12-06-2012, 02:29 PM
STUDY OF JOSEPHSON EFFECTS
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DC JOSEPHSON EFFECT
According to Brian Josephson, if two superconductors are separated by a sufficiently thin layer of insulator (10 A thick), weak superconducting currents can tunnel through the potential barrier without any applied voltage, that is, the resistance is zero. In a given superconductor, the pairs are represented by
Ψ= ΨO ejΦ
where Φ is the phase and is the same for every pair. They are said to be “phase coherent”.
The super current through the barrier is related to the phase difference on the left and right sides of the junction. If the superconductor on the left has phase Φ1 and that on the right has phase Φ2, Josephson showed.
CHARACTERISTICS:
The IV characteristics can be explained on the basis of the explanation for the DC Josephson effect. It can be seen that when the current is below IC, there is no voltage drop. As the current increases just above IC, the voltage drop is Π ∆/2e.
AC JOSEPHSON EFFECT
Josephson predicted that when a finite voltage V is applied across the junction there will be a flow of current. Solution to Schrödinger’s equation gives the expression for current as
I = Ic sin [∂ (0) + [(2eV/h) t] (6.4)
This shows that current oscillates with frequency ωj=2eV/h where V is the applied voltage, h is the constant; e is the basic electronic charge. If V is a constant then the phase difference can be given by the equation ∂=∂ (0) + ωj t. Then d∂/dt = ωj. That is the ratio of change of phase difference is directly proportional to the applied voltage.