JAHN-TELLER EFFECT IN EPR SPECTRUM OF CU++ - MGO AT 1.2 DEGREES K

The EPR spectrum of Cu++:MgO at 1.2°K is described and shown to be characteristic of a Kramers quartet of states which have anisotropic transitions characteristic of a one-ion-per-unit-cell system. This is an octahedral symmetry property which may be realized either by the vanishing of the Jahn-Teller effect at 1.2°K or by the presence of tunneling between equivalent tetragonally distorted states. The tunneling process generates a Kramers doublet and quartet separated by a small tunneling (inversion) splitting, as has been shown by Bersuker and O'Brien. A theoretical g-value formula for the tunneling quartet is derived for the magnetic field in an arbitrary direction. The experimental g values are shown to be in good agreement with this formula, but they are not in agreement with a similar formula for the static octahedral symmetry quartet. The hyperfine structure is also shown to be better explained as a property of the tunneling quartet. The tunneling doublet has not been detected at 1.2°K and is presumed to be thermally depopulated. It is concluded that the paramagnetic resonance properties of Cu ++:MgO at liquid-helium temperatures measured at 9×10 9 cps are determined by a dynamic tunneling effect. We have found no evidence for a frozen-out" tetragonal distortion."