Possible absence of a Jahn-Teller distortion critical thickness in geometrically designed LaMnO3 films

We investigated the Jahn-Teller phonon modes in ${\mathrm{LaMnO}}_{3}$ film grown on an orthorhombic (110)-oriented ${\mathrm{GdScO}}_{3}$ substrate using Raman spectroscopy, which enabled us to verify the presence of the cooperative Jahn-Teller distortion in the ultrathin limit. A characteristic ${B}_{1g}$ phonon mode with preserved symmetry was observed down to the two-unit-cell thickness. This finding suggests the absence of a film-thickness-driven critical transition of the cooperative Jahn-Teller distortion and the corresponding orbital order when the two-dimensional Jahn-Teller-distortion plane is designed to be parallel to the substrate by misfit strain.