Raman study of spin and orbital order and excitations in perovskite-typeRVO3(R=La, Nd, and Y)

Abstract

The orders and excitations in spin and orbital sectors have been investigated for perovskite-type $R{\mathrm{VO}}_{3}$ ($R=\mathrm{La}$, Nd, and Y) by measurements of Raman scattering spectra. The orbital excitation bands emerge in the $C$-type spin- and $G$-type orbital-ordered phases commonly for the three $R{\mathrm{VO}}_{3}$ compounds. The investigation of resonance effect on the Raman scattering spectra indicates the quasi-one-dimensional orbital excitation corresponding to the exchange of the occupied orbital state ($yz$ or $zx$ state) on the neighboring sites, to be termed a two-orbiton in analogy to two-magnon in antiferromagnetically ordered states. Among the three compounds, ${\mathrm{YVO}}_{3}$ alone undergoes the phase transition from the $C$-type spin- and $G$-type orbital ordered state to $G$-type spin- and $C$-type orbital-ordered one, as temperature is lowered. In the lowest-temperature ordered phase, the Raman band assigned to two-magnon excitation is observed. In these compounds, structural distortions coupled with the respective orbital orders activate the specific Raman bands assigned to oxygen stretching modes. In the $G$-type orbital-ordered phase in ${\mathrm{YVO}}_{3}$, the phonon mode shows a two-peak feature, while that in other compounds the single-peak one. This suggests that the nominally $G$-type orbital-ordered state in ${\mathrm{YVO}}_{3}$ involves the short-range correlation or fluctuation of the orbital $C$-type, making this phase distinct from the prototypical $G$-type orbital ordered state as in ${\mathrm{LaVO}}_{3}$ and ${\mathrm{NdVO}}_{3}$.