David W. Reagor

Tunable and adaptive bandpass filter using a nonlinear dielectric thin film of SrTiO3

We have prepared an electrically tunable and adaptive 3‐pole half‐wave bandpass coplanar waveguide filter incorporating a 1.2‐μm‐thick paraelectric SrTiO3 bottom layer and a 0.4‐μm‐thick superconducting YBa2Cu3O7−x top electrode layer on a LaAlO3 substrate. By applying a separate bias voltage on each pole and also on each coupling capacitance of the device at 4 and 76 K, the filter response is not only fine tuned to achieve symmetric and optimized filter characteristics with less than 2% bandwidth centered around 2.5 GHz, but also broadband tuned to shift the passband by more than 15%.

Electrically tunable coplanar transmission line resonators using YBa2Cu3O7−x/SrTiO3 bilayers

We have prepared electrically tunable microwave resonators incorporating superconducting YBa2Cu3O7−x (YBCO) and paraelectric SrTiO3 (STO) layers on LaAlO3 substrates. The top YBCO layer for each sample was patterned into a 8 mm long coplanar transmission line with a 40 μm gap and a 20 μm center line width. The microwave transmission through the coplanar transmission line exhibits resonances corresponding to standing microwaves along the coplanar transmission line. These resonances are modulated by applying a bias voltage between the center line and the ground planes. Samples with a 0.5 μm thick (2 μm thick) bottom STO layer show, for a resonance at around 8 GHz (5 GHz), a frequency modulation of about 4% (24%) and a quality factor Q of about 200 (50) under 100 V bias at 80 K.

High frequency dielectric studies of hydrated Nafion

Abstract This study reports high frequency dielectric measurements (0.045–30 GHz) on Nafion® 117 at various states of hydration. A novel technique to measure the broad band frequency dependent real and imaginary parts of the relative permittivity is described. The basic experimental configuration and numerical data analysis are reported along with a discussion of several difficulties encountered and experimental validation of the method. The preliminary results show a strong dependence of the dielectric constant of the Nafion® 117 membrane with water content. The dielectric constant for all hydrated membrane samples was observed to be constant over the initial part of the frequency span, ranging from a maximum of 20 in the samples with 13 waters per sulfonate to a minimum of four in very dry samples. The results reflect the decreasing polar environment of the water at low water contents as well as the increasing extent of binding of the water at the fixed ionic site at low water content. A ‘roll off’ in the dielectric constant, the extent of which was dependent on water content, was observed also in the frequency spectrum. Several possible origins for this ‘roll-off’, including real effects of dipolar relaxations occurring in the hydrated polymers and experimental effects due to the similarity of the wavelength of radiation to the sample dimensions are discussed. Finally, the conductivity of the membranes at various water contents and at frequencies below 5 GHz was extracted from the loss factor spectra. These values agreed well with previous conductivity measurements (obtained at lower frequencies) suggesting that no relaxations are observed in the intervening frequency range (roughly 5–50 MHz).

Improvement in performance of electrically tunable devices based on nonlinear dielectric SrTiO3 using a homoepitaxial LaAlO3 interlayer

Improved structural and dielectric properties of nonlinear dielectric SrTiO3 thin films on LaAlO3 substrates were accomplished by incorporating a homoepitaxial LaAlO3 interlayer between the substrate and the dielectric film. Using this interlayer, the quality factor of SrTiO3 films with high-temperature superconducting YBa2Cu3O7−x electrodes on LaAlO3 substrates was improved by more than 50% at 4.2 GHz and 4 K. This improvement, combined with no change in nonlinearity, led to greater than a 50% enhancement of the finesse factor (defined as the product of the quality factor and the fractional shift resonant frequency) for the coplanar waveguide microwave resonators. The reduced planar defect density in the SrTiO3 films was attributed to this improvement.

Large magnetoresistance in La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3 ramp-edge junctions

We report on the fabrication of ferromagnet–insulator–ferromagnet junction devices using a ramp-edge geometry based on (La0.7Sr0.3)MnO3 ferromagnetic electrodes and a SrTiO3 insulator. The maximum junction magnetoresistance (JMR) as large as 23% is observed below 300 Oe at low temperatures (T<100 K). Our ramp-edge junctions exhibit JMR of 6% at 200 K with a field less than 100 Oe. The device performance at room temperature is believed to be limited by both the nearly equivalent coercive fields in the electrodes and the magnetization process, rather than by the insulating barrier.

