A large thermal hysteresis ( > 100 K) of elastic modulus was measured for coarse grained Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O. Whereas, only a small thermal hysteresis (2∼3 K) near the velocity minima at 240 K, and 150 K was observed in Bi2 Sr2 CaCu2 O8 single crystal, which were found to be associated with phaselike transitions (PLT). The investigations of isothermal recovery of modulus for coarse grained YBCO and BSCCO disclose that the large thermal hysteresis may be attributed to the defects of pore size generated in the interconnecting region between grains as a result of PLT of weak first order but covering a wide temperature range. The mechanism of a large thermal hysteresis for coarse grained samples has been discussed in detail.

We have examined the photoresponse of two-dimensional granular superconducting films of high Tc superconductors (Y-Ba-Cu-O). We see evidence for the existence of a twodimensional Kosterlitz-Thouless transition. The photoresponse and resistance have been measured as a functions of temperature, excitation power, bias current, and photon energy. The response to photons having energies above and below the superconducting energy gap(s) has been measured. Photoresponse features have been observed that are not explainable in terms of a conventional phonon bolometric mechanism. Comparison will be made with similar observations made on low temperature superconducting films (NbN-BN).

The microstrip resonator technique has been applied to study the temperature dependence of the magnetic penetration depth in high quality YBa2Cu3O7−δ thin films. The temperature dependence at low temperatures comes out directly from measured data, with no assumptions about transmission line geometry, dielectric properties, or a model for the temperature dependence of the penetration depth. One can interpret the data in terms of either an exponential decay of λ(T) at low temperatures or as a power law decay. The energy gaps obtained from the exponential decay at low temperature are found to be significantly smaller than weak coupled BCS theory and power-law exponents are in the range of 1.3 to 3.2. These results will be discussed in terms of microscopic theories and the possibility that materials properties dominate the measurement.

In this paper an experimental study of YBa2Cu3O7−x/silver composites is presented and the relationship between microstructure, superconducting and mechanical properties is discussed. Silver contents ranging from 0 to 17 vol% fell all below the percolation treshold. X-ray and Wavelength Dispersive Spectrometry analyses indicate that Ag remains in a free state, and is located mainly at the grain boundaries. A tenfold increase in Jc. and a more than twofold increase in strength are the benefits of the improved microstructure of silver composites. The Tc remains unaffected.

We review recent work on the dynamical behavior of coupled resistively-shunted Josephson junctions, with emphasis on our own calculations. We present a model which allows for the inclusion of finite temperature, disorder, d.c. and a. c. applied currents, and applied magnetic fields. We discuss applications to (a) calculations of critical currents and IV characteristics; (b) harmonic generation and microwave absorption by finite clusters of Josephson junctions; (c) critical energies for vortex depinning; and (d) quantized voltage plateaus in arrays subjected to combined d.c. and a. c. currents. Possible connections to the behavior of granular high-temperature superconductors are briefly discussed.

We study the zero temperature phase transitions of a chain of Josephson junctions, taking into account the quantum fluctuations due to the charging energy and the effects of an Ohmic dissipation. We map the problem onto a generalized Coulomb gas model, which then is transformed into a sine-Gordon field theory. Apart from the expected dipole unbinding transition, which describes a transition between globally superconducting and resistive behavior, we find a quadrupole unbinding transition at a critical strength of the dissipation. This transition separates two superconducting states characterized by different local properties.

A 1/z expansion is used to study the coupling of phase fluctuations in an array of superconducting grains. By lowering Tc. and increasing the critical grain diameter αc,fluctuations suppress the long-range order in the material. When the temperature T is of order U = e2/C, where C is the grain capacitance, peaks appear in the fluctuation corrections to both the specific heat and the short-range order parameter. These peaks are induced by the coupling between thermal fluctuations of the charge and quantum fluctuations of the phase on neighboring grains.

Films of tin condensed onto 4K pyrex substrates can be driven from “insulating” (i.e. dR/dT < 0) to superconducting behavior by low temperature annealing. Global superconductivity occurs when the normul state sheet resistance, R□, shrinks to a value very near R□ ≡ h/4ea, as in experiments where R□ is changed by increasing the film thickness. When R□ > R□, conduction is decidedly non-ohmic, with R□(T,V) = R□(T) + R1 (T)exp(−m(T)|V|).

Quench-condensed films of simple superconductors can be deposited with a morphology which is either granular (grain sizes on the scale of tens of angstroms) or homogeneous. Transport and tunneling studies of the normal and superconducting states of these films as a function of sheet resistance have revealed profound differences between them. In uniform films Tc. and the energy gap decrease continuously with decreasing film thickness. Superconductivity is destroyed by the reduction of the amplitude of the order parameter. In granular films, the grains are sufficiently large to support superconductivity in each grain. Long range phase coherence is destroyed by intergrain phase coherence breaking.

The resistivity of sputter deposited Snx (SiO2)100−x films has been measured for Sn concentration x between 50 and 90 atomic percent. Films with more than 53 percent Sn are superconducting at low temperatures with transition temperatures slightly above that of bulk Sn. The enhancement is 0.11 K for the 90 percent film, increasing to 0.24 K in the 55 percent film. As the metal concentration decreases, the transition region broadens, with the resistance beginning to decrease about 1 K above the transition in the highest resistance films. The films remain superconducting in substantial magnetic fields. In the 55 percent film, a magnetic field of 1 T reduces the transition temperature by 1 K and a field greater than 2 T is needed to eliminate superconductivity. At the largest field of 6 T, there is no sign of superconductivity in any sample, and the resistances increase down to the lowest temperatures measured (100 mK).

