Class of Molecular and Solid State Systems with Correlated Magnetic and Dielectric Bistabilities Induced by the Pseudo Jahn-Teller Effect

Based on the earlier revealed hidden pseudo Jahn-Teller effect which essentially involves excited electronic states we show that, with extension to solids, polyatomic systems with half-filled degenerate shells, $e^2$ or $t^3$, form a novel (practically inexhaustible) class of molecular and solid state materials with correlated and switchable magnetic and dielectric bistabilities. The latter are confirmed by state-of-the-art ab initio calculations, including estimation of lifetimes, for several representatives of this class, which also reveal a giant magnetoelastic effect in ${\mathrm{LiCuO}}_2$ and ${\mathrm{NaCuO}}_2$ crystals.

Figure 1

The $e^2$ electronic configuration spans the states ${}^{3}A_2$, ${}^{1}A_1$, and ${}^{1}E$. While the magnetic ${}^{3}A_2$ state is lowest in energy and stable for the high-symmetry configuration (left) the excited states interact through the PJT effect leading to a low-energy distorted equilibrium configuration with different properties (right).

Figure 2

Ab initio calculated energy levels of ${\mathrm{CuF}}_3$ as a function of the angle $\alpha$ ($e$ mode distortion) showing the formation of the two equilibrium configurations typical for HFS systems.

Figure 3

Calculated relative stability of the ${\mathrm{LiCuO}}_2$ and ${\mathrm{NaCuO}}_2$ crystals in the magnetic to diamagnetic transition as a function of the in-plane lattice parameter: at $\sim 35\%$ of strain in ${\mathrm{NaCuO}}_2$ and $\approx 50\%$ in ${\mathrm{LiCuO}}_2$ the two phases become approximately degenerate. The distortion along the $c$ axis is also illustrated.

Figure 4

Temperature dependence of lifetimes estimated for the higher in energy high-spin states of ${\mathrm{Si}}_3$, ${\mathrm{CuF}}_3$ and an isolated octahedral center in ${\mathrm{NaCuO}}_2$ by means of model calculations following Ref. 4.