We investigated the complexation of thermoresponsive anionic poly(N-isopropylacrylamide) (PNiPAM) microgels and cationic ε-polylysine (ε-PLL) chains. By combining electrophoresis, light scattering, transmission electron microscopy (TEM) and dielectric spectroscopy (DS) we studied the adsorption of ε-PLL onto microgel networks and its effect on the stability of suspensions. We show that the volume phase transition (VPT) of microgels triggers a large polyion adsorption. Two interesting phenomena with unique features occur: a temperature-dependent microgel overcharging and a complex reentrant condensation. The latter may occur at fixed polyion concentration, when temperature is raised above the VPT of microgels, or by increasing the number density of polycations at fixed temperature. TEM and DS measurements unambiguously show that short PLL chains adsorb onto microgels and act as electrostatic glue above the VPT. By performing thermal cycles, we further show that polyion-induced clustering is a quasi-reversible process: within the time of our experiments large clusters form above the VPT and partially re-dissolve as the mixtures are cooled down. Finally we give a proof that the observed phenomenology is purely electrostatic in nature: an increase of the ionic strength gives rise to polyion desorption from the microgel outer shell.

We systematically investigate the effect of doping on the temperature dependence of the c-axis resistivity in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }.$ We present simultaneous measurements of the resistivity tensor components ${\rho }_c$ and ${\rho }_{\mathrm{ab}}$ at different doping from the underdoped to the overdoped regime. The c-axis resistivity behaviors as a function of temperature are interpreted, in the normal state $(T>\mathrm{}{T}^{*}),$ through a single phenomenological model based on the existence of two energy barriers with different heights and widths. Two complementary processes are considered for each barrier: incoherent tunneling and thermal activation. The resistive measurements at different doping are well fitted with a small number of free parameters with well defined physical meanings. The analysis of the behaviors of the fitting parameters as a function of doping gives support and consistency to the two-barrier model.

We have used a multiterminal technique in order to measure the $\mathrm{}(\mathit{a},\mathit{b})\mathrm{}$ plane excess conductivity $\Delta \sigma$ in several ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ single crystals. We find that the experimental $\Delta \sigma$ does not follow a simple power law $\Delta \sigma \sim {\epsilon }^{-\alpha },$ with $\epsilon =\ln {(T/T}_c),$ and that it drops faster than the two-dimensional Aslamazov-Larkin law, α=1, with increasing temperature. In addition, data for samples with different doping do not scale on a universal curve. We discuss our data in terms of microscopic and Ginzburg-Landau theories, where high-momentum fluctuations are either not excited, or phenomenologically cut off. The experimental $\Delta \sigma$ drops even faster than the prediction of the extended microscopic theory. However, we can accurately describe all our data up to $T\approx 1.3$ $T_c$ with the GL theory, assuming a sample-dependent cutoff value. We relate the cutoff parameter to the doping level of our samples.

We present measurements of the critical current in an epitaxial ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ film as a function of the magnetic field, the temperature, and of the angle ϑ between the field and the a,b planes. The orientational study reveals an increasing anisotropy in ${\mathit{J}}_{\mathit{c}}$ with increasing temperature. The isothermal curves ${\mathit{J}}_{\mathit{c}}$(H) taken at various angles 0°≤ϑ≤90° are found to collapse to a single curve when the magnetic field is normalized to a scaling function f(ϑ): ${\mathit{J}}_{\mathit{c}}$(H,ϑ)=${\mathit{J}}_{\mathit{c}}$[H/f(ϑ)]. The best fit for f(ϑ) is obtained using an expression originally derived for ${\mathit{H}}_{\mathit{c}2}$(T,ϑ) by Tinkham. The increase in ${\mathit{J}}_{\mathit{c}}$(0°)/${\mathit{J}}_{\mathit{c}}$(90°) with increasing temperature is reflected by the increase in the anisotropy ratio f(0°)/f(90°), consistent with the quasi-two-dimensional Tinkham expression. Consistently, the same model quantitatively describes the increasing anisotropy with increasing temperature.