Angle-resolved photoemission spectroscopy of superconducting LiFeAs: Evidence for strong electron-phonon coupling

By applying a state-of-the-art angle-resolved photoemission to LiFeAs, the only stoichiometric pnictide superconductor without magnetic ordering, we identify a clear fingerprint of the phonon spectrum in the fermionic self-energy and estimate the electron-phonon coupling strength, which appears to be sufficient to mediate the superconductivity. This result suggests that the superconductivity in pnictides could be based on the conventional phonon pairing enhanced by the van Hove singularity in the electronic density of states and by the strong electron-electron interaction.

Figure 1

(a) The Fermi surface (FS) map of LiFeAs: the thinnest contour around the $\Gamma$ point (dotted line) corresponds to the holelike FS sheet, which is formed by the two-dimensional band formed by Fe-3$d_{\mathit{xy}}$ orbitals. (b–d) Different cuts of this band. The position of each cut is shown in the insets. Red squares on (c) and (d) trace the experimental dispersion.

Figure 2

Fingerprints of low-energy interaction. Experimental (or renormalized) dispersions: (a) The positions of MDC maxima $k_m$ as a function of energy $\omega$ for cut 1 at different temperatures, for the same cut but temperature integrated and smoothed, and for cut 2. The last two curves are shifted up by 0.05 and 0.1 Å${}^{-1}$, respectively. The inset shows the second derivative of the dispersion vs the same energy axis. (b) MDC width (the half-width at half maximum) which is proportional to the inversed lifetime of the fermionic quasiparticles. The similarly integrated curve is shifted down. The inset shows the differential scattering rates for cuts 1 and 2 on top of the phonon density of states derived from Ref. 5. On both panels, the vertical dashed lines indicate the positions of three optical-phonon modes: 15 meV (the lowest one), 30 meV, and 44 meV (the highest one).

Figure 3

An estimate of the phonon undressed dispersion of the Fe-3$d_{\mathit{xy}}$ band and the corresponding band velocity.

Figure 4

Scattering rate on large-energy scale. MDC width and the imaginary part of fermionic self-energy for the cuts 1 and 2. MDC width for (Bi,Pb)${}_2$Sr${}_2$Ca${}_2$CuO${}_{8-\delta }$ (BSCCO)[8] is shown for comparison. The EDC width at the bottom of the band and the imaginary part of the self-energy for BSCCO refer to the right axis.