Forbidden Transitions in Molecular Nanomagnets*

Selection rules typically arise from the constraints imposed by conservation laws. In electron-spin resonance spectroscopy, the standard single-photon-transition selection rule Δm = 0,±1 arises from angular momentum conservation. Such selection rules can be lifted when the magnetic quantum number of the spin system under study is not a good quantum number. In molecular nanomagnets (MNMs), magnetic anisotropy can lead to a preferred (easy) axis of orientation for the molecule’s spin, giving rise to an anisotropy barrier between “up” and “down” states. In addition, transverse anisotropy breaks the system’s symmetry, leading to state mixing and tunneling between spin states. Thus, tunneling can lead to the observation of forbidden transitions that strongly violate selection rules. I will report studies by my group of two MNMs that exhibit such highly forbidden transitions. In the Ni₄ MNM, a spin-4 system, we observe single-photon transitions in which the magnetic moment changes by as much as ~7 hbar, nearly reversing the molecule’s spin [1]. In addition, we observe direct transitions between tunnel-split states in this MNM, allowing us to precisely determine its transverse anisotropy [2]. In the Cr₇Mn MNM, a spin-1 system, there is a large tunnel splitting lifting the zero-field ground-state degeneracy. The two lowest states then have the structure of an atomic-clock transition in which the transition frequency is to first order independent of magnetic field, making the system largely immune to the decohering effects of external-field fluctuations. We find that the decoherence time T₂ is enhanced by a factor of three in the vicinity of this clock transition in Cr₇Mn.

[1] Y. Chen, et al., Phys. Rev. Lett. 117, 187202 (2016).
[2] C. A. Collett, et al., Phys. Rev. B 94, 220402 (2016).