Electronic Dispersion Anomalies in Iron Pnictide Superconductors

Recently, experimental studies of the spin excitation spectrum revealed a strong temperature dependence in the normal state and a resonance feature in the superconducting state of several Fe-based superconductors. Based on these findings, we develop a model of electrons interacting with a temperature dependent magnetic excitation spectrum and apply it to angle resolved photoemission in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. We reproduce in quantitative agreement with experiment a renormalization of the quasiparticle dispersion both in the normal and the superconducting state, and the dependence of the quasiparticle linewidth on binding energy. We estimate the strength of the coupling between electronic and spin excitations. Our findings support a dominantly magnetic pairing mechanism.

Figure 1

(a) Dominant orbital contributions at the $k_z=0$ Fermi surface cut (top); momentum dependence of the spin-fluctuation mode (bottom). The mode is centered at wavevectors $\pm {\mathit{Q}}_1=(0,\pm \pi )$ and $\pm {\mathit{Q}}_2=(\pm \pi ,0)$. (b) Energy-momentum dependence of the spin susceptibility in the normal state ($T=50\mathrm{K}$, bottom) and in the superconducting state ($T=15\mathrm{K}$, top). (c) Spectral function ${\chi }^{\prime \prime }(\omega ,\mathit{Q})$ for $T=50\mathrm{K}$ (dotted) and $T=15\mathrm{K}$ (solid). The dashed line shows the behavior for $T=15\mathrm{K}$ if the material were in the normal state. (d) The ratio $w_T/{\chi }_T$ as function of temperature.

Figure 2

On top: spectral function in the normal state for $T=50\mathrm{K}$ along a cut in the 1st Brillouin zone for $k_z=0$. At bottom: the same for the superconducting state at $T=15\mathrm{K}$ for the square regions indicated (a) and (b) in the top panel. The black lines show MDC-derived dispersions for the (a) ${\alpha }_1$ and ${\alpha }_2$ band and the (b) ${\beta }_1$ band.

Figure 3

(a) and (c) MDC-derived dispersions at the ${\alpha }_1$, ${\alpha }_2$ and ${\beta }_1$ band at $T=15\mathrm{K}$ (solid) and $T=50\mathrm{K}$ (dashed). The insets show the real part of the effective self-energy ${\Sigma }^{\prime }(ϵ)={\Sigma }_{ϵ,{\mathit{k}}_{ϵ}}^{\prime }$. (b) Imaginary part of the effective self-energy at $T=15\mathrm{K}$ at the ${\alpha }_1$ (solid) and the ${\alpha }_2$ (dashed) band. (d) Temperature dependence of the imaginary effective self-energy at the ${\alpha }_1$ band and the real part at the ${\beta }_1$ band.