Nano Focus: Proton irradiation enhances critical features simultaneously in iron-based superconductors
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esearchers at the Brookhaven National Laboratory in New York succeeded in simultaneously enhancing two counteracting properties of superconductors which are important for applications, the critical temperature and the critical current density. The phenomenon was observed in iron-based superconductors (IBSCs) irradiated with low-energy protons. The study, led by Qiang Li at Brookhaven, was published in Nature Communications (doi:10.1038/ncomms13036). According to Toshinori Ozaki, now at Kwansei Gakuin University in Japan, the results open new possibilities for the production of low energy accelerators, which will be less expensive to purchase and maintain than currently available, as well as for new transmission and distribution cables that can be used for the upgrade of power grids. In Type I materials, superconductivity is suppressed not only when temperature rises above the critical temperature (Tc), but also when the magnetic field rises above a critical value. Type II superconductors on the other hand, like the iron-based ones used by Li’s team, can tolerate even strong fields, by allowing quanta of magnetic flux to penetrate them in the form of vortices. The penetration takes place near defects, around which non-superconductive areas are created. The density of these non-superconductive regions, which reflects the ability of the defects to pin the vortices, is expressed through the critical current density J c. Enhancing T c is a challenging task, thus, much of the research for the improvement of superconductors has instead focused on raising J c. But the more tolerant a material becomes to higher magnetic fields, the more its T c is reduced. Increasing T c and J c at the same time, although highly desirable, is very difficult to realize. Li’s group has previously achieved very high J c (>105 A/cm2 under 30 tesla magnetic fields) in epitaxial thin films
of FeSe 0.5Te 0.5 (FST), a type of IBSC. However, Li’s group soon found that further controlling the defects and increasing J c was increasingly difficult. The team decided to choose ion irradiation for the introduction of defects in the films. “The low energy proton irradiation which we used to bombard the superconductor material, as well as the aluminum foil, turned out to be the keys in this study,” Ozaki says. The researchers used pulsed laser deposition to grow FST films of 100– 130 nm thickness with An electron microscope image of FeSe0.5Te0.5 iron-based a CeO 2 buffer layer on superconducting films irradiated with low-energy protons. SrTiO3 single-crystalline Yellow arrows designate the resulting chains of defects. The inset image shows a zoomed-in view of a typical defect chain. substrates, and covered Credit: Toshinori Ozaki and Qiang Li. the films with a 1.5-mmthick aluminum foil. The samples were subsequentthe idea that the stress generated by imly irradiated with 190-keV protons at a plantation enhances Tc looks reasonable dose of 1×1015 p cm–2. The low energy irradiation created casif one keeps in mind the high sensitivcade defects on the films
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