Superconducting fluctuations in small grains – The Universal Hamiltonian and the reduced BCS model

Small superconducting grains are discussed in the frameworks of both the reduced BCS Hamiltonian and the Universal Hamiltonian. It is shown that fluctuations of electrons in levels far from the Fermi energy dominate superconducting properties in small and ultrasmall grains. Experimental consequences related to the spin susceptibility and persistent currents of grains and rings with weak electron–electron interactions are discussed.