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We report the first study on the significant effect of in-situ hydrostatic pressure on the magnetic relaxation in Fe1−xCoxSe0.5Te0.5 single crystals. We find that vortex creep rates are significantly suppressed by pressure, and a crossover from elastic to plastic creep is observed. The pressure also induces vortex creep to move from the large bundle to the small bundle region. Our study indicates that in-situ hydrostatic pressure is very effective for not only significantly increasing the pinning energy and the critical current density, but also reducing the size of flux bundles to suppress the decrease in current density from vortex motion.

Unlike monolayer Fe-Chalcogenide (Fe-Ch)/SrTiO3 (STO), which possesses the potential for high-temperature superconductivity (HTS), a regular Fe-Ch thin film grown on a non-STO substrate by the pulsed laser deposition method shows totally different superconducting behavior and a different mechanism. Although regular Fe-Ch thick films grown on CaF2 generally show the highest superconducting transition temperature (Tc) compared with any other substrates, the disappearance of superconductivity always takes place when the thickness of the Fe-Ch film is reduced to a critical value (∼20nm for Fe-Se and ∼30nm for Fe-Se-Te) with the reason still under debate. Here, we report an enhanced Tc≈17.6K in a 7-nm-FeTe/7-nm-FeSe bilayer heterostructure grown on CaF2 substrate. Generally, the Fe-Ch film on CaF2 is supposed to be one order of magnitude greater in thickness to achieve similar performance. Hall measurements manifest the dominant nature of hole-type carriers in the films in this work, which is similar to the case of a pressurized bulk FeSe single crystal, while in sharp contrast to heavily electron-doped HTS Fe-Ch systems. According to the electron energy loss spectroscopy results, we observed direct evidence of nanoscale phase separation in the form of a fluctuation of the Fe-L3/L2 ratio near the FeTe/FeSe interface. In detail, a several-unit-cell-thick Fe(Se,Te) diffusion layer shows a higher Fe-L3/L2 ratio than either an FeTe or an FeSe layer, indicating low Fe 3d electron occupancy, which is, to some extent, consistent with the hole-dominant scenario obtained from the Hall results. It also implies a possible relationship between the state of Fe 3d electron occupancy and the enhanced Tc in this work. Our work clarifies the importance of the FeTe/FeSe interface in reviving the superconductivity in Fe-Ch ultrathin films, contributing to a more unified understanding of unconventional Fe-Ch superconductivity.

Magnesium diboride (MgB2) superconducting wires have demonstrated commercial potential to replace niobium-titanium (NbTi) in terms of comparable critical current density. Its higher critical temperature makes MgB2 wire suitable for liquid-helium-free operation. We recently reported boron-11 isotope-based low-activation Mg11B2 superconducting wire with decent critical current density appropriate for low-cost superconducting fusion magnets. In this study, we have mainly focused on the neutron diffraction technique to measure the residual stress in Mg11B2 superconducting wire for the first time. The residual stress state was given qualitative and quantitative interpretation in terms of micro- and macrostress generation mechanisms based on the isotropic model confirmed by neutron texture measurements. The relationship between the stress/strain state in the wire and the transport critical current density is also discussed. This investigation could pave the way to further enhancement of the critical current density of low-activation Mg11B2 superconducting wires suitable for next-generation fusion grade magnets.

To develop powerful wind turbine generators using superconducting technology, highperformance superconducting racetrack coils are essential. Herein, we report an evaluation of a multifilamentary magnesium diboride (MgB2) conductor-based racetrack coil cooled and impregnated simultaneously by solid nitrogen (SN2). The coil was wound on a copper former with 13 mm winding width, an inner diameter of 124 mm at the curvature, and 130 mm length of the straight section. An in situ processed S-glass-insulated 36-filament MgB2 wire was wound on the former in two layers with 19.5 turns, and heat treated via the wind and react method without any epoxy resin. The coil was evaluated for critical temperature and transport critical current in the SN2 environment at different temperatures up to 31.3 K in self-field. The coil was able to carry 200 A transport current at 28.8 K in self-field. During coil charging and operation, SN2 effectively acted as an impregnation material. The test results demonstrate the viability to use MgB2 racetrack coil potentially with SN2 impregnation in advanced rotating machine applications.

