Nuclear spin coherence and RKKY interaction in superconducting <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Nb</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:mi>Sn</mml:mi> </mml:mrow> </mml:math>

We have investigated the normal and superconducting states of the technologically important compound Nb3⁢Sn using 93 Nb nuclear magnetic resonance where spin-lattice relaxation measurements indicate strong suppression of the zero-temperature superconducting gap by magnetic field. We found an anomalously large indirect nuclear spin exchange interaction from spin-spin relaxation measurements, an order of magnitude beyond that of the nuclear dipolar coupling. This unusually high degree of spin coherence enables measurement of the RKKY interaction showing how it evolves from normal to superconducting states, becoming essentially Lorentzian in the low-temperature limit of the superconductor.