# Josephson Effect

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In this lecture, we are going to learn about the Josephson junction, Josephson Effect, D.C. Josephson Effect, A.C Josephson Effect, and applications of Josephson junction in a very detailed manner. So let’s start from what is Josephson junction?

## What is Josephson junction?

Two superconductors separated by a very thin strip of an insulator forms a Josephson junction.

## Josephson Effect

The wave nature of moving particles makes the electrons tunnel through the barrier(insulator), i.e., the electrons can tunnel from one superconductor to the other. As a consequence of the tunneling of electrons( Cooper pairs) across the insulator, there is net current across the junction. This is called as D.C. Josephson Effect. The current flows even in the absence of a potential difference.

The magnitude of the current depends on the thickness of the insulator, the nature of the material nature, and the temperature.

On the other hand, when a potential difference V is applied between the two sides of the junction, there will be an oscillation of the tunneling current with angular frequency v=\frac{2eV}{h}. This is called the A.C. Josephson Effect.

Thus, according to A.C. Josephson Effect, the junction generates an a.c. current at a frequency of \frac{2eV}{h} Hz per volt.

Note: Cooper pair is bound pair of electrons formed by the interaction between the electrons with opposite momenta and spin in a phonon field.

## Application of Josephson junction

Josephson junctions are used in sensitive magnetometers called SQUID-Superconducting Quantum Interference Device.

A SQUID is formed by connecting two Josephson junctions in parallel.

When current is passed into these arrangements, it splits flowing across the two opposite arcs. The current through the circuits will have a periodicity that is very sensitive to the magnetic flux passing normally through the closed circuits, As a result, extremely small magnetic flux can be detected with this device.

This device can also be used to detect voltages as small as 10-15 V.

Magnetic field changes as small as 10-21T can be detected.

The weak magnetic field produced by biological currents such as those in the brain can also be detected using SQUIDs.

SQUID detectors are used to measure the levels of iron in the liver so that iron built-up can be treated before much harm is done to the body.

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