5. The Dynamic Jahn-Teller Effect in Cu(II)/MgO

A true dynamic Jahn-Teller effect in the solid state has proven to be quite elusive, as pure compounds suffer from cooperative effects, while doped systems are susceptible to small crystal imperfections that lock the system into static distortions. Cu(II) doped into the cubic host MgO represents a rare example of such a dynamic Jahn-Teller system and has been the subject of numerous experimental and theoretical studies. Recently we have presented high resolution low temperature Electron Paramagnetic Resonance (EPR) spectra of Cu(II)/MgO as a function of the applied field direction. These spectra indicate that at temperatures as low as 1.8 K the Cu(II) centre is in a degenerate vibronic state of E symmetry that is delocalized over the ground potential energy surface, indicating a true dynamic Jahn-Teller effect. The experiments also show us that this system has a potential energy surface with three equivalent minima, each at three equivalent tetragonally elongated geometries, separated by low barriers. Relaxation from the anisotropic E type spectrum to an isotropic spectrum occurs at temperatures above 6 K. The observation of the dynamic Jahn-Teller effect in this system is due to small barrier heights between the minima and the random crystal strain, which is small when compared to the tunneling splitting. We examine the limitations of the experiment in being able to determine these quantities separately and suggest future experiments that may shed further light on this fascinating system.