Combining in-depth neutron diffraction and systematic bulk studies, we discover that the √ 5 × √ 5 Fe vacancy order, with its associated block antiferromagnetic order, is the ground state with varying occupancy ratios of the iron 16𝑖 and vacancy 4𝑑 sites across the phase-diagram of K𝑥Fe2−𝑦Se2. The orthorhombic order, with one of the four Fe sites vacant, appears only at intermediate temperatures as a competing phase. The material experiences an insulator to metal crossover when the √ 5 × √ 5 order is highly developed. Superconductivity occurs in such a metallic phase.
Five surfactants [sodium dodecyl benzene sulfonate (SDBS), sodium deoxycholate (NaDC), Triton X‐100 (TX10), Arabic gum (AG), and cetyltrimethyl ammonium bromide (CTAB)] were employed separately or jointly as superficial active agents (SAAs), to enhance solubilization/dispersion of multi‐walled carbon nanotubes (MWNTs) in aqueous solution and cement matrix. The stabilizations of MWNTs in 12 SAAs solutions were estimated through sedimentation time of each suspension with centrifugation or sitting in 60 days. The mechanical and electrical properties as well as microstructures of MWNTs/cement composites (MNT/CCs) using the SAAs of SDBS, NaDC, AG, or the mixture of SDBS and TX10 were investigated. It was found that the capability of SAAs in dispersing MWNTs roughly decreases in the order as SDBS&TX10, SDBS, NaDC&TX10, NaDC, AG, TX10, and CTAB. The SAA of SDBS and TX10 with a mixing ratio of 3:1 by weight exhibits the best solubilization/dispersion capability. The delaminating time of the suspension of the above SAA with or without centrifugation is 80 min and 60 days, respectively. The microscopy observation indicates that the nanotubes form a stable network in the MNT/CC using SDBS&TX10 SAA. In addition, compared to the Plain/C, the corresponding MNT/CC increases in flexural strength, compressive strength, and the electrical conductivity by 29.10%, 20.8%, and two orders of magnitude, respectively. The enhancements are attributed to the uniform and stable distribution of MWNTs with the suitable surfactants and the ultrasonic dispersion method.
A sufficiently large supercurrent can close the energy gap in a superconductor and create gapless quasiparticles through the Doppler shift of quasiparticle energy caused by finite Cooper pair momentum. In this gapless superconducting state, zero-energy quasiparticles reside on a segment of the normal state Fermi surface, whereas the remaining Fermi surface is still gapped. Here we use quasiparticle interference to image the field-controlled Fermi surface of Bi2Te3 thin films proximitized by the superconductor NbSe2. A small applied in-plane magnetic field induces a screening supercurrent, which leads to finite-momentum pairing on the topological surface states of Bi2Te3. We identify distinct interference patterns that indicate a gapless superconducting state with segmented Fermi surface. Our results reveal the strong impact of finite Cooper pair momentum on the quasiparticle spectrum.
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