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ac susceptibility measurements are reported for single crystals of the layered organic superconductor $\ensuremath{\kappa}{\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}.$ The value of the out-of-plane penetration depth ${\ensuremath{\lambda}}_{b}(0)$ was found to be 133 $\ensuremath{\mu}\mathrm{m},$ consistent with earlier results. The temperature dependence of the in-plane superfluid density, with the value of in-plane component ${\ensuremath{\lambda}}_{\mathrm{ac}}(0)$ of the order of 1 $\ensuremath{\mu}\mathrm{m},$ is strongly linear in T, whereas the out-of-plane superfluid density varies as ${T}^{2}$ at low temperatures. The observed behavior is fully consistent with the one expected for d-wave superconductivity.

We report on our experimental evidence of a substantial geometrical ingredient characterizing the problem of incipient dissipation in high-${T}_{c}$ superconductors (HTS's): high-resolution studies of differential resistance-current characteristics in the absence of magnetic field enabled us to identify and quantify the fractal dissipative regime inside which the actual current-carrying medium is an object of fractal geometry. The discovery of a fractal regime proves the reality and consistency of a critical-phenomena scenario as a model for dissipation in inhomogeneous and disordered HTS's, gives the experimentally based value of the relevant finite-size scaling exponent, and offers some interesting new guidelines to the problem of pairing mechanisms in HTS's.

A detailed study of dissipation in weak link networks of high-{ital T}{sub {ital c}} superconductors has been performed. The model for dissipation has been elaborated assuming a general correspondence with the problems of inhomogeneous charge transport. The model treats the basic experimental ingredients, current-voltage ({ital I}-{ital V}) and current-differential resistance ({ital I}-{ital dV}/{ital dI}) characteristics, as a current-induced critical phenomenon. The validity of a simple relationship between non-Ohmic weak link network and Ohmic classical percolation networks has been proved, allowing a quantitative interpretation of {ital I}-{ital V} characteristics. These characteristics are primary determined by dynamical features of transport on percolation networks. The model focuses on both aspects of of the scaling-related power law, its dynamical exponent and its prefactor, and gives specific predictions for magnetic field and temperature dependencies. The experimental results are in full agreement with both predictions. In particular, the experimental values of dynamical scaling exponents ({ital t}=2, {ital s}=0.7) coincide with generally accepted literature values for three-dimensional systems. {copyright} {ital 1996 The American Physical Society.}

Broadening of the resistive transition of sintered ${\mathrm{GdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ superconductor has been studied varying the measuring current over the range of five orders of magnitude. The observed well-defined branching points in the resistive transition coincide with the quasi-Ohmic saturation of V-I curves and scale with the average grain size. The results suggest the validity of a simple percolation model for dissipation in systems whose global superconductivity is realized by Josephson coupling of the superconducting grains. A common origin of structured resistive transition, observed in many superconducting systems, has been also discussed.

The time dependence of the dc voltage that remains on a sintered ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ sample after the application of a magnetic field has been systematically studied in various magnetic fields and at three different temperatures. The obtained logarithmic time dependence has been related to the intragranular flux creep assuming that the dissipative part of the sample, the subsystem of weak links, is controlled by local magnetic fields. A one-dimensional model for the dissipation has been introduced and solved numerically, showing that the inverse of the relaxation rate approaches the intragranular pinning potential in the limit of high applied fields. The experimental values of appropriately defined pinning potentials increases with temperature.

The variation of critical currents and their distributions with thickness has been investigated for two high-quality ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ samples with different average grain size (10 and 30 \ensuremath{\mu}m for samples ${\mathrm{S}}_{1}$ and ${\mathrm{S}}_{2}$, respectively) in the temperature range 78\char21{}90 K and in the applied magnetic field H50 Oe. The critical-current density (${\mathit{J}}_{\mathit{c}}$) for ${\mathrm{S}}_{1}$ initially increased but later on leveled off on reducing the thickness, whereas for ${\mathrm{S}}_{2}$ remained essentially unchanged even after threefold reduction in thickness. Since the other parameters related to macroscopic homogeneity have not changed on reducing the thickness of the samples, the variations of ${\mathit{J}}_{\mathit{c}}$ are interpreted in terms of thickness and grain-size-dependent self-field effects. The same model explains well the changes of critical-current distribution curves with thickness and may also explain the variation of ${\mathit{J}}_{\mathit{c}}$ with the grain size, as reported recently for ceramic Y-Ba-Cu-O samples.

