Superconductivity is the result of the formation of a quantum condensate of paired electrons (Cooper pairs). In small particles, the allowed energy levels are quantized and for sufficiently small particle sizes the mean energy level spacing becomes bigger than the superconducting energy gap. It is generally believed that superconductivity is suppressed at this point (the Anderson Criterion)12. This loss of superconductivity has been explored in nanoparticles and in nanowires, but more complex systems exhibit many exciting and novel effects. In particular, metal –insulator – superconductor transitions occur in both lithographically defined13,14 and nanoparticle systems19,20,15,16 due to the interplay between disorder and superconductivity. The observation of strange highly insulating ‘superresistive’25,26,17 and ‘Cooper pair insulator’ states 18 is particularly intriguing percolating - tunneling systems allow the sizes of the particles and sizes of clusters of particles (see Fig. 1) to be controlled independently. The key question that remains unresolved is the way in which the relative strength of the coupling between particles and between clusters controls the ground state of the system.
- P. Anderson, ‘Theory of dirty superconductors’, J. Phys. Chem. Solids 11, 26 (1959).
- D. Harris et al, ‘Effect of random disorder on the critical behavior of Josephson Junction Arrays’, Phys. Rev. Lett. 67, 3606 (1991).
- Y. Yun, I. Baek and M. Choi, ‘Phase Transition and Critical Dynamics in Site-Diluted Josephson-Junction Arrays’, Phys. Rev. Lett. 97, 215701 (2006).
- S. Bose et al, ‘Mechanism of the Size Dependence of the Superconducting Transition of Nanostructured Nb’, Phys. Rev. Lett. 95, 147003 (2005).
- M. Stewart et al, ‘Superconducting Pair Correlations in an Amorphous Insulating Nanohoneycomb Film’, Science 318, 1273 (2007).
- M. Tian et al, ‘Evidence of local superconductivity in granular Bi nanowires fabricated by electrodeposition’, Phys. Rev. B 78, 045417 (2008).
- V. Vinokur et al, ‘Superinsulator and quantum synchronization’, Nature 452, 613 (2008).