Abstract
The low-field microwave absorption of the Nd1.85Ce0.15CuO4–y electron-carrier superconducting system has been investigated. From the temperature dependence of the absorption signal intensity, it is suggested that superconductivity is achieved in two steps. As the temperature is decreased below the superconducting transition temperature, individual grains in the ceramic sample first become superconducting via intragranular currents. A further temperature decrease then induces improved Josephson couplings between individual superconducting grains to generate intergranular currents which cause the bulk sample to become superconducting and maximize the microwave absorption. Still lower temperature decreases the Josephson coupling efficiency and the microwave absorption signal decreases. The effects of variable microwave power and magnetic-field modulation amplitude on the absorption signal lineshape and intensity are reported and interpreted in terms of a model involving the pinning and depinning of magnetic fluxons at the surface of the granular material. Magnetic-field hysteresis of the absorption signal is found to be weak, even at low modulation amplitudes.
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