Angle-Resolved Photoemission Spectroscopy of the Fe-Based ${\mathrm{Ba}}_{0.6}{\mathbf{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ High Temperature Superconductor: Evidence for an Orbital Selective Electron-Mode Coupling

We have performed an angle-resolved photoemission spectroscopy study of the new superconductor ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ in the low energy range. We report the observation of an anomaly around 25 meV in the dispersion of superconducting ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ samples that nearly vanishes above $T_c$. The energy scale of the related mode ($13\pm 2\mathrm{meV}$) and its strong dependence on orbital and temperature indicates that it is unlikely related to phonons. Moreover, the momentum locations of the kink can be connected by the antiferromagnetic wave vector. Our results point towards an unconventional electronic origin of the mode and the superconducting pairing in the Fe-based superconductors, and strongly support the antiphase $s$-wave pairing symmetry.

Figure 1

(a) ARPES intensity plot in the superconducting state (15 K) along a cut crossing the $\alpha$ band. The inset shows the schematic FS of ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ with the location of the cut (red). $\Gamma$ (0,0), $X$ ($\pi /2,\pi /2$) and $M$ ($\pi$,0) are high symmetry points described in the unreconstructed BZ. (b) Corresponding EDCs. (c) Corresponding MDCs, in the 0–60 meV binding energy range. Grey dots indicate the maximum position of the peaks. (d) ARPES intensity plot at 150 K divided by a Fermi-Dirac function, recorded along the same cut as in (a). (e) Real and imaginary parts of $\Sigma (\omega )$. Fade colors are used for binding energies smaller than 17 meV since $\Sigma (\omega )$ in this range is complicated by particle-hole mixing due to superconductivity. The inset compares the partial DOS along the cuts measured at 15 K (blue) and 150 K (red), respectively.

Figure 2

(a) ARPES intensity plot in the superconducting state (15 K) along a cut crossing the $\beta$ band. (b) EDCs corresponding to the cut shown in panel (a). (c) Schematic Fermi surface of ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ with the location of the cuts presented in panel (a). We also traced the AF wave vector ${\mathbf{Q}}_{\mathrm{AF}}$, which connects the $\alpha$ and $\beta$ bands. (d) Partial DOS associated with the $\alpha$, $\beta$, and $\gamma$ bands. The dots represent the position of the dip.

Figure 3

(a) ARPES intensity plot in the normal state (50 K) along a cut crossing the $\alpha$ band [see inset of Fig. 1a]. (b) Comparison of the MDC fit dispersion for the $\alpha$ band in the superconducting state (15 K) and in the normal state (50 K) for the cut presented in panel (a). The inset shows a zoom around the kink. (c) Temperature dependence of the real part of the self-energy referred from the MDC fit dispersion at 150 K. (d) Maximum value of the real part of the self-energy (blue) plotted as a function of temperature. The ARPES results are compared to the neutron scattering intensity of the 14 meV spin resonance (red) located at the AF wave vector [27]. The dashed line indicates the critical temperature.