Thin-film superconductive edge-transition bolometers are modeled with a one-dimensional analytic thermal model with joule heating, film and substrate materials, and the physical interface effects taken into consideration. The results from the model agree well with the experimental results of samples made of large-meander-line Yba(2)Cu(3)O(7-x) films on crystalline SrTiO(3), LaAlO(3), and MgO substrates up to 100 kHz, the limits of the experimental setup. Compared with the results of the SrTiO(3) substrate samples, the results from the model of the LaAlO(3) and the MgO substrate samples deviate slightly from the measured values at very low modulation frequencies (below ~10 Hz). The deviation increases for higher thermal-conductive substrate materials. When the model was used, the substrate absorption and the thermal parameters of the devices could also be investigated.
The phase and amplitude of the response to infrared radiation of superconducting bolometers was investigated. The detectors were fabricated with 120–550 nm Y1Ba2Cu3O7−δ films on MgO, SrTiO3, and LaAlO3 substrates. A model for the response is developed and compared to the experimental results. The response versus frequency of the samples shows bolometric behavior, in agreement with the measured time dependence of the signals at low frequencies. Different techniques were developed to measure the key parameter in the response of the bolometers, G, the thermal conductance. Several anomalies were observed in the study that provide insight into heat conduction in these devices. The dc or low modulation frequency thermal conductance, G(0), of the samples is found to be limited by the substrate/cold-head thermal boundary resistance. It was also found to decrease with increasing substrate thickness, but still limited by the substrate/cold-head interface. This thickness dependence of G(0) is attributed to scattering of phonons within the substrate, that changes their transmission rate through the substrate/cold-head interface. The results from simultaneous measurements of the IR response and dR/dT (using R versus T) at low modulation frequencies (20 Hz) show that the magnitude of the response differs by up to 30% from the dR/dT curve. The discrepancy is found to be frequency dependent, increasing with decreasing modulation frequency.
This can be treated by use of temperature-dependent thermal constants (G and the heat capacity, C) in the model for the bolometric response. The discrepancy is also observed to be dependent on the superconducting transition, which suggests a possible correlation between the heat conduction through the substrate (and its interfaces) and the absorption mechanism in the superconductor. The phase of the response versus temperature shows an abrupt change at the transition. This is evidence for a change in thermal constants in the bolometer as it goes through the superconducting transition, affecting both the phase and magnitude of the response. Joule heating at even high bias currents has little effect on the response (and its deviation from the dR/dT curve) in our samples, and the effect of noise on the response is significant only at very low bias currents.
Step edge grain boundary (GB) junctions and rf-SQUIDs have been made using pulsed laser deposited Y-Ba-Cu-O films on crystalline LaAlO3 substrates. The steps were developed using various ion-beam etching processes resulting in sharp and ramp type step structures. Sharp step based GB junctions showed behavior of serial junctions with resistively shunted junction (RSJ) -like I-V characteristics. The ramped type step structures resulted in relatively high critical current, Ic, junctions and noisy SQUIDs. The sharp steps resulted in low noise rf-SQUIDs with a noise level below 140 fT/Hz1/2 down to few Hz at 77 K while measured with conventional tank circuits. The Ic of the junctions and hence the operating temperature range of the SQUIDs made using sharp steps was controlled by both the step height and the junction widths. The junction properties of the SQUIDs were also characterized showing RSJ-like characteristics and magnetic field sensitivities correlated to that of the SQUIDs. Two major low and high background magnetic field sensitivities have been observed for our step edge junctions and the SQUIDs made on sharp steps. High quality step edge junctions with low magnetic field sensitivities made on clean sharp steps resulted in low I/f noise rf-SQUIDs proper for applications in unshielded environment
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