Arthur W. Lichtenberger

Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices

Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

A Broadband Quasi-Optical Terahertz Detector Utilizing a Zero Bias Schottky Diode

A quasi-optical broadband terahertz detector using a zero bias Schottky diode mounted on a self-complimentary sinuous antenna has been developed. Design and characterization of this detector are described. Measurements show that a responsivity of 300-1000 V/W covering the frequency range of 150-440 GHz has been achieved. The detector performance has been compared to waveguide detectors covering four frequency bands up to 600 GHz. A recent measurement at 600-900 GHz yielded the same output voltage as a waveguide detector. The noise equivalent power level of this detector is estimated to be 5-20 pW/√(Hz) based on the measurements of similar detectors.

Fabrication of Nb/Al-Al/sub 2/O/sub 3//Nb junctions with extremely low leakage currents

Nb/Al-Al/sub 2/O/sub 3//Nb trilayer films were deposited using DC magnetron sputtering guns in a UHV (ultrahigh vacuum) system which is capable of 5*10/sup -10/ Torr. SIS (superconductor-insulator-superconductor) junctions as small as 3.2*3.2 mu m/sup 2/ were isolated from the trilayer by standard photolithography. The junctions typically have V/sub m/=70-90 mV at 4.2 K, while at 2.0 K, V/sub m/ is as large as 1 V. This corresponds to a subgap current of 0.15% of the quasiparticle current rise. The subgap leakage current is compared to the predictions of the BCS (Bardeen-Cooper-Schrieffer) theory. The specific capacitance is preliminarily measured to be 45+or-5 fF/ mu m/sup 2/. >

Progress on tunerless SIS mixers for the 200-300 GHz band

An integrated superconductor-insulator-superconductor (SIS) mixer for the 200-300-GHz band with no adjustable tuning is described. A waveguide input is coupled to a coplanar mixer circuit with six individually tuned SIS junctions in series. Using the best mixer, the receiver noise temperature is 45-80 K DSB over 215-275 GHz. The mixer noise temperature at 230 GHz is 12 K DSB, and the conversion loss is 2.5 dB DSB.<<ETX>>

A 200-300 GHz SIS mixer-preamplifier with 8 GHz IF bandwidth

A 200-300 GHz SIS mixer-preamplifier with an IF bandwidth of 8 GHz Is described. The mixer uses Nb/Al-oxide/Nb tunnel junctions in a circuit with low IF capacitance and inductance. The mixer block is designed to mount directly on the body of the 4-12 GHz InP HFET preamplifier and mixer bias is provided through the input circuit of the preamplifier. At a LO frequency of 230 GHz, the mixer-preamp gain is 30-35 dB, and the DSB receiver noise temperature is 45-57 K across the whole IF band. This is the largest instantaneous bandwidth reported to date for an SIS receiver.

Insights Into the Fuel Oxidation Mechanism of La0.75Sr0.25Cr0.5Mn0.5O3 − δ SOFC Anodes

Solid oxide fuel cells (SOFCs) were fabricated by applying reproducible thin, dense La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3-δ (LSCM) anode and La 0.8 Sr 0.2 MnO 3-δ (LSM) cathode films to yttria-stabilized zirconia (YSZ) electrolytes by ultrasonic spray pyrolysis. A gold grid was applied to the anode film by lithography and plating, enabling adequate electrical contact while maintaining an open surface area for reaction. Alignment of the two electrode films and correct placement of the reference electrode enabled the separation of the anode and cathode impedance responses. Although the open-circuit voltage (OCV) of the LSCM|YSZ|LSM fuel cell at 700°C with humidified H 2 fuel was close to that predicted by the Nernst equation, the anode polarization resistance was large (23 Ω · cm 2 ). The addition of Pd to the anode film led only to a small decrease in polarization resistance, indicating that oxygen ion conductivity was the limiting factor for H 2 fuel. The OCV for CH 4 was close to that when an inert gas (He) was supplied to the anode. This OCV increased to 0.87 V upon addition of Pd to the LSCM film surface. This indicates that the bare LSCM film is limited by a low CH 4 activation activity.

