Ultrafast Momentum-Dependent Response of Electrons in Antiferromagnetic ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ Driven by Optical Excitation

Employing the momentum sensitivity of time- and angle-resolved photoemission spectroscopy we demonstrate the analysis of ultrafast single- and many-particle dynamics in antiferromagnetic ${\mathrm{EuFe}}_2{\mathrm{As}}_2$. Their separation is based on a temperature-dependent difference of photoexcited hole and electron relaxation times probing the single-particle band and the spin density wave gap, respectively. Reformation of the magnetic order occurs at 800 fs, which is 4 times slower compared to electron-phonon equilibration due to a smaller spin-dependent relaxation phase space.

Figure 1

(a) Simplified electronic band structure of Fe pnictides along $\Gamma \mathrm{\text{−}}X$ in the paramagnetic state. (b) Below $T_N$, electronlike bands are folded back to $\Gamma$, hybridize with holelike bands and open SDW energy gaps near $E_F$. Boxes indicate the integration used in Fig. 3. (c) Sketch of the pump-probe experiment. (d) Laser ARPES intensity of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ taken at $T=30\mathrm{K}$ without optical excitation.

Figure 2

(a) Pump-induced change of spectral weight $\Delta I$ in a false-color map for $t=100\mathrm{fs}$. Blue color marks depletion of spectral weight (hole excitations), while red marks increased $\Delta I$ (electron excitations). Boxes represent integration areas for $\Delta I$ used in Fig. 3. (b) Energy distribution curves (EDCs) for $k_{\parallel }=0.25{\AA }^{-1}>k_F$ before (black solid circles) and $t=100\mathrm{fs}$ after laser excitation (red [medium gray] diamonds) for various temperatures. Increase and depletion of spectral weight are marked by red [medium gray] and blue [dark gray] areas, respectively. Spectra are vertically offset for clarity. (c) EDCs for $k_{\parallel }=0.00{\AA }^{-1}$ ($\Gamma$).

Figure 3

(a) Time-dependent spectral weight integrated over the energy-momentum intervals in Fig. 2a, at $\Gamma$ (upper panel, increase) and $k_{||}=0.25{\AA }^{-1}$ (lower panel, decrease) for various temperatures. Data are vertically offset for clarity. Solid lines are exponential fits. (b) Relaxation time constants ${\tau }_{+}$ and ${\tau }_{-}$ as a function of temperature obtained from the fits. Error bars represent 99.7% confidence intervals.

Figure 4

trARPES intensity in a false-color representation at $\Gamma$ for $T=210\mathrm{K}$ (a) and $T=30\mathrm{K}$ (b), as a function of energy and pump-probe delay. Red dots mark the position of the leading edge midpoint (LEM) of the spectra. (c) Upper panel: LEM position around zero pump-probe delay. The pulse integral scaled to the data is shown as a red solid line, and the green dashed line is an exponential fit (see text). Lower panel: Pulse envelope obtained from electrons at $E\mathrm{\text{−}}{E}_F>1\mathrm{eV}$.