Investigations of AC response in certain type II superconductors

Abstract

We have intentionally varied the microstructures of polycrystalline YBCO samples and studied their AC loss behavior. A close correlation between AC losses and the microstructures of the superconducting samples is observed. The AC loss measurements in the superconductor YBCO by nonresonant RF power absorption show a well-defined increase in the AC loss in the granular superconductor with decreasing temperature. The AC loss response below the critical temperature of the granular superconducting sample is found to be mainly due to the mechanism of frequent Josephson junction (JJ) decoupling. This effect is also seen in other high Tc superconductors such as BSCCO, LSCO, and MgB both in the polycrystalline and single-crystal forms. The AC loss due to the frequent JJ decoupling is directly proportional to the applied AC frequency, so it is expected that it will give a bigger contribution at microwave frequency. These studies provide both qualitative as well as quantitative understanding of the AC losses in superconductors and would be useful for various AC applications of high Tc superconductors.

To summarize, a close correlation between the AC loss and the microstructures of the superconducting samples is observed. The temperature-dependent AC loss measurements in the superconductor MgB show two well-defined branches below the critical temperature. We argue that this behavior in MgB is related to the two-gap nature of its superconducting state and Josephson junction decoupling. AC loss in granular MgB sample reduces with increasing magnetic field at certain temperatures below Tc. Sintering at 750°C for 5 hours, increasing pressure, or decreasing grain size reduces the AC loss in granular MgB sample in the RF range. Increasing the RF amplitude above a certain critical value can make the AC loss in the superconducting state more than its nominal state.

In conclusion, we have studied the superconductor MgB and MgB alloyed with 10% by weight of MgO using the technique of nonresonant microwave absorption (NRMA). The NRMA signals from MgB are very different from that of high-Tc superconductors (HTSC). Low-field derivative-like narrow signals characteristic of the Josephson junctions are absent, while the rectangular hysteresis loops indicative of large critical current densities due to strong pinning are observed. The lower critical field is observed to decrease linearly with temperature. MgB when alloyed with MgO, however, shows the presence of intergranular contacts acting as weak links and a weaker pinning as compared to pure MgB. The absence of phase reversal of the NRMA signal at microwave frequency is noted as another unique feature of this system.

In conclusion, a strong correlation between the kinetic inductance and the applied magnetic field is observed in the superconducting strip of BSCCO-2223. It is interpreted that the phenomenon is due to the increase of Campbell length with increasing magnetic field. This phenomenon is used for designing a superconducting modulator circuit. Our preliminary result shows that the phenomenon is also observed in other superconducting strips made of high Tc superconductors like YBCO, LSCO, and MgB. This phenomenon can be used for device applications and designing other superconducting electronic circuits like superconducting power amplifiers, superconducting transmitters, and superconducting AC current controllers.

In summary, we have reported a novel phenomenon with respect to the division of current in parallel conducting paths, in which the total current is found to divide equally among the paths irrespective of their inductances and the normal state resistances as the system is cooled to and below the common superconducting transition at Tc. This equipartition should be relevant to system geometries where the divided (series-parallel) superconducting circuit is cooled to and below Tc in the presence of a transport current. It also provides an interesting laboratory example for self-organization without fine-tuning and for the application of the principle of extremal dissipation, which has been attracting considerable attention lately.