The elastic properties of La1−xCaxMnO3 (x»1/3) are anomalous, particularly in the paramagnetic phase, where values of the longitudinal sound velocity at ~10¹⁰ Hz, in the zero-sound regime, are more than 50% larger than those from ultrasound experiments at 10³–10⁶ 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. Our results suggest that the physics of phase separation in manganites is more complex than that of a stationary mixture of competing phases.The zero-sound velocity was determined for longitudinal acoustic (LA) phonons propagating along [001] using an ultrafast optical technique that applies to opaque substances and is sensitive to sound at GHz frequencies. The data reported here used a standard pump-probe setup in the reflection geometry and 100-fs laser pulses of energy density ~ 1 J/m² from a mode-locked Ti-sapphire laser and a 250-kHz optical parametric amplifier operating, respectively, at ~ 700-850 nm and ~ 500-600 nm.Coupling of acoustic waves to slow relaxation processes leads to dissipation and elastic dispersion at frequencies w<< kBQD/h, where kB is Boltzmann’s constant and QD is the Debye temperature. For w<< 1/t (t is the relaxation time), waves move with the ordinary adiabatic or first-sound velocity c1 while in the so-called zero-sound region w>> 1/t, sound propagates at a higher quasi-isothermal velocity, c0. In conventional solids, these velocities differ only slightly. Typically, D = (c0-c1)/(c0+c1) < 0.01. We have shown that one of the most studied CMR compounds, La1-xCaxMnO3, exhibits a significantly larger dispersion, particularly at high temperatures where D » 0.2. The manganites might then represent a new state of ‘hard’ matter characterized by unusually sluggish fluctuations which strongly couple to sound, as in ‘soft’ viscoelastic solids and crystals in the vicinity of structural phase transitions. We believe that the slow dynamics is the temporal manifestation of the diversity of nearly-isoenergetic phases, which is ultimately responsible for CMR and leads to nanoscale phase separation and textured states where competing phases coexist.

References:

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