Superconducting single crystals show about 50% increase of the superconducting critical temperature after Ar Ion implantation

Abstract

Single crystals of a Fe-based superconductor, Ba(Fe_0.943Co_0.057)₂As₂, with ∼200 μm thickness (t) have been irradiated along the thickness (c-axis) of by 1.5 MeV Ar⁶⁺ ions, having ion range (R) of 0.903 μm (SRIM calculation). Fast ion irradiations damage the lattice of the target, and usually decrease the superconducting critical temperature (T_c), with a few exceptions of small increases. However, our irradiation, a case of ion implantation (R > t), presently to a fluence of 2.5 × 10¹⁵ Ar⁶⁺/cm², increased the onset T_c of the single crystal superconductors by ∼ 8 K from its initial value of ∼ 17 K as measured from the real part of the magnetic susceptibility, matching results from our additional measurements of the imaginary part, electrical resistivity and magnetization. These self-supporting T_c results ensure correctness of the result. To our knowledge, such huge increase of the superconducting critical temperature by ion irradiation has not been reported earlier. Fortunately, we find following explanation, and consider it most appropriate. The insoluble inert gas atoms precipitate often as high-pressure miniature bubbles as is well known from various experiments and also from observations in nuclear reactor walls. This exerts a large compressive strain on the lattice. Our novel explanation is that this additional strain increases T_c by ∼ 49 %. Moreover, the same Ar implantation in non-superconducting BaFe₂As₂ induced 20 K superconductivity. Depending on such bubble formation and their stability in other target-ion pairs, such large T_c enhancement may be discovered in other superconductors, and used with the advantages of higher T_c.