Structural and electronic properties of LaFeAsO${}_{0.85}$F${}_{0.15}$ superconductor modified under neutron irradiation

Abstract

The effect of atomic disorder induced by neutron irradiation on the crystal structure and electronic states near $E_F$ of the lightly overdoped LaFeAsO${}_{0.85}$F${}_{0.15}$ ($T_c=21K$) was studied by X-ray diffraction and ${}^{75}$As NMR. The irradiation of the polycrystalline sample by "moderate" neutron fluence of $\Phi =(0;0.5;1.6)\cdot 10^{19}c{m}^{-2}$ at $T=50^{\circ }C$ leads to the suppression of superconductivity. It is shown that neutron irradiation produces an anisotropic expansion of the tetragonal lattice almost due to an increase of the Fe-As distance. A partial loss of the 2D character of the FeAs layer is accompanied with a suppression of the gap-like feature in temperature dependence of the spin susceptibility. In the most disordered state the ${}^{75}$As spin-lattice relaxation rate follows the Korringa law ${}^{75}{T}_1^{-1}\sim T$, the thermal behavior being typical for an isotropic motion of the conducting electrons.