Figure 1

(a) SEM image of the measured single crystals from the same batch. (b) Right panel: STM topographic image of an as-grown surface depicting growth terraces with a rms roughness smaller than 0.1 nm. Left panel: Height profile along the path indicated with an arrow: terrace-valley height differences are either one or two crystal unit-cell parameters ($c=8.47\AA$). (c) Local $dI/dV$ spectra (vertically shifted) acquired at 4.2 K along an 8 nm path. Spectra are color-coded according to their experimental gap values calculated as half the peak-to-peak or kink (black arrows) energy separation. The color convention is that of the local ${\Delta }_p$ histogram of the insert resulting from 4500 local spectra mapped in a $40\times 40{\mathrm{nm}}^2$ field-of-view. Green arrows indicate the high-bias kink located at $E_2$ and green circles the local dip at $E_{\mathrm{dip}}$. (d) Left panel: Zero-field-cooled magnetic moment of a crystal yielding $T_c=45\mathrm{K}$ and a transition width of 5 K [22]. Right panel: temperature evolution of average $dI/dV$ curves (in a $40\times 40{\mathrm{nm}}^2$ field-of-view). In the STM measurements an electrochemically-etched Ir tip served as the ground electrode and thus negative (positive) bias refers to occupied (empty) sample states. The tunnel-junction regulation parameters were of 80 meV and 1 nA. Spectra were acquired by means of a lock-in technique [14] with a bias modulation of $1.5\mathrm{m}{\mathrm{V}}_{\mathrm{rms}}$ and an energy resolution of 0.6 mV.Reuse & Permissions

Figure 2

(a) Schematics of the 1111-pnictides Fermi-surface pockets ($\alpha$-hole and $\beta$-electron sheets) including information on possible scenarios for the momentum structure of the gap. The orange (red) locations represent a positive (negative) gap. Upper panel: sign-reversing $s\pm$ symmetry. Lower panel: nodal $d$ symmetry with the dotted lines depicting the location of the gap nodes. (b) Average normalized spectra at 4.2 K obtained from hundreds of local $dI/dV$ curves with same ${\Delta }_p$ (open symbols). Full lines are fits considering the sign-reversing $s\pm$ (orange, purple, blue) and nodal $d$ (black) symmetries. A 4.2 K thermal broadening and a finite quasiparticle’s lifetime $\tau =\hslash /\Gamma$, with $\Gamma$ the scattering rate, were considered. Sign-reversing $s\pm$ fits yield $\Delta$ 4.2, 4.3 and 4.8 meV for the average spectra with increasing ${\Delta }_p$. In the case of nodal-$d$ fits the fitted $\Delta$ values are of 5.1, 5.2, and 5.8 meV, respectively. On increasing ${\Delta }_p$ the scattering rate obtained in the sign-reversing-$s\pm$ fits are 0.9, 0.7 and 1.5 meV, roughly twice the values obtained in the nodal-$d$ fits, 0.3, 0.3, and 0.9 meV, respectively. Insert: Evolution of $2\Delta /k_B$ with ${\Delta }_p$ for the two models. The dotted line indicates the weak-coupling BCS limit of $3.5T_c$ holding for classical superconductors.Reuse & Permissions

Figure 3

(a) Two archetypical local $dI/dV$ curves (vertically shifted) illustrating the spectral shape of the high-energy feature detected at $E_2$ (black arrows) and of the diplike feature observed at $E_{\mathrm{dip}}$ (blue arrow). (b) Histograms of the energy location of the coherence peaks or kinks, ${\Delta }_p$, for empty (orange) and occupied (blue) sample states. (c) Histogram of the local $E_2$ values: the distribution for the empty and occupied-sample states have mean values displaced in $\sim 3\mathrm{meV}$. Histograms obtained from 4500 spectra acquired over a $40\times 40{\mathrm{nm}}^2$ field-of-view.Reuse & Permissions

Figure 4

(a) Average $dI/dV$ curves for local spectra with the same ${\Delta }_p$ plotted as a function of the sample bias minus ${\Delta }_p$ (for the empty sample states). On increasing ${\Delta }_p$ the dip located at $E_{\mathrm{dip}}$ shifts towards the coherence peak. (b) ${\Delta }_p$ dependence of the collective-mode energy measured as the peak-to-dip energy separation, $\Omega =E_{\mathrm{dip}}-{\Delta }_p$. The darker region indicates the 1 meV uncertainty in determining $\Omega$.Reuse & Permissions