Figure 1

Direct spectroscopic evidence for two-gap superconductivity in $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$: (a) left panel: normalized tunneling conductance ($dI/dV$) vs bias voltage ($V$) spectra taken at $T=6$, 10, and 15 K for the sample with $x=0.06$ and $T_c=14\mathrm{K}$. The solid lines represent theoretical fittings to spectra using the Dynes formula in Eq. (1) modified for two-gap BCS superconductors. Two distinct tunneling gaps ${\Delta }_{\Gamma }$ and ${\Delta }_M$ can be identified from the spectrum at $T=6\mathrm{K}$. Right panel: the tunneling gaps ${\Delta }_{\Gamma }$ and ${\Delta }_M$ as a function of the reduced temperature ($T/T_c$) are shown by the symbols and solid lines. The error bars indicate the widths of the gap distributions obtained from the fitting using Eq. (1). (b) Left panel: ($dI/dV$) vs ($V$) spectra taken at $T=6$, 14, and 21 K for the sample with $x=0.12$ and $T_c=20\mathrm{K}$. Right panel: ${\Delta }_{\Gamma ,M}\mathrm{\text{−}}\mathrm{vs}.\mathrm{\text{−}}(T/T_c)$.Reuse & Permissions

Figure 2

Superconducting gap maps and histograms at $T=6\mathrm{K}$: (a) left to right: the first two panels correspond to the ${\Delta }_M$ and ${\Delta }_{\Gamma }$ maps for the underdoped sample ($x=0.06$), and the right two panels represent the corresponding histograms for both the quasiparticle (solid bars) and quasihole (shaded bars) branches, showing particle-hole symmetry and the mean values of $⟨|{\Delta }_M|⟩=4\mathrm{meV}$ and $⟨|{\Delta }_{\Gamma }|⟩=8\mathrm{meV}$. (b) The left two panels are, respectively, the ${\Delta }_M$ and ${\Delta }_{\Gamma }$ maps for the sample with $x=0.12$. The right two panels are histograms of ${\Delta }_M$ and ${\Delta }_{\Gamma }$, showing particle-hole symmetry and $⟨|{\Delta }_M|⟩=5\mathrm{meV}$, $⟨|{\Delta }_{\Gamma }|⟩=10\mathrm{meV}$.Reuse & Permissions

Figure 3

Correlation of atomically resolved surface topography with tunneling conductance spectra: (a) Surface topography of the overdoped sample ($x=0.12$) over a $(5.4\times 5.4){\mathrm{nm}}^2$ area. (b) The height histogram of the area in (a), showing height variations within one lattice constant along the $c$ axis. (c) Spatial evolution of the normalized $(dI/dV)\mathrm{\text{−}}\mathrm{vs}\mathrm{\text{−}}V$ spectra across a horizontal line slightly below the middle of the topography image in (a), which reveals consistent two-gap features.Reuse & Permissions

Figure 4

Tunneling conductance maps of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ samples at three constant bias voltages $V=0$, ($⟨{\Delta }_M⟩/e$), ($⟨{\Delta }_{\Gamma }⟩/e$), and for $T=6\mathrm{K}$: (a) $x=0.06$ and (b) $x=0.12$. (c) Fourier transformed (FT) tunneling conductance maps at $V=(⟨{\Delta }_M⟩/e)=4\mathrm{meV}$ (left panel) and $V=(⟨{\Delta }_{\Gamma }⟩/e)=8\mathrm{meV}$ (right panel) for $x=0.06$. (d) FT conductance maps at $V=(⟨{\Delta }_M⟩/e)=5\mathrm{meV}$ (left panel) and $V=(⟨{\Delta }_{\Gamma }⟩/e)=10\mathrm{meV}$ (right panel) for $x=0.12$. The thin white lines define the first and second Brillouin zones, and we have used the lattice constant $a=0.53\mathrm{nm}$ for the length of the first Brillouin zone ($2\pi /a$). All three QPI wave vectors $q_1$, $q_2$, and $q_3$ are slightly energy and doping dependent.Reuse & Permissions