Granular Metal Films Research Articles

Quantum fluctuations in two-dimensional superconductors

For granular metallic films or two-dimensional arrays of Josephson junctions, fluctuations in the number of Cooper pairs are conjugate to the superconducting phase and can lead to zero-point destruction of phase coherence at zero temperature. In two dimensions the resulting critical behavior is shown to be that of the $\ensuremath{\lambda}$ point of $^{4}\mathrm{He}$ in three dimensions, with the charging energy playing the role of temperature. The nature of the quantum \ensuremath{\rightarrow} classical crossover and its effect on the frequency dependence of precursor diamagnetism near the quantum critical point are discussed.

Multipole oscillations in small hemispherical particles

The peculiar optical properties of thin granular metal films deposited on transparent substrates can usually be explained by assuming ellipsoidal metal particles, which allow for pure dipole oscillations along their principal axes. Additional resonances have been observed, however, for alkali metal films. These structures are attributed to the coupling of all multipole oscillations, excited simultaneously in small truncated spheroids. Numerical calculations, performed for small hemispherical particles on various substrates with different values for the electronic mean free path, lead to satisfactory agreement with knowm experimental results.

Optical properties of metal aggregates in Li+ and K+ irradiated MgO crystals

AbstractOptical absorption spectra of MgO crystals irradiated with high energy (250 to 450 keV) alkali ions (Li+, K+) present two bands of colloidal nature after thermal annealing of the crystal. Mie theory cannot represent all the features of this behaviour which are related to the very high dose (5 × 1016 to 1017 ions/cm2) possible to get with such a technique (up to 1022 cm−3 implanted ions). The results are interpreted using Maxwell‐Garnett theory applied to a granular metal film embedded in a dielectric host and it is possible to attribute the two observed absorption bands to plasma resonance and optical conduction absorption of the metal precipitate.

Multipole oscillations in small hemispherical particles

The peculiar optical properties of thin granular metal films deposited on transparent substrates can usually be explained by assuming ellipsoidal metal particles, which allow for pure dipole oscillations along their principal axes. Additional resonances have been observed, however, for alkali metal films. These structures are attributed to the coupling of all multipole oscillations, excited simultaneously in small truncated spheroids. Numerical calculations, performed for small hemispherical particles on various substrates with different values for the electronic mean free path, lead to satisfactory agreement with known experimental results.

Percolation conductivity in granular metal films

Abeles, Pinch, and Gittelman have fitted the conductivity of annealed W-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ granular metal films to a percolation threshold. They find ${C}^{*}=0.47\ifmmode\pm\else\textpm\fi{}0.05$. By numerical simulation of conductivity in resistor networks we demonstrate that these conductivity data can be explained if there is a bias towards the formation of isolated metallic regions.

Effect of charging energy on superconductivity in granular metal films

Previous model calculations of the superconducting properties of aggregates of superconducting grains coupled by Josephson tunnel barriers did not take into account the charging energy associated with small metal grains. It is pointed out that in granular superconductors, in which the grain size is 20--100 A, the charging energy can be orders of magnitude larger than the Josephson coupling energy and superconducting coupling between the grains is quenched unless the grains are in intimate electrical contact.

Temperature dependence of the electrical conductivity of granular metal films

Temperature dependence of the electrical conductivity of granular metal films

Size effects in the electronic heat capacity of small platinum particles embedded in silica

Heat-capacity measurements on sputtered granular metal films of Pt-Si${\mathrm{O}}_{2}$ were carried out between 1 and 12 \ifmmode^\circ\else\textdegree\fi{}K. Mean sizes of the Pt particles range from 10 to 26 \AA{}; this small size coupled with these low temperatures makes the average electronic-energy-level spacing $\ensuremath{\delta}$ in the Pt metal large compared to ${k}_{B}T$. A quantum size effect in the electronic heat capacity of a metal has been predicted in this limit, $\ensuremath{\delta}\ensuremath{\gg}{k}_{B}T$, and is observed here for the first time, although in a very limited temperature range. Difficulity in observing this size effect was caused by a large heat-capacity enhancement over bulk values in the insulating vitreous silica. This enhancement is ascribed to the Pt particle inclusions in the host Si${\mathrm{O}}_{2}$ matrix augmenting the low-frequency Einstein modes first observed in the specific heat of Si${\mathrm{O}}_{2}$ by Flubacher et al.

