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Josephson Junction
Two superconductors separated by a thin insulating layer can experience tunneling of Cooper pairs
of electrons through the junction. The Cooper pairs on each side of the
junction can be represented by a wavefunction similar to a free particle wavefunction.
In the DC Josephson effect, a current proportional to the phase
difference of the wavefunctions can flow in the junction in the absence
of a voltage. In the AC Josephson effect, a Josephson junction will
oscillate with a characteristic frequency which is proportional to the
voltage across the junction. Since frequencies can be measured with
great accuracy, a Josephson junction device has become the standard measure of voltage.
The
wavefunction which describes a Cooper pair of electrons in a
superconductor is an exponential like the free particle wavefunction.
In fact, all the Cooper pairs in a superconductor can be described by a
single wavefunction in the absence of a current because all the pairs
have the same phase - they are said to be "phase coherent" (Clarke). If
two superconductors are separated by a thin insulating layer, then
quantum mechanical tunneling can occur for the Cooper pairs without
breaking up the pairs. Clarke envisions this condition as the
wavefuntions for Cooper pairs on each side of the junction penetrating
into the insulating region and "locking together" in phase. Under these
conditions, a current will flow through the junction in the absence of
an applied voltage (the DC Josephson effect). |
Index
Superconductivity concepts
Reference Ohanian
Clarke |
