Anomalous first-to-zero sound crossover in ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x\mathrm{Mn}{\mathrm{O}}_3$

The elastic properties of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x\mathrm{Mn}{\mathrm{O}}_3$ $(x\approx 1∕3)$ are anomalous, particularly in the paramagnetic phase, where values of the longitudinal sound velocity at $\sim {10}^{10}\mathrm{Hz}$, in the zero-sound regime, are more than 50% larger than those from ultrasound experiments at ${10}^3–{10}^6\mathrm{Hz}$. Such a behavior, indicative of coupling to relaxation processes, is similar to that of liquids and soft viscoelastic matter, but quite unusual for a hard solid. These results suggest that the physics of phase separation in manganites is more complex than that of a stationary mixture of competing phases.

Figure 1

Differential reflectivity for ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ (single crystal) at $T∕T_C=1.29$. The central wavelength of the probe pulse is denoted by ${\lambda }_{\text{probe}}$. For clarity, the traces have been shifted vertically.

Figure 2

Ultrafast optical data for a single crystal of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ at ${\lambda }_{\text{probe}}=510\mathrm{nm}$, corresponding to LA frequencies at $\sim 50\mathrm{GHz}$ (full squares). Open circles and squares are ultrasound results for polycrystalline samples of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ at, respectively, $10\mathrm{MHz}$[18] and $1.3\mathrm{kHz}$.[15]

Figure 3

LA dispersion for a thin film of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ at $T∕T_C=1.12$. Full squares are ultrafast optical results obtained in this work. The dashed line denotes the sound velocity determined from ultrasound measurements[16] and the black curve is a fit to Eq. (2).