We use electron tunneling to measure electronic energy levels in individual nm-scale Al particles. For sufficiently large particles ( $\gtrsim 5$ nm in radius), the eigenstate energies reveal the existence of a superconducting excitation gap $\Omega$ which is driven continuously to zero by an applied magnetic field. The presence of $\Omega$ increases the voltage threshold for tunneling in a particle with an even number of electrons in its ground state, but decreases the tunneling threshold for an odd-electron particle. We discuss the roles of spin and orbital pair breaking in the magnetic-field transition.

• Received 31 August 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.688