In 1962
B.D. Josephson was the first who predicted that a super current consisting of correlated pairs of electrons can be made to
flow across an insulating gap between two superconductors, provided the gap is small enough.
This
is termed as Josephson Effect which rewarded him with noble prize in 1975.
He made a sandwich of superconductor-insulator-superconductor.
i.e. A very thin insulator of thickness less than 2nm (two nanometer) is placed in between two super conductors. And when
a super conductor passes through it current flows across the insulator also! (i.e. insulator becomes a superconductor). This
occurs due to the tunneling of Cooper pairs.
[Cooper Pairs- We find electrons repelling each other because of Columbic interaction. Bu for some other reason
if the electron attracts each other they form a bound state, and as a result the electrons are bound o form a single system
and their motions are correlated. This pair of electrons is called Cooper pair.]
The
wave functions associated with Cooper pair of electrons are penetrating through the insulator and as a result these pairs
are tunneling through it which is the reason for the flow of supercurrent.
Such
a junction with a very thin insulator between two superconductors is called Josephson junction.
DC
Josephson Effect
Consider a circuit as shown in the fig.
A current
is made to flow in a bar of super conductor. A voltmeter is connected across the ends of the bar. The voltmeter indicates
a drop in voltage as zero. But if the bar is cut into two pieces and kept apart at a distance of 1cm the voltmeter indicates
a voltage equal to open circuit voltage of current source. When the distance is reduced to 1mm the voltmeter suddenly shows
zero voltage showing that a current flows across the gap in a superconducting way. This is known as dc Josephson effect.
Since there is no voltage across the junction.
Supercurrent,
Ij = Im.sinφ
Where,
Im is the maximum junction current which depends upon the thickness of the insulator.
φ
is the phase difference between the wave functions of Cooper pair on one side.
AC
Josephson Effect
In the above experiment when a direct voltage V
is applied across the junction phase difference φ changes with time.
υ=dφ/dt
=2ev/h
[Charge of Cooper pair =2etherefore 2ev=h υ]
Hence the
Josephson current varies sinusoid ally with time. This produces ac Josephson Effect. This occurs due to the presence of a
very high frequency alternating current in the gap.
Attainments
of Josephson Effect:
1) A dc current flows across a junction in the absence of any magnetic
or electric field.
2) When a dc voltage is applied across a junction, it causes high frequency
(radio frequency-rf) current oscillations across the junction. This effect has been utilized for the precise determination
of the value of h/e. Further, an rf voltage applied with the dc voltage can then cause a flow of dc current across a junction.
3) When a dc magnetic field is applied through a super conducting circuit,
containing two junctions, it causes the maximum supercurrent to show interference effects as a function of magnetic field
intensity. The interference pattern is shown in the fig.