Materials with sheet-like morphologies are highly desirable candidates for energy storage and conversion applications, due to the confined atomic thickness and high surface area, which would largely improve the electrochemical reaction kinetics. In this work, the sodium storage performance of TiSe 2 nanosheets and corresponding sodiation/desodiation reaction mechanism are studied for the first time. TiSe 2 nanosheets are readily exfoliated from bulk TiSe 2 after quick ultrasonication or grinding. The TiSe 2 nanosheets exhibit a reversible capacity of 147mAhg -1 at 0.1Ag -1 , and show excellent rate capability with a capacity of 103mAhg -1 at an ultra-high current density of 10.0Ag -1 . The combined in situ XRD and ex-situ HRTEM results suggest that sodium storage in TiSe 2 is achieved through a multi-step intercalation/deintercalation mechanism. Besides, TiSe 2 might be a promising 2D nanomaterial platform for other energy and electronic applications due to its easy exfoliation and unique physicochemical properties.

Superconductivity is one of the most attractive research projects in condensed matter physics. Among various types of different superconductors, the family of ironbased superconductors has always been a lively topic since the discovery of the superconductivity in LaFeAsO1-xFx in 2008. The conflict between the necessity of Fe element in this type of superconductor and the superficial thinking on the detrimental effect of ferromagnetism on superconductivity makes iron-based superconductivity both intriguing and unconventionally elusive. In material science, iron selenide (FeSe) is a very promising candidate for exploring the mechanism of iron-based superconductivity due to its having the simplest binary composition and much less toxicity than iron-based superconductors containing As. Since the sign of a dramatically high superconducting transition temperature (Tc) over 77 K in a 1 unit-cell FeSe layer grown on SrTiO3 (STO) substrate was reported in 2012 by a group from China, a flourishing upsurge of FeSe thin films was triggered, and the popularity of superconducting research was rejuvenated. The veil between the scientists and the mystery of iron-based superconductivity has never been so thin. However, plenty of issues still remained to be solved, such as why the same thin film material, FeSe, shows totally different superconducting behaviors in the monolayer and the film with typical thicknesses. In this thesis work, the research subject is FeSe superconducting thin film fabricated by the pulsed laser deposition (PLD) method. The emphasis is on the interface effect and the influence of the external coating process on the superconductivity of FeSe thin films. Enhanced superconducting performance was obtained, and the corresponding mechanism was clarified, based on various characterization methods such as physical property measurements and high-resolution electron microscopy.

This paper reports the fabrication, transport current testing, and finite element analysis of magnesium diboride (MgB2) racetrack coils made by a wind and react method at 4.2 K in self-field. This type of coil can be potentially appropriate for applications aimed at building block coils that can serve as wind turbine generator coils and correction coils for fusion magnets. We have fabricated racetrack coils using an in situ monofilament MgB2/Niobium/Monel wire; each coil had a wire length of 45 m and 80 turns. The best measured critical current of the coil was 134.9 A at 4.2 K and self-field of 0.55 T. In addition, the magnetic field and the Lorentz force contribution in the coil were estimated using finite element analysis.

