C. T. Wu

Crystal orientation and thickness dependence of the superconducting transition temperature of tetragonal FeSe1-x thin films.

Superconductivity was recently found in the tetragonal phase FeSe. A structural transformation from tetragonal to orthorhombic (or monoclinic, depending on point of view) was observed at low temperature, but was not accompanied by a magnetic ordering as commonly occurs in the parent compounds of FeAs-based superconductors. Here, we report the correlation between structural distortion and superconductivity in FeSe(1-x) thin films with different preferred growth orientations. The films with preferred growth along the c axis show a strong thickness dependent suppression of superconductivity and low temperature structural distortion. In contrast, both properties are less affected in the films with (101) preferred orientation. These results suggest that the low temperature structural distortion is closely associated with the superconductivity of this material.

Heterojunction of Fe(Se1−xTex) superconductor on Nb-doped SrTiO3

We report the fabrication of heterojunctions formed by the FeSe0.5Te0.5 (FeSeTe) superconductor and Nb-doped SrTiO3 semiconducting substrate and their properties. At high temperature when FeSeTe is in its normal state, the forward bias I−V curves behave like a metal-semiconductor junction with a low Schottky barrier. Direct tunneling through the thin depletion layer of the junction dominates the reverse bias I−V curves. When FeSeTe film becomes superconducting at low temperature, we observed that the Schottky barrier height of the junction increased but was suppressed by an external magnetic field. This deviation provides an estimate of the superconducting energy gap of the FeSeTe film.

Weak localization in FeSe1-xTex superconducting thin films

We have investigated the magneto-resistivity (MR) of FeSe1 xTex superconducting films on MgO substrate. The MR of pure FeSe and slightly Te-substituted films demonstrates regular Lorentz-type magnetic field dependence, MR B 2 . In highly Te-substituted samples, however, negative MR contribution due to the weak-localization effect gradually dominates at low temperature, which is consistent with the evolution of the temperature dependence of resistivity from a metallic to a weakly semiconductor-like behavior. Furthermore, the negative MR weakens and turns positive as temperature approaches the superconducting transition temperature, which is evidence for the Maki‐Thompson correction in the weak-localization regime. The experimental data can be described very well by the weak-localization theory with the existence of scattering by some magnetic moments. The fitting parameters demonstrate that disorder most likely comes from the excess iron. (Some figures may appear in colour only in the online journal)

The Preparation Effect of Li1+xTi2O4 and Its Aging Effect

We have successfully prepared the spinel Li1+xTi2O4, which is a type II superconductor with Tc∼12K, by solid-state reaction process. Owing to the volatility of lithium, excess lithium precursor up to molar fraction 0.15 was added during the synthesis to result in single phase Li1+xTi2O4 without any impurity phase in high resolution XRD pattern. The normal state temperature dependence of resistivity shows an initial semiconductor-like behavior, but turn into metallic-like at about 200K. Magnetization measurements show bulk superconductivity in the fresh samples. The sample is unstable to the exposure of air and humidity. It is observed that the resistive transition quickly diminishes with the exposure to air. However, the magnetic transition remains as sharp, only with the decrease in its superconducting volume fraction, even when the resistive transition completely disappears. An anomalous metal-insulator transition in the aged sample is observed at the same temperature as the superconducting transition temperature of the fresh sample.

The vortex state of FeSe1 − xTex superconducting thin films

We report the vortex dynamics of tellurium substituted FeSe1-xTex superconducting thin films. The electric field versus current density (E-J) curve for films with low Te substitution is still governed by the thermally activated flux flow model at temperatures as low as 0.5T(C)(offset). In contrast, we clearly observed a vortex liquid-glass transition in films with high Te substitution. The E-J curves of these samples fit nicely to the scaling relations based on the 3D vortex glass theory. Our results reveal an enhancement of the vortex pinning as more Te content is introduced, which probably originates from the excess Fe at the interstitial site.

The Transport Properties of Li(1+x)Ti2O4 in High Magnetic Field

The spinel oxide superconductor LiTi2O4 shows intricate transport properties in both normal state and superconducting state. The Hall effect measurements exhibit an abrupt change in carrier density near T=170 K where the temperature derivative of resistivity changes sign from negative to positive on cooling. In addition, the sample shows significant positive magnetoresistance below 40 K, e.g., ΔR(H)/R(H=0)∼3% at T=14 K and H=16 T. A most interesting feature is the magnetic field effect on the resistivity near the superconducting state. In addition to the field-induced broadening of R(T), there exhibits an anomalous upturn in R(T) near the superconducting regime when magnetic field is above 3T. The temperature T1 at which R(T) shows upturn linearly correlates with magnetic field H and can be expressed as T1/TC(H=0)=1−0.04H for 3T<H<12T. The superconductivity is completely suppressed at field above 14T and the low temperature resistance behaves semiconductor-like.