Nanoscale superconducting-gap variations and lack of phase separation in optimally doped ${\text{BaFe}}_{1.86}{\text{Co}}_{0.14}{\text{As}}_2$

We present tunneling data from superconducting ${\text{BaFe}}_{1.86}{\text{Co}}_{0.14}{\text{As}}_2$ and its parent compound, ${\text{BaFe}}_2{\text{As}}_2$. In the superconductor, clear coherencelike peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, $\Delta$. The average peak-to-peak separation gives a $2\Delta$ of $\sim 7.4k_B{T}_c$, which exceeds the BCS weak coupling value for either $s$- or $d$-wave superconductivity. The characteristic length scales of the deviations from the average gap value and of anticorrelations observed between the gap magnitude and both the zero-bias conductance and coherence peak strength match well with the average separation between the Co dopant ions in the superconducting FeAs planes.

Figure 1

(Color online) (a) Co-doped Ba122 shows a sharp superconducting transition $\left(\Delta T_c\sim 0.5\text{K}\right)$ at 22 K (red curves). The inset displays the full temperature range, with data from the parent compound (top trace). (b) Optical micrograph of the very flat cleavage surface typically obtained.

Figure 2

(Color online) (a) Constant current image $\left(V_{\text{sample}}=-35\text{mV},I_{\text{setup}}=40\text{pA}\right)$ of Co-doped Ba122. The inset shows LEED from the same surface with $E_0=114\text{eV}$. (b) Zoom of panel (a): the surface atoms can be seen as bright dots. (c) Further zoom of (b) showing a row of four surface atoms separated by the tetragonal cell dimension of $3.9\text{Å}$. (d) Fourier transform of panel (a), whereby the yellow X and arrow indicate real-space distances of 8 and $3.9\text{Å}$, respectively. All data (with exception of the LEED) were recorded at 4.2 K.

Figure 3

(Color online) (a) Constant current image from a different sample of ${\text{BaFe}}_{1.86}{\text{Co}}_{0.14}{\text{As}}_2$ $\left(V_{\text{sample}}=-88\text{mV},I_{\text{setup}}=40\text{pA}\right)$. (b) Analogous topography from ${\text{BaFe}}_2{\text{As}}_2$ $\left(V_{\text{sample}}=-100\text{mV},I_{\text{setup}}=33\text{pA}\right)$. (c) Line scans from the superconducting (top) and parent (bottom) compounds taken along the arrows. Typical distances between the bright rows in the images is $12\text{Å}$ indicated by gray ticks in the line scans. (d) Overlay of $400dI/dV$ conductance spectra from pristine Ba122, together with their average (bold line). All data were recorded at 4.2 K.

Figure 4

(Color online) (a) STS spectra from ${\text{BaFe}}_{1.86}{\text{Co}}_{0.14}{\text{As}}_2$ (averages of four adjacent pixels) displaying different values of $\Delta$, taken from the region marked in panel (b). (b) Gap map with identical FOV as Fig. 2b, taken with the same setup conditions. (c) Binned spectra for five ranges of $\Delta$ (Ref. 20), plotted in colors matching those in the gap map. (d) Map of the zero-bias conductance on an inverted color scale from the same data set as panel (b). (e) Correlation functions. From top to bottom: autocorrelation of the gap map shown in panel (b) (red squares) and of an analogous data set recorded from the FOV shown in Fig. 3a (black triangles). Yellow dots: cross correlation of the gap map of panel (b) with the corresponding topograph [Fig. 2b]. Blue (green) diamonds: cross correlations between the gap and ZBC (peak-strength) maps. (f) Map of the coherence peak strength [for half of the FOV of panels (b) and (d), and on an inverted color scale], extracted from a comparison of the coherence peak weight to the high energy background. (g) ZBC map of pristine Ba122 recorded from half the FOV shown in Fig. 3b. The conductance scale (shown on the right) is the same as that used in panel (d).