Richard Newrock, a professor of physics at the University of Cincinnati, has studied the physics of superconducting materials for 20 years. Here is his explanation.
A Josephson junction is made by sandwiching a thin layer of a nonsuperconducting material between two layers of superconducting material. The devices are named after Brian Josephson, who predicted in 1962 that pairs of superconducting electrons could "tunnel" right through the nonsuperconducting barrier from one superconductor to another. He also predicted the exact form of the current and voltage relations for the junction. Experimental work proved that he was right, and Josephson was awarded the 1973 Nobel Prize in Physics for his work.
To understand the unique and important features of Josephson junctions, it's first necessary to understand the basic concepts and features of superconductivity. If you cool many metals and alloys to very low temperatures (within 20 degrees or less of absolute zero), a phase transition occurs. At this "critical temperature," the metal goes from what is known as the normal state, where it has electrical resistance, to the superconducting state, where there is essentially no resistance to the flow of direct electrical current. The newer high-temperature superconductors, which are made from ceramic materials, exhibit the same behavior but at warmer temperatures.
What occurs is that the electrons in the metal become paired. Above the critical temperature, the net interaction between two electrons is repulsive. Below the critical temperature, though, the overall interaction between two electrons becomes very slightly attractive, a result of the electrons' interaction with the ionic lattice of the metal.
This very slight attraction allows them to drop into a lower energy state, opening up an energy "gap." Because of the energy gap and the lower energy state, electrons can move (and therefore current can flow) without being scattered by the ions of the lattice. When the ions scatter electrons, it causes electrical resistance in metals. There is no electrical resistance in a superconductor, and therefore no energy loss. There is, however, a maximum supercurrent that can flow, called the critical current. Above this critical current the material is normal. There is one other very important property: when a metal goes into the superconducting state, it expels all magnetic fields, as long as the magnetic fields are not too large.
In a Josephson junction, the nonsuperconducting barrier separating the two superconductors must be very thin. If the barrier is an insulator, it has to be on the order of 30 angstroms thick or less. If the barrier is another metal (nonsuperconducting), it can be as much as several microns thick. Until a critical current is reached, a supercurrent can flow across the barrier; electron pairs can tunnel across the barrier without any resistance. But when the critical current is exceeded, another voltage will develop across the junction. That voltage will depend on time--that is, it is an AC voltage. This in turn causes a lowering of the junction's critical current, causing even more normal current to flow--and a larger AC voltage.
The frequency of this AC voltage is nearly 500 gigahertz (GHz) per millivolt across the junction. So, as long as the current through the junction is less than the critical current, the voltage is zero. As soon as the current exceeds the critical current, the voltage is not zero but oscillates in time. Detecting and measuring the change from one state to the other is at the heart of the many applications for Josephson junctions.