Electrical Transport Properties In Granular Superconductor Films

Granular superconductor films are composite materials consisting of immiscible superconductors(usually metals)and insulators.Generally,the mean grain size of the superconductor granules varies from a few to several hundreds nanometer.Thus granular superconductor films are typical mesoscopic system.The mean size and volume fraction of the superconducting granules can be controlled easily,which make the system become suitable for investigating mesoscopic physics.Granular superconductor films can be divided into dielectric,transition and superconductor regions according to metal volume fraction.In the dielectric and transition regions,the experimental results in electrical transport properties are still inconsistent in a large extent below bulk superconducting transion temperature and the discrepancies also exist in the explainations of conduction mechanisms.In superconductor region,a better understanding of the electrical transport properties in a magnetic field is still absent.In this dissertation,the transport properties of Pb_x(SiO₂)1-x and Pb_x(TiI2)1-x granular superconductor films,which span the dielectric,transitional,and metallic regions,have been investigated systematically.In addiation,the influence of the mean size of Nb grain on the electrical transport properties of Nb films has also been investigated.The Pb_x(SiO₂)1-x granular films(0.46?x?0.74)with thickness of?1000 nm have been fabricated by magnetron sputtering method.It is found that the percolation threshold xc lies between 0.57 and 0.60.For the x ?0.50 insulating films,the resistivities begin to increase abruptly and then tend to saturation with further decreasing temperature below the bulk Pb superconducting transition temperature Tc.The temperature behavior of the resistivity in the sharply decreasing region obeys a thermal activation law.The superconducting gap of Pb obtained from the thermal activation law is almost equal to that obtained via single electron tunneling spectra measurement(the discrepancies are less than 10%).Therefore,the single electron(not the Cooper pair)hopping is the dominant electrical transport mechanism below Tc.For the 0.50<x<0.57 insulating films,the "reentrant" phenomena have been observed below Tc.Through analyzing the p-T curves at different magnetic field H and p-H curves at different temperature T,we have found that the "reentrant"phenomena is a consequence of the competition between resistance decrease due to the occurrence of superconductivity on isolated Pb grains and the enhancement of excitation resistance due to the opening of the energy gap on the grains.For the 0.57 ?x?0.72 films,the resistivities sharply decrease with decreasing temperature just below Tc,and then show a dissipation effect with further decreasing temperature.Treating the conducting paths composed of Pb particles as nanowires,we have found that the R(T)data below Tc can be well explained by a model that includes both thermally activated phase slips and quantum phase slips.The Pb_x(TiO₂)1-x granular films(0.16?x?0.80)with thickness of?1000 nm have also been fabricated by magnetron sputtering method.The percolation threshold xc lies between 0.24 and 0.30.For insulating films,the resistivities increase with decreasing temperature in higher temperature regions and then decrease quickly when the temperature is decreased below the superconducting transition temperature of Pb,This variation trend is just opposite to that of Pb_x(SiO₂)1-x granular films,which indicates that the hopping or tunneling of Cooper pairs dominates the conducting process.This may be the main reason for absence of "reentrant" phenomena in resistivity versus temperature curves in Pb_x(TiO₂)1-x granular films.For the global superconducting films,when a small magnetic field being perpendicular to the film plane is applied,the resistivities decrease quickly with decreasing temperature near Tc,and then tend to saturation with further decreasing temperature.In addition,the resistivity versus the reciprocal of temperature obeys an activation law,which indicates that the quantum metallic states may appear below Tc.The saturation effect at lower temperatures may caused by the macroscopic quantum tunneling of vortices.We have discussed our experimental data in the frame work of bosonic and fermionic models,respectively.The insulating,superconducting tendency and global superconducting Nb films at low temperature have been fabricated on quartz substrates via changing film thickness and substrate temperature.It is found that the mean grain size D increases with increasing substrate temperature.The superconducting transition temperature Tc of the superconducing films increases with increasing D,which can be explained by proximity effect proposed by Cooper.The upper critical magnetic field at 0 K[Hc2(0)]for the global superconducting films increases with decreasing D.It is found that Hc2(0)is almost varies linearly with D-1.In the superconducting tendency films,the critical magnetic field Bc for the superconductor-insulator transition satisfies with a scaling law.The insulating and superconducting regions are divided into two branches in the R versus |B-Bc|/T1/zv curves,where z and v are the dynamical critical exponent and the correlation length exponent,respectively.The scaling relationship has been discussed in detail.