Anita Guarino

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.

Comparison of Superconducting Properties of

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.

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

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.

Thin Films Grown on Different Substrates

Two-dimensional materials, such as graphene, topological insulators, and two-dimensional electron gases, represent a technological playground to develop coherent electronics. In these systems, quantum interference effects, and in particular weak localization, are likely to occur. These coherence effects are usually characterized by well-defined features in dc electrical transport, such as a resistivity increase and negative magnetoresistance below a crossover temperature. Recently, it has been shown that in magnetic and superconducting compounds, undergoing a weak-localization transition, a specific low-frequency 1/f noise occurs. An interpretation in terms of nonequilibrium universal conductance fluctuations has been given. The universality of this unusual electric noise mechanism has been here verified by detailed voltage-spectral density investigations on ultrathin copper films. The reported experimental results validate the proposed theoretical framework, and also provide an alternative methodology to detect weak-localization effects by using electric noise spectroscopy.

Pinning mechanism in electron-doped HTS Nd

We find an unambiguous relationship between disorder-driven features in the temperature dependence of the resistance and the behavior, as functions of the temperature, of the parameters necessary to describe some of the relaxation processes in the photoinduced differential time-resolved reflectivity of three samples of Nd

_{1.85}

_{1.83}

Ce

Ce

_{0.15}

_{0.17}

CuO

CuO

_{4-\delta }

_{4\pm\delta}