Critical currents and pinning forces in Nd2−xCexCuO4− thin films

Critical current density, Jc, and flux-pinning force density, Fp, have been investigated at different temperatures in electron-doped Nd2−xCexCuO4− thin films for magnetic fields, H, applied parallel to the c axis. The reduced pinning force density fFp /Fp max shows a clear scaling behavior when H is normalized to the irreversibility field H, indicating the presence of the same pinning mechanism in the investigated temperature range. Moreover the maximum of Fp as function of the field at each temperature depends linearly on H. The experimental data, interpreted using a modified Anderson-Kim description of the flux-creep theory, imply a magnetic field dependence of the activation energy UHH− with =0.8. This value indicates that in Nd2−xCexCuO4− a quasi-two-dimensional vortex system is present, intermediate between Bi-based and Y-based hole-doped compounds

Stability mechanisms of high current transport in iron-chalcogenides superconducting films

The improvement in the fabrication techniques of iron-based superconductors have made these materials real competitors of high temperature superconductors and MgB$_2$. In particular, iron-chalcogenides have proved to be the most promising for the realization of high current carrying tapes. But their use on a large scale cannot be achieved without the understanding of the current stability mechanisms in these compounds. Indeed, we have recently observed the presence of flux flow instabilities features in Fe(Se,Te) thin films grown on CaF$_2$. Here we present the results of current-voltage characterizations at different temperatures and applied magnetic fields on Fe(Se,Te) microbridges grown on CaF$_2$. These results will be analyzed from the point of view of the most validated models with the aim to identify the nature of the flux flow instabilities features (i.e., thermal or electronic), in order to give a further advance to the high current carrying capability of iron-chalcogenide superconductors.Comment: 4 pages, 3 figure

Piezoelectricity and charge trapping in ZnO and Co-doped ZnO thin films

Piezoelectricity and charge storage of undoped and Co-doped ZnO thin films were investigated by means of PiezoResponse Force Microscopy and Kelvin Probe Force Microscopy. We found that Co-doped ZnO exhibits a large piezoelectric response, with the mean value of piezoelectric matrix element d33 slightly lower than in the undoped sample. Moreover, we demonstrate that Co-doping affects the homogeneity of the piezoelectric response, probably as a consequence of the lower crystalline degree exhibited by the doped samples. We also investigate the nature of the interface between a metal electrode, made up of the PtIr AFM tip, and the films as well as the phenomenon of charge storage. We find Schottky contacts in both cases, with a barrier value higher in PtIr/ZnO than in PtIr/Co-doped ZnO, indicating an increase in the work function due to Co-doping

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Self-formed LaAlO3/SrTiO3LaAlO_3/SrTiO_3 Micro-Membranes

Oxide heterostructures represent a unique playground for triggering the emergence of novel electronic states and for implementing new device concepts. The discovery of 2D conductivity at the $LaAlO_3/SrTiO_3$ interface has been linking for over a decade two of the major current research fields in Materials Science: correlated transition-metal-oxide systems and low-dimensional systems. A full merging of these two fields requires nevertheless the realization of $LaAlO_3/SrTiO_3$ heterostructures in the form of freestanding membranes. Here we show a completely new method for obtaining oxide hetero-membranes with micrometer lateral dimensions. Unlike traditional thin-film-based techniques developed for semiconductors and recently extended to oxides, the concept we demonstrate does not rely on any sacrificial layer and is based instead on pure strain engineering. We monitor through both real-time and post-deposition analyses, performed at different stages of growth, the strain relaxation mechanism leading to the spontaneous formation of curved hetero-membranes. Detailed transmission electron microscopy investigations show that the membranes are fully epitaxial and that their curvature results in a huge strain gradient, each of the layers showing a mixed compressive/tensile strain state. Electronic devices are fabricated by realizing ad hoc circuits for individual micro-membranes transferred on silicon chips. Our samples exhibit metallic conductivity and electrostatic field effect similar to 2D-electron systems in bulk heterostructures. Our results open a new path for adding oxide functionality into semiconductor electronics, potentially allowing for ultra-low voltage gating of a superconducting transistors, micromechanical control of the 2D electron gas mediated by ferroelectricity and flexoelectricity, and on-chip straintronics.Comment: 8 pages, 4 figure

Correlation between structural and transport properties in epitaxial films of Nd2−xCexCuO4±δ

We present here a study on the influence of the oxygen reduction process on the structural and transport properties of epitaxial thin films of the electron-doped cuprate Nd2−xCexCuO4±δ. As is well known, the gradual removal from as-grown samples of a tiny percentage of excess oxygen ions leads to a drastic improvement of the metallic character of this system, which eventually becomes superconducting for suitable values of the cerium concentration, with amaximal critical temperature Tc≃25 K. We find that the oxygen loss occurring in thermal treatments in the temperature range 500–850 °C leads to a reduction of the disorder hindering conductance processes, but is insufficient to make the system become superconducting. On the other hand, as soon as the annealing temperature is raised above 850 °C, superconductivity appears, and at the same time a systematic variation of the length of the unit cell along the c-axis direction is detected. This is a clear indication that the transition to the superconducting phase is always accompanied by a structuralmodification. A further salient feature characterizing samples annealed at high temperatures is the emergence of a linear contribution in the normalstate resistivity, which superimposes to the quadratic one already present in samples which are oxygenreduced below 850 °C. This contribution is probably associated with the formation of hole-like carriers located at hole pockets developing at the Fermi energy along the nodal direction in the Brillouin zone. We conjecture that the evolution of the electronic stateswith oxygen removal for a given ceriumconcentration close to optimal doping, is similar to the one taking place in optimally annealed samples where cerium concentration is raised from the underdoped to the lightly overdoped regime value