Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron Pnictide Superconductor ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$

We present an atomic resolution scanning tunneling spectroscopy study of superconducting ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ single crystals in magnetic fields up to 9 T. At zero field, a single gap with coherence peaks at $\overline{\Delta }=6.25\mathrm{meV}$ is observed in the density of states. At 9 and 6 T, we image a disordered vortex lattice, consistent with isotropic, single flux quantum vortices. Vortex locations are uncorrelated with strong-scattering surface impurities, demonstrating bulk pinning. The vortex-induced subgap density of states fits an exponential decay from the vortex center, from which we extract a coherence length $\xi =27.6\pm 2.9\AA$, corresponding to an upper critical field $H_{c2}=43\mathrm{T}$.

Figure 1

(a) A $20\times 20{\mathrm{nm}}^2$ constant-current topographic image of the cleaved surface of ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$. The interatomic spacing is $a=3.96\AA$. (b) Zoom in ($\times 2$ magnification) on a $5\times 5{\mathrm{nm}}^2$ area within (a). The black circles denote the positions of the $2\times 1$ stripe structure. (c) Fourier transformation of (a). Peaks $A$ and $A^{\star }$ represent the atomic spacing between and within rows, respectively. Peaks $B$ and $C$ represent the alternating row characteristics: $B$ shows intensity modulations attributable to height or density of states variations, while $C$ shows in-plane shifts along the row direction. Both images are recorded using $V_{\mathrm{sample}}=-20\mathrm{mV}$ and $I=40\mathrm{pA}$, at $T=6.15\mathrm{K}$ in zero magnetic field.

Figure 2

(a) A series of $dI/dV$ spectra taken along an 11 nm line, illustrating the shape of the superconducting gap and the low differential conductance at the Fermi energy. To reduce noise, each spectrum shown is the average of all spectra acquired within a 5 Å radius. The vertical dashed lines are guides to the eye located at $\pm 6\mathrm{meV}$. (b) A $20\times 20{\mathrm{nm}}^2$ gap map, revealing the spatial variation of the gap magnitude $\Delta$. The gap has $\overline{\Delta }=6.25\mathrm{meV}$ and fractional variation only 12%. A color-coded histogram of $\Delta$ is shown below the gap map. The six spectra in (a) (from top to bottom) are taken at the locations of the black points indicated in (b) (from upper left to lower right). Data were acquired at $T=6.25\mathrm{K}$ in zero magnetic field.

Figure 3

(a) A $100\times 100{\mathrm{nm}}^2$ topographic image recorded in 9 T magnetic field with $V_{\mathrm{sample}}=-5\mathrm{mV}$ and $I=10\mathrm{pA}$. The bright spots are impurities. (b) Differential conductance map recorded simultaneously with the topographic image in (a), revealing the sample DOS at 5 meV in 9 T magnetic field at $T=6.15\mathrm{K}$. Vortices are visible as dark features due to the local suppression of the coherence peaks. The vortices form a disordered lattice. Impurities, also visible in (a), appear as smaller, sharper dark features. (c) Differential conductance map recorded at 6 T in the same field of view and with the same parameters as (b), except $T=6.21\mathrm{K}$. (d),(e) A Voronoi overlay on impurity-filtered data emphasizes the vortices and demonstrates the procedure used to compute the average area associated with each vortex.

Figure 4

(a) A $40\times 40{\mathrm{nm}}^2$ map of the zero bias conductance in 9 T magnetic field at $T=6.15\mathrm{K}$, showing $\sim 8$ vortices (broader, lighter objects) and 8 strong-scattering impurities (sharper, brighter objects). (b) A series of spectra along a 14.5 nm trajectory through one of the vortices, indicated by the red or gray line in (a). The superconducting gap is completely extinct inside the vortex core; only the V-shaped background remains. The three thick red or gray lines emphasize the spectra at the vortex core and far from it on both sides. The 75 spectra shown are offset by a total of 3 nS for clarity. (c) Azimuthally averaged radial dependence of the differential conductance $g(r,0)$ around a single vortex as measured (black squares) and fit by an exponential decay (red or gray line). The constant background $g_{\infty }$ has been removed in order to emphasize the exponential decay on the logarithmic scale of the $y$ axis. The exponential fit leads to a coherence length of $\xi =30.8\pm 1.4\AA$ for this vortex and to an average of $\xi =27.6\AA$ with standard deviation 2.9 Å for all vortices investigated.