Probing microscopic variations of superconductivity on the surface of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ single crystals

The spatially resolved electrical transport properties have been studied on the surface of optimally doped superconducting $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ single crystal by using a four-probe scanning tunneling microscopy. While some nonuniform contrast appears near the edge of the cleaved crystal, the scanning electron microscopy (SEM) reveals mostly uniform contrast. For the regions that showed uniform SEM contrast, a sharp superconducting transition at $T_C=22.1\text{K}$ has been observed with a transition width $\Delta T_C=0.2\text{K}$. In the nonuniform contrast region, $T_C$ is found to vary between 19.6 and 22.2 K with $\Delta T_C$ from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co concentration remains 7.72% in the uniform region, but changes between 7.38% and 7.62% in the nonuniform region. Thus the variations of superconductivity are associated with local compositional change.

Figure 1

(Color online) Topographic images of cleaved surfaces of ${\text{BaFe}}_{1.8}{\text{Co}}_{0.2}{\text{As}}_2$ single crystal. (a) SEM image showing uniform contrast with some dark regions near the edge of crystal, as marked by a rectangular box. (b) Zoom-in SEM image showing domain structures. Marked regions by symbols and a dash line in (a) and (b) indicate where the transport measurements and composition probing are carried out. (c) STM images showing the cleaved $ab$ plane of single-crystal surface with an atomic step. Image size: $100\times 100{\text{nm}}^2$, set point: 500 mV, 200 pA, measured at room temperature. Inset: quasiatomic resolution STM image of the cleaved surface.

Figure 2

(Color online) Resistance $\left(R\right)$ versus temperature $\left(T\right)$ measured at four different sample locations as marked by symbols in Fig. 1. The inset shows the measurement configuration of four STM probes. The separation between two voltage probes is $3\mu \text{m}$.

Figure 3

(Color online) (a) Temperature dependence of critical current at different sample locations. A $0.1\mu \text{V}$ voltage cutoff, corresponding to an electric field criterion of $3\times {10}^{-4}\text{V}/\text{cm}$, was used to define the $I_C$. The lines show fits to $I_C\left(T\right)=I_C\left(0\right){\left(1-T/T_C\right)}^n$. Inset: normalized $I_C\left(T\right)$ data superimposed onto the same curve of $I_C\left(T\right)/I_C\left(0\right)={\left(1-T/T_C\right)}^n$. (b) Critical current density derived from remanent magnetization of bulk crystal. The solid line fits to $j_C=j_C\left(0\right){\left(1-T/T_C\right)}^n$ of the corresponding data set.

Figure 4

(Color online) Location-dependent Co contents analyzed with WDS both in uniform and domain regions where local transport measurements were carried out. Areal scan was averaged from the elemental maps along a $5\mu \text{m}$ wide line. The line scan curve has been shifted downward by 0.1% for clarity.

Figure 5

(Color online) Location-dependent resistance measured with four-probe STM at 21.1 K, which is just below bulk transition temperature (22 K). Nonzero resistances indicate that there is nonsuperconducting or normal metal phase at 21.1 K. Inset shows measurement configurations in domain regions.