Charles F. Carter

HTS sensors for NQR spectroscopy

Using a high temperature superconductor (HTS) sensor, we have detected a /sup 14/N nuclear quadrupole resonance (NQR) signal from a room temperature sodium nitrite (NaNO/sub 2/) sample. This demonstrates the feasibility of using such sensors for the NQR detection of contraband, e.g. explosives. The sensor is composed of two high Q-value, self-resonant HTS devices strongly coupled to each other that detect the induced magnetization of the target compound. We briefly describe our techniques for making, adjusting the resonant frequency and spoiling the Q-value of the HTS sensors.

High T/sub c/ superconductor and III-V solid state microwave hybrid circuits

Several high-temperature superconductor (HTS)/III-V solid-state hybrid microwave circuits were designed, fabricated, and tested. The I-V curves, S-parameters, and noise behavior for several solid-state devices at cryogenic temperatures were measured. Several high-electron-mobility transistor (HEMT) and heterojunction bipolar transistor (HBT) C-band low-noise amplifiers were fabricated and tested at cryogenic temperatures with an additional gain of 3 dB when compared to their room-temperature gain. These amplifiers were also used in low-phase-noise oscillators stabilized by a HTS lambda /2 microstrip line resonator with a loaded Q-value of 3*10/sup 3/ or a sapphire-HTS resonator with a loaded Q-value of 1.2*10/sup 6/, both measured at 80 K. Preliminary measurement at 70 K indicates that the phase noise of an oscillator stabilized with a sapphire-HTS resonator was below -125 dBc/Hz at 10-kHz offset and limited by the test setup.<<ETX>>

Raman microprobe analysis of patterned Tl-2212 thin films

We have used MicroRaman spectroscopy to evaluate the effects of ion-milling on the exposed edges of patterned Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ (Tl-2212) lines. Raman microprobe has previously been used to evaluate oxygen loss at the edges of patterned YBCO lines. The results indicated that appreciable oxygen loss was caused by ion-milling under certain conditions. oxygen loss at the edges will decrease non-uniformly the effective width of the superconducting line. This can, in turn, impact the electrical characteristics of patterned devices. Using the 633 nm lined of a HeNe laser, we have measured the Raman spectrum scanning across patterned and unpatterned regions of several Tl-2212 films with a mapping stage. It is well known that the Raman peak at /spl ap/497 cm/sup -1/ corresponds to the Cu-O(2) stretching mode and is correlated with the superconducting transition temperature of the material. No appreciable variation in the center frequency of the Cu-O(2) peak was observed indicating thallium cuprate films are not degraded by our patterning process, even at the edges. Variations in the power handling of Tl-2212 co-planar lines, as determined by measurement of the Third Order Intercept, were not correlated with the Raman results.

Superconducting and normal metal multilayers of thallium-lead cuprates

We report the growth of c-axis oriented, epitaxial thin films of (Tl,Pb)Sr/sub 2/CuO/sub 5/ on (100) oriented LaAlO/sub 3/ substrates by off-axis sputter deposition in the presence of thallium oxide vapor. These films display a resistivity which crosses over from metallic to semiconducting behavior at low temperature. In addition, we have shown that multiple layers of superconducting (Tl,Pb)Sr/sub 2/Ca/sub 0.8/Y/sub 0.2/Cu/sub 2/O/sub 7/ (1212) and nonsuperconducting (Tl,Pb)Sr/sub 2/CuO/sub 5/ (1201) can be deposited sequentially without compromise of the superconducting properties of the 1212 layer. Bilayers of both 1212 on 1201 and of 1201 on 1212 have been fabricated. In either case, both materials grow with their c-axis normal to the substrate surface and with their a-axes aligned to each other and to the major axes of the substrate. Technologically interesting trilayer structures of 1212/1201/1212 have also been fabricated. Both 1212 layers are superconducting with transition temperatures of up to 94 and 93 K for the upper and lower layers respectively. We believe that this is an important step towards the development of thallium cuprate-based multilayer technology.<<ETX>>

S-Parameter, I-V Curve and Noise Figure Measurements of III-V Devices at Cryogenic Temperatures

We have measured the electrical operating parameters for several three-terminal semiconductor III-V devices at both room temperature and liquid nitrogen temperature, 80 K. The fundamental performance parameters, I-V curve, S-parameters and noise figure, change quite dramatically upon cooling. It is necessary to know these parameters if an optimal cryogenic device, e.g. high-temperature superconducting/III-V hybrid, is to be designed and fabricated. The electrical operating parameters for a Fujitsu GaAs high electron mobility field effect transistor, HEMT, and a Litton GaAs metal semiconductor field effect transistor, MESFET, were measured at room temperature and at 80 K. The data collected on these devices were used to design a III-V C-band amplifier with optimum operation at 80 K. This amplifier was used to construct a high-temperature superconducting/III-V hybrid oscillator, a major step towards the high degree of circuit integration required for a useful HTS/III-V hybrid subsystem or system.

High Power Intermodulation Measurements up to 30 W of High Temperature Superconducting Filters

We have demonstrated a high power intermodulation measurement set-up capable of delivering 30 W in each of two fundamental tones. For closely spaced frequencies (< 35 MHz), the dynamic range of the measurement is limited by the nonlinear performance of the mixer in the front end of the HP71210C spectrum analyzer. A tunable TE011 mode copper cavity was fabricated in which one of the endwalls could be adjusted shifting its resonant frequency between 5.7 and 6.6 GHz. Since the Q-value of this cavity is high, > 104, and its bandwidth is small, < 1 MHz, it can be used to attenuate the two fundamental tones relative to one of the harmonic tones, which greatly enhances the dynamic range of the measurement. This set-up can be used to measure the two-tone intermodulation distortion of any passive microwave device, e.g. a HTS filter, a connector, a cable, etc., over a frequency range of 5.9 to 6.4 GHz and a power range of 0.1 to 30 W. The third order intercept (TOI) of a prototype HTS filter measured at powers up to 30 W was +81.3 dBm.