on the electronic structure calculations for the compound; Discussion on magnets; Details of the spin and orbital magnetic moments obtained in the unconstrained calculation for the compound.

Magnetic and structural transitions were found to coincide by substituting Cu for Mn in Mn1-xCuxCoGe, leading to a giant magnetocaloric effects associated with the first-order magnetostructural phase transition from the ferromagnetic to paramagnetic state. Maximum magnetic entropy changes of -ΔSM = 52.5 and 53.3 J/kg K for a magnetic field change of ΔH = 5 T have been observed at 302 and 316 K, for x = 0.080 and 0.085, respectively. The giant magnetocaloric effects with tunable phase transition temperatures through subtle variations in composition make this system promising for room-temperature magnetic cooling applications. [ABSTRACT FROM AUTHOR]

Ac magnetization measurements were carried out for a gallium nanocomposite consisted of a metal loaded porous glass with 3.5 nm pore size to study dynamics in the vortex system and H-T diagrams. Variations of ac magnetization with temperature and bias magnetic field were obtained at different frequencies and amplitudes of ac field. Double peaks in the imaginary part of ac susceptibility associated with doubly structured real parts with distinct dependences on the amplitude of ac field were observed. Activation barriers were evaluated from variations with frequency of the upper-temperature peak in χ″ at different bias fields. The field dependence of the activation barrier followed the power law with exponents equal to 0.12 and 1.0 below and above 20 kOe, respectively. A pronounced positive curvature was observed at low magnetic fields for the Hc2(T) line with the crossover to a common negative curvature near 20 kOe, also. Both the change of the exponent in the power law for activation barriers and the crossover to the negative curvature in the upper critical field line were suggested to occur due to transition in the vortex system from the geometry restricted to space between gallium particles into more homogeneous distribution with increasing field. [ABSTRACT FROM AUTHOR]

The MnCoGe alloy can crystallize in either the hexagonal Ni2In- or the orthorhombic TiNiSi-type of structure. In both phases MnCoGe behaves like a typical ferromagnet with a second-order magnetic phase transition. For MnCoGeBx with B on interstitial positions, we discover a giant magnetocaloric effect associated with a single first-order magnetostructural phase transition, which can be achieved by tuning the magnetic and structural transitions to coincide. The results obtained on the MnCoGe-type alloys may be extensible to other types of magnetic materials undergoing a first-order structural transformation and can open up some possibilities for searching magnetic refrigerants for room-temperature applications. [ABSTRACT FROM AUTHOR]

The authors study within a first-principles approach the band structure, vibrational modes, and electron-phonon coupling in boron-, aluminum-, and phosphorus-doped silicon in the diamond phase. Their results provide evidences that the recently discovered superconducting transition in boron-doped cubic silicon, with a transition temperature of ∼0.34 K, can be explained within a standard phonon-mediated mechanism. The importance of lattice compression and dopant related stretching modes are emphasized. The authors find that TC can be increased by one order of magnitude by adopting aluminum doping instead of boron. [ABSTRACT FROM AUTHOR]

A magnetic field of up to 12 T was applied during the sintering process of pure MgB2 and carbon nanotube (CNT) doped MgB2 wires. The authors have demonstrated that magnetic field processing results in grain refinement, homogeneity, and enhancement in Jc(H) and Hirr. The extent of improvement in Jc increases with increasing field. The Jc for a 10 T field processed CNT doped sample increases by a factor of 3 at 10 K and 8 T and at 20 K and 5 T, respectively. Hirr for the 10 T field processed CNT doped sample reached 9 T at 20 K, which exceeded the best value of SiC doped MgB2 at 20 K. Magnetic field processing reduces the resistivity in CNT doped MgB2, straightens the entangled CNTs, and improves the adherence between CNTs and the MgB2 matrix. [ABSTRACT FROM AUTHOR]

We report on electrical spin injection measurements on multiwall carbon nanotubes (MWNTs). We use a ferromagnetic alloy Pd1-xNix with x≈0.7 which allows us to obtain devices with resistances as low as 5.6 kΩ at 300 K. The yield of device resistances below 100 kΩ, at 300 K, is around 50%. We measure at 2 K a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads. The relative difference between the resistance in the antiparallel (AP) orientation and the parallel (P) orientation is about 2%. [ABSTRACT FROM AUTHOR]

Laser-ablation studies of highly-oriented thin films of the electon-doped infinite-layer copper-oxide compounds Sr[sub 1-x]La[sub x]CuO[sub 2] are reported. We observe significant variations in film properties with substrate or buffer layer material. X-ray diffraction, atomic force microscopy (AFM), Rutherford back-scattering (RBS), and electrical resistivity were used to characterize the films. Films were deposited on strontium titanate (001) or on buffer layers of T'-phase copper oxides (Ln[sub 2]CuO[sub 4] with Ln = Pr, Nd, Sm), Sr[sub 3]FeNb[sub 2]O[sub 9], and La[sub 1.8]Y[sub 0.2]CuO[sub 4] on SrTiO[sub 3] (001). The in-plane lattice constants of such buffer layers (a = 0.390-0.400 nm) should provide the bond tension required for electron doping. Extremely flat, epitaxial buffer layers with X-ray rocking curves as narrow as 0.08° were obtained from stoichiometric targets of Ln[sub 2]CuO[sub 4]; the other buffer layers yielded poor epitaxy. A linear dependence of infinite-layer c-axis plane spacing on substrate or buffer-layer in-plane a-axis lattice constant is observed. [ABSTRACT FROM AUTHOR]

Provides information on a study that explored the magnetic properties of the FeVSn and CoVSn intermetallics. Methodology of the study; Results and discussion on the study; Conclusions.

