Point contact junctions made from two pieces of MgB2 can be adjusted to exhibit either superconductor-insulator-superconductor (SIS) or superconductor-normal metal-superconductor (SNS) current-voltage characteristics. The SIS characteristics are in good agreement with the standard tunneling model for s-wave superconductors, and yield an energy gap of (2.02+/-0.08) meV. The SNS characteristics are in good agreement with the predictions of the resistively-shunted junction model. DC superconducting quantum interference devices made from two SNS junctions yield magnetic flux and field noise as low as 4 mu Phi (0) Hz(-1/2) and 35 fT Hz(-1/2) at 19 K; Phi (0) is the flux quantum. (C) 2001 American Institute of Physics.
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