Magnetic flux in a superconductor is quantized in units of h/2e where h is Plank's constant and e is the charge of the electron. Superconducting circuits operate using Josephson junctions, and if the flux is quantized, or trapped, near the Josephson junctions then the circuits can not operate.
One strategy to deal with this problem is to design holes in the superconducting ground plane to attract and trap flux in places it will do the least harm. However, there has been a great deal of disagreement on the best design for these holes. Many circuit designers use small holes close to the Josephson junctions, other use moats, or elongated holes surounding the superconducting circuit to be protected.
We have therefore designed a chip with superconducting circuits surounded by various configurations of moats and holes in the superconducting ground plane. The chip has then been cooled in various magnetic fields and imaged using a high resolution scanning SQUID microscope at IBM. The images clearly show the location of the flux quanta and demonstrate that in weak fields continuous moats more effectivelly trap flux, protecting the circuits, than small holes in the same configuration.
Following are various magnetic images of superconducting circuits. Just click on the small picture to see the full image and learn more.
The scanning SQUID miscroscope consists of a 4 micron diameter pickup loop with 8 micron line width. The loop is integrated into a dc SQUID and scanned across the sample.
In this figure holes and moats have been patterened surounding the two MVTL OR gates. Isolated flux quanta are observed as dots, and the holes and moats are clearly seen when the flux is trapped inside. These data demonstrate that continuous moats more effectively protect the circuits than small holes in the same configuration.
These three images show small continuous moats surounding the inside of the MVTL circuits. We find that the smaller the area of the continuous moat, the higher the field that the circuit is protected.
This is a magnetic field image of a large MVTL divide by eight circuit. Each of the components is surrounded by a continuous moat.
Mark Jeffery, T. Van Duzer, J. R. Kirtley and M. B. Ketchen, Magnetic Imaging of Moat-Guarded Superconducting Electronic Circuits, Appl. Phys. Lett. 67 (12), pp1769-1771, 18 September 1995.
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