Direct Observation of Electron-Pairing Effect in Type-II Superconductors by Positron Annihilation

We have performed angular-distribution measurements of the two gammas produced by annihilation of positrons in a type-II superconductor (${\mathrm{Nb}}_3$Sn) to detect the superconducting smearing effect on the electrons momentum distribution function. Our results give, for the first time, a direct experimental evidence of the redistribution of $K$-space states at the superconducting transition.