Granular metal-semiconductor vidicon

A novel low-dark-current high-sensitivity vidicon target has been made using a semiconductor ‘n-type CdSe’ coated with a granular metal ‘Au-SiO2’ film. The granular metal-semiconductor interface forms a Schottky barrier which blocks effectively the beam electrons. The high lateral resistance of the granular metal film precludes the need of an array structure to achieve good resolution.

Structural and electrical properties of granular metal films

Abstract Granular metal films (50–200,000 A thick) were prepared by co-sputtering metals (Ni, Pt, Au) and insulators (SiO2, Al2O3), where the volume fraction of metal, x, was varied from x = 1 to x = 0.05. The materials were characterized by electron micrography, electron and X-ray diffraction, and measurements of composition, density and electrical resistivity at electric fields e up to 106 V/cm and temperatures T in the range of 1.3 to 291 K. In the metallic regime (isolated insulator particles in a metal continuum) and in the transition regime (metal and insulator particles in a metal continuum) and in the transition regime (metal and insulator labyrinth structure) the conduction is due to percolation with a percolation threshold at x⋍0.5. Tunnelling measurements on superconductor-insulator-granular metal junctions reveals that the transition from the metallic regime to the dielectric regime (10–50 A size isolated metal particles in an insulator continuum) is associated with the breaking up of a metal ...

Permittivité effective dans l'infrarouge des métaux à l'état divisé en vue du calcul des forces intergranulaires de coalescence

In this work we have studied the effective permittivity of granular metal films under the influence of an electromagnetic field in the light of the theories of Drude and Nordheim. We have shown that thin metallic films with an island structure possess an effective permittivity in the infra-red very much different from that of the bulk metal.

Optical properties of electrically insulating granular metal films

By successive evaporation of metal and dielectric we have made films consisting of many layers of small metal particles insulated from each other. Such films are opaque in the visible and near infrared, but electrically insulating. We have measured the dependence of the optical properties on metal particle size and metal-to-insulator volume ratio. The results can be qualitatively understood in terms of the Maxwell-Garnett theory. Possible technical applications are discussed.

Transmission Electron Microscopy of Granular Nickel/SiO2 Cermet Films

Granular nickel films have been prepared by co-sputtering nickel and silica onto glass substrates at room temperature. Seven films of (Ni)1−x(SiO2)x, where x is the volume fraction of silica, were examined by transmission electron microscopy. For all samples, the microstructure consisted of nickel grains embedded in a matrix of silica. The grain size is observed to decrease linearly with x; the distribution of grain sizes varies from 170 to 310 Å for x≃0.1 and from 40 to 55 Å for x≃0.75. The thickness of the intergranular silica is about 22 Å and is independent of composition. The nickel lattice is observed to expand with increasing x; the fractional expansion is inversely proportional to grain size and attains values as large as 10% for large x. These results will prove useful in the interpretation of the magnetic, electrical, and optical properties of granular metal films. In particular, these results have been applied to the interpretation of the depression in magnetic Curie temperature observed in the (Ni)1−x(SiO2)x system.

The effect of frequency on the electrical resistivity of granular metal films : A. A. Hirsh, N. Friedman and S. Bazian. Vacuum, Vol. 14, pp. 393–394

The effect of frequency on the electrical resistivity of granular metal films : A. A. Hirsh, N. Friedman and S. Bazian. Vacuum, Vol. 14, pp. 393–394

The effect of frequency on the electrical resistivity of granular metal films

A resistance method is given for the determination of the thickness of thin granular metal films based on the use of impedance measurements at frequencies in the order of magnitude of 1 Mc/sec. At these frequencies the intergranular gap resistance can be seen as short circuited by the inter-granular capacitance, so that the measured resistance is that of the metal grains. As shown the resistivity of the individual grains is in general several times greater than the bulk resistivity of the metal. This difference depends strongly on the vacuum conditions and the rates of evaporation.