Coherent Structural Dynamics of a Prototypical Charge-Density-Wave-to-Metal Transition

Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$ over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.

Figure 1

(a) Linear chains of high conductivity composed of ${\mathrm{MoO}}_6$ octahedra, separated by layers of K-atoms. The plane corresponds to the $(20\overline{1})$ cleavage plane. (b) Experimental setup; parameters as described in the text. The incidence plane of x-ray probe and 800 nm pump beam is vertical. The detector is positioned to measure the intensity of a SL reflection, providing a direct view of the structural symmetry of the system.

Figure 2

Time evolution of the normalized peak diffraction intensity of the $\mathbf{(}1\hspace{0.17em}\hspace{0.17em}(4-q_b)\hspace{0.17em}\hspace{0.17em}\overline{0.5}\mathbf{)}$ reflection for a pump fluence $F=0.3\mathrm{mJ}/{\mathrm{cm}}^2$, well below the melting threshold of the CDW condensate. The error bars correspond to photon counting statistics. The solid line is a fit to the data using a displacive excitation model [see Eq. (1)].

Figure 3

(a) Time evolution of the normalized SL diffraction peak intensity for increasing fluence $F$. The horizontal dashed line in the higher fluence scan represents the saturation background. Solid lines are fits according to the model as described in the text. (b) Photoinduced changes of the double-well potential as assumed in our model simulation. The grey potential corresponds to the unperturbed CDW state (where the excitation parameter $\eta =0$), the green circle represents the displacement along the structural coordinate of the Peierls distortion. The transient quasiequilibrium position after excitation is labeled $X_{\mathrm{eq}}$. (c) Schematic of the inhomogeneously excited crystal in the case of a high fluence excitation that leads to ${\eta }_0>1$, the $z$ axis corresponds to the axis perpendicular to the surface plane. (d) Simulated diffraction signal of a thin layer for certain values of $\eta$ (at $t=0$), where ${\delta }_L$ corresponds to the laser penetration depth.