Orbital fluctuations in the $R$VO$_3$ perovskites

TL;DR

This paper develops a spin-orbital superexchange model for RVO3 perovskites, explaining orbital fluctuations and anisotropies, and analyzes how structural distortions influence magnetic and orbital transition temperatures.

Contribution

The paper introduces a detailed model for RVO3 perovskites that accounts for orbital fluctuations and structural distortions, providing insights into magnetic and orbital ordering phenomena.

Findings

Orbital fluctuations along the c axis cause magnetic and optical anisotropies.

GdFeO3 distortion influences the second magnetic transition in YVO3.

Orthorhombic distortion controls orbital fluctuations and transition temperatures.

Abstract

The properties of Mott insulators with orbital degrees of freedom are described by spin-orbital superexchange models, which provide a theoretical framework for understanding their magnetic and optical properties. We introduce such a model derived for $(xy)^1(yz/zx)^1$ configuration of V$^{3+}$ ions in the $R$VO$_3$ perovskites, $R$=Lu,Yb,$\cdots$,La, and demonstrate that $\{yz,zx\}$ orbital fluctuations along the $c$ axis are responsible for the huge magnetic and optical anisotropies observed in the almost perfectly cubic compound LaVO$_3$. We argue that the GdFeO$_3$ distortion and the large difference in entropy of $C$-AF and $G$-AF phases is responsible for the second magnetic transition observed at $T_{N2}$ in YVO$_3$. Next we address the variation of orbital and magnetic transition temperature, $T_{\rm OO}$ and $T_{N1}$, in the $R$VO$_3$ perovskites, after extending the spin-orbital model by the crystal-field and the orbital interactions which arise from the GdFeO$_3$ and Jahn-Teller distortions of the VO$_6$ octahedra. We further find that the orthorhombic distortion which increases from LaVO$_3$ to LuVO$_3$ plays a crucial role by controlling the orbital fluctuations, and via the modified orbital correlations influences the onset of both magnetic and orbital order.