Temperature-dependent pseudogap in the oxypnictides ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ and ${\text{LaFePO}}_{1-x}{\text{F}}_x$ seen via angle-integrated photoemission

We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical-transition temperatures ($T_c$’s) of 26 and 5 K, respectively. We find that high-$T_c$ LaFeAsO:F exhibits a temperature-dependent pseudogaplike feature extending over $\sim 0.1\text{eV}$ about the Fermi level at 250 K, whereas such a feature is absent in low-$T_c$ LaFePO:F. We also find $\sim 20\text{-meV}$ pseudogaplike features and signatures of superconducting gaps in both LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogaplike features through comparison with the high-$T_c$ cuprates.

Figure 1

(Color) Temperature dependence of the laser-PES spectra of (a) As26, (b) P05, and (c) Au. The spectra were normalized to the intensity at 40 meV below $E_F$. The upper-right insets of (a) and (b) show enlarged plots near $E_F$. The lower-left insets of (a) and (b) show wide-range spectra recorded at $T\le 100\text{K}$ and normalized to the intensity at $-40\text{meV}$. The spectral densities of states (DOSs) of As26, P05, and Au derived from the spectra shown in the main panels of (a)–(c) are shown in (d)–(f), respectively. The inset of (c) shows a platelike area of As26 (indicated by arrows) on which we recorded the spectra.

Figure 2

(Color) Superconducting-gap feature of As26. (a) Near-$E_F$ spectra of As26 recorded during a $T$ cycling. The sequence of the $T$ cycling is indicated by arrows. The inset shows the spectra in a wide energy range. (b) Spectral DOS weights of As26 (left panel) and P05 (right panel) at several energies, derived from the data shown in (a) (crosses) and in Figs. 1d, 1e (circles). The dotted lines are guides for the eyes. The inset shows the depression rate of the spectral DOS weights with decreasing $T$ from 70 to 30 K.

Figure 3

(Color) Superconducting-gap feature of P05. (a) Laser-PES spectra recorded on fractured P05. The inset shows wide-energy-range spectra normalized to the intensity at 40 meV below $E_F$. (b) Spectral DOS of P05 showing signatures of the superconducting gap and the pseudogap.

Figure 4

(Color) Laser-PES spectra of (a) As26 and (b) P05 recorded in a wide-$T$ range, and the spectral DOSs of (c) As26 and (d) P05 derived from the spectra shown in (a) and (b), respectively. The spectra were normalized to the intensity at $E-E_F=-180\text{meV}$, well beyond the energy region affected by the pseudogaps and the Fermi-Dirac broadenings.