Raman spectroscopy of the Jahn-Teller phonons in LaMnO3 thin films

Abstract

Jahn-Teller (JT) distortion plays a significant role in determining physical properties in complex oxides.In this work, we investigate the symmetry analysis of JT-originated phonon modes in LaMnO$_3$ (LMO) film grown on a (001)-oriented KTaO$_3$ (KTO) and (110)-oriented GdScO$_3$ (GSO) substrate via Raman spectroscopy. In the first case, a 23-nm-thick epitaxial LMO thin film was deposited on KTO substrate using pulsed laser deposition. We extracted the pure film spectra from the composite film and substrate spectra to study the JT phonons. Angle-resolved Raman spectroscopy enables us to verify the presence of the JT distortion and the symmetry of the JT phonons within the film. We then correlate the symmetry of the JT phonons with that of the JT distortion based on Raman tensor analysis. Based on symmetry analysis of the phonon with B1g (Ag) symmetry near ~610 cm$^{‒1}$ (~490 cm$^{‒1}$), we conclude that the orientation of the JT plane of LMO is parallel to the KTO substrate. In the second case, we investigate on the JT distortion in ultrathin limit. We prepare a series of ultrathin films of LMO on GSO substrate. Based on the symmetry analysis of the Raman spectra for a thick film, we conclude that the orientation of the JT plane of LMO is parallel to the GSO substrate for the thick LMO thin film. In ultrathin limit, a noticeable hardening of the Raman shift is observed from the Raman spectra under 5-unit-cell thickness. The characteristic B1g mode is still observed in the 2-unit-cell thickness indicating the existence of JT phonons. In addition, the angle-resolved Raman spectra of the 2-unit-cell thickness film still shows the preserved symmetry of JT distortion. This finding suggests the possible absence of the critical thickness of the JT distortion when the JT plane is placed to be parallel to the substrate.