Picosecond Trajectory of Two-Dimensional Vortex Motion in FeSe_{0.5}Te_{0.5} Visualized by Terahertz Second Harmonic Generation.

We have investigated the vortex dynamics in a thin film of an iron-based superconductor FeSe_{0.5}Te_{0.5} by observing second-harmonic generation (SHG) in the terahertz frequency range.

Anisotropy of upper critical fields and interface superconductivity in FeSe/SrTiOgrown by PLD.

In this study, we grow FeSe/SrTiOwith thicknesses of 4-19 nm using pulsed laser deposition and investigate their magneto-transport properties. The thinnest film (4 nm) exhibit negative Hall effect, indicating electron transfer into FeSe from the SrTiOsubstrate. This is in agreement with reports on ultrathin FeSe/SrTiOgrown by molecular beam epitaxy.

Superconducting-Gap Anisotropy of Iron Pnictides Investigated via Combinatorial Microwave Measurements.

One of the most significant issues for superconductivity is clarifying the momentum-dependent superconducting gap Δ([Formula: see text]), which is closely related to the pairing mechanism. To elucidate the gap structure, it is essential to investigate Δ([Formula: see text]) in as many different physical quantities as possible and to crosscheck the results obtained in different methods with each other. In this paper, we report a combinatorial investigation of the superfluid density and the flux-flow resistivity of iron-pnictide superconductors; LiFeAs and BaFe(AsP) (x = 0.

Direct measurement of resistivity in destructive pulsed magnetic fields.

A simple method for measuring electrical resistivity under destructive pulsed magnetic fields is presented. This method uses pick-up voltage as the power source to allow the measurement of the absolute value of resistivity in ultra-high magnetic fields above 100 T. The experimental setup and its operation are described in detail, and its performance is demonstrated using critical field measurements of thin-film FeSeTe samples.

Superconductivity at 38 K at an electrochemical interface between an ionic liquid and FeSeTe on various substrates.

Superconducting FeSeTe thin films on SrTiO, LaAlO and CaF substrates were electrochemically etched in an ionic liquid, DEME-TFSI, electrolyte with a gate bias of 5 V. Superconductivity at 38 K was observed on all substrates after the etching of films with a thickness greater than 30 nm, despite the different T values of 8 K, 12 K and 19 K observed before etching on SrTiO, LaAlO and CaF substrates, respectively. T returned to its original value with the removal of the gate bias.

Control of structural transition in FeSeTe thin films by changing substrate materials.

Iron chalcogenide superconductors FeSeTe are important materials for investigating the relation be-tween the superconductivity and the orbital and/or electronic nematic order, because the end member material FeSe exhibits a structural transition without a magnetic phase transition. However, the phase separation occurs in the region of 0.1 ≤ x ≤ 0.

Low-temperature-compatible tunneling-current-assisted scanning microwave microscope utilizing a rigid coaxial resonator.

We present a design for a tunneling-current-assisted scanning near-field microwave microscope. For stable operation at cryogenic temperatures, making a small and rigid microwave probe is important. Our coaxial resonator probe has a length of approximately 30 mm and can fit inside the 2-in.

Suppression of phase separation and giant enhancement of superconducting transition temperature in FeSe(1-x)Te(x) thin films.

We demonstrate the successful fabrication on CaF2 substrates of FeSe(1-x)Tex films with 0 ≤ x ≤ 1, including the region of 0.1 ≤ x ≤ 0.4, which is well known to be the "phase-separation region," via pulsed laser deposition that is a thermodynamically nonequilibrium method.

Broadband method for precise microwave spectroscopy of superconducting thin films near the critical temperature.

We present a high-resolution microwave spectrometer to measure the frequency- dependent complex conductivity of a superconducting thin film near the critical temperature. The instrument is based on a broadband measurement of the complex reflection coefficient, S 11, of a coaxial transmission line, which is terminated to a thin film sample with the electrodes in a Corbino disk shape. In the vicinity of the critical temperature, the standard calibration technique using three known standards fails to extract the strong frequency dependence of the complex conductivity induced by the superconducting fluctuations.

Direct observation of rectified motion of vortices in a niobium superconductor.

Using Lorentz microscopy to directly image vortices, we investigate vortex motion control and rectification in a niobium superconductor. We directly observe a net motion of vortices along microfabricated channels with a spatially asymmetric potential, even though the vortices were driven by an oscillatory field. By observing the individual motion of vortices, we clarify elementary processes involved in this rectification.