Third-harmonic susceptibility for studying dissipation in heavy ion irradiated (Cu,C)Ba₂Ca₃Cu₄O_12−y high-temperature superconductors

A slice of the (Cu,C)Ba₂Ca₃Cu₄O_12−y bulk superconductor grown by high-pressure synthesis was polished down to a thickness of 50 μm, and then irradiated with high-energy (240 MeV) Au¹⁵⁺ ions (10¹¹ cm⁻²). Since the length of the ion tracks was estimated to be 12.2 μm, our sample consists of two different types of grain, with and without columnar defects, having very different critical current densities. Using the third-harmonic response of the sample, we were able to separate the small dissipation in the irradiated grains from the much larger dissipation in the non-irradiated grains, in a certain experimental range of temperature, field and current density. The critical current density was estimated from critical state models. For temperatures in the range of 85–105 K, we observed a peak effect at high dc magnetic fields, suggesting the possibility of a record irreversibility field at 77.3 K.