Angle-Resolved Photoemission Spectroscopy of the Iron-Chalcogenide Superconductor ${\mathrm{Fe}}_{1.03}{\mathrm{Te}}_{0.7}{\mathrm{Se}}_{0.3}$: Strong Coupling Behavior and the Universality of Interband Scattering

We have performed angle-resolved photoemission spectroscopy of the iron-chalcogenide superconductor ${\mathrm{Fe}}_{1.03}{\mathrm{Te}}_{0.7}{\mathrm{Se}}_{0.3}$ to investigate the electronic structure relevant to superconductivity. We observed a holelike Fermi surface (FS) and an electronlike FS at the Brillouin zone center and corner, respectively, which are nearly nested by the $Q\sim (\pi ,\pi )$ wave vector. We do not find evidence for the nesting instability with $Q\sim (\pi +\delta ,0)$ reminiscent of the antiferromagnetic order in the parent compound ${\mathrm{Fe}}_{1+y}\mathrm{Te}$. We have observed an isotropic superconducting (SC) gap along the holelike FS with the gap size $\Delta$ of $\sim 4\mathrm{meV}$ ($2\Delta /k_B{T}_c\sim 7$), demonstrating the strong-coupling superconductivity. The observed similarity of low-energy electronic excitations between iron-chalcogenides and iron-arsenides strongly suggests that common interactions which involve $Q\sim (\pi ,\pi )$ scattering are responsible for the SC pairing.

Figure 1

(a) ARPES spectra of ${\mathrm{Fe}}_{1.03}{\mathrm{Te}}_{0.7}{\mathrm{Se}}_{0.3}$ measured at 20 K with $h\nu =44\mathrm{eV}$. (b),(c) MDCs in the vicinity of $E_F$ measured along cut 1 and cut 2 in (f), respectively. Blue dots denote the peak positions of MDCs. (d),(e) Second derivative plot of MDCs along cut 1 and cut 2, respectively, as a function of binding energy and wave vector. (f) Schematic FS (green circles) with the locations of momentum cuts (red and blue arrows). (g) Intensity plot of (a) together with the calculated energy bands for FeTe at $k_z=0$ (red curves) [9]. Near-$E_F$ band dispersions extracted from the MDC peak positions are also shown by blue dots.

Figure 2

ARPES intensity plot at $E_F$ of ${\mathrm{Fe}}_{1.03}{\mathrm{Te}}_{0.7}{\mathrm{Se}}_{0.3}$ as a function of the two-dimensional wave vector measured at 20 K with 44 eV photons. The intensity at $E_F$ is obtained by integrating the spectra within $\pm 10\mathrm{meV}$ with respect to $E_F$. Solid and dashed red circles show experimentally determined $k_F$ points and schematic FSs, respectively. There are sizable experimental uncertainties on the $k_F$ points, mainly due to weak intensity around the $M$ point.

Figure 3

(a) ARPES spectra of ${\mathrm{Fe}}_{1.03}{\mathrm{Te}}_{0.7}{\mathrm{Se}}_{0.3}$ with the He $\mathrm{I}\alpha$ resonance line, measured at point $A$ displayed in (b). The inset shows the expansion in the vicinity of $E_F$. (b) Schematic holelike FS at the $\Gamma$ point with the location of the $k_F$ points $A$ and $B$. (c) Temperature dependence of symmetrized ARPES spectra (bottom) at point $A$, and the same but divided by the spectrum at $T=25\mathrm{K}$ (top). (d) Comparison of the symmetrized spectra at $k_F$ points along $\Gamma \mathrm{\text{−}}M$ (blue curve) and $\Gamma \mathrm{\text{−}}X$ (red curve) high-symmetry lines measured at 5 K divided by the 25 K spectrum. Dashed lines at 4 meV in (c) and (d) represent the energy scale of the SC gap $\Delta$.