Superconducting quantum interference devices, or SQUIDs, are fascinating gadgets. Take a superconducting loop with two weak links (e.g., tunnel junctions, or constrictions with a lower critical current). Now thread magnetic flux through the loop. The superconducting wavefunction, which includes a phase factor that involves the vector potential, must be single-valued around the loop. That means that the phase factor must return to itself modulo 2 pi going around the loop. The phase factor is proportional to the line integral of the vector potential, which itself is the magnetic flux through the loop. Therefore, the total magnetic flux through the loop must be quantized. If the external magnetic field doesn't give an integer number of flux quanta, then the superconductor must generate screening currents around the loop that produce flux and make up the difference. If you had connected the loop to an external current source and run that external current (which splits itself around the two branches of the loop) up to the edge of the critical current, you would find that the screening currents would drive the loop normal and lead to a detectable voltage drop that is periodic in magnetic flux through the loop. This periodicity allows SQUIDs to be phenomenally good magnetic field detectors. One can integrate a tiny SQUID onto a movable probe, and make a scanning SQUID microscope, and do amazing things like figure out the pairing symmetry of high-Tc superconductors.
This week a paper appeared on the arxiv relevant to scanning SQUID microscopy:
arxiv:1002.1529 - Koshnick et al., Design concepts for an improved integrated scanning SQUID
Here, Koshnick, together with scanning SQUID experts Kirtley and Moler, lay out ideas that they have in the works for refining the technology of these gadgets. Neat stuff.
Almost simultaneously, a new paper appeared in Nano Letters on an implementation of an aluminum scanning SQUID microscope. The basic concept, involving the use of a drawn optical fiber tip as a template for deposition of an aluminum ring and leads, hearkens back to the scanning single-electron transistor charge detector worked on previously by one of the coauthors.