The dynamic Jahn-Teller effect in Cu(II) doped MgO.

Area of Science:

• Solid State Physics
• Quantum Mechanics
• Spectroscopy

Background:

• Electron paramagnetic resonance (EPR) is a technique used to study materials with unpaired electrons.
• The Jahn-Teller effect describes the geometric distortion of a molecule or crystal due to electronic degeneracy.
• Copper(II) ions in MgO crystals are known to exhibit Jahn-Teller distortions.

Purpose of the Study:

• To re-examine the EPR spectra of Cu(II) doped MgO single crystals.
• To model these spectra within the framework of a dynamic Jahn-Teller effect.
• To elucidate the role of crystal strain and vibronic interactions in the observed spectral features.

Main Methods:

• Detailed analysis of experimental 1.8 K X-band EPR spectra.
• Modeling using a cubic spin Hamiltonian and a four-state model for the lowest vibronic energy levels.
• Application of the eigenfield method to precisely treat Zeeman, hyperfine, quadrupole, tunneling, and strain terms without approximations.

Main Results:

• The spectra are well-modeled by a dynamic Jahn-Teller effect, including vibronic reduction factors and random crystal strain.
• The first excited singlet state possesses A(2) symmetry, consistent with the expected Jahn-Teller potential energy surface.
• Accurate spectral reproduction required specific hyperfine values, a nuclear quadrupole term, and consideration of tunneling splitting (~4 cm⁻¹).

Conclusions:

• The Cu(II)/MgO system is characterized as an almost pure dynamic Jahn-Teller case.
• Random crystal strains significantly influence spectral features, requiring averaging over angular directions.
• The findings provide insights into the intrinsic Jahn-Teller coupling parameters of the potential energy surface.