Yao Cai

Microstructure and structural phase transitions in iron-based superconductors

Crystal structures and microstructural features, such as structural phase transitions, defect structures, and chemical and structural inhomogeneities, are known to have profound effects on the physical properties of superconducting materials. Recently, many studies on the structural properties of Fe-based high-Tc superconductors have been published. This review article will mainly focus on the typical microstructural features in samples that have been well characterized by physical measurements. (i) Certain common structural features are discussed, in particular, the crystal structural features for different superconducting families, the local structural distortions in the Fe2Pn2 (Pn = P As, Sb) or Fe2Ch2 (Ch = S, Se, Te) blocks, and the structural transformations in the 122 system. (ii) In FeTe(Se) (11 family), the superconductivity, chemical and structural inhomogeneities are investigated and discussed in correlation with superconductivity. (iii) In the K0.8Fe1.6+xSe2 system, we focus on the typical compounds with emphasis on the Fe-vacancy order and phase separations. The microstructural features in other superconducting materials are also briefly discussed.

Realization of practical level current densities in Sr0.6K0.4Fe2As2 tape conductors for high-field applications

122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields Hc2 (> 100 T) and low anisotropy γ (<2). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large critical current density Jc, which is limited by poor grain coupling and weak-link behavior at grain boundaries. Here, we report our recent achievement in the developing Sr0.6K0.4Fe2As2 tapes with transport Jc up to 0.1 MA/cm2 at 10 T and 4.2 K. This value is by far the highest ever recorded for iron based superconducting wires and has surpassed the threshold for practical application. The synergy effects of enhanced grain connectivity, alleviation of the weak-link behavior at grain boundaries, and the strong intrinsic pinning characteristics led to the superior Jc performance exhibited in our samples. This advanced Jc result opens up the possibility for iron-pnictide superconducting wires to win the race in high-field magne...

The effect of symmetry on resonant and nonresonant photoresponses in a field-effect terahertz detector

The effect of the symmetries in the terahertz (THz) field distribution and the field-effect channel on THz photoresponse is examined. Resonant excitation of cavity plasmon modes and nonresonant self-mixing of THz waves are demonstrated in a GaN/AlGaN two-dimensional electron gas with symmetrically designed nanogates, antennas, and filters. We found that the self-mixing signal can be effectively suppressed by the symmetric design and the resonant response benefits from the residual asymmetry. The findings suggest that a single detector may provide both high sensitivity from the self-mixing mechanism and spectral resolution from the resonant response by optimizing the degree of geometrical and/or electronic symmetries.

Cu/Te substitution effects on superconductivity and microstructure of phase-separated K0.8Fe1.75Se2

Two series of K0.8Fe1.75−xCuxSe2 () and K0.8Fe1.75Se2−yTey () single crystals of nominal composition have been prepared and their physical properties and microstructural features have been studied. Resistivity measurements demonstrate that the superconducting transition temperature decreases gradually with the increase of the substitution level and zero resistivity finally disappears in both systems. Systematic TEM, SEM and XRD structural analyses, in combination with the magnetic experimental data, reveal a rich variety of structural phenomena resulting from different types of substitution. Cu substitution gives rise to the volume of a new non-superconducting Cu-rich phase with modulation coexisting with the superconducting stripe domain. With the increase of the ratio of the new non-superconducting phase along with doping, the superconducting path is finally cut off, and results in the absence of zero resistivity. In contrast, the absence of superconductivity in Te-substituted materials is correlated with the complete disappearance of the q2 superconducting phase due to the suppression of phase separation.

Fe-site substitution effects on superconductivity and microstructure of phase-separated K0.8Fe1.75Se2 superconductor

Series of Fe-site–substituted K0.8Fe1.75−xMxSe2 (M = Cu, Ni, Mn, Zn, Co, Cr) single crystals of nominal composition have been prepared and microstructural features have been studied. Systematic Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) investigations reveal that the substitution effect can be classified into two types: Ni, Co and Cr atoms can be homogenously doped into the superconducting stripes of the phase-separated K0.8Fe1.75Se2 system, while the Cu, Mn and Zn atoms can hardly be introduced into the superconducting stripes, which instead give rise to the formation of new non-superconducting K-M-Se phases embedded in the sample. The complex physical measurement data reported in the literature can be nicely explained by the microstructure picture.

Microstructural properties of K0.8 Fe1.6 S2, K0.8 Fe1.75 Se2-y Sy(0???y???2) and K0.8 Fe1.5+x S2(0?<?x???0.5) single crystals

The structural features of the antiferromagnetic K0.8Fe1.6S2 have been studied in the temperature range from 300K up to 700K by means of in situ transmission electron microscopy (TEM). The superstructure with a wave vector q(1) = 1/5(3a* + b*) originating from a Fe-vacancy order has been clearly observed; moreover, the structural analysis shows that K0.8Fe1.6S2 undergoes a transition from the Fe-vacancy order to disorder at about 585K. The S substitution effect on the phase separation and superconductivity in the K0.8Fe1.75Se2-ySy materials has been systematically investigated by SEM and TEM structural analyses, as well as by electrical resistivity measurements. Our experimental results reveal that the S element adopts a homogeneous distribution in all investigated materials, and the essential phase-separation nature is very similar to what was observed in the K0.8Fe1.75Se2 superconductor. A phase-separated state formed by the coexistence of two Fe-vacancy orders with wave vectors q(1) = 1/5(3a*+b*) and q(3) = 1/4(3a*+b*) in K0.8Fe1.5+xS2 (0 < x < 0.1) has been briefly discussed. Copyright (C) EPLA, 2013

