The Electrical Transport Properties Of Pb_x?SiO₂?_1-x Nanogranular Films

Superconducting granular films,consisting of nanometer superconductors and insulators,have rich physical phenomena.such as enhancement of the superconducting transition temperature and critical magnetic field.In order to investigate the superconductor-insulator transition and the critical field in three-dimensional(3D)superconducting granular films,a series of Pbx(SiO2)1-x(0.26?x?0.87)granular films with thickness>500nm were deposited at room temperature by cosputtering method.The microstructure,composition and electrical properties were systematically studied.The Pb-SiO2 films show the typical granular structure in which the crystalline Pb particles are embedded in the amorphous SiO2 matrix.The superconductor-insulator transition was observed with decreasing Pb volume fraction x.The films with x>0.60 show the usual monotonic metal to superconductor transition with decreasing temperature;When x<0.53,the films show insulator behavior in the whole measured temperature range(300-2 K).In addition,when the temperature decreases to?7 K(near the superconducting transition temperature of pure Pb),the resistivity begins growing more quickly,indicating a more insulating state appears below?7 K;when 0.57?x?0.54,the samples lie in the transition range.Specifically,the quasireentrant superconductivity behavior was observed in the film x?0.54.The sample x = 0.57 just show“local superconductivity”and global superconductivity is only present in the film with metallic normal-state.From the results above,we get that the critical metal volume fraction xcs for the global superconductivity is almost identical to the percolation threshold xc for the metal-insulator transition.And our experimental data don't support the idea that there is a universal normal state critical tunneling resistance above which the global superconductivity could not be present in 3D granular superconductors.We measured the temperature dependence of the resistivity under different magnetic fields.The critical field at a certain fixed temperature increases with decreasing x.For the films near the metal-insulator transition,the superconducting states can preserve to relative high temperatures even at a magnetic field with magnitude of 9 T,and the critical field at zero temperature is estimated to be close to the Pauli limit(?15 T).Thus,embedding the elemental superconducting metal into insulating matrix in nanoscale is an effective way to improve the critical field of superconducting material.We have also studied electronic transport properties of Pbx(SiO2)1-x nanogranular films lying in the insulating region with x?0.53.We found that the activated behavior R = R0 exp(T0/T)in the low temperature(below 7 K)transforms into variable range hopping upon increasing temperature,which is contrary to the common expectations.We speculate that the thermally activated conductivity is governed by the large collective coulomb blockade gap for Cooper-pair propagation,not by the usual single electrons,indicating that the Cooper pairs are totally localized in single Pb granules.And from the TEM results,the insulating samples also have local superconductivity,for that the size of the Pb granular(5 nm)is larger than the critical size(2 nm)of Pb for existing superconductivity.Thus,the local superconductivity exists in the insulating samples.