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The Effect of Interfaces on Heat Transport Mechanism of Y-Ba-Cu-O Thin Film Superconducting Bolometer

Nazifi Takantapeh, Rana | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 54080 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Vesaghi, Mohammad Ali; Fardmanesh, Mahdi
  7. Abstract:
  8. In this thesis, we are focusing on the heat transport mechanism in one of the most applicable kinds of superconducting detectors called, YBa2Cu3O7-x (YBCO) transition-edge bolometer. The Time Domain Thermoreflectance (TDTR) method is described as a tool for investigating the thermal boundary conductance and thermal conductivity of the materials, and the measured thermal conductivity of our YBCO films is reported here. Although some recent papers have shown a dependence of the thermal conductivity of the YBCO films to their thickness, our results on many samples with different substrates didn’t show the reported thickness dependency and we have obtained the thermal conductivity of the YBCO films equal to 1.2 W/m.K on average. However, for the thick films that have a rough surface, we have observed smaller thermal boundary conductance between the YBCO and Aluminum thin films, which is associated to be more due to the surface roughness of the film. We have observed that changing the percentage of the oxygen in the structure of our YBCO crystal does not change the thermal conductivity in the c-axis direction either. But, for the films fabricated with two methods of sputtering and thermal co-evaporation, a significant difference in their thermal conductivity is observed.The response of the fabricated bolometers on three substrates including SrTiO3, MgO, and YSZ, and the effect of the substrate roughness and radiation source wavelengths on the response of the bolometers are also investigated to better understand the heat transfer mechanism in the fabricated bolometers. For the first time, the frequency response of a monolithic YBCO transition-edge bolometer without coupled antenna, to the millimeter waves is investigated and reported here, and we have observed dependence on the polarization of the 3-millimeter wave radiation. We speculate that when the polarization of the incoming 3-mm waves is parallel to the bolometer meander lines, the bolometer pattern itself acts as an absorbing antenna so that the absorption takes place in the YBCO film. In the other case, when the polarization of the incoming waves is perpendicular to the meander lines, the antenna absorption effect and consequently the YBCO absorption is reduced so that we observe a different frequency response behavior. In the latter case, it is speculated that the waves are absorbed in the substrate and then the generated heat increases the temperature of the YBCO film. Hence in this case, the direction of heat flow to the interface is reversed and results in a significant decrease in the amplitude of response with a large phase lag. For investigating the antenna effect on the 3-mm wave absorption mechanism, we have further measured the response of the detector at five different temperatures within the Normal-Superconductor transition region corresponding to five different electrical conductivities associated with different ratios of the normal electrons to the Cooper pairs of the YBCO film. The measurements show that there is an optimum conductivity for having the highest absorption in the meander line patterned YBCO film. Based on this analysis, the finite element modeling of the absorption in the detector structure is also compatible with the experimental results. Also, we have seen that the frequency response of the bolometer fabricated by the YBCO films on rough substrates is higher than that of the similar bolometer without the substrate surface roughness at high modulation frequencies. We interpret this effect to be a consequence of different interface properties and higher loss of the surface currents at the interface in this sample. The responsivity value for the bolometer labeled YSZ3 at 110 Hz modulation frequency and 87 µA bias current is obtained as 13 V/W and this number has the potential to be increased two orders of magnitude.These detectors fabricated by the economical Metal-Organic Deposition method working at temperatures above the liquid nitrogen temperature are very good candidates for imaging applications as for large area single-pixel or arrays of detectors
  9. Keywords:
  10. Detector ; High Temperature Superconductor ; Millimeter Wave ; Thermal Conductivity ; Heat Transfer ; Bolometer ; YBCO Superconductor

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