We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2(001) substrate. High critical current density values - larger than 1 MA/cm(2) in self field and liquid helium - are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through transmission electron microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what was observed on SrTiO3, where defects parallel to the c-axis enhance pinning in that direction

Using a field-effect transistor (FET) configuration with solid Li-ion conductor (SIC) as gate di-electric, we have successfully tuned carrier density in FeSe0.5Te0.5 thin flakes, and the electronic phase diagram has been mapped out. It is found that electron doping controlled by SIC-FET leads to a suppression of the superconducting phase, and eventually gives rise to an insulating state in FeSe0.5Te0.5. During the gating process, the (001) peak in XRD patterns stays at the same position and no new diffraction peak emerges, indicating no evident Li + ions intercalation into the FeSe0.5Te0.5. It indicates that a systematic change of electronic properties in FeSe0.5Te0.5 arises from the electrostatic doping induced by the accumulation of Li + ions at the interface between FeSe0.5Te0.5 and solid ion conductor in the devices. It is striking that these findings are drastically different from the observation in FeSe thin flakes using the same SIC-FET, in which Tc is enhanced from 8 K to larger than 40 K, then the system goes into an insulating phase accompanied by structural transitions. PACS numbers: 74.25.F-, 74.70.Xa, 74.78.-w Tuning carrier concentration is one of the most powerful approaches in the condensed matter physics for the explorations of novel quantum phases and exotic electronic properties as well as their underlying physical mechanics [1-8]. To overcome the inherent doping limit in the material synthetic methods, field effect transistor (FET) configurations have been applied to tune material properties using gating by electric field [9]. Two types of FET, metal-insulator-semiconductor (MIS) FET and electric double layer (EDL) FET, are widely used to control the charge carrier density on the surface of materials [10, 11]. In order to change the carrier density in the bulk, the so-called ionic field-effect transistor (iFET) with gel-like electrolyte as the gate medium has been used to drive Li + ions into layered materials. This type of FET configuration can effectively modulate 1T-TaS 2 electronic properties by the tunable Li + ion intercalation [12]. However, the heavily-doped electronic states in all these FET configurations are confined at the interfaces or overlaid with electrolyte, which prevents them from being characterized by many physical measurements. On the other hand, conventional MIS-FET devices cannot provide sufficient carriers to induce novel phases, such as superconductivity, by electrostatic doping, and the liquid or gel-like electrolyte is not compatible with modern solid electronics and may react with samples when gating voltage is applied [11, 13, 14]. Recently, we have fabri-[ †] These authors contributed equally to this work.

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(Se -x 1 Te 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, U H , 0 ( ) whereas the activation energy as a function of temperature, U T , ( ) 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.

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We report on the magnetic field and angular dependence of the critical current density of epitaxial FeTe0.5Se0.5 thin films. The films exhibit high critical current values and weak dependence on the applied magnetic field. The Jc is larger for field parallel to the c-axis, which is the opposite behavior of what expected from the critical field anisotropy. The analysis of the activation energy for vortex motion indicates that the single pinning regime holds up to 9 T, suggesting that correlated pinning centers are more effective than the vortex-vortex interaction even at the largest applied fields. Scanning tunneling microscope analysis indicates threading dislocations as possible pinning centers.