Fig. 1. Electronic structure of LiFeAs measured under different polarization geometries. (a) Overall Fermi surface of LiFeAs measured with a photon energy of 21.218 eV. Two Fermi surface sheets observed around Γ are labeled as β (blue circle) and γ (pink circle), while two crossing elliptical Fermi surface sheets observed around M point are labeled as δ (black ellipses). (b)–(d) Fermi surface of LiFeAs measured by using laser ARPES with a photon energy of 6.994 eV under different polarization geometries. The directions of the electric field vector E corresponding to the three polarization geometries are marked by double arrows in the bottom-right corner of each panel. We note that, while the electric field vector E in (c) fully lies in the sample plane, there is some component of the electric field vector E that is outside of the sample plane in (b) and (d). In (b), the α band is also marked (dashed green circle) around Γ in addition to the β and γ Fermi surfaces. (e)–(g) The band structure of LiFeAs measured along the Γ–X direction under three different polarization geometries that correspond to (b)–(d), respectively. The location of the momentum cut is marked in (b) by a red line. The green, blue, and pink arrows point to the α, β, and γ bands, respectively.

Fig. 2. Electron dynamics of the γ band of LiFeAs measured along the Γ–X direction at 20 K. (a) The γ band measured along the Γ–X direction. The location of the momentum cut is marked by the red line in the inset. (b) The second derivative image of (a) with respect to energy. (c) Second derivative image of the simulated single-particle spectral function which considers electron coupling with two bosonic modes at 20 meV and 34 meV. (d) Momentum distribution curves (MDCs) at several representative binding energies. The MDCs are fitted by Loretzians that are overlaid as dashed lines on the measured data. (e) Representative energy distribution curves (EDCs) at several momenta. (f) Dispersion relation obtained by MDC fitting. The dashed red and blue lines represent empirical bare bands that are used to get the effective real parts of the electron self-energy Re Σ (red line and blue line) shown in (g). The observed features are marked by pink, green, and orange strips. (h) Corresponding MDC width (full width at half maximum, FWHM) of the γ band in (a) from the MDC fitting.

Fig. 3. Electron dynamics of the β band of LiFeAs measured along the Γ–X direction at 20 K. (a) The β band measured along the Γ–X direction. The location of the momentum cut is marked by the red line in the inset. (b) The second derivative image of (a) with respect to energy. (c) MDCs at several representative binding energies. The MDCs are fitted by Loretzians that are overlaid as dashed lines on the measured data. (d) Representative EDCs at several momenta. (e) Dispersion relation obtained by MDC fitting. The dashed red and blue lines represent empirical bare bands that are used to get the effective real parts of the electron self-energy, Re Σ, (red line and blue line) shown in (f). The observed features are marked by pink, green, and orange strips. (g) Corresponding MDC width (FWHM) of the β band in (a) from the MDC fitting.

Fig. 4. Temperature dependence of the electron dynamics for the β and γ bands in LiFeAs. (a) Temperature dependent effective real part of electron self-energy of the γ band. For clarity, the curves are offset along the vertical axis. (b) Corresponding MDC width of the γ band measured at different temperatures. The upper-right inset shows the MDC width near the E_F region. (c) EDCs measured at the k_F point of the γ band at different temperatures. The EDC at 12 K is also multiplied by 5 times to show the dip structure near 34 meV as marked by an arrow. The upper-left inset shows the temperature dependence of the EDC width (FWHM) of the γ band. (d) Temperature dependent effective real part of electron self-energy of the β band. The curves are offset along the vertical axis for clarity. (e) Corresponding MDC width of the γ band measured at different temperatures. The upper-right inset shows the MDC width near the E_F region. (f) EDCs measured at the k_F point of the β band at different temperatures. The upper-left inset shows the temperature dependence of the EDC width (FWHM) of the β band.