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In bulk superconducting systems the large intrinsic range of the pair coherence length implies an extremely narrow critical region. However, as is well known, the size of the critical region becomes larger as the dimensions of the system decrease below the coherence length. An interesting limiting case corresponds to particles with all dimensions less than a coherence length which form essentially zero-dimensional systems. Here we calculate the thermodynamic properties of small superconducting particles using a static approximation. Within this approximation, both the order parameter and the quasiparticle contribution are explicitely studied.
Mühlschlegel et al. (Fri,) studied this question.