Effects of 250 MeV Au-ion Irradiation on the Superconducting Properties of Ba 1-x K x Fe 2 As 2 Single Crystals
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Effects of 250 MeV Au-ion Irradiation on the Superconducting Properties of BaíxKxFe2As2 Single Crystals Laura Gozzelino1,2, Roberto Gerbaldo1,2, Gianluca Ghigo1,2, Francesco Laviano1,2 and Tsuyoshi Tamegai3 1
Department of Applied Science and Technology, Politecnico di Torino, I-10129 Torino, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Torino, I-10125 Torino, Italy 3 Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 1138656, Japan 2
ABSTRACT In this work, we present an approach towards achieving a detailed knowledge on the effects of irradiation in iron-based superconductors with high-energy heavy ions. BaíxKxFe2As2 single crystals (x = 0.42) were irradiated with 250 MeV Au-ions. The crystals were shaped to be thin plates with thickness lower than the ion implantation depth and bases perpendicular to the c-axis. By means of the quantitative magneto-optical imaging technique we attained the local current density in pristine and irradiated sample regions without any model assumption. After irradiation a strong critical current density (Jc) enhancement was measured, overcoming a factor 3 at T = 4 K with an irradiation fluence I = 9.7 u 109 cm-2. Moreover, correlating point by point Jc and induction magnetic field maps, a contribution of the Au-ion induced defects to the anisotropic out of plane pinning (i.e. pinning in the direction parallel to the ion path) turned up. This contribution shows a maximum at applied fields quite lower than the nominal dose equivalent field. Furthermore, the temperature dependence of the penetration depth was evaluated before and after high-fluence irradiations by investigating the high-frequency behavior of these crystals using a microwave coplanar resonator technique. Fitting the experimental curves with a power law, we found a decrease of the power exponent, more pronounced at the higher fluences, that can supports the s± wave model. INTRODUCTION Ion irradiation has been proved to be an efficient tool for tailoring the structural and electromagnetic properties of several superconducting materials. Defects produced by irradiation can be controlled by choosing the particle and its energy [1]. Indeed, depending on the kind of ions, randomly distributed point defects [2,3], small cascades [4,5], correlated defects or columnar defects with nanometric section [6,7] can be created. Moreover, the same particle can produce completely different defects depending on its energy: a few MeV energy protons generate point defects in cuprate superconductors whereas nearly GeV energy protons create splayed columnar defects, which result from induced fission of the target atoms [8]. In addition, tuning the irradiation fluences, it is possible to better understand the effects of disorder on the superconductor properties as a function of the number of defects, even characterizing the same sample during a series of irradiation steps. Changes induced by irradiation in the structural and electromagnetic properties of the superconductors have been proved to be perma
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