Development of Integrated Terahertz Broadband Detectors Utilizing Superconducting Hot-Electron Bolometers

We report on the development of terahertz broadband detectors utilizing superconducting hot-electron bolometers and planar sinuous antennas. In this work, sinuous antennas designed to cover the frequency range of 50 GHz to nearly 900 GHz have been fabricated on semi-insulating silicon substrates. To maintain a self-complementary structure, four antenna arms are used, leading to a frequency-independent input impedance of 74 Omega. Two squares of superconducting niobium HEBs are required at the feed point of the antennas as the sheet resistance of a 10 nm thick niobium thin film is 35 Omega/square. Two sizes of HEBs (120 nm times 240 nm, and 2 mum times 4 mum) have been fabricated using e-beam lithography (EBL), and standard photolithography processes, respectively. A quasi-optical mount with high-resistivity silicon lens has been employed for coupling input power to the sinuous antenna. With a close-cycled cryocooler, the detector performance is studied and evaluated. Detector responsivity at 585 GHz has been presented and compared to waveguide Schottky diode detectors. Noise equivalent power (NEP) measurement is discussed and will be soon performed to the detectors developed in this paper.

A Micromachined Terahertz Waveguide 90

Waveguide twists are often necessary to provide polarization rotation between waveguide-based components. At terahertz frequencies, it is desirable to use a twist design that is compact in order to reduce loss; however, these designs are difficult if not impossible to realize using standard machining. This paper presents a micromachined compact waveguide twist for terahertz frequencies. The Rud-Kirilenko twist geometry is ideally suited to the micromachining processes developed at the University of Virginia. Measurements of a WR-1.5 micromachined twist exhibit a return loss near 20 dB and a median insertion loss of 0.5 dB from 600 to 750 GHz.

^{\circ}

We have developed 585-GHz quasi-optical mixers and focal-plane arrays (FPAs) comprised of planar annular slot antennas (ASAs) with integrated niobium hot-electron bolometers for imaging applications. In order to optimize the single-element mixer design, the embedding impedance of the single ASA presented to the bolometer is analyzed using the induced electromotive force (EMF) method by including the antenna feed contribution. This approach has been further expanded to analyze the ASA self-impedance and mutual impedance in an array by utilizing the even-odd mode analysis. In addition, the far-field radiation patterns of the ASAs mounted to an extended hemispherical high-resistivity silicon lens have been calculated using the ray-tracing techniques. The details of circuit design and fabrication are presented in this study. Single mixer element measurement results have shown that a conversion gain of -11.9 dB and a double-sideband (DSB) receiver noise temperature of ~650 K have been achieved. Initial array imaging experiment results are presented and show excellent agreement with theory and simulation data. A spatial resolution of ~2.75 mm has been demonstrated at 585 GHz for a 1-D mixer FPA that is capable of diffraction-limited imaging. Y-factor measurements show DSB mixer noise temperatures of 1675 and 3517 K with mixer conversion gains of -14.73 and -17.74 dB, respectively, have been obtained for two adjacent elements in a mixer array, which is comparable to the results reported in the literature.

Twist

An improved micromachined on-wafer probe covering frequencies 500-750 GHz is demonstrated in this paper to address sub-millimeter-wave integrated-circuit testing. Measurements of a prototype WR-1.5 micromachined on-wafer probe exhibit a return loss better than 12 dB and a mean insertion loss of 6.5 dB from 500 to 750 GHz. The repeatability of on-wafer measurements with the micromachined probe is investigated. Monte Carlo simulations are used to identify the dominant error source of on-wafer measurement and to estimate the measurement accuracy. The dominant error source is positioning error, which results in phase uncertainty. Reliability tests show the probe is robust and can sustain over 20 000 contacts.