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We have measured the thermopower S(T,H) and resistivity ρ(T,H) of laser ablated La0.67Ca0.33MnO3 films, as a function of temperature and external magnetic field. On heating, a metal–insulator transition occurs at temperatures below the resistivity peak, observed via a jump between a low T regime, where S∝T, and a high T regime, where S∝1/T. An applied magnetic field shifts the transition in a postannealed sample to higher temperatures, causing a giant magnetothermopower effect 100×(S8T−S0T)/S8T=−1400%. Both S(T) and ρ(T) are activated at high temperatures, but with significantly different activation energies. We interpret this as an evidence of small polarons at high temperatures.

The effect of a postdeposition anneal on the structure and dielectric properties of epitaxial BaxSr1−xTiO3 (BST) thin films (x=0.35–0.65) have been measured. The films were grown by pulsed laser deposition on LaAlO3 (001) substrates. The films were single phase and (001) oriented with a lattice parameter larger than the bulk. The dielectric properties of the x=0.35 film exhibited a broad temperature dependence and a peak at 168 K, which is 36 K below the peak observed in bulk BST (x=0.35). Annealing films for 8 h in flowing oxygen at 900 °C caused the lattice parameter to decrease and dielectric properties to become more like the bulk. Annealing also resulted in an increased electric field dependent dielectric tuning without increased dielectric loss.

We report on the design, fabrication, and testing of novel YBa2Cu3O7−δ/SrTiO3/LaAlO3 (YBCO/STO/LAO) and Au/SrTiO3/LaAlO3 (Au/STO/LAO) coupled microstrip line phase shifters (CMPS). These CMPS were tested at frequencies within the Ku and K bands (12–20 GHz), at temperatures from 24 to 77 K, and at dc voltages (Vdc) from zero to 350 V. A relative insertion phase shift (Δφ) of 390° was measured for an eight-element YBCO/STO/LAO CMPS at Vdc=350 V, 16 GHz, and 40 K. At 77 K, a Δφ ∼260° was obtained for the CMPS at the same bias and frequency. Both results correspond to an effective coupling length of 0.33 cm. At both temperatures, the phase shifter exhibits a figure of merit of ∼30°/dB. To our knowledge, these are the best results published so far at these frequencies where miniaturization, insertion loss, and phase delay are key considerations.

Colossal magnetoresistance of the variable range hopping regime in the manganites

Solid-state precursor reactions have been investigated as a general synthetic route to binary 111-V (1 3-1 5) compounds.The generic reaction scheme MXs + Na3Pn -MPn + 3 NaX (M = Al, Ga, In; X = F, C1, I; Pn = pnictogen = P, As, Sb) has been used to prepare crystalline powders of the 111-V semiconductors. The reaction mixtures can be either heated in sealed tubes or ignited with a hot filament, and the byproduct salts are simply removed by washing with an appropriate solvent. The ignited reactions are self-propagating and highly exothermic, owing to the formation of 3 mol of sodium halide. Products from both types of reactions have been characterized by powder X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and solid-state NMR. In some cases, the products of the ignited solid-state metathesis (SSM) reactions differ from those of the sustained heating reactions. These differences provide clues as to reaction pathways in the solid-state precursor reactions.

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Thin films of a new, metastable phase of superconducting niobium nitride have been grown using pulsed laser deposition (PLD). This new NbN phase is stabilized by heteroepitaxial growth on (100) Mgo and is shown to be a primitive cubic (Pm3m) distortion from the typical B1, or rocksalt structure. Structural and electrical characterization reveals that this NbN phase has a highersuperconducting critical temperature and a larger lattice parameter when compared with films of B1-NbN. Growth of this new phase demonstrates that PLD can be used as a synthesis tool to deposit new, metastable materials.The next generation of microelectronic devices will be based on the discovery of new materials and the development of related processing technology. The traditional methods of synthesis and preparation, typically involving high temperatures and long reaction times, have been used to investigate a large number of combination of elements. These reactions usually occur near chemical equilibrium, producing the thermodynamically stable products. The materials of the future will require the production of metastable compounds, synthesized far from chemical equilibrium by new reaction methods. Currently, high-powered lasers are being incorporated in a new physical vapor deposition technique called pulsed laser deposition (PLD). ' PLD is being used in the development of new synthesis routes to the formation of metastable compounds so that their intrinsic properties can be accurately measured and evaluated for use in future applications. The deposition of thin films of metastable compounds, such as the infinite layer oxides Ca, "Sr"CuOz,cubic boron nitride (c-BN), carbon nitride (CN"), and amorphous Ge, C",using PLD has been reported.In this paper, we report on the synthesis of a new phase of superconducting NbN by PLD and the structural and electrical characterization of this material. Niobium nitride is a refractory material with a bulk superconducting transition temperature (T, ) of -16 K.Thin films of NbN, typically deposited by sputtering methods, ' also are being studied for applications in electronics. The low chemical reactivity, mechanical durability, high T, ( T, ))4.2 K), and ease with which Josephson junctions can be reproducibly manufactured make NbN a good candidate for use in low-temperature digital electronics. We have demonstrated that, depending on the pressure of the reactive gas atmosphere [Nz/H2 (10%)], any one of several different NbN"(0(x (1.4) phases could be grown on 600'C (100) MgO substrates by ablating niobium targets. ' '" At deposition pressures between 1 and 20 mTorr, metallic Nb2N was deposited, and at pressures greater than 100 mTorr, insulating Nb3N~was grown. Superconducting NbN films were deposited at 60 mTorr. The NbN films grew in one of two different structures. One of these structures has never been reported. In this paper, we describe this new phase of NbN and compare it to the well-known NbN phase with the B1 structure.The films were grown on heated (100) MgO by ablating Nb foil with a pulsed ...