Fractional Charging and Reentrance in Granular Superconductors.
Abstract
Quantum mechanical states corresponding to non integer numbers of superconducting pairs are discussed within the context of granular superconductors. There are two cases of noninteger quantization discussed. The first correspond to states that appear to be halfpairs. The physical consistency of such states is established on the basis of the Anderson pseudospin picture of superconductivity. In the second case, it appears that the number of pairs in a given system may range over a continuum. In this case, the continuum arises in connection with a modification the usual HubbardStratonovich transformation used to discuss superconductivity. Physically, we have a situation in which coupling to external degrees of freedom break the symmetry associated with an integer counting of pairs. A formal device, called a frustrated HS transformation, allows us to discuss this symmetry breaking as a charge continuum. A pair sensitive device making a measurement on such a continuum, would always measure an integer pair number, the average over an ensemble of identical systems yielding a new quantum number, which we call the crystal charge, corresponding to the continuum. These continuous states give rise to a reentrant phase transition in a granular superconductor. It is shown that intergranular charge phase fluctuations that wipe out or severely diminish reentrance in discrete charge models, only weakly effect reentrance in the continuous model. We also show that under appropriate circumstances, the presence of dissipation may enhance reentrance. Finally, we argue that the reentrance reported in BaPb_{.75}Bi _{.25}O_3 is due to the existence of continuous charging states, augmented by dissipation.
 Publication:

Ph.D. Thesis
 Pub Date:
 1988
 Bibcode:
 1988PhDT........21C
 Keywords:

 Physics: General;
 Broken Symmetry;
 Granular Materials;
 Superconductivity;
 Superconductors;
 Continuum Mechanics;
 Quantum Mechanics;
 Transformations (Mathematics);
 SolidState Physics