The absorption of microwave radiation by the high-temperature superconducting ceramic Y1Ba2Cu3Oyin nearly zero magnetic field is extremely sensitive to the applied magnetic field. The spectra reported in the literature are very noisy. The authors show that this noise is not inherent to the superconducting system but is due to environmental magnetic noise. This noise can easily be removed by proper magnetic shielding which allows the ceramic to be used, without any further materials processing, as a very low magnetic field sensor. They also report the first measurements of this absorption in a single-crystal sample and show why certain explanations about the nature of this absorption can be ruled out.
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