why at critical temperature in superconductors resistance becomes zero

Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics.

For conductors as the temperature increases their rasistance increases and as temperature decreases the resistance decreases. If we continously decrease the temperature then a stage comes that the resistance of conductor becomes zero. The corresponding temperature is called critical temperature. So at critical temperature the resistance becomes zero.

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