Figure 1

Temperature dependence of the in-plane resistivity $\rho (T)$ of the four single crystals of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ studied in this work, with Co concentrations $x$ as indicated. The curves are normalized to their room-temperature value ${\rho }_{\mathrm{RT}}$ and rigidly offset by 0.05 for clarity. Inset: phase diagram showing the orthorhombic phase below $T_S$, the antiferromagnetic phase below $T_M$, and the superconducting phase below $T_c$ (from [7, 8]), with arrows indicating the composition of the four samples.Reuse & Permissions

Figure 2

Left panels: Temperature dependence of the thermal conductivity, plotted as $\kappa /T$ vs $T$, for three Co concentrations, each measured in magnetic fields as indicated. The lines are a power-law fit of the form $\kappa /T=a+b{T}^{\alpha }$ to the $H=0$ and $H=15\mathrm{T}$ data, used to extract the residual linear term ${\kappa }_0/T\equiv a$ in the $T\to 0$ limit (see text). Right panels: field dependence of $\kappa /T$ plotted for three temperatures, as indicated. The $T\to 0$ data (red squares) are obtained from the power-law extrapolations, with a typical error bar as shown.Reuse & Permissions

Figure 3

Left panel: Residual linear term ${\kappa }_0/T$ in the thermal conductivity of Co-Ba122 (full symbols) as a function of magnetic field $H$, plotted on normalized scales as $({\kappa }_0/T)/({\kappa }_N/T)$ vs $H/H_{c2}$, where ${\kappa }_N$ is the normal-state thermal conductivity in the $T\to 0$ limit, obtained from the Wiedemann-Franz law (see text), and $H_{c2}$ is the upper critical field, for the four Co concentrations $x$ as indicated. For comparison, we also show the behavior of pure (Nb) and dirty (InBi) $s$-wave superconductors (from 30), and of the $d$-wave superconductor Tl-2201 ($T_c=15\mathrm{K}$ and $H_{c2}\simeq 7\mathrm{T}$) (from 13). The data for the borocarbide superconductor ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$ ($T_c=16\mathrm{K}$ and $H_{c2}=7\mathrm{T}$) are also shown (from 19). Right panel: $({\kappa }_0/T)/({\kappa }_N/T)$ at $H/H_{c2}=0.25$ in Co-Ba122 as a function of $x$. This measures the extent to which zero-energy quasiparticles are excited by a relatively small magnetic field compared to $H_{c2}$. The Co-Ba122 data show a degree of excitation typical of $s$-wave superconductors in the region of coexisting antiferromagnetic or orthorhombic order at low doping, rising rapidly to a level typical of a $d$-wave superconductor at higher doping.Reuse & Permissions