Gaia Grimaldi

Influence of artificial pinning on vortex lattice instability in superconducting films

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.

Vortex pinning properties in Fe-chalcogenides

Among the families of iron-based superconductors, the 11-family is one of the most attractive for high field applications at low temperatures. Optimization of the fabrication processes for bulk, crystalline and/or thin film samples is the first step in producing wires and/or tapes for practical high power conductors. Here we present the results of a comparative study of pinning properties in iron-chalcogenides, investigating the flux pinning mechanisms in optimized Fe(SeTe x ) and FeSe samples by current–voltage characterization, magneto-resistance and magnetization measurements. In particular, from Arrhenius plots in magnetic fields up to 9 T, the activation energy is derived as a function of the magnetic field, whereas the activation energy as a function of temperature, is derived from relaxation magnetization curves. The high pinning energies, high upper critical field versus temperature slopes near critical temperatures, and highly isotropic pinning properties make iron-chalcogenide superconductors a technological material which could be a real competitor to cuprate high temperature superconductors for high field applications.

Continuous reel-to-reel measurement of critical currents of coated conductors

We demonstrate that magnetization measurements by Hall probe arrays are well suited for the reel-to-reel determination of critical currents along long lengths of coated conductors. The technique is contact-free, fast, simple, and it has a high spatial resolution. A quantitative measure of the critical current as a function of position was obtained by fitting a simple critical state model to the field distribution. The technique is demonstrated for YBa2Cu3O7 films on single-crystal substrates and then applied to samples of coated conductors.

Speed limit to the Abrikosov lattice in mesoscopic superconductors

We study the instability of the superconducting state in a mesoscopic geometry for the low pinning material Mo

Controlling flux flow dissipation by changing flux pinning in superconducting films

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Comparison of Superconducting Properties of

Ge characterized by a large Ginzburg-Landau parameter. We observe that in the current driven switching to the normal state from a nonlinear region of the Abrikosov flux flow, the mean critical vortex velocity reaches a limiting maximum velocity as a function of the applied magnetic field. Based on time dependent Ginzburg-Landau simulations we argue that the observed behavior is due to the high velocity vortex dynamics confined on a mesoscopic scale. We build up a general phase diagram which includes all possible dynamic configurations of Abrikosov lattice in a mesoscopic superconductor.

\hbox{FeSe}_{0.5}\hbox{Te}_{0.5}

We study the flux flow state in superconducting materials characterized by rather strong intrinsic pinning, such as Nb, NbN, and nanostructured Al thin films, in which we drag the superconducting dissipative state into the normal state by current biasing. We modify the vortex pinning strength either by ion irradiation, by tuning the measuring temperature or by including artificial pinning centers. We measure critical flux flow voltages for all materials and the same effect is observed: switching to low flux flow dissipations at low fields for an intermediate pinning regime. This mechanism offers a way to additionally promote the stability of the superconducting state.

Thin Films Grown on Different Substrates

We have shown that the superconducting properties of FeSe_0.5 Te_0.5 thin films are strongly dependent on the growth conditions and, in particular, the in-plane lattice constant of the substrate influences the crystallographic lattice parameters of the films, affecting the strain and is responsible for strong enhancements of the critical temperature <i>T</i>_c and for the introduction of different pinning mechanisms. Here we compare the structural, electrical and transport properties of superconducting Fe(Se_0.5, Te_0.5) epitaxial films deposited through pulsed laser ablation on three different substrates namely lanthanum aluminate (LaAlO₃), strontium titanate (SrTiO₃), and calcium fluoride (CaF₂). We analyze in particular the critical current density <i>J</i>_c as a function of the temperature and magnetic field, and its anisotropy, which is related to the different pinning mechanisms in play. The film grown on SrTiO₃ exhibits a higher critical current when the field is perpendicular to the film surface, opposite to what happens in the sample grown on LaAlO₃ due to the presence of extrinsic pinning along the <i>c</i>-axis, while we observe almost no anisotropy on the thin film grown on CaF₂.

Quasiparticle scattering time in niobium superconducting films

We study the quasiparticle energy relaxation processes in superconducting Nb films of different thicknesses corresponding to different electron mean free paths in a state far from equilibrium, that is the highly dissipative flux-flow state driven up to the instability point. From the measured currentvoltage curves we derive the vortex critical velocity v∗ for several temperatures. From the v∗(T ) values, the quasiparticle energy relaxation time τǫ is evaluated within the Larkin-Ovchinnikov model and numerical calculations of the quasiparticle energy relaxation rates are carried out to support the experimental findings. Besides the expected constant behavior of τǫ(T ) for the dirty samples, we observe a strong temperature dependence of the quasiparticle energy relaxation time in the clean samples. This feature is associated with the increasing contribution from the electron-phonon scattering process.

Pinning mechanism in electron-doped HTS Nd

The electrical transport properties of c-axis oriented Nd1.85Ce0.15CuO4 ? ? superconducting films have been investigated to analyze the pinning mechanism in this material. The samples were grown on SrTiO3 substrates using the dc sputtering high-pressure technique, whereas a detailed analysis of the structure and local composition of the films has been achieved using high-resolution electron microscopy and x-ray microanalysis. Magneto-resistance and current-voltage measurements, in the temperature range from 1.6 to 300 K and in magnetic field up to 9 T, have been reported. In particular, the anisotropic coefficient defined as the ratio between the parallel upper critical field, ab, and the perpendicular one, c, has been evaluated, pointing out the high anisotropy of this compound. Furthermore, the vortex activation energy as a function of the applied magnetic field, parallel and perpendicular to the CuO2 planes, has been derived and compared with the flux-pinning forces to enlighten the peculiar nature of pinning centers in this material.