Pronounced anisotropy of magnetic properties and complex magnetic order of a new oxi-halide compound Co-7(TeO3)(4)Br-6 has been investigated by powder and single-crystal neutron-diffraction, magnetization, and ac susceptibility techniques. Anisotropy of susceptibility extends far into the paramagnetic temperature range. A principal source of anisotropy are anisotropic properties of the involved octahedrally coordinated single Co2+ ions, as confirmed by angular-overlap-model calculations presented in this work. Incommensurate antiferromagnetic order sets in at T-N=34 K. The propagation vector is strongly temperature dependent reaching k(1)=[0.9458(6),0,0.6026(5)] at 30 K. A transition to a ferrimagnetic structure with k(2)=0 takes place at T-C=27 K. The magnetically ordered phase is characterized by very unusual anisotropy as well: while M-H scans along the b axis reveal spectacularly rectangular but otherwise standard ferromagnetic hysteresis loops, M-H studies along other two principal axes are perfectly reversible, revealing very sharp spin-flop (or spin-flip) transitions, such as in a standard antiferromagnet (or metamagnet). Altogether, the observed magnetic phenomenology is interpreted as an evidence of competing magnetic interactions permeating the system, in particular of the single-ion anisotropy energy and the exchange interactions. Different coordinations of the Co2+ ions involved in the low-symmetry C2/c structure of Co-7(TeO3)(4)Br-6 render the exchange-interaction network very complex by itself. Temperature-dependent changes in the magnetic structure, together with an abrupt emergence of a ferromagnetic component, are ascribed to continual spin reorientations described by a multicomponent, but yet unknown, spin Hamiltonian.

{ital V}-{ital I} curves of several Y-Ba-Cu-O samples have been measured in the temperature range 77--91 K in weak magnetic fields ({ital H}{lt}150 Oe). For the well prepared low-resistivity samples {ital R}{sub {ital f}}=={ital dV}/{ital dI} reaches a value of about 0.1{ital R}{sub {ital n}} ({ital R}{sub {ital n}} is the normal resistance just above transition) in the linear {ital V}-{ital I} regime. This is consistent with a picture of decoupled superconducting grains and does not yield any direct information about the flux-flow effects. Because of self-field effects, critical-current density ({ital J}{sub {ital c}}) obtained from transport (and magnetization) measurements does not provide clear insight on the nature of intergrain coupling. Information on the distribution of critical currents, obtained from {ital d}{sup 2}{ital V}/{ital dI}{sup 2}, supports the tunneling and/or proximity-effect coupling between the grains and seems to rule out large enhancements of {ital J}{sub {ital c}} in sintered samples with nonoriented grains.

Polarized and unpolarized neutron-diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl, Br). A model of the magnetic structure associated with the propagation vectors k(') (Cl) approximate to (-0.150,0.422, (1)/(2)) and k'(Br) approximate to (-0.172,0.356, (1)/(2)) and stable below T-N = 18 K for X= Cl and T-N = 11 K for X = Br is proposed. A feature of the model, common to both the bromide and chloride, is a canted coplanar motif for the four Cu2+ spins on each tetrahedron which rotates on a helix from cell to cell following the propagation vector. The Cu2+ magnetic moment determined for X = Br, 0.395(5)mu(B), is significantly less than for X= Cl, 0.88 (1)mu(B) at 2 K. The magnetic structure of the chloride associated with the wave vector k' differs ;from that determined previously for the wave vector k approximate to (0.150,0.422, (1)/(2))

