Ultrafast Modulation of the Chemical Potential in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ by Coherent Phonons

Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ around the high-symmetry points $\mathrm{\Gamma }$ and $M$. A global oscillation of the Fermi level at the frequency of the $A_{1g}(\mathrm{As})$ phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the $A_{1g}(\mathrm{As})$ phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling.

Figure 1

(a) Schematic illustration of time-resolved pump-probe ARPES of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. The photoexcited surface region, which is out of equilibrium with respect to the bulk, is indicated by shading. (b) Sketch of the Fe-As layer in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ illustrating the $A_{1g}(\mathrm{As})$ phonon mode. (c) 10-orbital tight-binding band structure of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ at low energies along $\mathrm{\Gamma }–M$ computed for various Fe-As bond angles $\alpha$ ($\alpha =34.5°$ is the equilibrium value). (d) Corresponding density of states as a function of $\alpha$. Note the modification of the occupied bandwidth implying a change in the Fermi energy.

Figure 2

Time-dependent ARPES spectra of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ taken at Fermi momenta near (a) $\mathrm{\Gamma }$ and (b) $M$. Data were recorded with $s$-polarized 22.1-eV probe pulses at $T=100\mathrm{K}$ and $F_{\text{abs}}=0.47\mathrm{mJ}/{\mathrm{cm}}^2$. Photoemission intensity was integrated over a momentum window of $0.18{\AA }^{-1}$. (c),(d) Comparison of spectra taken near $\mathrm{\Gamma }$ at specific pump-probe delays. Solid lines represent best fits with the model described in the text.

Figure 3

(a) Effective electron temperature and (b) chemical potential shift in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ as a function of pump-probe delay, extracted from trARPES data taken near $\mathrm{\Gamma }$ and $M$ with $s$- and $p$-polarized 22.1-eV probe pulses ($T=100\mathrm{K}$, $F_{\text{abs}}=0.47\mathrm{mJ}/{\mathrm{cm}}^2$). Thick gray lines represent best fits with the models described in the text. In (b) oscillatory and nonoscillatory components of the temporal response are indicated. (c) Extracted time constants and (d) amplitudes as a function of absorbed pump fluence. Lines serve as guides to the eye. In (d), filled gray symbols represent amplitudes obtained from data taken at a higher temperature and of the optimally hole-doped compound ${\mathrm{Ba}}_{0.65}{\mathrm{K}}_{0.35}{\mathrm{Fe}}_2{\mathrm{As}}_2$.