Plasmonic terahertz modulator based on a grating-coupled two-dimensional electron system

Electrically driven broadband modulator with large modulation depth and high speed is in high demand to meet the technical advancing and applications in terahertz fields recently. So far, the single-particle non-resonant absorption mechanism described by the Drude conductivity has been utilized in most of the related researches but is still not efficient enough. Here we proposed and demonstrated a terahertz modulator based on the collective electron plasma excitations (plasmons) in a grating-coupled two-dimensional electron gas in GaN/AlGaN heterostructure. By switching between the resonant and non-resonant conditions of the 2D plasmon excitation enabled by applying proper gate biases, the transmission of terahertz electromagnetic waves can be efficiently manipulated. Taking advantage of its resonant characteristic combined with the strong electric field enhancement in the active region, we experimentally achieved a maximum intensity modulation depth of 93%, a 3 dB operation bandwidth of ∼400 kHz, and a s...

Direct imaging of incommensurate wave pockets in the charge-density-wave state of LaTe2

We herein report the study of the atomic structure for a fully incommensurate CDW in LaTe2 using Cs-corrected scanning transmission electron microscopy (STEM). It is directly revealed for the first time that the atomic displacements adopt an incommensurate wave-pocket structure along each Te chain. This pocket structure has a long periodicity determined by the CDW incommensurability. We can use the sinusoidal waves as the first-order approximation to characterize the atomic motions within the pocket pattern, which can yield atomic displacements in good agreement with the theoretical model commonly used for studying CDW. These facts demonstrate that the incommensurate pocket patterns should be an essential structural nature in the CDW states and play a critical role for developing the mechanism of the CDW transitions.

Superconducting dome and microstructure properties of Rb0.8Fe1.6+xSe2 superconductors

High quality Rb0.8Fe1.6+xSe2 single crystals are grown by a one step melting growth method. Superconductivity has been observed in all samples with x more than 0.05 and the maximum critical temperature Tc~32 K has been obtained in samples with x from 0.1 to 0.2 from electronic resistivity measurement. A superconducting dome in the electronic phase diagram of Tc as a function of Fe content has been observed for the first time in this AxFeySe2 (A=K,Rb) superconducting system. Two coexisting phases with structural modulation vectors of q1=1/5 [3a*+b*] and q2=1/2[a*+b*] have been observed in all superconducting samples by TEM observations. Moreover, phase separation in the superconducting crystals, which can be recognized as micro-stripes in the ab plane, are found to be highly dependent on the Fe content.

Investigation of J c -Suppressing Factors in Flat-Rolled Sr 0.6 K 0.4 Fe 2 As 2 /Fe Tapes Via Microstructure Analysis

Pnictide superconductors will be very promising for applications if wires with high critical current density Jc can allow reel-to-reel large-scale fabrication at low costs. To understand the mechanism(s) that limited Jc in flat-rolled Sr0.6K0.4Fe2As2(Sr122) tapes, microstructure analysis has been considered the most direct and efficient way. Here, we report on high-resolution microstructure imaging and analysis on Fe-sheathed flat-rolled Sr122 tapes, which have a Jc as high as 2.3 × 104 A/cm2 at 10 T and 4.2 K. The overlapping nature of the Sr122 plates was clearly observed. Transmission electron microscopy/scanning transmission electron microscopy analysis showed that, besides the cracks formed during the fabrication process, the SrO2 phase and cavities caused by the inhomogeneously dispersed Sr and K are the other important factors suppressing Jc. The wetting phase FeAs at the grain boundaries can be partially substituted by Sn in Sn-added samples. Our findings provide insights that pave the way to further enhance the critical current of the rolled 122 tapes up to the practical level.Pnictide superconductors will be very promising for applications if wires with high critical current density Jc can allow reel-to-reel large-scale fabrication at low costs. To understand the mechanism(s) that limited Jc in flat-rolled Sr0.6K0.4Fe2As2 (Sr122) tapes, microstructure analysis has been considered to be the most direct and efficient way. Here we report on a high-resolution microstructure imaging and analysis on Fe-sheathed flat-rolled Sr122 tapes, which have a Jc as high as 2.3×10 A/cm at 10 T and 4.2 K. The overlapping nature of the Sr122 plates was clearly observed. TEM/STEM analysis showed that, besides the cracks formed during the fabrication process, the SrO2 phase and cavities caused by the inhomogenously dispersed Sr and K are the other important factors suppressing the Jc. The wetting phase FeAs at the grain boundaries can be partially substituted by Sn in Sn added samples. Our findings provide insights which pave the way to further enhance the critical current of rolled 122 tapes up to the practical level.