Critical behavior of the spin density wave transition in underdopedBa(Fe1−xCox)2As2(x≤0.05):As75NMR investigation

Abstract

We investigate the nature of the SDW (spin density wave) transition in the underdoped regime of an iron-based high-${T}_{c}$ superconductor ${\mathrm{Ba(Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$ by $^{75}\mathrm{As}$ NMR, with primary focus on a composition with $x=0.02$ (${T}_{\mathrm{SDW}}=99$ K). We demonstrate that critical slowing down toward the three-dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition ${T}_{s}=105$ K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between ${T}_{s}$ and ${T}_{\mathrm{SDW}}$, the dynamical structure factor of electron spins $S(\mathbf{q},{\ensuremath{\omega}}_{n})$ measured with the longitudinal NMR relaxation rate $1/{T}_{1}$ exhibits a divergent behavior obeying a power law $1/{T}_{1}\ensuremath{\propto}{\ensuremath{\Sigma}}_{\mathbf{q}}S(\mathbf{q},{\ensuremath{\omega}}_{n})\ensuremath{\sim}{(T/{T}_{\mathrm{SDW}}\ensuremath{-}1)}^{\ensuremath{-}\ensuremath{\delta}}$ with the critical exponent $\ensuremath{\delta}\ensuremath{\sim}0.33$.