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We report on magnetic properties of cubic compound Cu3TeO6 studied by $ac$ and $dc$ susceptibility and neutron powder diffraction for the first time. A novel magnetic lattice, three dimensional spin web, composed of almost planar regular hexagons of Cu2+, S =1/2 spins, defines the properties of Cu3TeO6. The behavior of the magnetic susceptibility in the paramagnetic state at approximately 170 K is suggestive for a competition between local anisotropies of Cu2+ hexagons. The resulting frustration is weaker than the antiferromagnetic nearest-neighbor interaction which leads to a collinear (or slightly canted) spin arrangement (k=0,0,0) and formation of magnetic domains below TN=61 K.

We have measured two structurally similar superconducting systems, RuSr2GdCu2O8 and GdBa2Cu3O7 by means of high-resolution ac susceptibility. The real and the imaginary part of ac susceptibility of both bulk-ceramic and powdered samples have been studied down to the very small magnetic-field levels. We show that there are significant differences in the evolution of superconductivity in the two studied superconducting systems. In particular, we show that the superconducting transition in the grains in RuSr2GdCu2O8 system is masked with intrinsic magnetism of complex origin.

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.

s74.30C 74.70V74.70574.70Y Some diffusive properties of oxygenin Y-Ba-Cu-O systems D. Djurek(~), V. Manojlov16(~), Z.Medun16 (1), M. Prester(2), E. Bab16(3) and K.Zadro(3) (1) Volta Applied Ceramics, 41000 Zagreb, Croatia, Yugoslavia (2) Instituteof Physics oftile UniversityZagreb, 41000 Croatia, Yugoslavia (3) Department of Physics,Faculty Natural Sciences,41000 Zagreb,Croatia,Yugoslavia(Received 21 August 1991, accepted 4October I991) Abstract.Wehavestudiedtilediffusive properties ofoxygeninY-Ba-Cu-O systems at 940°C by measurements oftileresistiveresponsetime r the samplessubjected to step-lflce increaseoftileoxygenpressure Apo~~ I to 5 bars. The superconducting(SC) transitionwidtlis AT~ plotted versus inverse diffusion constantD r obey the universal linear dependence. Thecharacteristic protruded tailofthereal part of AC susceptibility whichexhibitsthefeature at 86Kisabsentin samples withthenominal compositionY~Ba~CU~O~(Y-336) the regime r < 3 sas resultof weakly connected

An inelastic neutron scattering study of Cu2Te2O5X2 (X=Cl, Br) shows strong dispersive modes with large energy gaps persisting far above T-N, notably in Cu2Te2O5Br2. The anomalous features: a coexisting unusually weak Goldstone-like mode observed in Cu2Te2O5Cl2 and the size of the energy gaps cannot be explained by existing theories, such as our mean-field or random-phase approximation. We argue that our findings represent a new general type of behavior due to intercluster quantum fluctuations and call for development of a new theoretical approach.

{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))

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.

The onset of dissipation in thin, tape shaped form of high-Tc superconductors has been studied by the use of differential resistance characteristics in absence or in small magnetic fields. The studies reveal two characteristic currents: the onset current defining a sharp onset of dissipation and the critical current, a parameter of power-law dependencies. A broad range of intermediate currents is characterized by an anomalous saturation at the very low level of dissipation. The observations are interpreted inside the percolation model for the onset of dissipation as a crossover from the range dominated by finiteness of the sample on the scale of current-dependent correlation length to the range of power laws that corresponds to scaling regime of critical phenomena.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Method in which lock-in detectors process signals from standard ac susceptometers, particularly in the detection of higher harmonics, is analyzed in details. The exact formulas have been derived and checked experimentally by measurements on soft ferromagnetic sample, using several available lock-in amplifiers. The reasons why the proper phase adjustment has to be implemented in the protocol of higher harmonics measurements have been elaborated. The procedure of the lock-in phase adjustment is described, enabling separation of Fourier or Taylor components of hysteretic ac susceptibility into real and imaginary sectors.

Universal aspects of inhomogeneous transport was studied in natural Josephson junction networks. The weak link networks of polycrystalline high-Tc superconductors were found perfectly suitable for quantitative investigation of cluster growth phenomena in percolation (or fractal) networks. We report the experimental results of our studies of I-V characteristics of the two high-Tc families, YBCO and BSCCO. The onset of dissipation, represented by these characteristics, is interpreted as a critical phenomenon, i.e., as a current-induced phase transition. The cluster growth which underlies this phase transition is analyzed by the use of an appropriate model. The model links the non- Ohmic weak link network with classical Ohmic percolation networks (e.g., random resistor network). As a result, we were able to determine the values of appropriate dynamical exponents (exponent t with high precision, t equals 2 +/- 0.1, and exponent s approximately, s approximately equals 0.7), as well as to assign the dissipative ranges inside which the corresponding cluster dynamics takes place. It is therefore concluded that the high-Tc weak link networks represent a natural mesoscopic-scale system convenient for systematic investigation of the transport critical phenomena.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Initial dissipation of weak-link-diminated high-T c superconductors was investigated by systematic studies of I-V characteristics at different temperatures and in small magnetic fields. We propose a model for dissipation which focuses the quasi-Ohmic local dissipation at grain boundary Josephson junctions as an elemental dissipative process. According to this model the overall dissipation is organized as a global percolation in close relation with the problem of conduction in electrically inhomogeneous networks. As an experimental support we present the measured I-V characteristics which strictly obey the power-law behaviour above well-defined critical current, i.e., percolation threshold concentration. The experimental values of temperature and magnetic field independent power-law exponent strongly support its relation to conductivity exponent of percolation theory.

We present an experimental method enabling measurements of time dependence of ac susceptibility down to 1 ms after a steplike change of the applied dc magnetic field. An analog-digital converter has been used to collect the transient of the current, signaling the change of the dc magnetic field. The time window for investigation of magnetic time relaxation phenomena, as studied by the ac susceptibility technique, thus has been expanded in the range of short time scales for three orders of magnitudes.

The onset of dissipation has been studied in sintered (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10+x samples by investigating I–V characteristics in small applied magnetic fields. The observed universal features, similar to those of vortex glass model, have been attributed and quantitatively related to critical exponents of percolation theory.

Dielectric function of quasi-one-dimensional conductor NbSe3 has been measued in the vicinity of Peierls transition near Tp=145K by use of novel technique. The low frequency noise in V/I/ characteristics is attributed to the Gaussian type of position fluctuations of pinned charge density wave (CDW) and fundamental frequency of the noise is found to be identical with spring frequency of restoring forces.

We have examined the effect of a pulsed electric current on the resistive properties of the high temperature superconductor Ba2YCu3O9−δ. In the range of pulsed current densities 3–5 A/mm2 with rise time 0.6 μs, at room temperature, the semiconducting phase is converted to the stable metallic state.

We have observed a low resistance state up to 210 K in a mixed system of the nominal composition Y1.2Ba0.8CuO4-δ prepared under simultaneous action of a pulsed dc electric field up to 30 kV/cm and uniaxiakl pressure of ≈ 1.5 GPa. An onset of a decrease of the resistance at 281 K is observed in repeated cycles and is accompanied by diamagnetism in the temperature interval 150 K