Supercritical antisolvent precipitation (SAS) is based on the fast dissolution of a liquid solution in a supercritical fluid. This technique has been tested to produce nanoparticles of yttrium, samarium, and neodymium acetates to be used as precursors of high-temperature superconductors. Particles of various morphologies were produced at different expansion levels of the liquid solution. Nanoparticles down to about 100 nm were obtained at very large expansion levels, whereas very large aggregates (balloons) with the diameter of several microns were produced at intermediate expansion levels. The influence of several process parameters on particle diameter and particle size distribution was studied. Among SAS process parameters, pressure, temperature, concentration of the liquid solution, and different liquid solvents were tested.

In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an abrupt transition from low to high resistance. We argue, based on a nonuniform distribution of vortex velocity in the sample, that in strongly disordered systems the mean critical vortex velocity for flux-flow instability (i) has a nonmonotonic dependence on magnetic field and (ii) decreases as the pinning strength is increased. These findings challenge the generally accepted microscopic model of Larkin and Ovchinnikov (1979 J. Low. Temp. Phys. 34 409) and all subsequent refinements of this model which ignore the presence of pinning centers. 8

The magnetic properties of the triple-layered Sr(4)Ru(3)O(10) have been investigated by means of neutron scattering diffraction. At zero field we find that the magnetic moments are ferromagnetically coupled and oriented along the c-axis with no signatures of either long-range antiferromagnetic order or ferromagnetic components in the ab-plane. The field dependence of the reflection intensity points to a metamagnetic response involving only the planar magnetic moments. The structural refinement indicates a distinct rearrangement of the unit cell as a function of both temperature and in-plane applied field. We show that at the temperature T* ~/= 50 K, below which the metamagnetic behavior is observed, the c-axis lattice parameter exhibits a rapid increase while the in-plane amplitude saturates. A similar upturn of the in-plane lattice parameter after the quench of the c-axis amplitude occurs above a critical magnetic field.

We study the instability of the superconducting state in a mesoscopic geometry for the low pinning material\ud Mo3Ge characterized by a large Ginzburg-Landau parameter. We observe that in the current-driven switching to\ud the normal state from a nonlinear region of the Abrikosov flux flow, the mean critical vortex velocity reaches a\ud limitingmaximum velocity as a function of the appliedmagnetic field.Based on time-dependent Ginzburg-Landau\ud simulations, we argue that the observed behavior is due to the high-velocity vortex dynamics confined on a\ud mesoscopic scale. We build up a general phase diagram which includes all possible dynamic configurations of\ud the Abrikosov lattice in a mesoscopic superconducto

The complex ac magnetic susceptibilities (χ n = χ' n + iχ n ") of high T c superconductors in absence of dc fields have been studied by numerically solving the non-linear diffusion equation for the magnetic flux, where the diffusivity is determined by the resistivity. In our approach the parallel resistor model between the creep and flux flow resistivities is used, so that the crossover between different flux dynamic processes (thermally activated flux flow, flux creep, flux flow) can naturally arise. For this reason we remark that, as the frequency increases, the presence of a different non linearity in different regions of the I-V characteristic determines nonuniversal temperature dependencies of the χ n , i.e. the χ n are found to be not universal functions of a frequency and temperature dependent single parameter. Moreover, the actual frequency dependent behavior is also shown to be strictly related to the particular pinning model chosen for the simulations. Indeed, for large values of the reduced pinning potential (U/KT≥220) and for increasing frequency, a transition has been observed between dynamic regimes dominated by creep and flux flow processes. On the other hand, for smaller reduced pinning potentials, a transition from the thermally activated flux flow (Taff) to the flow regime occurs. In qualitative agreement with available experimental data but in contrast with previously used simpler models, the amplitude of the peak of the imaginary part of the first harmonic is shown to be frequency dependent. Moreover the frequency dependence of its peak temperature shows large discrepancies with approximated analytical predictions. Finally, the shape of the temperature dependencies of the higher harmonics are found to be strongly affected by the frequency.