Fig. 10-1 High-Tc superconductor under a magnetic field The crystal structure of High-Tc superconductors is like a sandwich composed of a CuO2 plain realizing superconductivity and an insulating one. When a magnetic field is applied parallel to the plain, Josephson vortices enter and form a triangular lattice.

Fig. 10-2 Voltage vs. magnetic field The simulation results for the magnetic field dependencies of the voltage under three transport current values show oscillating behavior, which has been observed in the experimental results.

Fig. 10-3 The change in vortex dynamics caused by increasing the magnetic field Josephson vortex density increases with the magnetic field and Josephson vortices move in the direction indicated by the green arrow. Generally, the sample edges provide a barrier for the vortex motion, as schematically shown in the top, while the vortices form a lattice to retain the vortex distance and the lattice slides. As a result, when the vortices simultaneously enter and leave at the sample edges, the resistance for the lattice motion increases (see the top and bottom situations) and the voltage drops according to Faraday's law of electromagnetic induction. This effect regularly appears when the magnetic field increases, as shown in Fig. 10-2.

Since the discovery of High-Tc superconductivity, many investigations have been completed and various unique properties have been clarified. For example, the crystal structure stacked along the c-axis shown in Fig. 10-1 results in a weak Josephson coupling between neighboring CuO2 plains and forms naturally stacked Josephson junctions. To date, the Josephson junction has been expected to be an alternative device showing high-speed electromagnetic response as compared to a semiconductor. Thus, High-Tc superconductors are now regarded as promising materials which have a great advantage in mass-production of homogeneous Josephson junctions since the crystal itself is a natural Josephson junction. Recently, an experimental research group in the National Institute for Materials Science reported that the electrical resistivity in High-Tc superconductors periodically oscillates when an applied magnetic field is varied and that the periodicity is very precise for changes of the magnetic field. We have investigated this phenomenon by performing computer simulations because this feature may be quite useful for detecting tiny magnetic field variations induced by defects inside materials. Consequently, we succeeded in reproducing the experimental results, as shown in Fig. 10-2, and have found that the voltage depression occurs when Josephson vortices simultaneously enter and leave at the sample edge. The origin of the depression is as follows. Generally, Josephson vortices form a triangular lattice and move under transport currents keeping the lattice structure, while they feel a surface barrier at the sample edge. Thus, the lattice encounters strong resistance for its motion when Josephson vortices simultaneously enter and leave as shown in Fig. 10-3. Furthermore, the fact that the lattice constant is proportional to the magnetic field indicates that the lattice feels strong resistance only when the magnetic field satisfies a condition that the lattice constant is commensurable with the crystal width. Now, we are studying how far the periodic quality holds with respect to the temperature and the magnetic field and examining the possibilities of using this phenomenon in a practical device.

Reference M. Machida, Dynamical Matching of Josephson Vortex Lattice with Sample Edge in Layered High-Tc Superconductors, Phys. Rev. Lett., 90, 037001-4(2003).

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