Discusses a study which examined the electronic structure of strained and unstrained gadolinium grown on molybdenum surface. Background on strain; Materials and methodology used; Results and conclusions.

The effect of hydrostatic pressure P up to 1 GPa and applied magnetic fields H up to 1.5 T on the magnetic and transport properties of La[sub 0.91]Mn[sub 0.95]O[sub 3] doped with 7 at. % Zn at the temperature range of 15–350 K is reported. It was found that the doping by Zn changes the low temperature conductance from a metallic-like to a nonmetallic one. Doping by Zn of the initial self-doped sample shifts the Curie temperature from 252 to 204 K. In both compounds the applied pressure enhances the ferromagnetic coupling and Curie temperature. The pressure coefficients of the Curie temperature are equal for both compounds being dT[sub C]/dPapprox. 13 K/GPa. Above the Curie temperature the resistivity of Zn-doped manganite obeys Mott’s law rather than the Arrhenius behavior. Both parameters, A and T[sub 0] of Mott localization law decrease under pressure indicating a reduction in the localization energy. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses a method for measuring magnetization under high pressure and pulsed high magnetic field for a Drickamer-type cell using diamond anvils and insulator gaskets. Effects of an increase in pressure on magnetization; Indications of a transition to a paramagnetic state.

Describes the epitaxial growth of single-crystalline films of La[sub 2-x]Sr[sub x]CuO[sub 4] by laser ablation. Exhibition of values of T[sub co] above 38 K by the films; Display of a linear variation of the resistivity with T and excellent crystal quality; Aid of the process to oxygen deficiency in the LSCO films.

Examines the superconductivity and microstructure of n-type Ln[sub 1.85]Ce[sub 0.15]CuO[sub 4-y](Ln=Pr, Sm, and Eu) polycrystals from as-prepared materials. Difference of the samples from conventionally synthesized materials; Use of transmission electron microscopy.

The discovery of a semiconducting ground state XyYZ (y = 0.8 or 0.75) in nominally 19‐electron half‐Heusler materials warrants a closer look at their apparently metallic properties that often make them good thermoelectric (TE) materials. By systematically investigating the temperature dependence of off‐stoichiometry (x) in V0.8+xCoSb, Nb0.8+xCoSb, and Ti0.75+xNiSb it is found that x invariably increases with increasing temperature, leading to an n‐type self‐doping behavior. In addition, there is also a large phase width (range of x) associated with each phase that is temperature dependent. Thus, unlike in typical 18‐electron half‐Heuslers (e,g, TiNiSn), the temperature dependence of vacancy and carrier concentration (n) in nominally 19‐electron half‐Heuslers links its transport properties to synthesis conditions. The temperature dependence of x and n are understood using density functional theory based defect energies (Ed) and phase diagrams. Ed are calculated for 21 systems which can be used in predicting solubility in this family of compounds. Using this simple strategy, suitable composition and temperature synthesis conditions are devised for obtaining an optimized n to engineer TE properties in phase‐pure V0.8+xCoSb, and the previously unexplored Ta0.8+xCoSb. Defect and doping concentrations in nominally 19‐electron half‐Heuslers are strongly dependent on the annealing temperature. A simple strategy based on calculated defect energies to exploit this temperature degree‐of‐freedom for engineering thermoelectric transport is realized. [ABSTRACT FROM AUTHOR]

Recent studies on new materials crystallized with the sp3 open framework of Si and other group IV elements are reviewed. The synthesis and predicted properties of a new allotrope of silicon, i.e., orthorhombically structured silicon Si24, are investigated. Si24 can be formed by heating a Na4Si24 precursor at temperatures as low as 47 °C. The quasi-direct band-gap nature with a gap of ∼1.3 eV is predicted on the basis of a first-principles calculation. We also review investigations on clathrate materials having Si, Ge or Sn framework atoms for which pressure plays an important role. The phenomena and characteristics of clathrates under high pressures, i.e., the volume collapse phase transition and amorphization, are discussed on the basis of a survey over various clathrates with types I, II, III, and VIII structures. [ABSTRACT FROM AUTHOR]

Recent studies on type-II Si/Ge clathrates for photovoltaic (PV) applications are reviewed. The band-gap energies (Eg) experimentally estimated for Si and Ge clathrates are discussed on the basis of the comparison with theoretical calculation. For the Si and Ge clathrates, Eg = 1.7–1.9 and 0.6–0.8 eV are acceptable values, respectively. Thin films of type-II Si clathrates have been successfully synthesized on Si wafers. A device was fabricated using a Si clathrate thin film, and its PV response was observed. For the Ge clathrate, a single-crystal-like thin film was identified to epitaxially grow on a Ge(111) wafer. The PV response and epitaxial growth of group IV clathrates found in recent studies strongly suggest that these new materials will be the next-generation platform for semiconductor technology. [ABSTRACT FROM AUTHOR]