Percolation Conductivity in W-Al2O3Granular Metal Films

Current Percolation and Anisotropy in PolycrystallineMgB2

“…This has been confirmed by SEM investigations. Other experimental data range from 0.17 in pressed mixtures of conducting spheres and Teflon powder [24], to 0.31 in systems consisting of silver coated and uncoated glass beads [22] or 0.47 in granular W-Al 2 O 3 films [23]. Experimental data on the unirradiated bulk sample at higher temperatures and the fitted J c (B) curves are plotted in Fig.…”

“…During this transition the system can be considered as a mixture of normal and superconducting grains. Systems consisting of two materials with different resistances (conducting/insulating [21,22,23,24] or normal/superconducting [25,26]) have been investigated experimentally and are well understood in the framework of percolation theory. In mixtures of normal and superconducting powders the resistivity remains finite as long as the probability p, that a grain is superconducting (i.e.…”

“…There, the current in a network consisting of equal resistors with a current voltage law U ∝ I α is given near the percolation threshold by I = σ ef f U 1/α , with the conductivity σ ef f ∝ (p − p c ) t [29] and t = (d − 1)ν + (ζ − ν)/α [30], where d denotes the dimensionality of the system, ζ is about one and the correlation exponent ν is 0.88 in 3D systems [31]. Most experimental data were obtained on systems consisting of normal conducting (α = 1) and insulating particles [8,22,23,24]. In this case, the above power law for σ ef f was found to be an excellent approximation and t to be in reasonable agreement with the theoretical prediction (t = 1.88).…”

Temperature dependence of the resistivity and tunneling magnetoresistance of sputtered FeHf(Si)O cermet films

“…4, plotted as log vs T Ϫ1/2 . This proportionality should represent a thermally activated tunneling current flowing from grain to grain through an insulating matrix as calculated by Sheng et al, [14][15][16] viz. ϰexp 2ͱ(C/k B T), with activation energy Cϭ(2m / ប 2 ) 1/2 sE c ͑where m is the electron mass, the effective barrier height, s the separation between the grains, and E c the charging energy͒.…”

“…For a large Fe concentration a metallic conductance arises because part of the Fe grains are connected. For lower Fe content the percolation concentration will be approached [14][15][16] and the grains become electrically isolated by an insulating amorphous FeHf͑Si͒O phase resulting in tunnel-type conductivity with orders of magnitude larger resistivities as compared to metallic iron ( Fe Ϸ10 ⍀ cm). Note the difference in scale between the resistivity of FeHfO and the FeHfSiO films.…”