Effects of Disorder on Iron Based Superconductors

Abstract

AbstractThis chapter deals with the effects of disorder on the properties of IBS. Structural disorder enhances charge carriers scattering, affecting the material at the fundamental level, besides yielding to other application-relevant effects, such as vortex pinning. Ion irradiation is one of the most powerful tool to introduce disorder in a controlled way. Here, we analyze the effects of irradiation-induced disorder in specific IBS families. First, we present a brief overview on Ba122 crystals irradiated with different ions and energies (3.5-MeV protons, 250-MeV Au ions, and 1.2-GeV Pb ions) that allowed us to deduce general scaling laws for penetration depth, critical current, and normal state resistivity as a function of disorder, quantified in terms of displacements per target ion (dpa). The linear behaviors we have found for these quantities are valid in the regime of \(low-\)to\(-moderate\) disorder (within dpa \(=\) 0.005). The second case discussed in this chapter is the effect of disorder on the penetration-depth anisotropy \(\gamma _\lambda \) of CaKFe\(_4\)As\(_4\). The response to introduction of defects by 3.5-MeV protons (again, at a moderate level) is compared to the effects of disorder induced by chemical substitution of Ni in the site of Fe. Generally, a decrease of \(\gamma _\lambda \) with disorder was found, but with different effects on its temperature dependence. While irradiation-induced disorder preserves the increasing temperature dependence of \(\gamma _\lambda (T)\), chemical disorder results in a \(\gamma _\lambda (T)\) decreasing with temperature. The role of \(\gamma _\lambda \) in determining the anisotropy of application-relevant parameters is discussed, together with possible optimal strategies to tailor its value by disorder. Finally, the London penetration depth of Ba(Fe\(_{1-x}\)Rh\(_x\))\(_2\)As\(_2\) crystals is considered, showing evidence of a disordered-driven transition from s\(_{\pm }\) to s\(_{++}\) order parameter symmetry, when higher disorder levels are explored (3.5-MeV protons, dpa\(>0.005\)). Signatures of the symmetry transition were found as a drop in the low-temperature penetration depth, and as peculiar features in the quasiparticle conductivity and surface impedance. We show how these observations can be described by multi-band Eliashberg calculations in which the effect of disorder is accounted for in a suitable way, ranging from the Born approximation to the unitary limit.