No abstract

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