This presentation will focus on the potential utility of mononuclear HoIII (4f 10) and TbIII (4f 8) molecular nanomagnets (MNs) encapsulated in polyoxometalate (POM) cages as molecular spin qubits. Multi-frequency continuous-wave (cw) and pulsed electron paramagnetic resonance (EPR) measurements on single-crystals demonstrate that the lowest-lying pair of Ligand-field (LF) states of these non-Kramers (i.e. integer moment) ions exhibit a series of optimal operating points, or atomic clock-transitions at which the quantum spin dynamics become protected against most sources of dipolar decoherence. Much of the presentation will focus on the HoIII system, where a significant magnetic anisotropy arises due to a splitting of the spin-orbit coupled total angular momentum (J = L + S = 8) ground state in the POM LF. The approximate D4d symmetry of the molecule results in a low-lying pair of mJ = ±4 singlets. High-frequency (~50 GHz) cw EPR studies reveal a highly anisotropic eight line spectrum corresponding to transitions between the mJ = ±4 states, split by a strong hyperfine interaction with the I = 7/2 Ho nucleus (100% natural abundance). Meanwhile, lower-frequency studies reveal a series of avoided level crossings (tunneling gaps) between the 16 [(2I + 1) × 2] lowest-lying electron-nuclear levels, close to zero-field, leading to a highly non-linear field-dependence of the low-frequency EPR spectrum [1]. Electron-spin-echo (ESE) measurements at X-band allow detailed studies of the coherent, coupled electron-nuclear spin dynamics, including Rabi oscillations, ESE-envelope-modulation (ESEEM), and evaluation of the phase coherence time T2. Remarkably long T2 times are found, even for the most concentrated samples, and at relatively high temperatures (5 K). Results are analyzed for different hyperfine transitions (i.e. different ΔmI selection rules) and [HoxLn1‑x] concentrations. The long T2 times are attributed to the non-linear field effects, which give rise to points in the EPR spectrum that are insensitive (to first order) to the applied field and, hence, immune to dipolar field fluctuations. These are the so-called atomic clock transitions. Finally, we note that the POM encapsulation offers the possibility of preserving the intrinsic properties of these lanthanide qubits outside of a crystal, e.g., on surfaces or in single-molecule devices [2]. [1] S. Ghosh et al., Dalton Trans. 41, 13697 (2012). [2] J. Lehmann et al., Nat. Nanotech. 2, 312 (2007).

Ligand-field engineering of atomic clock transitions in molecular spin qubits

249th National Meeting (2015)

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