Effect of Co Doping on the In-Plane Anisotropy in the Optical Spectrum of Underdoped $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$

We investigate the anisotropy in the in-plane optical spectra of detwinned $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$. The optical conductivity spectrum of $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$ shows appreciable anisotropy in the magnetostructural ordered phase, whereas the dc ($\omega =0$) resistivity is nearly isotropic at low temperatures. Upon Co doping, the resistivity becomes highly anisotropic, while the finite-energy intrinsic anisotropy is suppressed. It is found that anisotropy in resistivity arises from anisotropic impurity scattering due to the presence of doped Co atoms, and it is extrinsic in origin. The intensity of a specific optical phonon mode is also found to show striking anisotropy in the ordered phase. The anisotropy induced by the Co impurity and that observed in the optical phonon mode are hallmarks of the highly polarizable electronic state in the ordered phase.

Figure 1

Temperature dependence of ${\rho }_a$ and ${\rho }_b$ for annealed $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ with (a) $x=0$, (b) 0.02, and (c) 0.04. Solid and broken vertical lines indicate values of $T_s$ and $T_N$, respectively, determined from the resistivity of a twinned crystal [6, 25].

Figure 2

Temperature evolution of ${\sigma }_a(\omega )$ and ${\sigma }_b(\omega )$ of detwinned $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ ($x=0$, 0.02, and 0.04). The crystalline $b$ axis corresponds to the direction of applied compressive pressure. The conductivity ${\sigma }_a$ (${\sigma }_b$) of undoped $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$ in the AFO phase is reduced below $600(850){\mathrm{cm}}^{-1}$, and, by piling up the reduced spectral weight, a peak develops at $950(1050){\mathrm{cm}}^{-1}$. For $x=0.02$ and 0.04, the low-energy conductivity below $500(700){\mathrm{cm}}^{-1}$ and $400(500){\mathrm{cm}}^{-1}$ is suppressed, thereby leading to the formation of peaks at $900(950){\mathrm{cm}}^{-1}$ and $750(800){\mathrm{cm}}^{-1}$, respectively.

Figure 3

Anisotropic optical conductivity spectra at $T=5\mathrm{K}$ in low-energy region below $1500{\mathrm{cm}}^{-1}$ for (a) $x=0$, (b) 0.02, and (c) 0.04. (d)–(g) Decomposition of ${\sigma }_a(\omega )$ and ${\sigma }_b(\omega )$ for $x=0.02$ and 0.04. (h) Residual resistivity of ${\rho }_a$ and ${\rho }_b$ for $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ plotted against the Co content $x$. The broken lines serve as a visual guide. (i) Drude weight ($N_D$) and (j) width ($1/\tau$) extracted from the conductivity spectrum at 5 K plotted against $x$. For $x=0$, the value of $1/\tau$ is $\sim 5{\mathrm{cm}}^{-1}$ for both directions [22]. The broken lines serve as a visual guide. (k) Schematic picture of an anisotropic impurity state around a doped Co atom.

Figure 4

Intensities (${\Omega }_0^2$) of optical phonon mode at various temperatures for $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$. The solid vertical lines indicate $T_{\mathrm{s}}$ values for each composition.