Ultrafast melting of spin density wave order in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ observed by time- and angle-resolved photoemission spectroscopy with extreme-ultraviolet higher harmonic generation

The transient single-particle spectral function of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin density wave (SDW) ordered state within $\sim 1$ ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above $E_F$ at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From a comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are $\sim 0.60\mathrm{ps}$ both around the zone center and the zone corner.

Figure 1

(a) Temporal evolution of angle-integrated photoemission intensity around the $(0,0,6.5)$ point of the Brillouin zone (BZ). (b) Difference spectra of (a) from the average intensity before the arrival of the pump pulse. (c) First BZs of the paramagnetic (black) and antiferromagnetic (red) states. We shall use the paramagnetic notation below. Solid and dotted arrows indicate the momentum cuts for the $(0,0,6.5)$ and $(1,1,6.1)$ points, respectively. (d) Temporal evolution of the leading edge width (LEW). LEW is evaluated by fitting the photoemission intensity to the Fermi-Dirac distribution function convoluted with the instrumental Gaussian function. The black dotted line shows the fit to a decay function of the form $\text{LEW}\left(t\right)=A+B\mathrm{\Theta }\left(t\right)exp(-t/{\tau }_{\text{decay}})$, where $\mathrm{\Theta }\left(t\right)$ is the Heaviside step function.

Figure 2

(a) Time-resolved angle-resolved photoemission (TR-ARPES) spectra around the $(0,0,6.5)$ point. (b) Difference TR-ARPES spectra from the spectrum before the pump arrival $(t=-0.57$ ps).

Figure 3

(a) Difference TR-ARPES spectrum at 0.12 ps. The dotted curve is a guide to the eye, indicating a characteristic parabolic intensity decrease. (b), (c) Density-functional theory (DFT) band structure around the $(0,0,6.5)$ point for paramagnetic (PM) [(b)] and spin density wave (SDW) [(c)] states. For the SDW state, band structures for two inequivalent domains are plotted. (d), (e) Equilibrium ARPES spectra around the $Z$ point for the PM (200 K) and SDW (20 K) states, respectively. The black dotted line in (e) indicates a parabolic feature which appears only in the SDW state.

Figure 4

(a) Temporal evolution of angle-integrated PES spectra around the $(1,1,6.1)$ point. The momentum cut is indicated by a dotted arrow in Fig. 1. (b) Difference spectra of (a) from the average intensity before the arrival of the pump pulse. (c) Temporal evolution of LEW.

Figure 5

(a) TR-ARPES spectra around the $(1,1,6.1)$ point. (b) Difference TR-ARPES spectra from the spectrum before the pump arrival $(t=-1.02$ ps). (c), (d) DFT band structures around the $(1,1,6.1)$ point for the PM and SDW states, respectively.