**1. Introduction**

Iron-based superconductors have a reasonably high superconducting transition temperature *T*c, very high upper critical magnetic fields *H*c2, quite a small anisotropy *γ* and larger critical grain boundary angle than cuprate superconductors, which make them promising for high-field applications such as superconducting magnet and generators [1–5]. The use of superconducting materials for high field applications is limited by the critical current density *J*<sup>c</sup> in magnetic fields, which can be sustained by pinning the vortices (flux pinning) at structural defects with nano-meter sizes such as cracks, voids, grain boundaries and secondary phases [6,7]. The ion irradiation is a useful tool to generate the desired defect structure. Depending on the ion species, ion energy and the properties of the target materials, ion irradiation enables the creation of defects with well-controlled morphology and density, such as point [8], cluster [9–12] and columnar [13–15] defects. Early works on the ion irradiation of cuprate (Cu–O based) high-*T*<sup>c</sup> superconductors (HTS) for improving *J*<sup>c</sup> in the magnetic field have mostly focused on the high-energy, over hundreds of MeV, heavy ion irradiation [13–15]. At this energy range, the irradiation of superconducting materials by the swift heavy ion mainly causes electronic excitation and ionization of the target atoms. As a result, continuous amorphous tracks are formed in a process that can be described as the rapid melting and solidification of nm-sized columns in the path of an ion. Even though the heavy ion tracks proved to be very effective pinning defects, this approach has been limited to fundamental studies of the vortex matter.

Recently, ion irradiation of HTS with a low energy has received a renewed interest as a practical method for increasing *J*<sup>c</sup> in magnetic fields, due to the compact accelerator, lower radioactivity and less costly operation [9–12]. Low-energy ion irradiation utilizes a

**Citation:** Ozaki, T.; Kashihara, T.; Kakeya, I.; Ishigami, R. Effect of 1.5 MeV Proton Irradiation on Superconductivity in FeSe0.5Te0.5 Thin Films. *Quantum Beam Sci.* **2021**, *5*, 18. https://doi.org/10.3390/ qubs5020018

Academic Editors: Akihiro Iwase and Lorenzo Giuffrida

Received: 31 March 2021 Accepted: 25 May 2021 Published: 4 June 2021

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different mechanism for the creation of vortex pinning defects. The electronic excitation and ionization are low enough so the heat can dissipate without damaging the materials. The low-energy ion irradiation leads to the collision of the ion with the target atom nuclei, resulting in cascade, point and cluster defects. Matsui et al. demonstrated that 3 MeV Au2+ ion irradiation to 700 nm thick YBCO films yielded an enhancement in the in-field *J*<sup>c</sup> at 77 K of up to a factor of 4 [9]. Equally impressive results in YBCO commercial tape have been reported by Jia et al. using 4 MeV proton [10]. Recently, we reported a route to raise both *T*<sup>c</sup> and *J*<sup>c</sup> in iron-based superconducting FeSe0.5Te0.5 (FST) thin films by low-energy (190 keV) proton irradiation [16,17]. The 190 keV proton irradiation yields the increase in *T*<sup>c</sup> due to the nanoscale compressive strain induced by cascade defects. The irradiation also induced a near doubling of *J*<sup>c</sup> at 4.2 K from the self-field to 35 T through strong vortex pinning by the cascade defects and surrounding nanoscale strain.

In this paper, we report the effect of 1.5 MeV proton irradiation on iron–chalcogenide FST superconducting films. We report the performance of irradiated samples at different temperatures in a magnetic field up to 9 T. We show that 1.5 MeV protons clearly enhance *J*<sup>c</sup> in magnetic fields <1 T with no subsequent reduction in *T*c. However, we did not observe a reproducible positive effect in the magnetic fields >1 T. The results are discussed in terms of the spatial distribution of defects produced by fast protons.
