In layered superconductors with very weak coupling between the layers the concept of a flux-line lattice breaks down when the field is oriented parallel to the superconducting planes. For an arbitrary field orientation we propose that the formation of an Abrikosov lattice is only related to the perpendicular field component. The parallel field component penetrates as if the superconducting planes were completely decoupled. This model explains recent experiments which have questioned the driving mechanism for dissipation in the superconducting phase of the high-temperature oxide superconductors.

Detailed measurements of the critical current density $j_c$ of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ films grown by pulsed laser deposition reveal the increase of $j_c$ as a function of film thickness. Both this thickness dependence and the field dependence of the critical current are consistently described using a generalization of the theory of strong pinning of Ovchinnikov and Ivlev [Phys. Rev. B 43, 8024 (1991)]. From the model, we deduce values of the defect density ${(10\mathrm{}}^{21}\hspace{0.5em}{\mathrm{m}}^{-3})$ and the elementary pinning force, which are in good agreement with the generally accepted values for ${\mathrm{Y}}_2{\mathrm{O}}_3$ inclusions. In the absence of clear evidence that the critical current is determined by linear defects or modulations of the film thickness, our model provides an alternative explanation for the rather universal field dependence of the critical current density found in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ films deposited by different methods.

We present direct evidence of a "peak effect" as a precursor to vortex lattice melting in a single crystal of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ containing only two twin planes. The peak effect occurs just below the vortex liquid to solid phase transition in a narrow field region between 0.3 and 1.5 T for $H\parallel \mathbf{c}$ and is clearly observed as a function of both magnetic field and temperature. We demonstrate that the peak effect is due to enhanced vortex pinning by the twin boundaries, indicative of lattice softening prior to melting. In addition, we show evidence for a non-Ohmic behavior in the vortex liquid state due to depinning of the vortices from the two twin boundaries with increasing Lorentz force.

The response of the vortex system in the mixed state of type-II superconductors to ac fields is discussed from a unique macroscopic point of view. Linear and nonlinear response are derived as two opposite limits of the same diffusionlike equation for magnetic flux. ac susceptibility experiments carried out in the limit of strong nonlinearity are entirely equivalent to magnetic-relaxation or field-ramp experiments. The magnitude and frequency dependence of the nonlinearity threshold, or amplitude of the ac field where one crosses from essentially linear to strongly nonlinear response, are found and discussed for both the vortex-liquid and vortex-glass states.