Meissner effect in colloidal Pb nanoparticles.

Monodisperse colloidal lead nanoparticles with diameters ranging from 4.4 to 20 nm are prepared by a self-limiting growth method. The nanoparticles are monodispersed and protected from oxidation by an amorphous tin-lead oxide shell of 1.5-2 nm thickness. The magnetic susceptibility of the particles is measured as a function of size, temperature, and magnetic field. The Meissner effect is observed indicating the superconducting transition. For the 20 and 16 nm particles, the critical temperature is suppressed to 6.9 K from the bulk value of 7.2 K and is further reduced for smaller particles. Depending on the size of the particles, the critical field is enhanced by 60-140 times. The superconducting properties agree closely with the theoretical expectations.