Picosecond strain dynamics in ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ monitored by time-resolved x-ray diffraction

Coherent phonons (CPs) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, nonthermal switching in phase-change materials such as ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100-ps x-ray pulses from the European Synchrotron Radiation Facility (ESRF) storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal; however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone, implying the presence of excited-state effects in addition to temperature-driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase transitions in GST.

Figure 1

A series of snapshots showing the time evolution of the GST(222) diffraction peak. From the top left-hand corner across each row, the times are $-150$ ps, $-100$ ps, $-50$ ps, $-10$ ps, 10 ps, 50 ps, 100 ps, 300 ps, and 1 ns, respectively. The $Q_z$ axis corresponds to the reciprocal space direction ${\langle 111\rangle }^{*}$, while the $Q_x$ axis corresponds to the reciprocal space ${\langle 110\rangle }^{*}$ direction. The laser fluence used was $16\mathrm{mJ}/{\mathrm{cm}}^2$.

Figure 2

(top) Intensity distribution for a few prototypical values of $\delta t$ for $16\mathrm{mJ}/{\mathrm{cm}}^2$. (middle) Integrated intensity of the GST(222) peak as a function of $\delta t$ for different fluences. (bottom) Fractional shift in the (222) plane spacing (in percent) as a function of $\delta t$ for different fluences. To obtain the pseudo-one-dimensional spectra for all $\delta t$ values and fluences, the diffraction peaks were fit by Gaussian functions, and the results were tabulated.

Figure 3

(top) Measured time-$Q_z$ intensity plot of the GST(222) diffraction peak for a fluence of $16\mathrm{mJ}/{\mathrm{cm}}^2$. (bottom) Simulated time-$Q_z$ intensity plot of the GST(222) diffraction peak carried out using a ball-and-spring model (see text for details). The $Q_z$ axis corresponds to the reciprocal space direction ${\langle 111\rangle }^{*}$, while the $Q_x$ axis corresponds to the reciprocal space ${\langle 110\rangle }^{*}$ direction.

Figure 4

(left) Simulated time-$Q_z$ intensity plot of the GST(222) diffraction peak carried out using a ball-and-spring model (see text for details) in the vicinity of $t=0$ without convolution effects. The intensity scale is the same as in Fig. 2. (right) One-dimensional strain (in percent) as a function of time as calculated by a ball-and-spring model.