Kevin P. Roche

Aerosol flow reactor production of fine Y1Ba2Cu3O7 powder: Fabrication of superconducting ceramics

An aerosol flow reactor operating at 900–1000 °C is used to prepare high‐purity Y1Ba2Cu3O7 powders with a uniform chemical composition and a submicron to micron average particle size by thermally decomposing aerosol droplets of a solution consisting of the nitrate salts of Y, Ba, and Cu in a 1:2:3 ratio. The powders were at least 99% reacted based on thermogravimetric analysis, and the x‐ray diffraction pattern is essentially that of Y1Ba2Cu3O7. Magnetic susceptibility measurements showed the powders to be superconducting with a transition at 90 K even for average reactor residence times as short as 20 s. Sintering cold‐pressed pellets between 900 and 1000 °C provides dense, fine grained (average size on the order of 1 μm) superconducting ceramics with sharp 90 K transitions. The grain size and shape of a final sintered part could be varied depending on powder production, processing, and sintering conditions.

Subnanosecond incubation times for electric-field-induced metallization of a correlated electron oxide

Strong interactions, or correlations, between the d or f electrons in transition-metal oxides lead to various types of metal-insulator transitions that can be triggered by external parameters such as temperature, pressure, doping, magnetic fields and electric fields. Electric-field-induced metallization of such materials from their insulating states could enable a new class of ultrafast electronic switches and latches. However, significant questions remain about the detailed nature of the switching process. Here, we show, in the canonical metal-to-insulator transition system V₂O₃, that ultrafast voltage pulses result in its metallization only after an incubation time that ranges from ∼150 ps to many nanoseconds, depending on the electric field strength. We show that these incubation times can be accounted for by purely thermal effects and that intrinsic electronic-switching mechanisms may only be revealed using larger electric fields at even shorter timescales.

Increased metal-insulator transition temperatures in epitaxial thin films of V2O3 prepared in reduced oxygen environments

Thin films of V2O3 were grown epitaxially on c-plane sapphire substrates by oxygen plasma-assisted thermal evaporation. Reducing the amount of oxygen supplied during growth led to a nearly 50 K increase in V2O3’s metal-insulator transition temperature to a temperature as high as 184 K. By systematically varying the oxygen pressure the transition temperature monotonically increased, which was accompanied by a concomitant increase in the room-temperature resistivity. These trends are consistent with a continuous change in the stoichiometry of V2O3.

Substrate-induced disorder in V2O3 thin films grown on annealed c-plane sapphire substrates

We investigate the structural and electronic properties of V2O3 thin films deposited by oxygen plasma-assisted molecular beam epitaxy onto annealed and unannealed c-plane sapphire substrates. Annealing the substrates before growth to produce ultra-smooth surfaces improved initial epitaxy, according to in situ reflection high-energy electron diffraction. Surprisingly, films deposited on annealed substrates had a more island-like surface, broader x-ray diffraction peaks, and an increased resistivity of V2O3’s normally metallic high-temperature phase. We attribute these results to enhanced strain coupling at the interface between the substrate and film, highlighting the vulnerability of V2O3’s strongly correlated metallic phase to crystalline defects and structural disorder.

Magnetoresistance of self-assembled lateral multilayers

The angular-dependent magnetoresistance and magnetization of epitaxial FeηAg1−η self-assembled lateral multilayers (SALMs) grown on Mo(110)/Al2O3(11-2 0) template layers has been examined for an optimum stoichiometry of η=0.38. The low-temperature anisotropic magnetoresistance (AMR) and low-field magnetoresistance (MR) are measured as a function of field angle for two nearly orthogonal current directions. The SALM structures are observed to display a significant AMR (roughly 1.6% maximum for the entire structure) and a pronounced field-dependent MR with a maximum ΔR/R of 0.88% at 2.7 K.

Electronic and crystalline structures of zero band-gap LuPdBi thin films grown epitaxially on MgO(100)

Thin films of the proposed topological insulator LuPdBi—a Heusler compound with the C1b structure—were prepared on Ta-Mo-buffered MgO(100) substrates by co-sputtering from PdBi2 and Lu targets. Epitaxial growth of LuPdBi films was confirmed by X-ray diffraction and reflection high-energy electron diffraction. The root-mean-square roughness of the films was as low as 1.45 nm, even though the films were deposited at high temperature. The film composition is close to the ideal stoichiometric ratio. The valence band spectra of the LuPdBi films, observed by hard X-ray photoelectron spectroscopy, correspond very well with the ab initio-calculated density of states.

Epitaxial Growth of Rare Earths on Rare Earth Fluorides and Rare Earth Fluorides on Rare Earths: Two New Epitaxial Systems Accessed by MBE

We report two new epitaxial systems, prepared by MBE: basal plane epitaxy of the rare earth metal Dy onto LaF 3 films and vice versa. SQUID magnetometry studies indicate that buried epitaxial Dy films, of ∼300A thick, order ferromagnetically at similar temperatures to bulk Dy crystals.These epitaxial systems are one member of a new family of epitaxial systems of basal plane epitaxy of rare earth metals on rare earth fluorides and vice versa. Such systems may be used to probe the effects of strain and dimensionality on magnetic ordering in rare earth metal films and multilayers.

Effect of atmospheric composition and pressure on the laser ablation of (GeTe)(85)Sn(15) chalcogenide thin films.

Laser ablation of (GeTe)(85)Sn(15) thin films as a function of atmospheric exposure was monitored in real time by transient reflectivity. The observed optical changes were correlated with microstructural analysis. Among the key findings were that the presence of water in the atmosphere during laser irradiation of a thin-film structure reduced the incident laser power required for ablation by as much as a factor of 2. The magnitude of the effect was dependent on both H(2)O vapor pressure and duration of exposure to the vapor. The reduction of laser power necessary to ablate was partially reversed by exposure of the thin-film structure to vacuum. Significantly, exposure to other (dry) gases such as N(2) did not change the ablation threshold from that observed in vacuum. We determined that dome formation and ablation occurred at lower temperatures in the presence of water. In addition, the power necessary to crystallize the amorphous chalcogenide layer in the structure was independent of atmospheric composition or pressure. Microstructure analysis showed the presence of H(2)O fostered the formation of a nonuniform distribution of the chalcogenide material in the ablated region. The experimental results are consistent with our model that ablation is assisted by high pressures produced by vaporization of absorbed liquid water.