Role of adiabatic vibronic coupling and "inverted" Jahn-Teller effect in Kramers doublets on spin dynamics in half-integer spin systems

The Jahn-Teller (JT) effect embraces phenomena arising from electron-vibrational coupling in degenerate electronic states. Its presence was demonstrated in molecules and impurities in crystals as well as in extended materials where it is manifested in structural, magnetic and superconducting phases. The JT theorem predicts high-symmetry nuclear configurations to be unstable in the presence of electronic degeneracy. The distortions removing this degeneracy are often dynamic resulting in strongly non-harmonic nuclear behavior. However, this rule does not apply to the Kramers degeneracy realized e.g. in S = 1/2 orbitally non-degenerate terms (spin degeneracy) and twofold degenerate multiplets for odd number of electrons. In such states a net separation of degenerate electronic and harmonic nuclear subsystems is expected. Here we demonstrate that Kramers doublets exhibit the JT effect as well. Contrary to conventional JT effect, the instability of nuclear subsystem develops now after nuclear momenta rather than nuclear coordinates. Such JT effect leads to essential dynamical mixing of two KD components in a high symmetry environment implying its crucial role in the dephasing rate of spin qubits.