Ultrafast Decoherence of Charge Density Waves in K$_{0.3}$MoO$_{3}$

Abstract

Recent works have suggested that transient suppression of a charge density wave (CDW) by an ultra-short excitation can lead to an inversion of the CDW phase. We experimentally investigate the dynamics of the CDW in K$_{0.3}$MoO$_{3}$ by time resolved x-ray diffraction after excitation with optical pulses. Our results indicate a transient inversion of the CDW phase close to the surface that evolves into a highly disordered state in less than one picosecond. Numerical simulations solving the Ginzburg-Landau equation including disorder from strong pinning defects reproduce our main observations. Our findings highlight the critical role of disorder in schemes for coherent control in condensed matter systems.

Figures (3)

• Figure 1: Time-resolved diffraction data from the $(3, 7.252, -2.5)$ reflection. (a) Transient evolution of the diffracted x-ray intensity for various absorbed fluences (vertically shifted for clarity). The black vertical lines indicate the three delays $t_1, t_2, t_3$ at which the RSMs were constructed. The insets show the RSM along the surface normal at each of the three delays for the unexcited sample (black) and at $f_\text{pump}=1.3 \,mJ/\,\square cm$ (blue) and $f_\text{pump}=6.4 \,mJ/\,\square cm$ (red). (b) Width of the fit of Eq. \ref{['eq_Lorentzian']} to the projection of the RSM along the surface normal as function of the fluence $f_\text{pump}$ for the three delays. (c) Offset of the $\Delta I_o$ extracted from the fit for the three delays. In both plots (b) and (c), the fluence $f_\text{pump}=0 \,mJ/\,\square cm$ corresponds to times when the x-ray pulse arrives before the pump.
• Figure 2: Sketch of possible order-parameter dynamics: vertical axis represents the free energy, the radial distance the amplitude of the CDW and the angle (with respect to the x-axis) the phase. (a) Initial "sombrero" potential where the order parameter is denoted by a red dot. A strong excitation (1) turns the sombrero potential into a single well potential (b), where the order parameter moves through $\Psi =0$ and overshoots (2). The potential relaxes within an oscillation period (3), but defects along different chains favor different phases (c) such that the order parameter loses long-range coherence.
• Figure 3: Simulated transient diffraction data, using the model described in the text. (a) The transient evolution of the diffracted x-ray intensity for various excitations $\Delta T$. The black vertical lines indicate the three delays $t_1, t_2, t_3$ at which the RSMs were evaluated. (b) The width of the fit of the RSM projection as function of the excitation levels $\Delta T$. (c) The offset of the RSM for the three delays. (d) and (e) show the absolute value and phase of the order parameter as function of the depth below the surface ($z=0$) and delay for the excitation $\Delta T = 250\,K$ at an arbitrary in-plane position. The initial condition is taken from a thermalization process supplement Laser excitation occurs at $t=0$. The phase inversion close to the surface is visible in (d) as a dark line at $t_1$ and as a change of color from violet/red to green in (e).