We present the first direct experimental access to the actual surface charge number Z of colloidal particles under conditions of strong electrostatic interaction. We further calculate a renormalized charge number Z *(Z) using the modified DLVO approximation and the dependence of the shear modulus G(Z*) on the concentration of neutral electrolyte ns. The excellent agreement of predicted and measured values provides an experimental verification of the renormalization concept under variation of the salt concentration. The link between the true surface charge number and experimental results via Z* is extended to the conductivity σ(Z*). On the basis of the number of dissociable surface groups N and numerical solutions of the nonlinearized Poisson-Boltzmann equation we further provide a qualitative model for incomplete dissociation under conditions of overlapping double layers.

We here review some of our recent results on fluid-like and crystalline ordered colloidal suspensions. We have investigated the structure and the dynamical properties of strongly interacting charged latex particles. The interaction can be varied experimentally over a wide range and in a precisely controlled way. It gives rise to pronounced correlations between the particles, leading to ordered mesosocpic structures which bear striking similarities with atomic or molecular liquids and solids. These colloids therefore can be envisaged as model systems for condensed matter. Nevertheless, typical colloidal time and length scales as found in weakly interacting systems are retained throughout the whole phase diagram upon the formation of fluid, supercooled, and crystalline phases. We pay particular attention to demonstrate this in the structure-dependent self-diffusion dynamics. Due to the mesoscopic interparticle spacing the suspensions are easily probed by Bragg-scattering of visible light. This is used to determine the crystallization kinetics and the elastic properties of the resulting colloidal solids. The rigorous control of experimental parameters allows for quantitative tests of theoretical concepts. In the case of elasticity and diffusion, we detect strong systematic influences of the sample morphology. We also discuss how the determined equilibrium properties may contribute to the understanding of recent experiments in the presence of strong shearing fields.

Electrostatically interacting colloidal suspensions at medium to very low salt concentrations were prepared in differently ordered phases using the method of continuous deionization. Equilibrium phase states include fluid, mono- and polycrystalline material as well as coexistence between fluid and monocrystal. A nonequilibrium supercooled fluid state is reproducibly reached by shear melting. In these phases the long time self-diffusion coefficient DL was measured by forced Rayleigh scattering, while the potential of interaction was systematically varied by changing salt concentration cs and volume fraction φ. In the equilibrium fluid DL decreases by roughly an order of magnitude as the interaction increases. This trend extends continuously into the supercooled state. In all cases crystallization is observed for DL/D0≤0.10(1). In the polycrystalline phases self-diffusion coefficients are still 1 to 2 orders of magnitude lower than in the supercooled state. Here self-diffusion increases with increasing interaction. For the monocrystalline case upper limits of DL are given. These data on the solid phases are discussed in terms of grain boundary and vacancy diffusion.

Superconducting YBa2Cu3O y samples were doped with hydrogen and investigated by the muon spin rotation technique. For hydrogen concentrations above a thresholdx, which depends on the oxygen stoichiometry, we find a well defined precession signal in zero external field. This is a clear indication of magnetic ordering in these samples. In samples with a smaller hydrogen content no magnetic ordering was found. For these samples however, the depolarization rate σ(T→0) as measured in a transverse external field depends strongly on the hydrogen content. Our data are consistent with the assumption that hydrogen acts as an electron donor, filling the hole states in YBa2Cu3O y .

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In this work, forced Rayleigh scattering is combined with a new deionization technique to measure the self-diffusion coefficient of strongly interacting charged polystyrene particles. For each measurement the, actually continuous, deionization process is stopped, allowing the suspension to relax from a sheared state to an equilibrium state. Conductivity and diffusion coefficient are monitored, both during this relaxation to equilibrium, and in dependence of the total deionization time. Relaxational behavior in the diffusion coefficient is observed in samples of both fluid and crystalline equilibrium state, while the conductivity remains unchanged. With increasing total deionization time the conductivity reaches a minimum and rises again to a stable plateau. The self-diffusion coefficient in equilibrium (i.e., at rest) also drops with increasing deionization time for monodisperse samples to unmeasurably low values. On the other hand, a finite, but small diffusion coefficient is found for the charge bidisperse sample.

The muon spin rotation (MUSR) study of local magnetism of Sr-doped La2CuO4 is reviewed. Emphasis is placed on magnetic order as detected by local and bulk probes with local atomic environments studied by x ray absorption fine structure (XAFS). Correlations between the MUSR study of local magnetic ordering and the bulk magnetization study are presented along with a discussion of the dependence upon oxygen stoichiometry. Results are presented for both superconducting phases and magnetic phases. Recent data which reveals the existence of local magnetic ordering in the hydrogen-doped YBa2Cu3O7 system are also discussed.

Superconducting YBa2Cu3O6.6 samples (TC=40K) were investigated by the muon spin rotation technique in zero external field. Below 3 K we find a fast depolarization of the muon signal which indicates that local magnetic fields believed to originate from Cu electronic moments are present. For temperatures above 3 K and for higher oxygen concentrations these fields are absent. We suggest that dynamic spin correlations exist in YBa2Cu3Oy in a wide temperature and concentration range but only for y < 6.6 and low temperatures the fluctuations are sufficiently slow to be seen in a μSR experiment.

Muon spin rotation experiments performed on superconducting ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ samples clearly show that internal magnetic fields coexist at low temperatures ($Tl2$ K) with superconductivity for $x\ensuremath{\le}0.15$. The magnetic fields in the superconducting state are an order of magnitude smaller than the corresponding fields in undoped ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$. The data are discussed in the context of a magnetic pairing mechanism in high-${T}_{c}$ superconductors.

A reply to the comment on coexistence of superconductivity and magneti ordering in LaSrCuO is given.the argument that the samples are electronic- ally inhomogeneous and that magnetic ordering and superconductivity occur in spatially separated regions is argued against.The authors believe that the density of charge carriers is the relevent parameter for both superc- onductivity and magnetism. (AIP)