The rf residual surface resistance Rres is an important figure of merit for superconducting microwave devices and for establishing the homogeneity of superconducting surfaces. In granular superconductors not only are high residual rf surface resistance values Rres observed, but also strong field dependencies of Rres(T,ω, Hrf) and the penetraton depth λres(T, ω, Hrf) are encountered even at very low field levels (of order 1 Oe). These observations can be explained in terms of granularity consisting of weak links and of insulating interfaces, which provide additional degrees of freedom due to fluxoid motion and electromagnetic strip line modes. These degrees of freedom cause additional interaction mechanisms with applied rf fields.

Measurements of the resistive superconducting transition and magnetoresistance of indium metal in the restricted geometry of 50 Å size porous Vycor glass are reported. We have mapped out the upper critical magnetic field versus temperature phase diagram for these transitions for fields up to 20 T and temperatures down to 100 mK. The results are consistent with magnetization measurements by previous authors and can be generally interpreted in terms of the description of inhomogeneous type-II superconductors. The field-induced resistive transitions exhibit unusual behavior not probed by previous bulk-type measurements which may be related to the microstructure of the composite.

Coherent excitations of localized states, represented by a selfsimilar Cayley tree, produce a gap in the density of states, which is deeper as the number of generations increases.

The model can suitably describe systems in which an optimized concentration of “defects” allows self similarity of the state-tree and coherent coupling between clusters of localized states.

A renormalization of the state-tree leads to a hierarchy of the energy eigenvalues.

Recent studies of granular magnetic solids involving Fe and its alloys embedded in either an insulating medium (e.g. SiO2 A12O3) or a metallic medium are described. Below the percolation threshold, isolated magnetic granular exhibit single-domain properties and superparamagnetic relaxation. Giant coercivity has been uncovered in granular Fe-SiO2, with Hc- as high as 3 kOe at 5 K and 1.1 kOe at 300 K. Some recent results on granular Fe in a metallic medium, and other granular magnetic systems will also be discussed.

We have produced thin film composite matrices containing granular transition metals in the zero valent state and have found onset of percolation at a much lower volume fraction than was the case for Au, albeit for significantly smaller transition metal particles. At volume fractions below percolation these granular composites are superparamagnetic down to 6°K. Remanent magnetization and hysteretic ferromagnetic behavior was not observed until we were slightly above the onset of electrical percolation. We interpret this result as indicative of ferromagnetic ordering among the transition metal grains. No significant reduction in magnetic moment due to size effects or to chemical interaction with the matrix was observed. A very small size dispersion of the metal clusters was indicated both from magnetic a well as electron microscopy measurements.

Several samples of granular material consisting of Fe small particles dispersed in an alumina matrix were studied by means of various techniques. Structural results show that the particle surface is partially oxidized with occurrence of bonds with the matrix. Depending on the particle size, one observes b.c.c. a-Fe or amorphous structure. Magnetic properties are relevant of superparamagnetism phenomena, but the strong magnetic interactions between particles (due to the close compact arrangement) lead to a phase transition at zero temperature. It is also briefly discussed the magnetic volume determination and the influence on the properties of the magnetic disorder arising at the particle surface.

Homogeneous gelled composites of iron and silica containing 5-40 wt.%Fe prepared by low temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst) were heated to 380°C in the presence of hydrogen gas. X-ray diffraction and M6ssbauer effect measurements, and transmission electron microscope (TEM) observations show these materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. Magnetic susceptibility data indicate the materials became either superparamagnetic or ferromagnetic at room temperature. On cooling, the magnetization data furthermore show that the hydrogenated materials containing ll-30% Fe become magnetic spin glasses at temperatures less than 30 K. Magnetic history effects are observed in addition to displaced hysteresis loops below their spin freezing temperatures (Tf). For field-cooled materials at 10 K, the displacement of the hysteresis loops along the field axis indicates the presence of a unidirectional anisotropy which decreases with the cooling field. Both superparamagnetic-to-spin glass and ferromagnetic-to-spin glass transitions are observed in these nanocomposites. Tf varies with the Fe content from ∼30 K for the 11%Fe nanocomposite to ∼10 K for a content near 33%Fe.

The mechanical properties of sputtered Ni-Al2O3 granular thin films were investigated by low load microharaness testing. It was found that the microhardness of these films displayed a percolation threshold at a nickel volume fraction of about 0.6, below which the hardness is greatly enhanced. This behavior is qualitatively similar to the electrical and magnetic properties of these types of films. A percolation threshold in hardness can be understood as due to a change in the mechanism for plastic deformation.

Fe-SiO2 granular films were produced by co-sputtering from separate Fe and SiO2 targets. Films deposited on silicon nitride membranes were thin enough to observe directly by transmission electron microscopy (TEM). The coercive force (Hc) was strongly dependent on temperature and volume fraction of Fe as observed in previous investigations, but with lower Hc. Two features of the films with low Fe concentrations were remarkable. The hysteresis loops show high remanence, and the room temperature TEM images show stable domain structure and ripple patterns. This behavior is inconsistent with a model of non-percolating, non-interacting superparamagnetic Fe grains.

We have synthesized a series of nanostructured Fe/Ag composite metal films. The much enhanced magnetic coercivity was found to scale with the particle size which is process controlled. At a low substrate temperature(< 200°C), the film consists of small clusters of Fe(< 100Å) which behave superparamagnetically with a blocking temperature above 100K.