The investigations into the interfaces in iron selenide (FeSe) thin films on various substrates have manifested the great potential of showing high-temperature-superconductivity in this unique system. In present work, we obtain FeSe thin films with a series of thicknesses on calcium fluoride (CaF2) (100) substrates and glean the detailed information from the FeSe/CaF2 interface by using scanning transmission electron microscopy (STEM). Intriguingly, we have found the universal existence of a calcium selenide (CaSe) interlayer with a thickness of approximate 3 nm between FeSe and CaF2 in all the samples, which is irrelevant to the thickness of FeSe layers. A slight Se deficiency occurs in the FeSe layer due to the formation of CaSe interlayer. This Se deficiency is generally negligible except for the case of the ultrathin FeSe film (8 nm in thickness), in which the stoichiometric deviation from FeSe is big enough to suppress the superconductivity. Meanwhile, in the overly thick FeSe layer (160 nm in thickness), vast precipitates are found and recognized as Fe-rich phases, which brings about degradation in superconductivity. Consequently, the thickness dependence of superconducting transition temperature (Tc) of FeSe thin films is investigated and one of our atmosphere-stable FeSe thin film (127 nm) possesses the highest Tconset/Tczero as 15.1 K/13.4 K on record to date in the class of FeSe thin film with practical thickness. Our results provide a new perspective for exploring the mechanism of superconductivity in FeSe thin film via high-resolution STEM. Moreover, approaches that might improve the quality of FeSe/CaF2 interfaces are also proposed for further enhancing the superconducting performance in this system.

Superconducting wires are widely used for fabricating magnetic coils in fusion reactors. Superconducting magnet system represents a key determinant of the thermal efficiency and the construction/operating costs of such a reactor. In consideration of the stability of 11B against fast neutron irradiation and its lower induced radioactivation properties, MgB2 superconductor with 11B serving as the boron source is an alternative candidate for use in fusion reactors with a severe high neutron flux environment. In the present work, the glycine-doped Mg11B2 bulk superconductor was synthesized from isotopic 11B powder to enhance the high field properties. The critical current density was enhanced (103 A·cm−2 at 20 K and 5 T) over the entire field in contrast with the sample prepared from natural boron.

The best Jc performance (8 x 104 A·cm¿ 2 at 10 T, 4.2 K) has been obtained in our work among all the in-situ powder-in-tube (PIT) MgB2 wires reported so far by tailoring their boron and magnesium precursors, as well as employing appropriate high pressure during the manufacturing process. Moreover, the second stage densification using the special HIP has been applied to commercial long-length wire. Our work suggests that the economical starting precursors and the combination of cold and hot densification techniques could represent a promising alternative for industrial and economical production of practical MgB2 wires with excellent Jc.

Superconducting wires are widely used for fabricating magnetic coils in fusion reactors. Superconducting magnet system represents a key determinant of the thermal efficiency and the construction/operating costs of such a reactor. In consideration of the stability of B-11 against fast neutron irradiation and its lower induced radioactivation properties, MgB2 superconductor with B-11 serving as the boron source is an alternative candidate for use in fusion reactors with a severe high neutron flux environment. In the present work, the glycine-doped (MgB2)-B-11 bulk superconductor was synthesized from isotopic B-11 powder to enhance the high field properties. The critical current density was enhanced (10(3) Acm(-2) at 20 K and 5 T) over the entire field in contrast with the sample prepared from natural boron., China National Funds for Distinguished Young Scientists; National High Technology Research and Development Program of China; International Scientific Partnership Program; Australian Research Council; Australiam- India Early Career Researcher Fellowship; Australian Academy of Science; Australian Government Department of Industry; Innovation and Science; Department of Atomic Energy Scientific Research Council; Government of India Grant

An efficient cooling system and the superconducting magnet are essential components of magnetic resonance imaging (MRI) technology. Herein, we report a solid nitrogen (SN2) cooling system as a valuable cryogenic feature, which is targeted for easy usability and stable operation under unreliable power source conditions, in conjunction with a magnesium diboride (MgB 2) superconducting magnet. The rationally designed MgB 2/SN2 cooling system was first considered by conducting a finite element analysis simulation, and then a demonstrator coil was empirically tested under the same conditions. In the SN2 cooling system design, a wide temperature distribution on the SN2 chamber was observed due to the low thermal conductivity of the stainless steel components. To overcome this temperature distribution, a copper flange was introduced to enhance the temperature uniformity of the SN2 chamber. In the coil testing, an operating current as high as 200 A was applied at 28 K (below the critical current) without any operating or thermal issues. This work was performed to further the development of SN2 cooled MgB 2 superconducting coils for MRI applications.