Granular superconductor

The rather low critical current densities expected in bulk samples of these granular superconductor materials (103)(104), and their relatively low magnetic critical fields (50)(78)(105)(106), allied with the relatively low Tc s observed would appear to hinder development of superconducting applications for these materials. Even the critical current of 5 X 105 A/cm2 observed for single crystalline thin films (39) is now considered low for a superconductor at 4.2K. However, when considering the applicability of a material to a task,... 

Belous, N.A., Gabovich, A.M., Moiseev D.P. and Postnikov, V.M., Detection of the Influence of Disorder and of Frustration of Weak Links on the Critical Current in a Three-Dimensional Granular Superconductor BaPb0 7BBi0 26(V Sov. Phys. Solid State 28(9) 1615 (1986). 

Summary. Various experimental observations of the superconductor-insulator transition are described and compared with two theoretical models one based on boson-vortex duality and the other where the superconducting fluctuations at low temperatures in the magnetic field are calculated. The latter shows that the superconducting fluctuations in dirty but homogeneous superconductor act as grains in a granular superconductor. 

For the quantum generalization of the APR model (QAPR) the system corresponding to the all-J case also shows reentrance. This was first predicted by mean-field theory, and then verified via quantum Monte Carlo calculations as well as quantum Monte Carlo calculations analyzed via finite size scaling.Reentrance was also found in the corresponding model of granular superconductors. ... 

It is known that many bulk, low-temperature superconductors have dramatically enhanced (sometimes by several orders of magnitude) superconductive transition temperatures, Tc, when they are rendered into a disordered or granular state. Even more striking is the fact that several other systems whose constituents usually have no, or unmeasurably low, Tc s, have been shown to have measurable, and sometimes, even reasonably high... 

The examples discussed so far are all magnetic quantum phase transitions. Our last example in this section on quantum-to-classical mapping is a quite different transition, viz. the superconductor-insulator transition in two-dimensional dirty boson systems. Experimentally, this transition can be realized in helium absorbed in a porous medium or in granular superconducting films as an example. 

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