Ultrafast near-infrared spectroscopic study of coherent phonons in the phase-separated manganiteLa1/4Pr3/8Ca

Abstract

We report the generation of coherent optical and acoustic phonons in mixed valent manganite La 1/4 Pr 3/8 Ca 3/8 MnO 3 using femtosecond infrared pump-probe spectroscopy. Temperature-dependent measurements of the time-resolved optical reflectance, obtained over a range of 5-300 K, revealed that the energy of the photoexcited electrons dissipated during relaxation to acoustic phonons, in the high-temperature paramagnetic phase, and to optical phonons, in the low-temperature charge-ordering phase. Analysis of the temperaturedependent behavior reveals that the modal amplitudes of the coherent phonons appear strongly correlated with the charge-ordering phase.

Femtosecond dynamics of the structural transition in mixed valence manganites

“…44,47 The LPCMO crystal shows similar behavior as the LCMO film. The coherent oscillation with a frequency of 2.5 THz disappears when heated over T CO ≈ 210 K. This is in accordance with the findings of Jang et al 48 measured with a pump fluence of 0.1 mJ/cm 2 . Figure 2 shows the reflectivity changes measured at 60 K with laser fluences ranging from 0.5 − 7.7 mJ/cm 2 for LCMO and 1.0 − 14.0 mJ/cm 2 for LPCMO.…”

The influence of charge and magnetic order on polaron and acoustic phonon dynamics in LuFe2O4

“…11)), and c-axis sound velocity, respectively, [27][28][29][30][31] and the measured oscillation period s p $ 48 ps. In our experiments, doubling the probe photon energy caused the oscillation frequency to double, supporting coherent acoustic phonon generation as the origin of the oscillations.…”

“…9 More generally, similar trends have been observed using femtosecond spectroscopy to study other charge-ordered materials, such as La 0.5 Ca 0.5 MnO 3 and La 1/4 Pr 3/8 Ca 3/8 MnO 3 , although changes in the phonon temperature were only considered on longer time scales. 28,29 These systems also show a strong decrease in the amplitude of acoustic phonon oscillations as the temperature is reduced below T CO , with a corresponding increase in the amplitude of optical phonon oscillations. This could also be explained in a similar manner to LuFe 2 O 4 , as the lattice constant can also change through electronic transitions for T > T CO in those manganites, thus decreasing the measured acoustic phonon oscillation amplitude as the temperature is varied below T CO .…”

“…The observed oscillations are typically attributed to coherent acoustic phonons, which can be manifested as propagating strain waves, generated through the dynamic stress induced by absorption of the femtosecond laser pulses within the penetration depth. 26 These strain waves have often been observed in ultrafast optical experiments on correlated electron materials [27][28][29] and semiconductors, 30,31 with an oscillation period that depends on the probe wavelength, given by…”

“…This excitation is clearly delocalized in space and therefore will initially generate coherent phonons with momentum close to k $ 0. If the time resolution is high enough, then both k $ 0 coherent optical 14 and acoustic phonons [27][28][29] can be detected. However, we were unable to detect any coherent optical phonons, due to our relatively low time resolution ($75 fs) and their frequencies of several terahertz.…”