Evidence of a Spin Resonance Mode in the Iron-Based Superconductor ${\mathrm{Ba}}_{0.6}{\mathbf{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ from Scanning Tunneling Spectroscopy

We used high-resolution scanning tunneling spectroscopy to study the hole-doped iron pnictide superconductor ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($T_c=38\mathrm{K}$). Features of a bosonic excitation (mode) are observed in the measured quasiparticle density of states. The bosonic features are intimately associated with the superconducting order parameter and have a mode energy of $\sim 14\mathrm{meV}$, similar to the spin resonance measured by inelastic neutron scattering. These results indicate a strong electron-spin excitation coupling in iron pnictide superconductors, similar to that in high-$T_c$ copper oxide superconductors.

Figure 1

(a) Typical tunneling spectra measured below (black) and above (red) $T_c$. (b) Normalized spectrum averaged over an area of $16.7\mathrm{nm}\times 8.4\mathrm{nm}$. The features of two superconducting gaps are indicated by gray vertical lines. Black arrows indicate the hump-dip structures mentioned in the text, and the green line is a fit to two-gap weak-coupling BCS theory. Spectra were taken with a fixed junction resistance of $0.25\mathrm{G}\Omega$. (c) Second derivative of the tunneling current calculated from the data presented in (b). The green line is a BCS calculation. (d) The black curve is the relationship between $d^{2}I/d{V}^2$ vs. ($\mathrm{eV}-\Delta$) obtained on a conventional strong-coupling superconductor Pb in an energy window outside the superconducting gap $\Delta$ [3]. The green line is a BCS curve. (e) Electron-phonon spectral function of Pb [3].

Figure 2

(a) Spatially-resolved tunneling spectra recorded along a line of 16.7 nm on the cleaved surface of a single crystal of ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$. (b) A histogram of the occurrences of the larger gap $\Delta$ (red) and the dip $E_{\mathrm{dip}}$ (black), which were observed in the same region mentioned in Fig. 1. (c) Schematic diagram of the Fermi surface pockets of ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($\alpha$-hole and $\beta$-hole pockets centered at $\Gamma$ point and electron pocket around $M$ point). The black arrow indicates the nesting vector between the $\alpha$-hole pocket and the electron pocket.

Figure 3

(a) Temperature dependence of averaged tunneling spectra. (b) Spatial dependence of tunneling spectra taken along a line cutting through a local impurity, indicating the close relationship between the strength of the hump-dip structure and the superconducting order parameter.

Figure 4

(a) Image of the superconducting gap values, or gap map [$\Delta (r)$]. (b) Image of the mode energy $\Omega (r)=E_{\mathrm{dip}}(r)-\Delta (r)$. Both (a) and (b) were obtained in the same region as mentioned in Figs. 1, 2. (c) Histogram of the occurrences of $\Omega$ plotted as a local ratio $\Omega (r)/2\Delta (r)$ against $\Delta (r)$.