Photoinduced dynamics of nematic order parameter in FeSe

Formation of electronic nematicity is a common thread of unconventional superconductors. We use ultrafast electron diffraction to probe the lattice interactions with electronic degrees of freedom in superconducting FeSe and find a significant lattice response to local nematicity. We observe that a perturbation by a laser pulse leads to a surprising enhancement of the high-symmetry crystalline order as a result of suppression of low-symmetry local lattice distortions, which are signatures of nematic fluctuations. The distortions are present at temperatures both below and above the nematic phase transition, as corroborated by our x-ray pair distribution function analysis and transmission electron microscopy measurements. Nonequilibrium lattice behavior of FeSe reveals two distinct time scales of nematic response to photoexcitation, 130(20) and 40(10) ps, corresponding to diffusive and percolative dynamics of nematic fluctuations respectively.

Figure 1

Decay of the Bragg intensity during the first 5 ps for $\langle 200\rangle$, $\langle 020\rangle$ (a) and $\langle 400\rangle$, $\langle 040\rangle$ (b) reflections measured with UED at 27 K at fluence $1.24\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$. Solid blue curves are exponential decay fits for the experimental data. (c) The peaks’ dynamics during 1000 ps. Inset shows the FeSe diffraction pattern. The orange line in the inset shows the diffraction intensity profile integrated within the indicated frame. Frames A (red) and B (blue) show the time regions where averaged intensity differences (d) were calculated. (d) Changes of the intensity profile, shown in the inset (c), after the pump pulse with respect to the profile of the unpumped sample. Red (blue) line corresponds to changes averaged over the time frame A (B) highlighted in (c).

Figure 2

(a) PDF at 84 K with the fit assuming an orthorhombic structural model. (b) PDF at 300 K with the fit assuming a tetragonal structural model. Blue circles show the experimental data, red line is the fit to the respective model, orange line shows the misfit. The plots contain green (Fe-Se), blue (Fe-Fe), and red (Se-Se) tick marks below the residual, which indicate the different unique pair distances from refining the respective models.

Figure 3

(a) Electron-diffraction pattern at 300 K (b) Electron diffraction from the same area as (a) at 88 K. (c) Typical HRTEM image. (d) FFTs taken from the respective areas as shown in (c). The peaks forbidden by the orthorhombic and tetragonal symmetries are highlighted by red circles.

Figure 4

(a) Dynamics of $\langle 080\rangle$, $\langle 800\rangle$ peaks obtained with UED at different temperatures for the incident fluence of $1.65\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$. Dynamics of the same peaks at different excitation fluences for the full measurement time range (b) and during the first 200 ps (c) at 27 K. The gray dashed line in (c) is a guide to eye. Insets show schematics of unequal atomic bonds at the corresponding time intervals. Open circles are the experimental data and solid lines are the fits [23].