Ming-Jye Wang

Gap opening and orbital modification of superconducting FeSe above the structural distortion.

We utilize steady-state and transient optical spectroscopies to examine the responses of nonthermal quasiparticles with respect to orbital modifications in normal-state iron-chalcogenide superconductors. The dynamics shows the emergence of gaplike quasiparticles (associated with a ~36 meV energy gap) with a coincident transfer of the optical spectral weight in the visible range, at temperatures above the structural distortion. Our observations suggest that opening of the high-temperature gap and the lattice symmetry breaking are possibly driven by short-range orbital and/or charge orders, implicating a close correlation between electronic nematicity and precursor order in iron-based superconductors.

Recent advances in β-FeSe1−x and related superconductors

Abstract It has been more than four years since the discovery of β-FeSe1−x superconductors. Through the efforts of many outstanding research groups, unprecedented advances in the field have been achieved. High-quality single crystals of β-FeSe1−x and related compounds have been prepared by various techniques, allowing us to explore in detail the physical properties of this class of materials. Detailed characterizations of the structure and properties of these crystals have helped us to understand the origin of superconductivity in β-FeSe1−x. The occurrence of superconductivity is associated with the low-temperature structure distortion, which is accompanied by several anomalies. Recent measurements on quasiparticle and acoustic phonon dynamics with respect to the orbital modification in β-FeSe1−x suggest the opening of an energy gap below 130–140 K, accompanied by a coincident transfer of optical spectral weight in the visible range and alterations in transport properties. These observations provide convincing evidence that the modification of the electronic structure occurs prior to the lattice distortion. They further suggest that the high-temperature gap and the lattice symmetry breaking are driven by short-range orbital and/or charge orders.

CCT- and CRI-tuning of white light-emitting diodes using three-dimensional non-close-packed colloidal photonic crystals with photonic stop-bands

This study exhibited the correlated color temperature (CCT)- and color-rendering index (CRI)-tuning behavior of light emission from white light-emitting diodes (WLEDs) using three-dimensional non-close-packed (3D NCP) colloidal photonic crystals (CPhCs). The CCT of approximately 5300 K (characteristic of cold WLEDs) of white light propagated through the NCP CPhCs dropped to 3000 K (characteristic of warm WLEDs) because of the photonic stop-bands based on the photonic band structures of NCP CPhCs. This study successfully developed a novel technique that introduces lower-cost CCT- and CRI-tuning cold WLEDs with a CRI of over 90 that of warm WLEDs by using 3D NCP CPhCs.

Transport properties in FeSe0.5Te0.5 nanobridges

FeSeTe nanobridges of different widths have been fabricated on MgO substrates using focused ion beams. These nanobridges exhibit the Josephson effects. The current-voltage curves of junctions with 248–564 nm wide follow the resistively and capacitatively shunted junction model. Shapiro steps under microwave radiation were clearly observed in these nanobridges. The products of the critical current and normal state resistance (IcRn) are remarkably high. The temperature dependence of IcRn product followed the Ambegaokar-Baratoff (A-B) relation. The value of energy gap of FeSeTe calculated from the A-B relation is 3.5kBTc. The nanobridge junctions have a strong potential for high frequency applications.

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.

Design and Performance of a 3-Junction Series Distributed SIS Mixer for Wide IF Applications

Series distributed superconductor-insulator-superconductor mixers have been used to provide very wide instantaneous bandwidth of operation. In this paper, we explore a design based on a 3-junction array in which the junctions themselves form part of the RF tuning network of the mixer. We have developed mixers based on these designs for use in the 200 and 300 GHz bands. Receivers incorporating these wideband mixers demonstrate very low noise temperature. At a local oscillator frequency of 300 GHz, the lowest measured intermediate frequency noise temperature is about 42 K and the usable intermediate frequency bandwidth extends from 6 to 16.5 GHz. Both the design theory and experimental results are presented.

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.

An overview of the Fe-chalcogenide superconductors

This review intends to summarize recent advancements in FeSe and related systems. The FeSe and related superconductors are currently receiving considerable attention for the high critical temperature (T C) observed and for many similar features to the high T C cuprate superconductors. These similarities suggest that understanding the FeSe-based compounds could potentially help our understanding of the cuprates. We begin the review by presenting common features observed in the FeSe- and FeAs-based systems. Then we discuss the importance of careful control of the material preparation allowing for a systematic structure characterization. With this control, numerous rich phases have been observed. Importantly, we suggest that the Fe-vacancy ordered phases found in the FeSe-based compounds, which are non-superconducting magnetic Mott insulators, are the parent compounds of the superconductors. Superconductivity can emerge from the parent phases by disordering the Fe vacancy order, often by a simple annealing treatment. Then we review physical properties of the Fe chalcogenides, specifically the optical properties and angle-resolved photoemission spectroscopy (ARPES) results. From the literature, strong evidence points to the existence of orbital modification accompanied by a gap-opening, prior to the structural phase transition, which is closely related to the occurrence of superconductivity. Furthermore, strong lattice to spin coupling are important for the occurrence of superconductivity in FeSe. Therefore, it is believed that the iron selenides and related compounds will provide essential information to understand the origin of superconductivity in the iron-based superconductors, and possibly the superconducting cuprates.