Particle-Size Effects on Nuclear Magnetic Resonance in Superconductors

The Knight shift in the superconducting state has been observed in samples containing various sizes of small tin particles. The three main samples were constructed by vacuum evaporation of alternate layers of metal and dielectric (silicon monoxide). The particles are in the form of platelets whose diameter and thickness were measured with an electron microscope. The geometric means of the particle dimensions were 170, 300, and 570 Å. The fractions of the normal-state shift (referred to $\alpha$ tin) remaining at zero temperature were 84, 74, and 62%, respectively. Three other samples produced by a variety of processes gave results consistent with those of the main samples. The data fit well the theory of spin-reversing scattering through spin-orbit coupling which had been proposed to explain the nonzero Knight shift in superconductors. Two other possible contributions, a shift due to Van Vleck-type orbital susceptibility and crystalline-field spin-orbit coupling, cannot be ruled out theoretically, but do not appear to be important experimentally.