London penetration depth of zero-dimensional Pb nanoparticles

The London penetration depths of zero-dimensional Pb nanoparticles were studied by measuring the ac magnetic susceptibility with a weak probing magnetic field. Both the diamagnetic screening effect that results in negative values for χ′ and the dissipation peaks in χ″ that signify the loss of the probing field to the screening current are clearly seen. The London penetration depth becomes shorter for smaller nanoparticles. A plot of the variation of the London penetration depth with particle size shows that the surface effect is less significant than the quantum size effect in reducing the penetration depth, and the quantum size effect becomes significant for particles smaller than 6 nm.