Electron energy levels in superconducting and magnetic nanoparticles

We describe electron-tunneling measurements of the spectrum of discrete energy levels for electrons in metal nanoparticles, for metals with sufficiently strong electron interactions to form superconducting or magnetic states. For aluminum nanoparticles, superconducting pairing interactions produce an energy gap for tunneling, which affects even- and odd-electron spectra differently. As a function of an applied magnetic field, we observe that the correlated electron ground state is disrupted by breaking one Cooper pair at a time. Energy levels in ferromagnetic cobalt particles exhibit effects of strong exchange and magnetic anisotropy forces, with a non-linear, hysteretic dependence on an applied magnetic field.