Work function of the mixed-valent manganese perovskites

We have performed resistivity and work function measurements on a series of samples with Ln0.7D0.3MnO3 composition, where Ln is a lanthanide (La, Pr, or Nd) and D is a nominally divalent ion (Sr, Ca, or Pb). The resistivity measurements are, within some small variation, in agreement with those previously reported for polycrystalline samples of similar composition. The measurement of a work function in air is often difficult to reproduce, mainly owing to the fact that it is a surface probe and can be modified significantly by attachment of contaminants from the atmosphere. Here we have obtained reproducible results that are slightly more stable than the reference metals used in the measurements. This required the introduction of a higher order correction to a common Kelvin probe experimental procedure. Another factor contributing to the high quality of the results is the low carrier density of the conducting oxides. This allows greater penetration of quasistatic electric fields. The work function results h...

Molecular modeling of trifluoromethanesulfonic acid for solvation theory

Abstract Reported here are theoretical calculations on the trifluoromethanesulfonic (triflic) acid with and without an additional water molecule, establishing molecular scale information necessary to molecular modeling of the structure, thermodynamics, and ionic transport of Nafion® membranes. The optimized geometry determined for the isolated triflic acid molecule, obtained from ab initio molecular orbital calculations, agrees with previous studies. In order to characterize side chain flexibility and accessibility of the acid proton, potential energy and free energy surfaces for rotation about both carbon–sulfur and sulfur–oxygen(hydroxyl) bonds are presented. A continuum dielectric solvation model is used to obtain free energies of electrostatic interaction with the solvent. Electrostatic solvation is predicted to reduce the free energy barrier to rotation about the F3C–SO3 bond from 3.5 kcal/mol to about 2.7 kcal/mol. This electrostatic effect is associated with slight additional polarization of the CF bond in the eclipsed conformation. The energetic barrier to rotation of the acid hydroxyl group away from the sulfonic acid oxygen plane, out into the solvent is substantially flattened by electrostatic solvation effects. The maximum free energy for those solvent accessible proton conformations is about 1.0 kcal/mol. We carried out additional ab initio electronic structure calculations with a probe water molecule interacting with the triflic acid. The minimum energy structures found here for the triflic acid molecule with the probe water revise results reported previously. To investigate the reaction path for abstraction of a proton from triflic acid, we found minimum energy structures and energies for isolated molecular fragments, and solvation free energies for: (a) a docked configuration of triflate anion and hydronium cation and (b) a transition state for proton interchange between triflic acid and a water molecule. Those configurations are structurally similar but energetically substantially different. The activation free energy for that proton interchange is predicted to be 4.7 kcal/mol above the reaction end-points.

Superconductor/nonlinear-dielectric bilayers for tunable and adaptive microwave devices

Microwave devices using coplanar waveguide (CPW) structures which incorporate bilayers of superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ and nonlinear dielectric SrTiO/sub 3/ on 1 cm/spl times/1 cm/spl times/0.5 mm LaAlO/sub 3/ substrates. The microwave devices include voltage-tunable phase shifters and delay lines, voltage-tunable mixers, and voltage-tunable and adaptive bandpass filters. Under dc voltage bias, these compact CPW devices exhibit more than 360/spl deg/ relative phase shift at 2 GHz, more than 30 dB tunability in mixed microwave power between 0.7 GHz and 5.9 GHz, and fine-tunable symmetric filter profile at around 2.5 GHz with less than 2% bandwidth and more than 15% adaptive range.

Observation of spin-dependent transport and large magnetoresistance in La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3 ramp-edge junctions

We have fabricated ferromagnet-insulator-ferromagnet junctions using a ramp-edge geometry based on (La0.7Sr0.3)MnO3 ferromagnetic electrodes and a SrTiO3 insulator. Pulsed laser deposition was used to deposit the multilayer thin films and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance is dependent on the relative orientation of the magnetization in the electrodes. A junction magnetoresistance (JMR) as large as 30% is observed at low temperatures and low fields. In addition, we have found that JMR is reduced at high temperatures (T>100 K) and decreases monotonically with increasing field at high fields (0.5 T

DEVELOPMENT OF A RELIABLE MATERIALS BASE FOR SUPERCONDUCTING ELECTRONICS

Careful studies of the corrosion, redox, galvanic and oxygen evolution/uptake reactions associated with YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and related compounds have been completed. These studies have led to an understanding of the many factors that contribute to the poor material characteristics exhibited by these popular high-T{sub c} phases. With knowledge of the structure-reactivity relationships, a powerful crystal engineering approach has been developed that is capable of producing cation substituted versions of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}; the resulting compounds therefrom produced exhibit markedly improved processability, oxygen stability, and durability characteristics. These materials have been combined in thin film structures so as to make prototype SNS junctions and SQUID sensors which exhibit promising device performance characteristics. {copyright} {ital 1997 Materials Research Society.}