Quantum size effects on the superconducting parameters of zero-dimensional Pb nanoparticles

The superconducting parameters of fine Pb powders, having mean particle diameters much smaller than the London penetration depth and the coherence length of bulk Pb, were measured using conventional means. The crystalline structures of the nanoparticles remain in fcc symmetry. The quantum size effects become significant for particles smaller than ∼6 nm, below which the superconducting transition temperature decreases and the critical magnetic field increases abruptly. Surprisingly, the superconducting coupling strength was found to increase considerably for smaller particles. The T_C of 4.5-nm particles reduced to 4.6 K, which was found to be extremely insensitive to the applied magnetic field. The critical magnetic field of Pb nanoparticles was found to be at least two orders of magnitude higher than that of bulk Pb. No signs of superconductivity were observed in the 2-nm particles.