Clock transitions in orientationally disordered samples of the [VO(TPP)] molecular nanomagnet 

Molecular nanomagnets (MNMs) have the ability to exhibit long coherence times, making them promising qubit candidates. At atomic clock transitions (CTs), where the transition frequency is independent of the magnetic field to the first order, the decohering effect of magnetic-field fluctuations are reduced, enhancing coherence. Using ESR, we have previously characterized a zero-field CT in a crystalline sample of the molecular nanomagnet [VO(TPP)], but did not observe the predicted CTs at finite fields, likely because the CT has a strong dependence on sample orientation. Here we characterize CT behaviour in a powder sample of [VO(TPP)] – a random ensemble of orientations – in which some portion of the ensemble is correctly aligned with the field. We observe evidence of a zero-field CT in powder [VO(TPP)], the results of which are consistent with simulations of [VO(TPP)]’s clock transition behavior. We find a marked enhancement in coherence around zero field, with T₂ = 1.9 ± 0.1 μs at zero field reducing to 0.72 ± 0.03 μs at 15 Oe. These findings show that the zero-field CT of [VO(TPP)] enhances coherence, and also suggests that a powder sample may be a viable platform for investigating CTs away from zero field.