Observation of shell effects in superconducting nanoparticles of Sn

In a zero-dimensional superconductor, quantum size effects(QSE) not only set the limit to superconductivity, but are also at the heart of new phenomena such as shell effects, which have been predicted to result in large enhancements of the superconducting energy gap. Here, we experimentally demonstrate these QSE through measurements on single, isolated Pb and Sn nanoparticles. In both systems superconductivity is ultimately quenched at sizes governed by the dominance of the quantum fluctuations of the order parameter. However, before the destruction of superconductivity, in Sn nanoparticles we observe giant oscillations in the superconducting energy gap with particle size leading to enhancements as large as 60%. These oscillations are the first experimental proof of coherent shell effects in nanoscale superconductors. Contrarily, we observe no such oscillations in the gap for Pb nanoparticles, which is ascribed to the suppression of shell effects for shorter coherence lengths. Our study paves the way to exploit QSE in boosting superconductivity in low-dimensional systems

Phase diagram of hole-doped high-Tc superconductors: Effects of Cooper-pair phase fluctuations within fluctuation-exchange theory

Manske D, Dahm T, Bennemann KH. Phase diagram of hole-doped high-Tc superconductors: Effects of Cooper-pair phase fluctuations within fluctuation-exchange theory. PHYSICAL REVIEW B. 2001;64(14): 144520.Using the Hubbard model Hamiltonian we study spin-fluctuation exchange-induced superconductivity of d-wave symmetry. Results are presented for the characteristic temperature T* at which a gap appears in the spectral density, for T*(c) at which Cooper-pairs are formed, for T-c at which Cooper pairs become phase coherent, and for the superfluid density n(s). We find that, with increasing doping, for x>0.15 the phase coherence energy becomes larger than the Cooper-pair condensation energy. Accordingly, one has T(c)proportional ton(s) for xDelta for the overdoped cuprates. We use our results to discuss dynamics and recent dynamical conductivity and ultrafast nonequilibrium measurements