High-resolution ac susceptibility and thermal conductivity measurement on ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{X}_{2}(X=\mathrm{Br},\mathrm{Cl})$ single crystals are reported. For Br-sample, sample dependence prevents one from distinguishing between possibilities of magnetically ordered and spin-singlet ground states. In Cl-sample a three-dimensional transition at 18.5 K is accompanied by almost isotropic behavior of susceptibility and almost switching behavior of thermal conductivity. Thermal conductivity studies suggest the presence of a tremendous spin-lattice coupling characterizing Cl- but not Br-sample. Below the transition Cl-sample is in a complex magnetic state involving AF order but also the elements consistent with the presence of a gap in the excitation spectrum.

We report high-sensitivity ac susceptibility measurements of the penetration depth in the Meissner state of the layered organic superconductor $\ensuremath{\kappa}\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}.$ We have studied nominally pure single crystals from the two different syntheses and employed controlled cooling procedures in order to minimize intrinsic remnant disorder at low temperatures associated with the glass transition, caused by ordering of the ethylene moieties in BEDT-TTF molecule at ${T}_{\mathrm{G}}=75\mathrm{K}.$ We find that the optimal cooling procedures (slow cooling of $\ensuremath{-}0.2\mathrm{K}/\mathrm{h}$ or annealing for 3 days in the region of ${T}_{\mathrm{G}})$ needed to establish the ground state depend critically on the sample origin, indicating different relaxation times of terminal ethylene groups. We show that, in the ground state, the behavior observed for nominally pure single crystals from both syntheses is consistent with unconventional d-wave order parameter. The in-plane penetration depth ${\ensuremath{\lambda}}_{\mathrm{in}}(T)$ is strongly linear, whereas the out-of-plane component ${\ensuremath{\lambda}}_{\mathrm{out}}(T)$ varies as ${T}^{2}.$ In contrast, the behavior of single crystals with long relaxation times observed after slow $(\ensuremath{-}0.2\mathrm{K}/\mathrm{h})$ cooling is as expected for a d-wave superconductor with impurities [i.e., ${\ensuremath{\lambda}}_{\mathrm{in}}(T)\ensuremath{\propto}{\ensuremath{\lambda}}_{\mathrm{out}}(T)\ensuremath{\propto}{T}^{2}]$ or might be also reasonably well described by the s-wave model. Our results might reconcile the contradictory findings previously reported by different authors.

We report ac susceptibility data on RuSr2(Eu,Ce)(2)Cu2O10-y (Ru-1222, Ce content x=0.5 and 1.0), RuSr2GdCu2O8 (Ru-1212), and SrRuO3. Both Ru-1222 (x=0.5, 1.0) sample types exhibit unexpected magnetic dynamics in low magnetic fields: logarithmic time relaxation, switching behavior, and "inverted" hysteresis loops. Neither Ru-1212 nor SrRuO3 exhibit such magnetic dynamics. The results are interpreted as evidence of the complex magnetic order in Ru-1222. We propose a specific multilayer model to explain the data, and note that superconductivity in the ruthenocuprate is compatible with both the presence and absence of the magnetic dynamics.

We measured polycrystalline samples of SrRuO3 and both superconducting and nonsuperconducting (hydrogen loaded) ruthenocuprates-Eu1.5Ce0.5RuSr2Cu2O10-x (Ru-1222)-samples using ac susceptibility, Ru 3p x-ray photoemission and Ru 2p x-ray-absorption measurements. The superconducting-and metallic-ruthenocuprate samples exhibit significant ferromagnetic fluctuations above the highest magnetic ordering temperature. The (Ru 3p) core level of the SrRuO3, and superconducting ruthenocuprate samples indicate that the (Ru) layers are metallic.

Microwave-absorption measurements using a conventional electron-spin resonance spectrometer provide a novel leadless technique for the detection of Josephson junctions and their size in superconductive Ba-Y-Cu-O compounds.