Granular superconductivity

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. 

Yoshino, Zakhidov and co-workers [175,176] reported that doping the conjugated polymer/C o composites with alkali atoms results in granular superconductivity of the Cfio component embedded in the polymer matrix. Photoinduced electron transfer may increase the number of superconducting domains in these composites. 

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,... 

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. 

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... 

In summary, the difiusion of K into C o films produces a minimum resistivity phase that is barely metallic with p = 2.2 milliohm-cm at composition K3.oo o.o5C6o, and a high resistivity (max) phase at composition Ks.ooio.osCjo. Intermediate compositions yield activated conduction with x-dependent activation energy, which can be explained by a simple model with immiscible phases where the metallic phase maximally avoids percolation. The recognition that the normal state has a granular microstructure will also be important in understanding superconductivity in thin films. 

Naugle and Porter [2.83] have proposed using the increase in inductance of a superconducting thin film as the transition temperature is approached from below to detect radiation. Their analysis reveals high sensitivity for a granular aluminum thin film superinducting bolometer operated at 2 K. 

NbN/BN granular films are sensitive, high-speed detectors for pulsed far infrared radiation. These films are deposited on sapphire or quartz substrates by reactive radiofrequency sputtering of a split Nb/BN target in an Ar/N2 atmosphere. These devices can also be operated in the superconducting or resistive region critical temperatures range between 1.6 to 15 K,... 

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