Exchange narrowing of dipolar broadened EPR lines in superconductors

The exchange narrowing of dipolar broadened paramagnetic resonance (EPR) lines by the long range exchange interaction is considered. That is the actual situation in magnetically diluted superconducting alloys, where a sharp decrease of the EPR linewidth under the superconducting transition was observed. These calculations show that the EPR linewidth drop below the superconducting transition temperature is a clear indication of the long wave-length electronic susceptibility decreasing in the superconducting state.