# Characteristics of Coherent Optical Phonons in a Hexagonal \(\mathbf{YMnO_{3}}\) Thin Film
## 4 Results and Discussions
### Temperature Dependence of Coherent Optical Phonons
The change in intensity of coherent phonons around the Neel temperature has reported in hexagonal \(R\)MnO\({}_{3}\) compounds with \(R\) = Y, Lu and Ho [23; 24; 25; 26]. On the other hand, they reported that coherent phonons are completely damped when the temperature passes through the Neel temperature. In contrast to these reports, the present coherent optical phonon can be observed in the temperature range from 10 to 300 K including the Neel temperature. Thus, the present data is of great importance for understanding the characteristics of the coherent phonon dynamics in hexagonal \(R\)MnO\({}_{3}\) compounds. The intensity of the coherent phonon generated by the DECP mechanism reflects the population of excited states. As described in Figure 2, the absorption band has the strong temperature dependence. Accordingly, the population of photogenerated electrons can be changed by temperature under the fixed pump photon energy of 1.72 eV. The amount of the lower energy shift of the absorption peak is \(\sim\)50 meV as increasing temperature from 10 to 70 K, while the profile of the absorption band is almost unchanged. Therefore, we simply evaluated the contribution of the absorption band shift to the coherent phonon intensity by decreasing the pump photon energy by 50 meV at 70 K. Figure 10a shows the \(\Delta R/R_{0}\) signals at the pump photon energy of 1.67, 1.70 and 1.72 eV at 70 K, where the maximum of the pump photon energy difference, 50 meV, corresponds to the energy shift of the absorption peak from 10 to 70 K. The time-domain oscillatory signals extracted from the \(\Delta R/R_{0}\) signals are shown in Figure 10b, where the right-side inset shows the integrated intensities of the FT band of the time-domain oscillatory signals. It is evident that the pump-photon energy dependence of the FT intensity is negligibly small in comparison with the temperature dependence of that from 10 to 70 K as shown in Figure 9b. This fact demonstrates that the intensity drop of the coherent optical phonon around the Neel temperature is not related to the temperature dependence of the absorption band. Figure 9: (**a**) Peak frequency of the FT spectrum in Figure 8b as a function of temperature, where the bar indicates FWHM of the FT band. (**b**) Integrated intensity of the FT band as a function of temperature, where the vertical dotted line indicates the Néel temperature. The solid curves in (**a,b**) are guides to the eye. It is considered from the above discussion that the change in intensity of the coherent optical phonon is related to the antiferromagnetic ordering. In YMnO\({}_{3}\), the Y atom is nonmagnetic while the Mn atom is directly related to the magnetic property. It is noted that the A\({}_{1}\)-symmetry optical phonon includes only the vibrations of Y atoms [28]. It seems strange that the oscillation intensity of the A\({}_{1}\)-symmetry optical phonon correlates with the antiferromagnetic ordering. This behaviour can be explained by the isostructural transition around the antiferromagnetic ordering as follows [14]. As decreasing temperature to the Neel temperature from a higher temperature range, Mn atoms displace from their ideal positions to get antiferromagnetic ordering and the other atoms also displace within the unit cell. The variation of the atom positions can change the characteristics of phonon properties. In YMnO\({}_{3}\), there are two types of Y atoms locating at different lattice sites. The displacements of two Y atoms are not identical as a function of temperature, whereas they have extrema values at the same temperature of \(\sim\)90 K. This temperature almost agrees with the temperature at which the oscillation intensity of the coherent optical phonon has the minimum value. On the assumption that the relative amount of the displacement of Y atoms from their position at 10 K corresponds to the decrement of the intensity of the coherent optical phonon, the dip-shaped dependence in Figure 9b can be qualitatively explained.