Susie Speller

© 2019 IOP Publishing Ltd. The discovery in 2008 of the iron-based superconducting pnictide and chalcogenide compounds has provided an entirely new family of materials for studying the crucial interplay between superconductivity and magnetism in unconventional superconductors. The alkali-metal-intercalated iron selenide (A x Fe 2-y Se 2 , A = alkali metal) superconductors are of particular interest owing to their relatively high transition temperatures over 30 K and the co-existence of the superconducting state with antiferromagnetic ordering. Intrinsic phase separation on the mesoscopic scale is known to occur in 'single crystals' of these compounds, adding to the complexity of interpretation of bulk property measurements. In this study, we investigate the local electronic structure and chemistry of Rb x Fe 2-y Se 2 crystals using scanning microscopy techniques. Nano-focussed angle-resolved photoemission spectroscopy has enabled the band structure of the minority superconducting phase and the non-superconducting matrix to be measured independently and linked to the surface chemistry from the same regions using core level spectroscopy. Valence band mapping reveals the characteristic microstructure of these crystals, but does not have sufficient spatial resolution to enable the precise morphology of the superconducting phase to be elucidated. Cryogenic magnetic force microscopy has shown that the superconducting phase has a fine-scale stripey morphology that was not resolved in the scanning photoemission spectroscopy experiment. The correlation of these findings with previous microstructural studies, bulk measurements and first-principles density functional theory calculations paves the way for understanding the intriguing electronic and magnetic properties of these compounds.

© 2002-2011 IEEE. Bi-2212/Ag superconducting wires are being considered as key conductor candidates for the development of high-field magnets (up to 25 T), but it is recognized that a reliable joining process between these wires is a critical challenge for most high-field applications. This work focuses on the design and testing of jointing processes between multifilamentary Bi-2212 wires to make persistent mode joints using soldering techniques. The ability of several different superconducting solder alloys from the PbBi and SnInBi systems to remove and replace the Ag matrix in fully reacted Bi-2212 wires has been explored. The SnInBi lead-free solder was found to be relatively effective at removing the Ag matrix, but does not make good contact with the Bi-2212 filaments. PbBi solder wets the Bi-2212 filaments more effectively, but is slower at removing the Ag matrix. Therefore, a two-stage process has been developed using molten Sn to first remove the Ag matrix followed by PbBi to replace the nonsuperconducting Sn. Joints made using this two-stage process have higher critical currents (133 A in self-field at 4 K) than joints made using PbBi alone (120 A in self-field at 4 K).

© 2018 American Physical Society. Higherature superconducting (HTS) cuprate materials, with the ability to carry large electrical currents with no resistance at easily reachable temperatures, have stimulated enormous scientific and industrial interest since their discovery in the 1980's. However, technological applications of these promising compounds have been limited by their chemical and microstructural complexity and the challenging processing strategies required for the exploitation of their extraordinary properties. The lack of theoretical understanding of the mechanism for superconductivity in these HTS materials has also hindered the search for new superconducting systems with enhanced performance. The unexpected discovery in 2008 of HTS iron-based compounds has provided an entirely new family of materials for studying the crucial interplay between superconductivity and magnetism in unconventional superconductors. Alkali-metal-doped iron selenide (AxFe2-ySe2, A=alkalimetal) compounds are of particular interest owing to the coexistence of superconductivity at relatively high temperatures with antiferromagnetism. Intrinsic phase separation on the mesoscopic scale is also known to occur in what were intended to be single crystals of these compounds, making it difficult to interpret bulk property measurements. Here, we use a combination of two advanced microscopy techniques to provide direct evidence of the magnetic properties of the individual phases. First, x-ray linear dichroism studies in a photoelectron emission microscope, and supporting multiplet calculations, indicate that the matrix (majority) phase is antiferromagnetic whereas the minority phase is nonmagnetic at room temperature. Second, cryogenic magnetic force microscopy demonstrates unambiguously that superconductivity occurs only in the minority phase. The correlation of these findings with previous microstructural studies and bulk measurements paves the way for understanding the intriguing electronic and magnetic properties of these compounds.

© 2017 IOP Publishing Ltd. Superconducting NbTi alloys have been successfully fabricated by a simple powder processing route involving ball-milling, pressing and annealing. The microstructure and superconducting properties of the NbTi alloys after each processing step have been characterised and compared to the microstructure and performance of NbTi wire manufactured by a conventional thermomechanical process. At the early stages of milling, a lamellar structure of pure Nb and Ti regions is formed, which is gradually refined by further milling leading to the introduction of a high density of microstructural defects. After 20 h milling, diffusion of Ti into the Nb generates a matrix of β-Nb-50 wt%Ti alloy, with a small grain size (50 nm) and high strain (1.8%), containing an even distribution of thin Ti flakes (10-40 nm). In some regions, these Ti flakes contain a supersaturation of Nb as a result of the energetic ball-milling process. The T c (8.1 K) and B c2 (9.8 T at 4.2 K) values of this as-milled material are slightly lower than those reported for Nb-47 wt%Ti due to the impurity content and lattice disorder. Sintering at 400 C leads to well consolidated, high density bulk samples, but annealing at temperatures above 600 C decreases J c values due to excessive grain growth and strain release. Annealing at lower temperatures results in higher J c values, a shift of the pinning force density peak towards higher fields and the presence of thermodynamically stable α-Ti precipitates which are effective pinning sites leading to critical current density values comparable with those of commercial NbTi wires.